The role of spin in the kinetic control of recombination in organic photovoltaics

Abstract

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

“…The impact of fullerene nanocrystallinity on charge separation has been noted previously, where it was attributed to high local electron mobility within the traditional Onsager framework 11,30 as well as to shifts in electron affinity of the fullerene crystallites 31 . In contrast, our results here ARTICLE point to the role of delocalization, analogous to recent findings for organic-inorganic HJs 32 , and are consistent with a growing number of recent reports in which charge separation is driven by accessing band-like CT states that subsequently decay on a sub-50 ps timescale (an upper limit based on the resolution of our PL transient data) into separated, free charge carriers 3,4,29,33 . In summary, these results support a model of delocalized CT state dissociation and show that this process depends critically on nanoscale fullerene crystallinity.…”

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…We understand that charge recombination is equally important as charge generation in PSCs, and recent findings suggest that kinetics (rather than thermodynamics) can prevent charge recombination and hence increase the device performance. 27,28 We start with experimental observations of CT states from both absorption and emission measurements. This is followed by spectroscopic evidence that free charge carriers are generated in the range of 100 femtoseconds in highly efficient blends.…”

Ultrafast charge separation and nongeminate electron–hole recombination in organic photovoltaics

“…In efficient devices, either re-trapping must be suppressed or the trapped CT states that form must themselves be able to separate into free charges. In a recent experiment, Rao et al noted the absence of nongeminate triplet excitons at open circuit in an efficient PIDT-PhanQ:PC 60 BM device 17 . Since three quarters of nongeminate CT states should have triplet character, they concluded that such CT states were able to separate long after exciton dissociation first occurs, thus avoiding the formation of triplet excitons.…”