The need to tackle climate change is leading to an intensified global interest in energy generation based on renewable sources. Compared to other renewable resources, the harvesting of solar irradiance is one of the most widespread, practical, and economical ways to generate energy [1]. Because of the absence of mechanical motion components, the low maintenance costs, and the abundance of solar energy that reaches the Earth’s surface every year (1.8 × 10¹¹ MW) [2], photovoltaics (PVs) represent one of the most promising solutions to fulfill the increasing world power consumption. In fact, the improvement in the cell conversion efficiency coupled with the decrease in manufacturing costs allowed for the installation of 1185 GW by the end of 2022 [3]. According to the International Energy Agency’s (IEA) “Renewable Energy Market Update” [4], the global solar PV capacity is envisioned to be able to produce almost 1 TW in 2024, which is sufficient to meet the yearly demand of 650 GW by 2030 and the net-zero emissions by 2050 scenario. The commitment to decreasing greenhouse gas (GHG) emissions related to building energy demands, which accounts for 40% of the global energy supply, is leading to the powering of several operations that extend beyond mere electricity production with solar energy, such as ventilation, air conditioning and refrigeration, water desalination, decontamination, and heating [5,6]. Figure 1 summarizes the possible applications for which a PV façade can be designed.

In particular, buildings’ air conditioning and electricity needs consume more than 55% of the global electricity supply, with an estimated annual growth rate of 2.5% [7]. This, coupled with land acquisition constraints and the vast impact that PV plants have on biodiversity and the natural environment, has limited the growth of such plants in favor of smaller-scale distributed installations, especially in urban environments [8,9]. Article 2 of the amended Energy Performance of Buildings Directive (EPBD) (2018/844/EU) [10] aims to support the renovation of the national stock of buildings, residential and non-residential, into highly energy-efficient and decarbonized buildings by 2050, facilitating their transformation towards net-zero energy buildings (NZEBs). Building-integrated photovoltaic (BIPV) technologies constitute interesting and alternative solutions to the production of electricity, eliminating the necessity of areas exclusively dedicated to PV plants [11]. Since BIPV devices directly replace building elements, the surface area for PV installations is provided by the building envelope, and the building’s electrical system acts as an interface between the PV panels and the public utility grid [12,13,14]. Happle et al. [15] established that, if properly installed, BIPVs can limit GHG emissions by up to 50%. This represents a cost-effective method to considerably reduce the environmental impact of a sector that accounts for 39% of total GHG emissions [16].

In this regard, De Boeck et al. [17] identified the increase in a building envelope’s insulation efficiency as a key parameter for increasing its energy performance. Furthermore, BIPV technologies are designed to combine power production and insulation efficiency, improving a building’s architectural expression [18]. Indeed, according to IEA Task 15, to actually consider PV modules as building-integrated elements, they must provide additional functions other than energy production, such as mechanical rigidity or structural integrity for the building, impact protection against atmospheric agents (rain, snow, wind, and hail), shading, daylighting, or thermal insulation, as well as fire and noise protection.

A critical issue is represented by glazed surfaces since they highly impact a building’s lighting, heating, and ventilation [19]. The design of windows plays an important role in ensuring low cooling and lighting loads for NZEBs [20,21], leading to the implementation of smart windows or dynamic shading systems that increase the resilience of a glazed envelope in terms of energy savings [22,23]. The attention of the scientific community is therefore focused on the study of active glazed surfaces that could improve the building energy demand with respect to conventional static windows, especially for commercial buildings with large transparent façades [24]. De Masi et al. [25] analyzed the impact of windows on building performance in different European climates, showing that, depending on specific factors, such as window typology and orientation, climatic conditions, and window-to-wall ratio, the optimized design of a glazed building envelope may lead to an energy saving varying from 27% to 62%. Similarly, the U.S. Department of Energy reported that window-related energy losses account for up to 25% of a household’s energy consumption [26].

