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.

We report a total-area power conversion efficiency of 15% for a copper indium gallium disulfoselenide (CIGSSe) solar cell fabricated from a copper indium gallium disulfide (CIGS) nanoparticle ink based process. Careful optimization of the fabrication process has resulted in a significant improvement in efficiency compared to our previously reported efficiency of 12%. This efficiency ranks among the highest reported in the literature for solution processed CIGSSe. Despite having an absorber thickness of approximately 700-800 nm, which is less than half the thickness of high efficiency devices grown by both coevaporation and solution processes in the literature, our devices show good short-circuit current (32.1 mA/cm 2 ). Surprisingly, the sintered film shows lateral composition fluctuations, which have not been reported for other high efficiency devices and may be responsible for the lower open circuit voltage (636 mV) observed here. This suggests an avenue for further improvement through optimization of the nanoparticle selenization process to better control composition in the sintered film.

Detailed quantum efficiency (QE) analysis of a nanoparticle-based Cu2ZnSnSe4 (CZTSe) solar cell has been conducted to understand photogenerated carrier collection in the device. Specifically, voltage-dependent analysis has been considered to characterize both diffusion limitations and recombination limitations to carrier collection. Application of a generalized QE model and corresponding experimental and analytical procedures are presented to account for non-ideal device behavior, with specific consideration of photogenerated charge trapping, finite absorber thickness, back-surface recombination, and recombination of photogenerated carriers via interface, space-charge-region limited, and/or band tail limited recombination mechanisms. Analysis of diffusion limited collection results in extraction of the minority carrier diffusion length, mobility, back surface recombination velocity, and absorption coefficient. Additionally, forward bias QE measurements afford analysis of the dominant recombination mechanism for photogenerated carriers. For the analyzed CZTSe device, diffusion limitations are not expected to play a significant role in carrier collection in forward bias. However, voltage-dependent carrier collection, previously identified to contribute to open-circuit voltage limitations, is attributed to high recombination rates via band tail states/potential fluctuations in forward bias. A consideration of the assumptions commonly applied to diffusion length, band gap, and band tail extraction is also discussed.

A simple solution-based approach for the deposition of Cu 2 ZnSn(S,Se) 4 using an amine−thiol mixture is presented. The versatility of this solvent mixture in dissolving different cation sources and chalcogens opens the door for a variety of metal chalcogenide molecular precursor designs. The process involves incorporating the metal sources and chalcogens into an amine−thiol solvent mixture at room temperature, spin coating a precursor film, and heat-treating precursor films in both an inert gas and selenium atmosphere. With this solution approach, high-quality kesterite CZTS and CZTSSe thin films were formed after low-temperature annealing and selenization. Solar cells were fabricated based on these CZTSSe thin films, resulting in a total area power conversion efficiency of 7.86% for a cell area of 0.47 cm 2 under standard AM 1.5 illumination.

Detailed electrical characterization of nanoparticle based Cu2ZnSn(SxSe1−x)4 (CZTSSe) and Cu2Zn(SnyGe1−y)(SxSe1−x)4 (CZTGeSSe) solar cells has been conducted to understand the origin of device limitations in this material system. Specifically, temperature dependent current-voltage analysis has been considered, with particular application to the characterization of solar cells with non-ideal device behavior. Due to the presence of such non-ideal device behavior, typical analysis techniques—commonly applied to kesterite-type solar cells—are found to be insufficient to understand performance limitations, and an analysis methodology is presented to account for the non-idealities. Here, the origin of non-ideal device behavior is chiefly considered in terms of electrostatic and band gap potential fluctuations, low minority carrier lifetimes, temperature dependent band edges, high surface/bulk recombination rates, and tunneling enhanced recombination. For CZTSSe and CZTGeSSe, the main limitations to improved device performance (voltage limitations) are found to be associated with significant EA deficits (EA–EG) at 300 K, large ideality factors, and voltage-dependent carrier collection, which we associate with the bulk material properties of the absorbers. The material origin of these non-ideal electrical properties is considered. Additionally, for CZTGeSSe, the effect of Ge-incorporation on the electrical properties of the solar cells is discussed, with improvements in the electrical properties characterized for the Ge-alloyed devices.

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Probe-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the inter-and intraparticle compositional inhomogeneity of multinary Cu 2 ZnSnS 4 (CZTS) nanoparticles. CZTS nanoparticles were prepared following three distinct synthesis protocols described in the literature. Strong fluctuations in composition were observed for Cu and Zn in individual nanoparticles, independent of the synthesis method. Certain particles have regions that have compositions close to that of Cu 2 SnS 3 , as well as, in the extreme case, the presence of nearly pure ZnS species. This is an observation that has not been reported in prior studies of these systems and underscores the need to both more carefully study the polydispersity of multinary semiconductor nanoparticles (MSNs) and to improve synthetic protocols and characterization of MSNs. Notablydespite the observation of compositional fluctuations in individual nanoparticlesreactive sintering in Se vapor was shown to reduce the nanoscale compositional fluctuations in the resulting sintered grains, facilitating the use of these heterogeneous particles in optoelectronic devices.

a Real-time energy dispersive x-ray diffraction (EDXRD) analysis has been utilized to observe the selenization of Cu-Zn-Sn-S nanoparticle films coated from three nanoparticle populations: Cu-and Sn-rich particles roughly 5 nm in size, Zn-rich nanoparticles ranging from 10 to 20 nm in diameter, and a mixture of both types of nanoparticles (roughly 1:1 by mass), which corresponds to a synthesis recipe yielding CZTSSe solar cells with reported total-area efficiencies as high as 7.9%. The EDXRD studies presented herein show that the formation of copper selenide intermediates during the selenization of mixedparticle films can be primarily attributed to the small, Cu-and Sn-rich particles. Moreover, the formation of these copper selenide phases represents the first stage of the CZTSSe grain growth mechanism. The large, Zn-rich particles subsequently contribute their composition to form micrometer-sized CZTSSe grains. These findings enable further development of a previously proposed selenization pathway to account for the roles of interparticle heterogeneities, which in turn provides a valuable guide for future optimization of processes to synthesize high quality CZTSSe absorber layers.