Application of Copper Indium Gallium Selenide Thin-Film Solar Technology in Green Retrofitting of Aging Residential Buildings

Abstract

The growing imperative for sustainable building retrofits has spurred significant interest in advanced photovoltaic (PV) solutions. This study evaluates the feasibility and competitiveness of incorporating CIGS thin-film photovoltaic (PV) modules into retrofit projects for aging buildings. By combining qualitative analyses of market and environmental factors with a quantitative multi-criteria index model, this research assesses CIGS performance across five critical dimensions: aesthetic, economic, safety, energy saving, and innovation. The weights assigned to each criterion were determined through expert evaluations derived from structured focus group discussions. The results demonstrate that CIGS exhibits substantial strengths in aesthetic, economic, safety, energy saving, and innovation while maintaining reasonable economic feasibility. The quantitative assessment demonstrated that CIGS thin-film solar cells received the highest overall score (88.92), surpassing silicon-based photovoltaics (86.03), window retrofitting (88.83), and facade cladding (82.21) in all five key metrics of aesthetics, economic feasibility, safety, energy efficiency, and innovation. The findings indicate that CIGS technology exhibits not only exceptional visual adaptability but also attains balanced performance with regard to environmental and structural metrics. This renders it a highly competitive and comprehensive solution for sustainable building retrofits.

1. Introduction

1.1. Background and Purpose of the Study

In the ongoing efforts to combat climate change, there has been a notable shift from the Kyoto Protocol to the Paris Agreement framework. In 2021, as the global economy recovers from the impact of the new crown epidemic, it is projected that global carbon dioxide emissions will increase by 1.5 billion tons, marking the second largest annual increase in history [1]. In response to this, many developed countries have promised to set more ambitious greenhouse gas emission reduction targets. South Korea, for its part, has submitted to the United Nations a target of reducing greenhouse gas emissions by 24.4% by 2030 compared to 2017. It is estimated that greenhouse gas emissions from the construction sector in South Korea will reach 197.2 million tons by 2030. In response, the South Korean government has established a national carbon emission reduction target (CERT), with the objective of reducing greenhouse gas emissions from the construction sector by 32.7% by 2030 [2]. This is indicative of the pivotal function of buildings in the nation’s decarbonization strategy.

In South Korea, the construction sector is responsible for a considerable proportion of energy consumption and carbon emissions. Indeed, over 75% of buildings in Seoul were constructed more than 20 years ago [3]. The absence of modern energy-saving facilities in older high-rise residential apartments, built decades ago, often results in suboptimal thermal performance and elevated energy consumption. The retrofitting of these structures with green technologies is imperative to address the disparity in energy efficiency between old and new buildings. This study aims to explore the application of CIGS thin-film solar cell technology in the green retrofit of old residential buildings, with the objective of improving energy performance and integrating on-site renewable energy generation. Such retrofits are expected to contribute to national carbon emission reduction targets by reducing greenhouse gas emissions in the building sector.

1.2. Research Problem, Objectives, Scope, and Method

A substantial corpus of research has corroborated the efficacy of green retrofits in enhancing the energy efficiency of buildings [4,5]. However, the majority of these studies have centered on energy comparisons and economic feasibility analyses [6,7,8], including the enhancement of envelope insulation [6,9], the replacement of windows [10,11], and the optimization of mechanical and electrical (MEP) systems [12,13]. Concurrently, a number of studies have also explored the impact of improving the indoor environment on living comfort, though the majority are constrained to public or residential facilities [14]. In contrast, studies on green retrofits for private sector buildings are scarce, especially in terms of the integrated application of new renewable energy technologies.

Current government green retrofit policies mainly target public buildings, with very few involving private residential and commercial facilities. In terms of renewable energy systems, solar cell technology used in existing buildings is almost monopolized by silicon-based solar cells. Conversely, the utilization of CIGS (copper indium gallium selenide) thin-film solar cells remains limited in existing buildings due to their nascent state in terms of application. The objective of this paper is to investigate the potential for CIGS thin-film solar cells in green retrofits within the private sector, with a particular focus on enhancing the sustainability performance of buildings through a systematic integration approach that does not necessitate destructive reconstruction.

Whilst traditional green retrofitting methods have been successful in enhancing energy efficiency, they have also exhibited limitations with regard to the utilization of renewable energy. In recent years, building-integrated photovoltaic (BIPV) systems have emerged as a pivotal avenue for the advancement of green buildings. BIPV systems involve the integration of solar modules into the building envelope (e.g., facades, roofs, or windows), thereby achieving a symbiotic integration of the building’s exterior maintenance structure and clean energy systems. However, the rigidity and thickness of traditional silicon-based photovoltaic modules limit their flexible deployment on complex facades and high-rise buildings, especially in the renovation of the facades of old buildings.

In contrast, CIGS thin-film solar cells have significant advantages such as flexibility, light weight, and high light absorption. These cells have the capacity to adapt to a variety of facade conditions, and they can be directly attached to windows, guardrails, and exterior walls without changing the building structure. This ability to integrate seamlessly into existing structures offers a multifaceted solution for enhancing the performance of green renovation projects across multiple dimensions. The feasibility and adaptability of CIGS thin-film solar cells in the green renovation of private buildings is verified through a systematic evaluation of their aesthetics, economy, safety, energy efficiency, and innovation.

The objectives of this study are as follows: (1) to assess the market competitiveness of CIGS thin-film solar technology in the field of green building retrofits using the Porter’s Five Forces Framework model; (2) to develop an index-based evaluation model that includes key performance indicators (aesthetics, economy, safety, energy conservation, and innovation) for comparing CIGS solar cells with traditional solutions; and (3) to quantitatively assess CIGS performance relative to that of silicon solar cells, windows, and exterior wall cladding materials commonly used in apartment retrofits. The scope of the study is limited to high-rise residential buildings in an urban environment. The focus is on retrofitting the exterior walls and windows, including the application of solar technology to the outside of buildings rather than upgrading the interior or mechanical systems.

2. Literature Review

2.1. Policy Trends in Green Renovation in Korea and the World

2.1.1. Korea

In order to reduce greenhouse gas emissions, the South Korean government is implementing a project at the central and local levels to support green retrofits. The policy will be applicable to private buildings from 2025 onwards. Given that 74% of existing buildings are designated as “old buildings”, the market for green retrofits is projected to expand in conjunction with the renewable energy sector. As of 2024, private apartments with over 30 households will be required to obtain ZEB (zero-energy building) level 5 certification, and as of 2025, private apartments with an area exceeding 1000 square meters will be subjected to the same certification requirement [15].

• Private Building Green Remodeling Loan Interest Support Project;
• Support for public buildings owned by public institutions more than 10 years after the completion of construction;
• Pre-investigation and consulting support for institutions willing to participate;
• More than 1000 square meters of private buildings to be completed after 2025, complying with ZEB grade 5.

2.1.2. The EU

The European Union has developed the concept of the Energy Performance Directive (EPBD) and Zero Energy Building (ZEB) with the aim of reducing the energy consumption of buildings. According to the revised version of the Economic Development Institute, EU member states are obligated to ensure that new construction occupied and owned by public institutions becomes NZEB by 31 December 2018, and that all new buildings become NZEB by 31 December 2020. In addition, each Member State acknowledges the significance of renovating existing structures to achieve energy efficiency and carbon neutrality objectives. The EU is obligated to adhere to the NZEB requirements for new buildings as defined at the regional level among Member States [16].

• EU Member States comply with NZEB requirements for new buildings defined at the regional level;
• After renovation, the building’s primary energy consumption decreased by 75% compared to its previous state;
• Global Buildings Performance Network (GBPN) energy consumption for ventilation energy consumption of heating/cooling, hot water, and building aids is less than 50–60 kWh/m² per year;
• Do not release less than 3 kg CO₂/m²/year [17].

2.1.3. The USA

In 2019, the United States accounted for USD 412 billion in energy expenditures for all buildings, constituting 40% of the nation’s total energy consumption and 73% of its electricity consumption. The U.S. Department of Energy (DOE) has established a target for the energy efficiency of buildings that will reduce the average energy use intensity of all U.S. buildings by 30% by 2030 compared to 2010 [18]. The U.S. Green New Deal policy encompasses a broad spectrum of areas, including the green remodeling of 4 million existing buildings over a four-year period, encompassing business buildings, warehouses, and public buildings, the reinforcement of the insulation performance of 2 million houses, the preservation of wetlands from disasters, and the restoration of green spaces.

• Green remodeling of 4 million existing buildings over four years, reinforcing the insulation performance of 2 million houses;
• Subsidies for upgrading high-efficiency home appliances and installing high-performance windows;
• Legislation of net zero carbon emissions for new commercial buildings by 2030;
• Improve indoor air quality of national and public school facilities, strengthen energy efficiency, and build climate resilience;
• Supply 1.5 million public housing units with guaranteed high-efficiency energy performance [19].

2.1.4. Japan

In the context of Japanese architecture, the terms “remodelling” and “renovation” are often used interchangeably. A significant number of buildings constructed during Japan’s economic boom of the late 1980s and early 1990s are now experiencing the effects of aging. In response, the Japanese government has articulated a strategic objective to expand the market to KRW 200 trillion by 2025. The government has set itself the ambitious target of reducing greenhouse gas emissions by 46% by 2030 compared to 2013, with a further 50% reduction [20]. The government has proposed architectural design guidelines for the rational use of energy in housing and has initiated the formulation of regulations in a comprehensive field. From 2020, all new homes must adopt a zero-energy system. Between 2002 and 2008, the government allocated KRW 150 billion to support the cost of BEMS, and energy conservation was made mandatory within three years of its introduction [21].

2.2. Challenges Facing Aging Residential Buildings in Korea

In addition, of the 7 million buildings in Korea, 3.96 million buildings have been completed for more than 20 years, accounting for 56.9%, and the demand for green renovation is increasing as the aesthetics of buildings deteriorate due to aging. A distinctive feature of the Korean context is the significant disparity in energy efficiency between older and newer buildings, attributable to the rapid evolution of energy efficiency standards [22]. Renewable energy sources, such as solar and wind power, hold immense potential for universal application in building energy systems. However, the utilization of wind power is constrained due to limitations imposed by Korea’s geographical and climatic conditions. In the field of building design, there is a clear preference for renewable energy systems utilizing solar energy. In addition, high-rise multi-family housing constitutes the predominant residential type in Korea, accounting for 61.4% of all households, and population density is predominantly concentrated in major metropolitan areas, with 50.1% of the total population residing in the metropolitan area. Consequently, multi-family housing is designed with high density, and the available space for the installation of new and renewable energy systems, such as parking lots, building exteriors, and rooftops, is severely constrained [23]. The Korean climate is characterized by four distinct seasons, with temperature variations ranging from −7 °C to 30 °C. During summer, the climate is marked by extreme heat and humidity, while winter brings extreme cold and dry conditions. This seasonal and climatic variability leads to a strong preference for south-facing houses in the Korean context, reflecting a cultural and environmental sensitivity to the optimal use of sunlight and energy [24].

