How to Achieve High Spatial Resolution in Organic Optobioelectronic Devices?

Light activated local stimulation and sensing of biological cells hold great promise for minimally invasive bioelectronic interfaces. Organic semiconductors are particularly appealing for these applications due to their optoelectronic properties and biocompatibility. This study examines the material properties necessary to localize the optical excitation and achieve optoelectronic transduction with high spatial resolution. Using photovoltage and photocurrent microscopy, we investigate spatial broadening of local optical excitation in Phthalocyanine/3,4,9,10-Perylenetetracarboxylic diimide (H₂PC/PTCDI) planar heterojunctions. Our measurements reveal that resolution losses are tied to the effective diffusion length of charge carriers at the heterojunction. For the H₂PC/PTCDI heterojunction, the diffusion length is determined to be λ_d = 1.5 ± 0.1 µm, attributed to reduced carrier mobility. Covering the heterojunction with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) improves the charge generation performance but increases the carrier diffusion length to λ_d = 7.0 ± 0.3 µm due to longer lifetime and higher carrier mobility. These findings elucidate the physical mechanisms underlying transduction and provide design principles for organic semiconductor devices aimed at achieving high efficiency and high spatial resolution for wireless and optically activated bioelectronics.