5:00 PM - EN01.08.02
Colorful Transparent and Flexible Silicon Based Transparent Solar Cells for BIPV Applications
Baurzhan Salimzhanov2,Sung Bum Kang1,Myeong Hoon Jeong2,Ju-Young Kim2,Kyoung Jin Choi2
University of Illinois at Urbana-Champaign1,Ulsan National Institute of Science and Technology2
BIPVs (building-integrated photovoltaics) are regarded as next-generation photovoltaic technologies that can generate electricity in urban areas with limited available land while also serving as aesthetic architectural elements. In terms of aesthetics, new functions are additionally required other than power-conversion efficiency, long-term stability, and price competitiveness of conventional solar cells, such as transparency, color tunability, and flexibility. There are two main approaches to achieving a balance between PCE and AVT (average visible transmittance).1, 2 The first approach is usage wavelength-selective active materials that absorb ultraviolet (UV) and near-infrared (NIR) radiation while transmitting light in the visible region. The second approach is control thickness of active material to decrease light absorption. Recently, Lee et. al.3 demonstrated TSC device based on opaque crystalline-Si. The devices demonstrated 6.7% PCE with 50% transparency, but the problem with rigidity and coloring remains unsolved. Kang et. al.4 demonstrated TSC device using Si microwires array. The device showed 50% transparency and flexibility, but efficiency was only 1.8%.
Here, we demonstrate a flexible, color-tunable, and large-area transparent solar cell (TSC) based on a 100 μm thick crystalline-Si wafer with a periodic square hole array embedded with PDMS. The periodic hole array was formed using a dry etching process (Bosch process). The ratio of etching gas (SF6) and deposition gas (C4F8) was optimized to reduce Si wall damage during etching. Due to that, the TSC devices exhibit PCE values of 7.38% and 5.52% at the average visible transparencies (AVT) of 45% and 60%, respectively. It should be noted that, following the formation of the periodic hole array, the flexibility of the device was dramatically enhanced, such that the minimum bending radius decreased from 100 mm to 6 mm; it further decreased to 3 mm after PDMS embedding. To explain the extreme enhancement of flexibility, finite difference time domain (FDTD) simulations were applied. The results of the FDTD simulations showed that the periodic hole array structure uniformly distributes the stress across the entire area. The highest stress appeared near hole edges, while the Si grid experienced relatively low stress. Such unequal distribution of stress helps to stop random cracks and their propagation in TSC devices under bending stress. PDMS encapsulation helped to decrease bending radius to 3 mm due to stress relief at the edges of holes and create a neutral plane. TSC device and PDMS maintained 95% of initial PCE after 1000 repeated bending with a radius of 8 mm. In addition, TSCs of various colors were obtained by dispersing the dye inside the PDMS. Blue, green, yellow semitransparent solar cells were fabricated using organic dyes. Despite parasitic absorption of the dyes, the efficiency of the colored devices was reduced only by 5% of colorless device. The PDMS-embedded TSCs demonstrate extremely high flexibility and long-term stability without significant degradation even after cycles bending deformations up to 1000 cycles and 1500 h of the standard damp heat test. We expect that the silicon-based TSCs with a square hole-array and the features of transparency, color tunability, flexibility, high efficiency, and long-term stability showed in the study will speed up the convenient application of BIPV technology.
1. Du, X. Y.; Li, X. R.; Lin, H.; Zhou, L.; Zheng, C. J.; Tao, S. L., A 2019, 7 (13), 7437-7450.
2. Gholami, H.; Rostvik, H. N.; Muller-Eie, D., Energy and Buildings 2019, 203.
3. Lee, K.; Kim, N.; Kim, K.; Um, H. D.; Jin, W.; Choi, D.; Park, J.; Park, K. J.; Lee, S.; Seo, K.. Joule 2020, 4 (1), 235-246.
4. Kang, S. B.; Kim, J. H.; Jeong, M. H.; Sanger, A.; Kim, C. U.; Kim, C. M.; Choi, K. J., Light-Science & Applications 2019, 8.