Grain Boundaries in Cu(In, Ga)Se₂: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

Abstract

Cu(In,Ga)Se 2 thin-film solar cells have attracted significant research interest in recent decades due to their high efficiency in converting solar energy into electricity for enabling a sustainable future. Although the Cu(In,Ga)Se 2 absorber can be grown as a single crystal, its polycrystalline form is dominating the market not only due to its lower costs, but also due to its unexpectedly higher cell efficiency. However, this absorber contains a high fraction of grain boundaries. These are structural defects where deep-trap states can be localized leading to an increase in recombination activity. This controversy is mirrored in the existing literature studies where two main contradictory believes exist: 1) to be crucial grain boundaries in Cu(In,Ga)Se 2 absorber are anomalous, being benign in terms of cell performance, and 2) grain boundaries are regions characterized by an increased recombination activity leading to deteriorated cell performance. Therefore, the present review tackles this issue from a novel perspective unraveling correlations between chemical composition of grain boundaries and their corresponding electronic properties. It is shown that features such as Cu depletion/In enrichment, segregation of 1-2at.% of alkali dopants, and passivation by a wide-bandgap or type inversion at grain boundaries are crucial ingredients for low open-circuit voltage loss and, hence, for superior cell performance.

Fingerprints Indicating Superior Properties of Internal Interfaces in Cu(In,Ga)Se2 Thin‐Film Solar Cells

“…Recently, we have implemented a parameter called the Cu factor (Δβ), [ 19 ] which is defined as the difference in composition changes (Δ = (composition at GB) − (composition in GI)) at a GB between Cu and Se + In + Ga: $$\[\Delta \beta = \Delta {\rm{Cu}} - (\Delta {\rm{Se}} + \Delta {\rm{In}} + \Delta {\rm{Ga}})\]$$…”

“…[35] GBs have been intensively studied for Cu-poor CIGS systems by various groups, and a recent comprehensive review summarizing all existing works can be found in ref. [19]. However, no publication exists yet on GBs in Cu-rich CIGS absorbers.…”

“…Recently, we have implemented a parameter called the Cu factor (∆β), [19] which is defined as the difference in composition changes (∆ = (composition at GB) − (composition in GI)) at a GB between Cu and Se + In + Ga:…”

“…Recent works have shown that not only the heterojunction, but also the grain boundaries (GBs) in CIGS absorbers play an essential role for the optoelectronic properties of the CIGS-based devices. [18,19] To date, various theories exist explaining the possible benign role of GBs in Cu-poor CIGS absorbers due to the passivation effect induced by alkalis [20][21][22][23][24][25][26] or by point defects. [27] However, not much is known about the GB properties of Cu-rich CIGS absorbers, which is essential for enhancing the performance of CIGS solar cells.…”

Correlative microscopy and techniques with atom probe tomography: Opportunities in materials science

“…Therefore, very recently, substantial efforts have been put forth to reveal if a correlation or a rather coincidence relationship exists between the composition and several materials properties, such as optical/electrical and structural properties. These efforts have led to novel methodologies called correlative EBIC-EBSD-APT, 24,25 where EBIC stands for electron-beam-induced current.…”