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We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH₃NH₃)PbI₃ as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI₂ and deposited onto a submicron-thick mesoscopic TiO₂ film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (J_SC) exceeding 17 mA/cm², an open circuit photovoltage (V_OC) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH₃NH₃)PbI₃ NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO₂ film. The use …