Voigt Bryan, Moore William, Manno Michael, Walter Jeff, Jeremiason Jeff D, Aydil Eray S, Leighton Chris
Department of Chemical Engineering and Materials Science , University of Minnesota , Minneapolis , Minnesota 55455 , United States.
Department of Physics and Astronomy , Carleton College , Northfield , Minnesota 55057 , United States.
ACS Appl Mater Interfaces. 2019 May 1;11(17):15552-15563. doi: 10.1021/acsami.9b01335. Epub 2019 Apr 22.
Pyrite FeS has long been considered a potential earth-abundant low-cost photovoltaic material for thin-film solar cells but has been plagued by low power conversion efficiencies and open-circuit voltages. Recent efforts have identified a lack of understanding and control of doping, as well as uncontrolled surface conduction, as key roadblocks to the development of pyrite photovoltaics. In particular, while n-type bulk behavior in unintentionally doped single crystals and thin films is speculated to arise from sulfur vacancies (V), proof remains elusive. Here, we provide strong evidence, from extensive electronic transport measurements on high-quality crystals, that V are deep donors in bulk pyrite. Otherwise identical crystals grown via chemical vapor transport under varied S vapor pressures are thoroughly characterized structurally and chemically, and shown to exhibit systematically different electronic transport. Decreased S vapor pressure during growth leads to reduced bulk resistivity, increased bulk Hall electron density, reduced transport activation energy, onset of positive temperature coefficient of resistivity, and approach to an insulator-metal transition, all as would be expected from increased V donor density. Impurity analyses show that these trends are uncorrelated with metal impurity concentration and that extracted donor densities significantly exceed total impurity concentrations, directly evidencing a native defect. Well-controlled, wide-range n-doping of pyrite is thus achieved via the control of V concentration, with substantial implications for photovoltaic and other applications. The location of the V state within the gap, the influence of specific impurities, unusual aspects to the insulator-metal transition, and the influence of doping on surface conduction are also discussed.
黄铁矿（FeS）长期以来一直被认为是一种潜在的储量丰富、成本低廉的用于薄膜太阳能电池的光伏材料，但一直受到低功率转换效率和开路电压的困扰。最近的研究发现，对掺杂缺乏理解和控制以及表面传导不受控制是黄铁矿光伏发展的关键障碍。特别是，虽然推测无意掺杂的单晶和薄膜中的n型体行为源于硫空位（V），但证据仍然难以捉摸。在这里，我们通过对高质量晶体进行广泛的电子输运测量提供了有力证据，表明V是块状黄铁矿中的深施主。通过化学气相传输在不同S蒸气压下生长的其他条件相同的晶体，在结构和化学上进行了全面表征，并显示出系统地不同的电子输运。生长过程中S蒸气压的降低导致体电阻率降低、体霍尔电子密度增加、输运活化能降低、电阻率正温度系数的出现以及接近绝缘体-金属转变，所有这些正如V施主密度增加所预期的那样。杂质分析表明，这些趋势与金属杂质浓度无关，并且提取的施主密度显著超过总杂质浓度，直接证明了一种本征缺陷。因此，通过控制V浓度实现了对黄铁矿的良好控制、宽范围的n型掺杂，这对光伏和其他应用具有重要意义。还讨论了V态在能隙中的位置、特定杂质的影响、绝缘体-金属转变的异常方面以及掺杂对表面传导的影响。
