• Energy Research

The initial interaction of atomic layer deposited films of Al2O3 at room temperature on CH3NH3PbI3 (MAPI) films is studied. Synchrotron radiation–based photoelectron spectroscopy is applied to analyze the initial changes in the Al‐derived features by comparing samples with different Al2O3 film thicknesses. It is found that polarons and excitons, both intrinsic defects of Al2O3, play a key role in the interface formation. The polaronic states uptake a charge from the MAPI substrate. This charge is transferred to and stabilized in the excitonic state of Al2O3 which is assigned to predominately tetrahedral coordinated Al sites. This charge transfer is initiated by vacancies present in the MAPI substrate and stabilizes a covalent bonding at the Al2O3–MAPI interface but also causes a roughening of the interface which may lead to the formation of grain boundaries. On top of the rough interface, 2D Al2O3 clusters with an increasing number of octahedrally coordinated Al—O bonds grow, and with increasing Al2O3 coverage, they introduce self‐healing of the structural defects.

AbstractIn this work, solar cells with a freshly made CH3NH3PbI3 perovskite film showed a power conversion efficiency (PCE) of 15.4 % whereas the one with 50 days aged perovskite film only 6.1 %. However, when the aged perovskite was covered with a layer of Al2O3 deposited by atomic layer deposition (ALD) at room temperature (RT), the PCE value was clearly enhanced. X‐ray photoelectron spectroscopy study showed that the ALD precursors are chemically active only at the perovskite surface and passivate it. Moreover, the RT‐ALD‐Al2O3‐covered perovskite films showed enhanced ambient air stability.

Schematic depiction of spiro-OMeTAD degradation causing perovskite instability under pseudo-operating conditions and its prevention using an ultra-thin ALD layer of alumina.