Selenization of KF Premixed CuInGa Precursor and Its Impact on Solar Cell Performance
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ORIGINAL ARTICLE - ENERGY AND SUSTAINABILITY
Selenization of KF Premixed CuInGa Precursor and Its Impact on Solar Cell Performance Ji‑A Oh1 · Yu‑Jin Song1 · Sung‑Wook Cho1 · Gyeong‑A Lee1 · A‑Hyun Kim1 · Chan‑Wook Jeon1 Received: 28 July 2020 / Accepted: 26 August 2020 © The Korean Institute of Metals and Materials 2020
Abstract K-doped Cu(In,Ga)Se2 absorber thin films were prepared by a two-step process (sputtering/selenization) using a metal precursor of a KF-premixed In/CuGa/Mo/glass structure and Se vapor. KF was deposited on top of In, between In and CuGa, and below CuGa by thermal evaporation. The properties were compared with those of a precursor without KF inserted. As an effect of KF insertion, a large electrical change was observed, i.e., the disappearance of deep-level defects, and the grain size and Ga composition distribution of the absorber layer thin film were changed slightly. Nevertheless, there was no benefit in terms of the solar cell performance compared to a precursor without KF inserted. The K inserted in the precursor occupied a position competitively with Na diffused from the substrate, resulting in an undesirable carrier density profile in the surface region of the absorber layer thin film. Graphic Abstract
Keywords Cu(In,Ga)Se2 · Cu-In-Ga precursor · Na · K · MoSe2 · Selenization
1 Introduction
* Chan‑Wook Jeon [email protected] 1
School of Chemical Engineering, Yeungnam University, Gyeongsan, Korea
Unlike crystalline silicon solar cells, thin-film solar cells are manufactured by depositing a thin absorption layer on a glass or flexible metal foil or plastic substrate. The manufacturing cost is low because the coated semiconductor thin film is very thin and uses a low-cost substrate material, such as glass or thin metal. [1] The CIGSe thin-film
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solar cell using a Cu(In,Ga)Se2 (CIGSe) thin film as a light absorber layer is suitable for low-cost solar cells because of its large area coating and high power conversion efficiency. [2, 3] The efficiency of CIGS thin-film solar cells was reported to be more than 23%. [4] To make a highefficiency solar cell, a process of doping alkali elements, such as Na, K, and Cs, into the absorber layer is essential. Na, which is essential in the CIGSe thin-film solar cell process, increases the free carrier concentration, improves the p-type conductivity, and improves the open-circuit voltage ( Voc).[5, 6] In addition, the roles of smoothing the surface morphology, [7] increasing the grain size, and reducing the Se vacancies present on the grain boundaries and surface of the CIGSe thin film during thin film growth have been reported. K is used widely as an alkali element other than Na, and K increases the free carrier concentration and induces Cu and Ga-deficient surfaces to promote the diffusion of Cd to the absorber layer surface, thereby inducing good CIGSe/CdS junction quality. [8, 9] The recently announced highest efficiency of 23.35% was attributed to the effects of doping with another alkali element, Cs.[4] The CI
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