A fully integrated model of a photoelectrochemical cell for water electrolysis is applied to the case of light-absorbing particles embedded in a membrane separator. Composition of the product gases is shown to be one critical measure of device performance. Not only must the composition be kept outside the explosive window for mixtures of H2 and O2, but also product purity is a concern. For the absorber-in-membrane geometry and the model assumptions used here, results show purely water-saturated H2 on the cathode side and water-saturated O2 on the anode side. Since it is possible to design devices that violate these assumptions, it should not be assumed that a polymer separator or an absorber-in-membrane geometry will be effective in preventing explosive mixtures in all cases. Net H2 collected, iH2,net, is the second essential performance metric, and it is shown to differ significantly from the more commonly reported total H2 produced and operating current density. Schemes which co-evolve H2 and O2 violate the first metric and do not provide the second. A composite of triple-junction silicon absorbers in a Nafion membrane is shown to have an optimum thickness of 30 ?m, dependent on the properties of the light absorber. Varying membrane properties reveals a tradeoff between conductivity, ?m, and gas permeabilities, ?H2 and ?O2, that can potentially be exploited differently than in a fuel cell. Modulating the relative humidity (RH) is insufficient. The maximum iH2,net is calculated to be 6.97 mA cm?2 at RH = 30% relative to a value of 6.92 mA cm?2 at RH = 100%. The model identifies target material properties for new polymers. If ? is dropped one order of magnitude below that of Nafion (?/?Nafion = 0.1), the optimum value for iH2,net increases by 63.5%. For ?/?Nafion = 0.01, the optimum iH2,net increases by 73.5%, which compares favorably to the 74.5% improvement that would result if Nafion were made impermeable (?/?Nafion = 0). Meanwhile, ?m can drop to a value of 1.2 × 10?3 S cm?1 (two orders of magnitude below liquid-equilibrated Nafion) with less than a 5% decline in iH2,net.