Design, development, and characterization of low distortion Design, development, and characterization of low distortion advanced semitransparent photovoltaic glass for buildings advanced semitransparent photovoltaic glass for buildings applications applications

: Aesthetic appearance of building-integrated photovoltaic (BIPV) products, such as semi-transparent PV (STPV) glass, is crucial for their widespread adoption and contribution to the net-zero energy building (NZEB) goal. However, the visual distortion signiﬁcantly limits the aesthetics of STPV glass. In this study, we investigate the distortion effect of transparent periodic-micropattern-based thin-ﬁlm PV (PMPV) panels available in the market. To minimize the visual distortion of such PMPV glass panel types, we design and develop an aperiodic micropattern-based PV (APMP) glass that signiﬁcantly reduces visual distortion. The developed APMP glass demonstrates a haze ratio of 3.7% compared to the 10.7% of PMPV glass. Furthermore, the developed AMPV glass shows an average visible transmittance (AVT) of 58.3% which is around 1.3 times higher than that of AMPV glass (43.8%). Finally, the measured CIELAB values (L* = 43.2, a* = − 1.55, b* = − 2.86.) indicate that our developed AMPV glass possesses excellent color neutrality, which makes them suitable for commercial applications. Based on the characterization results, this study will have a signiﬁcant impact on the areas of smart window glasses that can play a vital role in developing a sustainable environment and enhancing the aesthetical appearance of net-zero energy buildings (NZEB).


Introduction
Over the last couple of decades, worldwide energy consumption has increased significantly, mainly due to increases in world population and the industrial revolution.The world population is expected to be nearly 11 billion by the end of 2050.For this large population and ongoing industrialization, the energy demand will soar up to maintain a normal and sustainable lifestyle.It is predicted that the global energy demand will exceed 0.74 billion Tera joule (TJ) by 2040, and this requires the burning of 22.7 billion tons of fossil fuel, mainly anthracite coal [1,2].Modern buildings and infrastructures, both residential and industrial, are nowadays consuming more energy than what was consumed a decade ago.Further industrialization offers better social and financial benefits to the world population, however, it results in the migration of more and more people from rural to urban areas where a greater amount of global energy is already consumed for living by the buildings and infrastructures [3][4][5].In addition to this, around two-thirds of the world's population is predicted to live in metropolitan areas by 2050, where there will be a huge challenge for the governments around the world to meet the large energy demand of densely populated cities [4].On the other hand, urban planners, environmentalists, and architects are more concerned about the issues of environmental hazards and global warming caused by non-renewable fossil fuel-based energy sources [2].These non-renewable energy sources (Gas, Coal and Oil) have also been creating devastating challenges, namely the emission of greenhouse gases, pollutants (trace metals, particles, NO x , SO 2 ) into the atmosphere, pollution of water from coal, and uncontrollable ash wastes [5].These major challenges can be effectively overcome by employing sustainable renewable energy technologies [6].Solar photovoltaics (PV) is one of the most prominent renewable energy technologies, compared to other available technologies [7].
Recently, the utilization of solar panels in built environments has gained more popularity due to their capability of transforming a house into a power generator [8].This is performed by integrating PV modules in the building either during or after the construction period.A building-integrated photovoltaics (BIPV) system provides not only electrical and thermal energy but also technical and economical benefits to buildings [9,10].BIPV systems have recently gained more popularity after the net-zero energy buildings (NZEB) concept was announced by the Directive of the European Parliament (2010/31/EU) [11].For the NZEB concept, buildings must require extremely low energy that can be obtained from on-site renewable energy sources.The greatest potential of NZEB is the development of smart cities, where energy demand and supply come from renewable energy sources and storage systems implemented in the same city [3].Therefore, the NZEB goal to realize "a completely zero-carbon built environment" can be achieved using BIPV systems [3,[12][13][14].
Although BIPV systems can generate power cost-effectively when considering both the purpose of electricity generators and building materials [9,15], some modern societies still have a negative opinion regarding the implementation of BIPV technology, mainly because of the limitation of the visual and aesthetical appearance [16][17][18][19][20][21][22][23][24][25][26] that displays black or dark blue colors, depending on the photovoltaic technology used [19,27].BIPV products featuring visible transparency with acceptable aesthetical color (the apparent visual color captured by normal human eyes) are fascinating for the enhancement of public satisfaction and installation proportion [28][29][30][31][32]. Therefore, it is vital to investigate the problems associated with the development of aesthetically appealing BIPV products and to find innovative solutions for mass production that enables reaching the goal of NZEB, thus saving our environment by reducing the demands of fossil fuel.The two important reasons that limit the adoption of semitransparent BIPV glass products in architectural structures are (i) visual distortion and (ii) conversion efficiency.The most efficient semitransparent BIPV glass is so far made by Advanced Solar Power (ASP)-China [33], which is based on thin-film cadmium telluride (CdTe) PV materials.It has been achieved by partially removing the PV materials using laser micromachining technology to achieve transparency and sunlight-to-electricity conversion simultaneously.The semitransparent PV glass made by ASP is the best choice for BIPV application, however, this type of PV glass exhibits visual distortion which makes the glass blurry due to having narrow PV strips that act as a diffraction grating.To solve this issue, we propose using nonuniform spacing between the PV strips, i.e., aperiodic structure, where the visual distortion can be reduced up to a certain level that is commercially acceptable.No studies have so far been found where aperiodic-micropatterns-based semitransparent thin-film PV modules have been developed using laser micromachining.To the best of our knowledge, this is the first study aimed at designing, developing, and characterizing aperiodic-micropatterns-based semitransparent low distortive thin-film PV glass using laser micromachining.

