Effects of KF and RbF post deposition treatments on the growth of the CdS buffer layer on CIGS thin films - a comparative study

Tim Kodalle,Léo Choubrac,Ludovic Arzel,Rutger Schlatmann,Nicolas Barreau,Christian A Kaufmann

doi:10.1016/j.solmat.2019.109997

Abstract

In this contribution we are analyzing and comparing the impact of two different alkali-fluorine post deposition treatments (KF and RbF) on the growth of chemical-bath-deposited CdS buffer layers on Cu(In,Ga)Se2 absorber layers for thin film solar cells. By combining Raman scattering, scanning electron microscopy, current-voltage analysis and measurements of the internal quantum efficiency we provide a comprehensive picture of this issue on the material and device level. We find that both PDTs lead to a better CdS-coverage of the surface of the CIGS, which leads to an improved junction quality at early growth stages compared to untreated devices. Furthermore the growth rate of the CdS is enhanced on KF-treated absorber layers while it is decreased on those treated with RbF (compared to the reference). This leads to a more stable behavior of RbF-treated devices after longer duration of the CdS deposition, while the KF-treated devices suffer from reduced fill factor and open circuit voltage. Furthermore we show that not only both PDTs but also the growth of the CdS lead to a reduction of the amount of the so called ordered defect compound, which is initially present at the surface of our absorber layers. This behavior indicates either the formation of CdCu -anti-sites or of a secondary phase at the interface.

Highlights

By combining Raman scattering, scanning electron microscopy, current-voltage analysis and measurements of the internal quantum eciency we provide a comprehensive picture of this issue on the material and device level
Since most of the latest progress in increasing the eciency of solar cells based on Cu(In, Ga)(Se, S)2 (CIGSSe) absorber layers was achieved by application of an alkali-uoride post deposition treatment (PDT) using KF, RbF or CsF [1][2][3], a lot of recent studies are focusing on the mechanisms involved during the PDTs [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
We carried out a combinatory Raman scattering and scanning electron microscopy (SEM) study to investigate the PDT-induced changes on a fundamental level

Summary

Introduction

Since most of the latest progress in increasing the eciency of solar cells based on Cu(In, Ga)(Se, S) (CIGSSe) absorber layers was achieved by application of an alkali-uoride post deposition treatment (PDT) using KF, RbF or CsF [1][2][3], a lot of recent studies are focusing on the mechanisms involved during the PDTs [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. There is one study in which the authors could show that an alkali-PDT leads to a better coverage of the Cu(In, Ga)Se2 (CIGS) by the subsequently deposited CdS in early growth stages as well as to a faster growth rate of the CdS [16] The latter result was already mentioned in another study investigating the impact of a KF-PDT on CIGS grown on exible substrates [1]. To the Preprint submitted to Elsevier best of our knowledge no publication systematically investigating and comparing the eect of dierent PDTs on the growth of the CdS is published so far Targeting this issue, we present a comparison of the impact of an RbFPDT and a KF-PDT on the CdS growth. We prepared devices with dierent thicknesses of the CdS layer to combine those results with the optoelectronic properties of the nished solar cells

Methods

Discussion

Conclusion