Abstract
The development of semicrystalline polymer semiconductors with field-effect mobilities comparable to or even exceeding those of amorphous silicon has exposed limitations of describing charge transport in these materials with disorder-based models developed originally for more disordered, lower mobility polymers. Here, we show that the charge carrier density and temperature dependence of the field-effect electron mobility and Seebeck coefficient in the semicrystalline polymer P(NDI2OD-T2) with varying degrees of crystallinity are incompatible with a description of charge transport being limited by energetic disorder effects. We provide instead direct evidence of low disorder, narrow band conduction. A spatially inhomogeneous density of states and the inclusion of short range electron–electron interactions allow to consistently explain both the measured mobility and Seebeck coefficient. These results provide a rationale for improving thermoelectric efficiency of polymer semiconductors via increasing the extension of the crystalline domains.
Highlights
The development of semicrystalline polymer semiconductors with field-effect mobilities comparable to or even exceeding those of amorphous silicon has exposed limitations of describing charge transport in these materials with disorder-based models developed originally for more disordered, lower mobility polymers
We show that in order to explain the full set of experimental mobility and Seebeck data on such semicrystalline polymers consistently it is necessary to consider a spatially inhomogeneous density of states (DoS) and include electron–electron interaction through an explicitly charge density dependent DoS
The the mobility edge (ME) model accuracy of using a tail density of Ntail the fit is still too poor and the reason is that when the band tails extend in the forbidden energy gap the system cannot be considered as a non-degenerate semiconductor at the charge densities we are considering here, it is impossible for the model to predict the kB e lnð10Þ-slope we measure nor the lack of temperature dependence, regardless of the specific choice for the DoS. To show that this is a general issue and not specific to the model we chose, we have extended our comparison in Fig. 4a to other disorder based models which are commonly employed to describe charge transport in semicrystalline polymers including the one used in this work, P(NDI2OD-T2)[7,21,31,32]
Summary
The development of semicrystalline polymer semiconductors with field-effect mobilities comparable to or even exceeding those of amorphous silicon has exposed limitations of describing charge transport in these materials with disorder-based models developed originally for more disordered, lower mobility polymers. A spatially inhomogeneous density of states and the inclusion of short range electron–electron interactions allow to consistently explain both the measured mobility and Seebeck coefficient These results provide a rationale for improving thermoelectric efficiency of polymer semiconductors via increasing the extension of the crystalline domains. Most theoretical models of charge transport in conjugated polymers assume transport to be dominated by energetic disorder effects and describe the transport properties in terms of hopping between localized states or delocalized transport above a mobility edge (ME)[4,5,6] These models were initially developed by Mott for transport in strongly disordered inorganic semiconductors and provided an accurate prediction of the temperature and charge carrier dependence of the conductivity, the most prominent feature of amorphous inorganic semiconductors[7].
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