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.
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Enhanced thermoelectric performance of F4-TCNQ doped FASnI 3 thin films
In the past decade, great efforts have been devoted to the development of organic–inorganic hybrid perovskites for achieving efficient photovoltaics, but less attention has been paid to their thermoelectric applications. In this study, for the first time, we report the thermoelectric performance of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped NH 2 CHNH 2 SnI 3 (FASnI 3 ) thin films. It is found that the electrical conductivities of the F4-TCNQ doped FASnI 3 thin films increase and then decrease along with increased doping levels of F4-TCNQ. Systematic studies indicate that enhanced electrical conductivities are attributed to the increased charge carrier concentrations and mobilities and superior film morphologies of the F4-TCNQ doped FASnI 3 thin films, and decreased electrical conductivities originate from the cracks and poor film morphology of the F4-TCNQ doped FASnI 3 thin films induced by excess F4-TCNQ dopants. The quantitative thermal conductivity scanning thermal microscopy studies reveal that the F4-TCNQ doped FASnI 3 thin films exhibit ultralow thermal conductivities. Moreover, the thermoelectric performance of the F4-TCNQ doped FASnI 3 thin films is investigated. It is found that the F4-TCNQ doped FASnI 3 thin films exhibit a Seebeck coefficient of ∼310 μV K −1 , a power factor of ∼130 μW m −1 K −2 and a ZT value of ∼0.19 at room temperature. All these results demonstrate that our studies open a door for exploring cost-effective less-toxic organic–inorganic hybrid perovskites in heat-to-electricity conversion applications at room temperature.
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- Award ID(s):
- 1903303
- NSF-PAR ID:
- 10225824
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 8
- Issue:
- 47
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 25431 to 25442
- 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/D0TA09329G
