Organometal triiodide perovskites are promising, high-performance absorbers in solar cells. Considering the perovskite as a thin film absorber, we solve transport equations and analyse the efficiency of a simple heterojunction configuration as a function of electron-hole diffusion lengths. We found that for a thin film thickness of ≃1 μm the maximum efficiency of ≃31% could be achieved at the diffusion length of ∼100 μm.