Abstract-Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations

Sum TC1, Mathews N2,3, Xing G1, Lim SS1,4, Chong WK1,4, Giovanni D1,4, Dewi HA3.

http://www.ncbi.nlm.nih.gov/pubmed/26820796

• 1School of Physical and Mathematical Sciences, Nanyang Technological University , 21 Nanyang Link, 637371 Singapore.
• 2School of Materials Science and Engineering, Nanyang Technological University , Nanyang Avenue, 639798 Singapore.
• 3Energy Research Institute @NTU (ERI@N), Research Techno Plaza, Nanyang Technological University , X-Frontier Block, Level 5, 50 Nanyang Drive, 637553 Singapore.
• 4Energy Research Institute @NTU (ERI@N), Interdisciplinary Graduate School Nanyang Technological University , 639798 Singapore.

Lead halide perovskite solar cells are presently the forerunner among the third generation solution-processed photovoltaic technologies. With efficiencies exceeding 20% and low production costs, they are prime candidates for commercialization. Critical insights into their light harvesting, charge transport, and loss mechanisms have been gained through time-resolved optical probes such as femtosecond transient absorption spectroscopy (fs-TAS), transient photoluminescence spectroscopy, and time-resolved terahertz spectroscopy. Specifically, the discoveries of long balanced electron-hole diffusion lengths and gain properties in halide perovskites underpin their significant roles in uncovering structure-function relations and providing essential feedback for materials development and device optimization. In particular, fs-TAS is becoming increasingly popular in perovskite characterization studies, with commercial one-box pump-probe systems readily available as part of a researcher's toolkit. Although TAS is a powerful probe in the study of charge dynamics and recombination mechanisms, its instrumentation and data interpretation can be daunting even for experienced researchers. This issue is exacerbated by the sensitive nature of halide perovskites where the kinetics are especially susceptible to pump fluence, sample preparation and handling and even degradation effects that could lead to disparate conclusions. Nonetheless, with end-users having a clear understanding of TAS's capabilities, subtleties, and limitations, cutting-edge work with deep insights can still be performed using commercial setups as has been the trend for ubiquitous spectroscopy instruments like absorption, fluorescence, and transient photoluminescence spectrometers. Herein, we will first briefly examine the photophysical processes in lead halide perovskites, highlighting their novel properties. Next, we proceed to give a succinct overview of the fundamentals of pump-probe spectroscopy in relation to the spectral features of halide perovskites and their origins. In the process, we emphasize some key findings of seminal photophysical studies and draw attention to the interpretations that remain divergent and the open questions. This is followed by a general description into how we prepare and conduct the TAS characterization of CH3NH3PbI3thin films in our laboratory with specific discussions into the potential pitfalls and the influence of thin film processing on the kinetics. Lastly, we conclude with our views on the challenges and opportunities from the photophysical perspective for the field and our expectations for systems beyond lead halide perovskites.