Cu2ZnSnSe4Cu2ZnSnSe4 solar cell absorbers spin-coated from amine-containing ether solutions

A non-vacuum, solution-based method to deposit thin layers of Cu2ZnSnSe4Cu2ZnSnSe4 (CZTSe) as absorber layer for kesterite-based thin film solar cells was investigated. The concept is based on multiple spin-coated layers of ether solutions containing Cu, Zn, and Sn salts as precursors and monoethanolamine (MEA) as stabilizer, which are subsequently converted into CZTSe layers by annealing in selenium atmosphere. The influence of initial metal ratios and the number of spin-coated layers on the composition and homogeneity of the CZTSe layers was studied. Both zinc and, to a smaller extent, tin contents tend to decrease in selenized CZTSe layers, because of the partial evaporation of their volatile compounds during the selenization process. Using optimized metal ratios in the precursor solution, single phase kesterite layers of 1–2μm thickness were obtained. Increasing the number of spin-coating cycles caused the formation of “bubbles” between the CZTSe and the Mo layer, as a result of the compressive stress originating from the volume expansion during the selenization step. SEM micrographs and EDX compositional measurements of cross-sections also revealed a thin carbon-rich layer at the Mo back contact/CZTSe interface, which nevertheless provided a sufficient electrical contact between the metal contact and the absorber. The optical energy gap of the kesterite absorber determined from a quantum efficiency measurement is about 0.9 eV. Solar cells with efficiencies of up to 2.76%, with VocVoc of 381 mV, JscJsc of 15.8 mA/cm2 and fill factor of 42.1% were achieved for copper poor and zinc rich CZTSe absorbers.

► CZTSe layers obtained by spin-coating metal salt solutions and annealing in selenium. ► Metal ratios change during annealing in selenium atmosphere. ► CZTSe layers exhibit kesterite phase as confirmed by XRD, EDX, and Raman. ► Solar cell efficiency of 2.76% achieved despite a residual carbon layer.