Materials and Methods
The development of low distortion advanced semitransparent PV glass will begin with the design of micropattern-like periodic structures as shown in Figure 1a using Coreldraw, a professional vector-based designing software creating high-definition images [34].The micropattern shown in Figure 1 looks like a complete reddish color in 100% zooming, however, when the zooming percentage increases, it is clear that black gaps exist shown in Figure 1b,c, which are unseen to the naked eye.
It is well known that if the gap between two objects is less than around 50 micrometers, human eyes cannot distinguish the gap, and hence, it perceives the image as one object.This concept has been employed to develop semitransparent PV modules by removing micro-scale areas (red portions) from the thin film materials of the solar glass panel, thus transmitting a significant amount of light through the glass and converting the remaining portion to electricity, thus achieving transparency and electricity-generation simultaneously.In this article, a laser micromachining system (8020 Trotec Speedy 300 flexx), is used to remove selectively micro-scale areas (red areas) of thin-film PV materials, as illustrated in Figure 1.The fundamental process of laser engraving is a high-energy-density laser pulse irradiating the PV material surface is absorbed and thus increasing the temperature of the PV material, leading to their removal through vaporization [35,36].However, being transmissive to the laser wavelength, the glass surface on the thin-film PV panel does not get engraved [37].The laser-machined microstrips (black areas) are typically not seen by the naked eyes but absorb the light incident on them and convert it to electricity.However, periodic micropatterns act as a diffraction grating that typically results in light diffraction (a rainbow effect that causes visual distortion, e.g., haze).This undesired visual distortion can be overcome by using aperiodic micropatterns, as can be seen in Figure 2. Figure 2a illustrates an aperiodic micropatterns on the thin-film PV panel (shown in Figure 2b), where PV area, hence the conversion efficiency of the PV glass is maintained.The aperiodic patterns was developed by micromaching the PV area to generate PV microstrips of random widths, with the constraint that their overall area is the same as that of the periodic patterns, so that both generate the same electricity, but the aperiodic patterns result in lower grating effect, or distortion.This aperiodic micropatterning approach reduces the visual distortion to a practically adequate level.
A haze meter (Model: SKZ120A, Jinan, Shandong, China) is used to measure the amount of incident light diffracted (optical/visual distortion) when passing through the STPV glasses and these haze values are compared with that of clear glass.Moreover, the diffraction patterns or rainbow effect is visualized more clearly using a camera (Model: STC-P63CJ, SENTECH, Japan) based on the experimental setup shown in Figure 3. Finally, our developed semitransparent PV glasses are characterized using a UV-vis-NIR spectrophotometer (Model: Agilent Cary 5000) for the measurement of transparency, a color-difference meter (Konica Minolta 508D) to determine the color coordinates.These key characteristics' parameters determine the suitability of STPV glass in industrial applications.

