Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

James M. Cave, Nicola E. Courtier, Isabelle A. Blakborn, Timothy W. Jones, Dibyajyoti Ghosh, Kenrick F. Anderson, Liangyou Lin, Andrew A. Dijkhoff, Gregory J. Wilson, Krishna Feron, M. Saiful Islam, Jamie M. Foster, Giles Richardson, Alison B. Walker

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Abstract

The absorber layers in perovskite solar cells possess a high concentration of mobile ion vacancies. These vacancies undertake thermally activated hops between neighboring lattice sites. The mobile vacancy concentration N 0 is much higher and the activation energy E A for ion hops is much lower than is seen in most other semiconductors due to the inherent softness of perovskite materials. The timescale at which the internal electric field changes due to ion motion is determined by the vacancy diffusion coefficient D v and is similar to the timescale on which the external bias changes by a significant fraction of the open-circuit voltage at typical scan rates. Therefore, hysteresis is often observed in which the shape of the current-voltage, J-V, characteristic depends on the direction of the voltage sweep. There is also evidence that this defect migration plays a role in degradation. By employing a charge transport model of coupled ion-electron conduction in a perovskite solar cell, we show that E A for the ion species responsible for hysteresis can be obtained directly from measurements of the temperature variation of the scan-rate dependence of the short-circuit current and of the hysteresis factor H. This argument is validated by comparing E A deduced from measured J-V curves for four solar cell structures with density functional theory calculations. In two of these structures, the perovskite is MAPbI 3, where MA is methylammonium, CH 3 NH 3; the hole transport layer (HTL) is spiro (spiro-OMeTAD, 2,2 ′,7,7 ′- tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9 ′-spirobifluorene) and the electron transport layer (ETL) is TiO 2 or SnO 2. For the third and fourth structures, the perovskite layer is FAPbI 3, where FA is formamidinium, HC (NH 2) 2, or MAPbBr 3, and in both cases, the HTL is spiro and the ETL is SnO 2. For all four structures, the hole and electron extracting electrodes are Au and fluorine doped tin oxide, respectively. We also use our model to predict how the scan rate dependence of the power conversion efficiency varies with E A, N 0, and parameters determining free charge recombination.

Original languageEnglish
Article number184501
JournalJournal of Applied Physics
Volume128
Issue number18
Early online date9 Nov 2020
DOIs
Publication statusPublished - 14 Nov 2020

Bibliographical note

Funding Information:
We would like to thank Chris Fell and Laurie Peter for their careful reading of the manuscript. J.M.C. and N.E.C. acknowledge EPSRC funded studentships from the CDT in New and Sustainable Photovoltaics (No. EP/L01551X/1). This work has received funding from the Australian Renewable Energy Agency (ARENA) as part of ARENA’s Emerging Renewables Program via Grant No. P159394. D.G., A.B.W., and S.I. received funding from the European Union’s Horizon 2020 research and innovation programme under EoCoE (Grant Agreement No. 676629) and A.B.W. under EoCoE II (Grant Agreement No. 824158).

Publisher Copyright:
© 2020 Author(s).

Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.

ASJC Scopus subject areas

  • Physics and Astronomy(all)

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