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Effect of Cu Content on Post‐Sulfurization of Cu(In,Ga)Se 2 Films and Corresponding Solar Cell Performance
Author(s) -
Keller Jan,
Bilousov Oleksandr V.,
Wallin Erik,
Lundberg Olle,
Neerken Janet,
Heise Stephan,
Riekehr Lars,
Edoff Marika,
Platzer-Björkman Charlotte
Publication year - 2019
Publication title -
physica status solidi (a)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.532
H-Index - 104
eISSN - 1862-6319
pISSN - 1862-6300
DOI - 10.1002/pssa.201900472
Subject(s) - stoichiometry , copper indium gallium selenide solar cells , copper , annealing (glass) , ternary operation , analytical chemistry (journal) , solar cell , sulfur , grain boundary , chemistry , materials science , crystallography , microstructure , metallurgy , optoelectronics , chromatography , computer science , programming language
Herein, the effect of the initial copper content of co‐evaporated Cu(In 1− x ,Ga x )Se 2 (CIGS) absorber films on the impact of a post‐annealing step in elemental sulfur atmosphere is studied. The Cu concentration is varied over a wide range ([Cu]/[III] = CGI = 0.57–1.23), allowing to identify composition‐dependent trends in phase formation, chemical rearrangements, and solar cell performance after sulfurization. For all samples, a ternary CuInS 2 layer forms at the surface. In addition, sulfur 1) is incorporated in randomly distributed CuIn(S,Se) 2 mixed crystals underneath CuInS 2 ; 2) diffuses into multidimensional defects (e.g., dislocations and grain boundaries); and 3) is bound in Na–In–S surface plates. It is found that Cu‐poor absorber composition (CGI ≤ 0.82) favors CuInS 2 growth as compared with close‐stoichiometric CIGS films, driven by a faster diffusion of Cu toward the surface. For Cu‐rich absorbers (CGI > 1), Se—S exchange is significantly accelerated, presumably by the presence of Cu 2− x Se phases reacting to Cu 2− x S and eventually catalyzing CuInS 2 formation. Finally, open‐circuit voltage ( V OC ), fill factor (FF), and efficiency ( η ) of corresponding solar cells increase after sulfurization with increasing CGI until stoichiometry is reached. The result is explained by a mitigated Cu depletion of the absorber bulk after sulfurization for close‐stoichiometric CIGS.