Small (∼5 nm), Cu- and Sn-rich nanoparticles play a key role in initiating the growth of micrometer-sized Cu2ZnSn(S,Se)4 grains.

The search for alternative earth abundant semiconducting nanocrystals for sustainable energy applications has brought forth the need for nanoscale syntheses beyond bulk synthesis routes. Of particular interest are metal phosphides and derivative I–V–VI chalcogenides including copper phosphide (Cu3P) and copper thiophosphate (Cu3PS4). Herein, we report a one-pot, solution-based synthesis of Cu3P nanocrystals utilizing an in situ phosphorus source: phosphorus pentasulfide (P2S5) in trioctylphosphine. By injecting this phosphorus source into a copper solution in oleylamine, uniform and size controlled Cu3P nanocrystals with a phosphorous-rich surface are synthesized. The subsequent reaction of the Cu3P nanocrystals with decomposing thiourea forms nanoscale Cu3PS4 particles having p-type conductivity and an effective optical band gap of 2.36 eV. The synthesized Cu3PS4 produces a cathodic photocurrent during photoelectrochemical measurements, demonstrating its application as a light-absorbing material. Our process creates opportunities to explore other solution-based metal-phosphorus systems and their subsequent sulfurization for earth abundant, alternative energy materials.

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.

To understand and control the growth paths of kesterite based CZTSe films as prepared from kesterite nanoparticles we investigate films prepared from different chalcogenide ratios in the initial nanoparticles. To do so we introduce a new method for producing kesterite nanocrystals with controlled ratios of sulfur and selenium. Final CZTSe devices are prepared from these nanoparticles with total area solar power conversion efficiencies up to and exceeding eight percent (see Fig. 1). All devices made from selenizing nanoparticles that contained sulfur showed efficiency greater than six percent while pure selenide nanoparticles resulted in non-performing devices.

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 in the 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 through two simultaneous reactions: (1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu2ZnSnSe4; and (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 for the film quality are discussed. Additionally, the proposed growth model provides an explanation for the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.

Three-stage coevaporated Copper Indium Gallium diSelenide (CIGSe) solar cells have resulted in the highest efficiency thin-film devices[1]. This success is attributable to copper selenide formation during stage two, promoting grain growth[2]. Herein is the application of secondary copper selenide phases enhancing solar cells made from suspended CIGSe nanoparticles. Larger grains and higher efficiencies result when compared to devices of only CIGSe nanoparticles. Utilizing cupric selenide nanoparticles leads to the production of 4.3% total area power conversion efficiency under AM1.5 simulated radiation. Whereas, in absence of secondary copper selenide phase efficiencies of the CIGSe film is low from poor grain growth.

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. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3579996] All rights reserved.

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.

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