2, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, , Germany
3, University Freiburg, Freiburg, , Germany
4, KU-KIST Green School, Korea University, Seoul, , Korea (the Republic of)
Record efficiency of Perovskite/Silicon tandem solar cell 23.6% is approaching to highest silicon solar cell efficiency 26.6%. However, it still has lower efficiency than a single silicon device [1-2]. This is mainly because of the restriction at current matching between monolithically connected top and bottom solar cells . Since most Perovskite solar cells are made by solution process, every reported monolithic Perovskite/Silicon tandem was fabricated without front side texture of bottom silicon. Front side texture is essential to bust up current output and maximize tandem device performance. As a consequence, fabricating Perovskite above the textured silicon solar cells is essential.
We herein will talk about a novel method of fabricating Perovskite on the textured silicon surface and talk about the analysis of Perovskite above silicon texture. We used dry 2-step process which is deposit precursor materials and convert it into Perovskite. Conformal PbO precursor film firstly fabricated on the textured silicon surface with sputtering process. Then, exposed it to solid MAI source. As a result, we could construct conformal and uniform Perovskite film on the textured silicon surface. Films were investigated with XRD, μ-PL 3D mapping and SEM. XRD peak around 14.2 degree and PL emission around 770 nm were obtained. XRD peak shift about 0.1 degree was observed compared to conventional spin-coated Perovskite. This can be the evidence of stress induced by the structural characteristic of texture and volumetric expansion during conversion .
Due to arbitrary nature of random texture, it is not suitable for detail analysis for stress. For this reason, we fabricated patterned texture with photolithography technique and wet etching. Perovskite on patterned texture was analyzed by TRPL, XRD, PL 3D mapping, TEM and SEM. Carrier lifetime was measured with TRPL with high spatial resolution to analyze thin film characteristics. The average value of 5.7 ns was obtained with TiO2 layer, and the lifetime was uniform when measured over 10 points. About 0.1 degree XRD pick shift and 3.0 nm (~769 nm to 766 nm) PL peak position difference at the specific part of the pattern texture was observed. XRD pick shift represents changed d-spacing. Changes in d-spacing can be evidence of stress in the film, and this stress may shift the bandgap. As a way to mitigate XRD peak shift and PL peak difference, we also applied porous precursor and silicon texture rounding respectively and analyze it.
We presented a new method for manufacturing Perovskite on the textured surface of silicon and presented results of the analysis. Fabricating Perovskite on a textured silicon substrate is the substantial step in maximizing the efficiency of Perovskite/Silicon tandem solar cells.
 NREL Efficiency Chart. (accessed October 29, 2017).
 Bush, et al. Nature Energy 2 (2017): 17009.
 Chen, et al. Journal of the American Chemical Society 136.2 (2013): 622-625.