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Spectroscopic and electrical signatures of acceptor states in solution processed Cu2ZnSn(S,Se)4 solar cells

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posted on 10.11.2017 by Devendra Tiwari, Ekaterina Skidchenko, Jake Bowers, Michael V. Yakushev, Robert W. Martin, David J. Fermin
The nature and dynamics of acceptor states in solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) thin films are investigated by variable temperature photoluminescence (PL) and electrical impedance spectroscopy. Highly pure I-4 phase CZTSSe with the composition Cu1.6ZnSn0.9(S0.23Se0.77)4 is synthesized by sequentially spin coating of dimethyl-formamide/isopropanol solutions containing metal salts and thiourea onto Mo coated glass, followed by annealing in Se atmosphere at 540 oC. As-annealed films are highly compact with a thickness of 1.3 m and grain sizes above 800 nm, with a band gap of 1.18 eV. Photovoltaic devices of 0.25 cm2 with the architecture glass/Mo/CZTSSe/CdS/i-ZnO/Al:ZnO demonstrate a power conversion efficiency reaching up to 5.7 % in the absence of an anti-reflective coating. Under AM1.5G illumination at 296 K, the best device shows 396 mV open-circuit voltage (VOC), 27.8 mA/cm2 short-circuit current (JSC) and 52% fill factor (FF). The overall dispersion of these parameters is under 15 % for a total of 20 devices. In the near IR region, PL spectra are dominated by two broad and asymmetrical bands at 1.14 eV (PL1) and 0.95 eV (PL2) with characteristic power and temperature dependences. Analysis of the device electrical impedance spectra also reveal two electron acceptor states with the same activation energy to those observed by PL. This allows assigning PL1 as radiative recombination at localized copper vacancies (VCu), while PL2 is associated with CuZn¬ antisites, broadened by potential fluctuations (band tails). The impact of these states on device performance as well as other parameters such as barrier collection heights introduced by partial selenization of the back contact is discussed.

Funding

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) funded PVTEAM grant (EP/L017792/1). Impedance analysis is performed with Solartron analyzer procured under EPSRC CDT Capital grant EP/K035746/1. Microscopic imaging and analysis are done at the Chemical Imaging Facility, University of Bristol, with equipment funded by EPSRC Grant "Atoms to Applications" (EP/K035746/1).

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  • Mechanical, Electrical and Manufacturing Engineering

Published in

Journal of Materials Chemistry C

Citation

TIWARI, D. ... et al., 2017. Spectroscopic and electrical signatures of acceptor states in solution processed Cu2ZnSn(S,Se)4 solar cells. Journal of Materials Chemistry C, 5(48), pp. 12720-12727.

Publisher

Royal Society of Chemistry

Version

AM (Accepted Manuscript)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution 4.0 Unported Licence (CC BY 3.0). Full details of this licence are available at: https://creativecommons.org/licenses/by/3.0/

Acceptance date

25/10/2017

Publication date

2017

Notes

This is an Open Access Article. It is published by Royal Society of Chemistry under the Creative Commons Attribution 3.0 Unported Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/3.0/

ISSN

2050-7534

Language

en

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