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 of a solid hole conductor dramatically improved the device stability compared to (CH₃NH₃)PbI₃ -sensitized liquid junction cells.

Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I₃⁻/I⁻ redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)₂]^3+/2+(PF₆)_3/2 as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO₂, yielded a power conversion efficiency of over 10% at 100 mW cm⁻². This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I₃⁻/I⁻ redox shuttle.