Towards understanding the built-in-field in perovskite solar cells through layer by layer SPV measurements
Emilio Gutiérrez-Partida a
a Universität Potsdam, Soft Matter Physics, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Emilio Gutiérrez-Partida, presentation 154
Publication date: 20th April 2022

While the band diagram is well understood for most photovoltaic technologies, the distribution and magnitude of the built-in voltage (VBI) in the field of perovskite solar cells remains poorly understood. The VBI can be understood as an internal driving force for charges and is defined as the total voltage drop across all layers of the device. As such, the VBI strongly influences the charge transport and recombination processes in the absorber layer and at the interfaces and with that the open-circuit voltage (VOC) and fill factor (FF) of the device. Moreover, it is expected that a comparatively large VBI (similar to the VOC) is required to achieve well performing devices in the presence of significant interface recombination.

In this work, we have systematically studied the VBI in pin-type perovskite solar cells based on different hole transport layers (HTLs). To this end, we determined the surface photovoltage (SPV) of partial and complete device stacks layer-by-layer by measuring the work function (WF) under dark and light conditions with Kelvin probe and photoemission spectroscopy measurements in 3 different laboratories and compared the SPV to the VOC of full devices. Our results demonstrate that the SPV increases upon the addition of each additional layer until the SPV equals the VOC of the full device. This is consistent with the assumption that both electron and hole transport layers (HTL/ETL) increase the VBI of the device, however, the contribution of both the hole and electron transport layer (HTL/ETL) to the total SPV of the device is not really significant (only in the range of ≈100 – 200 meV). In contrast, in all cases, the largest contribution to the SPV originates from the addition of the top metal electrode (≈700 meV), upon completion of the device. The results suggest that the VBI of pin-type perovskite solar cells is largely a result of effectively work-function mismatched electrodes. Moreover, with regard to films (or incomplete cell stacks), our simulations  show that a large quasi-Fermi level splitting (>VOC) can be achieved without a significant internal field, which is consistent with the experimental data. Overall, this work presents a closer and more accurate look into the band diagram of pin-type perovskite solar cells and enables a better understanding of how the internal field impacts the non-radiative recombination losses and the overall performance of perovskite solar cells.

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