Graphene induced electrical percolation enables more efficient charge transport at a hybrid organic semiconductor/graphene interface.

Nicolas Boulanger,Victor Yu,David R Barbero,Michael F Toney,Michael Hilke

doi:10.1039/c7cp07871d

Abstract

Self-assembly of semiconducting polymer chains during crystallization from a liquid or melt dictates to a large degree the electronic properties of the resulting solid film. However, it is still unclear how charge transport pathways are created during crystallization. Here, we performed complementary in situ electrical measurements and synchrotron grazing incidence X-ray diffraction (GIXD), during slow cooling from the melt of highly regio-regular poly(3-hexylthiophene) (P3HT) films deposited on both graphene and on silicon. Two different charge transport mechanisms were identified, and were correlated to the difference in crystallites' orientations and overall amount of crystallites in the films on each surface as molecular self-assembly proceeded. On silicon, a weak charge transport was enabled as soon as the first edge-on lamellae formed, and further increased with the higher amount of crystallites (predominantly edge-on and randomly oriented lamellae) during cooling. On graphene however, the current remained low until a minimum amount of crystallites was reached, at which point interconnection of conducting units (face-on, randomly oriented lamellae and tie-chains) formed percolated conducting pathways across the film. This lead to a sudden rapid increase in current by ≈10 fold, and strongly enhanced charge transport, despite a much lower amount of crystallites than on silicon.

Highlights

Graphene is an outstanding material formed of a 2 dimensional layer of carbon atoms, in which the delocalization of p electrons, and its ultrafast charge transport, opens up the possibility to transfer charges to and from a conjugated polymer layer for organic hybrid opto-electronic applications.[1,2,3,4,5] Combining graphene with the semiconducting polymer P3HT into OFETs, OPVs and other electronic devices has been recently demonstrated.[3,5,6,7,8] the performance of such hybrid devices relies on the ability of the graphene/semiconducting polymer composite layer to transport charges efficiently
We propose a schematic model which illustrates the differences in crystallinity and charge transport of P3HT on graphene and silicon (Fig. 5a and b)
We have performed a parallel series of experiments to determine in situ the crystallinity and the charge transport on P3HT films on both silicon and graphene substrates as a function of the cooling temperature from the melt

Summary

Introduction

Graphene is an outstanding material formed of a 2 dimensional layer of carbon atoms, in which the delocalization of p electrons, and its ultrafast charge transport, opens up the possibility to transfer charges to and from a conjugated polymer layer for organic hybrid opto-electronic applications.[1,2,3,4,5] Combining graphene with the semiconducting polymer P3HT into OFETs, OPVs and other electronic devices has been recently demonstrated.[3,5,6,7,8] the performance of such hybrid devices relies on the ability of the graphene/semiconducting polymer composite layer to transport charges efficiently. We show the relationship between crystallization of P3HT on both graphene and silicon, and charge transport in the polymer film, by performing both GIXD and electrical measurement in situ as a function of temperature during cooling from a disordered (non-crystalline) state.

Results

Conclusion