Crystallization properties of Cu2ZnGeSe4

G Brammertz,T Kohl,J De Wild,M Meuris,B Vermang,J Poortmans

doi:10.1016/j.tsf.2018.12.015

Abstract

We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350 °C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu2ZnGeSe4, leading to small grains that have obtained their final size already after 3 min of selenization at 460 °C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu2ZnGeSe4, leading to a somewhat slower formation reaction and larger Cu2ZnGeSe4 grains, which obtain their final grain size only after 15 min of selenization at 460 °C.

Highlights

Thin film solar cells based on Kesterite absorbers have shown strong progress in conversion efficiencies in the past decade, with Sn-based Kesterite solar cells showing conversion efficiencies well in excess of 10 %. [1,2,3,4,5,6]
We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks
Studying the X-ray diffraction (XRD) spectra of the absorber at different times during the selenization process allowed to get insight into the crystallization reaction that happens during the selenization process

Summary

Introduction

Thin film solar cells based on Kesterite absorbers have shown strong progress in conversion efficiencies in the past decade, with Sn-based Kesterite solar cells showing conversion efficiencies well in excess of 10 %. [1,2,3,4,5,6]. Physical characterization has shown that it crystallizes readily in the Kesterite crystal structure [13,14] and shows a band gap in the 1.4 to 1.5 eV range [15,16] whereas recently solar cells with a conversion efficiency of about 6 % have been presented using a solution based process [17] and a vacuum process [18]. In this contribution we study the properties of CZGSe fabricated by using a two-step selenization process. The solar cell properties of the optimized CZGSe absorber layer are discussed in detail, trying to identify the causes limiting the conversion efficiency in these devices

Sample fabrication details

Crystallization study 2-step selenization

Properties of the optimized layer stack

Conclusions