Investigation of Local Dynamics on the Sub-micron Scale in Organic Blends Using an Ultrafast Confocal Microscope
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1270-HH01-03
Investigation of Local Dynamics on the Sub-micron Scale in Organic Blends using an Ultrafast Confocal Microscope
G. Grancini1, D. Polli1, J. Clark1, T. Virgili2, G. Cerullo1, G.Lanzani3 1
Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy
2
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy 3
Center for Nano Science and Technology of IIT@POLIMI, via Pascoli 70/3 20133 Milano, Italy
ABSTRACT We introduce a novel instrument combining femtosecond pump-probe spectroscopy and confocal microscopy for spatio-temporal imaging of excited-state dynamics of phase-separated polymer blends. Phenomena occurring at interfaces between different materials are crucial for optimizing the device performances, but are poorly understood due to the variety of possible electronic states and processes involved and to their complicated dynamics. Our instrument (with 200-fs temporal resolution and 300-nm spatial resolution) provides new insights into the properties of polymer blends, revealing spatially variable photo-relaxation paths and dynamics and highlighting a peculiar behaviour at the interface between the phase-separated domains.
INTRODUCTION Plastic optoelectronics and photonics are based on blends of organic semiconductors, properly engineered in order to obtain samples with enhanced or new properties respect to the characteristics of their constituents. In such samples, phenomena occurring at interfaces between the two materials ultimately determine the desired device performances but are extremely complex and yet poorly understood. The photogenerated singlet excitons can undergo a variety of processes, such as mono- and bimolecular decay, internal conversion, energy and charge transfer or intersystem crossing. Ultrafast optical spectroscopy provides rich information on the photophysics of organic semiconductors [1], but in standard setups it has a limited spatial resolution (≈100 µm), so that the experimental results are averaged over many mesoscopic domains. Microscopy techniques, on the other hand, can provide very high spatial resolution, but they have no access to the excited state dynamics and to the relaxation paths which dictate the ultimate device performance. For these reasons, we have developed an instrument combining broadband femtosecond pump-probe spectroscopy with confocal microscopy, delivering simultaneously high temporal and spatial resolution. This tool provides new insight into the properties of polymer blends by directly measuring the sample dynamics at the interfaces between the different domains.
EXPERIMENT The ultrafast confocal microscope setup is sketched in Figure 1. It is driven by 10-µJ, 150-fs pulses at 800 nm wavelength and 1-kHz repetition rate delivered by a commercial Ti:sapphire regeneratively-amplified laser. Pump pulses at 400 nm are produced by second harmonic generation, while probe pulses are created by filtering with 10-nm interference filters a white-light continuum generated in a s
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