Silicon solar cells are the most widely deployed modules owing to their low-cost manufacture, large market, and suitable efficiencies for residential and commercial use. Methods to increase their solar energy collection must be easily integrated into module fabrication. We perform a theoretical and experimental study on the light collection properties of an encapsulant that incorporates a periodic array of air prisms, which overlay the metallic front contacts of silicon solar cells. We show that the light collection efficiency induced by the encapsulant depends on both the shape of the prisms and angle of incidence of incoming light. We elucidate the changes in collection efficiency in terms of the ray paths and reflection mechanisms in the encapsulant. We fabricated the encapsulant from a commercial silicone and studied the change in the external quantum efficiency (EQE) on an encapsulated, standard silicon solar cell. We observe efficiency enhancements, as compared to a uniform encapsulant, over the visible to near infrared region for a range of incident angles. This work demonstrates exactly how a periodic air prism architecture increases light collection, and how it may be designed to maximize light collection over the widest range of incident angles.
© 2016 Optical Society of America
In solar cell technologies, efforts to increase power output while sustaining current low-cost production necessitate straightforward, scalable approaches to address losses. To this end, textured surfaces in the solar cell structure is one archetype that has led to increased light trapping that yields higher energy conversion [1, 2]. Specifically, v-groove structures reflect light at oblique angles, enabling light to remain confined in the solar cell, increasing the probability of conversion into electricity. V-groove structures have been predominately considered for incorporation into the surface of photovoltaic materials to mitigate reflective losses from its surface [1, 3, 4], or as rear reflectors [5–8] or concentrators . In standard, commercially available silicon solar cells, a specific cause of loss is due to shading and reflection by the metallic front contacts, or grid “fingers” as they are also known, which may account for losses up to 10% [10–12], and more so for incoming light at non-normal incident angles.
Several approaches have been pursued to reduce the optical losses associated with shading. One strategy is to redirect light towards the solar cell using deflecting elements that are incorporated into the protective encapsulant. Jaus et. al. realized this through roughening a glass surface above the metal contacts  in order to create light diffusers. A 3.3% increase in current was achieved in test modules . Jaus et. al. also covered contacts with diffuse reflective coatings, which yielded an average increase in the short circuit current of 2.5% . Cheng. et. al. introduced polystyrene beads into a thermally curable acrylic resin to achieve a 5.2% increase in the external quantum efficiency (EQE) relative to a conventional module . Mingareev et. al. employed ultrafast irradiation to create phase gratings within glass . This yielded a 2.8% increase in the photon flux reaching the solar cell when 8 stacks of gratings were employed. Recently, Kuna et al. applied ultrafast irradiation to create scattering and diffractive elements in crosslinked ethylene vinyl acetate (EVA) . This gave up to a 17% reduction in shadow losses, which corresponded to a calculated 0.7-1.2% increase in the achievable photocurrent.
Another approach to mitigate these losses is to incorporate a periodic array of air prisms into the encapsulant material , such that they are aligned with and overlay the contacts, as shown in Fig. 1. The prism interface and its slanted sides result in total internal reflection (TIR) that redirects light, which would otherwise scatter of the metal contacts, towards the solar cell. This is conceptually a structure that combines a v-groove surface with an additional layer of encapsulant material so as to trap air within the groove. Complete reduction of losses has been theoretically posited for such a structure by Korech et. al. . Current encapsulants, which are polymeric in composition, are uniform (bearing no optical structure) and usually applied (resin poured, then cured) as a processing step during the fabrication of photovoltaic modules, prior to installation. Incorporating an encapsulant containing air prisms offers a straightforward approach to improve performance that can be easily integrated into module fabrication. While the TIR mechanism employed by this structure is straightforward, this effect on the subsequent internal reflections of light rays in the encapsulant has not been considered, neither theoretically nor experimentally. This is necessary to more closely understand the conditions under which collection efficiency is further enhanced, or possibly reduced. This knowledge is critical for their proper design.
