Contributed Talk  - Thursday, 16 September I 10:50 AM (CEST)

In recent years, thin film perovskite solar cells have attracted extensive interests from researchers across the globe for their extensive advancement in a short period of time. Efficiencies of perovskite solar cells over 25% have been achieved, many of which can be processed or manufactured with cost effective solution processing method. Typically, solution processed thin film in planar perovskite solar cells are polycrystalline in nature having mean grain sizes ranging from nanometer to micrometers, which leads to variation in defects, orientation, and crystallinity within grains of the films. Therefore, local properties of perovskite thin films are likely variable that cannot be identified at macroscopic level using regular thin film and device characterization tools. Despite numerous applied research progresses, there still exists a lack of rigorous experimental studies in understanding the local fundamental properties of thin films in nanoscale regimes. So far, an explicit determination of the local charge carrier behaviors in these materials is absent. Local variations at spatially nanometer and temporarily nanosecond cannot be mapped using conventional AFM for perovskite and organic solar cells due to their limited spatial and temporal resolutions.  

In this talk, we have developed a Transient Photo-response AFM (TP-AFM) to map carrier recombination lifetime (τr), carrier transport time (τt) and carrier diffusion length (LD) in hybrid perovskite and organic solar cells. These spatially nanometer and temporarily nanosecond resolved parameters obtained from our in-house customized CS-AFM integrated with a signal oscilloscope through a breakout box and a green laser at 532nm wavelength that generates charge carriers, reveal substantial variations in charge carrier dynamics at grain boundaries (GBs) of perovskites and organic photovoltaic materials. Improved τr, τt and LD at GBs broaden the performance of these state-of-the-art mixed cation perovskite materials. Detail analysis of these parameters allow us to conclude that reduced density of trap states and recombination in mixed cation perovskites at GBs and its surrounding locations (extending to several nanometers into the grain interior) imply less ion migration. This is the first of its kind experimental realization of nanoscale mapping of charge carrier dynamics in perovskite and organic photovoltaic materials.

References from our published papers and patent

1. Science, 367 (2020) 1135-1140.

2. Nano Today, 33 (2020), 100874.

3. US patent: US2020371135A1

4. J. Am. Chem. Soc., 2020, 142, 1, 392-406