The Advancements and Challenges in Organic Photovoltaic Cells: A Focused and Spotlight Review Using the Proknow-C

Abstract

The global interest in environmental issues and sustainable energy has propelled extensive research in photovoltaic (PV) technologies. Brazil has emerged as one of the top ten solar energy producers and flexible PV suppliers in the world. In this context, organic photovoltaic cells (OPVs) have garnered attention due to their flexibility and ability to integrate into various surfaces, albeit facing challenges regarding lifespan and efficiency compared to silicon cells. This review examines the current state of research on OPVs and thin-film solar technologies, employing the systematic literature review methodology Proknow-C. The review includes an analysis of raw materials such as conductive polymers, fabrication processes including film deposition and encapsulation, and technological advancements that enhance the efficiency and stability of OPVs. Utilizing the Proknow-C methodology, a bibliographic portfolio was constructed to identify the current state of research in this field. Of 268 papers from major scientific databases, only 30 were deemed relevant to the theme, highlighting a significant research gap. This paper is a valuable resource for researchers, providing an updated overview and a foundation for future investigations in organic photovoltaics. The results emphasize the importance of materials such as conductive polymers and donor–acceptor molecules and the role of nanotechnology in advancing OPVs. Innovations in manufacturing techniques, such as inkjet and blade coating-based printing techniques, are shown to increase efficiency by providing precise control over film thickness and uniformity and reducing costs through lower material waste. Overall, this review highlights the necessity of further exploration and collaboration within the scientific community to address the challenges and propel advancements in organic photovoltaic cells. These advancements are crucial for transitioning to cleaner energy sources, reinforcing the ongoing significance of thin-film technologies in energy innovation.

1. Introduction

The growing global interest in environmental issues, including pollution, climate change, and the urgency to find sustainable energy sources, has propelled extensive research and development in renewable energies [1]. Among the highlighted sectors, photovoltaic technology, particularly solar cells, has gained prominence. For instance, Brazil has emerged as one of the leaders in solar energy generation, ranking among the top eight solar power producers globally, alongside China, the USA, Japan, Germany, India, Australia, and Italy. Additionally, Brazil is recognized as one of the top producers of flexible photovoltaics, with notable contributions from companies like Sunew in Minas Gerais. In this dynamic context, technological innovations such as organic photovoltaics (OPVs) have emerged, offering distinct benefits despite facing challenges [2].

The OPV field has made remarkable progress, with power conversion efficiencies (PCE) improving from 3% to over 19% in the past two decades. Composed of organic semiconducting films, commercial devices from organic cells have attracted interest due to their flexibility in terms of sizes and formats, albeit encountering limitations in their efficiency and lifespan compared to traditional silicon cells. In addition to the development of new technologies, identifying challenges and applying strategies to overcome them are crucial. A notable example is maximum power point tracking (MPPT), a vital methodology to address obstacles like partial shading [3]. Concurrently, advancements in the manufacturing processes of OPVs have been remarkable, with the adoption of techniques such as film deposition, lamination, and encapsulation. These improvements have strengthened the durability of OPVs, driving their rapid technological evolution. As a result, OPVs are emerging as a highly attractive alternative to conventional solar cell technologies thanks to their flexibility, capacity for integration into various surfaces, and reduced environmental impact [4].

This technical report provides an analysis of the current state of research on thin-film photovoltaics, more specifically on organic-based technologies, exploring the evaluation of existing technologies as well as the methodology employed to review the academic literature. By examining the latest studies, not only the progress achieved but also the challenges faced by the proposed solutions become evident. The panorama presented underscores the complexity and relevance of these technologies for the sustainable and effective development of the global energy sector.

In this review paper, the technological development and applicability of OPVs are analyzed using the systematic literature review method Proknow-C. This methodology aided in the development of knowledge regarding the following: (i) organic photovoltaic cells (OPVs): understanding their advantages, such as flexibility and capacity for integration into various surfaces, despite facing challenges of lifespan and efficiency; (ii) Proknow-C methodology: the systematic literature review using the Proknow-C methodology ensured a meticulous selection and critical analysis of relevant research and publications; (iii) manufacturing process: innovations in materials and manufacturing processes, such as conductive polymers and inkjet deposition, are improving the efficiency and reducing the costs of OPVs; and (iv) technological advancements: techniques like lamination and encapsulation are crucial to protect OPVs from adverse environmental conditions, highlighting the importance of thin-film technologies in transitioning to cleaner energy sources. Additionally, in a dedicated section, this review spotlights recent research from 2023 to 2024, focusing on both small-area organic cells (<1 cm²) and large-area OPV modules, showcasing the latest advancements and trends in this field.

2. Technological Roadmap of Organic Photovoltaic Cells (OPVs)

The bibliographic portfolio survey is a crucial step in any research, as it allows for the evaluation of scientific development and its relevance to advancing the state of the art. In this study, a scientific literature review on “advancements and challenges in OPV” was carried out using the Proknow-C methodology, which follows a scientific and constructivist approach [5].

The Proknow-C method consists of a sequence of procedures that range from selecting the search mechanism to filtering and choosing the bibliographic portfolio. The steps of this method include the following: (i) selecting a portfolio of articles relevant to the research topic; (ii) performing a bibliometric analysis of the portfolio; (iii) conducting a systematic analysis of the bibliographic portfolio; and (iv) defining the research question and objectives. These steps provide a systematic approach to identifying and analyzing the most pertinent articles on the research topic.

The bibliometric analysis assesses characteristics such as citation frequency, influential authors, and trends in the field. The systematic analysis, on the other hand, considers theoretical aspects, methodologies, and the results found in the selected studies. By applying the Proknow-C method, it is possible to establish a solid knowledge base on the subject, which contributes to supporting and guiding the research more efficiently and accurately.

