The most efficient perovskite solar cell devices use Pb(II); however, the toxicity of lead and its decomposition products represent possible obstacles to their widespread commercial and carbon impact.(1,2) Pb from hybrid perovskites may enter plants—and consequently the food cycle—10× more efficiently than do other Pb sources.(3) Although consumer solar modules are encapsulated, there remain concerns about their distribution, maintenance, and recycling.(4) Replacing lead with more environmentally benign metals, without compromising device efficiency, is of utmost importance. Recent high-throughput computational studies evaluating the electronic properties of thousands of ABX₃ perovskites revealed that only lead and tin perovskites can deliver solar cell relevant bandgaps and absorption coefficients.(5,6) Replacing Pb with Sn is a promising path; however, Sn(II) undergoes undesired oxidation forming Sn(IV) and charge carrier recombination centers, leading to a significant loss of photovoltage in solar cells.(7) Innovative approaches, including dopant additives, a reducing atmosphere, and comproportionation, have increased the record power conversion efficiency for Sn-based devices, approaching currently only half of that for Pb-based solar cells.(8−10) In this Viewpoint, we report that the solvent conventionally used in the fabrication of perovskite films, dimethyl sulfoxide (DMSO), oxidizes Sn(II) to Sn(IV). Using the combination of nuclear magnetic resonance (¹H NMR) and X-ray absorption near-edge spectroscopy (XANES), we find that the DMSO/Sn(II) pair undergoes an irreversible redox reaction forming dimethylsulfide (DMS) and Sn(IV) in solution. Recently, Hamill et al. showed that DMSO induces the conversion of methylammonium into dimethylammonium cation in CH₃NH₃I + PbI₂ solutions at 150 °C.(12) DMSO is also known as an oxidation agent for Sn(II) in acidic medium;(11) given that perovskite organic cations are Lewis acids, we hypothesized that DMSO can oxidize Sn(II) in perovskite solutions. To check this hypothesis and provide insight into reaction/decomposition pathways in Sn(II)-containing perovskite solutions, we performed a series of experiments. We prepared a perovskite solution in DMSO by dissolving FAI (where FA is formamidinium cation) and SnI₂, sealed the vial, and heated it at 120 °C for 5 h, all in an inert atmosphere. We noticed a significant change in color (Figure 1); the solution became much darker, indicating the oxidation of Sn(II).(13) On the other hand, the dimethylformamide (DMF)-based solution showed an only negligible change in color in the analogous experiment. Though perovskite solutions are generally not heated for such a long time, this accelerated test can provide insights about the processes that occur in solution when heating the solution to dissolve the precursors or annealing spin-coated film with the leftover DMSO. It does not take much Sn⁴⁺ doping (<0.1%) in ASnI₃ perovskites to turn the material’s p-type conductors and render them inactive as solar photovoltaic absorbers. Figure 1. Fresh and aged (120 °C for 5 h) formamidinium tin iodide solutions. ¹H NMR measurements of the corresponding solutions revealed the presence of DMS (at 2.07 ppm)(11) even after 10 min of heat treatment at 120 °C (Figure 2a,b). Longer heat treatment led to more DMS formation (Figure S1). A blank sample without FAI and SnI₂ was treated in the same way, and it showed no DMSO decomposition (Figure S2). Notably, FAI or PbI₂ DMSO-based heat-treated solutions under the same conditions exhibited trace amounts of DMS (Figures S3–S5). Variable-temperature ¹H NMR measurements showed that DMS formation commences at 100 °C in the presence of Sn²⁺ ions (Figures S6 and S7). Figure 2. ¹H NMR spectra of DMSO sample solutions (a) before and (b) after heat treatment at 120 °C for 10 min. The peak at 2.07 ppm is assigned to the methyl protons of DMS. The peaks at 8.82 ppm and 8.47 ppm correspond to the (−NH₂) protons, while the peak at 7.86 ppm corresponds to the (−CH−) proton of formamidinium cation.(18) XANES (c) Sn L-edge and (d) S K-edge spectra before and after heat treatment. We then conducted X-ray absorption near-edge spectroscopy (XANES) on the solutions before and after heat treatment. The XANES spectra of S K-edge confirmed that DMSO was partially reduced to DMS (emergence of a peak at 2470.5)(14) after annealing at 120 °C under an argon atmosphere (Figure 2c). Similarly, distinct peaks corresponding to Sn(IV) were observed in all the annealed solutions using the Sn L-edge,(15) indicating Sn oxidation (Figure 2d). These findings indicate that the following reaction took place in solution: Summary and Future Outlook. In sum, our work provides a piece of evidence that DMSO can oxidize Sn(II) at temperatures above 100 °C. DMSO has become a mandatory near-ubiquitous component in the fabrication of perovskite films, as it is known to be a good ligand in coordination chemistry, thus retarding crystallization and improving crystallite quality.(16,17) Even the most minute Sn⁴⁺ amount in the structure can compromise solar cell photovoltage to <0.5 V, much less than the ∼0.9 V expected from the bandgap. If Sn-based solar cells are going to match the performance of Pb-based ones, all potential oxidation pathways should be prevented, urging the development of new, DMSO-free solvent systems for the synthesis of Sn-based perovskites with suppressed defect densities. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.0c00402.

