Identifying triplet pathways in dilute pentacene films

Abstract

Building efficient triplet-harvesting layers for photovoltaic applications requires a deep understanding of the microscopic properties of the components involved and their dynamics. Singlet fission is a particularly appealing mechanism as it generates two excitons from a single photon. However, the pathways of the coupled triplets into free species, and their dependence on the intermolecular geometry, has not been fully explored. In this work, we produce highly ordered dilute pentacene films with distinct parallel and herringbone dimers and aggregates. Using electron paramagnetic resonance spectroscopy, we provide compelling evidence for the formation of distinct quintet excitons in ambient conditions, with intrinsically distinctive electronic and kinetic properties. We find that the ability of quintets to separate into free triplets is promoted in the parallel dimers and this provides molecular design rules to control the triplets, favouring either enhanced photovoltaic efficiency (parallel) or strongly bound pairs that could be exploited for logic applications (herringbone).

Growth, morphology and structure of mixed pentacene films

“…Both were used with no further purification as this has no effect on morphology or structure, as observed by comparing with previous work employing purified powder. 7 Pentacene and p-terphenyl were (co-)evaporated onto the silicon wafers from individual knudsen cells to a thickness of 200 nm using organic molecular beam deposition (OMBD) at a base pressure of 3x10 -7 mbar. The OMBD chamber used was a Kurt J. Lesker Spectros 100 system and the rates were monitored using quartz crystal microbalance sensors placed near the sources and at the substrate.…”

“…[1][2][3][4] P-terphenyl crystals, on the other hand, have been commonly implemented as scintillators for identifying particles for neutron energy spectrum measurements for their fast time response and high quantum yield. 5 This molecule has also been used as a matrix for the dilution of pentacene in crystal 6 and thin film 7 form.…”

“…By studying the texture of the films, we discover that the ratio of thin film and bulk phases is strongly dependent on the orientation of the films. The need to develop well-defined systems to investigate solidstate phenomena in optoelectronic and spintronic devices has already been highlighted in various studies 7,10,26,27 . Our work provides a methodology to grow the precise systems required in such studies.…”

Fluctuating exchange interactions enable quintet multiexciton formation in singlet fission

“…Whilst the initial and final states of SF are fairly simple and well understood, the dynamic pathways that allow transitions between them are complex [5][6][7] . Recent experimental results have revealed the presence of quintet excitons involved in the SF process [8][9][10] , an observation which has become even more interesting as these higher spin states are observed in more classes of molecular system which undergo SF [11][12][13] . This has raised questions related to the underlying mechanism which generate quintet states, and motivates the work described below.…”

Competition between Singlet Fission and Spin‐Orbit‐Induced Intersystem Crossing in Anthanthrene and Anthanthrone Derivatives

“…In addition, the difference in | D | and | E | values between SO‐ISC and SF‐ISC triplets (Table ) indicates different types of aggregate formation in toluene versus iodoethane. It has been shown that | D | and | E | values are different between herringbone and parallel orientations of dilute pentacene films due to a change in the dipolar coupling . The smaller | D | value of compound 2 in iodoethane indicates that the electron distribution in the triplet state is more delocalized in the aggregate formed in iodoethane than it is in toluene.…”