Development of low-bandgap (∼1.2 eV) Pb–Sn binary perovskites is exciting and has recently gained immense attention because of their high photovoltages, lowered Pb toxicity, and pivotal role in realizing perovskite tandem solar cells. Defect passivation in this class of perovskite alloys has immense potential to further reduce the photovoltage deficit but is relatively unexplored. Here, we investigate and report the passivation of defect sites in low-bandgap CH₃NH₃Pb_0.5Sn_0.5I₃ perovskite through the incorporation of fluoroalkyl-substituted fullerene (DF-C₆₀) via a graded heterojunction (GHJ) structure. Graded distribution of DF-C₆₀ successfully reduced the number of trap sites, and the resultant films had characteristically lower Urbach energy, dominant bimolecular recombination, and higher surface/bulk recombination resistance. The improved optoelectronic quality of films with GHJ structure was reflected in improved performance for corresponding photovoltaic devices, with the best PCE up to 15.61% and a remarkably high V_oc of 0.89 V. A V_oc of ∼92% of the Shockley–Queisser (SQ) limit achieved here is comparable to that of state-of-the-art inorganic technologies and is the best among perovskite solar cells (PVSCs) to date. Additionally, through stability studies, we find that though GHJ with DF-C₆₀ can slow down degradation due to moisture penetration, the oxidative susceptibility of Sn in binary perovskites sharply constraints overall stability.

中文翻译：

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低能隙Pb-Sn二元钙钛矿光伏电池中通过梯度富勒烯异质结的钝化钝化

低带隙（〜1.2 eV）的Pb-Sn二元钙钛矿的开发令人兴奋，由于其高的光电压，降低的Pb毒性以及在实现钙钛矿级联太阳能电池中的关键作用，最近受到了广泛的关注。在这类钙钛矿合金中，钝化缺陷具有巨大的潜力，可以进一步减少光电压的不足，但还没有得到充分的探索。在这里，我们调查并报告通过带隙异质结（GHJ）结构掺入氟烷基取代的富勒烯（DF-C ₆₀）的低带隙CH ₃ NH ₃ Pb _0.5 Sn _0.5 I ₃钙钛矿中的缺陷部位的钝化。DF-C _60的分级分布成功地减少了陷阱位点的数量，并且所得薄膜具有特征性的较低的Urbach能量，显性的双分子重组和较高的表面/本体重组抗性。具有GHJ结构的薄膜的光电质量提高反映在相应光伏器件的性能提高上，最佳PCE达到15.61％，V _oc达到0.89 V的显着高水平。Shockley-Queisser的V _oc约为〜92％（此处达到的SQ）极限与最先进的无机技术相当，并且是迄今为止钙钛矿太阳能电池（PVSC）中最好的。此外，通过稳定性研究，我们发现尽管GHJ与DF-C ₆₀ 可以减缓由于水分渗透而引起的降解，二元钙钛矿中锡的氧化敏感性极大地限制了整体稳定性。