Potential Effects of Various Optical Filtration Layers on the Techno-Economic Performance of Solar Photovoltaic/Thermal Modules: Status and Prospects

Abstract

This paper aims to review and summarize the performance assessment of PV/T modules with optical filtration layers and different materials designed to achieve full spectral utilization of sunlight through absorptive, refractive, reflective, and diffractive approaches. Different categories of optical filtration layers, including nanofluids, nano-enhanced phase change materials, the luminescent down-shifting technique, the radiative cooling technique, the colored optical technique, nanowires, and polymer materials, are examined and compared. Additionally, the cost-effectiveness of PV/T modules with optical filtration layers is evaluated by using the net present values, price-performance factor, least cost of energy, and life-cycle cost method in practical applications. This paper also discusses current challenges, future perspectives, recommendations, and potential applications aimed at overcoming the limitations for real-world implementation. Results conclude that the overall energy performance of the PV/T system with optical filtration layers can be enhanced by 85–90%, while the system payback period is reduced to less than 6 years compared to conventional PV/T modules.

1. Introduction

Fossil fuel energy consumption has become a progressively serious issue for carbon dioxide emissions and climate change. Based on the Paris Agreement, the Earth must realize a 45% carbon dioxide emission reduction by 2030 and reach net zero by 2050 [1]. Solar energy is one of the most ideal and extensively available replaceable energy sources for fossil fuel sources, which can reach the Earth in the forms of light and heat [2,3]. The solar spectrum wavelength varies in the range from 250 nm to 3000 nm, which consists of ultraviolet light (UV) from 250 nm to 380 nm, visible light (VIS) from 380 nm to 740 nm, and infrared light (IR) from 740 nm to 2500 nm [4]. Sunlight exhibits a high energy level and frequency at a short wavelength range [5,6].

Solar energy systems attract the attention of researchers because of providing economical usage and environmental cleanliness. Taking the restrictions of solar thermal collectors and PV technology into account, an integration technology of a PV module and solar thermal collector into a single module is exploited as a photovoltaic/thermal (PV/T) module for heat and electricity output simultaneously [7,8]. The PV/T module could harvest approximately 80% of the solar irradiation, which is much more efficient compared to the standalone PV module [9,10]. Nevertheless, all the incident photons are directed to the PV module surface in a traditional PV/T module, and the low-energy photons are not capable of being converted into power and are thus transferred into waste heat. Due to the band gap of PV module material, high-energy photons are not able to be transformed into electricity efficiently [11]. Consequently, in order to deal with the contradictions, the optical filtration layer is developed to resolve the pyrolytic coupling of photo-to-thermal and photo-to-electric processes [12,13]. The working principle of this technique is based on the fact that only photons in the solar spectrum with energy greater than the band gap of the semiconductor material can generate the photovoltaic effect, while the remaining photon energy is ultimately converted into heat. The optical filtration layer is regarded as a promising and advanced method to address the trade-off between thermal and electrical performance for the PV/T module. The portion of solar radiation aligned with the PV cell’s spectral response is directly incident upon the module for photoelectric conversion, whereas the remaining spectrum is directed via an optical filtration layer to solar thermal collectors for conversion [14,15]. Currently, the types of optical filtration layers mainly include nanofluid optical filtration layers, nano-enhanced phase change material (PCM) optical filtration layers, liquid luminescent down-shifting (LDS) optical filtration layers, radiative cooling optical filtration layers, water optical filtration layers, and colored optical filtration layers. Some previous research has reviewed the nanofluid optical filter in the solar PV/T module for solving the colloidal stability and agglomeration issues, and focused on the multipurpose application of the PV/T module. Specifically, Abdelrazik [16] provided a recent review regarding the influence of using a nano-liquid optical filtration layer on the solar PV/T performance based on experimental and numerical investigations. Pan et al. [17] reviewed systematically the development and application of nanofluid-driven multiple functional hybrid solar energy systems, such as the solar desalination system, solar thermal energy storage system, and solar lighting and heating system, based on the different bands of the solar spectrum and nanofluids’ thermal physical properties. Hong et al. [18] introduced briefly the solar-to-thermal, solar-to-electrical, and solar-to-chemistry energy conversions and heat recovery of the PV/T module with an optical filtration layer, and illustrated the technical merits and approaches to promote their applications. Ko et al. [19] summarized the merits and restrictions of diverse optically active materials on the solar electrical, thermal, optical, and chemical energy conversion performance based on different optical bands. Nevertheless, based on the research, it is found that few articles summarize the technical characteristics of the solar PV/T modules with various categories of optical filtration layers and solutions, such as the nano-enhanced phase change materials (PCM), liquid luminescent down-shifting (LDS) technique, radiative cooling technique, colored optical filter technique, as well as other types. Meanwhile, the effects of other liquid optical filtration layers on the energy conversion performance, efficiency enhancement, and absorption ratio enhancement are not mentioned. Furthermore, the economic assessment of the PV/T module with an optical filtration layer has not been discussed and analyzed.

Consequently, this paper aims to present different optical filtration layers based on spectral splitting theory for the solar PV/T module application, and review the latest research on optical filtration layers and the techno-economic performance of the hybrid PV/T module. An innovation comparison over the recent review papers is shown in

Table 1. Innovation comparison.

Item	Innovation	Key Findings
Kumar et al. [20]	This review focuses on developing the use of plasmonic nanofluids in solar PV/T module and test the plasmonic nanofluids for medium to high temperature.	The use of blended nanoparticles enables broadband absorption of the solar spectrum. PV/T with plasmonic nanofluids could significantly improve in the photo-thermal conversion efficiency.
Kumar et al. [21]	Summarized the technological and scientific advancement in PV/T modules using nanomaterials based on spectral beam splitting involving the experimental test, numerical simulation, and economical prospective.	Linear Fresnel mirror-based BSPV/T showed significant enhancement. The fluid temperature can reach up to 100 °C without much affecting the PV surface temperature. Nanofluids in BSPV/T system can achieve high temperatures compared to standalone thermal systems.
Yazdanifard et al. [22]	Summarize the configuration, effectiveness and advancement of the PV/T module integrated with nanofluids including. Investigate the effective parameters like the size and volume fraction of the nanoparticles, concentration on the system performance.	The system performance is more efficient in laminar regime than that of the turbulent one, when the nanoparticles are introduced. The water based nanofluids exhibit higher energy and exergy efficiency than that of the ethylene glycol water-based nanofluids.
Current paper	Summarizes the influence of different optical filtration layers integrated with PV/T module on energy conversion and economic performance.	The overall energy performance of the PV/T with optical filtration layers could be enhanced by 85–90%. The system payback period is less than 6 years in comparison to that of the normal PV/T module.

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The basic concepts of the PV/T module with optical filtration layers are introduced in Section 1. Then, the state-of-the-art optical filtration layer technologies, involving absorptive-, refractive-, reflective-, and diffractive-based PV/T modules, are illustrated in Section 2. Afterwards in Section 3, various layer materials and solutions are extensively reviewed based on thermal and electrical performance comparison between the PV/T modules with and without optical filtration layers under different environmental conditions. Furthermore, in Section 4, the economic performance of the PV/T module with optical filtration layers is explored to get the initial investment, overall cost savings, and dynamic payback period by using net present values (NPV), price-performance factor (PPF), least cost of Energy (LCOE), and life-cycle cost (LCC) methods in practical application. In Section 5, the technical challenges and prospective applications of the PV/T module with optical filtration layers are elaborated. Finally, the crucial outcomes are concluded in Section 6.

2. Optical Filtration Layer Based on Spectral Splitting Theory in the PV/T Modules

The design of conventional PV/T modules aims to minimize thermal resistance between the PV module, solar collector, and working fluid. To address this trade-off between power generation and waste heat utilization, a spectral splitting mechanism has been proposed as a promising solution [23,24]. Four main spectral splitting strategies—absorptive, refractive, reflective, and diffractive methods—have been implemented using various optical filtration layers, as summarized in the following section.

