Chemical Physics Seminar: Halide Perovskites: are they really defect-free semiconductors?
Dr. Igal Levine, Postdoc at Weizmann Institute of Science
Deep in-gap electronic states in solar cell absorbers serve as recombination centers and are detrimental for the cell’s photovoltaic performance, especially the open circuit voltage (Voc) of the cell, as well as the charge carrier diffusion length within the absorber.
Halides Perovskites (HaPs) have emerged as a new and promising class of materials for photovoltaic applications, with unprecedented small-area cell efficiencies already exceeding 23%, and their success is largely attributed to the relatively low ( < 1016 cm-3) concentration of defects. Specifically, Lead Bromide-based perovskites are of interest as parent composition for wide bandgap ( > 1.75 eV) absorbers for photoelectrochemical solar fuel synthesis.
To date, several studies showed that there is a general difficulty to observe deep in-gap states by direct optical absorption of sub-band gap photons in the HaPs.
In the first part of my talk, I will compare between derived HaP carrier diffusion lengths from pulsed measurements with actual, measured ones using the Steady State Photocarrier Grating (SSPG) method, under 1-sun equivalent steady state excitation, and show how the defect concentrations as well as the dominant e-h recombination mechanism can be determined.
In the second part of my talk, I will discuss the direct observation of deep defects in Lead Bromide-based perovskites using highly sensitive modulated Surface PhotoVoltage (SPV) measurements and low energy photoelectron spectroscopy (PES) . We reveal that four different, deep, defect levels exist in the bulk of the mix- cation Lead tribromide layers, and show that under steady-state 1-sun equivalent conditions, these sub-band gap defect levels play an active role and that the dominant e-h recombination mechanism is primarily trap-assisted.