Soe Chan Myae Myae, Stoumpos Constantinos C, Harutyunyan Boris, Manley Eric F, Chen Lin X, Bedzyk Michael J, Marks Tobin J, Kanatzidis Mercouri G
Department of Chemistry, Northwestern University, Evanston, IL, 60628, USA.
Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, 60208, USA.
ChemSusChem. 2016 Sep 22;9(18):2656-2665. doi: 10.1002/cssc.201600879. Epub 2016 Sep 15.
Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH NH PbI ) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal-halogen-metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonal→cubic phase transition is observed for CH NH PbI films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from in situ variable-temperature X-ray diffraction in the 25-55 °C (298-328 K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration.
尽管在钙钛矿太阳能电池中已采用反应性添加剂来改善薄膜形态并显著提高器件性能，但对于这些添加剂对钙钛矿结构和光学性质的影响却知之甚少。在此，我们报告了一项关于氢卤酸添加剂（HX；X = I、Br、Cl）在前驱体溶液中如何影响甲基碘化铅钙钛矿（CH₃NH₃PbI₃）性质的系统研究。详细的结构和光谱分析表明，所有这三种酸都会影响光学性质并改变晶胞晶格参数。根据HX的种类和浓度，光学带隙会变宽或变窄：添加HBr会产生更宽的带隙，而HI在高浓度时会压缩带隙；另一方面，HCl对带隙没有显著影响。通过将这些变化与多晶钙钛矿薄膜中存在的缺陷类型以及在非常小的微晶中诱导的结构应变相关联，可以理解这些变化。前驱体溶液中来自HX的额外卤化物的存在使得钙钛矿中的晶格空位得以填充，从而改变金属 - 卤素 - 金属键的连接性，进而改变晶胞体积和光学带隙。值得注意的是，对于用高浓度HX处理的CH₃NH₃PbI₃薄膜，观察到了室温下的四方相→立方相转变。通过在25 - 55°C（298 - 328 K）范围内进行原位变温X射线衍射，对这种异常相变有了进一步的了解，结果表明转变温度随着前驱体溶液HX浓度的增加而单调下降。
