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Title: Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices

Abstract

We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se)2 (CIGS) or Cu2ZnSn(S,Se)4 (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS devices. Sn may also preferentially incorporate on the S site as a deep acceptor in n-type ZnS, which suggests possible concerns with absorber-related interfacial compensation in CZTS devices with ZnS-derived buffers. Cu, Na, and K impurities are found tomore » all have the same qualitative behavior, most favorably acting as compensating acceptors when substituting on the cation site. Lastly, our results suggest one beneficial role of K and Na incorporation in CIGS or CZTS devices is the partial passivation of vacancy-related centers in CdS and ZnS buffers, rendering them less effective interfacial hole traps and recombination centers.« less

Authors:
 [1];  [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1440728
Report Number(s):
LLNL-JRNL-655930
Journal ID: ISSN 0021-8979; 776800; TRN: US1900766
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Applied Physics
Additional Journal Information:
Journal Volume: 116; Journal Issue: 6; Journal ID: ISSN 0021-8979
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS

Citation Formats

Varley, J. B., and Lordi, V. Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices. United States: N. p., 2014. Web. doi:10.1063/1.4892407.
Varley, J. B., & Lordi, V. Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices. United States. doi:10.1063/1.4892407.
Varley, J. B., and Lordi, V. Fri . "Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices". United States. doi:10.1063/1.4892407. https://www.osti.gov/servlets/purl/1440728.
@article{osti_1440728,
title = {Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices},
author = {Varley, J. B. and Lordi, V.},
abstractNote = {We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se)2 (CIGS) or Cu2ZnSn(S,Se)4 (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS devices. Sn may also preferentially incorporate on the S site as a deep acceptor in n-type ZnS, which suggests possible concerns with absorber-related interfacial compensation in CZTS devices with ZnS-derived buffers. Cu, Na, and K impurities are found to all have the same qualitative behavior, most favorably acting as compensating acceptors when substituting on the cation site. Lastly, our results suggest one beneficial role of K and Na incorporation in CIGS or CZTS devices is the partial passivation of vacancy-related centers in CdS and ZnS buffers, rendering them less effective interfacial hole traps and recombination centers.},
doi = {10.1063/1.4892407},
journal = {Journal of Applied Physics},
number = 6,
volume = 116,
place = {United States},
year = {2014},
month = {8}
}

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Cited by: 11 works
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Figures / Tables:

FIG. 1 FIG. 1: (a) Schematic of a configuration coordinate diagram showing the photoexcitation process that promotes an electron localized on a cadmium vacancy ($V^{⁻2}_{Cd}$) to the conduction band minimum (CBM), resulting in a $V^{—}_{Cd}$. The diagram illustrates how the absorption (purple arrow) and emission (green arrow) energies are related to themore » zero-phonon line (ZPL, black arrow) by the various relaxation energies associated with each charge state, $E^{—}_{rel}$ and $E^{⁻2}_{rel}$. An alternate depiction of the absorption (Abs.) and emission (Ems.) processes using the defect levels and conduction band edge is shown in (b), and the analogous energies for the ϵo excitations involving the valence band are shown in (c). (d) The calculated configuration coordinate diagram for VCd transitions involving the conduction band as in (a) and (b), shown for all stable charge states. The configuration coordinate $Q$ is taken as the average S–S distance surrounding the VCd.« less

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