2.1. Absorptive-Based Optical Filtration Layer Technology

Absorptive optical filtration layers could separate the spectrum by selecting the allowed wavelengths to traverse the PV/T module, but the residual solar radiation that cannot be converted to power is directly absorbed by the optical filtration layer in the form of heat, like enthalpy or internal energy. Henein, a typical PV/T module with an absorptive optical filtration layer contributes to increasing its enthalpy or internal energy. Specifically, Han et al. [25] experimentally explored the optical transmittance of a concentrated PV/T (CPV/T) module by adding various categories of glycols like glycerol, propylene glycol, and ethylene glycol, and found that all glycols display beyond 95% transmittance in the wavelength range from 350 nm to 885 nm. Later, Han et al. [26] added a sharp absorption edge of 650 nm (HB650) and propylene glycol (PG) into the liquid absorption optical filtration layer, and concluded that introduction of the HB650 and PG conduces to improving overall energy efficiency by 44.67% compared with that of the PV panel alone.

Yu et al. [27] developed a CPV/T module with a triangular and plate flat module, incorporating the Ag@ SiO₂ and ethylene glycol (EG) nanofluid as an absorptive optical filtration layer to enhance the thermal and electrical efficiencies, as exhibited in Figure 1a. It is observed that the average outlet temperature distributions of CPV/T with Ag@ SiO₂ and EG nanofluid filtering layers are 316.3 K for 0.01 kg/s and 302.0 K for 0.05 kg/s, respectively. El-Samie et al. [28] developed a 3D numerical module of the CPV/T with an absorption optical filter to evaluate the influence of the solar concentration ratio on the CPV/T energy conversion efficiency by using CFD 2020 R1 software. It is demonstrated from Figure 1b that the photo-to-electric conversion efficiency of the absorption optical filtration layer based on CPV/T is improved by 6.6% for 5 solar concentrations and by 7.8% for 10 solar concentrations, respectively. Elharoun et al. [29] developed an innovative compound parabolic concentrator (CPC) with a liquid absorption optical filtration layer to assess electricity conversion efficiency and the effect of air-layer thickness on overall system performance, as illustrated in Figure 1c. It is concluded that the electricity conversion efficiency of the CPC with an absorption optical filter is around 17% higher than that without an absorption optical filtration layer. Alnajideen and Min [30] developed a novel PV-thermo-electric (PV/TE) unit with a dichroic mirror as the absorption optical filtration layer in order to assess the overall system efficiency. The results demonstrate that the electrical output of the PV/TE module with a dichroic mirror improves from 12.41 mW to 62.04 mW. Meanwhile, the cut-off wavelength occurs at 700 nm, 712 nm, and 740 nm for 71°, 60°, and 45° of the incidence angle, respectively, implying that the optical performance of the dichroic mirror is sensitive to the incidence angle.

2.2. Reflective-Based Optical Filtration Layer Technology

Various types of thin membrane materials, owing to excellent optical properties, are usually utilized as the reflective optical filtration layers applied to the solar PV/T modules. To be more specific, Li et al. [31] designed a novel TiO₂–Na₃AlF₆ planar reflective optical filtration layer in the CPV/T module for assessing the reflectivity, optical efficiency, and energy conversion efficiency, as displayed in Figure 2a. It is demonstrated that the splitter spectral reflectivity of the TiO₂–Na₃AlF₆ planar layer could reach 62.4% when the included angle is between the generatrix and the axis of 45°. Zambrano-Mera et al. [32] numerically explored the effect of adding Zr-oxide multiple layers into the TiO₂@ SiO₂-reflective material on optical transmittance performance. As shown in Figure 2b, all of the Zr-oxide multiple layers exhibit excellent homogeneity and strong interfacial adhesion, contributing to reducing the light scattering and providing a better transmittance. As illustrated in Figure 2c, the average maximum transmittance of ARZr-0 reaches 94%, corresponding to a 6% increase in glass transmittance within the wavelength range of 400 nm to 750 nm. Kandil et al. [33] experimentally investigated a PV/PCM module combined with the reflective optical filtration layer for optimizing energy conversion, as shown in Figure 2d. It is obtained from Figure 2e that the energy output of the PV/PCM with a reflective optical filtration layer has a significant enhancement from 984.96 kJ to 1684.8 kJ. This means that the reflective optical filtration layer contributes to attaining higher electrical energy production than that of the standalone PV module.

2.3. Refractive-Based Optical Filtration Layer Technology

The refractive optical filtration layer works based on light refraction, which involves a change in the direction of incoming sunlight as it passes between media with different refractive indices. This optical element is typically implemented as an array of solid transparent prisms, each functioning independently [34]. Each prism is oriented in such a way that the light rays of a specific reference wavelength are mapped on the same target region, as described in Figure 3a. Zhao et al. [35] developed a luminescent solar concentrator (LSC) using a colloidal carbon dots (C-dots@) polyvinylpyrrolidone module to explore the influence of the refractive index on optical efficiency, as shown in Figure 3b. Results reveal that the LSC with the C-dots@ polyvinylpyrrolidone module is conducive to enhancing the optical efficiency by 27%, indicating that the optical efficiency depends on the refractive index. Liu et al. [36] verified a luminescent solar concentrator (LSC) module based on a “laminated glass” structure including two waveguide layers and a polymethyl methacrylate (PMMA)@ quantum dots (QDs) interlayer to enhance the external optical efficiency. It is concluded that the maximum external optical efficiency is 3.4% for the PMMA@ QDs-based LSC, which is 92% higher than that of a Cadmium selenide-based LSC.

Wu et al. [38] synthesized a gradient refractive index SiO₂ using the combined influences of catalysis to enhance electricity generation, and found that the average transmittance of the PV with SiO₂ sols achieves 97.57% in the wavelength range from 380 nm to 1800 nm, demonstrating significant improvement in broadband transmittance. Saura et al. [37] utilized the Helios 3198 CPV solar simulator to experimentally investigate the spectral non-uniformities’ distributions for the reflective and refractive systems, as illustrated in Figure 3c. Results obtained from Figure 3d–f show that the spectral uniformity ratio ranges from 1.54 to 1.77 for the refractive system, compared to only 1.23 to 1.36 for the reflective system. Additionally, the irradiance peak-to-average ratio of the reflective system reached 4.99 and 5.23 at the top and middle of the sub-cell, respectively. This means that the performance of both systems is highly sensitive to the concentrator-to-receiver distance. Consequently, selecting an appropriate distance is critical for mitigating performance degradation in concentrator photovoltaic (CPV) module design.

2.4. Diffractive-Based Optical Filtration Layer Technology

Diffractive optical filtration layer technology depends on the light diffraction through a diffractive optical element (DOE). Diffractive-based filters act not only as spectral splitting filters but also as concentrators. For example, Kim et al. [39] explored the influences of diffractive nanostructures of TiO₂ and SiO₂ layers on the short-circuit current density loss of the solar PV cell. The novel diffractive nanostructures are prepared based on the transfer lithography and nanoscale imprinting approaches. It is evidenced that the increase in the short-circuit current density loss of SiO₂ has little effect on the loss of color quality in comparison with TiO₂. Gün and Yüce [40] experimentally achieved simultaneous concentration and spectral splitting of broadband light using an optimized diffractive optical element (DOE) wavefront shaping approach. Results concluded that the spectral splitting efficiencies reach 66%, 57%, and 52% for the red, green, and blue diffractive layers, respectively. Similarly, Huang et al. [41] numerically investigated a DOE-based optical filter applied to a lateral multijunction solar cell, reporting an optical efficiency exceeding 80% at a thickness of 250 μm.

To sum up, optical filtration layers used in PV/T modules can be categorized into four types based on absorptive, reflective, refractive, and diffractive mechanisms for performance enhancement. Notably, only absorptive filters require specific thermal properties due to the use of liquid media such as water, nanofluids, or other working fluids. In contrast, refractive, diffractive, and reflective optical filtration layers—typically solid materials—exhibit significantly different thermal behavior. These solid filters generally exhibit low absorptivity, resulting in minimal solar heat absorption, and possess low thermal expansion coefficients, which contribute to reducing thermal stress. Currently, the absorptive and reflective optical filtration layers are extensively utilized in the PV/T module, whereas refractive and diffractive types have received limited numerical and experimental attention, particularly in PV/T applications. Research on refractive and diffractive layers has primarily focused on concentrated PV/T (CPV/T) systems for utility-scale applications, as they can function simultaneously as concentrators and spectral splitters—a promising direction for future development. However, PV/T modules integrating selectively-reflective optical filtration layers remain costly and limited in size, hindering large-scale engineering adoption. Therefore, developing scalable and cost-effective reflective optical filtration technologies is essential to facilitate the broader application of optically filtered PV/T systems.

