Safe and effective neural modulation has a range of therapeutic applications, including restoration of sensory function and treatment of neurological disorders and chronic pain. Traditional neural modulation technologies rely on the application of electrical pulses through implanted metallic electrodes. Non-invasive light-activated devices based on organic semiconductors are a promising alternative to conventional metallic electrodes, with potentially higher biocompatibility due to their softness, flexibility, and chemical similarity to biological systems. Light-induced neural modulation can be achieved through photothermal, photochemical, and photocapacitive mechanisms, with photocapacitance being preferred as no permanent change is induced in either the device or the biological environment. However, photoredox reactions with the complex biological environment have been shown to occur in parallel with photocapacitance, with rates depending on the semiconductor properties and device architecture. These reactions may modulate neural function but may also produce dangerous quantities of reactive oxygen species. In this talk, we discuss photochemical reactions and photoelectrical properties of thin films of polythiophene donors, fullerene-based acceptors, and non-fullerene acceptors in water and common biological buffer solutions. These reactions include hydrogen and oxygen production, peroxide production, and semiconductor degradation. We demonstrate how coupling accessible electrochemical techniques such as impedance spectroscopy and chronoamperometry with light stimulation can be used to guide the design of effective neural interfaces prior to time-consuming cell culturing and patch-clamp electrophysiology. We also present recommendations for promoting photocapacitance while avoiding reactions that degrade the semiconductor and produce cytotoxic species.