Chemical doping is widely used to manipulate the electrical and thermoelectric properties of organic semiconductors, yet intelligent design of polymer–dopant systems remains elusive. It is challenging to predict the electrical and thermoelectric properties of doped organic semiconductors due to the large number of variables impacting these properties, including film morphology, dopant and polymer energetics, dopant size, and degree of polaron delocalization. Herein, a series of dopants with varying sizes and electron affinities (EAs) are combined with polymers of differing ionization energies (IEs) to investigate how the difference between polymer IE and dopant EA influences the doping efficiency and electrical conductivity, and how the dopant size influences the thermoelectric properties. Our experiments demonstrate that at low doping levels the doping efficiency strongly depends on the difference between the polymer IE and dopant EA; the effectiveness of doping on increasing electrical conductivity drastically decreases at high loadings for the molybdenum dithiolene complexes, while FeCl 3 remains effective at high loadings; and the large molybdenum complexes lead to more delocalized polarons as compared to FeCl 3 . To take advantage of the complementary doping characteristics of the molybdenum complexes and FeCl 3 , both dopants are employed simultaneously to reach high power factors at relatively low dopant concentrations.
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Visualisation of individual dopants in a conjugated polymer: sub-nanometre 3D spatial distribution and correlation with electrical properties
While molecular doping is ubiquitous in all branches of organic electronics, little is known about the spatial distribution of dopants, especially at molecular length scales. Moreover, a homogeneous distribution is often assumed when simulating transport properties of these materials, even though the distribution is expected to be inhomogeneous. In this study, electron tomography is used to determine the position of individual molybdenum dithiolene complexes and their three-dimensional distribution in a semiconducting polymer at the sub-nanometre scale. A heterogeneous distribution is observed, the characteristics of which depend on the dopant concentration. At 5 mol% of the molybdenum dithiolene complex, the majority of the dopant species are present as isolated molecules or small clusters up to five molecules. At 20 mol% dopant concentration and higher, the dopant species form larger nanoclusters with elongated shapes. Even in case of these larger clusters, each individual dopant species is still in contact with the surrounding polymer. The electrical conductivity first strongly increases with dopant concentration and then slightly decreases for the most highly doped samples, even though no large aggregates can be observed. The decreased conductivity is instead attributed to the increased energetic disorder and lower probability of electron transfer that originates from the increased size and size variation in dopant clusters. This study highlights the importance of detailed information concerning the dopant spatial distribution at the sub-nanometre scale in three dimensions within the organic semiconductor host. The information acquired using electron tomography may facilitate more accurate simulations of charge transport in doped organic semiconductors.
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- Award ID(s):
- 1729737
- NSF-PAR ID:
- 10394392
- Date Published:
- Journal Name:
- Nanoscale
- Volume:
- 14
- Issue:
- 41
- ISSN:
- 2040-3364
- Page Range / eLocation ID:
- 15404 to 15413
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Achieving high electrical conductivity and thermoelectric power factor simultaneously for n‐type organic thermoelectrics is still challenging. By constructing two new acceptor‐acceptor n‐type conjugated polymers with different backbones and introducing the 3,4,5‐trimethoxyphenyl group to form the new n‐type dopant 1,3‐dimethyl‐2‐(3,4,5‐trimethoxyphenyl)‐2,3‐dihydro‐1
H ‐benzo[d]imidazole (TP‐DMBI), high electrical conductivity of 11 S cm−1and power factor of 32 μW m−1 K−2are achieved. Calculations using Density Functional Theory show that TP‐DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of −1.94 eV than that of the common dopant 4‐(1, 3‐dimethyl‐2, 3‐dihydro‐1H ‐benzoimidazol‐2‐yl) phenyl) dimethylamine (N‐DMBI) (−2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n‐type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N‐DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP‐DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm2 V−1 s−1than films with N‐DMBI doping, demonstrating the potential of TP‐DMBI, and 3,4,5‐trialkoxy DMBIs more broadly, for high performance n‐type organic thermoelectrics. -
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Abstract Achieving high electrical conductivity and thermoelectric power factor simultaneously for n‐type organic thermoelectrics is still challenging. By constructing two new acceptor‐acceptor n‐type conjugated polymers with different backbones and introducing the 3,4,5‐trimethoxyphenyl group to form the new n‐type dopant 1,3‐dimethyl‐2‐(3,4,5‐trimethoxyphenyl)‐2,3‐dihydro‐1
H ‐benzo[d]imidazole (TP‐DMBI), high electrical conductivity of 11 S cm−1and power factor of 32 μW m−1 K−2are achieved. Calculations using Density Functional Theory show that TP‐DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of −1.94 eV than that of the common dopant 4‐(1, 3‐dimethyl‐2, 3‐dihydro‐1H ‐benzoimidazol‐2‐yl) phenyl) dimethylamine (N‐DMBI) (−2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n‐type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N‐DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP‐DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm2 V−1 s−1than films with N‐DMBI doping, demonstrating the potential of TP‐DMBI, and 3,4,5‐trialkoxy DMBIs more broadly, for high performance n‐type organic thermoelectrics. -
Thin films of amorphous small molecule semiconductors are widely used in organic light emitting displays and have promising applications in solar cells and thermoelectric devices. Adding dopants increases the conductivity of organic semiconductors, but high concentrations of dopants can disrupt their structural ordering, alter the shape of the electronic density of states in the material, and increase the effects of Coulomb interactions on charge transport. Electrical doping of the solution processable hole-transport material 2,2′,7,7′-tetrakis[ N , N -di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) was studied with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) as a p-type dopant. Infiltration of F 4 TCNQ from the vapor phase into films of spiro-OMeTAD provided a route to highly doped films with up to 39 ± 2 mol% doping. Structural characterization confirmed that the films remain amorphous even at the highest doping levels with no apparent phase separation. We quantitatively determined the carrier concentration using UV-Vis spectroscopy to interpret the evolution of the electrical conductivity. Over the range of carrier concentrations (10 19 –10 20 1 cm −3 ), the electrical conductivity increased no more than linearly with carrier concentration, while the thermopower had a small increase with carrier concentration. The trends in conductivity and thermopower were related to the unique electronic structure of spiro-OMeTAD, which is able to support two carriers per molecule. Temperature-dependent conductivity measurements were used to further analyze the transport mechanism.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2NR03554E
