The authors investigate the electron-hole diffusion lengths in mixed halide (CH3NH3PbI3-xClx) and triiodide (CH3NH3PbI3) perovskite absorbers using transient absorption and photoluminescence quenching measurements. They find that the diffusion lengths in the mixed halide perovskite exceed 1 micron, significantly longer than the absorption depth, while the triiodide perovskite has diffusion lengths of about 100 nanometers. These findings explain the high efficiency of planar heterojunction perovskite solar cells and highlight a critical parameter for optimizing perovskite absorber development. The study also demonstrates that uniform solid perovskite films can achieve the highest efficiencies in thin-film configurations.The authors investigate the electron-hole diffusion lengths in mixed halide (CH3NH3PbI3-xClx) and triiodide (CH3NH3PbI3) perovskite absorbers using transient absorption and photoluminescence quenching measurements. They find that the diffusion lengths in the mixed halide perovskite exceed 1 micron, significantly longer than the absorption depth, while the triiodide perovskite has diffusion lengths of about 100 nanometers. These findings explain the high efficiency of planar heterojunction perovskite solar cells and highlight a critical parameter for optimizing perovskite absorber development. The study also demonstrates that uniform solid perovskite films can achieve the highest efficiencies in thin-film configurations.