Carrier Screening Controls Transport in Conjugated Polymers at High Doping Concentrations.

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 counterions 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-Hückel 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 versus 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.

Raising Dielectric Permittivity Mitigates Dopant-Induced Disorder in Conjugated Polymers.

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density-of-states (DOS). The authors examine the effects of dopant-induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller-Abrahams hopping rates, show that dopant-induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade-off curve. Increasing the dielectric permittivity of the polymer mitigates dopant-carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine-doped P3HT and P3HT blended with barium titanate (BaTiO3 ) nanoparticles. The addition of 2% w/w BaTiO3 nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant-charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.

Tuning charge transport dynamics via clustering of doping in organic semiconductor thin films.

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping - in particular, clustering of dopants within conjugated polymer films - has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2';5',2'';5'',2'''-quaterthiophen-5,5'''-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang-Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications.

Correction: Directed nucleophilic addition of phenoxides to cyclopropenes.

Correction for 'Directed nucleophilic addition of phenoxides to cyclopropenes' by Pavel Yamanushkin et al., Org. Biomol. Chem., 2017, 15, 8153-8165.

Directed nucleophilic addition of phenoxides to cyclopropenes.

The alkali metal-templated addition of aryloxides across the double bond of non-conjugated cyclopropenes is described. High cis-selectivity is achieved through a directing effect of a strategically positioned carboxamide functionality.