1. Introduction

With global energy demand still rising and pressure to reduce carbon emissions increasing, the building sector has become an important target for urban energy conservation and carbon reduction. In recent years, progress in energy and materials technologies has offered new ways to lower operational energy use and support low-carbon building energy systems. Energy efficiency and energy storage have also become major priorities for sustainable development as global energy consumption continues to grow. According to the Global Alliance for Buildings and Construction (GlobalABC), buildings and construction account for a significant share of global final energy use and carbon emissions. The sector is therefore important to urban low-carbon development and carbon neutrality^[1]. From a whole-life-cycle view, building-related carbon emissions mainly consist of operational carbon and embodied carbon. Operational carbon comes from heating, cooling, lighting, ventilation, and equipment use during building operation. Embodied carbon is produced during non-operational stages, including material production, transportation, construction, maintenance, renovation, demolition, and recycling^[2]. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that building decarbonization requires coordinated action on demand reduction, energy efficiency, low-carbon energy use, and material decarbonization^[3]. Deep decarbonization, therefore, cannot depend solely on improved envelope insulation and more efficient equipment. Low-carbon energy supply and life-cycle emission reduction strategies also need to be included. Renewable energy generation can be embedded in the building itself. Buildings can then act as both energy producers and consumers, reducing dependence on external energy supplies and supporting urban carbon reduction^[4].

Solar energy is clean, renewable, abundant, and widely available. It is an important alternative to fossil fuels and can support the development of low-carbon energy systems. Studies of terawatt-scale photovoltaic (PV) development suggest that PV technology could provide a larger share of global energy supply in the future^[5]. In recent years, continued progress has been made in crystalline silicon photovoltaics, thin-film photovoltaics, perovskite and tandem solar cells, as well as organic and flexible photovoltaics. Power conversion efficiency (PCE) has steadily increased, while their range of applications and suitability for building integration have also expanded^[6]. Terawatt-scale PV deployment, however, also requires systematic improvement in material selection, low-carbon manufacturing, resource use, recycling, and life-cycle environmental impact^[7]. In the built environment, photovoltaic technologies are no longer used only as separate power-generating devices. They are increasingly being developed as integrated building energy systems in which architectural expression, technical standards, economic feasibility, and digital design requirements are considered together^[4,8]. Compared with centralized PV power plants, building-integrated photovoltaics (BIPV) and building-attached photovoltaics (BAPV) can make use of rooftops, facades, curtain walls, windows, and shading elements in cities. Local electricity generation and use can therefore be achieved^[6,7]. BIPV can supply renewable electricity while also serving as an architectural component. Functions such as insulation, waterproofing, and daylighting can be provided, and integration with the facade can improve visual appearance^[9]. BIPV also plays an important role in supplementing building energy supply, reducing conventional energy demand, and supporting the decarbonization of residential buildings^[10]. For semi-transparent BIPV, daylighting, solar heat gain, glare control, and visual comfort need to be considered together. This balance is especially important for windows, curtain walls, and high-rise buildings^[11]. As the functional capabilities of building envelopes continue to expand, energy-saving strategies for building envelopes are gradually shifting from static thermal regulation toward dynamic optical-thermal management responsive to environmental changes^[12]. Envelope performance control is also shifting from single-spectral regulation to multi-spectral coordination, so that different photothermal needs across changing seasons and climates can be addressed^[13].

Recent deployment shows that BIPV is evolving into a broader family of commercial building-envelope products. Bonomo et al. report more than 200 BIPV products on the EU market, while BIPV represents only about 1-3% of PV installations, indicating a clear gap between product availability and construction-sector uptake^[14]. Faes et al. describe BIPV as technologically mature, but note that wider adoption is still constrained by limited awareness and expertise, perceived risks, and gaps in digitalization; further improvements in reliability, aesthetics, cost, and installation simplicity could strengthen its attractiveness^[15]. Field evidence is also growing. de Queiroz Corrêa et al. monitored a vertical organic photovoltaics (OPV) façade in São Paulo comprising 200 m² of OPV-integrated glass, 90 panels, and 76.65 m² of active area over four years. Energy generation remained broadly consistent, while global solar radiation and solar altitude were the key variables in the regression models^[16]. These studies support evaluating BIPV not only by energy yield, but also by building integration, reliability, maintenance, and service life.

Against this background, this paper reviews the development of photovoltaic materials and devices for improving building performance. Three areas are considered: the development of photovoltaic materials, building performance research, and future research directions. The development of silicon-based photovoltaics, conventional thin-film photovoltaics, functional semi-transparent photovoltaics, perovskites, and tandem solar cells is first outlined^[6,11]. Performance evaluation studies are then examined at several scales, including laboratory tests, outdoor tests, actual buildings, and urban districts. Particular attention is paid to power generation efficiency, building energy consumption, thermal performance, daylighting, and visual comfort^[17]. Several future research topics are also discussed. These include further improvements in efficiency and architectural adaptability, multi-criteria optimization, colored photovoltaic facades, and the integration of photovoltaics with passive thermal management technologies. Cross-disciplinary thermal management methods, such as radiative cooling used in agriculture, are considered for their possible relevance to building-integrated photovoltaics. The use of photovoltaic technologies in extreme environments and under resource-constrained conditions is also discussed^[18-20]. This review aims to provide a reference for the further application of building-integrated photovoltaics in low-carbon construction and urban energy transition.

2. Development of Photovoltaic Materials

Crystalline silicon has long dominated conventional photovoltaic development, while thin-film technologies such as hydrogenated amorphous silicon (a-Si:H), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) have broadened the range of absorber materials and device structures. As PV began to be considered for building use, more attention was given to transparent, semi-transparent, and colored materials. More recently, perovskites and multijunction tandem architectures have further expanded the options for combining high photovoltaic performance with the optical and mechanical requirements of building integration.

Crystalline silicon and thin-film cells were already used in early BIPV products, including PV tiles, roofing membranes, façade modules, and solar glazing. In these products, the PV component served as part of the building envelope while simultaneously generating electricity^[21]. Among conventional thin-film technologies, CIGS is particularly attractive because of its strong light absorption, tunable bandgap, good low-light response, and potential for lightweight and flexible devices. The performance of conventional thin-film materials has continued to improve through better control of composition and interfaces. For (Ag,Cu)(In,Ga)Se₂ cells, high-concentration Ag alloying, a steep Ga gradient near the back contact, and post-deposition treatment with rubidium fluoride (RbF) were used. A power conversion efficiency (PCE) of 23.64% was achieved over an aperture area of 0.90 cm², and the result was independently certified by the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE)^[22]. Flexible CIGS minimodules have demonstrated photovoltaic conversion efficiencies above 18%. Ishizuka et al. fabricated monolithically interconnected CIGS minimodules on 0.2-mm-thick flexible ceramic substrates, achieving an areal mass of approximately 1.3 kg m^-2 (Figure 1a). The two flexible monolithically interconnected CIGS minimodules reported in the study achieved independently certified efficiencies of 18.64% and 18.5%, respectively. The low-light harvesting capability of flexible CIGS modules was further demonstrated by modules measuring 8 × 10 cm² and 2 × 10 cm², which were able to power a green light-emitting diode under indoor fluorescent illumination of approximately 200 lx (Figure 1b). Although flexible CIGS technology has already achieved high photovoltaic performance, further improvements in large-area module efficiency require mitigation of damage introduced during cell separation and patterning. In particular, mechanically scribed P2/P3 edges can reduce shunt resistance and increase recombination-related losses. Therefore, improved scribing processes and effective edge passivation or termination represent important routes toward further enhancement of flexible CIGS module performance^[23].

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Figure 1. a. Photograph of CIGS solar minimodule. Reproduced from reference^[23]. CC BY 4.0; b. Photographs of Lightweight Flexible CIGS Minimodules. Reproduced from reference^[23]. CC BY 4.0; c. A Schematic fabrication of CPI films and flexible hydrogenated a-Si:H solar cells. Reproduced from reference^[24]. CC BY 4.0; d. Cross-sectional scanning electron microscopy image and schematic layered structure (thickness) of p-i-n a-Si:H in fabricated solar cells. Reproduced from reference^[24]. CC BY 4.0; e. Representative image of flexible TSC. Reproduced from reference^[24]. CC BY 4.0; f. Image showing setup used for repeated bending of flexible TSCs. Reproduced from reference^[24]. CC BY 4.0. CIGS: copper indium gallium selenide; CPI: colorless polyimide; a-Si:H: hydrogenated amorphous silicon; TSC: transparent solar cell; AZO: aluminum-doped zinc oxide; a-Si: amorphous-silicon.

