The concept of using core Cu nanowires (CuNWs) that are conformally encapsulated by a reducible fugitive material for transparent conducting electrodes (TCEs) with high oxidation stability is presented. By the chemical reaction of an acid with surface oxide and hydroxide, a uniform surface shell layer is readily obtained on each CuNW upon adding lactic acid to the CuNW dispersion. The Cu lactate shell prevents the core CuNW from oxidizing during storage and film formation, enabling the core Cu nanowires to maintain their characteristic optoelectronic properties. Through simple thermal annealing under a nitrogen atmosphere, the Cu lactate shell is easily decomposed to expose the underlying pure Cu, providing an effective way to produce a pure-CuNW-network TCE with a sheet resistance of 19.8 Omega sq(-1) and an optical transmittance of 85.5% at 550 nm. The application of the CuNW-based TCE to the transparent top electrode in organometallic halide perovskite solar cells is further demonstrated for the first time, yielding a power-conversion efficiency 9.88% as compared to that of 13.39% for conventional perovskite solar cells with an indium-tin-oxide electrode. This study proposes the high feasibility of these CuNWs as a vacuum-free and noble-metal-free transparent- window electrode in perovskite solar cells.