Directional charge separation in isolated organic semiconductor crystalline nanowires.

J A Labastide,M D Barnes,S R Marques,N S Colella,A L Briseno,H B Thompson

doi:10.1038/ncomms10629

Abstract

One of the fundamental design paradigms in organic photovoltaic device engineering is based on the idea that charge separation is an extrinsically driven process requiring an interface for exciton fission. This idea has driven an enormous materials science engineering effort focused on construction of domain sizes commensurate with a nominal exciton diffusion length of order 10 nm. Here, we show that polarized optical excitation of isolated pristine crystalline nanowires of a small molecule n-type organic semiconductor, 7,8,15,16-tetraazaterrylene, generates a significant population of charge-separated polaron pairs along the π-stacking direction. Charge separation was signalled by pronounced power-law photoluminescence decay polarized along the same axis. In the transverse direction, we observed exponential decay associated with excitons localized on individual monomers. We propose that this effect derives from an intrinsic directional charge-transfer interaction that can ultimately be programmed by molecular packing geometry.

Highlights

One of the fundamental design paradigms in organic photovoltaic device engineering is based on the idea that charge separation is an extrinsically driven process requiring an interface for exciton fission
Both the sign and magnitude of the matrix element, Vij, describing the nearest-neighbour coupling are important in aggregates of conjugated organic materials: the exciton diffusion coefficient scales as | Vij |, while the sign of the interaction determines the energy ordering of coupled states
The different intensities in vibronic features in PL indicate that the dominant coupling depends on aggregation state; in the small nanocrystallite shown in Fig. 2a, the emission spectrum is characterized by an intensity ratio of vibronic components close to unity, while the corresponding intensity ratio for the extended crystal PL spectrum (Fig. 2c) is B0.5, and is essentially indistinguishable from those of physical vapour transport (PVT) grown crystals[16]

Summary

Introduction

One of the fundamental design paradigms in organic photovoltaic device engineering is based on the idea that charge separation is an extrinsically driven process requiring an interface for exciton fission. Diffusional transport of localized excitons in organic semiconductors is mediated by coupling between neighbouring chromophores, where the coupling mechanism is usually taken to be a Coulombic dipole–dipole (Frenkel exciton, FE) coupling[9] Both the sign and magnitude of the matrix element, Vij, describing the nearest-neighbour coupling are important in aggregates of conjugated organic materials: the exciton diffusion coefficient scales as | Vij |, while the sign of the interaction determines the energy ordering of coupled states (the lowest energy state for Vij 40 contains N-1 nodes, where N is the number of coupled chromophores, whereas the lowest energy state for Vijo[0] is nodeless). We observed similar effects in isolated P3HT nanofibers where the interplay between CT and Frenkel exciton coupling interconverts inter- and intra-chain excitons at different rates depending on the degree of p-stacking registration between lamellar stacks.[17]

Methods

Results

Conclusion