Optically induced metastability in Cu(In,Ga)Se2

Abstract

Cu(In,Ga)Se2 (CIGS) is presently the most efficient thin-film photovoltaic technology with efficiencies exceeding 22%. An important factor impacting the efficiency is metastability, where material changes occur over timescales of up to weeks during light exposure. A previously proposed (V Se -V Cu) divacancy model presents a widely accepted explanation. We present experimental evidence for the optically induced metastability transition and expand the divacancy model with first-principles calculations. Using photoluminescence excitation spectroscopy, we identify a sub-bandgap optical transition that severely deteriorates the carrier lifetime. This is in accordance with the expanded divacancy model, which predicts that states below the conduction band are responsible for the metastability change. We determine the density–capture cross-section product of the induced lifetime-limiting states and evaluate their impact on device performance. The experimental and theoretical findings presented can allow assessment of metastability characteristics of leading thin-film photovoltaic technologies.

Electronic defects in Cu(In,Ga)Se2 : Towards a comprehensive model

“…This double vacancy has been predicted to present a shallow donor state [146]. This donor state was confirmed by recent GW calculations [147]. The defects related to the V Se vacancy are metastable, however [79,148].…”

Large metastability in Cu (In,Ga)Se2 devices: The importance of buffer properties

“…Many published studies have documented the behavior of metastable defects in the CIGS absorber . It is generally accepted that these defects act in two ways, typically both beneficial, when a device is light soaked.…”

“…Voltage metastability in CIGS devices A, for different buffer types with mean value marked as "X" and B, as a function of light soaking time for a typical sample set made on one absorber piece 4 | ABSORBER VARIATIONS Many published studies have documented the behavior of metastable defects in the CIGS absorber. 3,10,11,[32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] It is generally accepted that these defects act in two ways, typically both beneficial, when a device is light soaked. In the bulk, light soaking puts defects into an acceptor state, moving the Fermi level toward the valence band edge and thus increasing the built-in potential.…”

A short review on the advancements in electroplating of CuInGaSe2 thin films

“…Other defects such as copper interstitial (C i ), selenium vacancies (V Se ), and (V Se -In Cu ) di-vacancy defect complex are considered as origin of metastable-related effects such as persistent photoconductivity (PPC) and red-blue illuminations. During electrical bias or illumination, this di-vacancy complex (V Se -V Cu ) can shift from a donor into an acceptor configuration in p-type CIGS, increasing the hole concentration and thus acts as a recombination channel for the minority charge carriers (electrons) [27,28]. The prevalence of these intrinsic defects and compensations is considered to be the origin of potential fluctuations in the material.…”