As demonstrated in the above

Table 1. Average climate data for Korea (1991–2020) [30].

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Average temperature (°C)	−1.9	0.7	6.1	12.6	18.2	22.7	25.3	26.1	22.6	15	7.5	0.2
Daylight hours (h)	169.6	170.8	198.2	206.3	223.0	189.1	123.6	156.1	179.7	206.5	157.3	162.9
Precipitation (days)	6.1	5.8	7	8.4	8.6	9.9	16.3	14.7	9.1	6.1	8.8	7.8
Humidity (%)	56.2	54.6	54.6	54.8	59.7	65.7	76.2	73.5	66.4	61.8	60.4	57.8
Amount of snow (days)	7.1	5.1	2.8	0.2	0	0	0	0	0	0	2.3	6.4
Heat wave (days)	0	0	0	0	0.1	0.7	4.1	5.9	0.2	0	0	0
Fog (days)	0.7	0.9	0.9	0.9	1	1.2	2	0.3	0.3	0.2	1.2	0.7
Wind velocity (m/s)	2.3	2.5	2.7	2.7	2.5	2.2	2.2	2.1	1.9	2.2	2.2	2.3
Cold wave (days)	12.8	11.6	5.9	0.7	0	0	0	0	0	1.5	8.7	13

, the average climate of Seoul can be determined, and the data can be used to inform decisions regarding energy demand, the optimal timing for the introduction of new and renewable energy sources, and the potential for environmentally sustainable renovation [25]. Of the two CO₂ applications, buildings are one of the major energy consumers. A substantial body of research by GJ Levermore has revealed that over one-third of global CO₂ emissions are attributable to the combustion of fossil fuels for the purpose of meeting the energy demands of buildings [26]. In particular, Seoul, the capital of Korea, has a significant number of buildings that are more than 20 years old. In 2020, 42.3 million buildings were more than 20 years old, accounting for 75.4% of the total 59 million buildings [3]. Furthermore, the energy efficiency of most buildings designed before 2001 was not given consideration during construction due to the lack of relevant regulations at the time [27]. In accordance with the first Green Building Basic Plan (Ministry of Land, Infrastructure and Transport), a Green Building Basic Plan is established every five years to promote the creation of green buildings [28]. This involves the establishment of objectives and the direction of their implementation. In order to fulfil the obligation to reduce greenhouse gases, the first Green Building Basic Plan established and implemented standards for improving the performance of each part of the building. After 15 years, building permits and construction areas gradually decrease, while old buildings more than 20 years after completion account for 58.2% of all buildings. On a particular note is the finding that approximately 37% of buildings constructed 30 years ago, which had weaker insulation levels than today, account for approximately 37% of all buildings and have a significant impact on energy use in the building sector [29].

As illustrated in

Table 2. Classification of Seoul’s housing status by aging time in architecture [35].

Old Age of Buildings	Type of House	2015	2016	2017	2018	2019	2020
20 to less than 30 years	Sum	799,927	803,062	825,790	794,865	791,237	835,901
	detached house	146,537	148,713	147,480	137,760	123,743	107,657
	Apartment	418,161	428,085	451,450	433,254	455,777	520,612
	Townhouse	45,636	48,638	52,022	54,722	50,961	49,594
	Multiplex housing	176,350	165,121	162,107	157,406	150,483	149,293
	Non-residential house	13,243	12,505	12,731	11,723	10,273	8745
More than 30 years	Sum	373,416	416,145	428,331	508,928	551,001	588,320
	detached house	155,798	152,684	147,542	150,326	157,280	165,735
	Apartment	163,553	185,417	196,539	265,298	290,335	307,366
	Townhouse	29,448	32,423	33,439	34,577	37,808	39,725
	Multiplex housing	16,119	36,789	40,897	47,785	53,508	62,198
	Non-residential house	8498	8832	9914	10,942	12,070	13,296

, residential apartments represent the predominant category of housing. A comprehensive consideration of the factors influencing the energy independence of buildings is imperative. Previous studies have indicated that the facade, defined as the exterior of a building, exerts a substantial influence not only on the building’s energy supply but also on its visual aesthetics for occupants [31]. The facade represents a technically challenging, multifaceted, and interdisciplinary element of a building. From the perspective of building design, facades represent the most significant components in terms of aesthetic values and architectural expression. From an engineering perspective, facades play a pivotal role in safeguarding indoor thermal conditions and enhancing the sustainable performance of buildings [32]. Additionally, the direction, initial plan, height, window, floor, material, and circulation space of the building exert a substantial influence on energy efficiency [33]. According to a press release by the Ministry of Land, Infrastructure and Transport, residential buildings approved for use within the last 10 years have shown a 23% decrease in energy consumption (215 kWh/m²/year → 166 kWh/m²/year). This is in comparison to buildings approved for use prior to 1979, which saw a 36% decrease (225 kWh/m²/year → 144 kWh/m²/year) in energy use per unit area [34], as detailed in Figure 1.

The energy performance of older high-rise residential apartments, constructed decades ago, is often suboptimal due to the absence of contemporary energy-saving facilities, resulting in inadequate thermal performance and elevated energy consumption. The retrofitting of these structures with green technologies is imperative to address the disparity in energy efficiency between older and newer buildings.

2.3. Utilization of CIGS Thin-Film Solar Cells

CIGS (copper indium gallium selenium) solar cells have been shown to possess a number of advantageous properties, including strong light absorption, high power generation capacity and stability, low production costs, and a short energy recovery period. Consequently, there has been rapid advancement in various thin-film solar cell production technologies [36]. Concurrently, energy efficiency is also increasing. According to the German Solar and Hydrogen Energy Research Institute (ZSW), the laboratory efficiency of CIGS thin-film solar cells in 2021 was reported to be 21.7% [37]. This efficiency has been further enhanced to 23.4% by the Semiconductor Research Institute of the Chinese Academy of Sciences. The efficiency of the CIGS thin-film solar cell is a subject of ongoing research and review [38]. Presently, the laboratory efficiency of 23.5% is reported to be the highest to date [39], as detailed in Figure 2.

The visual elements and overall design of the product must not be overlooked. While silicon solar cells are predominant in the market, thin-film batteries exhibit superior overall characteristics, including flexibility in design, the capacity to accommodate arbitrary shapes and sizes, and exceptional curvature. Furthermore, the integration of color and transmittance into the design can enhance its harmonious integration with the surrounding architecture. In contrast to silicon solar cells, which exhibit a fixed transparency, the ability to adjust the transparency of thin-film batteries enables light to penetrate the room when installed in a window, thereby facilitating natural illumination [40].

2.3.1. The Stability of the Different Generations of Solar Cells

First generation: Crystalline silicon (c-Si) solar cells are characterized by their long-term stability. Their typical power degradation rate is less than 1% per year [41]. In fact, c-Si modules typically experience a loss of approximately 2–3% of their output in the initial one-year period, attributable to photo-induced degradation. Subsequent to this, the annual power loss typically ranges from 0.5% to 0.7% [42]. Consequently, a high-quality silicon panel is expected to retain approximately 95% of its initial power output after a period of five years and approximately 90% of its power output after a period of ten years. Most manufacturers provide a guarantee that after 10 years of operation, the power output of a silicon panel will be no less than 90% of its initial capacity, and after 25 years of operation, the power output will be no less than 80% [43].

The stability of silicon can be attributed to its well-established technology and durable packaging. However, some degradation modes can still occur over decades (for example, light-induced degradation (LID) in boron-doped cells, potential-induced degradation (PID) under high voltage stress, and package agent browning) [44]. It is noteworthy that the initial generation of c-Si cells has established a benchmark, demonstrating a lifespan that exceeds 25 years.

Second generation: The stability of thin-film solar cells is approaching that of silicon solar cells; however, some technical details still require improvement. For instance, the annual degradation rate of cadmium telluride (CdTe) modules is approximately 0.5%/year. However, with effective management of the copper content within the back contact and the implementation of robust packaging methodologies, it is anticipated that CdTe panels will be capable of preserving approximately 90% of their initial output capacity following a 10-year period and approximately 80% following a 20-year period, akin to c-Si panels. Copper indium gallium selenide (CIGS) technology also demonstrates excellent stability. The median power loss rate of contemporary CIGS modules is low, at approximately 0.5% per year, with the majority of modules demonstrating a power loss rate between 0% and 1% per year. It has been observed that some modules exhibit an initial performance increase due to the light soaking effect. Long-term field studies of CIGS have confirmed that if moisture ingress can be prevented and the module packaging can be kept intact, the efficiency degradation will be minimal [45]. In comparison, amorphous silicon (a-Si) thin-film cells exhibit reduced stability, being susceptible to the Staebler–Wronski effect, a light-induced metastability that can result in an initial efficiency drop and a total power loss of typically 10–15% over a period of 10 years. Multijunction amorphous silicon designs have been developed to reduce this initial photodegradation; however, long-term stability remains inferior to that of crystalline silicon [46]. In summary, second-generation thin films (in particular, cadmium telluride and copper indium gallium selenide) can now achieve a service life of 20–30 years with minimal power loss, while amorphous silicon requires careful design to achieve a similar service life.

Third generation: Photovoltaic technologies in this category have been shown to exhibit high efficiency; however, the stability of these technologies remains a challenge. In particular, the long-term stability of perovskite solar cells (PSCs) is yet to be demonstrated. Under continuous light and elevated temperatures, a typical perovskite cell will rapidly degrade due to inherent instabilities (e.g., ion migration and phase separation) and external factors (e.g., moisture, oxygen ingress, and UV irradiation). The stability metrics reported thus far indicate that even the most stable perovskite devices exhibit a decline in performance of 80% over a period of between 1000 h (approximately 42 days) and 10,000 h (approximately 14 months), even in specialized 2D/3D combinations [47]. While these figures represent a marked improvement, they are still an order of magnitude less than the estimated 200,000 h (25–30 years) that silicon photovoltaics are expected to last [48]. Additionally, the operational lifespan of organic photovoltaic (OPV) cells is significantly constrained by their inherent vulnerability to rapid degradation. Early OPV devices experienced a 50% reduction in output power after months of outdoor exposure, and their lifetimes were usually less than two years. The instabilities experienced by OPV cells are attributed to photo-oxidation of the organic layer, moisture intrusion, and electrode reactions. However, recent advances in materials science, particularly the development of non-fullerene acceptors, and improvements in device packaging have led to significant enhancements in the stability of OPV [49]. While the power loss of third-generation cells continues to exceed that of first- and second-generation cells, the discrepancy is gradually diminishing, owing to advancements in stability-related factors, such as the utilization of advanced packaging materials, interface engineering, and the incorporation of robust inherent materials.

