Time-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence

Thomas Paul Weiss,Romain Carron,Max H Wolter,Johannes Löckinger,Enrico Avancini,Susanne Siebentritt,Stephan Buecheler,Ayodhya N Tiwari

doi:10.1080/14686996.2019.1586583

Abstract

ABSTRACT Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley–Read–Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation.

Highlights

Time-resolved photoluminescence (TRPL) is generally applied to measure the minority carrier lifetime of Cu(In,Ga)Se2 (CIGS)-based p-type semiconducting absorbers and a good correlation to device efficiency is observed [1,2,3,4,5]
The impact of the double-graded CIGS absorber on the temperature dependence of the PL decay time has been demonstrated
technology computer-aided design (TCAD) simulations were carried out, which show that the conduction band increase toward the front contact imposes a barrier for the excess electrons and, reduces front surface recombination

Summary

Introduction

Time-resolved photoluminescence (TRPL) is generally applied to measure the minority carrier lifetime of Cu(In,Ga)Se2 (CIGS)-based p-type semiconducting absorbers and a good correlation to device efficiency is observed [1,2,3,4,5]. Experimental evidence has been presented that trapping of minority charge carriers may impact these measured lifetimes, which results in measured effective lifetimes larger than the bulk lifetime [6,7]. These trapping states are incorporated into several simulation models presented in literature to describe experimental data [8,9]. The main experimental evidence for trapping is a temperature dependence of the measured photoluminescence (PL) decay times, which cannot be Supplemental data for this article can be accessed here. Redinger et al showed that a degradation of the absorbers surface might cause a strongly temperature dependent PL decay time, which can be suppressed when measuring in N2 atmosphere instead of ambient air [10] and the authors concluded that trapping is not present in their CIGS absorber

Results

Discussion

Conclusion