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

Abstract

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 mA 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.