Herein, we report both a theoretical and experimental investigation on the light collection and conversion efficiency of a standard silicon solar cell encapsulated by a coating consisting of a periodic array of air prisms. We perform ray tracing simulations for light impinging on the encapsulant, and consider two different geometric shapes for the air prisms. Collection efficiency calculations show enhancements, compared to a uniform film, for a range of incidence angles. The calculations also indicate a strong dependence on the air prism shape and angle of incidence (θ), which affect the light ray’s path inside the encapsulant, resulting in enhanced or reduced collection. Similar ray-tracing simulations have been used by others to assess the effect of different encapsulants , measure overall module performance and losses , and test new anti-reflection coatings .
We corroborate our findings by fabricating and employing a silicone encapsulant consisting of a 1-D array of air prisms, and experimentally measure the EQE over a range of incident angles. The encapsulant enables higher efficiencies over a uniform encapsulant for a wide angular range. Our simulation and experimental measurements show ~5% enhancement in both light collection efficiency and EQE. To put this enhancement into perspective, in the United States alone the current installed capacity is close to 30 GW . Hence, even this modest increase in light capture can provide significant benefits to total energy collection. Developing these encapsulants from commercial silicones provides the advantages of utilizing facile processing techniques for curing resins under ambient conditions, and scalably patterning materials over large areas.
2. Experimental methodology
2.1 Optical simulations
Ray tracing simulations were implemented by using the Advanced System Analysis Program (ASAP). As the air prism structure is a 1-D array, the incoming rays constituted a 1-D parallel light source, which included 10,000 rays impinging on the width of the simulated solar cell, which was set to 40.375 mm. Thus, the ray spacing is 4.0375 μm/ray, which is far less than the critical dimension of the air prism (200 μm base), such that the features of the prism can be accurately resolved by the rays. We considered rays impinging on the solar cell surface at θ values from −60° to + 60°.
We simulated a planar (i.e., non-textured) multi-crystalline silicon solar cell comprising of metal contacts spaced 2.375 mm apart, thereby including 17 contacts in the simulation cell. The contacts had a semi-ovular shape, with a 150 μm width and 100 μm height. These dimensions are based on the dimensions of the solar cell used in experimental work. The refractive index of the encapsulant was set to 1.4125 to match the measured value of the polydimethyl siloxane (PDMS) elastomer used to fabricate encapsulants in this work. The refractive index measurement was made with an Abbe Refractometer (Atago, NAR-1T SOLID, Accuracy of ± 0.0002). The refractive index of silicon was set to 4.15. All light transmitted into the silicon material was considered to be collected (i.e., no inherent absorbance loss). The metallic contacts were set to perfectly reflect light. The wavelength used in simulations was 532 nm. The collection efficiency was calculated as the percent of light transmitted through the silicon surface over the entire duration of the simulation. Ray tracing was run until all rays had left the simulation cell.
2.2 Film fabrication and encapsulation
A mold consisting of a 1-D periodic array of triangular prisms was micro-machined into a brass plate of 1 cm × 1 cm dimensions (Potomac, Maryland). The array had a periodicity of 2.375 mm, and the prisms had a 200 μm base and 241 μm height. A formulation of PDMS elastomer (Sylgard® 184) base and curing agent was thoroughly mixed and allowed to degas at room temperature under ambient conditions. The formulation was then poured over the brass mold contained in a plastic petri dish. The volume poured was such that it submerged the mold below ~1 mm of resin. The resin was allowed to cure at room temperature under ambient conditions for 24 hours. The cured film over the mold was then cut and carefully peeled off. This created a 1-D periodic array of v-grooves in the surface in the PDMS, which would form the slanted faces of the air prisms. We experimented with different ratios of base and curing agent to minimize the elasticity of the sample, in order for the v-groove structure to retain its shape, yet not to harden to an extent that the film was damaged during the peel off. Highly elastic samples skew the pattern, and highly hardened samples showed degradation of the pattern after peel off due to remnant resin on the brass mold. The optimal formulation was found to be 10:1.175 weight ratio of base to curing agent.