Flexible and transparent a-Si:H thin-film solar cells have also shown substantial progress. Choi et al. introduced light-scattering microstructures on colorless polyimide (CPI) substrates together with a dual n-type rear window layer to enhance light harvesting and bifacial photovoltaic performance. The optimized device exhibited an average transmittance of 36.9% over 500-800 nm and achieved a bifacial power conversion efficiency of 6.15% under simultaneous illumination of 1 sun from the front and 0.3 sun from the rear. The laser lift-off (LLO) fabrication route was used to transfer the light-scattering structure onto the CPI substrate while minimizing laser-induced thermal damage (Figure 1c). Under the optimized LLO condition, the transferred CPI film retained an average transmittance of approximately 88.3% over 400-800 nm. The cross-sectional structure consists primarily of a CPI/aluminum-doped zinc oxide (AZO)/p-i-n a-Si/AZO stack, including an approximately 280-nm-thick intrinsic a-Si absorber layer and a 40-nm-thick n-μc-SiO_x layer (Figure 1d). The representative flexible transparent solar cell corresponds to the optimized bifacial device, which achieved a PCE of 6.15% while maintaining an average transmittance of 36.9% over 500-800 nm (Figure 1e). Furthermore, the repeated-bending test demonstrated good mechanical reliability: the flexible transparent solar cells sustained 500 bending cycles at radii of 8 and 6 mm and 300 cycles at a radius of 4 mm, highlighting their potential for curved and flexible building-integrated photovoltaic applications (Figure 1f)^[24].

Earlier studies of conventional photovoltaic materials mainly examined conversion efficiency, stability, and manufacturing cost. Once these materials were used more widely in buildings, additional properties became important. Power-to-weight ratio, flexibility, light transmittance, and compatibility with building components are now also used as evaluation criteria^[25]. When photovoltaic technology is integrated into building envelopes and windows, material performance must be considered together with daylighting, thermal insulation, solar heat gain control, and architectural appearance. In photovoltaic glazing and building curtain walls, part of the incoming solar radiation must be absorbed for electricity generation. Adequate daylight, visual transparency, and color reproduction should still be maintained. Transparent, semi-transparent, and colored photovoltaic materials have therefore become important research directions. According to their optical absorption strategy, transparent photovoltaics can generally be divided into non-wavelength-selective and wavelength-selective systems^[26]. In the first type, light transmission is mainly achieved by reducing the coverage of opaque cells, decreasing the thickness of the absorber layer, or introducing transparent openings. These structures are relatively simple, but lower efficiency, uneven visual appearance, or color shifts may occur. Wavelength-selective systems operate differently. Ultraviolet or near-infrared radiation is preferentially absorbed, while visible light is allowed to pass through. This makes it possible to consider visual transparency and electricity generation at the same time. Conversion efficiency alone is not sufficient for evaluating these materials. Other parameters also need to be considered, including average visible transmittance (AVT), spectral selectivity, color rendering index (CRI), haze, and color coordinates^[27,28]. Perovskites have adjustable bandgaps and colors and can be processed from solution at relatively low temperatures. These properties make them important candidates for photovoltaic glazing^[28,29]. Laser micropatterning has been used in perovskite devices. Laser scribing produced evenly distributed transparent micro-areas with circular, linear, and square geometries within an otherwise opaque perovskite stack. Optimized features down to 25 μm supported devices with AVT up to 44%, CRI up to approximately 97, and haze down to 3%; a 4 cm² submodule reached a PCE of 9.0% at 32% AVT (Figure 2e)^[30]. Transparency gradients and decorative patterns can also be produced by this method, providing more options for building skin design.

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Figure 2. a. Facade-Integrated PV-DSF and PV-IGU. Reproduced from reference^[54]. CC BY 4.0; b. The visual effect of PV-DSF (left) and PV-IGU (right). Reproduced from reference^[54]. CC BY 4.0; c. Measurement instruments of the BIPV test bed. Reproduced from reference^[54]. CC BY 4.0; d. Outdoor testing rig for determining SAPM parameters. Reproduced from reference^[54]. CC BY 4.0; e. Light microscopy images of opaque perovskite solar cells and laser scribed transparent areas of different shapes. Reproduced from reference^[30]. CC BY 4.0; f. Energy-saving buildings and All-season used SPWs. Reproduced from reference^[76]. CC BY 4.0; g. Overall structure of novel CSTPV glazing. Reproduced from reference^[68]. CC BY 4.0; h. Picture of novel CSTPV glazing sample. Reproduced from reference^[68]. CC BY 4.0; i. Outdoor test scenario. Reproduced from reference^[68]. CC BY 4.0. PV-IGU: photovoltaic insulating glass units; PV-DSF: photovoltaic double-skin facades; BIPV: building-integrated photovoltaics; SPWs: Smart Photovoltaic Windows; CSTPV: coated semi-transparent photovoltaic; SAPM: Sandia Array Performance Model.

Although perovskite photovoltaics have a shorter development history than crystalline silicon and conventional thin-film technologies, their material compositions and device architectures have evolved rapidly. In 2009, CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ were used as visible-light sensitizers in mesoporous titanium dioxide (TiO₂) photoelectrochemical cells. The iodide system reached a conversion efficiency of about 3.80%, while an open-circuit voltage of 0.96 V was obtained with the bromide system^[31]. At that time, the devices still followed the structure of dye-sensitized solar cells. Their stability was poor because perovskites could readily dissolve in the liquid electrolyte. Despite this limitation, the photovoltaic function of organic-inorganic halide perovskites was demonstrated for the first time. In 2012, an all-solid-state mesoscopic superstructured device was reported. Insulating mesoporous aluminium oxide (Al₂O₃) was used as an inert scaffold, while CH₃NH₃PbI₃-xClx served as the absorber layer. The efficiency was increased to 10.90%^[32]. This result showed that perovskites could not only absorb light but also transport electrons and holes.

Further progress was then made in composition engineering, crystallization control, defect passivation, and transport-layer design. The tunable bandgap of perovskites began to be used in tandem solar cells. In 2017, a wide-bandgap perovskite based on mixed cesium/formamidinium (Cs/FA) cations was combined with a silicon heterojunction bottom cell optimized for infrared response. A buffer layer of tin oxide (SnO₂) prepared by atomic layer deposition (ALD) was used. Damage and parasitic absorption caused by transparent electrode deposition were reduced, and a conversion efficiency of 23.60% was achieved for a 1 cm² two-terminal monolithic perovskite/silicon tandem cell^[33]. The same window layer could also act as a diffusion barrier. Related perovskite devices completed a 1,000 h damp-heat test at 85 °C and 85% relative humidity. Perovskite photovoltaics thus developed from photosensitizers in liquid systems into semiconductor absorber layers in all-solid-state devices. They were later used as wide-bandgap top cells in tandem structures. This development was supported by continued improvements in material composition, crystallinity, interface energy levels, transparent interconnection layers, and environmental stability.

Over the past three to five years, photovoltaic materials research has no longer focused only on peak device efficiency. Three main directions can be identified. For mature materials such as crystalline silicon and CIGS, losses are being reduced through contact passivation, back-contact structures, and composition control^[22,34]. In perovskite/silicon tandem cells, greater attention is being paid to defect control, area scaling, and operational stability^[32,33]. All-perovskite devices are also being extended from double-junction structures to triple- and quadruple-junction designs^[35].

