Thin-film solar cells based on compound semiconductors consist of a multilayer structure with various interfaces and contain a multitude of elements and impurities, etc. A rapid progress of these photovoltaic technologies can only be achieved by an insight-driven optimization/development. Hence it is crucial to characterize and understand the relationship between the chemical and electronic properties of these components. This paper reviews some examples of our recent work characterizing compound semiconductor thin films using laboratory- and synchrotron-based electron and soft X-ray spectroscopic characterization methods. It is demonstrated how these different analytical techniques are extraordinarily powerful to reveal the material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of the related electronic and chemical properties. As examples, the paper will discuss the electronic surface structure of chalcopyrite thin-film solar cell absorbers, the chemical structure of the CdS/chalcopyrite interface, present the band alignment at the CdS/kesterite interface, and report on how post-deposition treatments cause chemical interaction/interdiffusion processes in CdTe/CdS thin-film solar cell structures. (C) 2012 Elsevier B.V. All rights reserved.

In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. Because of the high complexity of the Cu_2ZnSn(S,Se)_4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu_2ZnSnS_4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.

Air-exposed Cu2ZnSnS4 (“CZTS”) thin-film solar cell absorbers have been investigated by surface-sensitive x-ray photoelectron and x-ray-excited Auger electron spectroscopy, as well as by bulk-sensitive energy dispersive x-ray spectroscopy. We find a native surface oxidation of (mainly) tin, but also (to a lesser extent) of zinc and sulfur as well as evidence for a Cu-poor region at the surface of the absorber, best described by a Cu-free Zn-Sn-S surface layer.

The chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surface band gap increased from (1.53 ± 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 ± 0.15) eV.

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

The electronic structure of Cu2ZnSnS4 has been investigated by S L-2,L-3 x-ray emission and absorption spectroscopy. An empirical assignment of the most prominent spectral features of the valence and conduction band, respectively, can be made based on a comparison with spectra of corresponding binary compounds and with calculated densities of states. We find that the top of the valence band is dominated by hybridized antibonding Cu 3d-S 3p states, while the bottom of the conduction band is dominated by respective Sn 5s-S 3p states. We also determine the electronic band gap near the Cu2ZnSnS4 surface to be 1.5 (+/- 0.3) eV.

Multi-stage evaporation is a well-established method for the controlled growth of chalcopyrite thin films. To apply this technique to the deposition of Cu2ZnSnS4 thin films we investigated two different stage sequences: (A) using Cu2SnS3 as precursor to react with Zn-S and (B) using ZnS as precursor to react with Cu-Sn-S. Both Cu2SnS3 and ZnS are structurally related to Cu2ZnSnS4. In case (A) the formation of copper tin sulphide in the first stage was realized by depositing Mo/SnSx/CuS (1<x<2) and subsequent annealing. In the second stage ZnS was evaporated in excess at different substrate temperatures. We assign a significant drop of ZnS incorporation at elevated temperatures to a decrease of ZnS surface adhesion, which indicates a self-limited process with solely reactive adsorption of ZnS at high temperatures. In case (B) firstly ZnS was deposited at a substrate temperature of 150 degrees C. In the second stage Cu, Sn and S were evaporated simultaneously at varying substrate temperatures. At temperatures above 400 degrees C we find a strong decrease of Sn-incorporation and also a Zn-loss in the layers. The re-evaporation of elemental Zn has to be assumed. XRD measurements after KCN-etch on the layers prepared at 380 degrees C show for both sample types clearly kesterite, though an additional share of ZnS and Cu2SnS3 can not be excluded. SEM micrographs reveal that films of sample type B are denser and have larger crystallites than for sample type A, where the porous morphology of the tin sulphide precursor is still observable. Solar cells of these absorbers reached conversion efficiencies of 1.1% and open circuit voltages of up to 500 mV. (C) 2008 Elsevier B.V. All rights reserved.