A planar multi-crystalline Silicon screen-printed solar cell (Sundance Solar) was used in this study. The solar cell dimensions were 2 cm × 4 cm × 0.5 mm. The rated efficiency was 17%, and short circuit current (JSC) 55 mA. The solar cell as-received had no encapsulant. The solar cell was first coated with a thin “priming” layer (~0.12 mm) of PDMS to submerge the contacts, and to provide a flat surface onto which the structured encapsulant could be placed. The structured film was visually aligned and placed over the contacts, with adherence between it and the priming layer ensured by gently pressing the encapsulant against the cell. The structured encapsulant and priming layer were pressed together soon after they were cured, when both surfaces were sufficiently “sticky” so as to adhere to one another.
2.3 Solar cell measurements
External quantum efficiency measurements were carried out using a commercial measurement system (IQE 200B, Newport). The measurements were carried out according to the ASTM standard (E1021-12). The setup consisted of an arc lamp (broadband light source) with a chopper to produce a pulse source (80 Hz), an automated monochromator system, and electronics for measuring current output. The beam cross-section was circular with a sufficiently large diameter (~1 cm) to irradiate 5 contacts. To probe a single contact, the beam was reduced to a diameter of ~1 mm using an aspherical lens, and the beam was rastered over a contact by moving the solar cell using a mechanical linear motion stage. All measurements were conducted in the same region of the solar cell, in order to eliminate positional differences, attributed to the multi-crystalline nature of the cell, from affecting the comparison of different encapsulant performances.
3. Results and discussion
Two air prism geometries were considered in the ray tracing simulations, as shown in Fig. 2. In the first case, air prisms with isosceles cross-section were used, as shown in Fig. 2(a), which can direct light incident on the surface of the solar cell from either angular direction. In the second case, a right-angled triangles were used, as shown in Fig. 2(b). Isosceles shaped air prisms may be aligned such that they are parallel to either the yearly or daily diurnal trajectory of the sun, as shown in Figs. 2(c) and 2(d), respectively. Furthermore, in the former case, the surface normal would be aligned such that the sun at the midpoint of its yearly path is directly over the cell. The right-angled cross-section accounts for solar cells laid flat, on the Earth’s surface for example, with the angled side facing away from the equator (i.e., north for northern latitudes, south for southern latitudes), as shown in Fig. 2(e).
Figures 2(a) and 2(b) also show the operation of the architecture, indicated by a single ray from a simulation and its redirection via reflection off a prism towards the solar cell. We refer to the two different encapsulant structures as symmetric air prism structure (SAPS) and asymmetric air prism structure (AAPS) for isosceles and right-angled shapes, respectively. Both structures are characterized by their apex angle (A), located across from the base of the triangle. The base of both triangular cross-sections is fixed to 200 μm, which is slightly larger than the typical metal contact width (~150 μm). The apex angle is adjusted in the simulation by adjusting the height of the prism. The simulations account for the thin priming layer which was experimentally measured to be ~0.12 mm; hence, the total thickness of the encapsulant simulated was 1.12 mm. The base of the prism is formed by the surface of the thin priming layer, and hence the air prisms are elevated above the front contact surfaces. This is different from the encapsulants previous considered, in which the air prisms are laid directly on top of the front contacts [18, 19] so that the base of the air prism is the solar cell surface.
The ray tracing simulations were used to calculate the collection efficiency of the SAPS and AAPS over a range of apex angles and angles of incidence. Figure 3 shows contour plots of the collection efficiency both in absolution percent value, and difference in percentage relative to calculations for a uniform (unstructured) encapsulant. The contours display specific regions, labeled in Figs. 3(b) and 3(d), showing collection enhancements over range of θ for different air prism shapes, as well as regions with losses. These positive and negative changes in collection efficiency strongly indicate different light propagation paths as the determinative factor for efficiency, which changes based on the air prism shape and θ. The collection efficiency for SAPS is symmetric, as expected based on the isosceles cross-section.