• Sample preparation, characterization information, and additional NMR spectra (PDF)

Sample preparation, characterization information, and additional NMR spectra (PDF) M.I.S., I.S., and J.A. contributed equally. Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material, requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. This work was supported in part by the US Department of the Navy, Office of Naval Research (grant award no. N00014-17-1-2524 and N00014-17-1-2231). This work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN). The authors thank the Canadian Light Source (CLS) for support in the form of a travel grant. XANES data were collected at the Soft X-ray Microcharacterization Beamline (SXRMB) at the Canadian Light Source with the assistance of Dr. Yongfeng Hu, Dr. Qunfeng Xiao, and Dr. Mohsen Shakouri. This article references 18 other publications.

中文翻译：

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常规溶剂氧化钙钛矿油墨中的Sn（II）

最有效的钙钛矿太阳能电池设备使用Pb（II）。但是，铅及其分解产物的毒性可能会阻碍其广泛的商业和碳影响。（1,2）杂化钙钛矿中的Pb可能比其他Pb来源更有效地进入植物，因此进入食物循环的10倍。 （3）尽管消费型太阳能电池组件是封装的，但仍然存在着对其分布，维护和回收利用的担忧。（4）在不影响器件效率的前提下，用对环境更有益的金属代替铅是至关重要的。最近的高通量计算研究评估了数千种ABX ₃的电子性能钙钛矿表明只有铅和锡钙钛矿才能提供与太阳能电池有关的带隙和吸收系数。（5,6）用Sn代替Pb是一条有前途的途径；但是，Sn（II）会发生不希望的氧化，形成Sn（IV）和电荷载流子复合中心，从而导致太阳能电池中的光电压显着损失。（7）包括掺杂剂添加剂，还原性气氛和复合化在内的创新方法不断增加基于Sn的器件的功率转换效率达到了创纪录的水平，目前仅接近基于Pb的太阳能电池的功率转换效率的一半。（8-10）在此观点下，我们报告了钙钛矿薄膜制造中通常使用的溶剂二甲基亚砜（ DMSO），将Sn（II）氧化为Sn（IV）。结合使用核磁共振（¹1 H NMR）和X射线吸收近缘光谱（XANES），我们发现DMSO / Sn（II）对经历不可逆的氧化还原反应，在溶液中形成二甲基硫（DMS）和Sn（IV）。最近，哈米尔等。表明DMSO诱导CH ₃ NH ₃ I + PbI _2中的甲基铵转化为二甲基铵阳离子溶液在150°C下。（12）DMSO也被称为酸性介质中Sn（II）的氧化剂;（11）鉴于钙钛矿有机阳离子是路易斯酸，我们假设DMSO可以氧化钙钛矿中的Sn（II）解决方案。为了检查该假设并提供对含Sn（II）的钙钛矿溶液中反应/分解途径的了解，我们进行了一系列实验。我们通过溶解FAI（FA是甲ami阳离子）和SnI ₂制备了DMSO中的钙钛矿溶液。，密封小瓶，然后在惰性气氛中于120°C加热5小时。