Nanocrystal‐based Cu2Zn(SnyGe1‐y)(SxSe4‐x) (CZTGeSSe) thin‐film solar cell absorbers with tunable band gap have been prepared. Maximum solar‐conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu2ZnSn(SxSe4‐x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the absorber film during the high‐temperature selenization treatment, although some Ge loss from the absorber surface is still observed following this step. Despite limitations imposed by elemental losses present at the absorber surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage‐dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.

Thin‐film solar cells using Cu2ZnSn(S,Se)4 absorber materials continue to attract increasing attention. The synthesis of kesterite Cu2ZnSnS4 nanoparticles by a modified method of hot injection is explained. Characterization of the nanoparticles by energy dispersive X‐ray spectroscopy, X‐ray diffraction, Raman, and transmission electron microscopy is presented and discussed. When suspended in an ink, coated, and processed into a device, the nanoparticles obtained by this synthesis achieve a total area (active area) efficiency of 9.0% (9.8%) using AM 1.5 illumination and light soaking. This improvement over the previous efficiency of 7.2% is attributed to the modified synthesis approach, as well as fine‐tuned conditions for selenizing the coated nanoparticles into a dense absorber layer. Copyright © 2014 John Wiley & Sons, Ltd.

Large-grain absorber formation through selenization techniques is a promising route for high performance chalcogenide solar cells. Understanding and subsequently controlling such grain growth is essential in improving absorber quality and developing absorbers with unique optoelectronic and morphological properties. We explain the essential role of liquid selenium in the grain growth of Cu 2 ZnSnSe 4 (CZTSe) absorbers from Cu 2 ZnSnS 4 nanoparticles by proposing a liquid-assisted grain growth mechanism. Through the use of a multizone rapid-thermalprocessing furnace, control of liquid Se delivery to the film and the Se (g) atmosphere during processing is shown to result in novel absorbers with tunable properties. Additionally, the processing parameters necessary for high quality CZTSe absorbers, the role of nanoparticle properties, and the role of alkali metal dopants in the liquid-assisted growth mechanism are shown. Ultimately, record nanoparticle-based device performance of 9.3% is achieved for selenized CZTSe absorbers.

Wurtzite-derived copper−zinc−tin sulfide nanoparticle films were observed to undergo a phase transformation to a kesterite phase when exposed to Se vapor at 500 °C. The resulting dense and selenized Cu 2 ZnSn(S,Se) 4 (CZTSSe) films were found to have the same bilayer kesterite structure as absorber layers derived directly from kesterite Cu 2 ZnSnS 4 (CZTS) nanoparticles (Guo, Q.

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