3. Different Types of Optical Filtration Layers

3.1. Nanofluid Optical Filtration Layer

Nanofluids are appropriate and profitable materials for enhancing the photo-to-electric and photo-to-thermal energy conversions of a PV/T module. The thermophysical characteristics of nanofluid filters could be tuned and governed based on the band gap of PV cells. Studies have demonstrated that the overall energy efficiency of PV/T systems equipped with nanofluid-based optical filtration layers, at nanoparticle concentrations ranging from 0 to 200 ppm, is higher than conventional photovoltaic conversion efficiency. To be more specific, Xia et al. [42] studied the performance of a solar PV/T system integrated with various concentrations of carbon nanotube (CNT) Ag nanofluids as an optical filtration layer, as shown in Figure 4a. It can be seen from Figure 4b that the optical filtration efficiency based on the CNT@ Ag nanofluid could reach 18.3%; meanwhile, the system thermal and electrical efficiencies are 45% and 8.2%, respectively. This means that the optical synergistic influence is beneficial to the system performance. Meraje et al. [43] designed a solar concentrated photovoltaic/thermal (CPV/T) module integrated with a linear Fresnel lens and ZnO nanofluid to improve the system efficiency, as shown in Figure 4c. Results from Figure 4d show that the CPV/T system efficiency is 76.1%, which is higher than that of the normal CPV/T system with 56.47%. Elharoun et al. [44] investigated a compound parabolic concentrator (CPC) system combined with an optical filtration layer based on ZnO nanofluid to evaluate the influence of optical filter layer thickness on the system performance, as illustrated in Figure 4d. The homogeneous ZnO nanofluid is prepared using an ultrasonic stirrer operating at 20 Hz for 20 min with a maximum power of 700 W, as described in Figure 4e. Results demonstrate from Figure 4f that systems with optical filtration layers exhibit higher electrical efficiency than those without, with efficiency increasing from 6.2% to 7.9% as the layer thickness increases from 1 cm to 2 cm. Correspondingly, the electrical, thermal, and overall efficiencies also improved, rising from 6.5% to 8.3%, 23.6% to 45.9%, and 27.3% to 50%, respectively. Lee et al. [45] discovered a lab-scale de-coupled PV/T module using emulsion optical filtration filters, including fish oil, silicone oil, soybean oil, and mineral oil. The system consists of a heat exchanger, a filter layer, a tank, and a thermostat, as shown in Figure 4g. It is demonstrated that when the emulsion filter ultimately approaches the red ideal dotted line, a plan for high-energy conversion of solar sunlight could be achieved, as exhibited in Figure 4h. The results show that the overall efficiency of the de-coupled PV/T module with a fish oil emulsion filter is 84.4%, which is 6.4% higher than that of a module using a water filter. Furthermore, the overall energy efficiency of the fish oil emulsion-based system is 84.4%, representing an 18.3% increase compared to a traditional PV/T module, as shown in Figure 4i. These findings indicate that the emulsion nanofluid filter could achieve the highest thermal energy efficiency and enhance the overall energy output. Liang et al. [46] experimentally investigated the influences of different concentrations of ZnO@ glycol nanofluid on electrical and thermal energy efficiencies of a concentrating photovoltaic thermal (CPV/T) system with an optical filtration layer. Results reveal that the electrical energy production is 12.9 W, 13.3 W, 13.9 W, and 14.26 W, corresponding to the concentrations of the glycol-ZnO nanofluid of 13.1%, 13.55%, 13.14% and 14.49%, respectively. This indicates that the higher concentration of glycol-ZnO nanofluid allows more photon energy near the band gap of the Si semiconductor to be captured. In contrast, the thermal energy efficiencies of the system under the same concentrations are 10.97%, 9.19%, 8.17%, and 7.4%, suggesting that lower concentrations of glycol-ZnO nanofluid are more beneficial for achieving higher thermal energy output.

Hashemian et al. [47] designed and experimentally investigated an optical filtration layer with Ag@ Cr₂O₃ nanofluid to ameliorate the PV/T system performance. It is demonstrated that the Ag@ Cr₂O₃ nanofluid filter could obtain the maximum thermal energy conversion efficiency by 31.55% when the concentration of Ag@ Cr₂O₃ nanofluid is 80 ppm. By comparison, this value is merely 13.25% without the optical filtration layer in the wavelength range from 650 nm and 1075 nm. Han et al. [48] investigated the effect of introducing the cobalt sulfate (CoSO₄) and silver (Ag) solutions into the optical filtration layer on transmittance, merit function (MF), and energy conversion efficiency of the nanofluid filter. It is observed that Ag@ CoSO_4–1 with a high concentration of Ag contributes to capturing wavelengths between 350 nm and 550 nm; by comparison, the Ag@ CoSO_4–2 and Ag@ CoSO_4–3 nanofluids with a low concentration of Ag absorb less solar radiation during the period of the short wavelength spectrum. In addition, the mass fraction of Ag nanoparticles has a vital influence on the MF variation; the MF values with Ag@ CoSO₄ as nanofluid filters could achieve an approximately constant value of higher than 1.371 in the range from 24 ppm to 57 ppm. In comparison with the PV panel alone, the highest optical efficiency of the PV/T module with the Ag@ CoSO_4–1 nanofluid optical filtration layer could achieve 53.1%, resulting in 36.3% of economic saving, and also the MF values of all the designated optical filtration layers are greater than 1, indicating that the PV/T system using a nanofluid filtration layer displays superior performance. Later, Han et al. [49] studied the performance enhancement of a PV/T system by dispersing Ag nanoparticles into propylene glycol (PG) and CoSO₄. It is demonstrated that the system efficiency of optical filtration layers with Ag@ CoSO₄-PG nanofluid exhibits 9% greater compared to that of the system with Ag@ CoSO₄-water optical filters. Abdelrazik et al. [50,51] numerically explored the effects of the base-Ag@ water optical filtration length and nanofluid concentration on the transmittance of the PV/T system via the Rayleigh approach. It was found that the transmittance is superior at low concentrations and shorter path lengths than that without an optical filtration layer. Zhang et al. [52] numerically discovered the effects of Ag@ water nanofluids with particle sizes of 40 nm and 90 nm as an optical filtration layer on the PV/T system energy performance by using the Monte Carlo approach. Results concluded that the 90 nm Ag nanofluid has a broad-spectrum absorption, which is optimal for thermal energy output, while the 40 nm Ag nanofluid in the range from 700 nm and 1100 nm is more suitable for improving electrical energy production. Du et al. [53] designed an optical plasmonic nanofluid filtration layer to improve the exergetic efficiency of a PV/T system owing to absorbing most of the undesired photons and revealed that the exergetic efficiency of the PV/T module with a plasmonic nanofluid optical filter is improved by 13.3%.

3.2. Liquid Luminescent Down-Shifting Optical Filtration Layer

High-energy ultraviolet (UV) photons are not able to be fully employed owing to energy loss related to the above absorbed band gap photons, absorption losses from front glass and encapsulation materials, and high surface recombination loss. One of the feasible techniques to extend the short wavelength limit of PV cells is the luminescent down-shifting (LDS) technique, which is conducive to converting high-energy UV-photons into lower energy ones at longer wavelength, thus improving the spectral response of c-Si PV cells [54]. To be more specific, Alexandre et al. [55] discovered the UV degradation issues owing to the photo electrons of TiO₂ within the perovskite light trapping in the solar PV cell, as depicted in Figure 5a. Results verified that such optimized photonic resolution contributes to decreasing the detrimental UV photocarrier production and enhancing the perovskite solar cell efficiency by 80% compared to a normal solar cell module, as shown in Figure 5b.

