Energy Level Modulation in Diboron Modified SnO2 for High Efficiency Perovskite Solar Cells

英文原文与中文译文逐段对照 · English original with Chinese translation

Energy level modulation in diboron modified ${\mathrm{{SnO}}}_{2}$ for high efficiency perovskite solar cells

用于高效钙钛矿太阳能电池的二硼改性${\mathrm{{SnO}}}_{2}$中的能级调制

Nengxu Li, Xiuxiu Niu, Fengtao Pei, Huifen Liu, Yang Cao, Yuquan Liu, Haipeng Xie,

李能旭，牛秀秀，裴风涛，刘慧芬，曹洋，刘玉泉，谢海鹏

Yongli Gao, Qi Chen, Fanyang Mo*, Huanping Zhou*

高永利，陈琦，莫凡洋*，周欢萍*

N. Li, F. Pei, H. Liu, Prof. H. Zhou

N. 李，F. 裴，H. 刘，H. 周教授

Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials & Key

先进电池材料理论与技术北京市重点实验室及

Laboratory of Polymer Chemistry and Physics of Ministry of Education

教育部高分子化学与物理重点实验室

Department of Materials Science and Engineering

材料科学与工程系

College of Engineering

工程学院

Peking University

北京大学

Beijing 100871, P. R. China

中国北京100871

happy

X. Niu, Prof. Q. Chen

牛秀秀，陈琦教授

Department of Materials Science and Engineering

材料科学与工程系

Beijing Institute of Technology

北京理工大学

Beijing 100081, P. R. China

中国北京100081

Y. Cao, Prof. F. Mo

曹洋，莫凡洋教授

Department of Energy and Resources Engineering

能源与资源工程系

College of Engineering

工程学院

Peking University

北京大学

Beijing 100871, P. R. China

中国北京100871

Y. Liu, H. Xie, Prof. Y. Gao

刘Y，谢H，高Y教授

Hunan Key Laboratory for Super-microstructure and Ultrafast Process

湖南省超微结构与超快过程重点实验室

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/solr.201900217

本文已被接受发表并经过全面同行评审，但尚未经过编辑、排版、分页和校对过程，这可能导致本版本与记录版本之间存在差异。请将本文引用为doi: 10.1002/solr.201900217

Collage of Physics and Electronics

物理与电子学院

Central South University

中南大学

Changsha, Hunan 410012, P.R. China.

中国湖南长沙410012

Abstract

摘要

Energy level modulation between perovskite and carrier transport layers to obtain a promoted carrier extraction and reduced charge recombination is an effective way for high efficiency perovskite solar cells. Here, we employ diboron as an effective interfacial modifier between ${\mathrm{{SnO}}}_{2}$ and perovskite. By taking advantage of the higher Femi level on the surface of ${\mathrm{{SnO}}}_{2}$ after diboron treatment, we obtain a power conversion efficiency of 22.04% in a solar cell device based on two-step solution processed planar n-i-p structure. With the help of thorough characterizations, we argue that the diboron treated ${\mathrm{{SnO}}}_{2}$ possesses some ${\mathrm{{Sn}}}^{3 + }$ species, which serve as electron donors with more n-type nature, promoting electron extraction and reducing carrier recombination in ETL/perovskite interface. Further analysis speculate that the formation of surface diboron-oxygen Lewis pair induces a reducing state of diboron complexes, resulting the spontaneous electron redistribution and the formation of ${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$ species. This provides an effective chemical approach to tune the energy alignment between the oxide ETL and absorber.

在钙钛矿和载流子传输层之间进行能级调制以促进载流子提取并减少电荷复合，是实现高效钙钛矿太阳能电池的有效途径。在此，我们采用二硼作为${\mathrm{{SnO}}}_{2}$和钙钛矿之间的有效界面改性剂。通过利用二硼处理后${\mathrm{{SnO}}}_{2}$表面较高的费米能级，我们在基于两步溶液处理的平面n-i-p结构的太阳能电池器件中获得了22.04%的功率转换效率。借助全面的表征，我们认为经二硼处理的${\mathrm{{SnO}}}_{2}$具有一些${\mathrm{{Sn}}}^{3 + }$物种，它们作为具有更多n型性质的电子供体，促进电子提取并减少ETL/钙钛矿界面处的载流子复合。进一步分析推测，表面二硼-氧路易斯对的形成诱导了二硼配合物的还原态，导致自发的电子重新分布并形成${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$物种。这提供了一种有效的化学方法来调节氧化物ETL和吸收体之间的能量排列。

1. Introduction

1. 引言

In recent years, organic-inorganic halide perovskites have emerged as exciting new materials for optoelectronic devices such as solar cells ${}^{\left\lbrack 1\right\rbrack }$ , light emitting diodes (LEDs) ${}^{\left\lbrack 2\right\rbrack }$ , and photodetectors ${}^{\left\lbrack 3\right\rbrack }$ , mainly due to their remarkable properties, such as strong light absorption ${}^{\left\lbrack 4\right\rbrack }$ , superb charge carrier mobility ${}^{\left\lbrack 5\right\rbrack }$ , and low-cost fabrication ${}^{\left\lbrack 6\right\rbrack }$ . Power conversion efficiency (PCE) of perovskite solar cells (PSCs) have risen from 3.8% to a certified 24.2% ${}^{\left\lbrack 4,7\right\rbrack }$ , which is close to the state-of-the-art crystalline silicon (c-Si) solar cells. In the context of PCE, earlier work focus on the perovskites film quality, including component engineering ${}^{\left\lbrack 1c\right\rbrack }$ , fabrication optimization ${}^{\left\lbrack 8\right\rbrack }$ , and defects passivation ${}^{\left\lbrack 9\right\rbrack }$ . Meanwhile, the carrier transport layers (CTLs) are also intensively investigated. Taking the planar n-i-p PSCs as an example, tin dioxide (SnO ${}_{2}$ ) is widely used as electron transport layer (ETL) due to its high bulk mobility (up to ${240}{\mathrm{\;{cm}}}^{2} \; {\mathrm{V}}^{-1}\;{\mathrm{\;s}}^{-1}$ ) and suitable energy level ${}^{\left\lbrack {10}\right\rbrack }$ , 2,27,7’-tetrakis(N, N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) is one of the most adaptable hole transport layer (HTL) thanks to its suitable highest occupied molecular orbital (HOMO) energy level, excellent hole-extraction and transportation ability [11]. In addition, the interface between perovskite/ETL and perovskite/HTL are also crucial to the carrier extraction and transportation ${}^{\left\lbrack {12}\right\rbrack }$ . On one hand, the interface often induce defect formation and migration ${}^{\left\lbrack {13}\right\rbrack }$ , which lead to the observed current-voltage hysteresis and long-term device instability ${}^{\left\lbrack {14}\right\rbrack }$ . On the other hand, energy level mismatching at the interfaces will lead to non-effective charge carrier injection and excess recombination due to the carrier accumulation ${}^{\left\lbrack {15}\right\rbrack }$ . How to reduce the defects existed at the interface, and obtain a more favourable energy level alignment between perovskite and carrier transport layer are urgently needed to obtain better optoelectronic properties of PSCs.

