Low Temperature Atomic Layer Deposited TiO2 Compact Layers for Planar Perovskite Solar Cells

Abstract

Titanium dioxide (TiO2) thin films have been utilized in a wide range of applications in optoelectronics and solar energy conversion. In particular, TiO2 has been exclusively used in photovoltaic (PV) devices and photoelectrochemical (PEC) cells owing to its ability to efficiently transport electrons and passivate photoactive materials against aqueous media. However, TiO2 exhibits optimal transport properties in the crystalline anatase phase, and thus requires a high temperature annealing step at 500 °C, which significantly limits its application in temperature sensitive substrates (e.g. PET) or active materials (e.g. organic-inorganic hybrid perovskites). Moreover, while high resistance of TiO2 films necessitates the need for ultrathin films to reduce series resistance in PV devices, the synthesis of ultrathin pinhole-free TiO2 films remains a challenge through conventional deposition methods. To address these issues, TiO2 compact layers for planar perovskite PV devices were deposited via atomic layer deposition (ALD) – a chemically diverse vapor phase deposition technique with the capability to synthesize ultrathin pinhole-free films. We have investigated the influence of low temperature atomic layer deposited (ALD) compact TiO2 layers as a function of post-treatment (i.e. O3, UV-O3, 500 °C) on the performance of planar perovskite PV cells. The performance of devices indicated variations primarily in open circuit voltage (VOC) and fill factor (FF) upon various forms of post-treatment. Changes in the VOC was mainly associated with the density of states within the bandgap of TiO2 as confirmed by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Also, low temperature post-treatment (O3 and UV-O3) resulted in reduced FF compared to the conventional high temperature annealing treatment as a consequence of decreased shunt resistance. We attribute this phenomena to the amorphous nature of the low temperature post-treated compact TiO2 layers providing channels for hole conduction. However, the devices fabricated with UV-O3 treated TiO2 compact layers resulted in a comparable overall performance compared to those fabricated with the conventional post-annealing process at 500 °C, opening possibilities for its application in temperature sensitive substrates as well as active materials.