Photovoltaic conversion is regarded as the ultimate solution to the growing demand for energy, yet traditional silicon-based solar cells are expensive to produce, and production itself involves intensive energy consumption. Emerging hybrid organic-inorganic solar cells based on perovskite CH3NH3PbI3, on the other hand, are not only inexpensive to process but also flexible, and thus are widely pursued as one of the most promising next-generation photovoltaic conversion technologies.