近年来，有机无机卤化物钙钛矿已成为太阳能电池${}^{\left\lbrack 1\right\rbrack }$、发光二极管(LED)${}^{\left\lbrack 2\right\rbrack }$和光电探测器${}^{\left\lbrack 3\right\rbrack }$等光电器件中令人兴奋的新型材料，主要是由于它们具有显著的特性，如强光吸收${}^{\left\lbrack 4\right\rbrack }$、出色的载流子迁移率${}^{\left\lbrack 5\right\rbrack }$和低成本制造${}^{\left\lbrack 6\right\rbrack }$。钙钛矿太阳能电池(PSC)的功率转换效率(PCE)已从3.8%提高到经认证的24.2%${}^{\left\lbrack 4,7\right\rbrack }$，接近最先进的晶体硅(c-Si)太阳能电池。在PCE方面，早期的工作集中在钙钛矿薄膜质量上，包括组分工程${}^{\left\lbrack 1c\right\rbrack }$、制造优化${}^{\left\lbrack 8\right\rbrack }$和缺陷钝化${}^{\left\lbrack 9\right\rbrack }$。同时，载流子传输层(CTL)也受到了深入研究。以平面n-i-p PSC为例，二氧化锡(SnO${}_{2}$)因其高体迁移率(高达${240}{\mathrm{\;{cm}}}^{2} \; {\mathrm{V}}^{-1}\;{\mathrm{\;s}}^{-1}$)和合适的能级${}^{\left\lbrack {10}\right\rbrack }$而被广泛用作电子传输层(ETL)，2,27,7’-四(N, N-二对甲氧基苯胺)-9,9’-螺二芴(spiro-OMeTAD)由于其合适的最高占据分子轨道(HOMO)能级、出色的空穴提取和传输能力，是最适用的空穴传输层(HTL)之一[11]。此外，钙钛矿/ETL和钙钛矿/HTL之间的界面对于载流子提取和传输也至关重要${}^{\left\lbrack {12}\right\rbrack }$。一方面，界面常常会诱导缺陷的形成和迁移${}^{\left\lbrack {13}\right\rbrack }$，这会导致观察到的电流-电压滞后和器件长期不稳定性${}^{\left\lbrack {14}\right\rbrack }$。另一方面，界面处的能级不匹配会由于载流子积累导致无效的电荷载流子注入和过多的复合${}^{\left\lbrack {15}\right\rbrack }$。如何减少界面处存在的缺陷，并在钙钛矿和载流子传输层之间获得更有利的能级排列，对于获得更好的PSC光电性能来说是迫切需要的。

Both defects passivation and energy level alignment at the interface have been reported in recent work. For example, organic halide salt phenethylammonium iodide (PEAI) are formed on the perovskite surface to boost the efficiency of PSCs, which may passivate the iodine vacancy defects existed at the perovskite/HTL interface ${}^{\left\lbrack {16}\right\rbrack }$ ; Poly(vinylcarbazole) (PVCz)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are also used as interfacial passivation layer between perovskite and ${\mathrm{{TiO}}}_{2}$ , suppressing the carrier recombination at ${\mathrm{{TiO}}}_{2}$ /perovskite interface ${}^{\left\lbrack {17}\right\rbrack }$ . In addition, some strategies are also explored to modulate the Femi-level $\left( {\mathrm{E}}_{\mathrm{F}}\right)$ of ${\mathrm{{SnO}}}_{2}$ , such as using ethylene diamine tetraacetic acid (EDTA) treated ${\mathrm{{SnO}}}_{2}$ to obtain a better energy match with the conduction band of perovskite, eliminating the charge accumulation at the perovskite/ETL interface ${}^{\left\lbrack {18}\right\rbrack }$ . Moreover, a compositional gradient modulation at the rear interface between the perovskite and HTLs can improve the hole collection and reduce charge recombination, resulting a higher efficiency PSCs ${}^{\left\lbrack {19}\right\rbrack }$ . All the optimization indicate that the defects elimination and energy level modulation of CTLs/perovskite interface are the effective means to obtain high efficiency solar cells, although the profound mechanism of energy band modification between ${\mathrm{{SnO}}}_{2}$ and perovskite is still not clear.

近期的研究工作中均报道了界面处的缺陷钝化和能级对齐。例如，在钙钛矿表面形成有机卤化物盐苯乙铵碘(PEAI)以提高PSC的效率，这可能会钝化钙钛矿/HTL界面处存在的碘空位缺陷${}^{\left\lbrack {16}\right\rbrack }$；聚(乙烯咔唑)(PVCz)/[6,6]-苯基-C61-丁酸甲酯(PCBM)也被用作钙钛矿与${\mathrm{{TiO}}}_{2}$之间的界面钝化层，抑制${\mathrm{{TiO}}}_{2}$/钙钛矿界面处的载流子复合${}^{\left\lbrack {17}\right\rbrack }$。此外，还探索了一些策略来调节${\mathrm{{SnO}}}_{2}$的费米能级$\left( {\mathrm{E}}_{\mathrm{F}}\right)$，例如使用乙二胺四乙酸(EDTA)处理${\mathrm{{SnO}}}_{2}$以与钙钛矿的导带获得更好的能量匹配，消除钙钛矿/ETL界面处的电荷积累${}^{\left\lbrack {18}\right\rbrack }$。此外，在钙钛矿与HTLs之间的后界面处进行成分梯度调制可以改善空穴收集并减少电荷复合，从而得到更高效率的PSC${}^{\left\lbrack {19}\right\rbrack }$。所有这些优化表明，CTLs/钙钛矿界面的缺陷消除和能级调制是获得高效太阳能电池的有效手段，尽管${\mathrm{{SnO}}}_{2}$与钙钛矿之间的能带修改的深层机制仍不清楚。

