Speaker: Professor Thomas Kirchartz - University of Duisberg-Essen, Germany
Date: Wednesday, 7 May
Time: 11:00am
Venue: Rayleigh seminar room, Maxwell Centre
Abstract:
The efficiency of halide perovskite solar cells has been continuously rising over the past decade to values above 26%. Future technological development will have to deal with issues of device stability, but also strive to further minimise efficiency-limiting loss processes in the bulk and at interfaces within the cell stack. The identification and understanding of electrical losses will require the ability to characterise solar cells and multilayer stacks with a variety of steady-state, time-domain and frequency-domain techniques that are sensitive to the transport and recombination of charge carriers. Especially, time- and frequency-domain techniques offer a large amount of information on dynamic processes in the solar cell, while posing a substantial challenge in terms of the complexity of data analysis.
Here, I discuss our recent work related to transient photoluminescence (TPL) and photovoltage spectroscopy (TPV) applied to halide perovskites. I show that by using extremely low repetition rates and a gated CCD camera, we can obtain high dynamic range TPL data with continuously changing decay times that exceed 100µs. Furthermore, I show that by changing the repetition rate, basically any decay time can be extracted from one sample, whereby the extracted decay time is approximately the inverse repetition rate. I explain why this is the case both mathematically and physically. Further, I present recent results on the determination of diffusion lengths as a function of injection level using the reabsorption effect (as previously shown by Cho et al.). Finally, I show how to separate recombination from extraction by using TPV data combined with a novel analysis approach based on the determination of eigenvalues of a 2 × 2 matrix. The model provides two time constants (the inverse eigenvalues), one for the rise and one for the decay of the voltage after the pulse. These two time constants can be experimentally determined as a function of light intensity. By comparison of model and experimental data, I can then derive a time constant for recombination and one for charge extraction, whereby the ratio of these two time constants is directly correlated with solar cell efficiency.
Bio:
Thomas Kirchartz is currently a Professor of Electrical Engineering and Information Technology at the University of Duisburg-Essen and the head of the department of Analytics and Simulation and the group of Organic and Hybrid Solar Cells at the Research Centre Jülich (Institute for Energy and Climate Research). Previously, he was a Junior Research Fellow at Imperial College London. His research interests include all aspects regarding the fundamental understanding of photovoltaic devices, including their characterisation and simulation.