Figure 3 provides a visual classification of photovoltaic technologies into three distinct generations.

The incorporation of

Table 3. The stability of the different generations of solar cells.

Generation	PV Technology	Power Loss After 1 Year	After 5 Years	After 10 Years	${\mathbf{T}}_{80}$ (Time to 80% of Initial)
First	Crystalline silicon	~2–3% initial light-induced loss, then ~0.5%/year (~3% total) [42]	~5% (can be <5% with high quality) [42]	~8–10% (warranted max ~10%) [47]	~25–30 years (≈200,000 h) to 80% output [47]
Second	a-Si	Significant initial LID: ~2–5% in first year [46]	~7–8% (initial + ~1%/year) [46]	~12–15% (after 10 years) [46]	~10–15 years (unassisted) (Can reach 20+ years with multi-junction designs and light-soaking) [46]
	CIGS	~0% loss (some light-soaking gain initially) to at most ~1% in first year [45]	~2–3% (at ~0.5%/year typical) [45]	~5% (at ~0.5%/year) [45] (many modules ≥95% even at 10 y)	~25+ years (often >80% even at 25 y) [45]
	CdTe	~1–5% loss in first 1–2 years (e.g., Cu back-contact diffusion causes ~4–7% over 2 years) [45]	~5–7% (after initial stabilization, ~0.5–0.7%/year) [45]	~10–12% (median case) [45] (can be lower with improved design)	~20–25 years (field data shows ≥80% at 20 years) [45]
Third	Perovskite	Rapid decay: often >20% loss in weeks to months (unencapsulated) ${\mathrm{T}}_{80}$ typically within hundreds of hours [47]	Device usually fails before 5 years (encapsulated lab cells might last ~1–2 years max)	N/A (no sustained performance at 10 y without replacement)	${\mathrm{T}}_{80}$~0.1–1 year (≈103–104 h in best reports) [36]—far below the >20-year goal [48]
	Organic (OPV)	Unprotected: >20% loss in <1 year (rapid photo-oxidation) [49] Encapsulated: can achieve <5% loss in 1000 h (~6 weeks) [50]	Unprotected: cell often non-functional by ~2–3 years [49] Encapsulated: projected ~10% or less by 5 years (per extrapolated 30-year data) [50]	Encapsulated prototypes maintain ~80–90% up to 10 years (estimated) [50]	${\mathrm{T}}_{80}$ highly variable: a few months for older OPVs, now ~6–8 years (~50,000–70,000 h) extrapolated with state-of-the-art stabilizations [50]

, which seeks to furnish a systematic comparison of the long-term stability characteristics of first-, second-, and third-generation photovoltaic technologies, was pivotal in the selection of CIGS thin-film solar cells as the focal point of this study. This finding indicates that, in contrast to emerging third-generation technologies such as chalcogenide and organic photovoltaics, which demonstrate considerable instability over brief periods, CIGS modules exhibit an average annual decay rate of approximately 0.5%, a figure that is analogous to the reliability of crystalline silicon. This stability is a pivotal factor in building-integrated photovoltaic (BIPV) applications, where long-term durability and consistent output are paramount.

2.3.2. Principle of Power Generation of Bifacial CIGS Thin-Film Solar Cells

Copper indium gallium selenide (CIGS) technology has historically been utilized in single-sided module architectures. However, recent advancements have rendered the fabrication of bifacial CIGS solar cells feasible by integrating a transparent back contact and a double-glass package. This hybrid architecture integrates the high absorption efficiency and flexibility of CIGS films with the enhanced light collection capabilities of a bifacial system, thereby facilitating the generation of electricity from both direct and reflected light. In building-integrated photovoltaic (BIPV) applications, such as vertical facades or skylights, bifacial CIGS modules are particularly advantageous because they can collect ambient light from multiple angles while maintaining aesthetics and architectural compatibility.

Bifacial photovoltaic (Bifacial PV) technology functions in a manner analogous to traditional single-sided photovoltaic (mPV) technology, with both types of technology being founded on the photoelectric effect. In comparison with single-sided photovoltaic cells, the back surface field (BSF) structure located on the back of the cell is eliminated, and instead, an anti-reflective coating and a back electrode contact layer are employed. This configuration enables the back surface field (BSF) structure to absorb sunlight from both the front and back directions concurrently, as illustrated in Figure 4. When the photovoltaic cell is exposed to sunlight, light passes through the anti-reflective coating from both sides of the cell and enters the cell. Photons above the material’s bandgap are known to transfer some of their energy to electrons, which subsequently form electron–hole pairs [51]. In the event that these carriers are generated in close proximity to the depletion region in the semiconductor, they will not undergo recombination but will instead diffuse to the base and emitter regions. Under the influence of the built-in electric field, the carriers will be attracted to their respective regions—electrons to the N-type semiconductor and holes to the P-type semiconductor. Consequently, a voltage potential is established between the anode and cathode of the cell. Once a circuit is formed on both sides of the photovoltaic cell, electrons can flow via an external load, thereby generating current [52].

2.4. The Application Technology of Green Remodeling in Korea

The technical elements applied to green remodeling in Korea were examined. The main technologies for green remodeling of old buildings include roofing, waterproofing, windows, exterior wall insulation, and structure. Window replacement and exterior wall insulation are essential technologies for green remodeling. Depending on the region, technologies such as replacing heating and cooling equipment, repairing roofs, and strengthening seismic design are sometimes used [53].

2.5. Application of Solar Cells in Korea

Solar cells were widely distributed in line with the government’s supply of new and renewable energy. The Seoul Metropolitan Government has been supplying solar mini power plants since 2011. Energy efficiency can be improved in a short period of time through low amounts and government subsidies compared to remodeling. Veranda-type solar cells are the most widely distributed around apartments. However, due to reckless installation, there are also problems with stability and urban beauty [54]. In particular, the lower floors of apartments are hidden by apartments on the other side, so they cannot receive solar heat efficiently. In the case of apartments, as demonstrated in Figure 5, solar cells are installed on balconies, rooftops, and public facilities. In Korea, the maximum efficiency of solar cells is 30 degrees from the angle of inclination to the south, and since the sun is tilted from the center of the sky to the south, considering the latitude of Korea, the module must be erected at 35 degrees to receive solar energy vertically [55].

3. Materials and Methods

3.1. Research Design and Framework

In order to comprehensively analyze the competitive advantages and application prospects of CIGS thin-film solar cells in building renovation, this paper adopts a research strategy that combines Porter’s Five Forces Framework and the exponential model, supplemented by a mixed research method that parallels positivism and interpretivism. Initially, the Five Forces Framework was employed to methodically ascertain the competitive structure of the industry environment in which CIGS is situated, encompassing competitive rivalry, threat of new entry, threat of substitutes, supplier power, and buyer power. Nevertheless, as a qualitative analysis instrument at the strategic level, the Porter Model possesses inherent limitations in elucidating the technical performance and practical application advantages of CIGS technology.

Consequently, this paper proposes an index modelling method, beginning with the core pain points and differentiation elements identified in the Five Forces Framework and summarizing and constructing five key capability dimensions: aesthetics index, economic index, safety index, energy saving index, and innovation index. Experts were invited to conduct the first round of applicability confirmation. Subsequently, the author conducted desktop research, reviewing the relevant literature published between 2000 and 2024, with the aim of extracting core indicators and features related to building retrofit performance and solar material evaluation.

After identifying the performance indicators applicable to building retrofits, focus group discussions were conducted to assess the level of importance of each indicator. Through expert consensus, the key indicators were summarized and classified into the five main dimensions (aesthetics, economy, safety, energy savings, and innovation) mentioned above and included in the subsequent index modelling analysis. Finally, a weighted scoring model was used to quantitatively compare CIGS thin-film solar cells with other alternative materials, with the aim of verifying their applicability and competitive potential in the practice of green building retrofits.

3.2. Porter’s Five Forces Framework

Porter’s Five Forces Framework is employed as a preliminary analytical instrument to qualitatively assess the market attractiveness and competitiveness of CIGS thin-film solar technology in the building retrofit sector. The qualitative description is primarily derived from the following factors: (i) the threat posed by existing competitors, (ii) the threat of substitutes, (iii) the bargaining power of suppliers, (iv) the bargaining power of buyers, and (v) the threat of new entrants [56]. The framework is widely used to assess the attractiveness of an industry and the balance of power between the various participants [57]. This approach facilitates the identification of opportunities and challenges posed by new technologies.