Here we report an effective interface engineering approach by using the diboron $\left( {{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}}\right)$ as the interfacial modifier between ${\mathrm{{SnO}}}_{2}$ and perovskite. Based on the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment, we obtain a higher ${\mathrm{E}}_{\mathrm{F}}$ on the surface of ${\mathrm{{SnO}}}_{2}$ (meaning a more n-type nature), resulting a better energy matching at ETL/perovskite interface and a higher PCE of 22.04% with negligible hysteresis in planar n-i-p perovskite solar cells. The most visible improvement is found to be open-circuit voltage (Voc), which is mainly due to the higher carrier extraction and suppressed charge recombination at the ${\mathrm{{SnO}}}_{2}$ /perovskite interface. More importantly, we demonstrate that the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ possesses some ${\mathrm{{Sn}}}^{3 + }$ species, acting as electron donors to obtain a more n-type nature of ETLs. Finally, we speculate that the formation of surface diboron-oxygen Lewis pair induces a reducing state of diboron complexes, which further enables spontaneous electron redistribution for the formation of ${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$ species on the surface of ${\mathrm{{SnO}}}_{2}$ . This strategy could be extended to other similar molecules that can be used in optoelectronic devices. those for the BSO film are -7.93 eV and -4.14 eV, respectively, which are in agreement with previous report ${}^{\left\lbrack {20}\right\rbrack }$ . Although SO and BSO films exhibit the similar band structure, the BSO film shows a more n-type nature with the higher ${\mathrm{E}}_{\mathrm{F}}$ , which means its energy level matched well with perovskite materials, facilitating the electron extraction ability of ${\mathrm{{SnO}}}_{2}{}^{\left\lbrack {21}\right\rbrack }$ . In addition, the ${\mathrm{E}}_{\mathrm{F}}$ , VBM, Eg of perovskite film which fabricated by a typical two-step solution process are shown in Figure 1c, and the corresponding band structure is shown in Figure 1b. Compared the ${\mathrm{E}}_{\mathrm{F}}$ of perovskite, $\mathrm{{SO}}$ and $\mathrm{{BSO}}$ , the smaller gap between perovskite and $\mathrm{{BSO}}$ indicates the better band alignment between them and less energy loss while transporting carriers. As a result, ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ modified ${\mathrm{{SnO}}}_{2}$ , as an excellent n-type semiconductor, are more suitable to be the ETL of PSCs. of substrate that can influence the grain size of perovskite ${}^{\left\lbrack {21}\right\rbrack }$ , we therefore measured the contact angle of N, N-Dimethylformamide (DMF) on the top of the SO and BSO films. The obtained values are ${8}^{ \circ }$ for SO and ${28}^{ \circ }$ for BSO film, respectively, as shown in Figure S2, Supporting Information. This implies that the BSO film has lower wettability of DMF solvent, thus fewer nucleation sites are possessed, resulting in a reduced nucleation rate during perovskite formation. Hence, by fixing the growth rate, the slower nucleation rate based on BSO substrate results in a relatively larger grain size in the perovskite film, which is consistent with other reports ${}^{\left\lbrack {22}\right\rbrack }$ .

在此，我们报告了一种有效的界面工程方法，即使用二硼$\left( {{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}}\right)$作为${\mathrm{{SnO}}}_{2}$和钙钛矿之间的界面改性剂。基于${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理，我们在${\mathrm{{SnO}}}_{2}$表面获得了更高的${\mathrm{E}}_{\mathrm{F}}$(意味着更具n型性质)，从而在ETL/钙钛矿界面实现了更好的能量匹配，并在平面n-i-p钙钛矿太阳能电池中实现了22.04%的更高功率转换效率(PCE)，滞后现象可忽略不计。最明显的改善是开路电压(Voc)，这主要归因于在${\mathrm{{SnO}}}_{2}$/钙钛矿界面处更高的载流子提取和抑制的电荷复合。更重要的是，我们证明了经过${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$具有一些${\mathrm{{Sn}}}^{3 + }$物种，作为电子供体，使ETL具有更n型的性质。最后，我们推测表面二硼-氧路易斯对的形成诱导了二硼配合物的还原态，这进一步使得电子能够自发重新分布，从而在${\mathrm{{SnO}}}_{2}$表面形成${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$物种。这种策略可以扩展到可用于光电器件的其他类似分子。对于BSO薄膜，其值分别为-7.93 eV和-4.14 eV，这与先前的报告${}^{\left\lbrack {20}\right\rbrack }$一致。尽管SO和BSO薄膜表现出相似的能带结构，但BSO薄膜具有更高的${\mathrm{E}}_{\mathrm{F}}$，表现出更n型的性质，这意味着其能级与钙钛矿材料匹配良好，有利于${\mathrm{{SnO}}}_{2}{}^{\left\lbrack {21}\right\rbrack }$的电子提取能力。此外，通过典型的两步溶液法制备的钙钛矿薄膜的${\mathrm{E}}_{\mathrm{F}}$、价带顶(VBM)、带隙(Eg)如图1c所示，相应的能带结构如图1b所示。比较钙钛矿、$\mathrm{{SO}}$和$\mathrm{{BSO}}$的${\mathrm{E}}_{\mathrm{F}}$，钙钛矿和$\mathrm{{BSO}}$之间较小的带隙表明它们之间的能带排列更好，并且在载流子传输过程中的能量损失更小。因此，经过${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$改性的${\mathrm{{SnO}}}_{2}$作为一种优异的n型半导体，更适合作为PSC的ETL。由于能够影响钙钛矿${}^{\left\lbrack {21}\right\rbrack }$晶粒尺寸的衬底，我们因此测量了N，N-二甲基甲酰胺(DMF)在SO和BSO薄膜顶部的接触角。如图S2(支持信息)所示，获得的值对于SO薄膜为${8}^{ \circ }$，对于BSO薄膜为${28}^{ \circ }$。这意味着BSO薄膜对DMF溶剂的润湿性较低，因此具有较少的成核位点，导致在钙钛矿形成过程中的成核速率降低。因此，在固定生长速率的情况下，基于BSO衬底的较慢成核速率导致钙钛矿薄膜中晶粒尺寸相对较大，这与其他报告${}^{\left\lbrack {22}\right\rbrack }$一致。

Figure 1. a) Device structure of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ modified planar-heterojunction PSCs. b) Energy band diagram of ${\mathrm{{SnO}}}_{2}$ (SO), ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ (BSO) and perovskite (PVSK) thin films. c) The secondary electron cut-off region of ultraviolet photoelectron spectroscopy (UPS) spectra of SO, BSO and PVSK films. d) The valence band region of UPS spectra of SO, BSO and PVSK films. e) The relationship of (dn) ${}^{2}$ vs energy for SO, BSO and PVSK films.

图1. a) ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$修饰的平面异质结PSC的器件结构。b) ${\mathrm{{SnO}}}_{2}$(SO)、${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理后的${\mathrm{{SnO}}}_{2}$(BSO)和钙钛矿(PVSK)薄膜的能带图。c) SO、BSO和PVSK薄膜的紫外光电子能谱(UPS)的二次电子截止区域。d) SO、BSO和PVSK薄膜的UPS光谱的价带区域。e) SO、BSO和PVSK薄膜的(dn) ${}^{2}$与能量的关系。

The planar heterojunction PSCs are used in this study, whose device structure is tin-doped indium oxide (ITO)/ ${\mathrm{{SnO}}}_{2}/{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}/$ perovskite/Spiro-OMeTAD/Au (Figure 1a). When the ${\mathrm{{SnO}}}_{2}$ thin film is treated with ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ (for convenience, we denote it as BSO, detailed fabrication methods see the Supporting Information), the Fermi level $\left( {\mathrm{E}}_{\mathrm{F}}\right)$ of BSO shows an obvious up-shift compared to the reference (untreated ${\mathrm{{SnO}}}_{2}$ , we denote it as SO), as shown in Figure 1c. It can be found that the ${\mathrm{E}}_{\mathrm{F}}$ of the SO and BSO are -4.35 and -4.18 eV, respectively. In addition, the corresponding valence band maximum (VBM) are located at 3.64 and ${3.75}\mathrm{{eV}}$ below the ${\mathrm{E}}_{\mathrm{F}}$ (Figure 1d), respectively. In combination with the band gap (Eg) acquired from the UV-visible spectra (Figure 1e, 3.83 eV for SO, 3.79 eV for SBO), we then obtain the energy levels of the SO and BSO samples which are shown in Figure 1b. The VBM and conduction band minimum (CBM) of SO film are -7.99 eV and -4.16 eV, while

