The migration and accumulation of iodide ions create a modulation of the respective interfacial barriers causing the hysteresis in solar cells based on methylammonium lead iodide perovskites. Iodide ions are identified as the migrating species by measuring temperature dependent current-transients and photoelectron spectroscopy. The involved changes in the built-in potential due to ion migration are directly measured by electroabsorption spectroscopy.

BROADER CONTEXTPerovskite solar cells, which promise to deliver the highest efficiency, lowest cost nextgeneration PV technology, have been largely advanced over the last few years by improvements in the polycrystalline thin film quality. So far, improvements in film uniformity and smoothness, have mainly been at the expense of crystalline grain-size, and charge recombination losses at defect sites. High luminescence efficiency, which is an indication of better optoelectronic quality, has generally been found in films with polycrystalline grains of many micrometres in scale. This suggests a current compromise between ideal morphology and ideal optoelectronic quality.For traditional semiconductors and crystalline solids, the influence of impurity ion doping has been studied extensively and can either alter the crystallisation or induced electronic positive or negative type doping. However, in the perovskite community, impurity doping has been largely unexplored. Here, we show that doping the perovskite solution with Al 3+ , which has a much smaller ionic radius than Pb 2+ , has profoundly positive influenced on the crystalline and optoelectronic quality of the perovskite absorber layer: We demonstrate a two-fold increase in the photoluminescence quantum efficiency and a significantly reduced electronic disorder, despite the films still having polycrystalline grains on the order of one micrometer is scale. This largely overcomes the trade-off between film smoothness and optoelectronic quality, and these improvements translate into highly efficient planar heterojunction perovskite solar cells.Our work paves the way for further improvement of the optoelectronic quality of perovskite thin films, and subsequent devices, via highlighting a new avenue for investigation of the role of dopant impurities upon crystallisation and controlling the electronic defect density in the perovskite thin films. ABSTRACTRealizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries.Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a power-conversion efficiency (PCE) of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that...

Reversible photo-induced performance deterioration is observed in mesoporous TiO 2 -containing devices in an inert environment. This phenomenon is correlated with the activation of deep trap sites due to astoichiometry of the metal oxide. Interestingly, in air, these defects can be passivated by oxygen adsorption. These results show that the doping of TiO 2 with aluminium has a striking impact upon the density of sub-gap states and enhances the conductivity by orders of magnitude. Dye-sensitized and perovskite solar cells employing Al-doped TiO 2 have increased device effi ciencies and signifi cantly enhanced operational device stability in inert atmospheres. This performance and stability enhancement is attributed to the substitutional incorporation of Al in the anatase lattice, "permanently" passivating electronic trap sites in the bulk and at the surface of the TiO 2 .