3.2.1. Construction of a CIGS Performance Index System Based on Porter’s Five Forces Framework

The construction of a five-dimensional performance index system based on Porter’s Five Forces Framework has been undertaken. This system maps the five forces to the corresponding performance indicator dimensions. The correspondence and logic between each of the Porter’s Five Forces and the performance index are as follows:

• Threat of new entrants → innovation index: The threat of new entrants depends on the level of barriers to entry. These barriers encompass technology patents, capital requirements, and regulatory approvals for integrated building products. As governments worldwide have introduced incentives for green buildings, the barriers to entry have gradually decreased. Consequently, CIGS manufacturers must persist in fortifying their competitive edge through technological innovation and distinctive market positioning.
• Supplier influence → safety index: The bargaining power of suppliers has a direct impact on the market competitiveness of CIGS solar technology. The number of companies in South Korea that can provide the core technology of CIGS thin-film solar cells is extremely limited, resulting in a heavy reliance on foreign imports for raw material supplies and vulnerability to fluctuations in the international market [58].
• Buyer power → economic index: The impact of buyer power on the market penetration of CIGS thin-film solar cells is significant. Given the nascent stage of CIGS technology, the lifecycle and long-term benefits remain to be elucidated. The high installation cost has led to the primary purchasing groups being governments, state-owned enterprises, and private enterprises, while households and individual users tend to opt for more economical alternatives.
• Threat of substitutes → energy saving index: The threat of substitutes primarily originates from technologies and solutions that can meet similar needs. CIGS thin-film solar cells, for instance, have a variety of alternatives. These alternatives may include silicon-based CdTe, GaAs, and other technologies, as well as applications in fields such as windows and exterior wall materials. If these alternatives prove to be more cost-effective, possess superior quality, and result in lower user costs, competition is expected to intensify. To this end, CIGS products must possess substantial energy-saving advantages, including but not limited to annual power generation energy savings, carbon emission reductions, and energy independence.
• The intensity of competition among existing companies in this field is a primary factor. The most salient factor influencing the competitiveness of CIGS cells is the rivalry with preexisting competitors, encompassing silicon solar cells, other thin-film solar cells, windows, and exterior materials. The global CIGS market is dominated by a few mainstream manufacturers such as Solar Frontier (Tokyo, Japan), Avancis (Torgau, Germany) and MiaSolé (Santa Clara, CA, USA). And the South Korean domestic solar cell market has undergone a significant transformation due to the influx of inexpensive Chinese solar cells, resulting in intensified competition within the market. Competition is also influenced by the stage of technological development. The application of CIGS in buildings is still in the introductory stage, while silicon photovoltaics has reached a mature stage. This dynamic suggests that CIGS exhibits greater flexibility and aesthetic adaptability compared to crystalline silicon technology, indicating its strategic potential for integration into building facade applications. The strategic positioning of CIGS companies hinges on their ability to establish unique advantages in specific application domains, such as distinct aesthetic differentiation and seamless design integration. The successful establishment of these competitive market segments is predicated on the ability to leverage these unique strengths.

The foundation of our study is the Five Forces analysis, a comprehensive framework that offers a detailed assessment of the competitive landscape for CIGS thin-film solar cells. However, it is important to note that Porter’s Five Forces Framework has limitations and is a competitive analysis model based on a relatively fixed industry size. CIGS thin-film solar cells are in their infancy and require government support and private sector cooperation to secure additional resources and markets, as shown in Figure 6.

3.2.2. Applicability of Porter’s Five Forces Framework

Daniel et al. (2021) emphasized that “for most prospective Delphi studies, five to eight initial experts should be sufficient”, particularly in exploratory or early modeling efforts, where the depth of expertise is more significant than the number of experts [58]. A number of studies have also reported on the use of expert panels comprised of five members. When considered as a whole, the established precedent and the internal consistency of this methodology indicate that a panel of five members is both sufficient and appropriate, particularly in a study focused on a specific area like this one, where the quality of expert insight is paramount [59,60,61].

First, in the initial round of the Porter’s Five Forces model, a group of five experts with a minimum of 10 years of experience in the building and solar industries was invited to participate. This group included two engineers with over 20 years of experience in construction companies, one architect with 15 years of experience in the design of BIPV buildings, one professor of architectural engineering with more than 18 years of experience in academia, and one researcher with 12 years of experience in solar energy technology.

Five experts were invited to discuss the applicability of Porter’s Five Forces Framework in evaluating the competitiveness of CIGS thin-film solar cells. While some experts concurred that Porter’s Five Forces Framework, as a classic framework for industry competition analysis, can effectively assess the competitive environment of the solar window industry, such as supplier bargaining power, buyer bargaining power, threat of substitutes, threat of new entrants, and industry competition intensity, others expressed divergent views. However, this assertion is not universally accepted. In contradistinction, certain experts have posited an alternate viewpoint, contending that Porter’s Five Forces Framework utility is more aptly suited for macro-market competition analysis and that it may not be adequately equipped to directly assess the specific performance and value of individual green building technologies.

Following an exhaustive deliberation, the experts arrived at a consensus that Porter’s Five Forces Framework can be utilized as a reference for the macro-context of the study. However, it was determined that the model cannot directly inform the construction of specific evaluation indicators. Consequently, this study, drawing upon the Five Forces Framework, has refined performance dimensions that are more apt for technology evaluation. These refined dimensions include an aesthetic index, an economic index, an energy saving index, a safety index, and an innovation index.

3.3. Identifying Performance Indicators from the Literature

A four-stage approach was employed to review the literature for this study, following the protocol outlined by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). This approach entailed the following stages: (i) identification, (ii) screening, (iii) eligibility assessment, and (iv) inclusion. In the preliminary “identification” stage, the researchers employed the keywords “solar building”, “BIPV”, and “solar energy” to conduct a search of the Web of Science database, yielding a total of 2707 relevant publications. To further refine the search results, the research team filtered the results by subject categories such as “Green Sustainable Science Technology”, “Energy Fuels”, “Environmental Sciences”, “Construction Building Technology”, “Engineering Civil”, and other subject categories. They then combined these results with the Sustainable Development Goals (SDGs), including “07 Affordable and Clean Energy”, “13 Climate Action”, “11 Sustainable Cities and Communities”, “09 Industrial Innovation and Infrastructure”, and “06 Clean Water and Sanitation”. This process ultimately yielded 881 journal papers with high relevance.

In the subsequent stage of the screening process, researchers eliminated duplicate records, books, non-English literature, and publications for which full texts were not available. This procedure was implemented to ensure the rigor and accessibility of the included literature. Following this step, the number of remaining documents was 298.

In the third stage, which was designated as the “eligibility assessment”, researchers conducted a preliminary assessment of the relevance of the literature by reading the titles and abstracts of the papers. Papers that focused primarily on areas such as utility-scale photovoltaic power plants, solar water heating and PVT systems, nanophotothermal technology, grid integration, and other topics that were less relevant to the theme of building-integrated photovoltaics (BIPVs) were excluded. Consequently, 64 papers advanced to the fourth stage, entitled “inclusion”, where they underwent a more comprehensive review, extraction, and analysis of performance indicators.

The analysis of these studies revealed that the following indices were used as first-level indicators: the aesthetic index, the economic index, the energy saving index, the safety index, and the innovation index. A total of 34 second-level indicators were identified.

Table 4. Solar energy evaluation indicators.

Primary Indicator	Secondary Indicator	Meaning	Reference
Aesthetics	A1 Visible Light Transmittance	Transparency and potential for natural lighting are reflected by the percentage of visible light.	[62,63,64,65,66,67,68,69]
	A2 Color Rendering Index	The degree to which colors are reproduced (relative to a standard light source).	[64,65,69,70,71,72]
	A3 Unified Glare Rating	The degree of discomfort caused by direct or reflected sunlight.	[73,74,75,76]
	A4 Architectural Aesthetic Integration	The degree to which colors, patterns, and forms harmonize with the architectural design style and meet architectural aesthetic requirements.	[77,78,79]
	A5 Color Customizability	The ability to provide a variety of color or hue options without significantly reducing performance.	[80,81]
	A6 Visible Defect Rate	The frequency of visible defects (bubbles, cell grid lines, etc.)	[66,78,82]
Economic	E1 Initial Installation Cost	Initial investment cost per unit area (including materials and construction).	[72,83,84,85,86,87]
	E2 Energy Savings Costs	The annual reduction in building energy costs due to photovoltaic power generation and enhanced thermal insulation.	[86]
	E3 Payback Period	The length of time it takes to recoup the initial investment in energy savings.	[62,87,88]
	E4 Return on Investment	The percentage of net income generated by an investment relative to its cost over a specific period.	[72,89]
	E5 Lifecycle Cost	The total cost over the entire lifecycle minus the revenue from its power generation.	[90,91]
	E6 Levelized Cost of Energy	Average cost per kWh of electricity generated, considering lifecycle and total cost.	[81,88,91,92,93,94,95]
	E7 Property Value Appreciation	The installation of solar energy brings additional value to the building.	[63,82,90,95,96,97,98,99,100]
Safety	S1 Fire Resistance	Fire resistance and flame retardancy.	[101,102,103]
	S2 Impact Resistance	Ability to withstand impacts (e.g., hail, debris).	[102,104,105]
	S3 Wind Pressure Resistance	Maximum wind pressure that can be safely withstood without structural damage or excessive deflection.	[106]
	S4 Electrical Safety	Ability to protect against electrical hazards (e.g., insulation, grounding, arc protection measures).	[71,72,107]
	S5 Expected Service Life	Expected service life (in years) while maintaining performance and safety standards.	[79]
	S6 Thermal Safety (Prevent Overheating)	Ability to prevent excessive heat build-up within the system (e.g., through ventilation or temperature-controlled circuit breakers) to reduce the risk of fire or overheating.	[63,80,81,82,83,90,96,102,104,105]
Energy Saving	ES1 Photoelectric Conversion Efficiency	The proportion of solar energy converted to electricity under standard conditions.	[64,67,68,70,71,72,83,106,108,109,110,111,112,113,114]
	ES2 Annual Energy Output	The total amount of electricity generated per year (kilowatt-hours per square meter per year, depending on the local climate).	[63,74,82,84,86,87,88,90,92,93,96,97,100,115,116,117]
	ES3 Solar Heat Gain Coefficient	The lower the proportion of incident solar radiation that is used as heat inside the room, the lower the air conditioning load in summer.	[62,73,74,76,101,102,103,117,118,119,120]
	ES4 U Value (heat transfer coefficient)	The lower the heat transfer rate (W/m2·K), the better the insulation properties.	[77,79,118,121]
	ES5 Anti-glare	The ability to introduce sufficient light while reducing glare, usually achieved by embedding patterns or coloring.	[89,96,99,108,122,123]
Innovation	I1 Intelligent Dimming Control	Integrate intelligent dimming technologies, such as electrochromism and liquid crystal dimming, to adjust transparency as needed.	[70,74,124]
	I2 Digital Twin Monitoring	Use digital twin technology to monitor the operation of the photovoltaic facade in real time, diagnose faults, and manage optimization.	[72,75,85,124]
	I3 Building Energy Pipe System Integration	Integrate with the building energy management system (BEMS) to achieve optimal linkage control of lighting and air conditioning.	[106,125]
	I4 Adaptive Ventilation Design	Introduce innovative ventilation structures (ventilated double-skin photovoltaic windows) into the design to enhance heat dissipation and performance.	[110,113]
	I5 Lightweight Flexible Components	Lightweight and flexible CIGS modules are used to facilitate installation and reduce structural loads.	[84,97,100]
	I6 Perovskite/CIGS Stacking Technology	Cutting-edge tandem cell technology (perovskite on CIGS) is used to improve efficiency and transparency.	[65,66,69,75,78]
	I7 Self-cleaning Coating	Hydrophobic/photocatalytic self-cleaning coatings are applied to reduce dust accumulation and maintenance frequency.	[94,125]
	I8 Bioaffinity Design Fusion	Biophilic design combining photovoltaic elements with greenery (e.g., photovoltaic-powered vertical green facades) to enhance well-being and innovation.	[68,88,91,92,95,100,103]
	I9 Module Customized Design	Flexible customization of photovoltaic module shape/size to suit various window designs (e.g., curved, triangular).	[62,63,64,65,67,69,70,71,72,73,74,77,80,81,82,83,84,90,92,93,96,97,98,99,101,102,106,107,108,109,110,111,114,115,116,118,119,120,121,122,123,124]
	I10 Energy Storage Integration	Combination with local battery or supercapacitor energy storage to achieve energy balance and improve energy resilience.	[112]

provides a comprehensive list of these studies along with their respective references.