本研究中使用了平面异质结PSC，其器件结构为氧化铟锡(ITO)/${\mathrm{{SnO}}}_{2}/{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}/$钙钛矿/Spiro-OMeTAD/金(图1a)。当${\mathrm{{SnO}}}_{2}$薄膜用${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理时(为方便起见，我们将其表示为BSO，详细的制备方法见支持信息)，与参考样品(未处理的${\mathrm{{SnO}}}_{2}$，我们将其表示为SO)相比，BSO的费米能级$\left( {\mathrm{E}}_{\mathrm{F}}\right)$显示出明显的上移，如图1c所示。可以发现，SO和BSO的${\mathrm{E}}_{\mathrm{F}}$分别为-4.35和-4.18 eV。此外，相应的价带最大值(VBM)分别位于${\mathrm{E}}_{\mathrm{F}}$以下3.64和${3.75}\mathrm{{eV}}$处(图1d)。结合从紫外可见光谱获得的带隙(Eg)(图1e，SO为3.83 eV，SBO为3.79 eV)，我们得到了SO和BSO样品的能级，如图1b所示。SO薄膜的VBM和导带最小值(CBM)分别为-7.99 eV和-4.16 eV，而

Figure 2. a, b, c, d) The top-view field emission scanning electron microscopy (FE-SEM) images; e) The UV-visible absorption spectra; f) The X-ray diffraction (XRD) patterns; g) The steady-state photoluminescence (PL) spectra; h) The time-resolved photoluminescence (TRPL) spectra of perovskite films which fabricated on $0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}},{0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ films, respectively. The scale bar of SEM images is ${1\mu }\mathrm{m}$ .

图2. a、b、c、d) 顶视图场发射扫描电子显微镜(FE-SEM)图像；e) 紫外可见吸收光谱；f) X射线衍射(XRD)图谱；g) 稳态光致发光(PL)光谱；h) 分别在$0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}},{0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理后的${\mathrm{{SnO}}}_{2}$薄膜上制备的钙钛矿薄膜的时间分辨光致发光(TRPL)光谱。SEM图像的比例尺为${1\mu }\mathrm{m}$。

As the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ is treated on the top of ${\mathrm{{SnO}}}_{2}$ , we then attempted to understand how the BSO film affects the properties of perovskite films. To investigate this, the top-view field emission scanning electron microscopy (FE-SEM) measurement on the sample are firstly performed. It can be found that BSO based perovskite films exhibit slightly larger grain size than that of reference (SO based perovskites), regardless of the concentration of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ , as shown in Figure 2a-d. While the morphology of ${\mathrm{{SnO}}}_{2}$ and ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ have no significant difference (Figure S1, Supporting Information). In consideration of the wettability

由于在${\mathrm{{SnO}}}_{2}$顶部处理了${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$，我们接着试图了解BSO薄膜如何影响钙钛矿薄膜的性能。为了研究这一点，首先对样品进行了顶视图场发射扫描电子显微镜(FE-SEM)测量。可以发现，无论${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的浓度如何，基于BSO的钙钛矿薄膜的晶粒尺寸都比参考样品(基于SO的钙钛矿)略大，如图2a - d所示。而${\mathrm{{SnO}}}_{2}$和${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理后的${\mathrm{{SnO}}}_{2}$的形貌没有显著差异(图S1，支持信息)。考虑到润湿性

In addition, UV-visible spectra of the SO and BSO film fabricated on ITO-glass is displayed in the Figure S3, Supporting Information. The transmittance of them are almost the same, indicating that ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ has no extra self-absorbance. The absorbance of the perovskite films which fabricated on SO/BSO are also measured, as shown in Figure 2e, confirming that ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment has no significant optical influence on perovskite films. X-ray diffraction (XRD) patterns of perovskites are displayed in Figure 2f, and the peak intensity of perovskite phase is enhanced by increasing the concentration of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ , mainly due to the larger grain size induced by BSO based substrate. Moreover, neither shifts in the peaks, nor new peaks are observed in the XRD patterns (Figure S4, Supporting Information), indicating that BSO substrate does not alter the crystal structure of the perovskite films. We further performed the Fourier-transform infrared spectroscopy (FTIR) measurements to detect the diboron species. The result is shown in Figure S5, Supporting Information. When taking diboron powder as a reference, diboron thin film which deposited on quartz substrate and annealed at ${70}^{ \circ }\mathrm{C}$ for 5 min also exhibit similar signals. This results demonstrate the existence of diboron species after annealing process, which may play an important role on the interfacial modification.

此外，在ITO玻璃上制备的SO和BSO薄膜的紫外可见光谱如图S3(支持信息)所示。它们的透过率几乎相同，表明${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$没有额外的自吸收。还测量了在SO/BSO上制备的钙钛矿薄膜的吸光度，如图2e所示，证实${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理对钙钛矿薄膜没有显著的光学影响。钙钛矿的X射线衍射(XRD)图谱如图2f所示，通过增加${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的浓度，钙钛矿相的峰强度增强，这主要是由于基于BSO的衬底诱导的更大晶粒尺寸。此外，在XRD图谱中(图S4，支持信息)既未观察到峰的移动，也未观察到新峰，表明BSO衬底不会改变钙钛矿薄膜的晶体结构。我们进一步进行了傅里叶变换红外光谱(FTIR)测量以检测二硼物种。结果如图S5(支持信息)所示。以二硼粉末为参考，沉积在石英衬底上并在${70}^{ \circ }\mathrm{C}$退火5分钟的二硼薄膜也表现出类似的信号。这些结果证明了退火过程后二硼物种的存在，这可能在界面改性中起重要作用。

To clearly compare the electron extraction ability of SO and BSO film, we further performed the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements (Figure 2g-h) based on the samples structure: ITO/SO/perovskite or ITO/BSO/perovskite. The PL intensity of BSO based perovskite is reduced significantly compared to SO based perovskite, ascribed to the more efficient extraction of photo-generated electrons from the perovskite to the ${\mathrm{{SnO}}}_{2}{}^{\left\lbrack {23}\right\rbrack }$ . In addition, the faster fluorescence decay (Table

为了清楚地比较SO和BSO薄膜的电子提取能力，我们基于样品结构ITO/SO/钙钛矿或ITO/BSO/钙钛矿进一步进行了稳态光致发光(PL)和时间分辨光致发光(TRPL)测量(图2g - h)。与基于SO的钙钛矿相比，基于BSO的钙钛矿的PL强度显著降低，这归因于光生电子从钙钛矿到${\mathrm{{SnO}}}_{2}{}^{\left\lbrack {23}\right\rbrack }$的更有效提取。此外，基于BSO的钙钛矿样品的更快荧光衰减(表

S1, Supporting Information) for the BSO based perovskite sample indicates that ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment could further enhance the interfacial electron extraction.