In crystalline silicon research, the absorber itself is already relatively mature. Further efficiency gains now depend more strongly on reducing recombination at surfaces and contact regions. A double-sided nanocrystalline selective-contact silicon heterojunction cell reached a PCE of 27.0% on a 210 mm half-cut cell. The result was independently tested by the Institute for Solar Energy Research Hamelin (ISFH) Calibration and Test Center (CalTeC). A module with a mask area of 1.63 m² was then fabricated using the same technology and certified by Fraunhofer ISE CalLab, where a PCE of 25.44% was obtained^[34]. In silicon heterojunction back-contact cells, metal contacts are placed on the rear side. Interdigitated selective contacts are formed by laser patterning. Under standard test conditions (STC), ISFH measured a PCE of 27.09% for a specified illuminated area of 243.0 cm² and 26.74% for the total active area of 274.3 cm^2[36]. These results show that modern crystalline silicon research is paying more attention to local losses at selective contacts, polarity boundaries, and wafer edges rather than only to large-scale structural design. Conventional thin-film materials still have room for improvement as well. Silver-alloyed copper indium gallium selenide (ACIGS) was improved through silver alloying, a steep Ga gradient, and alkali metal post-treatment. A PCE of 23.64% was independently certified by Fraunhofer ISE. The result shows that further control of composition distribution and interface passivation can reduce open-circuit voltage losses^[22].

Recent progress in perovskite/silicon tandem cells has largely come from reducing non-radiative recombination in wide-bandgap perovskites. Binary two-dimensional perovskite passivation was applied to both the surface and the bulk of the absorber layer. With this absorber, a 1.0 cm² wide-bandgap single-junction device reached a steady-state efficiency of 20.79%, while a monolithic perovskite/silicon tandem cell obtained a third-party certified steady-state efficiency of 30.65%^[37]. Further improvement was achieved by combining phosphonic acids with piperidinium chloride for passivation. A 1.0 cm² tandem device reached a steady-state PCE of 31.60% certified by the Joint Research Centre-European Solar Test Installation (JRC-ESTI). A 60 cm² effective-area device achieved a Fraunhofer ISE CalLab-certified steady-state PCE of 28.90%^[38]. New limits appear when small-area cells are scaled to mini-modules. These include thin-film uniformity, series interconnection, busbar dead zones, and ion diffusion. With low-temperature crystallization control and a P1.5 interconnect design, a 14.61 cm² mini-module achieved a certified quasi-steady-state efficiency of 22.73%. More than 90.00% of its initial efficiency was retained after 1,000 hours of continuous illumination^[39].

All-perovskite tandem devices are also moving from two-junction structures toward three- and four-junction designs. In tin-lead (Sn-Pb) narrow-bandgap perovskites, tin(II) ions (Sn²⁺) are easily oxidized, and crystallization is difficult to control. Bifunctional amino acid salts can be used to regulate the colloidal state and crystallization of the precursor solution through carboxylic acid groups. Their ammonium groups can also improve the optoelectronic properties of the thin film. Film uniformity and semiconductor quality can therefore be improved^[35]. With the optimized Sn-Pb absorber layer, single-, double-, and triple-junction devices reached power conversion efficiencies of 23.90%, 29.70%, and 28.70%, respectively. The double-junction device was independently verified by an external laboratory at the Shanghai Institute of Microsystem and Information Technology (SIMIT). Its current density-voltage (J-V) test PCE reached 29.76%, and its steady-state PCE was 29.26%. The four-junction device achieved a PCE of 27.90% and an open-circuit voltage of 4.94 V^[35]. These results show that multi-junction photovoltaic research is increasingly focused on precursor chemistry, oxidation-state control, current matching between subcells, and complex interface control.

Photovoltaic materials research is therefore moving away from efficiency as the only target. Efficiency, stability, scalability, and functional adaptability are now being improved together. Crystalline silicon and conventional thin-film materials are expected to continue developing through contact passivation, interface control, and functionalization. Perovskites and their tandem systems still need better large-area uniform fabrication, long-term stability, transparent interconnection, and control of complex interfaces. For building applications, light transmittance, color, encapsulation reliability, and environmental safety also need to be considered^[27]. The material advantages seen in small-scale devices can be transferred to high-performance photovoltaic components for building envelopes only when high efficiency, long lifetime, low-cost manufacturing, and architectural functions are balanced.

These material-level advances acquire practical significance in the built environment when they can be translated into durable and constructible building-envelope products. BIPV has reached a relatively mature technological stage, although its penetration in the photovoltaic market remains limited. Current applications include roofs, façades, cladding, and glazing, where photovoltaic components can simultaneously generate electricity and provide building-envelope functions such as weather protection, thermal insulation, acoustic protection, and architectural expression^[15]. BIPV can also expand the available surface area for solar-energy harvesting in dense urban environments where roof space is limited. However, wider deployment is still constrained by high initial costs, uncertain economic returns, project-specific design requirements, installation complexity, reliability concerns, and limited professional experience. In addition, the integration of photovoltaic materials into buildings requires coordinated consideration of electricity generation, thermal performance, daylighting, visual comfort, and architectural appearance, since improvements in one aspect may affect the overall building performance^[4]. Further development therefore depends on cost reduction, improved long-term reliability, greater product standardization, and closer integration of photovoltaic systems with architectural and building-envelope design.

Overall, photovoltaic materials for building applications are being developed along two closely connected directions: higher photovoltaic performance and stronger architectural functionality. Future progress will depend on translating laboratory-scale advances into scalable, durable, safe, and standardized BIPV products whose electrical, thermal, optical, and visual performance can be evaluated at the whole-building level. The representative photovoltaic material technologies for BIPV are comparatively summarized in Table 1 in terms of their photovoltaic performance, applicable building components, building-related advantages, and major limitations (Table 1).

3. Photovoltaic Materials and Devices for Improving Building Performance

3.1 Lab-scale material and device research

Small-scale studies of photovoltaic materials and devices provide the basis for improving power conversion efficiency per unit area. Small-area cells or samples are usually tested under controlled conditions. The effects of absorber crystallinity and phase stability, bulk and interface defects, carrier recombination, and transport-layer matching can then be examined separately. Research has moved from changing a single component to a broader form of optimisation. Composition, defects, interfaces, and carrier dynamics are now considered together to improve device PCE, steady-state power output, and operational stability.

In perovskite photovoltaics, composition engineering is mainly used to improve the stability of the light-active phase, film uniformity, and device reproducibility. Saliba et al. added inorganic caesium to formamidinium/methylammonium mixed-cation perovskites and formed a tri-cation system. Thermal stability improved, phase impurities decreased, and device performance became less sensitive to changes in fabrication conditions. A stable power output of 21.10% was achieved, and about 18% was retained after 250 hours of operation under working conditions^[40]. The result shows that a suitable combination of cations can simultaneously support phase stability, efficiency, and batch consistency, providing a more stable material basis for later area scaling. Jeong et al. used formate as a pseudohalide anion. Anion vacancies at grain boundaries and film surfaces were suppressed, while film crystallinity was improved. The power conversion efficiency reached 25.60%. A 450-hour operational stability test was completed, and the external quantum efficiency of the electroluminescence exceeded 10%. Non-radiative energy loss in the device was therefore effectively reduced^[41]. Carrier extraction and transport also affect device efficiency. Yoo et al. adjusted the chemical bath deposition process of the SnO₂ electron transport layer to control its coverage, thickness, and chemical composition. Bulk passivation of the perovskite and interface passivation were designed separately so that carrier recombination and bandgap loss could be reduced. The resulting device achieved an external quantum efficiency of 17.20% for electroluminescence. Independent testing at Newport Corporation confirmed a steady-state PCE of 25.20%, which is 80.50% of the thermodynamic efficiency limit for the corresponding bandgap^[42]. When absorber performance is already high, further gains therefore require lower transport-layer resistance, less interface recombination, and smaller open-circuit voltage losses. Min et al. coupled chlorine-bonded SnO₂ with a chlorine-containing perovskite precursor. An atomically coherent interfacial layer was formed between the electron transport layer and the perovskite layer, reducing deep-level interface defects and improving charge extraction and transport. A PCE of 25.80% was obtained, and Newport Corp. independently verified 25.50%. Unencapsulated devices retained about 90% of their initial efficiency after 500 hours of continuous illumination^[43]. These results also show that interface quality directly affects open-circuit voltage, fill factor, and the light stability of small-area devices.