This section of the article presents a systematic literature analysis using the Proknow-C (Knowledge Development Process-Constructivist) methodology [5], a widely recognized scientific review process that has been extensively used in systematic reviews [6,7,8,9,10].

The methodological steps for conducting this review are detailed in the following: The first step is to perform a preliminary investigation on the topic. Next, the scientific literature was reviewed using the Proknow-C scientific approach methodology [5,11]. Pre-established criteria were applied to ensure the selection of studies relevant to OPVs. Once identified, the studies underwent a critical evaluation of their methodological quality. The reporting of the review followed specific guidelines, detailing each stage of the methodological process. Subsequently, periodic updates of the review to include new studies and maintain the relevance of the results can be considered. The complete process, including peer review, ensured methodological quality and result reliability. The Proknow-C methodology stands out for its approach, contributing to high-quality systematic reviews.

The elaboration of the theoretical review began with the selection of the database that would define the sample scope, resulting in a set of articles available for consideration during the selection process. For this study, the chosen data source was the IEEE Xplore database, and regarding the keywords employed, after determining the sample scope, their selection proceeded, playing the role of the first filtering criterion in the article selection, with the main keywords being “OPV”, “Thin films”, and “Photovoltaics cells”. Using the aforementioned keywords and publication date after the year 2000, the search in the chosen database returned 268 references, as illustrated in Figure 1.

Five references to be excluded from the sample were identified. After these exclusions, the library of articles was left with 263 references at this point in the selection process. From these references, the titles of the articles were read to assess their relevance to the research. Following this analysis, 196 references were excluded for not being aligned with the research theme, resulting in 67 references to be analyzed, as shown in Figure 2.

These references, aligned with the research theme based on their titles, were analyzed for their scientific recognition since the date of publication. For this analysis, all references were searched on Google Scholar regarding the number of citations and sorted in descending order. Based on this information, a cutoff value was created for the most cited articles. This cutoff value was set to select the minority of the most cited articles. With this determination, 39 articles were selected based on the number of citations and those with few or no citations were excluded, as evidenced in Figure 3. It is important to note that the less-mentioned articles must undergo additional analysis, considering other criteria, which may enable their inclusion in the final set of articles that will compose the theoretical basis of the research.

Once the articles with the highest scientific recognition were selected, they were analyzed for the alignment of their abstracts with the focus of the research in question. Out of the 39 abstracts analyzed, 16 were excluded and formed repository P due to a lack of alignment with the research objective, thus leaving 23 articles forming repository K, which demonstrated alignment during the title and abstract reading, had a number of citations, and had an accessible abstract.

These 23 scientifically recognized articles, in line with the research theme, were chosen to form the essential basis of the theoretical framework on performance evaluation from a strategic perspective. However, additional analysis is required for the 23 selected articles to evaluate less cited articles that may still be considered in the final set of articles. For any article with fewer citations to be included in the final research portfolio, the following criteria were considered: (1) articles published less than 2 years from the analysis, considering they may not have had time to receive a significant number of citations and (2) articles published more than 2 years ago must be authored by a researcher already present in the group of 34 articles aligned with the abstract and with scientific relevance. With these two conditions established, from the articles analyzed in the review, six were added to the portfolio of articles, thus totaling twenty-nine articles, as can be seen in Figure 4.

The bibliographic portfolio (BP) completed is presented in Table 1. It was possible to understand that the Proknow-C review method plays a fundamental role in academic and scientific research, enabling the identification, selection, and critical evaluation of available evidence, ensuring the validity of the review, and allowing for comparison among different studies, promoting a more focused synthesis of existing knowledge. This was essential to establish solid foundations for research on OPV technologies.

By exploring the current state of research in thin-film solar cells and, more specifically, OPV, significant contributions can be identified in studies such as the one by Łukasiak et al. [12] on the photoluminescence of porous silicon (PSi) coated with thin organic films. Their work reveals the interaction between organic materials and the structure of porous silicon while pointing to applications in silicon nanostructures for gas sensors and biomedical markers. Perkins et al. [13] investigate amorphous indium zinc oxide (a-IZO) films, highlighting the need for enhancements, especially in the Transparent Conductive Oxide (TCO) properties and processability of these films to advance beyond demonstration devices. Studies like those of Yang et al. [14] and Charfi et al. [15] emphasize the structural, chemical, and optoelectronic characteristics of thin-film solar cells and the electrochemical synthesis of transition metal oxides and polymeric layers for roll-to-roll (R2R) OPV manufacturing. These research efforts reflect the current state of organic photovoltaics, highlighting advancements in the properties and applications of these technologies.

As OPV technology progresses, a diverse set of challenges and innovations emerges, promoting a deeper and, at the same time, more focused on the understanding of this promising technology. For instance, Ito et al. [16] present an innovative approach to selective machining OPVs using an ultra-short pulse laser. By demonstrating the successful application of this technique in particular layers, the study highlights the feasibility of selective machining for the fabrication of “in-plane” tandem structure modules.

Continuing, the studies by David et al. [17], Dolara et al. [18], and Stoichkov et al. [19] introduce innovative approaches, such as the application of machine learning methods to model and predict diurnal variations in OPV performance parameters, experiments in real environmental conditions, and a new testing methodology that applies multiple stresses simultaneously. These studies not only contribute to a deeper understanding of the challenges and innovations in OPVs but also emphasize the complexity and multifaceted narrative of these devices. The proposal of a hybrid method for monitoring and controlling OPVs aligns with the innovations and challenges identified in the mentioned studies. Developing an online energy management system for OPVs not only contributes to the advancement of this area but also addresses crucial issues related to performance and degradation over their lifespan under extreme conditions. For instance, Stoichkov et al. [19] propose a new testing methodology that applies multiple stresses simultaneously, using a Design of Experiment (DOE) approach to predict OPV aging. The generalized log-linear life model is employed to predict the life of OPVs, achieving an accurate estimate of simulated lifetime, with up to 18% precision.