A possible solution to further decrease the cost of BIPV technologies is the manufacture of devices that have static or monoaxial concentrating components in the so-called building-integrated concentrating photovoltaic (BICPV) technology. Data from [27,28,29,30] estimated that systems with a low-concentration factor specifically designed for building integration can reduce the cost of the device by up to 40% with respect to similar non-concentrating devices. According to the most general definition, a solar concentrator is an object redirecting light rays from a larger area, namely the aperture area, to a smaller one, called the receiver of the absorber area [31]. The ratio between the aperture and the absorber area defines the concentration ratio, which is commonly used to classify optical concentrators. The concentrators enhance the luminous flux that reaches the active PV area by using relatively cheap optical concentrating elements. The goal is to maximize the power generated by the cells by a factor almost equal to the concentration ratio [32], while reducing the amount of PV cells per active area. However, optical concentration also leads to a higher nominal operating cell temperature; therefore, for a medium or high concentration factor, the presence of active cooling procedures is particularly important to avoid limitations in the system efficiency due to additional thermal losses within the PV cells [33,34].

A further strategy to maximize the utilization of solar radiation for building energy efficiency is its concurrent conversion into both thermal and electrical energy via hybrid photovoltaic–thermal (PVT) systems [35,36], also efficiently coupled with BICPVs. Building-integrated photovoltaic–thermal (BIPVT) devices are particularly indicated to support net-zero energy constructions. In addition to their insulation and electricity generation functions, they efficiently harvest a portion of the thermal energy related to solar irradiance [37]. The heat extracted by the thermal collectors of the PVT systems may be used in ventilation air pre-heating [38,39], underfloor heating systems [40], domestic hot water [41], passive and active cooling [42], and heat storage [43,44], thus increasing the overall building energy efficiency.

In terms of architectural and functional design, a façade’s optical properties depend on building energy and lighting needs. In Figure 2, an illustration that clarifies the effects of external skin transparency on indoor spaces is presented.

In light of the current surge in interest surrounding BIPV systems, numerous researchers conducted comprehensive reviews that delve into distinct PV technologies. In particular, Sirin et al. [45] reviewed BIPVT systems for green buildings, presenting the operation of these systems, their classification, and potential contributions to building energy efficiency. Several in-depth works on building-integrated and -attached photovoltaic solutions have been performed by A. Ghosh in recent years [46]. In their review on fenestration photovoltaic devices [47], the authors presented different PV-based fenestration-integrated photovoltaic systems to estimate their impact on building energy consumption. Romani et al. [48] further examined this topic by presenting the most common optical, thermal, and electrical models, combined with the modeling capabilities of the most common building simulation tools. In 2022, Massod et al. [49,50] investigated the developments and applications of concentrating photovoltaic technologies. They concluded that the best concentrators for integration in building façades are asymmetric compound parabolic concentrators (ACPCs) and 3D crossed compound parabolic concentrators (CCPCs). Li et al. [51] discussed the recent advances in BIPVs with a particular emphasis on colored technologies, presenting design principles, theoretical analysis, technical routes, and the corresponding demonstration studies. All these works focused on specific PV categories regardless of the building component in which they are integrated. As a further example, Taser et al. [52] provided a comprehensive analysis of BIPV systems, ranging from façade-integrated solutions to roof-mounted solutions. However, due to the breadth of the review, the authors neglected some technical parameters, such as the U-value, the SHGF, or the WWR, which may represent crucial information in the comparison of different PV technologies.

Cutting-edge BIPV solutions allow for the substitution of several envelope components, such as façades, windows, shading elements, or roofs [53]. However, for high-rise commercial or residential buildings, a rooftop PV alone is not sufficient to achieve the NZEB goal, as the building energy demand is greater than the energy produced by the roof PV installation. This forces a large part of the required PV installation to be placed on the façade, namely, a part of the building envelope having a significant architectural value and providing the building with a characteristic appearance and attractiveness. For this reason, innovative BIPV solutions present a tailored design in terms of transparency, color, or texture, thus increasing the building energy efficiency without compromising the façade aesthetics or functionality [54,55]. Moreover, a façade PV has the possibility of exploiting different orientations to guarantee a more uniform energy production during the entire day [56,57,58].