To move beyond the efficiency limit of single-junction devices, Lin et al. focused on the lead-tin narrow-bandgap subcell in all-perovskite tandem solar cells. Grain-surface passivation was carried out using 4-trifluoromethylbenzylammonium. The passivating agent showed stronger adsorption at surface defect sites, and the carrier diffusion length was doubled to more than 5 μm. Effective charge collection was maintained in an absorber layer about 1.20 μm thick. The lead-tin subcell reached an efficiency above 22%. For the full tandem device, a certified steady-state PCE of 26.40% was obtained in independent testing by the Japan Electrical Safety & Environment Technology Laboratories (JET). After 600 hours of maximum power point operation under ambient conditions, the encapsulated device retained more than 90% of its initial performance^[44]. High output per unit area in tandem devices therefore depends on several factors being optimized together, including bandgap matching, absorber thickness, defect passivation, and carrier diffusion length.

In organic photovoltaics, efficiency can be improved by reducing energy losses through control of donor-acceptor bulk-heterojunction morphology and crystallisation. Fu et al. used 1,3,5-trichlorobenzene as a crystallisation regulator. Molecular aggregation in the active layer initially increased and then decreased, thereby preventing excessive aggregation of the non-fullerene acceptor. The PM6 binary device reached a laboratory-tested PCE of 19.31%. Independent testing was also carried out at Enli Tech Optoelectronic Calibration Lab, which is accredited under the International Organisation for Standardization/International Electrotechnical Commission (ISO/IEC) 17025:2017. A PCE of 18.93% was measured, and the non-radiative recombination loss was reduced to 0.190 eV. The PM1 device reached a PCE of 19.10%, with the non-radiative recombination loss further reduced to 0.168 eV^[45].

Several routes are therefore used to raise power conversion efficiency per unit area in small-area devices. Stable and uniform light-absorbing phases can be formed by controlling composition and crystallisation. Deep-level defects in the bulk, at grain boundaries, and on surfaces need to be reduced to limit non-radiative recombination. Energy-level alignment and structural matching between the absorber and selective transport layers also need to be improved. The required balance depends on the device structure. For single-junction, tandem, and organic heterojunction devices, absorber thickness, carrier diffusion length, and microstructure must be coordinated rather than optimised separately. Standard laboratory tests are normally carried out under fixed spectral conditions, irradiance, and cell temperature. Photovoltaic devices installed on building surfaces operate under much less stable conditions: solar irradiance, ambient temperature, and device temperature change continuously outdoors. Small-scale outdoor testing is therefore needed between laboratory materials research and real building applications.

Comparable outdoor results require the test conditions to be reported clearly. Device structure, active area, encapsulation status, installation orientation and tilt, pretreatment, initial stabilised efficiency, and sample number should all be specified. Plane-of-array irradiance, ambient and device temperatures, relative humidity, and wind speed, where needed, should also be recorded. The stability testing consensus based on the International Summit on Organic Photovoltaic Stability (ISOS) recommends reporting stress conditions, sample number, test atmosphere, temperature, humidity, illumination, and electrical bias. Maximum power point tracking (MPPT) is preferred for evaluating stable output^[46]. The Perovskite PV Accelerator for Commercialising Technologies (PACT) outdoor testing protocol provides additional requirements for the physical installation, electrical connections, and data analysis of perovskite modules in field tests. Standard methods may need to be adjusted for differences among samples and for unusual electrical responses. These methods include periodic forward and reverse current-voltage scans, synchronised monitoring of environmental parameters, and degradation analysis based on accumulated irradiance or energy yield^[47]. For tandem devices, current mismatch caused by spectral changes should also be assessed.

Velilla et al. tested small perovskite modules outdoors under a real tropical climate. Samples with active areas of 17 cm² and 50 cm² were fabricated, encapsulated, and tested for 500 hours, while commercial silicon modules were used as a reference. During daylight hours, meteorological variables and in situ current-voltage curves were recorded every minute. Performance maps were then built in accordance with the International Electrotechnical Commission (IEC) 61853-1. The power output and short-circuit current of both photovoltaic technologies changed strongly with weather conditions. The perovskite modules, however, showed a nonlinear open-circuit-voltage response to temperature at high irradiance, which differed from the behaviour of conventional silicon devices^[48].

Outdoor temperature and changes in the solar spectrum can alter current matching between subcells in perovskite/silicon tandem devices. Aydin et al. tested a 25%-efficient two-terminal monolithic perovskite/silicon tandem cell in a hot environment with strong sunlight. Silicon and perovskite band gaps were found to respond to temperature in opposite ways. As a result, a device optimised under standard test conditions deviated from its best current match as the operating temperature increased. Under the field conditions used in the study, an operating temperature above 55 °C required that the perovskite bandgap under standard test conditions be below 1.68 eV^[49]. The best bandgap in the laboratory is therefore not necessarily the best one under the climate experienced by a building. Device operating temperature must be considered in both material selection and tandem design.

The translation of high-efficiency perovskite solar cells from small-area laboratory devices to large-area BIPV modules remains challenging. Increasing the device area makes it more difficult to achieve uniform perovskite crystallization and film quality, while defects, interconnection losses, and inactive areas introduced during module fabrication can reduce the PCE compared with small-area cells. Long-term outdoor stability is another major barrier, as perovskite materials and their interfaces are sensitive to moisture, oxygen, heat, UV irradiation, and thermal cycling. Effective encapsulation is therefore essential, but large-area modules impose more stringent requirements on barrier performance, mechanical integrity, and manufacturing reproducibility. These challenges are particularly important for BIPV, where modules are expected to operate reliably for extended periods under variable outdoor conditions. Further advances in scalable deposition, module interconnection, encapsulation, and long-term stability are therefore required to enable the widespread application of large-area perovskite modules in buildings^[50,51].

Small-scale photovoltaic research should therefore begin with material and interface optimization in the laboratory. The results then need to be checked by dynamic testing under natural outdoor conditions. This sequence provides a more reliable basis for increasing device area and for later studies at the building-component scale.

3.2 Full-scale building performance evaluation

Once BIPV components move from laboratory prototypes into real buildings, their operating temperature and power output are affected by more than the device itself. Installation location, building form, local climate, indoor loads, and control strategies all play a role. Full-scale evaluation is therefore necessary. Existing evidence comes from several levels: controlled comparison in full-scale test chambers or test buildings, seasonal or annual measurements of real building components, experimentally validated building energy models, and neighbourhood or urban simulations based on three-dimensional city models.

Early work by Peng et al. on photovoltaic double-skin facades provided a detailed experimental basis for full-scale evaluation. A full-scale test was carried out in Hong Kong using a semi-transparent amorphous-silicon photovoltaic outer layer, operable inner windows, and an air cavity of about 400 mm. Mechanical ventilation, buoyancy-driven ventilation, and sealed operation were compared. Ventilation removed heat from the cavity and reduced solar heat gain, while the sealed mode performed better in limiting winter heat loss. No single mode was best under all weather and seasonal conditions^[52]. Under mechanical ventilation, power generation was 1.90% higher than with buoyancy-driven ventilation and 3.00% higher than in the non-ventilated mode. These gains, however, need to be considered together with fan electricity use and changes in heating and cooling loads^[53]. Wang et al. conducted a comparative study of the thermal, electrical, and daylighting performance of a photovoltaic double-skin façade (PV-DSF) and a photovoltaic insulating glass unit (PV-IGU) using a full-scale south-facing test bed in Hong Kong. The two systems were evaluated under the same outdoor conditions (Figure 2a). The semi-transparent amorphous-silicon (a-Si) PV modules used in both systems had a rated efficiency of 6.3% and a transmittance of 20%. Both systems remained see-through and preserved visual contact with the outdoor environment (Figure 2b). The integrated monitoring system included MPPT controllers, an inverter, thermocouples, heat-flux meters, light meters, and data acquisition and logging equipment; all monitored variables except power output were recorded at 1-min intervals (Figure 2c). The outdoor test rig was used to measure I-V curves and key electrical parameters, which were then used to determine the parameters required for the Sandia Array Performance Model (SAPM) (Figure 2d). The measured average solar heat gain coefficients (SHGCs) of the PV-DSF and PV-IGU were 0.152 and 0.238, respectively, indicating that the ventilated PV-DSF was more effective in reducing solar heat gain. Their corresponding U-values were 2.535 W m^-2 K^-1 and 2.281 W m^-2 K^-1, demonstrating the better thermal insulation performance of the PV-IGU. Owing to the lower operating temperature provided by cavity ventilation, the average energy conversion efficiency of the PV-DSF was approximately 5.7%, compared with 5.6% for the PV-IGU, corresponding to a relative improvement of about 1.8%. When the experimentally validated models were applied to five representative climate zones in China, the average overall energy-saving potentials of the PV-DSF and PV-IGU relative to a conventional insulating-glass window were approximately 28.4% and 30.0%, respectively^[54]. These results show that photovoltaic facades should not be judged by electrical efficiency alone. Solar heat gain, insulation, and auxiliary equipment energy use also need to be included.