Table 1. Bibliographic portfolio after reviewing OPV scientific literature with Proknow-C.

Description	Ref.	Cit.
Classification and comparison of maximum power point tracking techniques for photovoltaic system: a review (Renewable and Sustainable Energy Reviews)	[20]	876
A review of renewable energy utilization in islands (Renewable and Sustainable Energy Reviews)	[1]	484
Power tracking techniques for efficient operation of photovoltaic array in solar applications (Renewable and Sustainable Energy Reviews)	[21]	236
Recent progress in inkjet-printed solar cells (Journal of Materials Chemistry A)	[22]	140
Benzodithiophenedione-based polymers: recent advances in organic photovoltaics (NPG Asia Materials)	[23]	110
The role of physical techniques on the preparation of photoanodes for dye sensitized solar cells (International Journal of Photoenergy)	[24]	96
Overview of high-efficiency organic photovoltaic materials and devices (Renewable and Sustainable Energy Reviews)	[25]	89
A review on emerging barrier materials and encapsulation strategies for flexible perovskite and organic photovoltaics (Advanced Energy Materials)	[26]	74
Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance (RSC Advances)	[27]	51
Metal–organic framework nanosheets for enhanced performance of organic photovoltaic cells (Journal of Materials Chemistry A)	[28]	41
Multistress testing of OPV modules for accurate predictive aging and reliability predictions (IEEE Journal of Photovoltaics)	[19]	14
Thin film solar cells for indoor use (37th IEEE Photovoltaic Specialists Conference)	[14]	12
Mechanism and analysis of thermal burn-in degradation of OPVs induced by evaporated HTL (IEEE Journal of Photovoltaics)	[29]	12
Amorphous indium-zinc-oxide transparent conductors for thin film PV (37th IEEE Photovoltaic Specialists Conference)	[13]	11
Outdoor assessment and performance evaluation of OPV modules (IEEE Journal of Photovoltaics)	[18]	10
Hierarchical modeling of OPV-based crossbar architectures (14th IEEE International Conference on Nanotechnology)	[30]	7
Métodos de aquisição experimental de curvas I-V de arranjos fotovoltaicos: uma revisão (Proceedings of the 11th Seminar on Power Electronics and Control)	[31]	5
Photoluminescence of Electrochemically Etched Porous Silicon (13th International Conference on Transparent Optical Networks)	[12]	4
Estudo e dimensionamento de um conversor para energy harvesting de luminosidade indoor utilizando painel solar fotovoltaico orgânico (Universidade Federal de Minas Gerais)	[32]	4
Optical modelling of semi-transparent OPV devices (2016 International Conference on Numerical Simulation of Optoelectronic Devices)	[33]	3
Selective machining of organic thin film photovoltaic cell by a ultra-short pulse laser (CLEO/Europe and EQEC 2009 Conference Digest)	[16]	2
Novel measurement method of ion impurity in OPV materials (26th International Workshop on Active-Matrix Flat Panel Displays and Devices)	[34]	2
A method to characterize OPV temperature, humidity and irradiance combined degradation-preliminary results (38th IEEE Photovoltaic Specialists Conference)	[35]	1
Forecasting OPV outdoor performance, degradation rates and diurnal performances via machine learning (47th IEEE Photovoltaic Specialists Conference)	[17]	1
Review of maximum power point tracking: history, developments and challenges (International Journal of Electrical, Electronics and Computer Engineering)	[36]	0
Induction of internal capacitance effect of organic photovoltaic device (OPV) by Real-Time One-Sweep Method (RTOSM) in I–V measurement (IEEE 40th Photovoltaic Specialist Conference)	[37]	0
Electrochemical synthesis of transition metal oxides and polymer layers for OPV fabrication (24th Microoptics Conference)	[15]	0
Simulação de um conversor boost com MPPT digital para otimizar a geração de energia de uma linha de um Filme Fotovoltaico Orgânico (OPV) perante mudanças de irradiância (Universidade Federal da Integração Latino-Americana)	[38]	0
Organic Photovoltaic (OPV) with Electronic Protection System: A Systematic Review (Brazilian Archives of Biology and Technology)	[39]	0

Table 1. Bibliographic portfolio after reviewing OPV scientific literature with Proknow-C.