Biyik et al. [59] presented the advantages of the implementation of BIPV systems in comparison to utility-scale solutions that can be summarized as an increment in PV deployment areas as well as a reduction in building thermal losses and capital costs.

The efficiency of PV modules, currently in the range of 12–23% for commercial devices [2], depends strongly on the panel technology and its working conditions [41]. Depending on the technologies on which they are based, cutting-edge solar modules can have either single or multiple active layers. These layers may perform the proper conversion of solar energy into electrical energy, or improve the absorption of solar radiation in different ranges, thus enhancing the entire device efficiency [60]. A widespread methodology to classify solar cells and PV panels is based on the technology used for solar cell fabrication: each PV module can be subdivided into three main generations strictly related to the amount of contained crystal silicon [61]. The different generations are depicted in Figure 3. The first generation is characterized by the presence of crystalline silicon (c-Si) solar cells [62], which can be subdivided into monocrystalline (mc-Si) [63,64] or polycrystalline (pc-Si) silicon solar cells [65,66]. Standard second-generation PV panels are thin-film (TF) solar cells that present several advantages over first-generation c-Si devices, especially for building integration, having a lower thickness with respect to the first-generation ones. Indeed, the possibility of achieving TF solar cells with a thickness that ranges from a few nanometers to a few microns opens the possibility to tailor the flexibility, weight, and transparency of these devices according to their function in the building envelope. In addition, as the cell cost accounts for roughly 50% of the total module price [67], if the manufacturing cost of TF technologies competes with first-generation PV solutions, their implementation may lead to a potential cost reduction [68]. For this reason, research activities aiming to improve the efficiency of TF devices based on different technologies, such as copper–indium–gallium–selenide (CIGS) [69,70], cadmium telluride (CdTe) [71,72], or amorphous silicon (a-Si) [73,74], have significantly increased.

According to the theoretical Carnot limit, if efficiently converted, solar radiation can generate electricity close to 95% [75]. However, single-junction cells of the first and second generations can only convert 31% due to the Schockley–Queisser limit, whereas third-generation solar cells are free from this limit [76]. The percentage of solar energy not converted into electricity is dissipated as heat. Third-generation solar cells are characterized by devices in which silicon-based solutions play a secondary role and have not yet achieved large-scale manufacturing as opposed to first- and second-generation PVs. Examples of third-generation solar devices are dye-sensitized solar cells (DSSCs) [77,78], perovskite solar cells (PSCs) [79,80], and organic photovoltaics (OPVs) [81,82]. DSSCs include four main components: the photoanode, the counter electrode, the dye sensitizer, and the electrolyte. The first is a wide-bandgap semiconductor that, combined with the dye sensitizer, is needed to actively harvest light, while the electrolyte contains the redox couple for dye regeneration [83]. PSCs are distinguished by perovskite-structured compounds acting as a light-harvesting active layer; the most common ones are hybrid organic–inorganic lead or tin halide-based materials [84]. OPV devices are based on conductive organic polymers or small organic molecules for light absorption and charge transport rather than semiconductor p–n junctions [85]. Despite the technology on which they are based, PV panels are very sensitive to environmental conditions, such as irradiance, dust, humidity, and ambient temperature. Numerous investigations have focused on the influence of the inclination angle and the orientation of PV panels on their performance [86].

The building-integrated concentrating photovoltaic (BICPV) systems represent a niche that is still commercially under-exploited, mainly because of the esthetical constraints related to the concentrating components. Optical concentrators commonly used in PVT systems can be broadly classified into two main families, namely, conventional and holographic concentrators [87]. These two families can be further subdivided into different types, such as refractive, reflective, hybrid, and luminescent concentrators [88], which can be in turn subdivided into compound parabolic concentrators (CPCs), V-trough reflectors, Fresnel reflectors (FR), or luminescent solar concentrators (LSC) [89,90,91].