Seasonal operation is another important issue in field tests of facade photovoltaics. Winter experiments by Liu et al. showed that ventilation can recover solar heat when irradiance is high enough, thereby allowing the air in the cavity to reach a useful temperature. Under low irradiance and low outdoor temperature, continuous ventilation may instead increase heat loss^[55]. Summer tests separated the effect of mechanical ventilation into three parts: extra electricity from module cooling, lower cooling load from heat removal, and fan electricity use. Switching ventilation based on irradiance and temperature differences was found to be more suitable than running it continuously throughout the day^[56]. In winter heating applications, spectral-split PV/thermal (T) systems use selective optical media to direct wavelengths suitable for photovoltaic conversion to the cells. Other wavelengths are converted into low-grade heat for space heating. Such heat has practical value only when both its temperature level and its available period match the building heating demand^[57].

Photovoltaic envelope research has recently expanded beyond electricity generation alone. Heat recovery, air treatment, and indoor environmental control are increasingly being combined with PV functions. Tang et al. assessed a semi-transparent BIPV/T curtain wall coupled with an air handling system using experiments and modelling. Active ventilation reduced the component temperature by up to 9.03 °C. Annual power output reached 5,530.87 kW·h. Compared with the baseline operating mode, total energy use for building heating, ventilation, and air conditioning (HVAC) and lighting fell by 1,399.73 kW·h^[58]. Ma et al. proposed a curved photovoltaic-thermal-catalytic roof, while Luo et al. developed a photovoltaic roof that can switch among air heating, cooling, and purification modes. These studies show that photovoltaic systems can also support the use of waste heat and improve indoor air quality^[59,60].

Long-term measurements in full-scale test houses and real buildings are useful because they show how photovoltaic components operate under varying seasons, weather conditions, and building loads. They can also be used to check whether control strategies and mode switching work as intended. A reliable long-term assessment should include electricity generation, changes in the thermal environment, and energy use by auxiliary equipment, such as fans.

Facade solar potential is not determined only by whether a surface faces east, south, west, or north. Building density, spacing, street-canyon dimensions, and the arrangement of building volumes also change the distribution of solar radiation. A global study of 120 representative cities combined 3D building outlines, meteorological time series, and dynamic shadow calculations. In the group of cities with relatively strong facade performance, the average ratio of facade to rooftop photovoltaic potential was about 68.20%. Facade potential was even higher than rooftop potential in about 17.50% of the sampled cities, reflecting the large amount of usable vertical surface area^[61]. LiDAR-based work in Lisbon also showed that using roofs and facades together can increase the share of urban electricity demand covered by PV. Rooftop systems generally had payback periods of less than 10 years, whereas a mixed system with equal roof and facade shares required about 15 years^[62]. Facade economics are affected by irradiance, mounting method, and maintenance difficulty. Results from 3D models therefore need to be screened further using installable area, system losses, load profiles, and cost. Temperature losses, inverter efficiency, soiling, and operational availability in urban models should also be calibrated with measurements from representative buildings where possible.

Existing studies now form a continuous evaluation path from full-scale measurements to annual building analysis and then to neighbourhood and city-scale estimates. Component and test-house experiments reveal the links among module temperature, ventilation, heat transfer, and power generation and provide data for operational control. Building simulations validated by measurements can then estimate year-round effects on heating, cooling, lighting, and net energy use. Three-dimensional neighbourhood and urban models are used to study building-to-building shading, available installation surfaces, and spatial differences. Long-term monitoring of real buildings is still limited. Continuous operating data are needed to correct assumptions about module degradation, dust accumulation, and control error. These parameters should also be carried over to city-scale assessments, and uncertainty ranges should be reported to make theoretical potential estimates more reliable.

3.3 Building performance analysis

Building performance analysis examines how operational energy use, energy efficiency, and indoor environmental quality change after PV systems are integrated into buildings. For opaque rooftop PV, electricity generation and system efficiency are usually the main indicators. Semi-transparent PV windows, PV facades, and PV/thermal systems also change solar heat gain, envelope heat transfer, indoor daylighting, and cooling loads. The optical, thermal, and electrical performance of PV envelope systems are coupled, and the actual energy-saving effect also depends on climate and building operation^[63]. Poor parameter choices can even offset part of the energy saved through PV generation. Building-scale evaluation therefore should not rely on a single device-efficiency value. Heating, cooling, lighting, auxiliary energy use, and PV generation need to be calculated separately under the same building geometry, weather conditions, occupancy and equipment schedules, and indoor setpoints. Net building energy use and the overall energy-saving rate can then be obtained. Indoor temperature, daylighting, glare, and colour quality should also be assessed to ensure that apparent energy savings are not achieved at the expense of indoor environmental quality.

For whole-building energy use, Leite Didoné et al. carried out year-round simulations of office buildings in Brazil. Electricity generated by semi-transparent PV windows was evaluated together with lighting and air-conditioning energy use^[64]. Chae et al. entered measured spectral and electrical properties of semi-transparent amorphous silicon cells into commercial building models. Energy use for power generation, heating, cooling, and lighting was compared across six climates. Changing the cell spectrum clearly affected total building energy use, and the optimal window parameters varied across climates^[65]. Ng et al. compared different transmittance levels, power conversion efficiencies, orientations, and window-to-wall ratios (WWR) under Singapore conditions. Lower transmittance reduced cooling loads but could increase electricity use for artificial lighting^[66]. These studies show that the building benefit of a semi-transparent PV window comes from electricity generation, shading, and daylighting together. A full building energy balance is therefore required. Tang et al. combined experiments and modelling to assess a semi-transparent BIPV/T curtain wall with an air handling system. Module temperature, PV output, daylighting, heating and cooling demand, and energy used for air treatment were included. Active ventilation reduced the module temperature by up to 9.03 °C, and annual power generation was 5,530.87 kW·h^[58]. Such systems may also recover waste heat from PV modules. The final net energy gain, however, depends on fan electricity use, airflow resistance, and whether the recovered heat is available when the building actually needs it.

Indoor thermal performance analyses primarily consider how photovoltaic envelope systems affect heat transfer and solar heat gain, and indoor thermal loads. Experiments by Wang et al. showed that ventilated photovoltaic double-skin facades are more effective at reducing solar heat gain, whereas sealed photovoltaic insulating glass units provide better insulation^[54]. Meng et al. combined perovskite solar cells with temperature-sensitive hydrogels and tested the window at different temperatures. Visible-light transmittance, solar regulation, power conversion efficiency, and long-wave emissivity were measured. The smart window had an AVT of 27.30% at room temperature, which fell to 10.40% above 40 °C. Its solar modulation ability (ΔT_sol) reached 15.70%, the peak power conversion efficiency was 17.50%, and low-emissivity behaviour was also observed^[67]. These values describe the window’s ability to control solar radiation and generate electricity. Their effect on actual heating and cooling demand still depends on the local climate, the window surface temperature, and the threshold at which the state changes.

