Charge‐Carrier Density Independent Mobility in Amorphous Fluorene‐Triarylamine Copolymers

Alasdair J Campbell,Abhimanyu Mukherjee,Ruth Rawcliffe,Maxim Shkunov,Donal D C Bradley,Martyn A Mclachlan,Alexander Guite,Jorge Costa Dantas Faria

doi:10.1002/adfm.201504722

Alasdair J Campbell, Abhimanyu Mukherjee + Show 6 more

Open Access

https://doi.org/10.1002/adfm.201504722

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Abstract

A charge‐carrier density dependent mobility has been predicted for amorphous, glassy energetically disordered semiconducting polymers, which would have considerable impact on their performance in devices. However, previous observations of a density dependent mobility are complicated by the polycrystalline materials studied. Here charge transport in field‐effect transistors and diodes of two amorphous, glassy fluorene‐triarylamine copolymers is investigated, and the results explored in terms of a charge‐carrier density dependent mobility model. The nondispersive nature of the time‐of‐flight (TOF) transients and analysis of dark injection transient results and transistor transfer characteristics indicate a charge‐carrier density independent mobility in both the low‐density diode and the high‐density transistor regimes. The mobility values for optimized transistors are in good agreement with the TOF values at the same field, and both have the same temperature dependency. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge‐carrier density dependent model, and does not follow the expected power‐law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge‐carrier density independent mobility, and this is discussed in terms of polaron‐dominated hopping and interchain correlated disorder.

Highlights

Organic semiconductors (OSCs) such as conjugated polymers can be used in a range of different optoelectronic devices, including organic light emitting diodes (OLEDs), organic photovoltaics (OPVs) and organic field-effect transistors (OFETs)
Despite a relatively long scan time, the diffractogram showed no diffraction peaks, the spectrum being dominated by the background due to the glass substrate (we note that F8BT films show clear diffraction peaks under the same experimental conditions with the same apparatus)
The dark injection (DI) and TOF mobility values at a given field for PFB do not vary with charge-carrier density; the smaller data set for PFMO is consistent with this

Summary

Introduction

Organic semiconductors (OSCs) such as conjugated polymers can be used in a range of different optoelectronic devices, including organic light emitting diodes (OLEDs), organic photovoltaics (OPVs) and organic field-effect transistors (OFETs). The power-law dependency of the transistor mobility is calculated for the Vissenberg and Matters model from the TOF Gaussian DOS width, and compared to the measured OFET values. To investigate hole transport in PFB and PFMO in the low-carrier density diode regime, we use the time-of-flight (TOF) technique to measure the nature of transport and its field dependence, and analyze transient SCLC dark injection and TOF results to find the charge-carrier density dependence over the same field range.

Results

Conclusion