3.4. Focus Group Discussions

Focus group discussions (FGDs) are considered a robust and versatile exploratory technique that has been extensively utilized within the extant literature [126]. This approach entails the collection of data on a particular subject or issue through the utilization of negotiation and co-production by a small group of individuals led by a facilitator (researcher). Among exploratory techniques such as structured and semi-structured interviews, focus group discussions (FGDs) are considered a superior technique because they collect and synthesize data through dynamic and interactive group discussions [127]. Consequently, focus group discussions (FGDs) enable the emergence of individual perspectives while concurrently offering insight into the shared cognizance and beliefs of the group [128]. Studies have shown that the optimum size of a focus group is usually between six and ten members [129].

In order to ensure a comprehensive and professional perspective, a team of ten experts from Korean architecture, engineering, and construction companies was recruited. The selection criteria stipulated that the experts should have a minimum of ten years’ experience in building renovation projects or solar energy integration, ensuring that they were familiar with the challenges and advantages of integrating photovoltaic systems into existing buildings. The composition of the expert group encompassed architects, project managers, and engineers, thereby ensuring representation from diverse fields of related disciplines. The indicators identified in the literature as applicable to evaluating the performance of CIGS thin-film solar cells were grouped into four categories: aesthetics, economics, safety and energy savings, and innovation. These indicators were then compiled into a questionnaire and distributed to the participants one week prior to the focus group meeting, thus allowing them to preview and familiarize themselves with the content prior to the meeting discussion.

3.5. Merge and Rename

The objective of this paper is to establish a scientific, practical, and operationally feasible performance evaluation model for CIGS thin-film solar cells used in buildings. To this end, the paper systematically optimized and streamlined the original five dimensions comprising 34 secondary indicators based on a review of the previous literature and combined with the Focus Group Discussion (FGD) method. In conclusion, a novel performance evaluation system was devised, comprising five primary indicators and eleven secondary indicators.

This indicator system retains the capacity to express key performance dimensions while concomitantly enhancing the model’s adaptability and universality, thereby ensuring a more precise alignment with the practical requirements of architectural design, project evaluation, and engineering decision-making.

During the focus group meetings, experts thoroughly discussed the applicability and data acquisition difficulty of the original 34 indicators. There was a unanimous consensus that the original indicator system exhibited clear issues, including overlap, conceptual coupling, and terminological complexity, which were specifically evidenced as follows:

(1)

The presence of superimposed indicator data gives rise to the possibility of double counting during the assessment process;

(2)

Most indicators are highly technical (e.g., “U-value”, “LCOE”, “Digital Twin Monitoring”), making them difficult for non-technical personnel (e.g., architects, developers, and investors involved in discussions about safety indicators) to understand and participate in;

(3)

Excessive indicators will significantly increase the actual assessment manpower costs and data collection burden, reducing the efficiency of rapid scheme selection;

(4)

Too many sub-indicators result in a complex scoring system, increasing the difficulty of weight allocation and affecting model balance.

It is, therefore, recommended by experts that the retention of core variables with the highest distinguishability and sensitivity be prioritized through structural consolidation and redundancy removal. This is to ensure the integrity of system performance expression, thus establishing a more efficient and implementable performance evaluation system.

In the discourse on aesthetic indicators, the experts present at the meeting observed that there was considerable conceptual overlap among the original six indicators. For instance, the metrics of “A5 Color Customizability” and “A2 Color Rendering Index” both reflect the performance of the color reproduction, whilst “A1 Light Transmittance” and “A3 Unified Glare Rating” are related to the optical brightness of the display. The integrative characteristics assessed by “A4 Architectural Aesthetic Integration” can essentially be summarized by the broader “harmony” indicator. Furthermore, it has been determined that the “A6 Visible Defect Rate” is not a critical variable in practical applications.

In the course of the discussion on economic indicators, the experts present at the meeting reached a consensus that “E1 Initial Installation Cost” reflects the upfront one-time investment per unit area for system construction, including costs for materials, components, construction, and integration. This indicator is characterized by its clear definition and uniform measurement standards, thus establishing it as a fundamental variable for assessing the economic feasibility of BIPV systems. Consequently, it is maintained as an autonomous core item.

Despite the original indicator system’s omission of a “Maintenance Cost” item, experts reached a consensus that its incorporation was essential to reflect actual maintenance expenses during system operation. Such expenses encompass component cleaning, inverter replacement, and electrical safety inspections. The original “E3 Payback Period”, “E5 Lifecycle Cost”, and “E6 Levelized Cost of Energy” were all considered maintenance expenses in certain cases, so their contents can be consolidated into the “Maintenance Costs” item. Consequently, they will no longer be retained as independent indicators.

Expected revenue is the primary indicator under the economic dimension’s Revenue Side, consolidating the previously separate “E2 Energy Cost Savings”, “E4 Return On Investment”, and “E7 Property Value Appreciation” indicators. The primary parameters for assessing the profitability of such systems are the electricity cost savings generated by photovoltaic systems through self-consumption and feed-in tariff subsidies. In contrast to the enhancement of building value, the literature on the subject indicates that in countries such as the United States, green building incentives are predominantly manifested as fiscal returns, including tax credits or development intensity rewards. This stands in contrast to the more conventional market value appreciation typically observed in traditional economic models. Consequently, rather than establishing a discrete “Value Enhancement” category, it is more appropriate to assimilate it into the “Expected Revenue” scope for comprehensive consideration [130].

Whilst “S1 Fire Resistance” is imperative in safety assessments, it is crucial to note that all photovoltaic façade materials must comply with international safety standards (e.g., IEC 61,730 prerequisites rather than differentiated performance indicators). Consequently, they are, therefore, excluded from scoring [131].

Consequently, fire safety is regarded as a regulatory prerequisite as opposed to a differentiated performance indicator and is excluded from the scoring process.

“S2 Impact Resistance” and “S3 Wind Pressure Resistance” are classified as compressive strength because both involve the ability to recover from external environmental loads. The “S4 Electrical Safety” standard is classified as mechanical strength, reflecting the importance of safe system integration for the effective prevention of risk.

The “S5 Expected Service Life” is integrated with the “S6 Thermal Safety” to form the concept of crack propensity. This integration encompasses the collective impact of thermal fatigue, moisture ingress, and material degradation over an extended timeframe, thereby encapsulating the phenomenon of long-term durability deterioration.

In the context of energy efficiency indicators, “ES1 Photoelectric Conversion Efficiency” and “ES2 Annual Energy Output” have been demonstrated to exert a direct influence on the system’s power generation performance. According to experts in the field, ES2 emerges as a more comprehensive and environmentally focused indicator, while ES1, though important during design, is an embedded parameter already reflected in annual energy output. Consequently, only annual energy output is retained, with conversion efficiency regarded as a derived input. While “ES3 Solar Heat Gain Coefficient” and “ES4 U Value” serve as indicators of thermal performance, their indirect influence on cooling and heating loads, in turn, affects building-level carbon emissions. Consequently, they are classified under the overarching category of “Carbon Reduction”.

Despite the absence of ES indicators that directly define energy self-sufficiency, “ES2 Annual Energy Output” functions as the numerator for self-sufficiency:

$$\mathrm{Energy} \mathrm{Independence} (\%)={\displaystyle \frac{\mathrm{Annual} \mathrm{PV} \mathrm{Output}}{\mathrm{Total} \mathrm{Energy} \mathrm{Demand}}}\times 1$$

(1)

ES2 demonstrates compatibility with the objectives of “Energy Conservation” and “Energy Independence”, contingent upon the denominator data.

In the innovation indicators, experts underscored the complexity of the concept of innovation, emphasizing its sensitivity, multidimensionality, and temporal dynamics. They noted that a unidimensional, single-point assessment of technological novelty is inadequate to fully capture the value of innovation in the context of building-integrated photovoltaics (BIPVs). It is evident that a number of these technologies are of significance. For instance, certain technologies (e.g., series-connected solar cells, smart coatings, and real-time monitoring) exemplify technical maturity and immediate performance advantages, which are most aptly reflected through “Technological Progress”. It is evident that alternative technologies and design strategies (e.g., bio-integrated facades and module customization) demonstrate strategic flexibility, system-level synergies, and learning potential. These attributes are subject to evolution over time and display variability based on the specific application, necessitating an assessment approach, “Learning-Driven Progress”.

The structure of the revised performance indicator system is presented in

Table 5. Revised evaluation criteria.

Primary Indicator	Secondary Indicator
Aesthetics	A1 Color
	A2 Brightness
	A3 Harmony
Economic	E1 Initial Cost
	E2 Maintenance Cost
	E3 Expected Revenue
Safety	S1 Strength
	S2 Pressure Resistance
	S3 Cracking
Energy Saving	ES1 Energy Saving
	ES2 Carbon Emissions
	ES3 Energy Independence
Innovation	I1 Technological Progress
	I2 Learning for Progress

, which provides a comprehensive overview of the five primary and eleven secondary indicators.

4. A Porter’s Five Forces Model Analysis of CIGS Thin-Film Solar Cells

To translate the results of the expert ratings into the final evaluation system, the study employed a combination of ratings and weighting to integrate the expert opinions. In light of the culminating round of Delphi results, the mean importance score was computed for each Level 1 dimension. The proportion of weight for each dimension in the evaluation system was subsequently ascertained. For instance, if a dimension obtained the highest average score, a moderately elevated weight was allocated to reflect the experts’ perception of its heightened importance. The weights were computed using the ratio of the average score of each dimension to the total average score, ensuring that the sum of the five Level 1 indicators equaled 100%. Consequently, the mean importance of each first-level dimension’s sub-indicators was standardized to determine the weights of the second-level indicators. It is important to acknowledge that, given the Likert scale’s ordinal nature and the fact that the mean is merely considered an estimate, our primary focus was on the ranking and relative variations in expert opinions rather than on the precise numerical values. During the expert discussion, the decision was made to merge sub-indicators that exhibited high correlation or analogous meanings. Consequently, the indicators of the final weight allocation have become more independent of each other.