The contours show, furthermore, that enhancements can be attained for a wide, tunable range of θ by changing the apex angle. For example, an ideal design that shows constant gains is a SAPS with an apex angle of 45°, which shows an approximate 6% enhancement over θ from −30 to 30°. Yet, for smaller apex angles, positive gains, although varying (minimum of ~4%, maximum of ~10%), can be obtained up to a range of −60 to 60°, indicating the potential for significant wide-angle light collection. For the AAPS, an apex angle of 35° yields a relatively constant ~6% gain for θ of approximately 0 to + 40°. The ray-tracing results for the SAPS, by simulating subsequent reflections, provide a more accurate evaluation of the reduction in shading losses than what has been previously determined by considering only TIR at the air/encapsulant interface [18, 19].
Figure 4 shows a plot of the average efficiency over the total angular range (−60°, + 60°) as a function of apex angle for both SAPS and AAPS encapsulants. The uniform encapsulant is also shown for comparison. The average efficiency was calculated by integrating the collection efficiency over the entire angular range simulated, then dividing by 120°, in order to provide an average efficiency over the range. The plots show that the collection efficiency for both SAPS and AAPS are higher as compared to the uniform encapsulant, with the former showing the highest efficiency of the three. The plots confirm that, despite regions in which the collection efficiencies are lower than for a uniform encapsulant, the overall efficiency gain is positive for the SAPS and AAPS.
Figure 5 shows exemplary ray-tracing diagrams that indicate the mechanisms for either gain or loss for the regions labelled in Figs. 3(b) and 3(d). These gains or losses are associated to whether or not the TIR condition is met for light at the prism interface, as well as, importantly, subsequent reflections and refraction of the light rays in the encapsulant. For Region I of the SAPS, as shown in Fig. 5(a), the light rays undergo TIR both at the prism interface and also at the air/encapsulant interface (i.e., encapsulant top surface), thereby becoming trapped in the film through internal reflection. This allows more chances for collection with every subsequent impingement of light onto the solar cell surface, advantageously increasing light collection. For region II, as shown in Fig. 5(b), TIR on only one side of the air prism produces rays that remain within in the encapsulant; rays reflected on the other side incur subsequent losses at the air/encapsulant interface associated with transmission through it (due to refraction), thereby reducing the total light collected.
For region III, as shown Fig. 5(c), light reflected off of both sides of the prism results in non-TIR conditions at the air/encapsulant interface, which results in further reduction in the overall collection. Nevertheless, the structure still provides a positive gain in collection over a uniform encapsulant by virtue of there being no reflection off the contacts. For region IV, as shown in Fig. 5(d), particularly at very large θ, light rays no longer undergo TIR at the air prism interface, which leads to scattering off the contact. When this scattered light consequently leaves the cell, then lower collection efficiencies occur. It is in such a case that a structured encapsulant can show an efficiency less than that for a uniform encapsulant, because this lost light would have remained trapped in the latter. The boundary between region I and IV represents this change from TIR to non-TIR conditions, and thus indicates the limit in the coverable range of incident angles by the structure over which consistent gains can be achieved.
For the AAPS, the 90° side of the prism does not meet the condition of TIR, which results in losses. Efficiency enhancement is observed when the TIR condition is met on the slant face of the air prism. This is shown in Fig. 5(e), which corresponds to Region I in the AAPS collection efficiency map shown in Fig. 3(d). For other incident angles neither side provides TIR; however, redirecting the light away from the contacts still provides positive gains in collection efficiency. This is shown in Fig. 5(f), which corresponds to Region II. Due to the asymmetry of the air prism, negative θ will refract through the slanted surface, and can impinge on the contact, as shown in Fig. 5(g), which will lead to losses. This corresponds to the low collection efficiency region III in Fig. 3(d). Of course, there are incident angles that lead to instances of light rays impinging onto another neighboring air prism, which can possibly lead to losses. This is one explanation for the smaller variations in the collection efficiency maps within each region. This theoretical insight shows the importance of the subsequent reflections in the encapsulant to understanding the mechanisms by which light collection is enhanced.
