Abstract: Cu(In,Ga)Se2 (CIGS) thin-film solar cell is the device with highest power conversion efficiency (~22.6%) of single junction. However, In and Ga in CIGS are scarce elements resource so that industrialization of CIGS solar cells are restricted. New material Cu2ZnSnS4 (CZTS) is very similar to CIGS in crystal structure and optoelectronic properties, both of them are the semiconductors with direct band gaps. CZTS thin film could replace CIGS absorber layer in CIGS solar cells, leading to a new CZTS thin-film solar cell. Adverse to the CIGS absorber, CZTS material is composed of earth-abundant non-toxic elements, only earth-abundant elements. Many studies indicate that CZTS solar cells with higher conversion efficiency and better stability could be fabricated by all solution-processing techniques. Therefore, CZTS thin-film solar cells should be low-cost environment-friendly industrialization-promising thin-film solar cells. CZTS solar cell is the same device structure as the CIGS cell, with the structure {SLG/Mo/CZTS/CdS/i-ZnO/n-ZnO}. At present, the CZTS cell with the highest conversion efficiency (~12.6%) are still used the CdS buffer layer in CIGS device, so that the industrialization processes and photovoltaic applications of CZTS cells could be faced to the danger of high-toxic heavy-metal Cd pollution. It is necessary to find Cd-free buffer materials to replace CdS for elimination of potential Cd pollution. Besids, compared to high efficiency {CIGS/CdS} cells, {CZTS/CdS} cells might not be optimized in band-alignment, so the conversion efficiency of CZTS cells is much worse than that of CIGS cells. New Cd-free buffer materials are required. When determining the new buffer materials, it must be taken into account the effect of the band-alignment effect of the {CZTS/new buffer layer} interface on device performance. There are 3 kinds of new materials for Cd-free buffer layers in CIGS and CZTS cells: sulfides, oxysulfide, oxides semiconductors. Their thin films could be prepared by chemical bath deposition (CBD) and other techniques. The selection of materials mostly depend on the conduction band offset on the interface between the material and CZTS or CIGS absorber, since the conduction band offset mostly affects the performance parameters of the cell. A large positive conduction band offset (spike) presents a barrier for minority carrier (electron) collection, reducing short-circuit current density Jsc. By contrast, negative offset (cliff) leads to increased recombination at the buffer-absorber interface, thereby reducing open-circuit voltage Voc. Ideally, the device would have a small 0—0.4 eV conduction-band offset spike, as found in CIGSSe devices employing a CdS buffer. In order to develop new buffer materials for the low-cost environment-friendly CZTS cells, the present paper reviews the development of Cd-free buffer materials for CZTS and CIGS cells. The selection conditions of Cd-free buffers, the properties and problems of some sulfides (such as ZnS and In2S3), oxysulfides (such as Zn(S,O), In(S,O,OH)) and oxides (such as ZnO, Zn1-xMgxOy, Zn1-xSnxOy)as the buffer in CZTS cell, especially their conduction band offsets, are discussed. For Se-contained CZTSSe devices, In2S3 and Zn(S,O) might be better for Cd-free buf-fer; for the more environment-friendly and low-cost all-sulfur CZTS devices, oxides Zn1-xMgxOy and Zn1-xSnxOy could be provided better buffer properties.
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2020, Vol. 34 
