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Transport properties of doped conjugated polymers (CPs) have been widely analyzed with the Gaussian Disorder Model (GDM) in conjunction with hopping transport between localized states. These models reveal that even in highly doped CPs, a majority of carriers are still localized because dielectric permittivity of CPs is well below that of inorganic materials, making Coulomb interactions between carriers and dopant counter-ions much more pronounced. However, previous studies within the GDM did not consider the role of screening the dielectric interactions by carriers. Here we implement carrier screening in the Debye-H¨uckel formalism in our calculations of dopant-induced energetic disorder, which modifies the Gaussian density of states (DOS). Then we solve the Pauli Master Equation using Miller-Abrahams hopping rates with states from the resulting screened DOS to obtain conductivity and Seebeck coefficient across a broad range of carrier concentrations and compare them to measurements. Our results show that screening has significant impact on the shape of the DOS and consequently on carrier transport, particularly at high doping. We prove that the slope of Seebeck coefficient vs electric conductivity, which was previously thought to be universal, is impacted by screening and decreases for systems with small dopant-carrier separation, explaining our measurements. We also show that thermoelectric power factor is underestimated by a factor of ∼ 10 at higher doping concentrations if screening is neglected. We conclude that carrier screening plays a crucial role in curtailing dopant-induced energetic disorder, particularly at high carrier concentrations.
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Charge separation boosts exciton diffusion in fused ring electron acceptors
Non-fullerene acceptors (NFAs) are highly promising materials for organic photovoltaics (OPVs). Exciton diffusion in NFAs is crucial to their photovoltaic performance, but is not yet well understood. Here we systematically examine exciton diffusion in a fused-ring electron acceptor (IDIC) based on a first-principles framework. We discover that low-energy excitons in disordered IDIC are charge-separated with electrons and holes residing on neighboring molecules, yielding long exciton lifetimes. With low energetic disorder, high exciton density of states (DOS) and long lifetimes, the disordered IDIC is predicted to exhibit large exciton diffusion lengths and high quantum efficiency. The temperature and energy dependences of exciton diffusion are explored and the manner in which various materials properties (exciton energy, DOS, energetic disorder, and phonon frequency) conspire to influence exciton diffusion is elucidated. Finally, we show that dilation could be an effective strategy to increase the exciton diffusion length in IDIC.
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- Award ID(s):
- 1828019
- PAR ID:
- 10226271
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 8
- Issue:
- 44
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 23304 to 23312
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0ta08666e
