亚稳相纤锌矿铜锌锡硫(WZ-CZTS)纳米晶的合成及光伏应用的研究现状与进展

Synthesis of Metastable Phase Wurtzite Copper Zinc Tin Sulfide( WZ-CZTS)Nanocrystals and Their Photovoltaic Applications: a Review

直接带隙半导体化合物铜锌锡硫(Cu_2ZnSnS_4,CZTS)具有光吸收系数高、组成元素安全无毒且含量丰富、光电性能优异等优势,是发展绿色、低成本、高效率薄膜太阳电池的理想核心材料。前期的研究主要集中于锌黄锡矿相(Kesterite-type,KT-CZTS)在CZTS基薄膜太阳能电池中的应用。随着研究的深入,CZTS的光电转换效率取得了重大突破,但是距离其理论预期值-32.2%的光电转换效率仍然存在很大的差距。2011年制备出的纤锌矿相(Wutzite-type,WZ-CZTS),与KT-CZTS和黄锡矿相(Stannite-type,ST-CZTS)相比,其具有更高的载流子浓度和更低的电阻率,因此其在光伏器件中具有很大的潜在应用价值。然而,WZ-CZTS是一种亚稳相,当外界条件发生变化时,它很容易转变为稳定态结构的KT-CZTS。此外,化学计量比的少量偏离,就极易产生二元或三元等化合物杂相,因此如何减少和控制杂相的生成,制备出纯相、形貌可控的WZ-CZTS纳米晶体并将之应用于薄膜太阳能电池显得至关重要。最近几年,研究者们除了研究WZ-CZTS对器件性能的影响外,主要从选择合适的制备方法以及与之有关的有机溶剂、硫源、前驱体配比和优化薄膜制备工艺方面不断尝试,并取得了丰硕的成果。在充分发挥WZ-CZTS高的载流子浓度优势的同时大幅提升了器件效率。目前,以WZ-CZTS为薄膜太阳能电池吸收层材料制备的太阳能电池的转换效率已从最初的2.44%快速提高到6.0%。WZ-CZTS粉体的制备最初主要采用热注入法,但此法制备过程复杂、所用设备较为昂贵,且反应温度较高(≥240℃)。近两年,研究者们引入一锅法制备WZ-CZTS粉体,该法具有反应温度低、不需要惰性气体保护、工艺流程简单的优点,为WZ-CZTS粉体的大规模工业生产提供了可能。目前,WZ-CZTS作为太阳能电池吸收层薄膜的制备方法主要为纳米晶体油墨涂覆法,该法具有结晶度较好、缺陷少、工艺相对简单、制备成本低、易于实现规模化生产的优点,引起了国内外学者的广泛关注。本文对WZ-CZTS材料的结构特性、制备工艺、光电性质及应用进行了综述,重点分析了溶剂、硫源、前驱体等因素对其物相和形貌的影响。同时,对WZ-CZTS在光伏领域的应用进行了探讨。最后对WZ-CZTS目前存在的挑战和今后的研究方向进行总结,展望了其未来可能的突破方向。

The direct bandgap semiconductor compound copper zinc tin sulphide(Cu2ZnSnS4,CZTS)is an ideal core for the development of green,low-cost,high-efficiency thin-film solar cells due to their high optical absorption coefficient,safe and non-toxic components,rich content as well as excellent photoelectric performance.In the early stage,the research was mainly focused on the application of zinc-tin-tin phase(kesterite-type,KT-CZTS)in CZTS-based thin-film solar cells.With the deepening of the research,a major breakthrough has been made in the the photoelectric conversion efficiency,yet there is still a big gap between its theoretical expected vaule-32.2%.The wurtzite-type(WZ-CZTS)prepared in 2011 exhibited a higher carrier concentration and a lower resistivity than the KT-CZTS and stannite-type(ST-CZTS),therefore it has a great potential for application in photovoltaic devices.However,WZ-CZTS is attributed to a metastable phase,which is easy to transform into a stable structure-KT-CZTS when the external conditions change.Besides,a small deviation of the stoichiometric ratio is inclined to produce binary or ternary compound heterophases,thus reduce the formation of heterophases,and prepare the pure,morphology-controlled WZ-CZTS nanocrystals are crucial as well as apply them in thin film solar cells.In recent years,except for investigating the effects of WZ-CZTS on device performance,researchers have been continuously attempting to search for a suitable preparation methods and related organic solvents,sulfur sources,precursor ratios,and optimized thin film preparation processes.And so far fruitful results have been achieved.The device efficiency is drastically improved while the high carrier concentration advantage of WZ-CZTS is fully utilized.At present,the conversion efficiency of solar cells prepared by using WZ-CZTS as the thin film solar cell absor-ber layer has rapidly increased from the initial 2.44%to 6.0%.Initially the preparation of WZ-CZTS powder mainly adopted hot injection method,while the complex preparation process,expensive equipment and high reaction temperature(≥240℃)restricted its widely application.In the past two years,researchers introduced one-pot method to prepare WZ-CZTS powder.This method has a series of merits including low reaction temperature,no need for inert gas protection and simple process flow,thus making it possible for large-scale industrial production of WZ-CZTS powder.At present,the nano-crystal ink coating method is mainly used for preparing WZ-CZTS solar cell absorber layer film,which has a good crystallinity,fewer defects,relatively simple process,low preparation cost and easy realization of large-scale production,and has attracted widespread attention.In this paper,the structural characteristics,preparation process,photoelectric properties and application of WZ-CZTS are reviewed.The effects of solvent,sulfur source and precursor on the phase and morphology are highlighted.In addition,the application of WZ-CZTS in photovoltaic field is discussed.Finally,it summarizes the current challenges and future research directions of WZ-CZTS and looks forward to possible future breakthroughs.