Description	Ref.	Cit.
Classification and comparison of maximum power point tracking techniques for photovoltaic system: a review (Renewable and Sustainable Energy Reviews)	[20]	876
A review of renewable energy utilization in islands (Renewable and Sustainable Energy Reviews)	[1]	484
Power tracking techniques for efficient operation of photovoltaic array in solar applications (Renewable and Sustainable Energy Reviews)	[21]	236
Recent progress in inkjet-printed solar cells (Journal of Materials Chemistry A)	[22]	140
Benzodithiophenedione-based polymers: recent advances in organic photovoltaics (NPG Asia Materials)	[23]	110
The role of physical techniques on the preparation of photoanodes for dye sensitized solar cells (International Journal of Photoenergy)	[24]	96
Overview of high-efficiency organic photovoltaic materials and devices (Renewable and Sustainable Energy Reviews)	[25]	89
A review on emerging barrier materials and encapsulation strategies for flexible perovskite and organic photovoltaics (Advanced Energy Materials)	[26]	74
Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance (RSC Advances)	[27]	51
Metal–organic framework nanosheets for enhanced performance of organic photovoltaic cells (Journal of Materials Chemistry A)	[28]	41
Multistress testing of OPV modules for accurate predictive aging and reliability predictions (IEEE Journal of Photovoltaics)	[19]	14
Thin film solar cells for indoor use (37th IEEE Photovoltaic Specialists Conference)	[14]	12
Mechanism and analysis of thermal burn-in degradation of OPVs induced by evaporated HTL (IEEE Journal of Photovoltaics)	[29]	12
Amorphous indium-zinc-oxide transparent conductors for thin film PV (37th IEEE Photovoltaic Specialists Conference)	[13]	11
Outdoor assessment and performance evaluation of OPV modules (IEEE Journal of Photovoltaics)	[18]	10
Hierarchical modeling of OPV-based crossbar architectures (14th IEEE International Conference on Nanotechnology)	[30]	7
Métodos de aquisição experimental de curvas I-V de arranjos fotovoltaicos: uma revisão (Proceedings of the 11th Seminar on Power Electronics and Control)	[31]	5
Photoluminescence of Electrochemically Etched Porous Silicon (13th International Conference on Transparent Optical Networks)	[12]	4
Estudo e dimensionamento de um conversor para energy harvesting de luminosidade indoor utilizando painel solar fotovoltaico orgânico (Universidade Federal de Minas Gerais)	[32]	4
Optical modelling of semi-transparent OPV devices (2016 International Conference on Numerical Simulation of Optoelectronic Devices)	[33]	3
Selective machining of organic thin film photovoltaic cell by a ultra-short pulse laser (CLEO/Europe and EQEC 2009 Conference Digest)	[16]	2
Novel measurement method of ion impurity in OPV materials (26th International Workshop on Active-Matrix Flat Panel Displays and Devices)	[34]	2
A method to characterize OPV temperature, humidity and irradiance combined degradation-preliminary results (38th IEEE Photovoltaic Specialists Conference)	[35]	1
Forecasting OPV outdoor performance, degradation rates and diurnal performances via machine learning (47th IEEE Photovoltaic Specialists Conference)	[17]	1
Review of maximum power point tracking: history, developments and challenges (International Journal of Electrical, Electronics and Computer Engineering)	[36]	0
Induction of internal capacitance effect of organic photovoltaic device (OPV) by Real-Time One-Sweep Method (RTOSM) in I–V measurement (IEEE 40th Photovoltaic Specialist Conference)	[37]	0
Electrochemical synthesis of transition metal oxides and polymer layers for OPV fabrication (24th Microoptics Conference)	[15]	0
Simulação de um conversor boost com MPPT digital para otimizar a geração de energia de uma linha de um Filme Fotovoltaico Orgânico (OPV) perante mudanças de irradiância (Universidade Federal da Integração Latino-Americana)	[38]	0
Organic Photovoltaic (OPV) with Electronic Protection System: A Systematic Review (Brazilian Archives of Biology and Technology)	[39]	0

David et al. [17] adopt machine learning methods to model and predict diurnal variations in OPV performance parameters, providing a valuable tool for determining the expected power output of modules. Furthermore, the degradation rate of OPV modules is predicted using a multivariate regression model, identifying key factors such as the irradiance, temperature, and operating point. Thus, considering the need for real conditions, the study by Dolara et al. [18] highlights a series of experiments conducted to evaluate the performance of OPV modules under real environmental conditions. By comparing the electrical performance of these modules with conventional photovoltaic technologies, such as crystalline silicon (c-Si) and CIS, the study reveals efficiencies below 4% for OPV modules under real conditions.

In the study by Sung et al. [29], environmental stability emerges as a critical barrier to the commercialization of OPVs. Despite advances in energy conversion efficiency, thermal degradation, termed “burn-in,” arises as a significant limitation. Strategies to stabilize the morphology of the photoactive layer are explored, but the main focus of future research must lie on the need to understand the direct relationship between thermal stress and efficiency losses.

The research by Upama et al. [33] expands the applications of OPVs, exploring their potential in energy-generating windows. By modeling the optics of semi-transparent OPV devices, the study highlights the importance of illumination direction and electrode layer thickness, outlining crucial considerations for optimizing the performance of these specific applications. In Kam-Lum’s work [35], the complexity of fundamental characterization of flexible OPV modules is highlighted, as well as the advantages of OPVs such as low cost, minimal pollution, and flexibility, as emphasized by Long et al. [37]. The interaction between multiple organic and inorganic layers, shown by Zahir et al. [30], and the potential ionic impurities, explained by Inoue et al. [34], challenge characterization methods. The research aims to develop a methodology to quantify degradations related to temperature, humidity, and combined irradiance, offering valuable insights for a comprehensive understanding of OPV aging. These studies, aligned with the former ones, paint a comprehensive picture of the complexities, innovations, and challenges faced by OPVs, consolidating knowledge that transcends efficiency boundaries to encompass the fundamental aspects, specific applications, and critical interaction with environmental factors.

Therefore, a complex and multifaceted evolution in the field of organic photovoltaics is highlighted, not only emphasizing challenges but also innovations and potential future paths, as well as efficient tracking techniques shown in Ahmad et al. [21] or in the study of different materials demonstrated in Ahmadi et al. [24]. The interdisciplinarity and integration of approaches, from materials synthesis to energy management strategies, represent a promising narrative for the sustainable development of solar energy and the consolidation of OPV cells as a viable and effective technology in the global energy scenario.

