Three-dimensional titanium oxide nanoarrays for perovskite photovoltaics: surface engineering for cascade charge extraction and beneficial surface passivation

Improved photovoltaic performance is reported for perovskite solar cells (PSCs) based on three-dimensional (3D) titanium oxide nanoarray electron transporting layers (ETLs). A solution- process is used to fabricate 3D nanoarrays that consist of rutile TiO2 nanorods (RTNR) with optimal lengths of 600 nm and an outside layer made of rutile, anatase or amorphous TiO2 nano- branches (referred to as RTB, ATB and Am-TB, respectively). These nanoarrays benefit perovskite precursor infiltration and increase the interfacial contact between the perovskite layer and ETL for efficient charge extraction. The PSCs based on the RTNR@ATB or RTNR@Am-TB arrays exhibit enhanced performance with improved open-circuit voltage (V-oc) and fill factor (FF), compared to the RTNR@RTB or pristine RTNR counterparts. This is assigned primarily to the appropriate band alignment between the nanorods and nano-branches forming a favorable charge extraction 'energy cascade', and to a beneficial surface passivation effect limiting charge recombination. PSCs (with minimal hysteresis) achieved 16.06% efficiency when the rutile trunks were modified with surface amorphous nano-branches, with a short-circuit photocurrent density (J(sc)) of 21.80 cm(-2), V-oc of 1010 mV and FF of 0.73. This unique morphological design provides an approach for constructing novel 3D electrodes for the fabrication of optoelectronic devices.