S1，支持信息)表明${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理可以进一步增强界面电子提取。

Figure 3. a) The open-circuit voltage (Voc); b) The short-circuit current (Jsc); c) The fill factor (FF); d) The power conversion efficiency (PCE) of perovskite solar cells which fabricated on $0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}}$ , ${0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ films, respectively. Each condition contains 14 devices.

图3。a)开路电压(Voc)；b)短路电流(Jsc)；c)填充因子(FF)；d)分别在$0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}}$、${0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$薄膜上制备的钙钛矿太阳能电池的功率转换效率(PCE)。每个条件包含14个器件。

Table 1. Photovoltaic parameters of the PSCs which fabricated on $0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}},{0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ films, respectively.

表1。分别在$0,{0.05}\mathrm{{mM}},{0.1}\mathrm{{mM}},{0.5}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$薄膜上制备的PSC的光伏参数。

Sample	Voc (V)	Jsc $\left( {\mathrm{{mA}}/{\mathrm{{cm}}}^{2}}\right)$	FF (%)	PCE (%)
Ref	1.126±0.009	23.52±0.21	76.60±1.33	20.29±0.33
0.05 mM	1.132±0.007	23.71±0.22	77.13±0.70	20.70±0.31
0.1 mM	1.144±0.006	23.69±0.23	79.29±0.72	21.48±0.26
0.5 mM	1.138±0.006	22.96±0.24	72.02±1.01	${18.82} \pm {0.38}$

To examine the impact of interface, we fabricate n-i-p planar PSCs using SO and BSO films as the ETL, by changing the concentration of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ from ${0.05}\mathrm{{mM}}$ to ${0.5}\mathrm{{mM}}$ , respectively. The device structure is shown in Figure 1a. As summarized in Table 1 and shown in Figure 3, the reference cells (SO based PSCs) give rise to an average PCE of 20.29%, while ${0.1}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ based cells exhibit the highest average PCE of 21.48%. It can be found that both Voc and FF have significant improvement while increasing the concentration of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ up to ${0.1}\mathrm{{mM}}$ . This indicates the less energy loss and more efficient electron extraction from perovskite to ETL, probably due to the well matched energy level between BSO and perovskite interface. The enhanced charge extraction also brings to the slight increase of short-circuit current (Jsc). When the concentration of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ increases up to ${0.5}\mathrm{{mM}}$ , Jsc and FF of corresponding PSCs drops below that of the reference. We speculate that such a concentration will induce an insulating ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ layer between ${\mathrm{{SnO}}}_{2}$ and perovskite, whose thickness is probably beyond the limitation of electron tunnelling. As a result, the PSC devices using ${0.1}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ exhibit the best photovoltaic performance among all the devices. the Voc and FF of devices are significantly improved by ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment. Moreover, both SO and BSO based PSCs show negligible J-V hysteresis (Figure S6, Supporting Information), meaning that the introduction of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ brings no extra defects at the interface. In addition, a stabilized efficiency of 21.11% is achieved by holding the voltage at maximum power point for BSO based device, as shown in Figure 4b, while the SO based device only exhibits the stabilized PCE of 20.43% (Figure S7, Supporting Information). Further, we conducted the EQE measurements on the reference (SO) and diboron treated (BSO) devices (Figure S8, Supporting Information). The integrated current values calculated by the EQE spectra for SO and BSO PSCs are 23.12 and ${23.46}\mathrm{\;{mA}}/{\mathrm{{cm}}}^{2}$ , respectively, which close to the J-V measurements. It is apparent that the BSO based device shows significantly higher EQE due to the faster carrier extraction and transportation, resulting less current loss. This further helps to explain the enhancement of device parameters by using diboron treatment.

为了研究界面的影响，我们分别通过将${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的浓度从${0.05}\mathrm{{mM}}$改变为${0.5}\mathrm{{mM}}$，使用SO和BSO薄膜作为电子传输层(ETL)来制备n-i-p平面型钙钛矿太阳能电池(PSC)。器件结构如图1a所示。如表1总结并如图3所示，参考电池(基于SO的PSC)的平均光电转换效率(PCE)为20.29%，而基于${0.1}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的电池表现出最高的平均PCE，为21.48%。可以发现，在将${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的浓度增加到${0.1}\mathrm{{mM}}$的过程中，开路电压(Voc)和填充因子(FF)都有显著提高。这表明从钙钛矿到ETL的能量损失减少且电子提取更有效，这可能是由于BSO和钙钛矿界面之间的能级匹配良好。电荷提取的增强也导致短路电流(Jsc)略有增加。当${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的浓度增加到${0.5}\mathrm{{mM}}$时，相应PSC的Jsc和FF低于参考电池。我们推测这样的浓度会在${\mathrm{{SnO}}}_{2}$和钙钛矿之间诱导出一个绝缘的${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$层，其厚度可能超出了电子隧穿的限制。因此，使用${0.1}\mathrm{{mM}}{\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$的PSC器件在所有器件中表现出最佳的光伏性能。${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理显著提高了器件的Voc和FF。此外，基于SO和BSO的PSC都显示出可忽略不计的J-V滞后(图S6，支持信息)，这意味着${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的引入在界面处没有带来额外的缺陷。此外，如图4b所示，通过将基于BSO的器件的电压保持在最大功率点，实现了21.11%的稳定效率，而基于SO的器件仅表现出20.43%的稳定PCE(图S7，支持信息)。此外，我们对参考(SO)和二硼处理(BSO)的器件进行了外量子效率(EQE)测量(图S8，支持信息)。通过EQE光谱计算得到的SO和BSO PSC的积分电流值分别为23.12和${23.46}\mathrm{\;{mA}}/{\mathrm{{cm}}}^{2}$，这与J-V测量值接近。显然，由于载流子提取和传输更快，基于BSO的器件显示出明显更高的EQE，从而导致电流损失更少。这进一步有助于解释使用二硼处理提高器件参数的原因。

Figure 4. a) J-V curves with reverse $\left( {{1.2}\mathrm{\;V}}\right.$ to $- {0.2}\mathrm{\;V})$ scan of the champion SO and BSO based PSC devices. The scan rate is ${100}\mathrm{{mV}}/\mathrm{s}$ . b) Stabilized PCE at the maximum power point (bias voltage is ${0.96}\mathrm{\;V}$ ) of BSO based PSC device. c) EIS measurements of SO and BSO based PSCs. d) The dark current-voltage characteristics of devices with ITO/SO or BSO/perovskite/PCBM/Au configuration.

