Biocompatible flexible photosensors based on 2D materials and crystalline organic semiconductors

Photoactive organic semiconductors combine effective light absorption and photogeneration with lightweight, flexibility and biocompatibility, making them highly attractive for biological photodetection applications, such as visual prosthesis. [1,2]

Organic single crystals (SCs) stand out due to an efficient and stable light sensitivity, granted by their long-range periodic molecular order and chemical purity, that reduce energetic disorder, enhance charge carrier mobility, and promote well-defined optical transitions. Solution-based processes, as spin-coating or spray deposition, have also been explored for the fabrication of photoactive media. [3,4]

This work investigates organic photodetectors based on small molecule semiconductors targeting different regions of the visible spectrum: rubrene for blue-green (~450–550 nm), PTCDA and PTCDI-C8 for yellow-red (~550–600 nm), and TCNQ and 9-Fluorenone for violet-blue (~400–450 nm).

Rubrene SCs devices, grown by Physical Vapor Transport (PVT), exhibited efficient photocurrent generation, with responsivity between 10⁻⁵ to 10⁻³ A/W near 460 nm. PTCDA could not be crystallized by PVT due to its high thermal stability, while PTCDI-C8 crystalline films yielded ohmic conductivity but failed to generate measurable photocurrent. TCNQ SCs showed responsivity and EQE values, respectively, in the range of 0.1–1 mA/W and 10⁻⁷ %. Spin-coated TCNQ and 9-Fluorenone films displayed partial crystallization, but low absorbance which hindered photosensitivity, likely due to insufficient thickness and solvent-induced charge traps.

Overall, the results confirm the potential of organic semiconductors, particularly single crystals, for biocompatible photodetectors. However, further optimization, especially of film deposition and device architecture, is necessary to enhance light absorption, carrier transport and reproducibility.