Notable advances in the industry, such as the new optimization techniques mentioned in Reisi et al. [20] and Soni et al. [36], and energy harvesting exemplified by Rios et al. [32], with special emphasis on OPVs, have been made. In this context, this segment aims to present the outcomes of an exhaustive analysis of thin films, focusing particularly on OPVs, covering topics such as raw materials, the manufacturing processes of the elements, and the current technology. The latest technologies indicate trends in this constantly evolving domain, including the emergence of new approaches for the experimental acquisition of photovoltaic system curves, as detailed by Treter et al. [31].

Zheng et al. [23] review the significant advancements in OPVs, focusing on the promising performance of benzodithiophenedione (BDD)-based materials due to their advantageous molecular structure and properties. On the other hand, metal–organic nanosheets (MONs) have great potential to improve OPV performance, demonstrating how their incorporation into a polythiophene–fullerene solar cell can significantly increase efficiency, as demonstrated by Sasitharan et al. [28]. The manufacturing of OPV layers has been the subject of intense evolution over the last decade. Despite the fact that spin coating, blade coating, and inkjet printing are widely used for organic thin-film deposition [22], techniques such as slot-die and roll-to-roll coating are necessary in order to enable more competitive large-area OPV processing. As discussed by Sutherland et al. [26], lamination and encapsulation are crucial in addressing operational stability challenges in both perovskite solar cells (PSCs) and OPVs. The rapid development of PSCs, in particular, has been driven by advancements in encapsulation strategies to prevent degradation. In addition to these strategies, using a thin polymer layer beneath the perovskite film has shown promise in enhancing stability. This polymer layer can interact with silver ions on the perovskite surface, leading to the passivation of under-coordinated defect sites and reducing the density of charge traps. The elimination of defects through polymer passivation has been shown to decrease photovoltage loss, extend the lifetime of charge carriers, and ultimately improve the efficiency and stability of PSCs. These developments highlight the importance of interface passivation, particularly at the hole transport layer/perovskite and electron transport layer/perovskite interfaces, in achieving higher performance and long-term stability in perovskite-based solar cells.

Solak et al. [27] provide a comprehensive overview of organic photovoltaic (OPV) cells, highlighting their potential to revolutionize the solar energy industry with lightweight, flexible, and cost-effective devices. They discuss the historical evolution and classification of PV cell technologies, comparing advancements in first-, second-, and third-generation solar cells based on diverse materials and structures. Meanwhile, Liu et al. [25] focus on recent breakthroughs in OPV technology, particularly in high-efficiency donor–acceptor polymers and small molecules. Their analysis emphasizes significant progress in optimizing device structures and understanding the structure–property relationships of photovoltaic materials. Finally, Izidoro et al. [39] revealed a significant research gap in the development of electronic protection systems for organic photovoltaic modules. The performed bibliographic research helped to lay a solid foundation for future research and advancements in OPV protection systems.

In summary, this technical report highlights the insights gained from the most recent findings and discoveries in the field of thin films, with a special focus on OPVs, covering topics such as raw materials, the manufacturing processes of the elements, and the existing technology. The ongoing advancements in these areas promise to play a crucial role in the transition to cleaner and more sustainable energy sources. Therefore, the current state of organic photovoltaics reflects a complex and multifaceted evolution, highlighting not only challenges but also innovations and potential future paths. The interdisciplinary nature and integration of approaches, from materials synthesis to energy management strategies, represent a promising narrative for the sustainable development of solar energy and the consolidation of OPV cells as a viable and effective technology in the global energy scenario.

3. Spotlight Research

Despite lying outside of the methodology used to construct this review, it is worth mentioning recent reports on high-efficiency OPVs in order to point to more accurate future prospects of this technology. Table 2 provides a performance summary of current state-of-the-art devices, ordered from the lowest to the highest observed performance. Note that these results are not from modules or panels, but from small-area single cells. In addition, even nowadays, research laboratories are still working with spin coating deposition technique.

By blending an elastomer to the donor–acceptor system, Li et al. [40] achieved both record-high stretchability and mechanical stability. Unfortunately, PCE decreases with the increasing weight ratio of the elastomer. A similar behavior was observed by Zheng et al. [41]. The best performance was achieved without the styrene-ethylene-propylene-styrene tri-block copolymer (SEPS). A ternary active layer (AL) was used in order to enhance efficiency. The PCE can be further improved by carefully moving to a quaternary active layer [42]. Even more audacious, a recent work proposed the use of hexanary blends [43]. In this case, however, Paleti et al. [43] intended to improve OPV thermal stability. The bulk-heterojunction active layer deposited by doctor blade was shown to be stable for at least 23 days at 130 °C in the dark and an inert atmosphere. An alternative to enhance performance is the deposition of self-assembled interlayers. Guan et al. [44] used 2-(9H-carbazol-9-yl) (2PACz) to replace poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on top of indium tin oxide (ITO). Additionally, 2PACz was dissolved with other active layer materials to form a ternary system. Finally, record efficiency can be observed in indoor environments with an efficiency as high as 36.3% [45]. Notice that, despite efforts towards improving stability, Wang et al. [46] have achieved a lifetime of just 1140 h, i.e., less than 2 months of continuous exposure, for a pristine 19.05% PCE OPV under one-sun illumination in a nitrogen atmosphere.

Table 2. Device parameters from high-performance and small-area ($\approx$1 cm²) organic solar cells.

