# Probing charge carrier movement in organic semiconductor thin films via nanowire conductance spectroscopy

## TL;DR

This paper investigates how silicon nanowire conductance spectroscopy can detect charge carrier movement in organic semiconductor thin films, revealing sensitivity limits and proposing methods for spatial charge distribution imaging.

## Contribution

It introduces a theoretical framework linking nanowire conductance changes to charge carrier dynamics in organic films, with quantitative sensitivity analysis and novel imaging proposals.

## Key findings

- Ultra-thin nanowires detect single-molecule charge changes at room temperature.
- Sensitivity increases with the number of charged molecules and nanowire size.
- Proposes noise spectroscopy and impedance tomography for charge state and distribution analysis.

## Abstract

Understanding the movement of charge within organic semiconducting films is crucial for applications in photo-voltaics and flexible electronics. We study the sensitivity of the electrical conductance of a silicon nanowire to changes of charge states within an organic semiconductor physisorbed on the surface of the nanowire. Elastic scattering caused by motion of charge carriers near the nanowire modifies the mean-free path for backscattering of electrons propagating within it, which we have mathematically expressed in terms of the causal Green's functions. The scattering potential has been computed using a combination of the polarizable continuum model and density functional theory with the range-separated exchange-correlation functional for organic molecules and the semi-empirical tight-binding model for silicon. As an example, the sensitivity to charge state changes in tetracene is computed as a function of operating temperature and geometrical parameters of a nanowire. For a single molecule, ultra-thin silicon nanowires with characteristic sizes of the cross-section below 2 nm produce a detectable conductance change at room temperature. For larger nanowires the sensitivity is reduced, however the conductance change grows with the number of charged molecules: with sub-4 nm nanowires being sensitive enough to detect several tens of charge carriers. We propose using noise spectroscopy to access the temporal evolution of the charge states. Information regarding the spatial distribution of charge carries in organic thin films can be obtained using a grid of nanowire resistors and electric impedance tomography.