Due to their low transparency, CPCs and V-trough reflectors are preferably employed in low-concentration systems, integrated into opaque roofs or façades [92,93], whereas FR or LSC are usually integrated into transparent or semi-transparent installations [94]. A further advantage of BICPVs is the higher working temperature of the PV cells, since the excess of thermal energy generated on the modules can be used in residential spaces or for water heating, drying and agricultural processes, as well as for other domestic or industrial applications [95,96].

Devices with a concentration ratio greater than 100× are defined as high-concentration systems, and they require both the implementation of high-precision dual-axis tracking and, in some cases, active cooling systems [89]. Systems with a concentration ratio ranging from 10× to 100× are called medium concentration devices, and if their concentration ratio is near 100×, they can still feature active cooling but the tracking is limited to a monoaxial one [88]. Objects with a concentration ratio lower than 10× are identified as low-concentration systems that do not require to be coupled with active sun tracking. However, single-axial tracking can be implemented to extend light collection hours during the day [28,97]. The system implemented in BICPV applications are usually medium and low concentrating collectors, as they are stationary or single axial tracking systems [98,99], manufactured to maximize the radiation that reaches the PV cells considering daily and seasonal changes in the solar position [100]. To increase sunlight harvesting, the development of concentrators with a large light acceptance angle is generally preferred to the coupling with a tracking system, as this practice not only decreases the overall system cost but also allows for the concentration of a larger portion of the diffuse radiation.

The ability to focus diffuse radiation makes this type of collector particularly suitable for BICPV applications, as in urban environments diffused contributions can be dominant, especially in high-population-density areas [101,102].

Solar concentrators can be clustered according to the optics, as imaging or non-imaging, or according to their concentration method, since they can be categorized as reflective, refractive, hybrid, or luminescent concentrators [88]. Additionally, CPCs can be subdivided into either two-dimensional or three-dimensional concentrators, depending on whether the concentration is performed on one or two planes [88].

The electrical power supplied by PV panels depends also on their temperature. In fact, the rating plate of these devices is measured under specific Standard Test Conditions (STCs): irradiance of 1000 W/m², ambient temperature of 25 °C, and wind speed of 1.5 m/s [103]. Typically, during standard test procedures, the modules are irradiated via flash lamps; thus, the ambient temperature corresponds to the cell temperature, even under STC irradiance. However, in on-field working conditions, if cells are exposed to 1000 W/m², they reach a considerably higher temperature, thus diminishing the power produced by the modules [104,105]. The prolonged increase in the panel surface temperature contributes to an increment in the panel shunt resistance as well as to the creation of hot spots, which not only decrease their performance, but also have an impact on their lifetime.

A further advantage of these devices is that the thermal collector is embedded in the system frame and placed behind the PV components, thus employing the same collector area for the extraction of the additional thermal energy. Despite that typical PVT modules are realized for low and medium temperature processes, with a fluid delivery temperature ranging from 20 °C to 80 °C [107,108], the better efficiency for both thermal and electrical components can only be attained at relatively low temperatures. For ideal operating conditions, the reference temperatures are the ambient temperature and 25 °C for the thermal and electrical components, respectively [109,110]. Considering the impact that BIPVT systems may have on NZEBs, various thermal collectors have been developed to enhance the overall system efficiency, such as air [111], water [112], bi-fluidic [113], or thermoelectric [114,115] cooling. Concerning extraction methodology, some of the new developments are nanofluids [116,117], evaporative cooling [118,119], and phase-change material (PCM) [120,121]. As for standard BIPV technologies, BIPVT solutions can be integrated into façades, rooftops, and shading devices [122]; however, in this article, only those related to building façades were analyzed.