Results and Discussion
Aesthetical appearance plays a critical role when developing a transparent PV panel.Transparent photovoltaic (TPV) is one of the dominant technologies which significantly enhance the visual appeal of the BIPV system.However, the highly efficient and transparent PV glass available in the market suffers from visual distortion due to the application of periodic micropatterned-based laser engraving [33].To solve the visual distortion problem, in this study, we have investigated the reason behind having high distortion on PMPV glass.It is found that a high haze value is mainly responsible for the visual distortion of any building-integrated glass products [38].In TPV application, the haze ratio is important because it reduces the clarity of the PV glass, resulting in blurred vision.To quantitatively evaluate the light diffusion through the TPV, the optical haze (H%) has been calculated using the ratio of the diffuse transmittance (Td) and total transmittance (Tt) [38][39][40].The haze value determines how much light can travel through a TPV (either dispersed or reflected) that creates blurring features on the backside of the PV glass.Therefore, a lower haze value makes the PV glass less distortive resulting in a clearer view [40].As a result, a TPV with a low haze value is required, as compared to the transmission haze ratio of glass (less than 1% [38]) for the current and future smart glass and window industries.To reduce the haziness, in this study, we have developed aperiodic micropatterned-based PV (AMPV) glass.The haze ratio for clear glass (CG), PMPV, and AMPV is measured by a Haze meter and presented in Table 1 and Figure 4.It can be seen that AMPV glass has a haze ratio of 10.7% which is significantly higher than AMPV glass (3.7%) and CG (0.9%).This is because the periodic micropatterns shown in Figure 5(a1) act as diffraction gratings.It is well known that diffraction gratings are basic optical elements with a specific groove pattern superimposed on them.These tiny, periodic, parallel structures diffract, or diffuse, incident light in such a way that individual wavelengths can be distinguished [41].Therefore, this diffracted light creates a rainbow effect similar to that of a prism resulting in a visually distorted (e.g., haze) image on the backside of PV glass.This undesired visual distortion is limited by applying aperiodic micropattern as illustrated in Figure 5(b1).The PMPV, and AMPV glasses and their corresponding diffraction patterns and visual images are shown in Figure 5. From Figure 5(a2,b2) which are captured by following the experimental setup shown in Figure 3, it is seen that how the light is diffracted and producing rainbow colors.PMPV glass produces more diffraction effects because periodic micropatterns within it behave as gratings that spread the incoming white light into different angles.
On the other hand, the higher diffraction pattern in Figure 5(a2) is drastically reduced as illustrated in Figure 5(b2) by the application of an aperiodic micropattern.Moreover, the photographs of natural scenery taken through PMPV and AMPV glasses are shown in Figure 5(a3,b3).It is noticeable that PMPV glass produces a blurry image that makes fatigue to the observer or human eyes.On the other hand, AMPV glass creates a visually clear image to the human eyes due to insignificant visual distortion compared to PMPV glass.Therefore, it is observed that AMPV glass reduced the visual distortion to a level (i.e., haziness is 3.7%) that is close to clear glass (i.e., haziness is 0.9%) and reported haze values of ref. [40].Additionally, the transmission haze ratio of our developed AMPV glass has compatibility with the values reported in refs [38,39].
The visual color appearances of semi-transparent solar cells as seen by human eyes are an important factor in assessing their applicability.The human perception of clarity and color differ from the experimental values obtained from the instruments due to the spectrally based reaction of the human eyes [42].However, optical transmittance in the visual spectrum determines the visual clarity of glass.Therefore, optical transmittance or average visual transmittance (AVT) plays a significant role to determine the suitability of window glass in buildings and vehicles.The accepted approach for the window industry to reporting AVT is to weigh the integration of the transmission spectrum against the human eye's photopic response [43,44], as follows: where T(λ) is the transmittance, P(λ) is the photopic response, λ is the wavelength and S(λ) is the solar photon flux (AM1.5G) for window applications or 1 for other applications.
