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Salman Manzoor1 Jakob Haeusele2 Zhengshan Yu1 Kevin Bush3 Axel Palmstrom3 Stacey Bent3 Michael McGehee3 Zachary Holman1

1, Arizona State University, Tempe, Arizona, United States
2, University of Konstanz, Konstanz, , Germany
3, Stanford University, Stanford, California, United States

Perovskite solar cells have gone through rapid improvement in their power conversion efficiency (PCE) in the last few years reaching up to 22.7% [1]. Due to superior optical properties (abrupt band-edge, IR transparency) and their easy bandgap tunability to wide energy ranges, perovskites are attractive candidates for top-cells in tandem applications [2].
We have previously demonstrated a 23.6% efficient monolithic, two-terminal perovskite/silicon tandem using Cs0.17FA0.83Pb(Br0.17I0.83)3 (CsFA) perovskite top-cell [3]. Though a world-record device, the PCE is still below the 30% that are achievable with two junctions in two-terminal devices with silicon as a bottom sub-cell [2]. Improving the PCE of the world-record device requires a detailed and systematic optical loss analysis that includes an analysis of parasitic absorption in layers other than perovskite and silicon in tandems, optimization of top and bottom cell thicknesses to achieve current matching, reduction of reflection losses etc. To perform this analysis, the first step is to accurately determine the optical constants of CsFA perovskite that are not available in the literature. We use multi-angle spectroscopic ellipsometry and spectrophotometry to uniquely determine the optical constants of two CsFA perovskites having different bandgaps of 1.61 and 1.68 eV, achieved by varying the Cs/Br ratio as 17/17 and 25/20 respectively. This method is verified on a common absorber, CH3NH3PbI3. Furthermore, we simulate the absorption and reflection of single junction perovskite cells with the obtained refractive indices and compare them to the measured values of real cells which shows excellent agreement. Our results reveal that the biggest losses in single-junction perovskite cells are front surface reflectance followed by parasitic absorption in the electron contact (C60) and front TCO. We further conclude that the front surface reflectance is very sensitive to the thicknesses of the top TCO, electron contact and perovskite absorber due to changes in the thin film interference pattern, hence the need for their optimization.
Further analysis will include the simulation of complete two-terminal perovskite/silicon tandems resulting in an understanding of various optical losses and approaches to minimize them.

[1] NREL Efficiency Chart (https://www.nrel.gov/pv/assets/images/efficiency-chart.png).
[2] Z.J. Yu, M. Leilaeioun, Z. Holman, Selecting tandem partners for silicon solar cells, Nature Energy, 1 (2016) 16137.
[3] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin, R.L. Hoye, C.D. Bailie, T. Leijtens, 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nature Energy, 2 (2017) 17009.

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