Double-Mesoscopic Hole-Transport-Material-Free Perovskite Solar Cells: Overcoming Charge-Transport Limitation by Sputtered Ultra-Thin Al <sub/>2</sub> O <sub/>3</sub> Isolating Layer

摘要： The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO2/carbon-graphite (CG) interface. Thick 1 μm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al2O3 which acts as highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way towards the development of next generation double-mesoscopic carbon-graphite based PSCs with highest efficiencies. Scanning electron microscope (SEM), energy dispersive x-ray analysis (EDX) and atomic force microscopy (AFM) measurements show the presence of fully oxidized sputtered Al2O3 layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned for the achievement of both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al2O3 not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO2/CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, stable VOC of 1 V using CH3NH3PbI3 perovskite has been achieved under full sun AM 1.5 G with stabilized device performance of 12.1%.