| AVS 61st International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN-TuP |
| Session: | Energy Frontiers Poster Session |
| Presenter: | Yongli Gao, University of Rochester |
| Authors: | Y. Gao, University of Rochester C.G. Wang, University of Rochester C.C. Wang, University of Rochester X.L. Liu, Central South University, Changsha, China J.S. Huang, University of Nebraska Lincoln |
| Correspondent: | Click to Email |
In a little over a year, there has been an unexpected breakthrough and rapid evolution of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite materials. This technology has the potential to produce solar cells with the very highest efficiencies while retaining the very lowest cost. We have measured the electronic density of states of CH3NH3PbI3 using ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and x-ray photoelectron spectroscopy (XPS). The valence band maximum (VBM) and conduction band minimum (CBM) positions are obtained from the UPS and IPES spectra, respectively, by liner extrapolation of the leading edges. With the Fermi level close to the VBM, the sample is slightly p-type, with the EVBM=0.76 eV, ECBM=-0.9 eV, and transport energy gap 1.7 eV. The ionization potential (IP) of 6.16 eV can be obtained from the sum of the UPS measured EVBM and the work function of 5.40 eV. The XPS spectra reveal an obvious deficiency of N is, whereas the concentrations of I and Pb are close to the expected values. The existence of O and excessive C indicate that the surface is contaminated, and the contamination is reduced by 12-20% by thermal annealing in vacuum. The interface between CH3NH3PbI3 and TiO2 is also investigated. As CH3NH3PbI3 layer thickness increases, one sees a gradual shift of the vacuum level cut-off. At the CH3NH3PbI3 thickness of 8.2 nm, the shift saturates, signaling the end of the interface dipole formation. The evolution of the VB is even more gradual, and it appears to mature only at the thickest layer. Given the UPS probing depth of ~1.5 nm and the typical interface formation in other systems, the saturation of the vacuum level cut-off at 8.2 nm in CH3NH3PbI3/TiO2 is surprisingly large. It points at the possibility that the films are not truly uniform. On the other hand, the interface dipole of 0.7 eV CH3NH3PbI3 and TiO2 is important for further understanding of the energy level alignment and charge transfer across the interface.