SiOxNy back-contact barriers for CZTSe thin-film solar cells.

Wenjian Chen,Levent Gütay,Teoman Taskesen,Jürgen Parisi,Jörg Ohland,Ulf Mikolajczak,Devendra Pareek,Hippolyte Hirwa

doi:10.1371/journal.pone.0245390

Abstract

The formation of molybdenum diselenide (MoSe2) is widely observed at the back-contact interface for copper zinc tin selenide (CZTSe) thin-film solar cells. Depending on individual selenium (Se) supply and thermal conditions for forming CZTSe absorbers on molybdenum (Mo) substrates, the thickness of MoSe2 can vary from a few hundreds of nanometers up to ≈ 1 μm, which is comparable to the commonly adopted thickness of 1 ~ 1.5 μm for CZTSe absorbers. In this study, for controlling the thickness of interfacial MoSe2, thin diffusion barrier layers of silicon oxynitride (SiOxNy) are deposited onto Mo layers prior to the growth of CZTSe absorbers in the fabrication process. As a result, a reduction in the thicknesses of MoSe2 layers is achieved. In terms of energy conversion efficiency (η), CZTSe solar cells grown on Mo/SiOxNy back contacts suffer a deterioration as the SiOxNy layers get thicker. CZTSe solar cells grown on Mo/SiOxNy/Mo back contacts preserve their efficiencies at ≈ 11% with thin 10 nm SiOxNy layers.

Highlights

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is considered as a promising substitution for chalcopyrite Cu(In,Ga)(Se,S)2 (CIGSSe) in thin-film solar cell technology due to its earth abundant and low-cost constituents [1, 2]
Compared to the reference M, which is grown on pure Mo back contact, all the solar cells from Mo/SiOxNy back contacts show a deterioration in all parameters, i.e., open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and energy conversion efficiency (η)
It cannot be excluded that a small amount of oxygen atoms from SiOxNy may diffuse into the copper zinc tin selenide (CZTSe) absorbers and lead to additional impurity states in the absorber, which could impact the defect landscape or influence the phase structure of kesterite material in relevant regions

Summary

Introduction

Kesterite Cu2ZnSn(S,Se) (CZTSSe) is considered as a promising substitution for chalcopyrite Cu(In,Ga)(Se,S) (CIGSSe) in thin-film solar cell technology due to its earth abundant and low-cost constituents [1, 2]. In terms of the energy conversion efficiency (η), CZTSSe solar cells reach only 12.6% while CIGSSe devices have an up-to-date record of 23.4% [3, 4]. For pure Cu2ZnSnS4 and Cu2ZnSnSe4 solar cells, the record efficiencies are reported to be 11% and 12.5%, respectively [5, 6]. In order to further improve kesterite solar cells, addressing the back-contact issues is important, especially for pure CZTSe devices. For the reported record 12.5% CZTSe solar cell, the MoSe2 thickness is at around 100 ~ 200 nm [6]. The limiting and/or control of MoSe2 thickness at the back

Methods

Results

Conclusion