Cu 2 ZnSnS(e) 4 (CZTS(e)) solar cells have attracted much attention due to the elemental abundance and the nontoxicity. However, the record efficiency of 12.6% for Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells is much lower than that of Cu(In,Ga)Se 2 (CIGS) solar cells. One crucial reason is the recombination at interfaces. In recent years, large amount investigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods. This paper gives a review of progresses on interfaces of CZTS(e) solar cells, including: (i) the band alignment optimization at buffer/CZTS(e) interface, (ii) tailoring the thickness of MoS(e) 2 interfacial layers between CZTS(e) absorber and Mo back contact, (iii) the passivation of rear interface, (iv) the passivation of front interface, and (v) the etching of secondary phases.

The interfaces are very important for kesterite‐structured solar cells. In this study, a facile room temperature chemical sulfurization process is developed to modify the surface of the CZTSe films, which can prevent the decomposition of kesterite film at high temperature during the traditional sulfurization process and thus introducing the deep defects in the absorber layer. It is found that the (NH4)2S vapor sulfurizes the surfaces of the CZTSe thin films previously etched by ammonia. Consequently, the surface defects are passivated by the incorporated sulfur, and the interfacial band alignment is improved at the junction. The open circuit voltage (VOC) of the solar cell device is improved from 364 mV to 406 mV, with the cell efficiency increased to 9.04% when the (NH4)2S vapor treatment time is optimized at 25 min. This study affords a new perspective for enhancing the performance of kesterite‐based thin film solar cells using a facile surface sulfurization approach.

Cd-free kesterite structured solar cells are currently attracting attention because they are environmentally friendly. It is reported that Zn(O,S) can be used as a buffer layer in these solar cells. However, the band alignment is not clear and the carrier concentration of Zn(O,S) layer is low. In this study, the band alignment of the Zn(O,S)/Cu 2 ZnSnSe 4 p-n junction solar cell and the effect of In 2 S 3 /Zn(O,S) double buffer layer are studied by numerically simulation with wxAMPS software. By optimizing the band gap structure between Zn(O,S) buffer layer and Cu 2 ZnSnSe 4 absorber layer and enhancing the carrier concentration of Zn(O,S) layer, the device efficiency can be improved greatly. The value of CBO is in a range of 0 eV-0.4 eV for S/(S + O)= 0.6-0.8 in Zn(O,S). The In 2 S 3 is mainly used to increase the carrier concentration when it is used as a buffer layer together with Zn(O,S).

Zn(O,S) film is widely used as a Cd-free buffer layer for kesterite thin film solar cells due to its low-cost and eco-friendly characteristics. However, the low carrier concentration and conductivity of Zn(O,S) will deteriorate the device performance. In this work, an additional buffer layer of In 2 S 3 is introduced to modify the properties of the Zn(O,S) layer as well as the CZTSSe layer via a post-annealing treatment. The carrier concentrations of both the Zn(O,S) and CZTSSe layers are increased, which facilitates the carrier separation and increases the open circuit voltage (V OC). It is also found that ammonia etching treatment can remove the contamination and reduce the interface defects, and there is an increase of the surface roughness of the In 2 S 3 layer, which works as an antireflection layer. Consequently, the efficiency of the CZTSSe solar cells is improved by 24% after the annealing and etching treatments. Simulation and experimental results show that a large band offset of the In 2 S 3 layer and defect energy levels in the Zn(O,S) layer are the main properties limiting the fill factor and efficiency of these CZTSSe devices. This study affords a new perspective for the carrier concentration enhancement of the absorber and buffer layers by In-doping, and it also indicates that In 2 S 3 /Zn(O,S) is a promising Cd-free hybrid buffer layer for high-efficiency kesterite solar cells.

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