Moisture-assisted post-annealing was performed on carbon-electrode based planar perovskite solar cells so as to improve the hole-extraction process. It was observed that, after being annealed at a relative humidity of 30% for 2 h, the short-circuit current density, fill factor, and open circuit voltage were all improved, leading to an improvement of 21.75% of power conversion efficiency [from 10.53 (±0.98)% to 12.82 (±1.07)%, with the optimized one at 14.77% for reverse scanning]. The transient photovoltage/photocurrent decay measurement and impedance spectroscopy study showed that, after being annealed at a relative humidity of 30%, the charge extraction rate was accelerated and charge transfer resistance was reduced, while recombination between photo-generated charges was retarded. Scanning electron microscopy studies indicated that voids were reduced between the perovskite film and the carbon electrode, which was ascribed to the re-coarsening process of the perovskite during the post-annealing process as revealed by the X-ray diffraction study. The improved contact accelerated hole-extraction between the perovskite film and the carbon electrode and then upgraded device performance.

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Carbon electrodes have been adopted widely in perovskite solar cells (PSCs). Due to its suitable work function (though not high enough), the carbon electrode itself could extract photogenerated holes and has helped to achieve a power conversion efficiency of ≈16% in the absence of hole‐transporting material. Meanwhile, due to the inert chemical nature and the micrometer‐sized film thickness (≈10 μm), carbon electrodes can prolong the stability of PSCs. These merits are appealing for the commercialization of PSCs. However, the efficiency of carbon‐electrode PSCs is relatively low. A gap of ≈30% remains when comparing with PSCs using evaporated metal films as the electrode. Herein, the progresses in the efficiency of the four kinds of carbon‐electrode based PSCs (mesoscopic, embedment, planar, and quasi‐planar) are reviewed and compared to metal‐electrode based PSCs. Then, the role of bulk engineering and interface engineering in the progress of efficiency is discussed. Finally, outlooks are described in accordance with the discussions.

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Polyvinyl pyrrolidone (PVP) is doped to PbI2 and organic salt during two‐step growth of halideperovskite. It is observed that PVP molecules can interact with both PbI2 and organic salt, reduce the aggregation and crystallization of the two, and then slow down the coarsening rate of perovskite. As doping concentration increases from 0 to 1 mM in organic salt, average crystallite size of perovskite decreases monotonously from 90 to 34 nm; Surface fluctuation reduces from 259.9 to 179.8 nm at first, and then increases; Similarly, surface roughness decreases from 45.55 to 26.64 nm at first, and then rises. Accordingly, a kind of “confinement effect” is resolved to crystallite growth and surface fluctuation/roughness, which helps to build compact and uniform perovskite film. Density of trap states (t‐DOS) is cut down by ≈60% at moderate doping  (0.2 mM). Due to the “confinement effect”, power conversion efficiency of perovskite solar cells is improved from 19.46 (±2.80) % to 21.50 (±0.99) %, and further improved to 24.11% after surface modification. Meanwhile, “confinement effect” strengthens crystallite/grain boundaries and improves thermal stability of both film and device. T80 of device increases to 120 h, compared to 50 h for reference ones.