Figures 3(d) and Figs. 5(e)-5(g) also justify the rationale for facing the slant side of the asymmetric air prisms away from the equator for yearly tracking: For many geographic locations, especially those above the 23.5°N and below the 23.5°S latitudes, the sun is predominantly in the southerly or northerly region of the sky, respectively. Consequently, light will be predominately incident from one side of normal incidence. The collection enhancements for positive θ in Fig. 3(d) correspond to light entering the solar cell from the 90° side of the prism, as shown in Figs. 5(e) and 5(f). Hence, a wider range of incidence angles will impinge on the slanted side of the prism at or above the critical angle for TIR, when it is faced away from the sun. Whereas, if the slanted side faces the sun, a case of which is shown in Fig. 5(g), most of the angular range would be below the critical angle, and no TIR will occur.
We experimentally confirmed collection enhancement by fabricating an encapsulant with a periodic array of air prisms (SAPS) with A = 45°. To create the structure, as shown in Fig. 6, a room-temperature curable PDMS resin was used to create an inverted impression of a mold. Optical microscopy shows well-defined, periodically spaced v-grooves embossed in the cured PDMS. The film had a total area of 1 cm2 and consisted of 5 grooves, in order to cover 5 contacts. Figure 7(a) shows the EQE measured as a function of wavelength of normal incident light for three cases: 1) a solar cell without any encapsulant, 2) a solar cell uniformly coated with PDMS, and 3) a solar cell covered with the fabricated, structured encapsulant. The structured encapsulant shows the highest EQE values over the VIS-NIR range (400-1000 nm). Using ambient light, visual comparison between solar cell contacts covered by the uniform encapsulant and structured encapsulant shows reduced brightness for the latter, indicating that less light is impinging and scattering off the contacts. Hence, the contacts have been “cloaked” to some degree from the incoming light.
Improvements in the EQE by using the air prism structured encapsulant was probed with a small (~1 mm) monochromatic beam (532 nm) normally incident to the film surface. We passed the beam over the contacts to probe the local EQE for the cases of uniform and structured encapsulants. Figure 7(b) corroborates that the structured encapsulant directs light away from the contact towards the solar cell, thereby enabling its collection, as evidenced by the EQE being maintained at a higher value over this region as compared to the uniform encapsulant. One explanation for the lower EQE, compared to the uniform encapsulant, at positions away from the contact (i.e., ~600 μm and onward from the center of the contact on both sides) is the slight surface roughness of the fabricated v-groove structure, owing to the peel-off process. As well, small air pockets may become trapped at the interface between the v-groove structure and the thin primer layer. Both of these artifacts may cause scattering effects that lower efficiency.
Over the visible range, the EQE enhancement for a structured encapsulant relative to a uniform encapsulant is ~5%, which matches well with the calculated increase of ~6% at A = 45° (at 532 nm). Using a spectrum of 1.5 AM, the calculated short circuit currents (JSC) for uncoated, uniform, and structured encapsulants were 354.7, 365.2, 386.1 A/m2, respectively. Hence, the structured encapsulant yields an approximate 5.7% enhancement. These evidences of efficiency enhancement corroborate the results of our ray-tracing simulations, as well as support and further elucidate the theoretical predictions by others [18, 19].
Figure 8 shows the EQE for different angles of incidence for solar cells employing a SAPS and uniform encapsulant. The EQE is sustained at higher values as compared to the uniform encapsulant over θ from 0 to + 40°. This demonstrates the capability for wide-angle collection, maintained power efficiency, and thus the potential to sustain energy output over the duration of the suns trajectory. Figure 8 also includes the collection efficiency values for the simulated SAPS (A = 45°). The drop in EQE below that for a uniform encapsulant for θ > 40° correlates to the drop in the collection efficiency enhancement below 0%. This corresponds to the boundary between regions I and IV, as shown in Fig. 3(b). Overall, the EQE and collection efficiency strongly correlate with one another (correlation coefficient = 0.921), indicating that trends in the EQE with angle of incidence are associated to the effects of the air prism structure. We expect the EQE to be a function of the collection efficiency as well as other factors, such as the internal quantum efficiency (IQE) of the solar cell. Hence, reflection effects both at the prism interface as well as thereafter at other interfaces is an important aspect for engineering air prism encapsulants. This must be considered in their design which depends on, for example, their conditions of operation, such as geographic position, or placement on a building or structure.