Yang et al. [56] studied the LDS technique to enhance the short-wavelength response and solve the spectral mismatch for a solar cell. As exhibited in Figure 5c, a 17 × 17 cm² of LDS film made of polymer PVA and ternary Eu³⁺ complex is attached on the top of a solar cell. Figure 5d shows that there is about a 15% enhancement in the external quantum efficiency (EQE) in the UV region, which indicates that the LDS layer displays an outstanding UV light-resistance stability. Meanwhile, the photo-electrical energy conversion efficiency of the solar cell is improved, ranging from 15.2% to 15.3% when the solar illumination is set up as AM1.5G. Singh et al. [57] studied various types of natural dyes involving rice husk, wheat husk, Moringa Oleifera leaves, and turmeric rhizomes to make the LDS material coating on the top of the PV cell for improving the electrical efficiency by using the short wavelength photons, as depicted in Figure 5e. The LDS natural dyes are amalgamated into the EVA encapsulation of the PV module, as shown in Figure 5f. Figure 5g shows that the improvement of the PV cell efficiency could attain 9% for moringa dyes and 6.8% for turmeric dyes. Razzaq et al. [58] synthesized the LDS materials by using the cerium-doped yttrium aluminum garnet (YAG: Ce³⁺) and ethylene-vinyl acetate (EVA) on the PV cell in order to transform short-wavelength photons to long-wavelength photons, as shown in Figure 5h. It can be observed from the SEM image that all particles exhibit approximately spherical shapes with a relatively narrow size distribution. Regards the YAG: Ce³⁺ particles of different diameters, the transmittance decreasing is attributed to the fixed mass ratio of phosphor particles to EVA, as illustrated in Figure 5i. Additionally, the external quantum efficiency (EQE) has a slight decline varying from 530 nm to 1100 nm owing to the growth of wavelength reflection, while the EQE is enhanced between 360 nm and 500 nm because of the creation of additional electron-hole pairs, as displayed in Figure 5j. This contributes to improving the electrical conversion efficiency, ranging from 16.99% to 17.94%. Coldrick et al. [59] also concluded that the electrical and thermal energy efficiencies of a PV/T system could reach 77% and 91%, respectively, when the luminescent imidazole-phenanthroline serves as the LDS optical filtration layer. Walshe et al. [60] demonstrated that the LDS optical filtration layer can provide 63% optical efficiency, resulting in an approximately 20% enhancement compared to the normal PV module. In summary, the LDS materials serve as semi-transparent solid layers that enhance conversion efficiency within the short wavelength range of 300–500 nm. The LDS is a passive method that involves applying a luminescent species within a layer prior to the cells, thus excluding the requirement to interfere with the active material of a PV module.

3.3. Water Optical Filtration Layer

Recently, water has been employed as the carrying fluid in the optical filtration layer due to its low cost, low viscosity, innoxious and strong absorption in the infrared region. Specifically, Al-Shohani et al. [61] experimentally investigated a solar PV/T module with a water-based optical filter to examine the effect of water layer thickness on optical and energy conversion efficiency. As shown in Figure 6a, a rectangular cell with an open top was formed using two glass panels sealed with silicone adhesive within a wooden frame; the water thickness was varied from 1 cm to 5 cm. Results in Figure 6b indicate that average transmittance and losses through the water filter were 87.85% and 12.14% for 1 cm, 81.81% and 18.19% for 2 cm, 88.34% and 11.66% for 3 cm, 86.25% and 13.75% for 4 cm, and 82.34% and 17.65% for 5 cm, respectively. Later, Al-Shohani et al. [62] continued to investigate the influences of different thicknesses of the water optical filtration layer on the heat accumulation of the PV panel. As exhibited in Figure 6c, a 30 × 20 cm optical filtration layer is installed at the top of the PV panel. Results concluded that the PV module temperature reductions with the water layer thickness reach 18.4 °C when the water layer thickness is 5 cm, as illustrated in Figure 6d. Lin et al. [63] studied the effects of bubbles within a water optical filter on the extinction rate and transmittance of the PV/T module, as illustrated in Figure 6e. A water layer of 1.4 mm thickness and a glass layer of 2 mm are used, as shown in Figure 6f. It is found that bigger bubbles significantly enhance light extinction—double and triple bubble diameters resulted in approximately fourfold and ninefold increases in extinction, respectively, as shown in Figure 6g. This means that there is an approximately quadratic relationship between the extinction effect and bubble diameter. In addition, in light of most wavelengths, when the number of bubbles is enhanced, the transmissivity is reduced, as displayed in Figure 6h. Almarzooqi et al. [64] evaluated a plexiglass-based water and air optical filter for PV/T performance under Sharjah climate conditions, as shown in Figure 6i. The average system efficiencies were 8.12% with the plexiglass water/air filter, 7.5% with plexiglass alone, and 7.02% for a standard PV module. The water in the filter reached temperatures up to 46 °C, suitable for preheating applications. These results demonstrate that water-based optical filters effectively absorb UV and partial IR radiation while transmitting visible and some IR light to the PV cell, thereby improving power conversion. This technique also helps transmit sunlight in the 0.37–1.18 μm range more uniformly, reducing hot spots and cyclic thermal stress compared to conventional PV/T modules.

3.4. Colored Optical Filtration Layer

Gupta et al. [65] made an experimental investigation of a solar PV/T module with a blue and red semi-transparent optical filtration layer in order to improve the thermal and electrical energy performance. Specifically, the sunlight is directed onto the semi-TSP module, where a portion is converted into thermal and electrical energy production. However, the remaining sunlight is further harvested by a large Fresnel lens and concentrated on a thermal collector module. As shown in Figure 7a, there are six segmented mirrors incorporated for redirection of the concentrated sunlight on the absorber system without tracking. Meanwhile, the energy performance of the PV/T module with a red optical filter is superior to that of the one with a blue optical filter, as illustrated in Figure 7b,c. Amara and Balghouthi [66] investigated the effect of colored optical filters on the electrical output of a PV module, as shown in Figure 7d. Results exhibited that the yellow-colored optical filter can provide the highest short circuit current of 4.70 A, whereas the blue one produced the lowest (3.46 A). The corresponding power outputs reached approximately 54% with the blue filter, 64% with the red, and 73% with the yellow. Wang et al. [67] explored the influences of various optical filtration layers, including the silver-cobalt sulfate/water, silver/water nanofluid, and deionized water, on the thermal and electrical efficiencies of the CPV/T modules, as exhibited in Figure 7e. Results reveal that the thermal and electrical efficiencies reached 5.6% and 5.8%, respectively, using silver@water nanofluid, and 8.5% and 8.0% with silver-cobalt sulfate@ water nanofluid, both surpassing the performance of the deionized water module, as illustrated in Figure 7f,g.

Chawrey et al. [68] examined a gold (Au) plasmonic nanofluid optical filter and reported an overall energy conversion efficiency of 42% for the PV/T module—higher than that with a deionized water filter (37%) or a standalone PV panel (14%). Eisler et al. [69] developed a polyhedral specular reflector (SPR) as an optical filter that can divide and concentrate incident sunlight, as depicted in Figure 7h. It is predicted that 50% efficiency of the seven-junction module could be achieved when the external radiative efficiency of the solar cell is set at 3%, as illustrated in Figure 7i. Cho et al. [70] fabricated a PV module with green, red, and blue color Zn (O, S) membrane optical filters to evaluate the color expression scale, as exhibited in Figure 7j. It was found that the performance of the short-circuit current density (JSC) varied with the color expression, ranging from 6.3% to 9.9%. Rudzikas et al. [71] built a one-dimensional photonic crystal structure by using TiO₂ and SiO₂ for ameliorating the PV cell coloring, as exhibited in Figure 7k. Experimental results evidenced that the efficiencies of PV cells with color optical filters are 16.52% for yellowish, 17.13% for greenish, 17.28% for light blue, as well as 18.75% for black, respectively, as shown in Figure 7l. These results demonstrate that optical filtration layers effectively remove infrared and ultraviolet portions of the solar spectrum, thereby mitigating overheating and enhancing module performance.

Figure 7. Colored optical filtration layers: (a) red and blue color filters of semi-TSP; (b) thermal efficiency; (c) electrical efficiency [65]; (d) experimental rig [66]; (e) images of the D-CPV/T module prototype based on various fluid filter materials; (f) thermal efficiency; (g) overall electric efficiency [67]; (h) conceptional design of PSR; (i) contactless device efficiency [69]; (j) images of PV cell with red, green, and blue color optical filters and reflectance values [70]; (k) photo of mini PV modules with color optical filters; (l) J_sc production [71].

Figure 7. Colored optical filtration layers: (a) red and blue color filters of semi-TSP; (b) thermal efficiency; (c) electrical efficiency [65]; (d) experimental rig [66]; (e) images of the D-CPV/T module prototype based on various fluid filter materials; (f) thermal efficiency; (g) overall electric efficiency [67]; (h) conceptional design of PSR; (i) contactless device efficiency [69]; (j) images of PV cell with red, green, and blue color optical filters and reflectance values [70]; (k) photo of mini PV modules with color optical filters; (l) J_sc production [71].