图4. a)基于冠军SO和BSO的PSC器件从反向$\left( {{1.2}\mathrm{\;V}}\right.$到$- {0.2}\mathrm{\;V})$扫描的J-V曲线。扫描速率为${100}\mathrm{{mV}}/\mathrm{s}$。b)基于BSO的PSC器件在最大功率点(偏置电压为${0.96}\mathrm{\;V}$)的稳定PCE。c)基于SO和BSO的PSC的电化学阻抗谱(EIS)测量。d)具有ITO/SO或BSO/钙钛矿/PCBM/Au结构的器件的暗电流-电压特性。

The best BSO based PSC device exhibits a Voc of 1.148 V, Jsc of 23.70 mA/cm ${}^{2}$ , FF of 80.98%, and a PCE of 22.04%. In comparison, the best reference (SO based sample) cgives rise to a Voc of ${1.134}\mathrm{\;V}$ , Jsc of ${23.45}\mathrm{\;{mA}}/{\mathrm{{cm}}}^{2}$ , fill factor (FF) of 77.86%, and a PCE of 20.71%. Figure 4a shows their current density-voltage (J-V) curves, which also indicates that

基于BSO的最佳PSC器件的开路电压(Voc)为1.148 V，短路电流密度(Jsc)为23.70 mA/cm ${}^{2}$ ，填充因子(FF)为80.98%，功率转换效率(PCE)为22.04%。相比之下，最佳参考样品(基于SO的样品)的开路电压为${1.134}\mathrm{\;V}$ ，短路电流密度为${23.45}\mathrm{\;{mA}}/{\mathrm{{cm}}}^{2}$ ，填充因子(FF)为77.86%，功率转换效率为20.71%。图4a显示了它们的电流密度-电压(J-V)曲线，这也表明

To further examine the effects of BSO on the carrier dynamics in the perovskite devices, we conducted Nyquist plots of the PSCs which tested in dark with applied bias (Figure 4c). From the two separate arcs we could derive the recombination resistance $\left( {R}_{rec}\right)$ at low-frequency (high resistance) and the contact resistance $\left( {R}_{co}\right)$ at high-frequency (low resistance) ${}^{\left\lbrack {24}\right\rbrack }$ . Obviously, BSO based device has smaller value of ${\mathrm{R}}_{\mathrm{{co}}}$ and higher value of ${\mathrm{R}}_{\mathrm{{rec}}}$ than the SO based cell. The smaller ${\mathrm{R}}_{\mathrm{{co}}}$ means the better electron extraction within the device, confirming the above results, while the larger ${\mathrm{R}}_{\mathrm{{rec}}}$ means that it can effectively suppress the charge recombination to improve the device performance.

为了进一步研究BSO对钙钛矿器件中载流子动力学的影响，我们对在黑暗中施加偏压测试的PSC进行了奈奎斯特图测量(图4c)。从两条独立的弧线中，我们可以得出低频(高电阻)下的复合电阻$\left( {R}_{rec}\right)$ 和高频(低电阻)下的接触电阻$\left( {R}_{co}\right)$ ${}^{\left\lbrack {24}\right\rbrack }$ 。显然，基于BSO的器件比基于SO的电池具有更小的${\mathrm{R}}_{\mathrm{{co}}}$ 值和更高的${\mathrm{R}}_{\mathrm{{rec}}}$ 值。较小的${\mathrm{R}}_{\mathrm{{co}}}$ 意味着器件内电子提取更好，证实了上述结果，而较大的${\mathrm{R}}_{\mathrm{{rec}}}$ 意味着它可以有效抑制电荷复合以提高器件性能。

To gain the insight into the electron mobility of PSCs, we then fabricated perovskite devices by using the structure: ITO/SO or BSO/perovskite/PCBM/Au, and characterized the evolution of the space-charge-limited current (SCLC) with different biases (Figure 4d). At low bias voltage, carriers from the perovskite films contribute the current, which lead to a linear relationship of I-V curve named “ohmic region”. When the bias voltage increases, injected carriers start to fill the trap states and finally reach the trap-filled limit (TFL), corresponding voltage named " ${\mathrm{V}}_{\mathrm{{TFL}}}$ “. Then with the bias voltage further increases, the current shows a linear relationship with the square of the voltage (I $\propto {\mathrm{V}}^{2}$ ) named “child region”. We thus obtain the carrier mobility from the Mott-Gurney law ${}^{\left\lbrack {25}\right\rbrack } : {J}_{D} = \frac{{9\varepsilon }{\varepsilon }_{0}\mu {V}^{2}}{8{L}^{3}}$ , where the $\varepsilon ,{\varepsilon }_{0}$ and L are the relative dielectric constant, the vacuum permittivity and the thickness of perovskite film, respectively. The electron mobility $\left( {\mu }_{\mathrm{e}}\right)$ of SO and BSO based devices are calculated to be ${0.106}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$ and ${0.071}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$ , respectively. The enhancement of electron mobility improves the resulting optoelectronic properties of the PSCs. This is also consistent with reduced PL spectral response and shorter lifetime by TRPL measurement (Figure 2g-h). To clarify whether diboron has passivation effects on the surface states of ${\mathrm{{SnO}}}_{2}$ , we calculate the carrier mobility (which can be influenced by defects scattering) of ETL by using SCLC measurements on the electron-only devices (Figure S9, Supporting Information). We estimate the carrier mobility to be ${1.15} \times {10}^{-3}$ and ${1.21} \times {10}^{-3}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$ for the SO and BSO devices, respectively. The similar values of carrier mobility among these devices indicate that diboron has no significant passivation effects on the surface states of ${\mathrm{{SnO}}}_{2}$ . Thus the suppressed charge recombination (higher value of ${\mathrm{R}}_{\text{ rec }}$ in BSO based device, Figure 4c) mainly attribute to the better electron extraction and transportation along perovskite $/{\mathrm{{SnO}}}_{2}$ interface.

为深入了解PSC的电子迁移率，我们随后采用ITO/SO或BSO/钙钛矿/PCBM/Au结构制备了钙钛矿器件，并表征了不同偏压下空间电荷限制电流(SCLC)的演变(图4d)。在低偏压下，来自钙钛矿薄膜的载流子对电流有贡献，这导致I-V曲线呈现线性关系，称为“欧姆区域”。当偏压增加时，注入的载流子开始填充陷阱态，最终达到陷阱填充极限(TFL)，对应的电压称为“${\mathrm{V}}_{\mathrm{{TFL}}}$”。然后随着偏压进一步增加，电流与电压的平方呈现线性关系(I$\propto {\mathrm{V}}^{2}$)，称为“子区域”。因此，我们根据莫特-古内定律${}^{\left\lbrack {25}\right\rbrack } : {J}_{D} = \frac{{9\varepsilon }{\varepsilon }_{0}\mu {V}^{2}}{8{L}^{3}}$获得载流子迁移率，其中$\varepsilon ,{\varepsilon }_{0}$和L分别是相对介电常数、真空介电常数和钙钛矿薄膜的厚度。基于SO和BSO的器件的电子迁移率$\left( {\mu }_{\mathrm{e}}\right)$经计算分别为${0.106}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$和${0.071}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$。电子迁移率的提高改善了PSC的光电性能。这也与TRPL测量中PL光谱响应降低和寿命缩短一致(图2g-h)。为了阐明二硼是否对${\mathrm{{SnO}}}_{2}$的表面态有钝化作用,我们通过对仅含电子器件进行SCLC测量(图S9，支持信息)来计算ETL的载流子迁移率(其会受到缺陷散射的影响)。我们估计基于SO和BSO的器件的载流子迁移率分别为${1.15} \times {10}^{-3}$和${1.21} \times {10}^{-3}{\mathrm{\;{cm}}}^{2}{\mathrm{\;V}}^{-1}{\mathrm{\;S}}^{-1}$。这些器件中载流子迁移率的相似值表明二硼对${\mathrm{{SnO}}}_{2}$的表面态没有显著的钝化作用。因此，电荷复合的抑制(基于BSO的器件中${\mathrm{R}}_{\text{ rec }}$值更高，图4c)主要归因于沿钙钛矿$/{\mathrm{{SnO}}}_{2}$界面更好的电子提取和传输。