我们注意到颜色发生了显着变化（图1）。溶液变得更黑，表明Sn（II）被氧化。（13）另一方面，在类似的实验中，基于二甲基甲酰胺（DMF）的溶液的颜色变化很小。尽管钙钛矿溶液通常不加热这么长时间，但该加速试验可以提供有关加热溶液以溶解前体或用剩余的DMSO退火旋涂膜时溶液中发生的过程的见解。在ASnI _3中不需要太多的Sn ⁴⁺掺杂（<0.1％）钙钛矿可转动材料的p型导体，使其不像太阳能光伏吸收剂那样工作。图1.新鲜和陈化（120°C，5小时）的碘化亚锡碘化铵溶液。相应溶液的¹ H NMR测量表明，即使在120°C热处理10分钟后，也存在DMS（2.07 ppm）（11）（图2a，b）。更长的热处理导致更多的DMS形成（图S1）。不含FAI和SnI _2的空白样品以相同方式处理，并且没有DMSO分解（图S2）。值得注意的是，在相同条件下，基于FAI或PbI ₂ DMSO的热处理溶液显示出痕量的DMS（图S3-S5）。恒温¹1 H NMR测量表明，在存在Sn ²⁺离子的情况下，DMS的形成始于100°C （图S6和S7）。图2. DMSO样品溶液（a）在120°C热处理10分钟之前和之后的¹ H NMR光谱。2.07 ppm处的峰归属于DMS的甲基质子。在8.82 ppm和8.47 ppm处的峰对应于（-NH ₂）质子，而在7.86 ppm处的峰对应于甲ami阳离子的（-CH-）质子。（18）XANES（c）Sn L-edge和（d）S K-edge谱图。然后，我们在热处理前后对溶液进行了X射线吸收近边缘光谱（XANES）。S K边缘的XANES光谱证实，在氩气氛下于120°C退火后，DMSO部分还原为DMS（在2470.5处出现峰）（14）（图2c）。同样，在所有退火溶液中，使用Sn L边缘观察到与Sn（IV）对应的明显峰（15），表明Sn发生了氧化（图2d）。这些发现表明在解决方案中发生了以下反应：摘要和未来展望 。总而言之，我们的工作提供了一个证据，表明DMSO在高于100°C的温度下可以氧化Sn（II）。DMSO已成为钙钛矿薄膜制造中必不可少的成分，因为它在配位化学中是良好的配体，从而阻碍了结晶并改善了微晶质量。（16,17）即使是最微小的Sn ⁴⁺结构中的最大数量会损害太阳能电池的光电压至<0.5 V，远低于带隙预期的〜0.9V。如果锡基太阳能电池要与铅基太阳能电池的性能相匹配，则应防止所有潜在的氧化途径，并敦促开发新的，无DMSO的溶剂系统，以合成缺陷密度受到抑制的锡基钙钛矿。可从https://pubs.acs.org/doi/10.1021/acsenergylett.0c00402免费获得支持信息。

• 样品制备，表征信息和其他NMR谱（PDF）

样品制备，表征信息和其他NMR谱图（PDF）MIS，IS和JA均贡献同等。本观点中表达的观点是作者的观点，不一定是ACS的观点。作者宣称没有竞争性的经济利益。无需订阅ACS Web版本即可获得电子支持信息文件。美国化学学会在任何可版权保护的支持信息中拥有版权权益。ACS网站上提供的文件只能下载供个人使用。未经美国化学学会许可，不得以其他方式允许用户以机器可读形式或任何其他形式全部或部分复制，重新发布，重新分发或出售ACS网站上的任何支持信息。为了获得复制，重新发布和重新分发此材料的许可，请求者必须通过RightsLink许可系统处理自己的请求。有关如何使用RightsLink权限系统的信息，请访问http://pubs.acs.org/page/copyright/permissions.html。这项工作得到了美国海军部海军研究办公室的部分支持（授权号N00014-17-1-2524和N00014-17-1-2231）。这项工作利用了西北大学的IMSERC，获得了软混合纳米技术实验（SHyNE）资源（NSF ECCS-1542205），伊利诺伊州和国际纳米技术研究所（IIN）的支持。作者感谢加拿大光源（CLS）以旅行补助金的形式提供的支持。XANES数据是在胡永峰博士，肖群峰博士和Mohsen Shakouri博士的协助下，在加拿大光源的软X射线微表征光束线（SXRMB）上收集的。本文引用了其他18个出版物。