Active Area Materials (w:w) 1	Deposition Techniques	Area (cm2)	JSC (mA/cm2)	VOC (V)	FF (%)	PCE (%)	Ref.
PM6/PYFT-o (1:1)	spin coating	-	23.75	0.902	70.72	15.23	[40]
PM6:Y6 2 (1:1.2)	spin coating	0.07	25.2	0.82	76.1	15.7	[47]
PM6:L8-BO:BTP-eC9 3 (1:0.6:0.6)	spin coating	1	25.67	0.86	71.40	15.71	[41]
PM6:Y6:Y7:Y18:N3: BTP-eC9 (1:0.24:0.24:0.24:0.24:0.24)	doctor blade	0.1	28.3	0.87	73.0	17.6	[43]
PM7-Thy10:L8-BO (1:1.2)	spin coating	0.04	25.64	0.88	76	17.05	[48]
PM6:L8-BO:BTP-eC9 (1:0:65:0.65)	spin coating	1.05	27.98	0.88	74.53	18.41	[44]
PM6:BTP-eC9 (1:1.2)	spin coating	~0.03	28.69	0.842	77.17	18.67	[46]
PM6:BTP-eC9:L8-BO-F	spin coating	~0.03	28.56	0.856	77.82	19.05	[46]
PM6:TAA-1 4 (1:1.2)	spin coating	0.06	27.2	0.890	80.0	19.3	[42]
PM6:Y6:2PACz (1:1.2:0.05)	spin coating	1	0.146 5	0.720 5	77.0 5	36.3 5	[45]

¹ w:w is a weight mass ratio. ² Device fabricated in an inverted solar cell structure. ³ The active layer has 5% SEPS content. ⁴ TAA-1 is BTP-S11:BTP-S12:BTP-S2 with 0.7:0.3:0.2 weight mass ratio. ⁵ Indoor illumination condition of LED 1000 lx (0.23 mW/cm²).

Table 2. Device parameters from high-performance and small-area ($\approx$1 cm²) organic solar cells.

Active Area Materials (w:w) 1	Deposition Techniques	Area (cm2)	JSC (mA/cm2)	VOC (V)	FF (%)	PCE (%)	Ref.
PM6/PYFT-o (1:1)	spin coating	-	23.75	0.902	70.72	15.23	[40]
PM6:Y6 2 (1:1.2)	spin coating	0.07	25.2	0.82	76.1	15.7	[47]
PM6:L8-BO:BTP-eC9 3 (1:0.6:0.6)	spin coating	1	25.67	0.86	71.40	15.71	[41]
PM6:Y6:Y7:Y18:N3: BTP-eC9 (1:0.24:0.24:0.24:0.24:0.24)	doctor blade	0.1	28.3	0.87	73.0	17.6	[43]
PM7-Thy10:L8-BO (1:1.2)	spin coating	0.04	25.64	0.88	76	17.05	[48]
PM6:L8-BO:BTP-eC9 (1:0:65:0.65)	spin coating	1.05	27.98	0.88	74.53	18.41	[44]
PM6:BTP-eC9 (1:1.2)	spin coating	~0.03	28.69	0.842	77.17	18.67	[46]
PM6:BTP-eC9:L8-BO-F	spin coating	~0.03	28.56	0.856	77.82	19.05	[46]
PM6:TAA-1 4 (1:1.2)	spin coating	0.06	27.2	0.890	80.0	19.3	[42]
PM6:Y6:2PACz (1:1.2:0.05)	spin coating	1	0.146 5	0.720 5	77.0 5	36.3 5	[45]

¹ w:w is a weight mass ratio. ² Device fabricated in an inverted solar cell structure. ³ The active layer has 5% SEPS content. ⁴ TAA-1 is BTP-S11:BTP-S12:BTP-S2 with 0.7:0.3:0.2 weight mass ratio. ⁵ Indoor illumination condition of LED 1000 lx (0.23 mW/cm²).

A similar investigation was performed for large-area OPVs and is summarized in

Table 3. Device parameters from high-performance and large-area ($\gg$1 cm²) organic photovoltaics.

Active Area Materials (w:w) 1	Deposition Techniques	Area (cm2)	JSC (mA/cm2)	VOC (V)	FF (%)	PCE (%)	Ref.
PM6-PBDBT(55):IPC1CN-BBO-IC2Cl (1:1) 2	blade coating	58.50	2.209	8.16	63.7	11.28	[49]
PM6:Y6 2	blade coating	216	1.90	12.75	48.5	11.72	[50]
PM6:Qx-1 (1:1.5) 2	slot-die coating	30	3.53	5.236	66	12.20	[51]
PM6:Y6-C12:PC61BM (1:1.2:0.24) 2	blade coating	204	0.608	31.5	76.0	15.08	[52]
PM6:BO-4Cl (1:1.2)	spin coating	19.31	3.66	5.945	72.39	15.74	[53]
PM6/BTP-eC9 3	blade coating	28.82	1.659	13.15	73.50	16.04	[54]

¹ w:w is a weight mass ratio. ² Device fabricated in an inverted solar cell structure. ³ A planar heterojunction is applied in the place of widely used bulk heterojunctions.

. One of the challenges in current OPV technology is scaling up without losing efficiency. A careful choice of materials and techniques are to be investigated in order to assess that problem. For instance, Zheng et al. [41] obtained a 19.26% PCE at 0.06 cm² from a PM6:L8-BO:BTP-eC9 active layer over glass/ITO/PEDOT:PSS. When transitioning to a larger-area (1 cm²) device over flexible TPU, efficiency decreased to 16.23%. On the other side, in a promising result, Gopikrishna et al. [49] demonstrated OPVs that were able to retain PCE with a minor change in the fill factor, even after an area extension from 1 cm² to 58.50 cm². Among the most used strategies is the application of an inverted device structure (i.e., anode on the top of the stack) with non-fullerene acceptors (NFA). This frequently demands ZnO as the electron transport layer (ETL) and MoO₃ as the hole transport layer (HTL). A strategy to enhance scalability and stability is the application of interfacial layers. Liao et al. [50] added polyethylenimine (PEI) between the ETL and the AL in an inverted structure. In addition, all the layers were deposited by blade coating. Photographs of an OPV module and the current vs. voltage characteristic curves obtained for different concentrations of PEI are shown in Figure 5a,b, respectively. Shen et al. [51] stored similar devices in a glove box for over 6000 h and the PCE remained at 103% of its initial values, pointing to enhanced shelf stability.

