Leilei Chen

7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers

Cu2ZnSnS4 (CZTS) thin films with fine control over composition and pure phase were fabricated by sulfurization of co-electroplated Cu–Zn–Sn–S precursors. We have systematically investigated that the concentration of Cu(II) ions can influence the properties of CZTS absorber layers and the photovoltaic performance of the resulting solar cell devices. The results indicate that an increase in Cu(II) concentration almost linearly increases the Cu content in the final CZTS thin films, greatly enhances the (112) preferred orientation, significantly improves the crystallinity of the absorber layer, remarkably reduces the ZnS secondary phase, and hence improves their photovoltaic performance. However, a further increase in the Cu(II) concentration degrades the crystal quality of the absorber layer, and forms the CuSx secondary phase, which is quite detrimental to the device photovoltaic performance. Here we introduce a novel sputtered CdS buffer layer for the CZTS solar cells. For the first time, co-electrodeposited CZTS solar cells exceed the 7% efficiency threshold. These findings offer new research directions for solving persistent challenges of chemical bath deposition of CdS in CZTS solar cells.

Co-electrodeposited Cu2ZnSnS4 thin-film solar cells with over 7% efficiency fabricated via fine-tuning of the Zn content in absorber layers

A simple and cost-effective co-electrodeposition process has been demonstrated to fabricate high-performance Cu2ZnSnS4 (CZTS) photovoltaic materials with composition tunability and phase controllability. Here we report a systematic investigation of the effects of the Zn(II) concentration on the properties of CZTS thin films and thus the performance of the as-resulted solar cells. These results indicate that increasing the concentration of Zn(II) linearly increases the Zn content in the final composition of CZTS thin films, significantly improves the grain size and morphology of the absorber layers, and consequently improves their photovoltaic properties, especially the response to the medium wavelength. In contrast, further increase of the Zn(II) concentration degrades the crystal quality of the absorber layer, and more ZnS phase appears on the surface of the CZTS thin film, forming a rather rough morphology, which is harmful to the photovoltaic performance of the device. When the concentration of Zn(II) is optimized to 30 mM, a power conversion efficiency of 7.23% is achieved, which, to the best of our knowledge, is the highest efficiency for a co-electrodeposited CZTS solar cell with a sputtered CdS buffer layer to date. Our findings offer a promising alternative approach towards the industrialization of CZTS solar cell modules.

Synthesis and characterization of earth-abundant Cu2MnSnS4 thin films using a non-toxic solution-based technique

Earth-abundant Cu2MnSnS4 (CMTS) thin films were fabricated through a non-toxic spin-coating technique. The precursor solution is based on a 2-methoxyethanol solvated thiourea complex with acetyl-acetone used as an additive agent, and the spin-coated films were post-annealed at 570 °C under a N2 atmosphere. The influence of annealing time on the structure, composition, morphology, and optical properties of the processed precursor films has been studied in detail. We found that a longer annealing time during CMTS growth can improve the phase purity, promote the preferred orientation along the (112) direction, and enhance grain growth in the micrometer range. Film annealed for 10 min gives a pure CMTS phase, whereas other films annealed for lower and/or higher than 10 min (especially 13 min) can form secondary phases (i.e., SnS, MnS). The band gap energy is estimated as 1.63–1.18 eV for post-annealed films depending on the heat treatment, compared to 1.69 eV for as-prepared film. An efficiency of 0.49% for the device fabricated here has been achieved with an open-circuit voltage of 308.4 mV, a short-circuit current density of 4.7 mA cm−2, and a fill factor of 33.9%. It offers a new research direction for the application of a CMTS absorber layer in low-cost solar cells.