Overcoming Zinc Oxide Interface Instability with Methylammonium-free Perovskites for High Performance Solar Cells
Kelly Schutt a, Pabitra Nayak a, Alexandra Ramadan a, Bernard Wenger a, Yen-Hung Lin a, Henry Snaith a
a University of Oxford, GB
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Kelly Schutt, presentation 168
Publication date: 11th February 2019

Perovskite solar cells have achieved the highest power conversion efficencies on metal oxide n-type layers, including SnO2 and TiO2. Despite ZnO having superior optoelectronic properties to these two metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI3, ZnO has largely been overlooked due to a chemical instability with the architypical MAPbI3, which leads to the rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether alternative metal halide perovskites also exhibit this instability with ZnO are yet to be undertaken. We develop experimental methods to elucidate the degradation mechanisms in ZnO-MAPbI3 interfaces. By substituting MA with formamidinium (FA) and cesium (Cs), we greatly enhance the stability of the perovskite-ZnO interface and find that stability compares favorably with SnO2-based devices after high intensity UV irradiation and 85°C thermal stressing. Devices comprising FA/Cs cations on ZnO reach 21.1% scanned power conversion efficiency and 18% steady-state power output, comparable to devices on SnO2. Our work demonstrates that, provided we move away from MA containing perovskites, ZnO is an equally feasible n-type charge extraction layer as SnO2 for use in perovskite solar cells, with many additional advantages.

We thank the EPSRC, UK for support. K.S. thanks the Marshall Aid Commemoration Commission. B.W. acknowledges funding from the European Commission via a Marie-Skłodowska-Curie individual fellowship (REA Grant Number 706552-APPEL).

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