Electro-optical characterization and device simulations of perovskite solar cells
Sandra Jenatsch a, Ennio Luigi Comi b, Antonio Cabas Vidani a, Evelyne Knapp b, Beat Ruhstaller a b
a Fluxim AG, 8400 Winterthur, Switzerland
b Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), 8401 Winterthur (Switzerland)
Proceedings of Device Physics Characterization and Interpretation in Perovskite and Organic Materials (DEPERO)
València, Spain, 2023 October 3rd - 5th
Organizers: Sandheep Ravishankar, Juan Bisquert and Evelyne Knapp
Invited Speaker, Sandra Jenatsch, presentation 003
Publication date: 14th September 2023

Thin-film opto-electrical devices based on perovskite and organic materials possess several interesting advantages compared to their inorganic counterparts. The continuous improvements in performance have led to commercial applications of OLEDs in displays and promising power conversion efficiencies of perovskite solar cells (PSCs) close to the established silicon photovoltaic technology. Progress in those technologies was also facilitated by the increased understanding of device operations and consequently targeted optimization of materials and device structures.

Electro-optical characterization techniques in steady-state, transient and frequency-domain are widely used to gain insights into operation mechanisms. Often, such characterizations are performed on systematically varied devices to understand trends. As a first example, we will show that excess formamidinium precursor in the perovskite layer leads to a faster ion dynamic in the device, as observed through electrical impedance spectroscopy.[1] Second, we analyse the impact of degradation by comparing open-circuit voltage decay (OCVD) measurements on fresh and degraded PSCs.

A quantitative analysis of material parameters by applying analytical formulas requires a careful assessment of the assumptions used in the model. In perovskite devices, the quantification of ionic charge carrier densities and mobilities is of outmost importance. Methods such as transient capacitance,[2] bias-assisted charge extraction[3] and OCVD[4] have been used to determine them. By using drift-diffusion simulations, we show that the traditional analysis of transient current measurements are significantly underestimating the mobile ion densities in systems with high ionic densities, in line with recent findings by Diekmann et al.[3] Based on these results, a slightly adapted method to obtain the correct mobility and density of the ionic charge carriers is proposed.

Combining electro-optical characterization and device simulations offers additional opportunities for the analysis of PSCs. In the last part of this contribution, we employed this strategy to measured electroluminescence (EL) images of a carbon-based mesoporous PSC. While taking transient EL images of 1.4 cm2 cells, we found very pronounced - spatially varying - dynamics in the EL signal. To understand the local temporal fluctuation in the EL signal, the complete device stack was modelled with the simulation software Setfos. Through parameter variation of transient simulations, it was found that an increased ion density could reproduce the stronger EL signal, experimentally observed at certain locations. As mobile ions are affecting the long-term stability of PSCs, spots with increased ionic concentrations could catalyse the degradation.

The authors acknowledge the Innosuisse project "Advanced Imaging and Machine Learning for PV Quality Assurance" (58054.1 IP-EE) as well as the SOLAR-ERA.NET project SCALEUP through funding from SFOE (SI/501958-01). This project has been partly funded by the European Union Horizon 2021 research and innovation programme under grant agreement No. 101075605(SuPerTandem)

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