A photoelectrochemical (PEC) reactor is a compromise engine: it must be a good electrochemical cell and a good optical system at the same time, and every design decision trades one against the other. Getting the three-way geometry right, electrodes, electrolyte path and light, is what makes the data worth publishing.
The working photoelectrode needs uniform illumination, so its plane faces the window at a fixed distance. The counter electrode belongs on the far side or in a separated compartment, close enough to keep solution resistance low, but never in the light path where it shades the working electrode or suffers photodeposition. The reference tip sits near the working electrode without touching it or distorting the current field.
Window material transmits the working band, quartz for UV work, glass for visible. Electrolyte depth between window and electrode is the optical path, and absorbing electrolytes argue for thin paths. The path must stay reproducible: a millimeter of repositioning changes measured photocurrents measurably.
Stirring or flow keeps products from accumulating at the electrode face, and trapped bubbles must have an escape route, a bubble parked on the photoelectrode is a dark spot and a false current drop. For gas-evolving chemistry, a headspace with outlet to analysis turns the reactor into a measurement system.
Stable contact resistances, uniform fields on both the electrical and optical side, and reproducible photocurrents run after run. When the same electrode gives the same number in the same reactor, the reactor is doing its job.
Perfectlight's photoelectrochemical reactors implement these geometry rules; see solutions.
