Optimization of grain size to achieve high-performance perovskite solar cells in vapor deposition

Abstract

<sec> Organometal halide perovskite is one of the most promising materials for high efficient thin-film solar cell. Solution fabrication process shows that the recorded power conversion efficiency (PCE) is 23.7%, however, large scale fabrication suffers the inevitable toxic solvent, preventing it from implementing the green commercialization. As one of the matured large-scale fabrication techniques, the vapor deposition is recently found to promise the green fabrication of perovskite thin film without toxic solvent. However, the PCE based on vapor deposition is considerably lower than that based on solution fabrication because of ineffective regulation methods of the perovskite films. So, there is intensive requirement for optimizing the growth of perovskite in vapor deposition for improving PCE, especially, developing a kind of quality regulation method of the perovskite films. </sec><sec>In this study, we provide a method of adjusting grain size in vapor deposition method. The grain size optimization of MAPbI<sub>3</sub> films is realized by simply modulating the reaction temperature between PbI<sub>2</sub> films and MAI vapor. We set the reaction temperature to be 140 ℃, 160 ℃, 180 ℃ and 200 ℃ separately and establish the relationship between reaction time and grain size during the complete conversion of PbI<sub>2</sub> film into MAPbI<sub>3</sub> film. We find that the average grain size of the film increases first with growth temperature increasing from 140 ℃ to 180 ℃ and then decrease at 200 ℃, giving an average grain size of 0.81 <inline-formula><tex-math id="M3">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182221_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182221_M3.png"/></alternatives></inline-formula> and a largest grain size of about 2 <inline-formula><tex-math id="M4">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182221_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182221_M4.png"/></alternatives></inline-formula> at 180 ℃. The defect density of perovskite film is deduced from the space charge limited current model, showing that it decreases from 5.90 × 10<sup>16</sup> cm<sup>–3</sup> at 140 ℃ to 2.66 × 10<sup>16</sup> cm<sup>–3</sup> at 180 ℃. Photovoltaic devices with structure FTO/TiO<sub>2</sub>/C<sub>60</sub>/MAPbI<sub>3</sub>/spiro-OMeTAD/Au are fabricated to demonstrate the performance. It is found that the devices with an active area of 0.045 cm<sup>2</sup> show that with the increase of grain size, the average PCE increases from 14.00% to 17.42%, and the best device shows that its PCE is 17.80% with 4.04% hysteresis index. To show the possibility of scaling up, we fabricate a uniform perovskite thin film with an area of about 72 cm<sup>2</sup>, and a device with an active area of 1 cm<sup>2</sup>, which gives a PCE of 13.17% in reverse scan. In summary, our research provides a method of regulating the grain size for the vapor deposition, which can improve device performance by reducing the trap density in perovskite film for suppressing the carrier recombination in grain boundary. Meanwhile, we prepare high performance devices and large area thin films, showing their potential in large area device fabrication and applications. </sec>