Grain Boundaries in Cu(In, Ga)Se<sub>2</sub>: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy
Oana Cojocaru‐Mirédin, Mohit Raghuwanshi, Roland Wüerz, Sascha Sadewasser
Abstract
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.