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Contributed Speaker

Luca Fabbri

Physics and Astronomy, University of Bologna, Italy

How to Achieve High Spatial Resolution in Organic Optobioelectronic Devices?


Luca Fabbri, Ludovico Migliaccio, Aleksandra Sirvinskyte, Giacomo Rizzi, Luca Bondi,
Cristiano Tamarozzi, Stefan Weber, Beatrice Fraboni, Eric Daniel Glowacki, Tobias Cramer*


This study explores the potential of light-activated local stimulation and sensing of biological cells through low-invasive bioelectronic interfaces, based on organic semiconductors. We investigate the fundamental physical mechanisms that determine spatial resolution in organic optobioelectronic devices and the properties that are essential to maintain a localized optoelectronic excitation. As a model system we use organic photocapacitors for cell stimulation containing a planar heterojunction of the semiconductors H2Pc and PTCDI (Rand et al., 2018). We use modulated off-resonance electrostatic force microscopy (schematic in Figure 1a) to demonstrate that resolution losses are directly linked to the effective diffusion length of charge carriers at the heterojunction. The AFM images of both surface morphology and photovoltage are reported in Figures 1b-e. With additional transient photovoltage measurements, we find that the H2Pc/PTCDI heterojunction offers a small diffusion length of λd = (1.4 ± 0.1) μm due to the small mobility of charge carriers along the heterojunction and short charge carrier lifetime. To understand the impact of device architecture we
compared these results to the devices with an additional PEDOT:PSS layer on top, finding an increased diffusion length λd = (8.3 ± 0.3) μm . Finally, we performed electrochemical photocurrent microscopy experiments to demonstrate micrometric resolution with the pn-junction under realistic aqueous operation conditions. Our study offers key insights into how excitation and transduction occur, providing design principles for future organic semiconductor junctions to achieve optical biotransducers with high efficiency and spatial resolution.

 


Figure 1: (a) Schematic of the AFM setup to perform modulated off-resonance electrostatic force microscopy.
(b,c) Morphology images respectively of the PN junction surface without and with the top PEDOT:PSS layer.
(d,e) Photovoltage images respectively of the PN junction surface without and with the top PEDOT:PSS layer.

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