Structure–Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

Abstract

Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure—function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure–function relationship in organic semiconductors.

Equilibrium Formation of Stable All‐Silicon Versions of 1,3‐Cyclobutanediyl

“…D has an inverse cubic dependence on the distance between the two unpaired electron spins, and hence gives information about the delocalization of the triplet exciton. Comparing the value of D with those obtained for naphthalene (D=2982 MHz) and anthracene (D=2154 MHz) seems to indicate that the triplet exciton is more delocalized in 3 a although such comparisons are to be treated with caution as the even lower value of the thermally excited triplet state of ( t Bu 2 MeSi) 2 Si=Si(SiMe t Bu 2 ) 2 shows (D≈1340 MHz) . Excitation at different wavelengths within the absorption spectrum resulted in identical spectra, both in terms of their shape as well as in the overall intensity if normalized to the number of incident photons.…”

Impact of Side Chains of Conjugated Polymers on Electronic Structure: A Case Study

“…Figure 3). One particular strength of EPR spectroscopy in general and TREPR spectroscopy in particular is not only its exclusive sensitivity to paramagnetic states, but also the clear distinction possible between triplet states and coulombically-bound polaron pairs, often termed charge-transfer complexes or radical pairs [25,73]. Both states consist of two unpaired electron spins interacting with each other via dipolar and exchange coupling.…”

“…A radical pair with its much larger separation of the two electron spins exhibited a much weaker dipolar and exchange interaction, the latter often negligible. Hence, its spectral width is dramatically reduced as compared to a triplet state [25,73].…”

“…Many different methods are available to probe the morphology of conjugated polymers, at least in films. Recently, we demonstrated (time-resolved) Electron Paramagnetic Resonance (EPR) spectroscopy [25,26,27] to be able to probe both film [28] and solution [29] morphology with molecular resolution. In contrast to morphology, direct access to the electronic structure of conjugated polymers is much more difficult to achieve.…”

Time‐Resolved X‐Ray Absorption Spectroscopy: Visualizing the Time Evolution of Photophysics and Photochemistry in Photocatalytic Solar Energy Conversion

“…Thus TR‐EPR is not suitable for fast photophysical and photochemical processes occurring normally on (sub)picosecond time scales. [ 6 ] By contrast, time‐resolved transient absorption spectroscopy (TR‐TAS) is an ultrafast laser pumping‐probe technique, by which the transitions between excited energy levels, including energy transfer, electron transfer, and other physical and chemical processes, can be analyzed on timescales reaching femtoseconds (fs). [ 7 ] In addition, time‐resolved photoluminescence (TR‐PL) experiments also provide a powerful tool to investigate the photoinduced charge transfer process on timescales ranging from picoseconds (ps) to seconds.…”