Dr. Xiaoming Wen
Professor, Center for Micro-Photonics, Swinburne University of Technology, Australia.

Speech title: Understanding photophysics of organic-inorganic halide perovskites for next generation high efficiency low cost solar cells - carrier and ion dynamics from femtoseconds to minutes

Abstract: Organic-inorganic hybrid perovskites have been shown very promising for next generation high efficiency low cost solution fabricated solar cells with quickly increased conversion efficiency over 22%1-3, which is usually attribute to their superb light absorption, decent ambipolar charge mobility and efficient exciton dissociation. 4-6 In addition, perovskites are promising candidate for photonic application, such as LED, light source, detector and sensors due to the high efficient luminescence. The device design and performance improvement critically depend on the physical understanding, in particular, the carrier dynamics at various timescale. More recently, two-dimensional perovskites are emerging as promising materials for photovoltaics and photonics.7 Despite the rapid progress in demonstrated efficiencies, there exists a gap in the fundamental understanding of the carrier dynamics and the instability and performance I-V hysteresis are the major technical barrier restrained its commercialization.8,9 It has been a consensus that mobile ion migration and accumulation is the physical mechanism for performance instability, I-V hysteresis and the changes of performance during light soaking.10-12 Dynamic information of mobile ions in organic semiconductors and ionic materials is considered as the ultimate influence on their performance in the photonic applications.13,14 Comprehensive understanding is urgently required forward highly stable high efficiency solar cells.
Here we systematically investigate the carrier and mobile ion dynamics in organic-inorganic halide perovskite using steady state and time-resolved spectroscopy combined with microscopy, in the various timescales from femtoseconds up to hours. In the picosecond range, ultrafast transient absorption measurement confirmed the slowed cooling of hot carriers in halide perovskites, which is promising for the next high efficiency hot carrier solar cells, with efficiency over the Shockley–Queisser limit.10,15,16 Recombination dynamics is investigated by nanoscale time-resolved photoluminescence, such as TCSPC and FLIM. Our investigation reveals both steady state PL and time-resolved PL are closely relevant to the excitation intensity. The carrier dynamics can be dominantly determined by the free carrier recombination, defect and surface recombinations. A nonlinear relationship between the excitation and emission intensity is also identified in both the steady state PL and time-resolved PL, confirmed the free electron-hole recombination is the major mechanism. Theoretical modelling reveals that the nonlinearity is determined by both the relaxation rate and density of the defect states; and thus relevant to the fabrication. Moreover, mobile ions have been proven significantly impact the carrier dynamics.11,12,14,17-19 Mobile ion migration and accumulation result in a much slow process in milliseconds to hours, and closely relevant to the characteristic hysteresis in I-V measurement and light soaking effects.