Other thermal management methods show the same need for multi-objective evaluation. Wang et al. introduced a highly transparent passive radiative-cooling coating onto the exterior surface of a semi-transparent CdTe photovoltaic window to form a coated semi-transparent photovoltaic (CSTPV) glazing system. The system integrates semi-transparent CdTe photovoltaic glazing, a passive radiative-cooling coating, and vacuum glazing to combine electricity generation, solar-heat control, and thermal insulation (Figure 2g). The fabricated CSTPV sample exhibited an average standard test conditions (STC) conversion efficiency of 4.009%, compared with 3.877% for the uncoated semi-transparent photovoltaic (STPV) glazing (Figure 2h). Its SHGC decreased from 0.244 to 0.196, while the U-values remained nearly unchanged at 0.695 and 0.693 W m^-2 K^-1 for CSTPV and STPV, respectively. Outdoor comparative tests were conducted for STPV, CSTPV, single glazing (SG), and vacuum glazing (VG) (Figure 2i). The CSTPV glazing exhibited an approximately 15% lower heat-gain rate than the uncoated STPV glazing. Simulations further indicated an approximately 3% increase in annual electricity generation, with the south-facing CSTPV producing about 97.0 kW·h, approximately 3.4% higher than the 93.8 kW·h generated by the corresponding STPV system. During the outdoor thermal test, the temperature difference between the internal and external glazing surfaces reached 22.22 °C for CSTPV, compared with 12.81 °C for STPV, further demonstrating the enhanced thermal-regulation capability provided by the transparent radiative-cooling coating. Daylighting, glare, CRI, and correlated color temperature (CCT) were also evaluated, indicating that thermal-management strategies for photovoltaic windows should be assessed together with electricity generation and indoor visual performance^[68]. Shi et al. used near-infrared-selective-absorption nanofluids to reduce solar heat entering CdTe photovoltaic windows. Daytime heat gain was lowered while natural daylight was maintained. The simulations, however, did not produce consistently positive overall energy savings for all concentrations and cities. In some cases, lighting and auxiliary energy use offset the thermal benefit^[69]. An improved heat transfer coefficient or solar heat gain coefficient, therefore, indicates only that the heat-transfer process has changed. Actual building energy savings must still be judged over a full year by including heating, cooling, lighting, and auxiliary system energy. Wang et al. also conducted thermal, electrical, and daylighting tests on a double-layer ventilated window with CdTe cells. Measured spectral data were used to predict indoor colour and daylighting, and annual energy performance was calculated with an experimentally validated thermal-electrical model. Solar heat gain coefficients of 0.28 in winter and 0.11 in summer were measured. The heat transfer coefficient was 2.05, and the daylight factor (DF) was about 0.60%^[70]. The ventilation cavity can therefore reduce solar heat gain in summer, although the low daylight factor may increase the need for artificial lighting.

Semi-transparent photovoltaic windows differ from opaque PV modules because their lighting and colour performance must also be evaluated. Liu et al. studied CdTe photovoltaic windows with transmittance levels of 20%, 30%, 40%, and 50%. Measured spectral data were used in annual RADIANCE simulations to assess useful daylight illuminance (UDI), illuminance uniformity, and glare probability. The colour quality of transmitted daylight was also examined using correlated colour temperature and colour rendering index^[71]. CdTe photovoltaic windows were found to improve indoor illuminance distribution and reduce glare risk. Higher transmittance, however, changed the balance among daylight availability, spatial uniformity, and electricity generation. Daylighting performance, therefore, cannot be represented by the average illuminance or visible-light transmittance at a single moment. Year-round useful daylight, excessive daylight, glare, and differences among indoor positions also need to be considered. Liu et al. later developed and tested a building-integrated concentrating photovoltaic (BICPV) smart-window prototype. Light transmittance, photovoltaic output, and temperature response were measured at different temperatures and material concentrations. Thermochromic dimming reduced transmittance at high temperature and changed both the radiation reaching the photovoltaic cells and their output power^[72]. In 2024, Li et al. combined ray tracing, heat-transfer calculations, computational fluid dynamics (CFD), and photovoltaic models. Transmission, absorption, local temperature, and electrical output of a complex photovoltaic window were predicted for different solar incidence angles, and the model was validated with experimental data^[73]. These studies show that the optical and electrical behaviour of complex photovoltaic windows changes with solar angle and local temperature. Fixed transmittance, solar heat gain coefficient, and conversion efficiency are therefore not suitable for whole-building annual simulation.

Selective absorption by photovoltaic materials can also change the colour of transmitted daylight indoors. Shi et al. measured the transmission spectra of semi-transparent photovoltaic glass with different colours. The transmitted light was evaluated using the general CRI, R_a, the TM-30 colour fidelity index (Rf), the gamut index (Rg), and CCT. Changes in photovoltaic coverage had only a limited effect on the colour quality of glass with the same colour. Differences among colours were much larger. R_a ranged from 48.5 to 98.1, Rf from 0.9 to 98.4, Rg from 76.4 to 170.8, and CCT from 1036.8 to 7948.8 K^[74]. New photovoltaic windows with self-powered, self-cleaning, luminescent concentrating, or multi-band tuning functions extend the ability to regulate lighting and thermal conditions dynamically. Wang et al. developed a tri-band, split-type smart photovoltaic window (SPW) for all-season thermal management by coupling a perovskite solar cell with a broadband thermal-management unit (Figure 2f). The unit integrates electrically responsive polymer-dispersed liquid crystals (PDLCs), a high-emissivity SiO₂ passive radiative-cooling layer, and an Ag low-emissivity layer, enabling coordinated regulation across the visible, near-infrared, and mid-infrared spectral regions. The perovskite solar cell can provide electrical power for switching the PDLCs, enabling solar-powered dynamic regulation. EnergyPlus simulations under Beijing climate conditions further showed that the SPW could reduce annual heating and air-conditioning energy consumption by approximately 16.7% compared with conventional glass, demonstrating its potential for all-season building thermal management and energy conservation^[75,76]. Most work in this area is still carried out at the material, device, or prototype level. Common tests include spectral tuning range, transmittance, power generation efficiency, surface properties, and temperature response.

Quantitative daylight metrics should be complemented by occupant-based evidence because semitransparent PV glazing can alter window views, perceived brightness and uniformity, colour perception, and non-visual light exposure. Yeom et al. combined building-energy simulation with a VR experiment involving 31 participants and STPV visible-light transmittance (VLT) levels of 10%, 20%, and 30%. Higher VLT improved psychological satisfaction and reduced task load, but decreased simulated energy performance; auditory/cognitive task performance showed no significant difference. Multi-objective optimization was therefore used to balance energy performance, psychological satisfaction, and task load^[77]. Wang and Xiang conducted a full-scale mock-up study with 36 participants, comparing hollow semitransparent PV glazing with clear glazing. Colour tasks and questionnaires were combined with equivalent melanopic lux (EML), electrodermal activity, and heart-rate measurements. The PV glazing produced more uniform perceived daylight distribution and lower glare potential, while colour-rendering performance remained comparable to clear glazing^[78]. These studies suggest that PV-glazing assessment should consider daylight and glare, view and colour quality, subjective comfort, task-related responses, and, where non-visual effects are relevant, melanopic and physiological indicators.

At the building-cluster scale, the energy performance of individual buildings needs to be combined with three-dimensional urban form. Zhao et al. used mixed-use block prototypes to study how combinations of building density, height, spacing, and volume affect facade solar radiation and electricity coverage^[79]. Chen et al. built 3D surfaces for buildings in Wuhan from multi-source remote sensing data and calculated solar resources and technical photovoltaic potential for roofs and facades^[80]. Yu et al. combined 3D building form, hourly weather data, and dynamic shading for 120 typical cities. Large regional and urban-form differences in the photovoltaic potential of facades were found^[61]. These studies estimate how much solar energy reaches building surfaces and how much surface area can be used for PV. Neighbourhood energy analysis requires an additional step. Hourly loads from residential, office, and commercial buildings need to be included to determine whether PV electricity can be used locally and how it affects peak demand and storage needs. Environmental benefits should also be assessed on this basis. Brito et al. used LiDAR data to study the photovoltaic potential of roofs and facades in two areas of Lisbon. Adding facades increased urban electricity coverage, but lower facade irradiance and more difficult installation and maintenance extended the payback period^[62]. Solar resources or technical potential from 3D city models therefore cannot be treated directly as actual energy savings or carbon reductions. Further assessment should include PV self-consumption, grid emission factors, storage and distribution needs, and life-cycle impacts from mounting structures, curtain-wall renewal, inverter replacement, and maintenance.