As indicated in the preceding focus group discussions, this study employed the expert scoring method to assign weights to each of the secondary indicators. The retrofit program competitors, silicon solar cells, windows and exterior materials, and CIGS thin-film solar cells were selected for comparison. The Analytic Hierarchy Process (AHP) is a structured multi-criteria decision-making method that breaks down complex decision-making problems into target levels, criteria levels, and sub-criteria levels. Through expert scoring, a pairwise comparison matrix is constructed, and Santy’s 1–9 scale method is used to calculate weights, thereby providing quantitative support for the construction and visualization of the performance evaluation model.

4.1. Index Modeling for the Competency of CIGS Thin-Film Solar Cell

An exponential modeling approach was selected to further analyze the capacity of CIGS thin-film solar cells. This decision was made based on the initial analysis of the Porter’s Five Forces Framework model. As indicated in the preceding focus group discussions, this study employed the expert scoring method to assign weights to each of the secondary indicators. The retrofit program competitors, silicon solar cells, windows and exterior materials, and CIGS thin-film solar cells were selected for comparison. The parameters selected for analysis include the aesthetic index, economic index, safety index, energy saving index, and innovation index. The hierarchical analysis method will be employed to conduct a detailed analysis. The Analytic Hierarchy Process (AHP) is a structured multi-criteria decision-making method that breaks down complex decision-making problems into target levels, criteria levels, and sub-criteria levels. The construction of a performance evaluation model is initiated with the implementation of an expert scoring system, which is then utilized to generate a pairwise comparison matrix. The subsequent calculation of weights is executed through the implementation of Santy’s 1–9 scale method, thereby providing quantitative substantiation for the model’s construction and visualization.

The signal questionnaire data are transformed into an individual comparison matrix, and the judgment matrix is first constructed for the m sub-indicators under a certain level of indicator A as follows:

$$\mathrm{A}={\left({\mathrm{a}}_{\mathrm{i}\mathrm{j}}\right)}_{\mathrm{m}\times \mathrm{n}}=(\begin{array}{ccc}{\mathrm{a}}_{11}& {\mathrm{a}}_{12}& \dots \\ \dots & \dots & \dots \end{array}\begin{array}{c}{\mathrm{a}}_{1\mathrm{n}}\\ \dots \end{array})$$

(2)

It is imperative that the elements of A satisfy three fundamental properties.

(1)

Positive Qualitative: ${\mathrm{a}}_{\mathrm{i}\mathrm{j}}>0$

(2)

Mutual Inversion: ${\mathrm{a}}_{\mathrm{i}\mathrm{j}}={\displaystyle \frac{1}{{\mathrm{a}}_{\mathrm{j}\mathrm{i}}}}$

(3)

Identity: ${\mathrm{a}}_{\mathrm{i}\mathrm{i}}=1$

The un-normalized weights are obtained by calculating the sub-square root of the product of all elements in the first row, as demonstrated by the following equation, where the elements of A are required to satisfy three basic properties:

$${\overline{\mathrm{w}}}_{\mathrm{i}}=\sqrt[\mathrm{m}]{{\prod }_{\mathrm{j}=1}^{\mathrm{m}}{\mathrm{a}}_{\mathrm{i}\mathrm{j}}},\mathrm{i}=\mathrm{1,2},\dots ,\mathrm{m}$$

(3)

Standardization is performed on $\overline{\mathrm{w}}=\left({\overline{\mathrm{w}}}_1,\dots ,{\overline{\mathrm{w}}}_{\mathrm{m}}\right)$ to obtain the final weight vector. Normalization is then implemented to achieve the final weight vector:

$${\mathrm{w}}_{\mathrm{i}}={\displaystyle \frac{{\overline{\mathrm{w}}}_{\mathrm{i}}}{{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\overline{\mathrm{w}}}_{\mathrm{j}}}},{\sum }_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{w}}_{\mathrm{i}}=1$$

(4)

In order to verify the logical consistency of the expert scoring matrix, the following indicators are introduced:

Maximum eigenvalue:

$${\mathsf{\lambda }}_{\mathrm{m}\mathrm{a}\mathrm{x}}={\displaystyle \frac{1}{\mathrm{n}}}{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\displaystyle \frac{{\left(\mathrm{A}\mathrm{w}\right)}_{\mathrm{i}}}{{\mathrm{w}}_{\mathrm{i}}}}$$

(5)

Consistency indicator:

$$\mathrm{C}.\mathrm{I}.={\displaystyle \frac{{\mathsf{\lambda }}_{\mathrm{m}\mathrm{a}\mathrm{x}}-\mathrm{n}}{\mathrm{n}-1}}$$

(6)

Consistency ratio:

$$\mathrm{C}\mathrm{R}={\displaystyle \frac{\mathrm{C}\mathrm{I}}{\mathrm{R}\mathrm{I}\mathrm{\prime }}}$$

(7)

Among them, RI is the random consistency index. When CR < 0.1, the consistency of the judgment matrix is considered to be within an acceptable range, and the weights used in this section are reliable. If CR ≥ 0.1, the ratios in the judgment matrix A should be reconsidered, and the weights should be recalculated and consistency verified.

4.1.1. Aesthetic Index

The main items in the aesthetic index are color, brightness, and harmony. As indicated by the data presented in

Table 6. Results of pairwise comparison matrix for aesthetic indicators.

Index	Color	Brightness	Harmony
Color	1	0.636	0.5
Brightness	1.571	1	0.5
Harmony	2	2	1

,

Table 7. Aesthetic indicators’ AHP hierarchical analysis results.

Index	Eigenvector	Weight (%)	Maximum Eigenroot	CI Value
Color	0.683	21.382	3.023	0.011
Brightness	0.923	28.901
Harmony	1.587	49.717

,

Table 8. Aesthetic index consistency test results.

Maximum Eigenvalue	CI Value	RI Value	CR Value	Consistency Test Results
3.023	0.011	0.525	0.022	PASS

and

Table 9. Aesthetics index.

Aesthetic Index	Score Index	Color	Brightness	Harmony
	Weight Factor (1)	0.21382	0.28901	0.49717
CIGS Solar Cell	Score	95	90	92
	Weighted Value	20.3129	26.0109	45.73964
Silicon Solar Cell	Score	88	85	82
	Weighted Value	18.81616	24.56585	40.76794
Window	Score	93	95	95
	Weighted Value	19.88526	27.45595	47.23115
Exterior	Score	90	90	90
	Weighted Value	19.2438	26.0109	44.7453

, harmony emerged as the predominant aesthetic factor, accounting for 49.717% of the total influence. This was followed by brightness, which accounted for 28.901%, and color, which accounted for 21.382%. The reliability of pairwise comparisons was confirmed through the application of the consistency ratio (CR = 0.022), which met the established threshold criterion (CR < 0.1). Silicon solar cells have an overwhelming share of 90% of Korea’s solar cells [132]. However, in order to install silicon solar cells in apartments, they must be installed in limited spaces such as rooftops and handrails. It can be seen by looking at the solar cell application form in 2.4 above. Windows can coexist with CIGS thin-film solar cells at the same time as competition. If the window-integrated CIGS thin-film solar cell is applied, there is no need to install unnecessary stands, so the exterior of the building will be more organized. In the case of exterior materials, various visual productions are possible using paint on concrete-based supports. In the case of CIGS thin-film solar cells, they are less affected to some extent by natural disasters such as rainwater erosion. In addition, various colors and patterns are customized and applied to buildings to enable various architectural designs, and design applications are possible because they have flexible characteristics.

4.1.2. Economic Index

The economy is a very important indicator of market competitiveness that includes all financial aspects such as investment, cost, and profit. As indicated by the data presented in

Table 10. Results of pairwise comparison matrix for economic indicators.

Index	Initial Cost	Maintenance Costs	Projected Income
Initial Cost	1	1	0.5
Maintenance Costs	1	1	0.357
Projected Income	2	2.8	1

,

Table 11. Economic indicators’ AHP hierarchical analysis results.

Index	Eigenvector	Weight (%)	Maximum Eigenroot	CI Value
Initial Cost	0.794	24.206	3.013	0.006
Maintenance Costs	0.709	21.637
Projected Income	1.776	54.157

,

Table 12. Economic index consistency test results.

Maximum Eigenvalue	CI Value	RI Value	CR Value	Consistency Test Results
3.013	0.006	0.525	0.012	PASS

and

Table 13. Economic index.

Economic Index	Score Index	Initial Cost	Maintenance Costs	Projected Income
	Weight Factor (1)	0.24206	0.21637	0.54157
CIGS Solar Cell	Score	68	85	85
	Weighted Value	16.46008	18.39145	46.03345
Silicon Solar Cell	Score	80	82	85
	Weighted Value	19.3648	17.74234	46.03345
Window	Score	95	95	73
	Weighted Value	22.9957	20.55515	39.53461
Exterior	Score	95	90	70
	Weighted Value	22.9957	19.4733	37.9099

, projected income emerged as the most influential economic factor, accounting for 54.157% of the total weight in the economic index. This was followed by initial costs at 24.206% and maintenance costs at 21.637%. The weights were derived through pairwise comparisons using the AHP method. The consistency of expert judgments was verified via the consistency ratio (CR = 0.012), which is well below the accepted threshold (CR < 0.1). This confirms the reliability of the evaluation. Windows and exterior materials are overwhelmingly superior in initial cost and maintenance compared to CIGS thin-film solar cells and silicon solar cells. They also have excellent thermal economy performance. However, from a long-term perspective, the installation of solar cells has excellent economic feasibility as it affects energy saving. According to a literature study, the price of silicon solar cells is USD 1~1.50 per watt, and the module is USD 0.18 per watt, down 45% and 33.3% from the high point in 2022, respectively. As of 2023, CIGS thin-film solar cells are priced between USD 0.75 and 1.10 per watt and USD 0.23 for modules [133]. Silicon solar cells are linked to the semiconductor industry, which has an economic advantage by reducing silicon production costs. However, we need a side material, such as a holder. On the other hand, CIGS thin-film solar cells do not require a separate holder and have a high initial cost, but power generation efficiency is currently increasing. Another advantage is that no additional maintenance is required. Based on the experience of partner construction companies, it costs an average of USD 298.10 per household to construct paint, an exterior material. Construction costs vary depending on the number of households and the number of acreages in apartments. According to the apartment management law in Korea, it is recommended to repaint the outer walls of apartments every five years [134]. In the case of windows, insulation decreases over time due to deformation, etc. Periodic inspections are required when it is over 15 years old. Windows and exterior materials are more of a one-time investment. In terms of power generation, the estimated income from windows and exterior materials is zero. In the case of silicon solar cells, the average solar power generation time in Seoul is 1314 h, assuming 150 W/m², and the annual power generation is 118.26 kWh/m². Converting this into electricity bills, the annual income per square meter is USD 15.92. In the case of a CIGS thin-film solar cell, the annual power generation is 126.14 kWh/m², assuming that it is 160 W/m². Converting this into electricity bills, the annual income per square meter is USD 16.97 per square meter. The price of electricity varies between countries and regions, but it is 1 kWh/USD 0.13 in Seoul [135].

