Earth abundant copper-zinc-tin-chalcogenide (CZTSSe) is an important class of material for the development of low cost and sustainable thin film solar cells. The fabrication of CZTSSe solar cells by selenization of CZTS nanocrystals is presented. By tuning the composition of the CZTS nanocrystals and developing a robust film coating method, a total area efficiency as high as 7.2% under AM 1.5 illumination and light soaking has been achieved.

This work combines experiments and computer models in order to understand the relationships between electrode microstructure and ionic transport resistances so that one may predict cell performance from fundamental principles. A scanning electron microscope (SEM) with focused ion beam (FIB) was used to image sections of commercially made porous electrodes utilizing LiCoO2 active material. The images reveal the existence of discrete porous carbon domains in the microstructure. Further experiments indicated that these carbon domains are highly tortuous and restrict to a large degree the overall ion transport in the cathode. Two types of 3D models for correlating and predicting the electrode microstructure were explored. The first, known as the dynamic particle packing (DPP) model, is based on aggregates of spheres that move collectively in response to interparticle forces. The second is a stochastic grid (SG) model closely related to Monte Carlo techniques used in statistical physics to study cooperative and competitive phase behavior. The models use a small set of fundamental interdomain and bulk interaction parameters to generate structures from a given electrode mass composition and porosity. Both models were able to semi-quantitatively reproduce experimental tortuosity measurements of cathodes at different porosity values.

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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We present, for the first time, versatile solutions of concentrated selenium, within an array of amines, in a fast and low temperature manner without contaminants. These solutions allow the unprecedented opportunity to synthesize a variety of pure selenium and selenide nanoparticles as well as mixed chalcogen sulfoselenide compounds.