Exciton Transport and Dissociation in Solar Energy Conversion Systems Studied by Ultrafast Pump-Probe Microscopy

Tong Zhu, Purdue University

Abstract

In solar energy conversion systems, crucial steps governing the performance of photovoltaic cells include exciton transport to the donor-acceptor interface and exciton dissociation into free carriers at the donor-acceptor interface for charge extraction. Understanding of these photophysical processes requires experimental tools interrogating the exciton properties with simultaneous high temporal and spatial resolution. In this thesis, we discuss about the development of transient absorption microscopy (TAM) with 200 fs time resolution and 50 nm spatial precision as a means to directly probe bright and dark Frenkel exciton and charge-transfer exciton transport in molecular semiconductors, as well as studying exciton dissociation, charge and energy transfer at the organic-inorganic interfaces. Long-range triplet exciton transport is necessary for the utilization of singlet fission (SF) materials for many solar cell applications. We first studied the interplay between SF dynamics and exciton diffusion in three polyacene organic semiconductors, namely, tetracene (Tc), rubrene (Ru) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). The effective exciton transport in Tc is found to be several times faster than that in Ru, and an order of magnitude faster than that in TIPS-Pn. We predict that tetracene and derivatives with a slight endothermic SF energy gap is favorable for a new singlet-mediated triplet transport mechanism while maintaining high singlet fission yield. Our findings point to a new direction in selecting and designing singlet fission materials for fabricating thicker bilayer solar cells. Then we reported our preliminary results on studying Frenkel exciton transport in a set of molecular aggregates. Artificial molecular aggregates are promising to enhance exciton transport length from tens of nanometers to up to several micrometers length scale by exploiting coherent transport effect. One dimensional, cofacially π-π stacked H aggregates are promising candidates for achieving the coherent exciton transport regime. By tuning the side chains of perylene diimide (PDI) molecules, they could be possibly crystallized into J type, intermediate and H type aggregates. Direct imaging of exciton transport in different types of PDI aggregates could allow us to gain detailed understanding of controlling coherent effect through manipulation of intermolecular coupling strength. Next, we investigated the formation, diffusion, dynamics and dissociation of charge-transfer (CT) excitons at tetracene and two-dimensional transition metal dichalcogenides (2D TMDCs) van der Waals (vdW) heterostructure interface. Electron and hole transfer is found to occur on the timescale of a few picoseconds and emission of interlayer CT excitons with a binding energy of ~ 0.3 eV has been observed. Transport of the CT excitons is directly measured by TAM, revealing coexistence of delocalized and localized states. Trapping-detrapping dynamics between the delocalized and localized states leads to stretched-exponential PL decay with an average lifetime of ~ 2 ns. The delocalized CT excitons are remarkably mobile with a diffusion constant of ~ 1 cm2s –1. These highly mobile CT excitons could have important implications in overcoming large interfacial binding energy to achieve charge separation for photovoltaic applications. Lastly, we studied triplet energy transfer between 2D TMDC material with molecular rubrene (WSe2/Ru and MoTe2/Ru). Triplet-triplet energy transfer (TTET) across the 2D heterostructure interface has been characterized by ultrafast pump-probe microscopy to occur on picosecond timescale, several orders of magnitude faster than the rate (~1µs) observed for the systems employing other inorganic sensitizers such as nanocrystals. Photoluminescence spectroscopy and PL imaging are employed to study triplet-triplet annihilation upconversion (TTAUC) in the 2D inorganic/organic vdW heterostructures. Heterostructures are also fabricated by interfacing Ru with different thickness of 2D layers to manipulate the driving force for TTET. This is a preliminary “proof-of-concept” study that demonstrates the potential of 2D TMDC as surrogates for sensitizing solid-state solar energy upconversion for device applications such as solar cells and photodetectors.

Degree

Ph.D.

Advisors

Huang, Purdue University.

Subject Area

Chemistry

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