How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics

Abstract

Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states. In one-dimensional co-linear and co-facial models of the interface, increasing local energetic disorder in one phase localizes the interfacial triplet charge-transfer ((3)CT) states and increases the rate at which these states relax to form lower-energy triplet excitons. In two dimensional BHJ models, energetic disorder within the fullerene phase plays little role in further localizing states pinned to the interface. However, inhomogeneous broadening introduces strong coupling between the interfacial (3)CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

“…Triplet Photosensitizers (PSs) are compounds showing strong absorption of UV or visible light, efficient intersystem crossing (ISC), appropriate excited state redox potential, and long triplet-state lifetimes. These compounds are widely used for photo-driven energy transfer or electron transfer processes, which are the fundamental photophysical processes in photocatalysis, such as catalytic H 2 evolution by water splitting (DiSalle and Bernhard, 2011;Gärtner et al, 2011Gärtner et al, , 2012, photocatalytic redox synthetic organic reactions (Shi and Xia, 2012;Xuan and Xiao, 2012;Hari and König, 2013), photoreduction of CO 2 (Sato et al, 2013), photodynamic therapy (PDT) (Awuah and You, 2012;Kamkaew et al, 2013;Stacey and Pope, 2013;Jiang et al, 2016;Li et al, 2017), photon upconversion (triplet-triplet annihilation upconversion) (Singh- Rachford and Castellano, 2010;Ceroni, 2011;Zhao et al, 2011;Monguzzi et al, 2012;Simon and Weder, 2012;Zhou et al, 2015), photovoltaics (Guo et al, 2006;Dai et al, 2012;Bittner et al, 2014;Cheema et al, 2014;Etzold et al, 2015), and photo-initiated polymerizations (Goessl et al, 2001;Ho et al, 2010;Rivard, 2012;Cengiz et al, 2017). It is highly desired to find a chromophore to develop a series of triplet PSs to meet these requirements.…”

Exciton–Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates

“…For example, recent studies have found that the interference between dipole–dipole couplings and short-ranged charge transfer interactions underlies the diversity in absorption spectra displayed by chemically near-identical molecular crystals, 34 and offers the possibility to control the exciton mobility in such materials. 35 Other studies demonstrated the importance of wave function delocalization to charge recombination at molecular heterojunctions, 36 and the crucial role of the signs of charge-transfer integrals in the suppression of this loss mechanism. 37 , 38 In the broader context, these studies form examples of a revived interest in coherent interference effects in molecular materials, with other prominent cases to be found in singlet exciton fission, 39 polaritons in microcavities, 40 charge currents in molecular junctions, 41 and excitation mobility in DNA.…”

Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells

“…However, in low energy offset polymer-polymer blends, triplet exciton formation can actually be enhanced relative to the neat materials 15 , and triplet exciton formation is thought to be mediated by spin mixing via charge separated states 11 14 17 . Nevertheless, in some polymer:fullerene blends, triplet formation does not appear to be a major loss channel, and the leading hypothesis is that delocalization of the CT states allows charge separation to kinetically outcompete triplet exciton formation 26 41 . Restricting the discussion to blends where triplet formation is energetically favorable, this model explains why polymer-polymer blends, which exhibit bound CT states, show major losses to triplet excitons 11 15 16 17 19 , while some high performing polymer:fullerene blends do not.…”