Kavli Affiliate: Graham Fleming
| First 5 Authors: Yuping Shi, Partha Roy, Naoki Higashitarumizue, Tsung-Yen Lee, Quanwei Li
| Summary:
A combination of ultrafast, long-range and low-loss excitation energy
transfer from the photo-receptor location to a functionally active site is
essential for cost-effective polymeric semiconductors. Delocalized electronic
wavefunctions along {pi}-conjugated polymer backbone can enable efficient
intrachain transport, while interchain transport is generally thought slow and
lossy due to weak chain-chain interactions. In contrast to the conventional
strategy of mitigating structural disorder, amorphous layers of rigid
conjugated polymers, exemplified by highly planar
poly(indacenodithiophene-co-benzothiadiazole) (IDT-BT) donor-accepter
copolymer, exhibit trap-free transistor performance and charge-carrier
mobilities similar to amorphous silicon. Here we report long-range exciton
transport in HJ-aggregated IDTBT thin-film, in which the competing exciton
transport and exciton-exciton annihilation (EEA) dynamics are spectroscopically
separated using a phase-cycling-based scheme and shown to depart from the
classical iffusion-limited and strong-coupling regime. In the thin film, we
find an annihilation-limited mechanism with a per-encounter annihilation
probability of much less than 100%, facilitating the minimization of
EEA-induced excitation losses. In contrast, excitons on isolated IDTBT chains
diffuse over 350 nm with 0.56 cm2 s-1 diffusivity, before eventually
annihilating with unit probability on first contact. We complement the
pump-probe studies with temperature dependent photocurrent and EEA measurements
from 295 K to 77 K and find a remarkable correspondence of annihilation rate
and photocurrent activation energies in the 140 K to 295 K temperature range.
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