The long-term performance of BIPV is significantly affected by installation configuration and operating temperature. Özkalay et al. compared the long-term performance of open-rack, partially ventilated, and non-ventilated BIPV systems and found that restricted rear ventilation increases module temperature and thermomechanical stress, potentially accelerating encapsulant discoloration, current loss, and damage to cells and interconnections. Appropriate material selection and improved rear ventilation can mitigate these degradation effects^[81]. Kim et al. conducted a long-term evaluation of a 142.654 kWp vertical BIPV system from 2012 to 2020. Among the 22 arrays, 19 operated normally and three experienced malfunctions. By 2020, the normally operating arrays showed a 19.12% performance degradation compared with 2012, while the malfunctioning arrays operated at approximately half the efficiency of the normal arrays; inverter efficiency remained relatively stable^[82]. Therefore, BIPV evaluation should consider not only initial power conversion efficiency but also long-term degradation, failures and maintenance, service life, and life-cycle environmental impacts, rather than relying solely on short-term efficiency to assess its value in building applications.

Building-integrated photovoltaic research has gradually developed an evaluation framework that includes electricity generation, heating, cooling, lighting, and the indoor thermal and visual environments. The analysis now extends from components and single buildings to neighbourhoods and cities. Actual BIPV performance is not determined solely by device conversion efficiency. Envelope heat transfer, natural daylight, operational control, building load patterns, three-dimensional urban shading, and available installation area also have strong effects. Future work needs more long-term and cross-seasonal monitoring in real buildings. Model boundaries, control methods, and auxiliary energy use should also be stated clearly so that different photovoltaic technologies can be compared under the same building service conditions. Table 2 provides a comparative summary of representative photovoltaic systems investigated for building applications, highlighting their coupled effects on daylighting and visual performance, thermal behavior, and electrical or building-energy performance (Table 2).

4. Future Research Directions

Improving power conversion efficiency remains a main direction for the next stage of BIPV research. More attention, however, needs to be given to the link between electricity generation and building energy demand. Semi-transparent photovoltaic windows are affected by electricity output, solar heat gain, natural daylight, envelope heat transfer, and climate. Their value to a building therefore depends on a balance between PV generation and operational energy use. Flexible photovoltaics and tandem devices are becoming more efficient while retaining lightweight, bendable characteristics useful for architectural integration (Figure 3a,b). High efficiency and structural compatibility can thus be developed together^[83]. Rated efficiency under standard test conditions does not fully describe PV performance in a building. During operation, non-uniform module temperatures and their interaction with electrical performance can change actual power output^[84]. Future studies should therefore include module temperature rise, building-envelope heat transfer, heating and cooling loads, and end-use energy patterns. A continuous evaluation method is needed across the material, device, component, and building scales. Transferable parameters across these scales would enable comparison of PV generation with hourly building demand. The real effect of higher device efficiency on annual electricity generation, operational energy use, and carbon emissions could then be evaluated more accurately. Device optimisation would, in this way, be more directly linked to whole-building energy and low-carbon performance.

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Figure 3. a. Structural schematic of flexible PSTs. Reproduced from reference^[83]. CC BY 4.0; b. Digital photo of the flexible tandem solar cells. Reproduced from reference^[83]. CC BY 4.0; c. A cross-sectional view of the perovskite microcell in the colored solar window. Reproduced from reference^[89]. CC BY 4.0; d. Photograph of a mosaic-patterned colored solar window with four different colors. Reproduced from reference^[89]. CC BY 4.0. PSTs: perovskite/silicon tandem solar cells.

At the building scale, BIPV design should not be optimised for electricity generation alone. Trade-offs among several performance indicators need to be studied. Research on BIPV windows in office buildings has used orientation, window-to-wall ratio, transmittance, and photovoltaic type as design variables. Annual net electricity cost and additional investment were also included, and multi-objective algorithms were used to obtain optimal solution sets for different cases. Clear trade-offs were found among design choices, operating benefits, and initial investment^[85]. Studies of BIPV shading systems reached a similar conclusion. Photovoltaic components affect electricity generation, natural daylighting, and radiant heating and cooling loads simultaneously, but the relative importance of each effect varies with climate and building geometry^[86]. Future BIPV assessments should therefore consider total building energy use, HVAC load, lighting demand, indoor environmental quality, payback period, and carbon emissions together. For semi-transparent photovoltaic windows, BIPV shading, and semi-transparent facades, changes in visible-light transmittance and solar heat gain must also be included in annual energy calculations. Overall building energy savings should be an important basis for design selection.

Coloured photovoltaic facade devices are another important direction, as building integration must meet both visual and functional needs. Structural colouration, photonic structures, luminescent conversion, and light-path control have been used in recent years to reduce the efficiency loss caused by colouring. Richer colours and patterns can therefore be produced while maintaining relatively high electricity-generation performance^[87,88]. These results show that architectural appearance and high efficiency do not necessarily conflict. Lee et al. developed colored solar windows by integrating patterned perovskite microcells with a metal-insulator-metal (MIM) resonant structure. Patterned semitransparent perovskite microcells serve as the photovoltaic component, while an Ag/SiO₂/Ag MIM resonator is incorporated on the rear side to generate wavelength-selective transmitted colors (Figure 3c). The perovskite microcells fabricated by the swelling-induced lift-off process achieved a PCE of 20.1%, nearly identical to that of the corresponding unpatterned device. After further optical optimization, the semitransparent photovoltaic device achieved an AVT of approximately 56.45%, a PCE of approximately 8.28%, a light-utilization efficiency (LUE) of 4.67, and a CRI of 97.5%. Four transmitted colors: red, green, blue, and purple, were obtained by adjusting the insulator thickness of the MIM resonant structure (Figure 3d). Because the MIM structure was positioned on the rear side of the semitransparent PV and primarily operated over the non-microcell regions, the electrical characteristics remained nearly unchanged among different colors^[89]. These results demonstrate that future BIPV development can combine photovoltaic performance with transparency, color quality, and customizable façade aesthetics through microstructural and optical design.

Future work on coloured photovoltaic facades should quantify the relationships among colour, reflection spectrum, conversion efficiency, long-term stability, and the manufacturing process. Colored PV could then be developed as a building-facade component with its own material expression. Product systems also need to suit different facade materials, urban appearances, and functional requirements.

Combining passive thermal management with PV systems is a promising way to improve real power output and building energy savings. As PV-thermal coupling has been studied in more detail, module temperature distribution and its effect on electrical performance have become important parts of operational evaluation^[84]. Low-energy and adaptive temperature control can be achieved through methods such as evaporative cooling and capillary transport, which can reduce high PV operating temperatures^[90]. Radiative cooling, moisture adsorption/desorption, and interfacial evaporation have also been combined. By strengthening water circulation and mass transfer, continuous cooling can be maintained, providing an additional material- and mass-transfer route for long-term PV cooling^[91]. Full-spectrum radiative cooling has been proposed for bifacial PV systems as well. Above-bandgap light absorption, below-bandgap light reflection, and thermal radiation are coordinated to control spectral energy flow. The trade-off between cooling benefits and electricity-generation gains across different climates is receiving increasing attention^[92]. In agricultural thermal management, radiative-cooling materials have been used to reduce local heat buildup caused by solar radiation. Passive cooling is obtained through high solar reflectivity and high emissivity in the atmospheric window^[20]. Switchable optical materials, including thermochromic and electrochromic coatings, are also being considered for seasonal adaptation. These approaches may offer useful insights into passive cooling and seasonal thermal management for building-integrated PV systems. Thermal management in BIPV requires coordinated consideration of PV operating temperature, heat transfer through the building envelope, and building heating and cooling demand. Ventilated rear cavities can enhance convective heat removal, while their effectiveness is strongly influenced by cavity depth, airflow rate, surface properties, and local climate. Phase change materials can provide passive thermal regulation by absorbing excess heat during periods of high solar irradiance, although their performance depends on melting temperature, thermal conductivity, enclosure design, and environmental conditions. Therefore, climate-responsive combinations of ventilation, insulation, thermal storage, and heat recovery should be further investigated to improve PV electrical performance while limiting unwanted heat transfer and maintaining indoor thermal comfort^[93,94].

