High Voc upon KF Post-Deposition Treatment for Ultrathin Single-Stage Coevaporated Cu(In, Ga)Se2 Solar Cells

Jessica De Wild,Marc Meuris,Jef Poortmans,Thierry Kohl,Thomas Schnabel,Dilara Gokcen Buldu,Marcel Simor,Guy Brammertz,Gizem Birant,Bart Vermang

doi:10.1021/acsaem.9b01370

Abstract

A simplified Cu(In, Ga)Se2 (CIGS) solar cell structure based on a 500 nm thin CIGS layer is presented. The absorber layers are grown with a single-stage coevaporation process, and various KF post-deposition treatments (KF-PDT) are performed. The KF-PDT leads to an efficiency increase from 7% to 12%. For all cells an increase in open circuit voltage (Voc) and fill factor is measured, which is attributed to an improved pn junction. By changing the annealing conditions, an additional Voc increase is measured. This increase is attributed to the reduction of light-induced defects at the CIGS/CdS interface in addition to the improved pn junction. A reduction of defects is confirmed by reduced sub band gap emission in the photoluminescence spectra, an increased decay time, and increased quasi Fermi level splitting. With SCAPS the results are simulated, and it is concluded that after KF-PDT the Voc is limited to 640 mV due to recombination at the back contact. A higher Voc can then only be achieved by applying a ...

Highlights

Thin film Cu(In, Ga)Se2 (CIGS) solar cells are a well-established marketable thin film technology
This is especially interesting for thin CIGS layers, since the interfaces become more limiting for thinner absorber layers
Absorber layers are prepared for glow discharge optical emission spectroscopy (GDOES), grazing incidence XRD (GI-XRD) and PL measurements under cryogenic conditions, of which one part underwent KF post deposition treatments (KF-PDT) and another part only the post-anneal

Summary

Introduction

Thin film Cu(In, Ga)Se2 (CIGS) solar cells are a well-established marketable thin film technology. Making these elemental gradients in the absorber layer makes the process complicated to implement on large scale, and often several steps are required to achieve this Another explored road towards cost reduction is decreasing the thickness of the absorber layer. The most recent improvements are related to the heavier alkali like K and Rb, of which K has proven to be beneficial for the pn junction properties This is especially interesting for thin CIGS layers, since the interfaces become more limiting for thinner absorber layers. Initial results of KF-PDT on thin single-stage absorber layers are published by de Wild and Hsu has performed KF-PDT on sputtered CIGS layers, both showing improved device efficiency.. The main requirement for improvements upon K treatment seems to be the presence of a copper poor surface layer. even when K is applied before absorber layer growth and reduces the grain size, improvements are observed. the precise impact of K in CIGS is still under investigation

Objectives

Methods

Results

Conclusion