Impact of Interfaces and Active Layer Thickness on the Assignment of Charge Carrier Recombination Dynamics in Thin Film Solar Cells.
Kristofer Tvingstedt a
a Experimental Physics VI, Julius Maximillian University of Würzburg, 97074 Würzburg, Germany
Proceedings of Interfaces in Organic and Hybrid Thin-Film Optoelectronics (INFORM)
València, Spain, 2019 March 5th - 7th
Organizers: Natalie Stingelin, Henk Bolink and Michele Sessolo
Invited Speaker, Kristofer Tvingstedt, presentation 017
DOI: https://doi.org/10.29363/nanoge.inform.2019.017
Publication date: 8th January 2019

Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells is of crucial importance for further improvements of this already promising photovoltaic technology. Understanding the order, rate and spatial whereabouts of the recombination processes as a function of excess carrier concentration is imperative towards identifying the remaining culprits behind the voltage deficit to the thermodynamic radiative limit (1.33 V for MaPbI3). With this objective, various electrical transient methods based on photovoltage decay have previously been employed. However, these techniques can be strongly influenced by the device capacitive response which overlays with the steady state relevant bulk carrier recombination. Herein, we will first outline the main limiting parameters in the assignment of correct and steady state relevant charge carrier lifetimes in both organic and perovskite thin film solar cells. To ascertain the identification of steady state relevant bulk charge carrier dynamics, we will highlight the benefit of evaluating thicker films at higher light intensity to minimize the impact of spatial capacitance artifacts. We focus our study on the electrical transient response in very efficient planar co-evaporated solar cells with an increased active layer thickness, up to 820 nm. While the increased capacitance for the thin cells leads to longer perceived decay times in the lower voltage regime, the higher voltage regime shows kinetics becoming independent of active layer thickness, allowing us to identify the transition from capacitance affected to the sought-after bulk charge carrier dynamics. Finally, we determine that the recombination order in thicker devices is ranging in between 1.6-2. These values are noticeably lower than what has previously been reported by these or equivalent electrical methods and are, more in line with dynamics observed in pure film, pointing towards trap-assisted and free carrier recombination under operating conditions in complete perovskite photovoltaic devices.

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