Perovskite cells pose an intriguing modelling challenge as the electrical cell properties are governed by both electronic and ionic charge transport and the optical cell properties need to be carefully optimized when seeking record efficiencies in tandem cell configurations with silicon wafer cells. In this contribution, we give an update on recent advances in both electrical and optical modeling and discuss experimental results.

Negative capacitance and inductive loops in impedance spectroscopy in perovskite solar cells have been described in several recent reports, though their origin remained unclear so far. The negative capacitance and inductive loop may be related to one another as they appear in the same samples but at different applied biases. Similarly, we have demonstrated that ion migration is present even in high-efficiency low-hysteresis perovskite cells [1]. We shed light on the likely physical mechanisms behind these observations and compare devices in the frequency domain at different applied bias by employing a mixed electronic-ionic device model that naturally produces inductive loops and negative capacitance allowing us to study correlations with relevant material parameters.

Moreover we present an optical model implemented in the software SETFOS 4.6 [2] for simulating perovskite/silicon monolithic tandem solar cells that exploit light scattering structures [3]. We validate the model with experimental data of tandem solar cells that either use front- or rear-side textures. The software is used to investigate the potential of different monolithic tandem structures. The p-i-n solar cell architecture is the most promising with respect to achievable photocurrent for both flat and textured wafers. Finally, cesium-formamidinium-based perovskite materials with several bandgaps were synthetized, optically characterized [4] and their potential in tandem devices was quantified by simulations. The most promising tandem has a potential of reaching a power conversion efficiency of 31% [5].

[1] M. Neukom, S. Züfle, E. Knapp, M. Makha, R. Hany, B. Ruhstaller, Solar En. Mat. & Solar Cells, 169 159ff (2017)

[2] T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, J. Appl. Phys. 110, 33111 (2011) and SETFOS 4.6 by Fluxim AG, https://www.fluxim.com, Switzerland

[3] S. Altazin, L. Stepanova, K. Lapagna, P. Losio, J. Werner, B. Niesen, A. Dabirian, M. Morales-Masis, S. de Wolf, C. Ballif, B. Ruhstaller, Proc. 32nd Eur. Photovolt. Sol. Energy Conf. 1276 (2016)

[4] J. Werner et al., ACS Energy Lett. 3, 742–747 (2018)

[5] S. Altazin, L. Stepanova, J. Werner, B. Niesen, C. Ballif, and B. Ruhstaller, Optics Express 26 (10), A579 (2018)