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Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

Deducing transport properties of mobile vacancies from perovskite solar cell characteristics
Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

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.

0021-8979
Cave, James M.
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Courtier, Nicola E.
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Blakborn, Isabelle A.
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Jones, Timothy W.
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Ghosh, Dibyajyoti
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Anderson, Kenrick F.
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Lin, Liangyou
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Dijkhoff, Andrew A.
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Wilson, Gregory J.
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Feron, Krishna
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Saiful Islam, M.
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Foster, Jamie M.
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Richardson, Giles
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Walker, Alison B.
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Cave, James M.
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Courtier, Nicola E.
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Blakborn, Isabelle A.
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Jones, Timothy W.
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Ghosh, Dibyajyoti
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Anderson, Kenrick F.
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Lin, Liangyou
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Dijkhoff, Andrew A.
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Wilson, Gregory J.
6cbba852-8a15-4a33-a509-d45a7b532a14
Feron, Krishna
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Saiful Islam, M.
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Foster, Jamie M.
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Richardson, Giles
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Walker, Alison B.
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Cave, James M., Courtier, Nicola E., Blakborn, Isabelle A., Jones, Timothy W., Ghosh, Dibyajyoti, Anderson, Kenrick F., Lin, Liangyou, Dijkhoff, Andrew A., Wilson, Gregory J., Feron, Krishna, Saiful Islam, M., Foster, Jamie M., Richardson, Giles and Walker, Alison B. (2020) Deducing transport properties of mobile vacancies from perovskite solar cell characteristics. Journal of Applied Physics, 128 (18), [184501]. (doi:10.1063/5.0021849).

Record type: Article

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.

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JAP20-AR-04499 - Accepted Manuscript
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Accepted/In Press date: 16 October 2020
e-pub ahead of print date: 9 November 2020
Published date: 14 November 2020

Identifiers

Local EPrints ID: 445793
URI: http://eprints.soton.ac.uk/id/eprint/445793
ISSN: 0021-8979
PURE UUID: 033b6a40-7117-47dc-af04-fc97f43d6bbd
ORCID for Nicola E. Courtier: ORCID iD orcid.org/0000-0002-5714-1096

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Date deposited: 07 Jan 2021 17:34
Last modified: 21 Nov 2021 21:12

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Contributors

Author: James M. Cave
Author: Isabelle A. Blakborn
Author: Timothy W. Jones
Author: Dibyajyoti Ghosh
Author: Kenrick F. Anderson
Author: Liangyou Lin
Author: Andrew A. Dijkhoff
Author: Gregory J. Wilson
Author: Krishna Feron
Author: M. Saiful Islam
Author: Jamie M. Foster
Author: Alison B. Walker

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