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High loadings of Cu were integrated on the light-facing side of Si microwire arrays used under simulated sunlight for the photoelectrochemical reduction of CO₂(aq) to hydrocarbons in 0.10 M KHCO₃(aq). Radial-junction n⁺p-Si microwire arrays decorated with Cu exhibited absolute photocurrent densities comparable to an uncovered Si surface. Moreover, with respect to a Cu foil electrode, the positive shift in the onset potential for hydrocarbon formation at n⁺p-Si/Cu microwire arrays was equal to or greater than the photovoltage of the semiconductor alone. Selective electrodeposition of Cu on the tips and sidewalls of Si microwires was responsible for the minimal parasitic reflection and absorption exhibited by the catalyst, such that light-limited, absolute current densities >25 mA·cm^–2 were sustained for 48 h under simulated sunlight. Photoelectrodes prepared from semiconductors with low diode quality factors and …