Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Publication date: 23rd October 2018
In the perovskite solar cell, the interfaces play crucial roles in the charge extraction, transfer and recombination. The properties of the interface of perovskite ABX3 should be strongly affected by the elements of the uppermost layer being either A, B, or X3.
Ohmann et al1 have reported STM images of an in-situ cleaved single crystal MAPbBr3 perovskite to be terminated with MA and Br. However, the surface elements of MAPbI3 thin film prepared from the solution, which is practically used in the most of solar cells, is unknown. In this work, we examined the surface elements of MAPbI3 prepared from the solution employing the easy and quick method of ultraviolet photoelectron (UPS) and metastable-atom electron (MAES) spectroscopies. UPS is an experimental technique to observe the valence electronic states; the ultraviolet photon excites the valence electron of the sample surface and the electron energy of the emitted photoelectron is analyzed. In MAES, the excitation source is replaced with the metastable atom. While the ultraviolet photon can penetrate into the sample, the metastable atom can only interact with the outer orbital of the sample2. Therefore, by comparing intensities of related features observed both in MAES and UPS spectra, we can obtain information about the elements at the uppermost surface.
The MAPbI3 film was prepared by the one-step method in N2 atmosphere. Perovskite precursor solution in DMF was spin-coated on ITO or PEDOT-PSS coated ITO substrates, and toluene was added as a poor solvent during the spin coating. The quality of the film was confirmed by the power conversion efficiency of the device (ITO/PEDOT-PSS/MAPbI3/PCBM/BCP/Ag) exceeding 12 %. UPS and MAES measurements were performed using a He Ia radiation light (21.218 eV) and a metastable He* (19.82 eV) atoms, respectively, at a pressure below 10-8 Pa.
We observed no discernible differences between the perovskite layers on the pristine and PEDOT-PPS coated ITO substrates. We assigned the features in the UPS and MAES spectra based on the reported DFT calculations3,4 and energy dependence of UPS measured in the beamline 2B of UVSOR in Okazaki, Japan. We observed the MA and I peaks in both UPS and MAES but Pb-derived peaks only in UPS. From the result, we conclude that the uppermost layer of perovskite films both on ITO and PEDOT: PSS/ITO substrates consist of MA and I.
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