3.5. Radiative Cooling Optical Filtration Layer

Radiative cooling for solar cells is a hot topic of research. The underlying principle involves deploying a transparent cooling layer atop PV cells to enhance thermal emissivity while maintaining effective solar absorption [72]. Therefore, radiative cooling optical filters are designed to exhibit high transmittance within the solar spectrum and strong thermal emission in the mid-infrared range. However, since commercial solar cells and encapsulation materials such as glass already possess high inherent thermal emissivity, the potential for further enhancing the cooling effect through this approach is limited [73].

For example, Wang et al. [74] synthesized a novel biomass-based passive radiative cooling (Bio-PRC) coating material using long-horned beetle forewings and polytetrafluoroethylene (PTFE) to regulate PV cell temperature, as shown in Figure 8a. The forewing exhibits a highly reflective, golden-shiny surface with favorable optical properties. Results demonstrate that the Bio-PRC coating can achieve an 88% absorptivity and over 92% of emissivity, resulting in a temperature reduction of 3.6 °C under direct sunlight. Zhu et al. [75] developed a novel optical filtration layer based on SiO₂ and polymethyl methacrylate materials, as exhibited in Figure 8b. Results show that the silicon absorber temperature can be decreased by 13 °C because of the radiative cooling. Later, Kiyaee et al. [76] revealed that the optical efficiency can reach 81% within the scope of 600–1150 nm spectral range, which is increased by 14% compared to the 400–1150 nm spectral range. Lee et al. [77] established a color-passive radiative cooling (CPRC) module with an optical filtration film made of Ag, SiO_2, and Si₃N₄ nanomaterials, as exhibited in Figure 8c. It can be observed that there are no significant changes in the reflected colors of the passive radiative cooling sample, metal–insulator–metal (MIM) structure, Ag, and traditional daytime cooler. Results demonstrate that the traditional cooler’s temperature is reduced by an average of 5.9 °C, with a maximum reduction of 9.3 °C compared to the ambient air. In contrast, the MIM module reached temperatures up to 4.5 °C above ambient, while the CPRC sample remained near ambient temperature under strong solar irradiation. Cui et al. [78] made a TPX@ SiO₂ coating on the front and rear surfaces of a mono-facial double-glass PV module to assess the influence of radiative cooling, as exhibited in Figure 8d. Experimental results reveal that the PV with TPX@ SiO₂ coating can achieve a 1 °C cooling effect and 0.21% of efficiency enhancement. Torgerson and Hellhake [79] placed a polymer coating on top of a PV module to enhance radiative cooling power, as illustrated in Figure 8e. The cooling power exceeded 110 W/m² when the emitter temperature was set to 30 °C. Jo et al. [80] fabricated a 3D Morpho butterfly wing-inspired structure using SiO₂ and TiO₂ membranes for PV applications, as shown in Figure 8f. The structure exhibits a blue, Christmas-tree-like morphology. The 3D module showed a 6% improvement in power conversion efficiency (PCE) and short-circuit current density (Jsc) compared to the 1D module across angles from 0° to 50°. Kecebas et al. [81] developed two types of passive radiative cooling structures using the periodic SiO₂ and TiO₂ films to improve the cooling performance. It is concluded that a 3–4% increase in mean reflectance across visible and near-infrared spectra leads to a 30–35 W/m² gain in cooling power. The maximum cooling power reached approximately 100 W/m².

3.6. Other Types of Optical Filtration Layer

Dorodnyy et al. [82] proposed a conception design using the multiterminal nanowire-based optical filtration layer to enhance the solar cell energy conversion efficiency, as exhibited in Figure 9a. Specifically, the optical filtration layer is categorized according to three distinct spectral absorption ranges. Its structure consists of a rectangular unit cell featuring nanowires with higher and lower band gaps on one side, and medium band gap nanowires on the opposite side. It is verified that high-energy photons are absorbed within the low-band-gap nanowires. Theoretical analysis indicates that the solar energy conversion efficiency of 48.3% can be achieved using the III−V material nanowire arrays. Furthermore, gallium arsenide (GaAs) exhibits the highest efficiency among the materials studied, as its light absorption aligns closely with the optimum region of the solar spectrum, as illustrated in Figure 9b. Huang and Markides [83] designed three types of PV/T modules featuring semi-transparent optical filtration layers, including a polymer solar cell (PSC), perovskite solar cell (PVSC), and cadmium telluride (CdTe), to analyze spectral distribution and electrical efficiency allocation, as shown in Figure 9c. It is seen from Figure 9d that the CdTe could capture most of the ultraviolet (UV) and visible spectrum (λ < 780 nm) and transmit most of the remaining spectrum (λ > 780 nm). A slight section of the visible spectrum (400 nm < λ < 780 nm) contributes to traversing the PVSC and achieving the highest transmittance, as depicted in Figure 9e. Moreover, some portions of the IR, visible, and UV spectrum could traverse the PSC and are directed to the thermal absorber module, as shown in Figure 9f. Thereby, it is concluded that the PVSC-based SSPVT collector could produce electricity with an efficiency of 13.8%, while the thermal efficiency is 22.5% for 50 °C and 21.1% for 150 °C, respectively. This means that the PV cell with a semi-transparent module is of great promise in this actual utilization, and could be used in next-generation, high-performance solar PV/T modules. Zheng and Xuan [84] developed an innovative photon management structure (PMS) comprising a one-dimensional photonic crystal (1DPC) combined with a biomimetic moth-eye structure (BMES) to enhance sunlight harvesting in the Vis–NIR range and address PV performance degradation and instability, as shown in Figure 9g. It is demonstrated from Figure 9h that when the BMES dimension exceeds 200 nm under UV illumination, sunlight diffraction occurs, leading to PV performance degradation. The structure improves solar weighted transmittance (SWT) from 39.43% to 40.75% within 800–1200 nm, and increases solar weighted reflectance (SWR) from 0.17 to 0.9 between 300 and 380 nm.

Zampiva et al. [85] investigated the function of using Erbium-doped forsterite (Mg₂SiO₄:Er³⁺) as the anti-reflection coating on PV cells to boost the overall efficiency. The atomic force microscopy (AFM) is utilized to analyze the topographies and roughness of the FoEr³⁺ and Fo for performing the saline environment tests with a high humidity, as displayed in Figure 9i. Results indicate that the FoEr³⁺-coated PV module has approximately 88% of transmittance under the Vis-NIR area, which means that there is about 7% of the dopant on the impact of transmittance.

3.7. Summary

From the view of the above summaries, it is evident that the nanofluid optical filters contribute to achieving high thermal and electrical efficiencies simultaneously. However, the large-scale application in practical engineering remains limited. Meanwhile, the LDS layers also exhibit some additional merits, including the probable elimination of demand for UV stabilizers in polymeric encapsulants, reducing thermalization losses of electron–hole pairs, promoting cooler device operation, and enhancing the aesthetic appearance of PV panels through coloration—all of which contribute to improving the external quantum efficiency (EQE). As a passive method, LDS involves incorporating a luminescent species into a layer placed before the cells, thereby avoiding the need to modify the active material of the PV module. Notably, the water optical filtration layer can absorb UV and part of IR radiation, while transmitting visible light and some IR to the PV panel, thereby improving power conversion in the PV/T system. Meanwhile, it is conducive to transmitting the solar radiation with wavelength ranging from 0.37 to 1.18 μm to the PV/T module avoiding thermal spots and lowering cyclic thermal stress compared to the normal PV/T module. In contrast, radiative cooling optical filters exhibit high transmittance within the solar spectrum and strong thermal emission in the mid-infrared range. A comprehensive summary of system configurations, parametric analyses including optical layer dimensions, fluid types, and filtered wavelength ranges as well as key findings, is provided in

Table 2. Summary of PV/T based on different types of optical filtration layers.

