Energy Transfer in Heterogeneous Organic Material Systems

Abstract

Rapid technological developments of photodiodes using organic semiconductors have gained significant attention over the past years. Innovative molecular and thin-film morphological designs of organic heterojunctions (HJs) are still required to enable devices such as organic photovoltaics (OPVs) approaching the market-entry-level performance. This thesis explores energy transfer properties of HJs as functions of molecular geometry, blend HJ structures, and material combinations, providing guidelines for future designs of organic optoelectronic material systems.

In the first part of this thesis, we investigate the charge transfer (CT) properties in small-molecule blend HJs comprising the nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70. A kinetic Monte Carlo model along with transmission electron microscopy and X-ray diffraction are used to quantitatively evaluate the crystallinity and percolation of the donor and acceptor at various blend ratios. A quantum confinement model is used to describe the dependence of both CT energy and lifetime on the C70 or DBP crystalline size. We identify that the delocalized CT2 state at the crystalline phase having >90% dissociation efficiency significantly contributes to efficient photogeneration in highly dilute (>80% C70) DBP/C70 HJs.

We also explore the loss mechanisms of OPVs during the transition from the photogenerated exciton energy of donor or acceptor (Eex) to the open circuit energy (qVoc). We derive universal relationships connecting the polarizabilities and exciton binding energies (EB) to molecular geometry. The design of large conjugated volumes along with the juxtaposition of electron donating and withdrawing groups characteristic of thiophene-based nonfullerene acceptors significantly decreases their EB and electron-phonon couplings, compared to more compact and symmetric fullerenes, leading to the decrease of energy loss by >0.2 eV and increase of efficiency for OPVs.

Furthermore, we explore the exciton energy transfer and annihilation at the high-exciton-density regime of an organic type-I HJ comprising 4-(dicyanomethylene)-2- methyl-6-julolidyl-9-enyl-4H-pyran (DCM2) and tris(8-hydroxyquinolinato) aluminum (Alq3). Interactions of singlet and triplet excitons show significantly different temperature dependence, suggesting their different energy transfer mechanisms, and eventually allowing for a separate management of singlet and triplet energy transport in organic material systems.

The second part of this thesis presents optoelectronic properties and applications of a new class of organic-inorganic HJs comprising a monolayer of transition metal dichalcogenide (TMDC) and a thin film of organic semiconductor. Both theoretical and experimental investigations of a HJ employing WS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) indicate that Frenkel states in PTCDA and two-dimensional (2D) Wannier-Mott states in the WS2 dissociate to form hybrid CT excitons at the interface that subsequently dissociate into free charges that are collected at opposing electrodes. Approximately 30% Förster resonant energy transfer is observed between excitons in the PTCDA and a monolayer MoSe2. This energy transfer from low mobility organic materials to higher mobility 2D semiconductors along with their extremely large oscillator strengths presents an attractive platform for developing new types of photodetection and energy harvesting systems.