Search

Functionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in-plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight-fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll-off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.

Chapter 4

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of approximate to 5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 mu W m(-1) K-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., Funding Agencies|STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; China Scholarship CouncilChina Scholarship Council; Center for Information Technology of the University of Groningen; Swedish Research CouncilSwedish Research Council [2016-03979]; Olle Engkvists Stiftelse [204-0256]; Advanced Functional Materials center at LiU [2009 00971]; King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) [OSR-CRG2018-3737]; NWO Exact and Natural Sciences [2020/ENW/00852342]

Chapter 4

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of approximate to 5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 mu W m(-1) K-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., Funding Agencies|STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; China Scholarship CouncilChina Scholarship Council; Center for Information Technology of the University of Groningen; Swedish Research CouncilSwedish Research Council [2016-03979]; Olle Engkvists Stiftelse [204-0256]; Advanced Functional Materials center at LiU [2009 00971]; King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) [OSR-CRG2018-3737]; NWO Exact and Natural Sciences [2020/ENW/00852342]

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of approximate to 5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 mu W m(-1) K-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., Funding Agencies|STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; China Scholarship CouncilChina Scholarship Council; Center for Information Technology of the University of Groningen; Swedish Research CouncilSwedish Research Council [2016-03979]; Olle Engkvists Stiftelse [204-0256]; Advanced Functional Materials center at LiU [2009 00971]; King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) [OSR-CRG2018-3737]; NWO Exact and Natural Sciences [2020/ENW/00852342]

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of approximate to 5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 mu W m(-1) K-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., Funding Agencies|STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; China Scholarship CouncilChina Scholarship Council; Center for Information Technology of the University of Groningen; Swedish Research CouncilSwedish Research Council [2016-03979]; Olle Engkvists Stiftelse [204-0256]; Advanced Functional Materials center at LiU [2009 00971]; King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) [OSR-CRG2018-3737]; NWO Exact and Natural Sciences [2020/ENW/00852342]

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of approximate to 5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 mu W m(-1) K-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics., Funding Agencies|STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; China Scholarship CouncilChina Scholarship Council; Center for Information Technology of the University of Groningen; Swedish Research CouncilSwedish Research Council [2016-03979]; Olle Engkvists Stiftelse [204-0256]; Advanced Functional Materials center at LiU [2009 00971]; King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) [OSR-CRG2018-3737]; NWO Exact and Natural Sciences [2020/ENW/00852342]

The molecular doping of organic semiconductors represents a key strategy for advancing organic electronic applications. However, the n-doping of organic materials is usually less efficient than p-doping and strategies toward the design of more efficient n-doping still remain less explored. In this contribution, the impact of electrostatic interaction is explored on the doping efficiency of fullerene derivatives. [6,6]-Phenyl-C 61-butyric acid methyl ester (PCBM) and a [60]fulleropyrrolidine with a more polarizable triethylene glycol type side chain (PTEG-1) are employed for a comparative study. It is found that the doping efficiency of lightly doped PCBM layers is limited to a few percent, while doped PTEG-1 films exhibit very high doping efficiency approaching 100%. The enhanced n-doping of PTEG-1 compared with that of PCBM is further substantiated by Raman and Fourier transform infrared spectroscopic studies. The activation energy for charge generation in doped PTEG-1 is much smaller than that of doped PCBM, which confirms a higher probability for dissociation of charge transfer complexes in the former compared to the latter. The enhanced molecular n-doping for PTEG-1 is attributed to the electrostatic interaction between the charge transfer complex and the polar environment offered by the triethylene glycol diether side chain.

The molecular doping of organic semiconductors represents a key strategy for advancing organic electronic applications. However, the n-doping of organic materials is usually less efficient than p-doping and strategies toward the design of more efficient n-doping still remain less explored. In this contribution, the impact of electrostatic interaction is explored on the doping efficiency of fullerene derivatives. [6,6]-Phenyl-C 61-butyric acid methyl ester (PCBM) and a [60]fulleropyrrolidine with a more polarizable triethylene glycol type side chain (PTEG-1) are employed for a comparative study. It is found that the doping efficiency of lightly doped PCBM layers is limited to a few percent, while doped PTEG-1 films exhibit very high doping efficiency approaching 100%. The enhanced n-doping of PTEG-1 compared with that of PCBM is further substantiated by Raman and Fourier transform infrared spectroscopic studies. The activation energy for charge generation in doped PTEG-1 is much smaller than that of doped PCBM, which confirms a higher probability for dissociation of charge transfer complexes in the former compared to the latter. The enhanced molecular n-doping for PTEG-1 is attributed to the electrostatic interaction between the charge transfer complex and the polar environment offered by the triethylene glycol diether side chain.

Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that su, Funding Agencies|German Ministry of Science and Education [FKZ 13N13719]; STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; National Natural Science Foundation of China (NNSFC)National Natural Science Foundation of China [21875286]

N-doping of conjugated polymers either requires a high dopant fraction or yields a low electrical conductivity because of their poor compatibility with molecular dopants. We explore n doping of the polar naphthalenediimide-bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole-dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high electrical conductivity of more than 10(-1) S cm(-1) for a dopant concentration of only 10 mol % when measured in an inert atmosphere. We find that the doped polymer is able to maintain its electrical conductivity for about 20 min when exposed to air and recovers rapidly when returned to a nitrogen atmosphere. Overall, solution coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelectric power factor of up to 0.4 mu W K-2 m(-1) compared to other NDI-based polymers., Funding Agencies|Swedish Research Council [2016-06146, 2016-03979]; Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship, and the European Research Council (ERC) [637624]; VINNOVA [2015-04859]

n/a

A new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk-heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin-coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near-infrared bias light illumination. Drift-diffusion calculations confirm that near-infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.