Authors	Methodology	Optical Layer Dimension	Optical Fluid Type	Filtered Wavelength Range	Findings and Proclaimed Outcomes
Meraje et al. [43]	Experimental and numerical study	The area and thickness are 0.84 m2 and 0.005 m	ZnO@ EG and water	400–1100 nm	The system energy efficiency with ZnO nanofluid optical flirtation layer could reach 76.1% that is higher compared to the conventional one reaching 56.47%. When the concentration ratios of ZnO nanofluid reaches 0.089 vol%, the efficiency has the maximum value of 85.7%.
Elharoun et al. [44]	Experimental and numerical study	Nanofluid layer thickness is setup as 1, 1.5 and 2 cm	ZnO@ water	400–2500 nm	The electrical efficiency is enhanced continuously from 6.2% for a thickness of 1 cm to 7.9% for a 2 cm thickness. The thermal efficiency is improved from 21.5% at 0 ppm to 31.4% at 200 ppm.
Liang et al. [45]	Experimental study	The dimension of Fresnel lens is 900 × 300 mm2	ZnO@ glycol	1100 nm	Photo-to-thermal conversion efficiency is enhanced by 47% when the concentration of ZnO nanoparticle varies from 11.2 ppm to 89.2 ppm. The energy conversion efficiency of CPV/T system with ZnO@ glycol is 0.218 and 0.05 greater compared to water polypyrrole and SiO2@ Ag nanofluid, respectively.
Jiang et al. [10]	Experimental study	The dimension is 200 (L) × 100 (W) × 1500 (H) with thickness of 1.0 mm	Indium tin oxide @ EG	380–1100 nm	The mean transmittance and absorbance could reach 69.1% and 30.9%, respectively, under the full spectrum. The optical efficiency of the entire system with ITOEG filter layer is 93.6%. The electrical and thermal output efficiencies are 17.7% and 18.5%, respectively.
Hashemian et al. [47]	Experimental study	The dimension is 110 (L) × 150 (W) × 10 (H) mm	Ag/Cr2O3 @ glycerol	650–1075 nm	The maximum photo-to-thermal energy conversion efficiency is 31.55% at the 80 ppm of the Ag@ Cr2O3-glycerol concentration. The entire thermal and electrical energy efficiency of Ag@ Cr2O3-glycerol could each 41% when the concentration is 40 ppm.
Han et al. [48,49]	Experimental study	The size is 90 (L) × 70 (W) × 10 (H) mm	Ag/CoSO4@ PG	325–670 nm	The overall efficiency of the PV/T system with Ag/CoSO4@ PG optical nanofluid filtration layer displays 9% superior to Ag/CoSO4@ water one. The PV/T with Ag/CoSO4@ PG nanofluid filter has the highest thermal capacity when the optical path length is 15 mm.
Abdelrazik et al. [50,51]	Experimental and numerical study	The size of optical lengths is defined as 2 (L) × 5 (W) × 10 (H) mm	Ag@ water	250–1400 nm	The transmittance of Ag@ water is higher in the spectral range from 250 nm to 1400 nm.
Zhang et al. [52]	Numerical study 2D Monte Carlo method	The thickness is setup as 10 mm	TiO2@ Ag	700–1100 nm	The maximum electrical and thermal output is obtained at the radius of Ag are 40 nm and 90 nm for wavelength between 700 nm and 1100 nm. The growth of optical layer thickness results in spectral transmittance reduction, but with nearly linear profile. The system exergy is improved by 13.3% than that of normal one.
Du et al. [53]	Numerical study based on 1D Monte Carlo method	The size is 1m length with 0.02m thickness	TiO2@ Ag	550–1100 nm	The exergy output of PV/T with system is enhanced by 13.3% compared with the traditional PV/T system. The TiO2@ Ag core-shell nanoparticles contribute to realizing an increasing in optical absorption rate because of LSPR effect.
Alexandre et al. [55]	Numerical study	The thickness layer is 1.5 cm	TiO2	350–900 nm	Optimized photonic resolution contributes to decreasing the detrimental UV photocarrier production and enhancing the perovskite solar cell efficiency by 80% compared to normal PV cell module.
Yang et al. [56]	Experimental and numerical study	LDS layer is 17 × 17 cm2	PLQY@ Eu3+	250–700 nm	EQE exhibits an increasing of 15% within the UV region.
Singh et al. [57]	Experimental study	The diameters vary from 3 to 30 µm	Yttrium aluminum garnet (YAG: Ce3+)	300–1100 nm	The YAG with a diameter of 15–20 µm displays the highest luminescence intensity, resulting in an improvement from 16.99% to 17.94%.
Almarzooqi et al. [64]	Experimental study	A plexiglass sheet of dimensions 0.645 × 0.54 × 0.01 m	Water	350–1100 nm	The electrical efficiency output could be enhanced by 8.12% for a plexiglass-air-box-covered PV module and 7.5% for a plexiglass-sheet-covered PV module, compared to a normal PV module by 7.02%.
Gupta et al. [65]	Experimental study	The radius of Fresnel lens is 100 cm with 0.4 cm thickness	Bule and red filter semi-TSP	350–1100 nm	Energy production and performance using the solar panel with red filter TSP is superior to the blue filter TSP one.
Amara and Balghouthi [66]	Experimental study	The optical dimension is 0.125 × 0.125 m	Bule, red and yellow filter semi-TSP	400–1200 nm	The electrical obtained of red, yellow and blue filters could reach 64%, 73% and 54%, respectively. Yellow filter could provide the highest short circuit current of 4.70 A, while blue one offers the least short circuit current of 3.46 A.
Eisler et al. [69]	Experimental study	The optical dimension is 649 × 407 × 5 mm	Polyhedral specular reflector	700–1200 nm	About 50% efficiency of the seven-junction module could be achieved when the external radiative efficiency of the solar cell reaches 3%. LSPR peak appears about 523 nm for synthesized Au nanoparticles with sizes 30 nm.
Zhu et al. [75] and Kiyaee et al. [76]	Experimental and numerical study	A single lens is setup as 20 × 10 cm	SiO2 @ polymethyl methacrylate	600–1150 nm	The optical efficiency could achieve 81% at 600 nm SSFL. The PV surface temperature decreases 73 °C, resulting in a 0.17% growth of electrical efficiency.
Cui et al. [78]	Experimental study	The size is 156 × 156 mm2	SiO2@ TPX	250–2500 nm	The PV with TPX@ SiO2 coating contributes to achieving 1 °C of cooling influence and 0.21% of efficiency enhancement.
Torgerson and Hellhake [79]	Experimental study	The dimension is defined as 15 × 15 × 250 cm	Polyethylene	350–1100 nm	The PE coating filter contributes to reducing temperature by 10 °C. The cooling power is beyond 110W/m2.
Kecebas et al. [81]	Experimental study	N/A	SiO2, TiO2 and Al2O3	800–1300 nm	The mean reflectance in the VIR as well as near-infrared spectra could be enhanced by 3–4% resulting in a cooling power enhancement of 30–35 W/m2. The power of radiative cooling could reach 100 W/m2.
Huang and Markides [83]	Numerical study	The dimension is 1000 × 1000 × 10 mm	N/A	300–900 nm	Results demonstrate that 27.2% the incident solar energy could be transmitted via using the PSC cell for 51.8%, CdTe for 27.2%, and PVSC for 33.3%, respectively. PV/T with PVSC module displays a better performance compared to traditional PV/T module.

. Additionally, a comparative assessment of the advantages and disadvantages of different optical filters is presented in

Table 3. Comparison of different optical filters.

Different Types	Advantages	Disadvantages
Nanofluid optical filtration layers	High transmission; High thermal efficiency; Broaden absorption spectrum.	Low well-dispersed stability; Less susceptible to changes in thermal and optical properties.
liquid (LDS) optical filtration layer	The cells are not directly exposed to the sun; The heat absorber also receives heat generated by the Stokes transfer.	Requires spectral splitting liquid to be stable in the long-term.
Water optical filtration layer	Avoiding the energy loss caused by secondary heat transfer; Flexible deployment; Help transmit sunlight in the 0.37–1.18 μm range more uniformly; Reduce the hot spots and cyclic thermal stress.	Relatively less energy conversion efficient.
Radiative cooling optical filtration layer	High transmittance; Strong thermal emission in the mid-infrared range.	The cooling effect is limited due to encapsulation materials.
Colored optical filtration layer	The power output of the solar PV/T module with color filters are higher than that of normal PV/T module.	Manufacturing process is complicated and time consuming.

.

