How to Achieve High Spatial Resolution in Organic Optobioelectronic Devices?

Light activated local stimulation and sensing of biological cells hold greatpromise for minimally invasive bioelectronic interfaces. Organicsemiconductors are particularly appealing for these applications due to theiroptoelectronic properties and biocompatibility. This study examines thematerial properties necessary to localize the optical excitation and achieveoptoelectronic transduction with high spatial resolution. Using photovoltageand photocurrent microscopy, we investigate spatial broadening of localoptical excitation in Phthalocyanine/3,4,9,10-Perylenetetracarboxylic diimide(H2 PC/PTCDI) planar heterojunctions. Our measurements reveal thatresolution losses are tied to the effective diffusion length of charge carriers atthe heterojunction. For the H 2 PC/PTCDI heterojunction, the diffusion lengthis determined to be λd = 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 generationperformance but increases the carrier diffusion length to λd = 7.0 ± 0.3 μmdue to longer lifetime and higher carrier mobility. These findings elucidate thephysical mechanisms underlying transduction and provide design principlesfor organic semiconductor devices aimed at achieving high efficiency and highspatial resolution for wireless and optically activated bioelectronics.