As for opaque BIPV solutions, the absence of mechanical motion components, the low maintenance costs, and the quantity of solar radiation filtering through traditional windows, make semi-transparent PV technologies the best candidates to turn traditional glazed surfaces into active building elements. In Figure 4, the heat and light transfer phenomena characterizing a standard window are presented. The optimization of fenestration-integrated PV performance is even more delicate with respect to opaque BIPV technologies since the primary purpose of PV windows is the visual and thermal comfort of the building occupants, whereas energy production represents an added value.

Indeed, as also testified by De Masi et al. [25], for a proper smart window design, additional parameters need to be taken into account, such as transparency, window-to-wall ratio, coverage, fresh air infiltration, and the solar-heat-gain coefficient (SHGC).

The estimation of indoor visual comfort is usually defined by the external daylight penetration [123,124], the correlated color temperature (CCT), and the color rendering index (CRI) [125]. The last two parameters are typically used to define the colorimetric properties of a light source. In this case, the PV glass is assimilated to a light source enlightened by solar radiation and having an emission spectrum equal to its transmission one. To ensure the occupants’ visual comfort, the CRI value should be higher than 80 out of 100 and the CCT should range between 3000 K and 7500 K [55]. In the literature, the external daylight that penetrates through fenestration BIPV devices is estimated by using the daylight glare probability (DGP) and the daylight glare index (DGI) [126,127] or by evaluating the illuminance of the building interior, which should be between 100 lx and 2000 lx to avoid discomfort due to glare [128].

All aspects related to thermal comfort are not of secondary importance, namely, the thermal energy transmission coefficient, U-value (W/(m²K)), SHGC, and solar factor (SF). The U-value quantifies the window heat-insulating properties, whereas the SHGC and SF are dimensionless coefficients, respectively, measuring the solar radiation and the energy transmitted to the window. Both the SHGC and SF range from 0 to 1, with 0 indicating a glaze with no transmittance of radiation or energy while 1 represents the full transmittance of the related quantities [129]. Despite having minor differences, the SHGC and SF are used as alternative parameters to evaluate solar penetration through transparent or semi-transparent surfaces.

Typical single-glazed windows have U-values of 3–5 W/(m²K), which decrease to 2–2.99 W/(m²K) if double-glazed air-filled windows are considered. This value can be further reduced by manufacturing multiple-layered windows or by increasing the glass thickness [130]. The adoption of multiple-glazed windows also allows the substitution of the air inside the cavity with inert gas, aerogel [131], or vacuum [132]. Due to the manufacturing process involving innovative techniques or materials, and the common utilization of a multilayer structure, which facilitates the PV component integration, smart windows typically have lower U-values when compared to traditional windows. Since a portion of the incoming radiation is absorbed by the integrated solar technology for energy production, the SHGC is usually lower, which is beneficial for hot climates as it decreases the building cooling load, but it negatively impacts the buildings in cold climatic areas. Smart windows can be subdivided into first, second, and third generation, according to the integrated PV technology.

As c-Si absorbs 90.5% of the incident radiation [133], it behaves as an opaque surface and the transparency of first-generation PV fenestration is obtained by spacing two adjacent cells. Semitransparent PV glazing is therefore realized by encapsulating crystalline PV cells between two glasses using standard encapsulant materials like EVA, PVB, or TPO. The transparency of these smart windows may be varied by modifying the glass transmittance or the cell interspacing. Thin-film PV technologies are particularly indicated for the integration in semi-transparent surfaces as they combine the reliability of first-generation PV cells with the possibility of creating devices that have uniform and continuous transparency. DSSCs, PSCs, and OPVs are becoming increasingly popular semi-transparent PV applications compared to those of previous generations due to their tunable transparency and their simpler fabrication processes [134]. Moreover, DSSCs and OPVs are characterized, respectively, by eco-friendly and low-toxicity properties [135].