Finally, we investigated the operational stability of unencapsulated SO and BSO devices under maximum power point (MPP) conditions with a continuous one sun irradiation (LED source) in nitrogen atmosphere (Figure S10, Supporting Information). BSO based device keeps 95% of its initial PCE, while the reference cell drops below 90% after 100 hours. This further confirms the interface treatment by diboron leads to an improvement in the long-term stability of PSCs. We attribute this to the more efficient carrier extraction during operational conditions in BSO based device, thus reducing carrier recombination and further ion migration.

最后，我们研究了未封装的SO和BSO器件在氮气气氛中连续一个太阳光照(LED光源)下最大功率点(MPP)条件下的工作稳定性(图S10，支持信息)。基于BSO的器件保持其初始PCE的95%，而参考电池在100小时后降至90%以下。这进一步证实了二硼的界面处理导致PSC的长期稳定性得到改善。我们将此归因于基于BSO的器件在工作条件下更有效的载流子提取，从而减少了载流子复合和进一步的离子迁移。

Figure 5. a) The X-ray photoelectron spectroscopy (XPS) spectra of SO and BSO film. b) The electron paramagnetic resonance (EPR) spectra of BSO film. c) The proposed mechanism diagram of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ , which contains some ${\mathrm{{Sn}}}^{3 + }$ species as electron donors.

图5. a)SO和BSO薄膜的X射线光电子能谱(XPS)光谱。b)BSO薄膜的电子顺磁共振(EPR)光谱。c)${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$的 proposed mechanism diagram，其包含一些作为电子供体的${\mathrm{{Sn}}}^{3 + }$物种。

To clearly identify the mechanism of the more n-type nature of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ films, we further conduct the X-ray photoelectron spectroscopy (XPS) measurements of SO and BSO films. The $\mathrm{{Sn}}3\mathrm{\;d}$ core-level spectrum of ${\mathrm{{SnO}}}_{2}$ nanoparticles is shown in Figure 5a, and the two peaks correspond to Sn $3{\mathrm{\;d}}_{5/2}$ and Sn $3{\mathrm{\;d}}_{3/2}$ , respectively. For SO film the peaks of Sn $3{\mathrm{\;d}}_{5/2}$ and $\mathrm{{Sn}}3{\mathrm{\;d}}_{3/2}$ are located at487.55and ${495.95}\mathrm{{eV}}$ , while this values of BSO film are 487.34 and ${495.76}\mathrm{{eV}}$ , respectively. It can be noted that the binding energy of $\mathrm{{Sn}}3\mathrm{\;d}$ core-level exhibits a slight shift toward to lower values for BSO films, indicating that the valence state of Sn is likely to be reduced by ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ . To further confirm the existence of low valence state of Sn in BSO films, electron paramagnetic resonance (EPR) measurement is conducted, wherein ${\mathrm{{Sn}}}^{3 + }$ with $5{\mathrm{\;s}}^{1}$ electron structure is electron paramagnetic. As shown in Figure 5b, the strong absorption signal of ${\mathrm{{Sn}}}^{3 + }$ is detected in the BSO film ${}^{\left\lbrack {26}\right\rbrack }$ , indicating that ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment surely induces some ${\mathrm{{Sn}}}^{3 + }$ species in the ${\mathrm{{SnO}}}_{2}$ film, which can serve as electron donors (Figure 5c). According to the equation which determines the space depletion width (W): $= \sqrt{\frac{{2\varepsilon }{\varepsilon }_{0}\left( {\mathrm{E} - \mathrm{{Ef}}}\right) }{eN}}$ , where $\mathrm{N},\varepsilon {\varepsilon }_{0}$ are the concentration of electron donors, relative dielectric constant, vacuum permittivity, respectively ${}^{\left\lbrack {27}\right\rbrack }$ . The higher donor concentration of BSO film is prone to induce a narrow depletion width, which leads to an enhanced carrier tunnelling probability from perovskite to ETL. These improvements are all beneficial to the carrier extraction and transportation in the entire PSC devices.

为了清晰地确定经${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$薄膜具有更多n型性质的机制，我们进一步对SO和BSO薄膜进行了X射线光电子能谱(XPS)测量。${\mathrm{{SnO}}}_{2}$纳米颗粒的$\mathrm{{Sn}}3\mathrm{\;d}$核心能级光谱如图5a所示，两个峰分别对应Sn$3{\mathrm{\;d}}_{5/2}$和Sn$3{\mathrm{\;d}}_{3/2}$。对于SO薄膜，Sn$3{\mathrm{\;d}}_{5/2}$和$\mathrm{{Sn}}3{\mathrm{\;d}}_{3/2}$的峰位于487.55和${495.95}\mathrm{{eV}}$处，而BSO薄膜的这些值分别为487.34和${495.76}\mathrm{{eV}}$。可以注意到，BSO薄膜的$\mathrm{{Sn}}3\mathrm{\;d}$核心能级的结合能向较低值有轻微偏移，表明Sn的价态可能被${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$降低。为了进一步证实BSO薄膜中Sn低价态的存在，进行了电子顺磁共振(EPR)测量，其中具有$5{\mathrm{\;s}}^{1}$电子结构的${\mathrm{{Sn}}}^{3 + }$是顺磁性的。如图5b所示，在BSO薄膜${}^{\left\lbrack {26}\right\rbrack }$中检测到${\mathrm{{Sn}}}^{3 + }$的强吸收信号，表明${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理确实在${\mathrm{{SnO}}}_{2}$薄膜中诱导了一些${\mathrm{{Sn}}}^{3 + }$物种，其可作为电子供体(图5c)。根据确定空间耗尽宽度(W)的方程:$= \sqrt{\frac{{2\varepsilon }{\varepsilon }_{0}\left( {\mathrm{E} - \mathrm{{Ef}}}\right) }{eN}}$，其中$\mathrm{N},\varepsilon {\varepsilon }_{0}$分别是电子供体浓度、相对介电常数、真空电容率${}^{\left\lbrack {27}\right\rbrack }$。BSO薄膜较高的供体浓度易于诱导较窄的耗尽宽度，这导致从钙钛矿到ETL的载流子隧穿概率增加。这些改进都有利于整个PSC器件中的载流子提取和传输。