However, when transparent photovoltaic (TPV) is applied in the buildings as window application, AVT is calculated according to the international organization of standardization (ISO) method (ISO 9050:2003), considering the response of the human eye to light in the visible light range (380-780 nm) as follows [39,45,46]: where τ(λ) represents the transmittance of PV, Dλ represents the spectral distribution of light incident on the PV, Vλ represents the sensitivity factor of the eye, and ∆λ represents the interval of the wavelength.The transmittance spectra of clear glass (CG), periodic micropatterned-based PV (PMPV), and aperiodic micropatterned-based PV (AMPV) are shown in Figure 6.The measurement was performed for the visible spectral wavelength range (380-780 nm).The AVT of these three samples is calculated based on the formula reported in ref. [47].It can be seen from Figure 6 that AMPV glass has more transparency (AVT = 58.7%)than the PMPV glass (AVT = 43.8%).It is reported in ref. [43,48] that any AVT value 60% and above looks clear, which is very close and compatible with the results of our developed AMPV glass.From the transparency point of view, AMPV glass is suitable for commercial uses because the AVT in residential windows can range from 15% for highly tinted glass up to 90% for common clear glass [48].In addition to the haze ratio and transmittance, color perception is also an important factor that needs to point out for the window industry [43].To accurately evaluate the color perception that is very similar to human vision color sensation, International Commission on Illumination (CIE) recommended several chromaticity diagrams namely CIE 1931 xy, CIE 1971 uv, and CIELAB, etc. for the window industry.However, CIELAB (CIE L*a*b*) color coordinate system is the most comprehensive color space that displays not only chromaticity but also brightness values [49,50].For this reason, we measured the color coordinates (shown in Figure 7) by color spectrophotometer in the CIELAB system.In Figure 7, the brightness (L*) represents the human sense of contrast, with a value of 0 representing complete black and 100 representing complete white, while green to red is represented by the value a*, and blue to yellow is represented by the value b*.It is reported in the Refs.[39,47,51]   It can be noticed that the brightness (L*) values for both PMPV and APMPV glasses are less than 50 since black microstrips are present in the PV glasses, although these are not visible to human eyes.However, the color coordinates (a*, b*) for both PV glasses are negative values and very close to (0,0).The color coordinates of our STPV glasses also have compatibility with the other chromaticity diagram of the semitransparent solar cells reported in refs.[42,52].It is seen that applying aperiodic micropatterns have no noticeable effect on the color coordinates except for little changes in the lightness value.These color values (a* and b*) indicated that our developed PV glasses possess excellent color neutrality which is suitable for industrial applications.This is because, in modern window applications, (yellow/red), high positive values of a* and b* are less frequent, while values near the origin (neutral/grey) and negative values of b* (blue) are more common if followed by high AVT [43].
Characterization is crucial after product development.However, in the case of semitransparent or transparent BIPV products, no standard characterization methods have so far been developed.Nevertheless, researchers agreed on the following three main perspectives that should be considered for the commercialization of BIPV products, namely: (1) high-power conversion efficiency at similar visible transmittance; (2) aesthetic factors, i.e., acceptable detraction, especially for construction and automobile applications; and (3) feasibility for real-world applications, including modularization and stability [39].Since in this study, we focus solely on the aesthetic aspect of TPV modules, characterization was focused on the aesthetical parameters, such as haze ratio, optical transparency, and color neutrality.From the characterization parameters, it is seen that our developed APMP-based PV glass samples possess very low haze, high AVT, and excellent color neutrality, and these make them suitable for BIPV applications.

Conclusions
In conclusion, we have demonstrated the design and development of a visually low distortive semitransparent photovoltaic (PV) glass by means of simple and cost-effective processes suitable for building integrated window applications.To reduce the visual distortion, we have applied aperiodic micropatterns that disrupt the grating properties and produced a very low distortion effect.To the best of our knowledge, this study can mitigate the visual distortion of periodic micropattern-based thin-film solar panels thus can benefit modern glass and window manufacturers.Furthermore, our developed AMPV glass possessed excellent optical transparency and color neutrality which is significantly important for the aesthetical appealing of BIPV products for various applications such as high-rise building construction, cars, greenhouses, and agrophotoviltaic.Additionally, it will play an important role in the widespread application of net-zero energy building (NZEB) that ensures environmental sustainability.

Figure 3 .
Figure 3. Block diagram of the experimental setup to see the diffraction patterns or rainbow effect.

Figure 4 .
Figure 4. Graphical representation of haze values for CG, PMPV glass and AMPV glass.