  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.
