Expanding Thermal Plasma Chemical Vapour Deposition of ZnO:Al Layers for CIGS Solar Cells

K Sharma,R E I Schropp,N J Bakker,H C M Knoops,B J Kniknie,A Mittal,M Creatore,W M M Kessels,B L Williams

doi:10.1155/2014/253140

Abstract

Aluminium-doped zinc oxide (ZnO:Al) grown by expanding thermal plasma chemical vapour deposition (ETP-CVD) has demonstrated excellent electrical and optical properties, which make it an attractive candidate as a transparent conductive oxide for photovoltaic applications. However, when depositing ZnO:Al on CIGS solar cell stacks, one should be aware that high substrate temperature processing (i.e., >200°C) can damage the crucial underlying layers/interfaces (such as CIGS/CdS and CdS/i-ZnO). In this paper, the potential of adopting ETP-CVD ZnO:Al in CIGS solar cells is assessed: the effect of substrate temperature during film deposition on both the electrical properties of the ZnO:Al and the eventual performance of the CIGS solar cells was investigated. For ZnO:Al films grown using the high thermal budget (HTB) condition, lower resistivities, ρ, were achievable (~5 × 10−4 Ω·cm) than those grown using the low thermal budget (LTB) conditions (~2 × 10−3 Ω·cm), whereas higher CIGS conversion efficiencies were obtained for the LTB condition (up to 10.9%) than for the HTB condition (up to 9.0%). Whereas such temperature-dependence of CIGS device parameters has previously been linked with chemical migration between individual layers, we demonstrate that in this case it is primarily attributed to the prevalence of shunt currents.

Highlights

IntroductionHave been demonstrated for CIGS solar cells [1]. The CIGS absorber layer has a direct band gap, and it is tunable from ∼1.02 eV for copper indium selenide (CuInSe2 ) to ∼
Very high conversion efficiencies of up to 20.8%have been demonstrated for CIGS solar cells [1]
Shunt-causing pinhole defects were intrinsically present in the CIGS layer, and their detrimental impact was exacerbated by high thermal budget zinc oxide (ZnO):Al deposition conditions, demonstrating that the processing restrictions for the front contact are highly dependent on the quality of the underlying films

Summary

Introduction

Have been demonstrated for CIGS solar cells [1]. The CIGS absorber layer has a direct band gap, and it is tunable from ∼1.02 eV for copper indium selenide (CuInSe2 ) to ∼. Low resistivities were achieved for ETP-CVD grown ZnO:Al, that is, as low as 6⋅10−4 ohm⋅cm for a film thickness of 300 nm, but the efficiency of the finished CIGS devices was seen to be sensitive to the substrate temperature reached during the ETP-CVD process. Whereas such temperaturedependence has been linked in literature [15] to thermally induced chemical migration between individual layers, we demonstrate that in our case it is primarily linked to the presence of macroscale defects, inducing the prevalence of shunt currents upon thermal exposure. Shunt-causing pinhole defects were intrinsically present in the CIGS layer, and their detrimental impact was exacerbated by high thermal budget ZnO:Al deposition conditions, demonstrating that the processing restrictions for the front contact are highly dependent on the quality of the underlying films

Experimental

Results and Discussion

Conclusions