aIn polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron-hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se 2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se 2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of -partially electronically active -planar defects in the {112} planes. Real-time X-ray diffraction
Broader contextThe development of thin-film solar cells has been a success story in recent years in terms of record efficiencies in the lab. Single junction solar cells based on compound semiconductor films have reached higher energy-conversion efficiencies than polycrystalline silicon. Despite this success and the prospects of novel applications such as flexible, lightweight solar panels, the market share of thin-film solar modules is stagnating. A major problem of compound thin-film solar cells, such as Cu(In,Ga)Se 2 , is the large gap between lab efficiencies and commercial module efficiencies. A large process parameter space makes trial-and-error optimization a time-consuming and expensive task. Therefore, understanding the underlying atomic-scale physics and chemistry is essential to identify the potential origins of efficiency losses in the transfer from lab-to large-scale fabrication. Even though Cu(In,Ga)Se 2 has been investigated for several decades, there is still a lack of fundamental knowledge of the quality-determining mechanisms during film growth. In this contribution we present results from an international collaboration that provides direct insight into defect formation and annihilation during the fabrication of Cu(In,Ga)Se 2 films. Consequences for process optimization and design are proposed. The presented approach can also be applied to understand other thin-film fabrication processes.