4.1.3. Safety Index

As demonstrated in the results presented in

Table 14. Results of pairwise comparison matrix for safety indicators.

Index	Strength	Pressure Resistance	Crack
Strength	1	1.6	1.125
Pressure Resistance	0.625	1	0.417
Crack	0.889	2.4	1

,

Table 15. Safety indicators’ AHP hierarchical analysis results.

Index	Eigenvector	Weight (%)	Maximum Eigenroot	CI Value
Strength	1.216	38.711	3.03	0.015
Pressure Resistance	0.639	20.322
Crack	1.287	40.967

,

Table 16. Safety index consistency test results.

Maximum Eigenvalue	CI Value	RI Value	CR Value	Consistency Test Results
3.03	0.015	0.525	0.029	PASS

and

Table 17. Safety index.

Safety Index	Score Index	Strength	Pressure Resistance	Crack
	Weight Factor (1)	0.40	0.30	0.30
CIGS Solar Cell	Score	92	92	95
	Weighted Value	36.8	27.6	28.5
Silicon Solar Cell	Score	90	85	80
	Weighted Value	36	25.5	24
Window	Score	95	95	95
	Weighted Value	38	28.5	28.5
Exterior	Score	90	88	75
	Weighted Value	36	26.4	22.5

, the safety performance of retrofit technologies was evaluated based on three criteria: structural strength, pressure resistance, and crack resistance. The pairwise comparison matrix (Table 14) was developed based on expert evaluations and the resulting AHP analysis. The analysis indicated that crack resistance (40.967%) and strength (38.711%) emerged as the two most significant safety factors, followed by pressure resistance (20.322%). The consistency ratio was calculated to be 0.029, which is less than the threshold requirement of CR < 0.1. This indicates that the pairwise comparisons are reliable. It compares and analyzes the stability index of CIGS thin-film solar cells, silicon solar cells, windows, and exterior materials through discussions and surveys with related industry workers. First, the window consists of 24–28 mm glass with a single-window and double-window structure. It is finished with a frame with a low deformation ability to prevent deformation of the glass. The strength varies depending on the single and double windows, and windows with a single window structure are usually used in old buildings. It is a structure that is directly coupled to the outer wall of concrete and is constructed by maintaining a gap with the outer wall of concrete. The reason for creating a certain gap is to prevent deformation of the window due to natural disasters such as earthquakes. Between the gaps, it is finished with silicon to prevent wind from seeping in, and insulation treatment is performed. In the case of exterior materials, corrosion prevention and damage to the outer wall of concrete are carried out. As the exterior material ages, cracks or water may touch the outer wall of the concrete, causing stress concentration, and the strength of the outer wall of the concrete decreases, reducing the lifespan of the building. Prior to 2021, there were many cases of spraying and painting, but after 2021, painting is carried out through a brush or roller. Usually, putty is used to fix the damaged area, and paint is applied using water-based paint. Silicon solar cells are mounted by fixing a separate frame on the outside of the building. Silicon solar cells are composed of a hard glass layer that is not easy to bend, so they must be mounted in a limited form. In Korea, strong typhoons land on a regular basis every year. Accordingly, the government regulates solar cells to be designed to withstand a maximum wind speed of 50 m/s. However, in the case of old apartments, they are often installed on handrails and rooftops of windows, and in the case of window handrails, since they are old, there is a possibility that the joint connected to the solar cell and the handrails themselves will be blown away by the typhoon. Usually, frame and silicon solar cells can be dangerous if they are blown away by the wind in high-rise apartments, which are around 20 kg. In fact, a typhoon with a maximum wind speed of 51.1 m/s landed in Korea in 2003, and the most powerful typhoon in recent years was a typhoon with a wind speed of 45 m/s in 2020. Korea’s summers are also humid and very hot, with temperatures of 39.6 degrees in 2018 [136]. In the case of summer, there is a risk of fire due to a short circuit of electrical wiring in the solar cell module due to the high radiant heat temperature. In the case of CIGS thin-film solar cells, they are very thin and light, and their stability is determined by the window or railing. There is no need for a separate frame, so it is attached directly to windows and handrails, and no separate stability is required for thin-film solar cells. In addition, ETFE or coating of CIGS thin-film solar cells can be safe from fire risks [137]. From the table below, it can be seen that the CIGS thin-film solar cell is excellent in the stability index. Safety indicators must be strictly observed in actual production and construction. Environmental factors should also be considered.

4.1.4. Energy Saving Index

As demonstrated in

Table 18. Results of pairwise comparison matrix for energy saving indicators.

Index	Power Saving	Carbon Emissions	Energy Independence
Power Saving	1	1.4	1.5
Carbon Emissions	0.714	1	1.286
Energy independence	0.667	0.778	1

,

Table 19. Energy saving indicators’ AHP hierarchical analysis results.

Index	Eigenvector	Weight (%)	Maximum Eigenroot	CI Value
Power Saving	1.281	41.904	3.004	0.002
Carbon Emissions	0.972	31.807
Energy independence	0.803	26.289

,

Table 20. Energy saving index consistency test results.

Maximum Eigenvalue	CI Value	RI Value	CR Value	Consistency Test Results
3.004	0.002	0.525	0.004	PASS

and

Table 21. Energy saving index.

Energy Saving Index	Score Index	Power Saving	Carbon Emissions	Energy Independence
	Weight Factor (1)	0.41904	0.31807	0.26289
CIGS Solar Cell	Score	95	95	85
	Weighted Value	39.8088	30.21665	22.34565
Silicon Solar Cell	Score	95	93	80
	Weighted Value	39.8088	29.58051	21.0312
Window	Score	85	85	80
	Weighted Value	35.6184	27.03595	21.0312
Exterior	Score	70	80	70
	Weighted Value	29.3328	25.4456	18.4023

, the energy-saving potential of each retrofit solution was evaluated based on three sub-criteria: power saving, carbon emissions reduction, and energy independence. The experts have developed a pairwise comparison matrix and AHP analysis. Among the three indicators, power saving emerged as the most influential factor, contributing 41.904% to the total weight, followed by carbon emissions at 31.807%, and energy independence at 26.289%. The consistency ratio (CR = 0.004) was found to be well below the acceptable limit of 0.1, thereby confirming the reliability of the expert assessments. Buildings consume a variety of resources throughout their lifecycles. There are energy, soil, water, materials, etc. The consumption of these resources should be minimized and made nature-friendly, and a comfortable and efficient space for users should be provided. Accordingly, the need for sustainable buildings is emerging. In the case of exterior materials, heat insulation can be provided temporarily through painting, but there is the inconvenience of periodically painting. In some cases, due to the deterioration of the exterior material, the insulation properties are poor, and more energy may be required. In the case of windows, insulation varies depending on single and double windows, but in the case of old buildings, most of them are single windows, and it is difficult to expect high insulation due to old age. Like exterior materials, this creates additional demand for energy. In the case of a silicon solar cell, if it is mounted on a window or railing weighing 20 kg, it creates a burden. This has the disadvantage of being able to further deteriorate old windows and handrails. On the other hand, CIGS thin-film solar cells have advantages in energy conservation and environmental protection. It can be attached anywhere in the sun and can be used with a light weight of around 2 kg, so it does not burden the windows or railings. Korea consumes 4502 kWh of electricity per household per year and emits 2146 kg of carbon emissions [138]. Taking the Seoul Federation of Korean Industries as an example, it produces an average of 780,000 kWh of electricity per year through solar cells. Converting this to carbon emissions can save 371,258 kg of CO₂. For example, a ZEB (Zero Energy Building) pilot private apartment in Incheon, Korea, produces an average of 96,675 kWh per year, and carbon emissions can be reduced by 45,066 kg of CO₂. This reduces land occupancy for power plants and energy storage facilities, enabling effective use of land. It also has the advantage of lowering air pollution by reducing carbon emissions. This is an important part of being able to meet Korea’s carbon emission reduction goal in 2030.

4.1.5. Innovation Index

As presented in

Table 22. Results of pairwise comparison matrix for innovation indicators.

Index	Advancement of Technology	Study for Progress
Advancement of Technology	1	1.125
Study for Progress	0.889	1

,

Table 23. Innovation indicators’ AHP hierarchical analysis results.

Index	Eigenvector	Weight (%)	Maximum Eigenroot	CI Value
Advancement of Technology	0.4705	47.05	2	0
Study for Progress	0.5295	52.95

,

Table 24. Innovation index consistency test results.

Maximum Eigenvalue	CI Value	RI Value	CR Value	Consistency Test Results
2	0	0	0	PASS

and

Table 25. Innovation index.

