Princeton University

School of Engineering & Applied Science

Photoelectron Spectroscopy of PbS(e) Quantum Dots and CH3NH3PbI3 Perovskite Thin Films

Dr. Elisa M. Miller, Postdoctoral Fellow at NREL
Engineering Quadrangle, B205
Thursday, August 21, 2014 - 11:30am

In recent years, PbS(e) quantum dots (QDs) and CH3NH3PbI3 perovskite have gained significant attention from the photovoltaics community; these materials have demonstrated the potential for efficient solar cell devices.  To have a better understanding of the band positions and what controls the energetics of the valence and conduction bands (VB and CB), we use x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) to study the VB to Fermi energy difference in these thin films.  We make conductive, thin films of PbS QDs and treat the surface with Na2S followed by PbCl2.  This sequential surface treatment transforms the surface from intrinsic to n-type and the change is probed with XPS and UPS.  Also, the surface band positions of solution-processed CH3NH3PbI3 perovskite thin films deposited on an insulating substrate (Al2O3), various n-type substrates (TiO2, ZrO2, ZnO, and F:SnO2 (FTO)), and various p-type substrates (PEDOT:PSS, NiO, and Cu2O) are studied with XPS and UPS. Theoretical many-body GW calculations with spin-orbit interaction of the VB density of states show a clear correspondence with our experimental spectra and are used to confirm our assignment of the VB maximum.  These surface-sensitive photoelectron spectroscopy measurements result in shifting of the CH3NH3PbI3 VB position relative to the Fermi energy as a function of substrate type.  Ultimately, we aim to make more efficient solar cells by understanding what controls the band positions within the device, leading to better device design.