Slow geminate‐charge‐pair recombination dynamics at polymer: Fullerene heterojunctions in efficient organic solar cells

Abstract

We explore charge recombination dynamics at electron donor‐acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC70BM), by means of combined time‐resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron‐hole pairs recombines with a temperature‐independent rate distribution spanning submicrosecond timescales to produce luminescent charge‐transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge‐pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long‐lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate‐charge recombination highlights the importance of the molecular nature of specific donor–acceptor electronic interactions in defining the relaxation pathways of trapped GPP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

“…Therefore, because photocarriers are generated with high yield in that system, there must be a direct mechanism for their generation that must bypass CT intermediates. Similarly, the present work points strongly to direct photogeneration (that is, no CT intermediate) of unbound polarons in PCDTBT:PCBM, although it is also clear that CTX must form eventually in this system since in a previous publication we observed geminate recombination through CTX emission on nanosecond to microsecond timescales 51 . We note from our work, however, that the direct polaron-generation mechanism is operative on ultrafast timescales in PCDTBT: PCBM, with no forthright evidence of early involvement of intermediate CT states, while both channels may contribute to photocurrent in P3HT:PCBM.…”

Particle plasmon‐induced charge trapping at heterointerfaces in PCDTBT:PC70BM blends

“…The negative feature denotes the bleaching of ground‐state absorption. The broad positive feature can be assigned to polaron pair absorption (PA), which has also been observed by Etzold et al and Provencher et al…”

“…). Similar to the classic model, the photo–electron conversion can be simply described by three processes: electron transition from polymer ground state (S 0 ) to excited state (S 1 ) under excitation; electron transfer (e‐transfer) to GPP state at the heterojunction; and polaron pairs dissociation into free charges . However, in our model, the interfacial GPP state has a broadband distribution, which includes untrapped and trapped states.…”

Visualizing excitations at buried heterojunctions in organic semiconductor blends

“…The system has been previously well characterised optically and structurally. [15][16][17][18] For instance several studies have shown that structurally all blends consist of intermixed regions of fullerene and polymer, the so-called mixed-phase. 12,13 Intercalation of fullerenes between the amorphous PCDTBT chains is observed [19][20][21] and strong similarities to fullerenes intercalating in-between MDMO-PPV are reported.…”

“…We note that these sites are different from CT states which exhibit transitions at much lower energies of around 1.46 eV (see Figure S1.1). 17 In unoptimised 4:1 polymer:fullerene blends, charges are formed at the lowest energy sites, and moving away from the interface entails an energetic penalty, leading to low EQEs and high geminate recombination. For the optimised 1:4 polymer:fullerene blend the interfacial energetics drive a very rapid, sub 150fs, motion of hole polarons away from the higher energy interfacial sites to low energy more highly ordered sites.…”