Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu₂ZnSnS₄ solar cells

Abstract

Although Cu2ZnSnS4 (CZTS) has attracted attention as an alternative absorber material to replace CuInGaSe2 (CIGS) in solar cells, the current level of understanding of its characteristic loss mechanisms is not sufficient for achieving high power conversion efficiency. In this study, which aimed to minimize the characteristic losses across the devices, we examined the relations between the compositional ratio distribution in the absorber layer, subsequent defect formation, and surface electrical characteristics. A high‐temperature sulfurization process was used to improve the crystallinity of the absorber layer, which increased the uniformity of the compositional ratio distribution and consequently suppressed the formation of a ZnS secondary phase on the CZTS/MoS2 interface. Because defects and defect clusters generated in the absorber layer are shallower when the compositional ratio distribution is uniform, the electron‐hole recombination loss is reduced. These characteristics were confirmed by measuring the defect energy level using admittance spectroscopy and by analyzing the surface potential and current characteristics. These measurements revealed that improving the compositional ratio distribution suppresses the formation of deep‐level defects and reduces the rate of carrier recombination. In addition, improving the compositional ratio distribution substantially contributes to improving the series resistance and short circuit current density characteristics. Copyright © 2015 John Wiley & Sons, Ltd.

Carrier transport and surface potential over phase variations in the surface and bulk of highly efficient Cu2ZnSn(S,Se)4 solar cells

“…Interestingly, we were able to observe SnO 2 phases near the Mo/CZTSSe interface; the SnO 2 peak intensity was higher near the MoSe 2 area, which indicated that Sn was reacting with oxygen from an unknown source. One of the most likely O sources, according to our previous studies, is SLG, which contains Na, Si, O, and so forth. We have already verified that the content of O in CZTS thin films diffused from SLG substrates is sufficient to be detected by secondary ion mass spectrometry (SIMS) and affect thin‐film fabrication .…”

“…the SnO 2 peak intensity was higher near the MoSe 2 area, which indicated that Sn was reacting with oxygen from an unknown source. One of the most likely O sources, according to our previous studies, 11,27,44 is SLG, which contains Na, Si, O, and so forth.…”

“…The narrow existence regions of CZTSSe, as determined by theoretical and experimental research, often cause difficulties in the deposition of high‐quality CZTSSe thin films. Excluding quaternary phases, unexpected secondary phases and accompanying defects have been known to hinder electron‐hole ( e‐h ) separation, which affects the open‐circuit voltage ( V OC ) and short‐circuit current density ( J SC ) . The secondary phases mainly observed in kesterite materials are Cu x (S,Se), Cu 2 Sn(S,Se) 3 , Sn(S,Se) x , and Zn(S,Se) .…”

“…Excluding quaternary phases, unexpected secondary phases and accompanying defects have been known to hinder electron-hole (e-h) separation, which affects the open-circuit voltage (V OC ) and short-circuit current density (J SC ). [9][10][11][12][13][14] The secondary phases mainly observed in kesterite materials are Cu x (S,Se), 15,16 Cu 2 Sn(S,Se) 3 , 10 Sn(S,Se) x , 17 and Zn(S,Se). 18 Cu-and Sn-related secondary phases typically have smaller energy band gaps (E g ) than CZTSSe quaternary phases.…”

Physical routes for the synthesis of kesterite

“…The progresses in these routes showed that the use of metallic stacks provides for the moment better efficiencies at least for the selenide compounds, with University of Oldenburg (EHF/LCP group) reporting 11.4% [9], IREC 11.8% [6] and DGIST a 12.6% efficiency device [7]. Furthermore, a certified efficiency of 13.04% has been obtained using sputtered precursors [10], whereas for pure sulfide kesterite compounds the best results are instead obtained using S-containing targets [19] and/or cosputtering approach.…”