4. Economic Assessments of PV/T System with Various Optical Filtration Layers

The economic feasibility of using a PV/T module with optical filtration layers is one of the most imperative elements for practical application. The essential economic parameters, like viability of design, capital investment, costs of nanofluid and different materials, expenses of installation and maintenance, payback period, and amount of CO₂ mitigation, could be clarified based on relevant methods, including life-cycle cost (LCC), levelized cost of energy (LCOE), cost of energy (COE), life-cycle savings (LCS), net present value (NPV), simple cost payback time (CPBT), and payback period (PBP) [84,85]. To be more specific, Hu et al. [86] implemented an economic investigation of an agricultural PV/T (APV/T) module with a reflective optical filter with and without a thermal energy storage (TES) system. Results showed that the NPV of the APV/T, APVT with TES/water, and APVT with TES/VP-1 systems are USD 77,860.38, USD 61,485.75, and USD 75,181.33, respectively, whereas the dynamic payback periods (DPP) are 3.18 years, 3.4 years, and 4.73 years, correspondingly. Zhang et al. [87] conducted the financial analysis of an agrivoltaics system and found that the system’s LCOE is USD 0.033/kWh. In light of the expense of nanofluid as a liquid optical filter, it is an imperative criterion in the practical application, which is based on the costs of materials and dosage itself. Yang et al. [56] performed an economic assessment of a PV/T module with an optical filter using various types of nanofluids, and demonstrated that the average expenses of Ag, polymer polypyrene, and Au nanomaterials are 35.7 USD/g, 167 USD/g, and 214 USD/g, which are 125, 585, and 750 times higher than those of ZnO nanomaterial in the market, respectively. This means the PV/T module with ZnO nanomaterial displays a huge price benefit for future commercial applications. Abdelgaied et al. [88] executed a complete financial investigation of a hemispherical solar evaporator by using paraffin and CuO@ water nanofluid. The overall costs of the solar evaporator with paraffin and CuO, CuO@ water, and only paraffin are 0.00645, 0.00719, and 0.00905 EGP/liter, respectively, while the expense of the solar evaporator alone is 0.01144 EGP/liter. It is evidenced that the integration of paraffin and CuO nanomaterial contributes to decreasing about 75% of the freshwater production price compared to the normal solar evaporator. Al-Waeli et al. [89] performed an economic assessment of a nanofluid-based PV/T module in Malaysia using a life-cycle cost approach. The results demonstrate that the system requires an initial investment of approximately USD 9300, with an energy cost of 0.196 USD/kWh and a payback period of around 8 years, indicating higher efficiency compared to other systems. Said et al. [90] implemented an economic assessment of a parabolic trough solar collector with MXene@ silicone oil, and confirmed that the MXene@ silicone oil is conducive to decreasing the overall expense, and increasing energy output by 1.51% in comparison with pure oil type. Kenfack et al. [91] clarified the economic performance of a solar PV/T system using palm oil and TiO₂ in Cameroon, and concluded that the system’s net present value and payback period are USD 568.45/year and 5.97 years, respectively.

Despite the nanofluids having excellent thermophysical properties, a vital restriction to their being used as thermal fluids in a wide range of fields is expense. Henein, some researchers have assessed the implications of both expense and thermal energy production by using the price-performance factor (PPF) method. For example, Adun et al. [92] carried out an economic study for a solar PV/T module with ternary nanofluids Fe₃O₄, ZnO, and Al₂O₃ by using the PPF, NPV, PBP, and LCOE approaches. Specifically, the PPF denotes the thermal conductivity ratio to the finished cost of the nanofluid, as shown in

Table 4. The calculation of the PPF.

Item	Equation
PPF for laminar flow	$PP{F}_{la\min ar}={\displaystyle \frac{\frac{C_k}{C_{\mu }}}{{\displaystyle {\sum }_n^{i=1}\mathrm{Pr}ice(\$/g)}}}$
PPF for turbulent flow	$PP{F}_{turbulent}={\displaystyle \frac{M_o>1}{{\displaystyle {\sum }_n^{i=1}\mathrm{Pr}ice(\$/g)}}}$
Laminar flow to whole thermal performance of nanofluid	${\displaystyle \frac{C_k}{C_{\mu }}}={\displaystyle \frac{({\mu }_{nf}-{\mu }_{bf})/{\mu }_{bf}}{(K_{nf}-K_{bf})/K_{bf}}}$
Turbulent flow to whole thermal performance of nanofluid	$M_o={({\displaystyle \frac{K_{nf}}{K_{bf}}})}^{0.67}\times {({\displaystyle \frac{{\rho }_{nf}}{{\rho }_{bf}}})}^{0.8}\times {({\displaystyle \frac{C_{pnf}}{C_{pdf}}})}^{0.33}\times {({\displaystyle \frac{{\mu }_{nf}}{{\mu }_{bf}}})}^{-0.47}$

. Results reveal that the overall yearly thermal and electrical energy output could reach 959,271 kWh and 220,276 kWh, leading to a cost saving of 3.98 USD/kWh and a PBP of 2.63 years.

Abadeh et al. [93] considered the economic aspects of various working fluids, including pure water, TiO₂, ZnO, and Al₂O₃-based water applied in the solar PV/T system. It was found that the shortest and longest payback periods are 4 years for the PV/T with ZnO@ water and 8 years for the PV/T with water. Esfe et al. [94,95] investigated the effectiveness of the mono and hybrid nanofluids based on their manufacturing costs via the price-performance factor method. It is evidenced that the utilization of hybrid nanofluid could be more cost-effective than that of using the mono-nanofluid. Alirezaie et al. [96] explored the thermal conductivity ratio (TCR) of MgO to the nanofluids’ finished expense based on the PPF approach. Results denote that the oxide nanofluid exhibits a lower cost compared to the metal nanofluid at all similar concentrations. Furthermore, several nanofluids are employed to replace traditional working fluids, but these do not directly decrease the system expenses, even though the operating expenses are able to be reduced by means of ameliorating the thermal physical property of nanomaterials, altering the synthesis approach, as well as boosting whole system performance. Concretely speaking, Navarro et al. [97] investigated the influence of the corrosive property of SiO₂ on the economic performance of the concentrating solar power plant, and demonstrated that the molten salt nanofluid could be utilized with Inconel 316L, 304 H, and 600 under high-temperature conditions for a long-term application, which contributes to enhancing the service lifetime. Yan et al. [98] performed a techno-economic assessment of the solar power plant with a solar salt@ white graphene system, and found that the thermal conductivity, liquid-phase-, and solid-phase-specific heat capacities are improved by 76.79%, 12.82%, and 29.8%, respectively, in comparison to those of the pure solar salt. This is conducive to decreasing the system volume and expense indirectly and avoiding the pipeline blockage. Farooq et al. [99] synthesized titanium nitride (TiN) nanoparticles via laser ablation for utilization in a PV/T system, demonstrating that the module’s thermal efficiency reaches 80%. They also found that this method becomes more cost-effective than wet chemical synthesis at nanoparticle production rates exceeding 550 mg/h.

To sum up, some economic models have been utilized to evaluate the financial aspects that impact the application of the PV/T module with an optical filtration layer in various districts and nations. The results demonstrate that the payback period for the PV/T system using nanofluid is approximately 6 years. Based on the calculations, the longest payback period observed in the study is about 8 years, corresponding to the system employing water as the heat-transfer fluid. It should be noted that this analysis pertains to a scenario in which the PV/T system operates for only 6–7 h per day and exclusively during the 6 months from April to September. For the remainder of the time, the system functions as a standard PV unit. Therefore, under typical continuous operation throughout the entire year, the payback period of the PV/T system could be significantly shortened. Moreover, as electricity prices for end-users increase, nanofluid-based PV/T systems can serve as cost-effective solutions for reducing energy expenses. Consequently, an economic factor comparison of different PV/T modules with optical filtration layers is illustrated in

Table 5. Impact factors of economic performance evaluation.

Authors	Nation and Region	Annual System Energy Obtained	Initial Capital	Operating Expense	Present Worth Factor	Inflation Rate	Annual Operating and Maintenance Cost	Working Fluid Price	Methods	NPV	PBP
Hu et al. [86]	China	67.45 MW	√	√	√	×	×	√	LCC	√	3.18 years
Zhang et al. [87]	China	201 kWh/m2	√	√	√	×	√	√	LCOE	×	5.6 years
Abdelgaied et al. [88]	Egypt	1296–2336 L/m2	√	√	√	√	√	√	LCC	×	×
Al-Waeli et al. [89]	Malaysia	47.25 MWh	√	√	×	√	√	√	LCC + COE	√	7–8 years
Kenfack et al. [91]	Cameroon	121.91 kWh	√	√	×	√	√	√	COE	√	5.97 years
Adun et al. [92]	Turkey	979,547 kWh	√	√	×	√	√	√	LCOE	×	2.63 years
Abadeh et al. [93]	Iran	10 kW	√	√	√	√	√	√	PPF	×	4–5 years

.

