Nanostructured electronic materials such as organic semiconductors and organic-inorganic metal halide perovskites have attracted attention both from the point of view of fundamental electronic properties and for applications in next-generation photovoltaics (PVs), light emitting diodes (LEDs) and other optoelectronic devices. In contrast to conventional semiconductors, one of the defining features of these materials is the spatial inhomogeneity of thin films on µm and sub-µm length scales. This has been widely studied by electron microscopy and X-ray techniques, yet an understanding of how these properties control and influence exciton and charge dynamics remains unknown. This lack of understanding can be traced back to the fact that transient electronic and vibrational spectroscopies are predominantly performed at the ensemble level, due to the necessity to generate a measurable signal in the presence of measurement noise. Although there are exceptions, most notably in single molecule spectroscopy, no currently available technique is able to spatially correlate transient spectroscopic data with local molecular structure and composition, thereby considerably limiting our ability to rationalise the observed spatial variations in electronic dynamics. Given the growing importance of these materials, there is a growing need for moving ultrafast electronic and vibrational spectroscopy beyond the ensemble average and towards high spatial resolution. I will introduce femtosecond wide-field transient absorption microscopy combining sub-10 fs pump and probe pulses covering the complete visible (500-650 nm) and near-infrared (650-950 nm) spectrum with diffraction-limited optical resolution. I will demonstrate the capabilities of our system by reporting the spatially- and spectrally-resolved transient electronic response of MAPbI3-xClx perovskite films across a grain boundary and reveal significant quenching of the transient bleach signal at grain boundaries. Furthermore, the unprecedented temporal resolution enables us to directly observe the formation of bandgap renormalization, completed in 25 fs after photoexcitation. In addition, we acquire hyperspectral Raman maps of TIPS pentacene films with sub-400 nm spatial and sub-15 cm-1 spectral resolution covering the 100-2000 cm-1 window. Our approach opens up the possibility of studying ultrafast dynamics on nanometer length- and femtosecond time-scales in a variety of two-dimensional and nanoscopic systems.

Ultrafast vibronic microspectroscopy of nanostructured electronic materials

253rd National Meeting (2017)

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