CZTS薄膜太阳能电池无镉缓冲层材料的研究进展

Development of Cd-free Buffer Materials for CZTS Thin-film Solar Cells

Cu(In,Ga)Se2(CIGS)薄膜太阳能电池是单结转换效率最高(~22.6%)的光伏器件,但In、Ga是稀缺元素,从而限制了CIGS电池的产业化。新型材料Cu 2ZnSnS 4(CZTS)是结构与光电性能均与CIGS十分相似的直接带隙半导体材料,它在CIGS器件结构中可替代CIGS吸收层,并得到新型CZTS薄膜太阳能电池。与CIGS相反,CZTS的原料丰富、无毒。大量研究表明,CZTS薄膜太阳能电池具有较高的转换效率和良好的稳定性,且可采用低成本、非真空的溶液法薄膜沉积技术来制造,因此CZTS器件是一种低成本、环境友好、极具产业化前景的薄膜太阳能电池。CZTS器件具有与CIGS器件一样的堆层结构{SLG/Mo/CZTS/CdS/i-ZnO/n-ZnO},目前转换效率最高(~12.6%)的CZTS器件仍沿用CIGS器件的CdS缓冲层,因而大规模生产与应用中存在高毒重金属镉污染的危险,寻找能替代CdS的无镉缓冲层材料来消除潜在的镉污染问题十分必要。此外,与高效率的{CIGS/CdS}器件相比,{CZTS/CdS}器件界面的能带匹配可能并不是最优,CZTS器件的转换效率还远不如CIGS器件,因此需要寻找新的无镉缓冲层材料。在确定新缓冲层材料时,必须考虑{CZTS/新缓冲层}界面的能级对齐效应。CIGS和CZTS器件的缓冲层新材料基本上可归纳为3种半导体材料:硫化物、硫氧化物、氧化物。这些材料的薄膜均可用化学浴(CBD)法等多种方法来制备。材料选取很大程度上取决于其与CZTS或CIGS吸收层接触所形成界面上的导带带阶情况,因为导带带阶对器件性能参数有很大的影响。大的正导带带阶(尖刺状带阶)对少子(电子)收集存在一个势垒而降低短路电流密度J sc;相反,负导带带阶(断崖状带阶)导致缓冲层与吸收层界面上的复合增大而降低了开路电压V oc;理想情况是器件有一个小(0~0.4 eV)的正导带带阶(尖刺状带阶),正如在使用CdS缓冲层的CIGSSe器件中所发现的那样。为了研发低成本、环境友好的CZTS电池器件的新型缓冲层材料,本文综述了CZTS和CIGS器件的无镉缓冲层材料的研究进展,讨论了无镉缓冲层材料的选用条件,以及多种硫化物(如ZnS和In2S3)、硫氧化物(如Zn(S,O)和In(S,O,OH))、氧化物(如ZnO、TiO2、Zn1-x MgxOy和Zn 1-x-SnxOy等)薄膜作为CZTS缓冲层的性能特点(特别是它们的导带带阶)以及存在的问题,探讨了其发展方向。对于含硒CZTSSe器件,In2S3、Zn(S,O)是良好的无镉缓冲层材料,而对于更环保、低成本的全硫CZTS器件,Zn1-xMgxOy和Zn1-xSnxOy可提供良好性能的缓冲层。

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 Cu 2ZnSnS 4(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 J sc.By contrast,negative offset(cliff)leads to increased recombination at the buffer-absorber interface,thereby reducing open-circuit voltage V oc.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-xMg xOy,Zn1-x SnxOy)as the buffer in CZTS cell,especially their conduction band offsets,are discussed.For Se-contained CZTSSe devices,In 2S3 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.