In comparison to other methods to reduce contact losses, our 5.7% enhancement in the JSC is about twice the enhancement achieved by Jaus et. al. [13, 14]. Our steady 5% increase in the EQE over the wavelength spectrum is comparable to the enhancements achieved by Cheng et. al. . We can infer from our collection efficiency results that the photon flux to the solar cell shows comparable enhancement to those achieved by Mingareev et. al., (~2%) , yet higher fluxes are possible in our case for different apex angles, particularly smaller ones. Our JSC enhancement is approximately 4.75 times greater than attained by Kuna et. al. .
Increasing the power output and efficiency of commercially available solar cell modules through an encapsulant, leaving the native solar cell material unchanged, is the most practical solution that is closest to commercial integration. We have conducted a theoretical investigation of light propagating in air prism structured encapsulants on a planar-surfaced silicon solar cell. We demonstrate increases and losses in light collection that depend strongly on the internal reflections in the encapsulant, changes with the air prism geometry, and incident angle of light. Collection efficiency maps may be used for designing the air prism geometry for optimizing/maximizing collection efficiency and wide-angle collection.
An experimental encapsulant made from a commercial silicone demonstrates a straightforward way of realizing the integration of the encapsulant onto standard silicon solar cells, with experimental EQE measurements confirming enhancement and wide-angle collection, as well as corroborating our theoretical findings. The approach of creating air prisms in elastomers is promising; however, further research on production techniques is required for their application. The simulations provide insight that can be leveraged to create higher complexity groove designs to enhance light collection and further mitigate losses. In order to further understand enhancements in light collection, future efforts will investigate the impact and influence of the encapsulant on a textured solar cell surface, including the anti-reflection coating and a textured front glass coating, under realistic illumination conditions.
References and links
1. T. I. Chappell, “The v-groove multijunction solar cell,” IEEE Trans. Electron Dev. 26(7), 1091–1097 (1979). [CrossRef]
2. P. Campbell and M. A. Green, “Light trapping properties of pyramidally textured surfaces,” J. Appl. Phys. 62(1), 243–249 (1987). [CrossRef]
3. S. Bailey, N. Fatemi, G. A. Landis, D. Brinker, and M. Faur, “Application of v-groove technology to InP solar cells,” in 2nd Indium Phosphide Relat. Mater.,Int. Conf. (1990). [CrossRef]
4. T. Yagi, Y. Uraoka, and T. Fuyuki, “Ray-trace simulation of light trapping in silicon solar cell with texture structures,” Sol. Energy Mater. Sol. Cells 90(16), 2647–2656 (2006). [CrossRef]
5. K. J. Weber, V. Everett, P. N. K. Deenapanray, E. Franklin, and A. W. Blakers, “Modeling of static concentrator modules incorporating lambertian or v-groove rear reflectors,” Sol. Energy Mater. Sol. Cells 90(12), 1741–1749 (2006). [CrossRef]
6. T. Uematsu, Y. Yazawa, K. Tsutsui, Y. Miyamura, H. Ohtsuka, T. Warabisako, and T. Joge, “Design and characterization of flat-plate static-concentrator photovoltaic modules,” Sol. Energy Mater. Sol. Cells 67(1–4), 441–448 (2001). [CrossRef]
7. T. Uematsu, Y. Yazawa, T. Joge, and S. Kokunai, “Fabrication and characterization of a flat-plate static-concentrator photovoltaic module,” Sol. Energy Mater. Sol. Cells 67(1–4), 425–434 (2001). [CrossRef]
8. T. Uematsu, Y. Yazawa, Y. Miyamura, S. Muramatsu, H. Ohtsuka, K. Tsutsui, and T. Warabisako, “Static concentrator photovoltaic module with prism array,” Sol. Energy Mater. Sol. Cells 67(1–4), 415–423 (2001). [CrossRef]
10. A. W. Blakers, “Shading losses of solar-cell metal grids,” J. Appl. Phys. 71(10), 5237–5241 (1992). [CrossRef]
11. M. F. Stuckings and A. W. Blakers, “A study of shading and resistive loss from the fingers of encapsulated solar cells,” Sol. Energy Mater. Sol. Cells 59(3), 233–242 (1999). [CrossRef]
12. T. Li, C. L. Zhou, Y. Song, H. F. Yang, Z. H. Gao, Y. Duan, Y. Z. Li, Z. G. Liu, and W. J. Wang, “Theoretical analysis and experimental study of optical loss of metal contacts of crystalline silicon solar cells,” Wuli Xuebao 60(9), 098801 (2011).
