Mesoscopic Oxide Double Layer as Electron Specific Contact for Highly Efficient and UV Stable Perovskite Photovoltaics

The solar to electric power conversion efficiency (PCE) of perovskite solar cells (PSCs) has recently reached 22.7%, exceeding that of competing thin film photovoltaics and the market leader polycrystalline silicon. Further augmentation of the PCE toward the Shockley-Queisser limit of 33.5% warrants suppression of radiationless carrier recombination by judicious engineering of the interface between the light harvesting perovskite and the charge carrier extraction layers. Here, we introduce a mesoscopic oxide double layer as electron selective contact consisting of a scaffold of TiO2 nanoparticles covered by a thin film of SnO2 , either in amorphous (a-SnO2), crystalline (c-SnO2), or nanocrystalline (quantum dot) form (SnO2 -NC). We find that the band gap of a-SnO2 is larger than that of the crystalline (tetragonal) polymorph leading to a corresponding lift in its conduction band edge energy which aligns it perfectly with the conduction band edge of both the triple cation perovskite and the TiO2 scaffold. This enables very fast electron extraction from the light perovskite, suppressing the notorious hysteresis in the current-voltage (J-V) curves and retarding nonradiative charge carrier recombination. As a result, we gain a remarkable 170 mV in open circuit photovoltage (V-oc) by replacing the crystalline SnO2 by an amorphous phase. Because of the quantum size effect, the band gap of our SnO2 -NC particles is larger than that of bulk SnO2 causing their conduction band edge to shift also to a higher energy thereby increasing the V-oc. However, for SnO2 -NC there remains a barrier for electron injection into the TiO2 scaffold decreasing the fill factor of the device and lowering the PCE. Introducing the a-SnO2 coated mp-TiO2 scaffold as electron extraction layer not only increases the V-oc and PEC of the solar cells but also render them resistant to UV light which forebodes well for outdoor deployment of these new PSC architectures.