In consideration of the chemical properties of diboron, previous study showed that diboron molecules can function as single reducing agents when they are bonded with Lewis basic oxygen atom ${}^{\left\lbrack {28}\right\rbrack }$ . Thus, we speculate that when the ${\mathrm{{SnO}}}_{2}$ films are treated with diboron, the bridging $\mathrm{O}$ or the adsorbed hydroxyl group on the surface of ${\mathrm{{SnO}}}_{2}$ can coordinate with dirboron, resulting a surface diboron-oxygen Lewis pair, inducing a reducing state of diboron complexes (Figure 5c). The single electron can transfer from diboron complexes to the adjacent $\mathrm{{Sn}}$ site, resulting in the formation of ${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$ species on the surface of ${\mathrm{{SnO}}}_{2}$ film. The presence of ${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$ species suggests a possible $\mathrm{n}$ type doping, and a high donor concentration on the ETL. It also well explains the up-shift of the Fermi energy of the BSO, which provides an effective means to tune the band alignment in the interfaces.

考虑到二硼的化学性质，先前的研究表明，二硼分子与路易斯碱性氧原子${}^{\left\lbrack {28}\right\rbrack }$结合时可作为单还原剂。因此，我们推测当用二硼处理${\mathrm{{SnO}}}_{2}$薄膜时，${\mathrm{{SnO}}}_{2}$表面的桥连$\mathrm{O}$或吸附的羟基可与二硼配位，形成表面二硼 - 氧路易斯对，诱导二硼配合物的还原态(图5c)。单电子可从二硼配合物转移到相邻的$\mathrm{{Sn}}$位点，导致在${\mathrm{{SnO}}}_{2}$薄膜表面形成${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$物种。${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{ - }$物种的存在表明可能存在$\mathrm{n}$型掺杂以及ETL上的高供体浓度。这也很好地解释了BSO费米能级的上移，这为调节界面中的能带排列提供了一种有效手段。

3. Conclusion

3. 结论

In summary, we have explored an effective interfacial modifier between ${\mathrm{{SnO}}}_{2}$ and perovskite by using the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ molecule. The higher ${\mathrm{E}}_{\mathrm{F}}$ on the surface of ${\mathrm{{SnO}}}_{2}$ (meaning a more n-type property) is obtained based on the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treatment, which close to the ${\mathrm{E}}_{\mathrm{F}}$ of perovskite, resulting a better energy alignment at ETL/perovskite interface and a higher PCE of 22.04% with negligible hysteresis in planar n-i-p perovskite solar cells. We attribute the improvement in optoelectronic properties to the more efficient carrier extraction and suppressed charge recombination at the ${\mathrm{{SnO}}}_{2}$ /perovskite interface. Moreover, we use XPS and EPR measurements to demonstrate that the ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ treated ${\mathrm{{SnO}}}_{2}$ induce some ${\mathrm{{Sn}}}^{3 + }$ species on the surface, which enable a more n-type nature at the ETL/perovskite interface. Considering the chemical properties of ${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$ , we speculate that the formation of surface diboron-oxygen Lewis pair enables a reducing state of diboron complexes, which induce spontaneous electron injection from ${\mathrm{O}}^{2 - }$ to adjacent ${\mathrm{{Sn}}}^{4 + }$ leading to an extremely stable surface ${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{- - }$ complex. This provides an effective chemical approach to tuning the energy level alignment at the ETL/perovskite interfaces.

总之，我们通过使用${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$分子探索了${\mathrm{{SnO}}}_{2}$和钙钛矿之间的有效界面修饰剂。基于${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理，在${\mathrm{{SnO}}}_{2}$表面获得了更高的${\mathrm{E}}_{\mathrm{F}}$(意味着更n型的性质)，其接近钙钛矿的${\mathrm{E}}_{\mathrm{F}}$，从而在平面n-i-p钙钛矿太阳能电池的ETL/钙钛矿界面处实现了更好的能量对准，并具有22.04%的更高PCE和可忽略不计的滞后现象。我们将光电性能的改善归因于在${\mathrm{{SnO}}}_{2}$/钙钛矿界面处更有效的载流子提取和抑制的电荷复合。此外，我们使用XPS和EPR测量来证明${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$处理的${\mathrm{{SnO}}}_{2}$在表面诱导了一些${\mathrm{{Sn}}}^{3 + }$物种，这使得ETL/钙钛矿界面具有更n型的性质。考虑到${\mathrm{B}}_{2}{\mathrm{{Cat}}}_{2}$的化学性质，我们推测表面二硼-氧路易斯对的形成使二硼配合物处于还原状态，这诱导了从${\mathrm{O}}^{2 - }$到相邻${\mathrm{{Sn}}}^{4 + }$的自发电子注入，导致形成极其稳定的表面${\mathrm{{Sn}}}^{3 + } - {\mathrm{O}}^{- - }$配合物。这提供了一种有效的化学方法来调节ETL/钙钛矿界面处的能级对准。

Acknowledgements

致谢

N.L. and X.N. contributed equally to this work. This work is supported by National Natural Science Foundation of China (51722201) (51672008) (91733301), National Key Research and Development Program of China Grant No. 2017YFA0206701, Natural Science Foundation of Beijing, China (Grant No. 4182026), Young Talent Thousand Program. National Key Research and Development Program of China Grant No. 2016YFB0700700, National Natural Science Foundation of China (51673025), Beijing Municipal Science and Technology Project No. Z181100005118002.

Conflict of Interest

利益冲突

The authors declare no conflict of interest.

作者声明不存在利益冲突。

Keywords

关键词

Diboron, perovskites, solar cells, interface engineering, high efficiency

二硼、钙钛矿、太阳能电池、界面工程、高效率

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IIIV Pordenov

IIIV 波德诺夫

TOC Text

目录文本

Diboron treated ${\mathrm{{SnO}}}_{2}$ possesses some ${\mathrm{{Sn}}}^{3 + }$ species, which serve as electron donors with more n-type nature, resulting the higher Femi level on the surface of ${\mathrm{{SnO}}}_{2}$ , promoting electron extraction and reducing carrier recombination in ETL/perovskite interface. A power conversion efficiency of 22.04 % is obtained in an n-i-p structure perovskite solar cell.

经二硼处理的${\mathrm{{SnO}}}_{2}$具有一些${\mathrm{{Sn}}}^{3 + }$物种，这些物种作为具有更多n型性质的电子供体，导致${\mathrm{{SnO}}}_{2}$表面的费米能级更高，促进了电子提取并减少了ETL/钙钛矿界面中的载流子复合。在n-i-p结构的钙钛矿太阳能电池中获得了22.04%的功率转换效率。