Fangzhou Zhou

Kesterite Cu2ZnSnS4 thin film solar cells by a facile DMF-based solution coating process

Kesterite Cu2ZnSnS4 (CZTS) thin films were fabricated using a low-cost and environmentally friendly route from a dimethylformamide (DMF) solution of a metal–thiourea complex. Thermal gravimetric analysis (TGA) has been performed to reveal the thermal decomposition behavior of the CZTS precursors for drying and sulfurization process design. A facile solution method of in situ introducing sodium dopant by adding NaOH into the precursor solution is presented. The sodium dopant improves the open circuit voltage (Voc) and fill factor (FF) and thereby enhances the power conversion efficiency from 4.47% to 5.68%. The enhanced performance is related to the increased grain size and increased minority carrier lifetime. A large number of large voids observed in the bulk absorber and at the absorber/back contact interface are considered to be the main reason for the low short circuit current density (Jsc).

Exploring the application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells by forming nearly micron-sized large grains

An innovative approach to overcome the main challenge of solution-based pure-sulfide Cu2ZnSnS4 thin film solar cells by sulfurizing quaternary Cu2ZnSnS4 nanocrystals into nearly micron-sized large grains in a few minutes is presented. We developed an efficient phase-transition-driven grain growth strategy to explore the application of metastable wurtzite Cu2ZnSnS4 nano-materials in the photovoltaic field. The obtained Cu2ZnSnS4 thin film has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. Clear variations of phase, morphology, and component redistribution of the Cu2ZnSnS4 thin film were identified after the sulfurization process, which is critical to get a dense large-grained Cu2ZnSnS4 layer from quaternary nanocrystals. By tuning the composition of the wurtzite Cu2ZnSnS4 nanocrystals, annealing conditions, and sodium-containing compound, laboratory-scale photovoltaic cells with 4.83% efficiency were demonstrated without anti-reflection coatings. These results suggest the potential application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells. This unique approach may also open up new opportunities to other optoelectronic devices, such as CuIn(S,Se)2, Cu2(In,Ga)Se4, CdTe, and Cu2ZnGe(S,Se)4 solar cells.

Lattice-matched Cu2ZnSnS4/CeO2 solar cell with open circuit voltage boost

We report a reproducible enhancement of the open circuit voltage in Cu2ZnSnS4 solar cells by introduction of a very thin CeO2 interlayer between the Cu2ZnSnS4 absorber and the conventional CdS buffer. CeO2, a non-toxic earth-abundant compound, has a nearly optimal band alignment with Cu2ZnSnS4 and the two materials are lattice-matched within 0.4%. This makes it possible to achieve an epitaxial interface when growing CeO2 by chemical bath deposition at temperatures as low as 50 °C. The open circuit voltage improvement is then attributed to a decrease in the interface recombination rate through formation of a high-quality heterointerface.