Innovation Index	Score Index	Advancement of Technology	Study for Progress
	Weight Factor (1)	0.5295	0.4705
CIGS Solar Cell	Score	95	95
	Weighted Value	50.3025	44.6975
Silicon Solar Cell	Score	95	95
	Weighted Value	50.3025	44.6975
Window	Score	80	85
	Weighted Value	42.36	39.9925
Exterior	Score	75	85
	Weighted Value	39.7125	39.9925

, the innovation index was assessed based on two key sub-criteria: advancement of technology and contribution to further study and progress. The pairwise comparison matrix and the AHP results indicate that the study on progress was considered to be marginally more influential (52.95%) than technological advancement (47.05%). Given that the matrix contained only two criteria, the maximum eigenvector was precisely 2, resulting in a consistency ratio of CR = 0. This indicates perfect consistency among expert judgments. The problem with old apartments is that there is a problem with heat loss due to the old age of buildings, resulting in an increase in power consumption and stability. First of all, heat loss can be reduced through the exchange of windows and has advantages such as sound insulation, insulation, moisture prevention, and wind pressure resistance. Exterior materials have the advantage of stability by repairing the waterproofing and concrete cracks of buildings. The innovative part of silicon solar cells and CIGS thin-film solar cells is to meet the electricity demand of buildings. It is an eco-friendly and energy-saving part. However, silicon solar cells are ahead of CIGS thin-film solar cells in terms of efficiency. The highest laboratory efficiency of silicon-based multi-junction solar cells was achieved (36.1%) [139]. The maximum efficiency of the CIGS thin-film solar cell in the laboratory is 23.5%. The efficiency of silicon solar cells and CIGS thin-film solar cells is continuously updated. In particular, CIGS thin-film solar cells in the non-silicon series have the advantage of having the highest energy conversion efficiency and excellent stability due to high light absorption. Double-sided light-receiving CIGS thin-film solar cells have also been developed, and they simultaneously absorb solar heat from the front and back of the battery, so they have excellent light conversion efficiency. It is well known for its high absorption coefficient, adjustable band gap, and flexibility. Nevertheless, the use of toxic regulatory Cd (cadmium) and its dependence on rare elements create great barriers to efforts to strengthen the commercial market of CIGS. Perovskite solar cells (PSCs) have increased scientific and technical interest due to their high efficiency. This is because they can play various roles, such as light absorption, charge separation, and hole and electron transport in the material. However, this technology is still limited due to its stability, scaling, and material composition. CIGS–perovskite series solar cells continue to show very high efficiency of over 24% [140].

5. Weights of the Competency Model

We analyze the superiority over CIGS thin-film solar cells, silicon solar cells, windows, and exterior materials for efficient green remodeling through comparison of five key strategic elements. Weights are determined from 0 to 1 by analyzing the five elements within the strategy. The sum of the weights is 1. The process for weight analysis was determined by expert in-depth discussion (FGD). The aesthetic index, economic feasibility, safety, energy saving, and innovation were 0.08182, 0.28786, 0.32505, 0.19258, and 0.11269, respectively. The calculated weights of the five key indices are visually represented in Figure 7.The above scores may vary depending on location, technology, and government policies and regulations. Relative ability strength for each factor was evaluated. It can be divided into 60 or less (weak), 60–70 (relatively weak), 70–80 (similar), 80–90 (relatively strong), and 90–100 (strong); the highest-scoring sample shows competitive advantage, and the difference in scores indicates relative competence.

By multiplying the evaluation value of each element by the corresponding weight, the competitiveness weight evaluation values of CIGS thin-film solar cells, silicon solar cells, windows, and exterior materials were 88.92094, 86.03364, 88.83937, and 82.20964, respectively. An evaluation of the two products reveals that CIGS thin-film solar cells and high-performance window retrofits have nearly identical overall performance scores (88.92 vs. 88.83, respectively). While this marginal difference of only 0.09 points may appear negligible at first glance, it carries significant meaning in the context of our Analytical Hierarchy Process (AHP) framework and retrofit decision-making. In an AHP-based multi-criteria evaluation, a minimal discrepancy in the composite score signifies a discernible ranking. In this instance, CIGS is identified as the optimal choice, with windows ranking second. The proximity of these scores indicates that both metrics demonstrate proficiency across the five criteria, resulting in an equitable alignment in performance. However, CIGS exhibits a modest superiority on an aggregate basis, a finding that merits attention in light of the established efficacy of high-efficiency window replacement as a retrofit strategy. In order to comprehend the significance of CIGS’s minute lead, it is necessary to consider the criteria trade-offs. The scoring breakdown (

Table 26. Competitive situation matrix.

	Key Factors	Aesthetic Index	Economic Index	Safety Index	Energy Saving Index	Innovation Index	Sum
	Weight Factor (1)	0.08182	0.28786	0.32505	0.19258	0.11269	1.00
CIGS Solar Cell	Score	92	79	93	92	95
	Weighted Value	7.52744	22.74094	30.22965	17.71736	10.70555	88.92094
Silicon Solar Cell	Score	85	82	85	89	95
	Weighted Value	6.9547	23.60452	27.62925	17.13962	10.70555	86.03364
Window	Score	94	87	95	83	82
	Weighted Value	7.69108	25.04382	30.87975	15.98414	9.24058	88.83937
Exterior	Score	90	85	84	73	80
	Weighted Value	7.3638	24.4681	27.3042	14.05834	9.0152	82.20964

) demonstrated that CIGS outperforms high-performance windows in certain areas, including energy savings and innovation. Conversely, windows demonstrated superiority in other areas, such as economic payback and perhaps safety and aesthetics.

The application of AHP weighting to the evaluation of criteria assumes paramount importance. While windows exhibited advantages under criteria with substantial economic and safety weighting, CIGS’s marked superiority in terms of its innovative and renewable energy contribution enabled it to narrowly exceed the window alternative in the weighted sum calculation. In practical terms, this means that CIGS can deliver a holistic benefit nearly equivalent to replacing windows while also introducing on-site renewable energy—a benefit that windows alone cannot provide. The negligible 0.09-point lead should be evaluated within the appropriate context. It suggests that if decision-makers prioritize innovation and long-term energy autonomy to a moderate extent (or if solar technology costs continue to decrease), CIGS would decisively establish itself as the preferred choice. From a retrofit feasibility perspective, the near parity of CIGS and window options is a positive development. This suggests that a novel technology such as CIGS, despite being in an early market stage, can already demonstrate equivalent performance to that of a mature retrofit solution.

This minor discrepancy is significant because it substantiates CIGS’s competitiveness. In the AHP prioritization framework, CIGS is assigned the highest priority, indicating that stakeholders who aspire to advance sustainability can confidently select CIGS without compromising overall performance. Furthermore, the proximity of the scores suggests that building owners possess a certain degree of flexibility in their decision-making process. They may consider either the well-established window upgrade or the cutting-edge CIGS panels, both of which can yield a comparable balanced sustainability outcome. The fundamental conclusion is that the incorporation of CIGS into the retrofit paradigm does not result in a significant disruption to the prevailing balance of criteria satisfaction. Instead, it introduces a renewable energy dimension to the retrofit strategy while preserving comparable benefits in terms of comfort, safety, and cost-effectiveness. Consequently, a marginal lead of 88.92 compared to 88.83 further validates the viability of CIGS. This suggests that CIGS technology is prepared to compete with conventional methods, a pivotal factor in gaining industry acceptance. In summary, although the sample size was modest, the observed discrepancy in scores substantiates the position of CIGS as the preeminent retrofit measure in the present study. This suggests that practitioners and policymakers may consider CIGS as a reliable alternative to, or even a superior option over, conventional upgrades, particularly as technological advancements continue to enhance its performance.

6. Discussion

The past and present energy-saving policies are very different. Old high-rise apartments in Seoul offer a critical context. Over 75% of buildings in Seoul are more than 20 years old, leading to stark differences in energy performance between new and aging buildings (as noted in the Introduction). As mentioned in the Introduction, there is a considerable difference in energy consumption between new and old buildings. Due to the complex intertwining of economic, policy, and resident interests, the process of obtaining approval for reconstruction and complete remodeling takes a long time, and the actual construction period takes a long time, ranging from 5 to 10 years. Additionally, some apartments in Seoul have been in the process of discussing reconstruction for 20 years due to various and complex issues. According to a study by Seunghoon Nam, energy consumption decreased by 20.6~67.4% when green remodeling was applied to public and educational buildings. However, in the field of energy conservation, it is too small for new and renewable energy to handle due to excessive energy consumption caused by the 24 h operation of computer rooms and the operation of many research equipment [141]. In light of these findings, our evaluation substantiates that the incorporation of CIGS thin-film solar cells can significantly enhance sustainability without necessitating extensive reconstruction. The multi-criteria assessment, which took into account factors such as aesthetics, economy, safety, energy savings, and innovation, revealed that CIGS retrofits attained the highest overall performance score (88.92). High-efficiency window upgrades closely followed, with a score of 88.83. This finding suggests that CIGS modules, despite being an emerging technology, can perform comparably to traditional envelope improvements in a holistic sustainability context. Silicon-based solar panels and exterior wall claddings received moderately lower scores (86.03 and 82.21, respectively), reflecting certain limitations in those options under the established criteria. The findings indicate that CIGS retrofitting is a competitive strategy for aging residential buildings, offering a balanced enhancement across multiple dimensions of building performance. Of particular significance are the implications for green retrofitting policy and practice. In Seoul, current energy-saving policies have predominantly targeted public buildings, while retrofits in the private sector have lagged. The findings of this study underscore a significant opportunity that has been overlooked: the incorporation of CIGS BIPV technology in private residential retrofits has the potential to address the performance disparity between older and newer buildings. The slight performance advantage of CIGS over conventional measures highlights its potential as a new focal point in retrofit initiatives, particularly in dense urban areas where on-site renewable energy generation is imperative. The capacity of CIGS modules to affix to facades and windows with malleability renders them a viable solution in scenarios where comprehensive renovations or reconstructions are impractical or exceedingly time-consuming. Consequently, while long-term solutions such as comprehensive remodeling or reconstruction are being pursued, CIGS retrofits can deliver immediate energy and carbon reductions, effectively buying time and functional improvements for aging buildings. The discourse surrounding Seoul’s case exemplifies this phenomenon. Given the protracted timelines and stakeholder negotiations inherent in urban redevelopment, CIGS retrofits offer a pragmatic interim solution that aligns with Korea’s 2030 carbon reduction goals.

7. Conclusions

In this paper, Porter’s Five Forces models for CIGS thin-film solar cells were used to analyze them, and to supplement them, they were further analyzed in competitive situations through exponential modeling. Quantitative and qualitative analyses were conducted on aesthetics, economics, safety, energy saving, and innovation. As a result of this study, the CIGS thin-film solar cell was 88.92094 points. Solar energy, which is renewable energy, does not use fossil fuels and is a nature-friendly energy. In a situation where it is necessary to reduce the use of fossil fuels, the traditional energy production method, there is no option other than solar cells due to Korea’s conditions. It also helps to implement carbon emission reductions in compliance with international agreements. In the case of green remodeling in Korea, various approaches are possible. However, in the private sector, development is slow, especially in Seoul, where there is a severe population density and many old buildings. Solar energy will be an important resource. Reconstruction and remodeling are influenced by residents’ interests and government policies, and the period is long. To compensate for this, it is expected that CIGS thin-film solar cells can be used to supplement time-consuming reconstruction and remodeling.