The internal resistance and quality of the interface in Cu 2 ZnSn(S, Se) 4 (CZT(S, Se)) solar cells were investigated using electrochemical impedance spectroscopy (EIS). We developed an equivalent circuit of the CZT(S, Se) solar cell involving the contact and material resistances R bulk , two parallel resistances, and a 'capacitance-like element' called a constant phase element (CPE) around the CZT(S, Se)/CdS heterojunction and Mo-Mo(S, Se) 2 /CZT(S, Se) back-contact interfaces. Using the EIS method, the states of Mo(S, Se) and the CZT(S, Se) layer could be digitised non-destructively. This result can provide guidelines for improving the performance of CZT(S, Se) solar cells or any semiconductor device.

A low-temperature colloid synthesis approach was applied to synthesize Cu2ZnSn(S,Se)4 (CZTSSe) nanoparticles for the first time. This method produced gram quantities of material with a chemical yield in excess of 90% within a short synthesis time. Average sizes of 80 nm nanoparticles with narrow diameter distribution were achieved using Na2S with Na2Se as a chalcogenide source. The formation reaction of kesterite CZTSSe fabricated from synthesized nanoparticles was investigated, and the results showed that the sufficiently high partial pressures of Sn with Se during the annealing process were important for the fabrication of high-quality CZTSSe films.

We characterized the microstructure of Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells fabricated from Cu-poor, Zn-rich Cu 2 ZnSnSe 4 (CZTSe) nanoparticles. Various sintering atmospheres of sulfur and selenium led to different microstructural properties. For the samples sintered under sulfur atmosphere, a large number of ZnS secondary phases were observed and the Zn/Sn ratio of the grains was significantly lower than 1. Regions of the ZnS secondary phases showed a dark contrast in electron-beam-induced current (EBIC) images, indicating that the existence of the ZnS phases reduced the minority carrier collection efficiency. In contrast, for the samples sintered under selenium atmosphere, Cu was found to accumulate near the grain boundaries to form the Cu-rich CZTSe phases and only a small number of ZnSe secondary phases were detected.

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