Future BIPV development should also place greater emphasis on new integration strategies for commercially available photovoltaic building materials. Existing PV products, such as opaque façade panels, roofing elements, and semi-transparent glazing, could be further integrated into prefabricated and modular building-envelope systems, enabling greater factory-based assembly, faster on-site installation, and improved construction quality. In parallel, digital design tools such as Building Information Modeling (BIM) can facilitate the early coordination of architectural design, photovoltaic layout, building-envelope performance, and electrical systems. Future research should therefore focus on modularized BIPV components, standardized mechanical and electrical interfaces, and design strategies that facilitate installation, replacement, and maintenance, thereby transforming BIPV from highly customized project-specific solutions into more scalable and adaptable building products^[14,95].

Taken together, future BIPV research should integrate high-efficiency device development with whole-building energy balance, multi-objective design, architectural color and visual quality, climate-responsive optical and thermal regulation, and modular construction. A common multiscale evaluation framework should connect material and device performance with annual energy use, indoor environmental quality, carbon performance, durability, safety, economic feasibility, and maintainability under representative operating climates. Such coordinated research would provide a stronger basis for translating advances in photovoltaic materials and smart-window technologies into reliable, scalable, and architecturally compatible building-envelope systems.

5. Conclusion

The development of building-integrated photovoltaics is closely linked to continued advances in photovoltaic materials, device structures, and building-envelope integration. Crystalline silicon and conventional thin-film technologies, including amorphous silicon, CdTe, and CIGS, have established an important technological basis for building applications, while perovskite, tandem, flexible, transparent, semi-transparent, and colored photovoltaic devices have further expanded the range of possible building-envelope configurations. Recent progress shows that power conversion efficiency remains a fundamental indicator, but it cannot fully describe the suitability of a photovoltaic technology for building integration. Stability, large-area fabrication, flexibility, weight, visible-light transmittance, spectral characteristics, color quality, encapsulation reliability, and compatibility with construction systems have become increasingly important. Perovskite and tandem devices offer opportunities for combining high conversion efficiency with tunable optical properties, while flexible and lightweight photovoltaic technologies provide additional possibilities for curved, lightweight, and prefabricated building components. Transparent, semi-transparent, and colored photovoltaic technologies further demonstrate that electricity generation can be coordinated with daylight transmission, architectural appearance, and facade design.

The reviewed studies also show that the performance of photovoltaic devices needs to be evaluated through a continuous sequence extending from laboratory characterization to outdoor exposure and full-scale building operation. Laboratory research has clarified the importance of composition engineering, crystallization control, defect passivation, interface optimization, carrier transport, and current matching for improving photovoltaic efficiency and stability. Outdoor studies further demonstrate that irradiance, spectral distribution, ambient temperature, module temperature, humidity, and other environmental conditions can substantially alter the electrical behaviour observed under standard test conditions. These effects become more complex when photovoltaic components are incorporated into buildings, where installation configuration, orientation, ventilation, shading, envelope construction, indoor loads, and operational control also influence actual performance. Consequently, rated conversion efficiency under standard test conditions should be complemented by measurements and models that represent realistic operating conditions, seasonal variations, and long-term exposure.

At the building scale, photovoltaic envelopes exhibit strong electrical, thermal, and optical coupling. Semi-transparent photovoltaic windows, photovoltaic double-skin facades, photovoltaic insulating glass units, and BIPV/T systems can simultaneously influence electricity generation, solar heat gain, envelope heat transfer, daylight availability, heating and cooling demand, and auxiliary energy consumption. The reviewed studies indicate that improvements in one performance indicator do not always produce a corresponding improvement in total building energy performance. Ventilation, for example, can reduce photovoltaic operating temperature and solar heat gain, but its benefits depend on season, airflow conditions, fan electricity consumption, and building demand. Similarly, lower window transmittance may reduce cooling loads and glare while increasing artificial-lighting demand. Smart photovoltaic windows using thermochromic materials, electrically responsive optical layers, spectral regulation, and passive radiative-cooling strategies provide additional opportunities for adapting envelope performance to changing environmental conditions. Their value should therefore be assessed through combined electrical, thermal, daylighting, visual-comfort, and energy analyses rather than through individual material parameters alone. Occupant-related indicators, including perceived brightness, glare, view quality, color rendering, psychological response, and non-visual light exposure, are also important when semi-transparent photovoltaic glazing is used in occupied spaces. The appropriate evaluation boundary further extends beyond individual components and buildings. At neighbourhood and city scales, building density, height, spacing, orientation, three-dimensional shading, and the distribution of available roof and facade areas strongly affect photovoltaic potential. Three-dimensional urban models can identify suitable installation surfaces and estimate spatial differences in solar resources, but technical solar potential alone does not directly represent actual energy or carbon benefits. Building load profiles, local electricity use, system losses, storage requirements, grid conditions, maintenance, and life-cycle impacts should also be incorporated. Long-term field studies are particularly important because degradation, module temperature, restricted ventilation, dust accumulation, component failure, and maintenance can progressively alter system performance. These factors are essential for evaluating whether the benefits observed in laboratory devices and short-term experiments can be maintained throughout the service life of a building-integrated photovoltaic system.

Future progress therefore depends on closer coordination between photovoltaic material development and whole-building performance requirements. Higher device efficiency should be pursued together with long-term stability, scalable fabrication, optical and architectural adaptability, thermal management, safety, economic feasibility, and maintainability. Multi-objective optimization is needed to balance electricity generation, HVAC and lighting energy use, indoor environmental quality, carbon performance, initial investment, and long-term operating benefits under different climates and building types. Colored photovoltaic facades and dynamically regulated photovoltaic windows should be developed with greater attention to spectral characteristics, color quality, durability, and architectural compatibility. Passive cooling, ventilation, thermal storage, radiative cooling, and other thermal-management approaches can further reduce photovoltaic operating temperatures and improve building energy performance when they are designed according to local climatic and operational conditions. At the construction level, prefabricated and modular BIPV components, standardized mechanical and electrical interfaces, digital design tools, and improved strategies for installation, replacement, and maintenance could help reduce the gap between technological development and practical deployment.

Overall, the future value of photovoltaic materials in buildings will depend on how successfully high-performance devices can be translated into durable, controllable, and constructible building-envelope systems. A multiscale evaluation framework linking material properties, device performance, outdoor operation, component behaviour, whole-building energy use, indoor environmental quality, long-term reliability, and urban context is therefore essential. Such integration can provide a more reliable basis for selecting photovoltaic technologies, optimizing smart photovoltaic envelopes, and assessing their contribution to low-carbon buildings. Continued coordination among materials science, photovoltaic engineering, building physics, architectural design, and construction technology will be critical for converting advances in photovoltaic materials and devices into scalable building solutions with sustained energy, environmental, and architectural benefits.

Acknowledgements

We confirm that Gemini 3.1 was used solely for language polishing during the manuscript preparation process, including improvements in grammar, wording, and readability. It was not used to generate or determine any core scholarly content. All research content, including literature selection and synthesis, scientific interpretation, conclusions, figures, tables, and other substantive academic content, was independently prepared and verified by the authors. The authors reviewed, revised, and approved the final manuscript and take full responsibility for its content.

Authors contribution

Shi S, Yuan Z: Conceptualization, literature review, writing-original draft.

Zhang J, Li J, Lin B: Supervision, writing-review & editing.

Conflicts of interest

The authors declare no conflicts of interest.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

Not applicable.

Funding

The study was supported by the National Natural Science Foundation of China (Grant Nos. 52394223, 52425801, 52130803), the Postdoctoral Fellowship Program of CPSF (Grant No. GZB20250413), and the China Postdoctoral Science Foundation (Grant No. 2025M771609).

Copyright

© The Author(s) 2026.