Another issue to be faced by the OPV industry is the use of non-halogenated solvents. Basu et al. [52] blade-coated a whole large-area module from non-halogenated solvents in ambient air. In addition, barely any performance loss was observed upon upscaling from 0.04 cm² cells to ca. 200 cm² modules. A photograph of one of the modules and the obtained record performance are shown in Figure 5c and d, respectively. Eco-friendly volatile additives are another way to go for the current devices. Jia et al. [53] demonstrated that the addition of menthol alleviates the disordered aggregation of acceptors and enhances crystallization, thus contributing to more efficient exciton dissociation and charge carrier transport. Liu et al. [54], on the other hand, opted to add the methyl nicotinate to the donor solution.

The chemical structure of some of the most used hole transporting molecules and active layer donors are summarized in Figure 6. 2PACz [44,45,53] and its derivatives obtained by chlorine substitution [46], as shown in Figure 6a, have emerged as a promising hole transporting material. Unlike PEDOT, which is often plagued by issues such as acidic nature and limited stability, 2PACz offers improved stability and compatibility with various active layers [44,45,53]. Additionally, 2PACz enhances the overall efficiency of OPV devices by providing better energy level alignment and facilitating more efficient charge extraction [44,45,53]. PM6, as shown in Figure 6b, is a widely used donor molecule, which features a backbone of benzo [1,2-b:4,5-b′]dithiophene (BDT) and thiophene units, offering well-balanced energy levels and strong absorption characteristics [40,41,42,43,44,45,46,47,49,50,51,52,53,54]. In addition, the presence of alkylthio side chains on the BDT units enhances its solubility and improve film morphology. Building on poly[[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl [5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo [1,2-c:4,5-c′]dithiophene-1,3-diyl]] (PBDB-T) [49], PM6 [40,41,42,43,44,45,46,47,49,50,51,52,53,54] and PM7 [48] features fluorine and chlorine substitutions, respectively, for optimizing molecular interactions and enhancing charge transport properties.

The chemical structure of some of the most used active layer acceptor molecules are given in Figure 7. Y6, also known as BTP-4F, is a widely used non-fullerene acceptor molecule in OPV technology due to its high efficiency in OPV and suitable charge transport properties [43,45,47,50,52]. This molecule has been a cornerstone for developing various derivatives. Among these derivatives is L8-BO, also known as L8-BO-2F, which shares the thienothienopyrrolo-thienothienoindole (TTP-TTI) core units and features branched 2-butyloctyl side chains that promote solubility and enhance film morphology [41,44,48]. Building on L8-BO, L8-BO-F is another highly efficient non-fullerene acceptor in the Y6 family. It retains the same TTP-TTI core units and incorporates mono-fluorinated peripheral 2-(5 or 6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile end-groups. Like L8-BO, L8-BO-F’s branched side chains contribute to improved packing, balanced charge transport, and reduced charge recombination, making it an excellent candidate for high-performance OPVs [46].

Despite these efforts, by comparing Table 2 and Table 3, it becomes clear that the performance of current commercial OPV modules lies far from benchmark results obtained from individual small cells. In Table 2, the top-performing cell under AM 1.5G spectra was fabricated on ITO-covered glass [44]. For instance, Wan et al. [48] achieved more than 17% PCE over glass, but 13.7% over stretchable thermoplastic polyurethane (TPU). The device structure of the intrinsically stretchable polymer solar cells (PSCs), an image, and current versus voltage characteristic curves are given in Figure 8a, b and c, respectively. In addition, as shown in Figure 8d, the efficiency decreased to ca. 80% of its initial value after 43% strain. Similarly, most results in Table 3 are based on modules processed on glass substrates. Once again, the highest PCE was observed on ITO glass [54]. There is just one exception that employed a polyethylene terephthalate (PET) substrate [51]. In addition, it is worth noting that slot-die coating used by Shen et al. [51] is usually the closest technique to the one used by the OPV industry, i.e., roll-to-roll processing.

4. Discussion

A focused review of scientific and technical literature on thin-film photovoltaics, specifically organic-based technologies, was conducted using various academic databases such as IEEE Xplore and Google Scholar, among other reliable sources. In this context, information on the raw materials used in OPV manufacturing was collected, covering conductive polymers, donor and acceptor molecules, transparent electrodes, and substrates. This data collection encompassed the physical and chemical properties of different elements and their associated costs. Additionally, manufacturing processes of OPVs, such as film deposition, roll-to-roll printing, lamination, and encapsulation, were explored. Information on relevant techniques, equipment, and operational parameters was also gathered.

Concurrently, a detailed survey of existing OPV technology was conducted, covering recent advances in efficiency, stability, and practical applications. This involved analyzing scientific articles, market reports, and technical documents describing procedures for analyzing the optical and electrical properties of the films. The bibliographies used led to an in-depth analysis of the collected data, identifying trends, correlations, and relevant insights, and enabling comparison among different materials, processes, and technologies to evaluate their advantages and disadvantages.

