Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
J Am Chem Soc. 2014 Dec 10;136(49):17163-79. doi: 10.1021/ja509142w. Epub 2014 Dec 1.
Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.
黄铁矿 (FeS2) 被认为是一种很有前途的丰富地球元素半导体，具有实现太瓦级部署的潜力，可用于太阳能转换。然而，尽管已经付出了广泛的努力并取得了进展，但黄铁矿的太阳能转换效率仍低于 3%，主要原因是开路电压 (VOC) 较低。在这里，我们报告了对 {100} 面 n 型黄铁矿单晶的全面研究，以了解其令人费解的低 VOC。我们利用了电输运、光学光谱、表面光电压、水溶液和乙腈电解质中的光电化学测量、紫外和 X 射线光电子能谱以及 Kelvin 力显微镜，来表征体相和表面缺陷态及其对黄铁矿单晶半导体性质和太阳能转换效率的影响。这些见解被用于开发电化学阻抗谱的电路模型分析，从而可以对体相和表面缺陷态进行全面表征，并构建黄铁矿晶体的详细能带图。整体评估表明，高密度的本征表面态不能令人满意地解释低光电压；相反，高密度体深施主态的电离，可能是由体硫空位引起的，会产生非恒定的电荷分布和非常狭窄的表面空间电荷区，从而限制总势垒高度，从而可以很好地解释黄铁矿单晶的光电压有限和光电转换效率低的问题。这些发现为改善单晶黄铁矿和纳米晶或多晶黄铁矿薄膜以成功应用于太阳能提供了建议。
