Observation of an Internal p–n Junction in Pyrite FeS2 Single Crystals: Potential Origin of the Low Open Circuit Voltage in Pyrite Solar Cells,ACS Materials Letters

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Observation of an Internal p–n Junction in Pyrite FeS2 Single Crystals: Potential Origin of the Low Open Circuit Voltage in Pyrite Solar Cells
ACS Materials Letters ( IF 9.6 ) Pub Date : 2020-06-22 , DOI: 10.1021/acsmaterialslett.0c00207
Bryan Voigt ₁ , William Moore ₁ , Moumita Maiti ₁ , Jeff Walter _{1,

2} , Bhaskar Das ₁ , Michael Manno ₁ , Chris Leighton ₁ , Eray S. Aydil _{1,

3}

Affiliation

Pyrite FeS₂ has long been considered a potentially ideal photovoltaic material, but solar cells utilizing pyrite exhibit low open-circuit voltages (V_OC) and have failed to achieve conversion efficiencies >3%. The recent discovery of a conductive p-type surface layer on n-type pyrite single crystals raises the intriguing possibility that the low V_OC results from a leaky internal p–n junction between the surface and interior. Here, we reveal this internal junction, for the first time, through horizontal electronic transport measurements on sulfur vacancy (V_S)- and Co-doped n-type pyrite single crystals. We observe a steep increase in resistance upon cooling heavily V_S-doped crystals below ∼200 K, as the dominant charge transport crosses over from interior to surface conduction. The frequently employed two-resistor equivalent circuit model for lightly-doped pyrite crystals cannot reproduce this steep rise, but it can be accounted for, quantitatively, and with high fidelity, by adding an internal Schottky junction resistance between the surface and the interior. The average extracted Schottky barrier height is 320 meV (varying from 130–560 meV), significantly below expectations from band bending calculations (>750 meV) but similar in magnitude to V_OC values reported for pyrite heterojunction solar cells. This internal p–n junction is thus implicated as the potential origin of the long-standing low-V_OC problem in pyrite.

中文翻译：

硫铁矿FeS ₂单晶体中内部p–n结的观察：黄铁矿太阳能电池中低开路电压的潜在来源

黄铁矿FeS ₂长期以来一直被认为是一种潜在的理想光伏材料，但是利用黄铁矿的太阳能电池显示出低开路电压（V _OC），并且未能实现> 3％的转换效率。最近在n型黄铁矿单晶上发现导电p型表面层的可能性增加了一个有趣的可能性，即低V _OC是由表面和内部之间的内部p–n泄漏连接引起的。在这里，我们首次通过水平电子传输测量硫空位（V _S）和Co掺杂的n型黄铁矿单晶。当主要的电荷传输从内部传导到表面传导时，我们观察到在将重掺杂V _S的晶体冷却至约200 K以下时，电阻会急剧增加。轻掺杂黄铁矿晶体常用的两电阻等效电路模型无法再现这种陡峭的上升，但是可以通过在表面和内部之间增加内部肖特基结电阻来定量地，高保真地解决。提取的肖特基势垒平均高度为320 meV（在130–560 meV之间变化），大大低于带弯曲计算的期望值（> 750 meV），但幅度与V _OC相似硫铁矿异质结太阳能电池的报道值。因此，该内部PN结被认为是黄铁矿中长期存在的低V _OC问题的潜在根源。

更新日期：2020-07-06

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