2, University of Maryland, College Park, Maryland, United States
3, Seoul National University, Seoul, , Korea (the Republic of)
4, Kyungpook National University, Daegu, , Korea (the Republic of)
The power conversion efficiency of perovskite solar cells is continuously improving, however, the fundamental operational principles and the degradation mechanisms are insufficiently revealed, causing the delay of commercialization. Electrical, physical, and chemical variations need to be carefully investigated to establish causes of the degradation, and some macro-/mesoscopic studies have been made up to date. Nonetheless, for in-depth understanding of the origin, observations in nanoscale where photo-excited carrier generations and collections actually take place should be made. In this presentation, we show nanoscale observations of photocurrent generations of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells, and uncover their origins of degradation mechanisms . For the first time, we employ a novel near-field scanning photocurrent microscopy (NSPM) technique to image nanoscale photocurrent generations of perovskite solar cells. We investigate how photocurrent generations vary with respect to the sample annealing temperature, and observe increased photocurrent generations at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C). However, we observe the opposite carrier generation patterns (i.e., reduced photocurrent generations at grain boundaries and increased photocurrent generations at grain interiors) in perovskite solar cells annealed at higher temperature (130 °C). By combining results from our novel NSPM with other characterization techniques including electron microscopy and x-ray diffraction, we reveal that the spatial pattern of photocurrent is caused by material inhomogeneity and dynamics of segregation of lead iodide. Next, we examine the nanoscale signatures of aging caused by continuous light exposure under normal operation to establish the degradation mechanisms of perovskite solar cells. We image multi-stage nanoscale photocurrent generation patterns of a perovskite solar cell as a function of time while the cell is degrading under the extended light exposure. It is found that the extended light exposure drives further structural and compositional changes of materials revealed by the nanoscale photocurrent imaging. The nanoscale observation of perovskite solar cells’ operation depending on the sample preparation temperature and on the degradation conditions would suggest pathways to further improve high-efficient perovskite solar cells with long-term stability.
 Dongheon Ha*, Yohan Yoon*, Ik Jae Park, Paul M. Haney, Sangwook Lee, and Nikolai B. Zhitenev, Nanoscale mapping of photocurrent generation in perovskite solar cells, Energy and Environmental Science (under review) (* indicates equal contribution)