13. J. Jaus, H. Pantsar, J. Eckert, M. Duell, H. Herfurth, and D. Doble, “Light management for reduction of bus bar and gridline shadowing in photovoltaic modules,” in 35th IEEE Photovoltaic Spec. Conf. (IEEE, 2010), 979–983. [CrossRef]
14. R. Ebner, M. Schwark, B. Kubicek, G. Újvári, W. Mühleisen, C. Hirschl, L. Neumaier, M. Pedevilla, J. Scheurer, A. Plösch, A. Kogler, W. Krumlacher, and H. Muckenhuber, “Increased power output of crystalline silicon pv modules by alternative interconnection applications,” in 28th Eur. Photovoltaic Sol. Energy Conf., Proc. Int. Conf. (2013).
15. T.-D. Cheng, Y.-P. Chen, and P.-C. Chen, “Efficiency improvement of photovoltaic module via grid diffusers in eva encapsulation layer,” 28th Eur. Photovoltaic Sol. Energy Conf., Proc. Int. Conf. (2012).
16. I. Mingareev, R. Berlich, T. J. Eichelkraut, H. Herfurth, S. Heinemann, and M. C. Richardson, “Diffractive optical elements utilized for efficiency enhancement of photovoltaic modules,” Opt. Express 19(12), 11397–11404 (2011). [CrossRef] [PubMed]
17. L. Kuna, G. C. Eder, C. Leiner, and G. Peharz, “Reducing shadowing losses with femtosecond-laser-written deflective optical elements in the bulk of EVA encapsulation,” Prog. Photovolt. Res. Appl. 23(9), 1120–1130 (2015). [CrossRef]
18. M. J. Nowlan, “Photovoltaic cell and process,” United States Patent US5076857 A (1991).
19. O. Korech, J. M. Gordon, E. A. Katz, D. Feuermann, and N. Eisenberg, “Dielectric microconcentrators for efficiency enhancement in concentrator solar cells,” Opt. Lett. 32(19), 2789–2791 (2007). [CrossRef] [PubMed]
20. K. R. McIntosh, J. N. Cotsell, J. S. Cumpston, A. W. Norris, N. E. Powell, and B. M. Ketola, “An optical comparison of silicone and eva encapsulants for conventional silicon pv modules: a ray-tracing study,” in 34th IEEE Photovoltaic Spec. Conf. (2009), 1649–1654. [CrossRef]
21. M. Winter, M. R. Vogt, H. Holst, and P. P. Altermatt, “Combining structures on different length scales in ray tracing: analysis of optical losses in solar cell modules,” Opt. Quantum Electron. 47(6), 1373–1379 (2015). [CrossRef]
22. S. Kann, M. J. Shiao, C. Honeyman, A. Perea, J. Jones, C. Smith, B. Gallagher, S. Moskowitz, J. Baca, S. Rumery, A. Holm, and K. O’Brien, Solar Market Insight Report 2016 Q3,” SEIA (2016).