Large bipolaron density at organic semiconductor/electrode interfaces

Rijul Dhanker,Sukrit Mukhopadhyay,Sean Nunez,Noel C Giebink,Christopher L Gray,Anatoliy N Sokolov,Chiao-Yu Cheng

doi:10.1038/s41467-017-02459-3

Rijul Dhanker, Sukrit Mukhopadhyay + Show 5 more

Open Access

https://doi.org/10.1038/s41467-017-02459-3

Copy DOI

Abstract

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Here we use charge modulation spectroscopy to reveal a bipolaron sheet density >1010 cm−2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic electronic devices than previously thought.

Highlights

Bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges
Using charge modulation (CM) spectroscopy[9,10,11] to directly monitor the density of TPD polarons and bipolarons in hole-only devices, we show that the latter can account for >1% of the interfacial charge under forward bias and that variations in the bipolaron density correlate closely with the magnetocurrent response in accord with the bipolaron model of OMAR2
While only a fraction of this polaron density is mobile[13], this result is notable because it shows that the bipolaron concentration can be significant in organic semiconductors—roughly 20% of the polaron concentration in this case

Summary

Introduction

In which two electrons or two holes occupy a single molecule or conjugated polymer segment, are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. Using charge modulation (CM) spectroscopy[9,10,11] to directly monitor the density of TPD polarons and bipolarons in hole-only devices, we show that the latter can account for >1% of the interfacial charge under forward bias and that variations in the bipolaron density correlate closely with the magnetocurrent response in accord with the bipolaron model of OMAR2 These results are understood by generalizing the interface energetic model of Oehzelt et al.[8], which indicates that significant bipolaron concentrations are likely to be common near contacts in the Fermi level pinning regime and that bipolaron-based OMAR may be more of an interfacial than a bulk effect

Methods

Results

Conclusion