The results of these inquiries were interpreted in light of the reviewed literature and analyses conducted, emphasizing the implications of the findings for the thin-film and OPV industry. Key findings and conclusions were summarized, highlighting essential points related to OPV raw materials, manufacturing processes, and existing technology. Based on these conclusions, the aim is to present recommendations for future research, technological developments, and practical applications in the field of OPV and thin films. It is essential to ensure that all data sources and information used are properly reviewed and referenced following appropriate standards.

Based on the detailed results presented in Section 3 and Section 4, significant advancements in understanding and applying thin-film technologies, with a specific focus on OPVs, can be observed. The comprehensive analysis of these results reveals valuable insights into raw materials, manufacturing processes, and existing technologies in this field. Recent studies indicate that materials such as conducting polymers, donor, and acceptor molecules are increasingly recognized for their potential to enhance the efficiency and stability of OPVs. Furthermore, advances in nanotechnology have played a crucial role in synthesizing more effective organic materials for use in OPVs. The results also highlight the evolution of manufacturing processes, with innovative techniques such as inkjet deposition promising to increase efficiency and reduce production costs of OPVs. The importance of lamination and encapsulation in protecting against adverse environmental conditions was also underscored.

In the broader context, the technological advancements discussed in this review have the potential to significantly drive the transition towards cleaner and more sustainable energy sources. The growing efficiency and diversification of OPV applications, including in wearable electronics, signify the crucial role of these technologies in innovation and the pursuit of more effective energy solutions. In summary, the results of this analysis reinforce the ongoing importance of development and research in thin film technologies, especially in the field of OPVs, to address global energy challenges and move towards a more sustainable future.

5. Conclusions

This review provides an in-depth overview of the thin-film PV industry, with a particular focus on OPVs, covering crucial topics such as raw materials, the manufacturing processes of the elements, and the current technology employed. It was shown that scaling OPV technology from laboratory research to industrial production faces several significant bottlenecks. One of the primary challenges is the stability and degradation of organic materials, which are susceptible to environmental factors such as oxygen, moisture, and UV light, necessitating effective encapsulation techniques to ensure long-term durability. Maintaining high PCE in large-scale production is another hurdle, as controlling the nanoscale morphology of the active layer consistently over large areas is difficult. The scalability of the production techniques poses additional challenges; while methods like spin coating are effective in laboratories, they are unsuitable for industrial-scale manufacturing, where techniques like roll-to-roll processing need optimization.

Ensuring uniform thickness and quality across extensive areas further complicates manufacturing. The high material and production costs also impede commercial viability, highlighting the need for affordable, high-performance organic materials and cost-effective manufacturing processes. Environmental and health safety concerns, such as the toxicity of some materials and the need for sustainable recycling and disposal methods, add to the complexity. The effective integration of OPV modules into various applications, such as building-integrated photovoltaics (BIPV), requires innovative design and standardization to ensure quality and performance consistency. Market acceptance and economic viability are also critical; building confidence among consumers, investors, and industry stakeholders, alongside demonstrating the economic benefits and environmental advantages of OPVs, is crucial for widespread adoption. Continuous research and development, involving collaboration between academia, industry, and government, is essential to address these technical and economic barriers, improve efficiency, and discover new materials.

6. Future Prospects

It was clearly evidenced that the strategic choice of raw materials exerts a crucial influence on the performance and efficiency of OPVs. The relentless pursuit of more effective conductive polymers and donor and acceptor molecules, coupled with the continuous refinement of the properties of these materials, significantly drives the progress of OPVs. Based on the latest findings, it becomes evident that raw material options continue to expand and improve, enabling increasingly pronounced optimization of OPVs.

The manufacturing processes of OPV elements have evolved significantly, incorporating advanced techniques of film deposition, lamination, and encapsulation. The results indicate that inkjet and blade coating-based printing techniques are streamlining large-scale production by allowing precise control over film thickness and uniformity, reducing costs through lower material waste, and enhancing efficiency by increasing production speed and scalability. Furthermore, the emphasis on the quality and reliability of the manufacturing processes is promoting the durability of OPVs, making them more suitable for practical applications. Existing OPV technology is progressing rapidly, resulting in substantial improvements in the efficiency and stability of these devices.

The conclusions of this report underscore the critical role of advancements in device engineering, interface optimization, and sustained research in enhancing energy efficiency. As a rapidly emerging alternative to conventional solar cell technologies, OPVs offer significant advantages, including flexibility, integration into a wide range of surfaces, and a smaller environmental footprint. Their flexibility and lightweight nature make OPVs ideal for integration into various applications, such as textiles, portable electronics, and BIPV. The future may see OPVs embedded in windows, walls, and even clothing, expanding their utility beyond traditional solar applications.

In sustainability, OPVs hold promise for eco-friendly manufacturing, leveraging non-toxic and abundant materials to minimize their environmental impact compared to silicon-based solar cells. As the push for more sustainable technologies grows, this aspect could significantly enhance the appeal of OPVs. Furthermore, there is potential for OPVs to be combined with other types of solar cells, such as perovskite solar cells, to create tandem or hybrid systems capable of capturing a broader spectrum of sunlight, thereby leading to higher overall efficiencies. Future research should prioritize achieving more competitive PCE, enhancing long-term stability, and reducing costs associated with both materials and the processing of large-area OPV modules.

In summary, this review highlights that thin films, especially OPVs, are destined to play a pivotal role, with potential applications in various sectors, from wearable electronics to integration into buildings and vehicles. This technical report underscores the study of the transformative potential of thin films, especially OPVs, and emphasizes the importance of continuous research, innovation, and collaboration for the advancement of these technologies. Thin films emerge as promising solutions that can significantly contribute to a more sustainable and energy-efficient future.