Influence of low-energy ammonia ion/group diffusion on electrical properties of indium tin oxide film

Abstract

In the case of methylammonium lead halide (MAPbH₃) perovskite solar cells, the indium tin oxide (ITO) film has been widely used as the transparent electrode. In the preparation process and service process of MAPbH₃ perovskite solar cells, the MAPbH₃ perovskite layer can decompose into the methyl, amino, methylammonium, halide ion/group, etc. Thus, the diffusion of ammonia ion/group into ITO film is inevitable, which can seriously deteriorate the electrical property of ITO transparent electrode. In this study, the ITO films with and without (100) preferred orientation are bombarded by a low-energy ammonia (NH<i>_x</i>) ion beam. After the bombardment, the electrical properties of ITO film without preferred orientation are deteriorated seriously, especially for carrier concentration, which is deteriorated down to an extent of about 5–6 orders of magnitude. The bombardment of low-energy NH<i>_x</i> ion/group has little influence on the electrical properties of ITO film with (100) preferred orientation. Such phenomena can be explained by the following reasons. Based on XPS measurement results, the low-energy NH<i>_x</i> ion/group diffuses into the ITO film surface after the bombardment. In the diffusion process, the low-energy NH<i>_x</i> ion/group is mainly bonded with O in ITO lattice, which results in the formation of In/Sn—O—N bond. Based on the crystal structure of ITO, the (100) lattice of ITO consists of In/Sn, and the calculated value of surface energy <inline-formula><tex-math id="M1">\begin{document}$ {\gamma }_{\left\{100\right\}/\left\{010\right\}/\left\{001\right\}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M1.png"/></alternatives></inline-formula> = 1.76 J/m². While the (110) and (111) lattices of ITO consist of In/Sn/O, in which the O atom percent on (110) and (111) lattices are 56 at.% and 25 at.% respectively. Besides, the calculated values of surface energy <inline-formula><tex-math id="M2">\begin{document}$ {\gamma }_{\left\{110\right\}/\left\{101\right\}/\left\{011\right\}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M2.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M3">\begin{document}$ {\gamma }_{\left\{111\right\}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M3.png"/></alternatives></inline-formula> are 1.07 and 0.89 J/m², respectively. Combining the XPS measurement results and crystal structure of ITO, it can be understood that in the diffusion process of low-energy NH_<i>x</i> ion/group into ITO film without preferred orientation, lots of In/Sn—O—N bonds are formed in the ITO lattices, which are rich in O and have lower surface energy <inline-formula><tex-math id="M4">\begin{document}$ \gamma $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M4.png"/></alternatives></inline-formula>. Then, after the low-energy NH<i>_x</i> ion/group bombardment, the electrical properties of ITO film without preferred orientation are deteriorated seriously. On the contrary, because of the absence of O and the highest surface energy <inline-formula><tex-math id="M5">\begin{document}$ \gamma $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200860_M5.png"/></alternatives></inline-formula>, it is hard for the low-energy NH<i>_x</i> ion/group to diffuse into ITO (100) lattice. Then, after the low-energy NH<i>_x</i> ion/group bombardment, the electrical properties of ITO film with (100) preferred orientation have little change. With all results, the ITO film with (100) preferred orientation can be an ideal candidate for transparent electrode in MAPbH₃ perovskite solar cells.