How to Achieve High Spatial Resolution in Organic Optobioelectronic Devices?

TL;DR

This paper investigates how material properties of organic semiconductors influence the spatial resolution of bioelectronic devices, demonstrating that diffusion length impacts excitation localization and proposing design principles for high-resolution applications.

Contribution

It identifies key material parameters affecting spatial resolution in organic optobioelectronic devices and introduces experimental methods to optimize device design for single-cell transduction.

Findings

Small diffusion length of 1.5 um in H2Pc/PTCDI heterojunction limits resolution.

Covering heterojunction with PEDOT:PSS increases diffusion length to 7.0 um.

Micrometric resolution achieved with electrochemical photocurrent microscopy under aqueous conditions.

Abstract

Light activated local stimulation and sensing of biological cells offers enormous potential for minimally invasive bioelectronic interfaces. Organic semiconductors are a promising material class to achieve this kind of transduction due to their optoelectronic properties and biocompatibility. Here we investigate which material properties are necessary to keep the optical excitation localized. This is critical to single cell transduction with high spatial resolution. As a model system we use organic photocapacitors for cell stimulation made of the small molecule semiconductors H2Pc and PTCDI. We investigate the spatial broadening of the localized optical excitation with photovoltage microscopy measurements. Our experimental data combined with modelling show that resolution losses due to the broadening of the excitation are directly related to the effective diffusion length of charge carriers generated at the heterojunction. With additional transient photovoltage measurements we find that the H2Pc/PTCDI heterojunction offers a small diffusion length of lambda = 1.5 +/- 0.1 um due to the small mobility of charge carriers along the heterojunction. Instead covering the heterojunction with a layer of PEDOT:PSS improves the photocapacitor performance but increases the carrier diffusion length to lambda = 7.0 +/- 0.3 um due to longer lifetime and higher carrier mobility. Furthermore, we introduce electrochemical photocurrent microscopy experiments to demonstrate micrometric resolution with the pn-junction under realistic aqueous operation conditions. This work offers valuable insights into the physical mechanisms governing the excitation and transduction profile and provide design principles for future organic semiconductor junctions, aiming to achieve high efficiency and high spatial resolution.