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 (JSC) exceeding 17 mA/cm2, an open circuit photovoltage (VOC) 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.

Ionic displacement modifying the electric field in the device is found as most likely reason for the hysteresis which is examined by separating fast and slow processes and comparing devices with and without blocking layer.

Hematite is a promising material for inexpensive solar energy conversion via water splitting but has been limited by the large overpotential (0.5-0.6 V) that must be applied to afford high water oxidation photocurrent. This has conventionally been addressed by coating it with a catalyst to increase the kinetics of the oxygen evolution reaction. However, surface recombination at trapping states is also thought to be an important factor for the overpotential, and herein we investigate a strategy to passivate trapping states using conformal overlayers applied by atomic layer deposition. While TiO 2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al 2 O 3 reduces the overpotential required with state-of-the-art nano-structured photo-anodes by as much as 100 mV and increases the photocurrent by a factor of 3.5 (from 0.24 mA cm À2 to 0.85 mA cm À2 ) at +1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co 2+ ions as a catalyst further decreases the overpotential and leads to a record photocurrent density at 0.9 V vs. RHE (0.42 mA cm À2 ). A detailed investigation into the effect of the Al 2 O 3 overlayer by electrochemical impedance and photoluminescence spectroscopy reveals a significant change in the surface capacitance and radiative recombination, respectively, which distinguishes the observed overpotential reduction from a catalytic effect and confirms the passivation of surface states. Importantly, this work clearly demonstrates that two distinct loss processes are occurring on the surface of high-performance hematite and suggests a viable route to individually address them.

Organic-inorganic hybrid perovskites have attracted attention as successful light harvesting materials for mesoscopic solid-state solar cells and led to record breaking effi ciencies. The photovoltaic performance of these devices is greatly dependent on the fi lm morphology, which in turn is dependent on the deposition techniques and subsequent treatments employed. In this work the perovskite fi lm is deposited by spin-coating a precursor solution of PbCl 2 and CH 3 NH 3 I (1 to 3 molar ratio) in dimethylformamide. Here, the role of the temperature used in the annealing process required to convert the as deposited solution into the perovskite material is investigated. It is found that the conversion requires suffi ciently high temperatures to ensure the vaporization of solvent and the crystallization of the perovskite material. However, increasing the annealing temperature too high leads to the additional formation of PbI 2 , which is detrimental to the photovoltaic performance. Furthermore, the effect of the annealing temperature on the fi lm formation, morphology, and composition is examined and correlated with the photovoltaic performance and device working mechanisms.