5. Techno-Economic Challenges of PV/T Module with Optical Filtration Layers

Overall, the PV/T module with optical filtration layers represents the most effective strategy to improve the module’s thermal and electrical energy output. Despite being a promising and vanguard technique, PV/T systems with optical filters still face several limitations and require further research.

• Currently, four types of PV/T modules with solid and liquid optical filtration layers—absorptive, refractive, reflective, and diffractive—have been studied both experimentally and numerically. Notably, only absorptive optical filters can function dually as spectral filters and working fluid media (e.g., nanofluids), while the others are typically solid. The performance of such PV/T systems can be tuned by adjusting the dimensions, materials, and thermal properties of the optical layers. Some promising liquids, such as HB650@ propylene glycol (PG), Ag@SiO₂@ ethylene glycol (EG), TiO₂@ SiO₂, and C-dots @ polyvinylpyrrolidone, exhibit excellent optical and thermal properties, high stability, and cost-effectiveness for long-term use. In contrast, solid optical layers serve only as filters, and their optical and thermal stability require further investigation. Despite the effectiveness of many composite materials, few studies have focused on the long-term performance of solid optical filters.
• Reflective and absorptive optical filtration layers have been extensively explored and applied in the PV/T modules. Currently, much less work has been committed to the development and testing of refractive- and diffractive-based optical filtration layers for PV/T utilization. However, diffractive- and refractive-based optical filtration layers are attractive in the CPV/T modules for large-scale deployment, since they are able to serve as sunlight splitters and concentrators simultaneously, which is a promising field for future investigation. Furthermore, current PV/T modules with selectively reflective optical filtration layers still have limited dimensions, which acts as a non-trivial impediment to the large-scale actual employment. Henein, a cost-effective and scalable optical filtration layer needs to be developed as a vital prerequisite for the PV/T with reflective filters.
• The thermal properties of refractive, diffractive, and reflective optical filtration layers are dissimilar from those of fluid-based absorptive ones. Solid phase optical filters usually exhibit low absorptivity; hence, they harvest little sunlight in the form of heat, whereas their low thermal expansions are also comparatively common, contributing to decreasing the thermal stresses. In the meantime, it is demonstrated that the liquid filtration layer filters with a high heat capacity contribute to transporting more heat per unit flow rate to the PV/T module. Nevertheless, the thermal fluid with a high heat capacity also has thermal inertia, as well as a start-up or transient time for achieving a desired temperature. Additionally, liquid-based optical filters in the PV/T modules have high thermal conductivity, which is conducive to attaining a homogeneous fluid temperature distribution for high electrical and thermal effectiveness output.
• More consideration should be given to the effects of exterior conditions, such as temperature, pressure, and magnetic field, on the stability of nanofluids. In particular, when the operating temperature is greater than 200 °C, it is a big challenge to sustain the nanofluid stability.
• One major challenge for nanofluid optical filtration layers in actual utilization is the long-time dispersion stability. Hence, more extensive investigations on stabilization approaches should be conducted in terms of ameliorating the usability of nanofluid in PV/T modules. Further exploration and optimization of concentration, dimension, thickness, and shape in nanoparticles should be implemented to realize the optimum filter based on the transmittance that most suits the PV cell.
• The price of composite material is dependent on the process of material preparation. Thus, in order to select the best nanofluid applied to the PV/T module from the financial viewpoint, a long-term and complete financial assessment involving initial cost and post-maintenance is required to be conducted to guarantee its market competitiveness. However, there is a limited number of studies on investigating the financial usefulness of PV/T modules with optical filtration layers. It is demonstrated that there is a shortage of exploration on optical analyses of some nano-materials, including NiFe₂O₄, Cu, MgO, Cr, and ionic liquids.

6. Conclusions

A comprehensive vision into various solid and liquid materials utilized in the solar PV/T with optical filtration layer modules is presented in this paper. It reveals the latest development propensities of the solar PV/T module with diverse materials and heat exchange configurations for attaining the optimum thermal, electrical, and optical performance, and tackles the impediments that limit the application of the PV/T module with optical filtration layers. The effects of key factors—including optical filter dimension, material composition, nanoparticle type, long-term stability, flow rate, and issues related to nanoparticle sedimentation and agglomeration—on external quantum efficiency and overall energy output are discussed. Furthermore, economic assessments based on life-cycle cost (LCC), levelized cost of energy (LCOE), cost of energy (COE), and power purchase agreement (PPF) are summarized, focusing on cost savings and payback periods. Henceforth, the following significant conclusions and recommendations are summarized:

• Only the PV/T module with absorptive filters could be integrated with optical filtration layers, which are typically made of solid materials. The performance of the PV/T modules with optical filtration layers could be regulated by altering the dimensions, material, and fluid thermal properties.
• The PV/T modules with optical filtration layers achieve higher thermal and electrical performance compared to conventional PV/T or standalone PV modules. By spectrally splitting incoming sunlight, these systems enable more efficient utilization of different bandwidths by the thermal collector and PV cell. The incorporation of nanomaterials further enhances thermo-optical properties, improving overall system performance. Optical efficiencies of reflective and refractive concentrators can reach approximately 80% at a concentration ratio of 10. However, performance degrades significantly at higher concentrations—for instance, decreasing to around 30% at a concentration ratio of 20.
• Absorptive and reflective filters have been widely studied in PV/T systems, whereas refractive and diffractive types remain less explored. The latter are particularly suitable for concentrated PV/T applications due to their dual function as spectral splitters and sunlight concentrators, making them promising for large-scale deployment. Results demonstrate that the refractive and diffractive filters can direct 50–60% of incoming sunlight to PV cells, achieving up to 70% energy conversion efficiency within the 600–1125 nm range. Moreover, reflective filters exhibit higher irradiance non-uniformity, with peak-to-average ratios reaching 4.99 and 5.23 at the top and middle of the sub-cell, respectively.
• Nanofluid-based filters can reduce PV surface temperature by 6–45%, contributing to overall system efficiencies of 80–85%. Using luminescent imidazole-phenanthroline as an LDS layer, optical, thermal, and electrical efficiencies can reach approximately 60%, 90%, and 80%, respectively.
• InGaP modules effectively reflect light beyond 865 nm, while CdTe modules exhibit semi-transparency in the visible and infrared regions. These properties inspire innovative optical filter designs tailored for PV/T systems. Additionally, novel composites like Mg₂SiO₄:Er³⁺ and Fo:Er³⁺ can achieve transmittance up to 88% in the Vis–NIR range.
• Different designs and materials entail varying initial costs, which must be considered during the design phase to ensure high overall energy efficiency. Material selection and system design should aim for a sufficiently high efficiency improvement to achieve a lower LCOE compared to conventional modules. The PBP for these systems typically ranges from 4 to 6 years.

7. Future Direction

• For the future development of PV/T modules with optical filtration layers, more research investigations should be made in these fields.
• Future research should focus on analyzing illumination unevenness in PV/T modules employing refractive and reflective optical filters, especially when integrated with secondary optical elements, as well as evaluating other factors causing heterogeneity, such as partial shading.
• The long-term operational stability of optical filtration layers also requires thorough assessment, as degradation of optical and thermal properties over time affects system durability. Further investigation should include long-term testing of nanofluids under varying concentrations, high temperatures, and magnetic fields to evaluate their impact on PV/T energy output.
• Composite materials with high heat capacity and thermal conductivity must be further developed to enhance the optical, electrical, and thermal performance of PV/T systems sustainably. Moreover, considering the environmental toxicity of nanomaterials, further research on the safe handling of nanofluids is essential for the practical deployment of integrated PV/T systems.
• The cost of working fluids and the additional expenses required to maintain stability under high solar radiation must be evaluated under varying conditions of temperature, concentration, and exposure time. Future studies should also examine the exergy performance, environmental impact, and social implications of optical filtration layer-based PV/T systems.
• Standardized testing procedures and performance metrics are essential to enable reliable technology benchmarking, guide R&D, and support commercialization. Priority should be shown to spectrally resolved testing, unified hybrid efficiency metrics—such as exergy-based standards—and durability criteria for optical materials. Reproducible test methods are also critical for industry adoption and certification, enabling investors and policymakers to objectively assess system eligibility for subsidies and incentives.
• A systems engineering approach addressing durability, environmental resilience, thermal management, and manufacturability should be adopted. Accelerated testing, long-term outdoor trials, and scalable integration methods are needed to realistically evaluate and implement optical filtering technologies in practical PVT applications.