The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.

The selenization of Cu-Zn-Sn-S nanocrystals is a promising route for the fabrication of low-cost thin film solar cells. However, the reaction pathway of this process is not completely understood. Here, the evolution of phase formation, grain size, and elemental distributions is investigated during the selenization of Cu-Zn-Sn-S nanoparticle precursor thin films by synchrotron-based in situ energy-dispersive X-ray diffraction and fluorescence analysis as well as by ex situ electron microscopy. The precursor films are heated in a closed volume inside a vacuum chamber under presence of selenium vapor while diffraction and fluorescence signals are recorded. The presented results reveal that during the selenization the cations diffuse to the surface to form large grains on top of the nanoparticle layer and the selenization of the film takes place in two simultaneous reactions: 1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu 2 ZnSnSe 4 ; 2) a slower selenization of the remaining nanoparticles. As a consequence of the initial formation of copper selenides at the surface, the subsequent formation of CZTSe starts under Cu-rich conditions despite an overall Cu-poor composition of the film. The implications of this process path on the film quality is discussed. Additionally, the proposed growth model provides an explanation of the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.

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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.