Mesoscopic Oxide Double Layer as Electron Specific Contact for Highly Efficient and UV Stable Perovskite Photovoltaics.
basic_science · Level V
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- Record sourced from PubMed, PMID 29528229.
- Also identified by DOI 10.1021/acs.nanolett.7b05469.
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Abstract
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 TiO<sub>2</sub> nanoparticles covered by a thin film of SnO<sub>2</sub>, either in amorphous (a-SnO<sub>2</sub>), crystalline (c-SnO<sub>2</sub>), or nanocrystalline (quantum dot) form (SnO<sub>2</sub>-NC). We find that the band gap of a-SnO<sub>2</sub> 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 TiO<sub>2</sub> 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 <sub>oc</sub>) by replacing the crystalline SnO<sub>2</sub> by an amorphous phase. Because of the quantum size effect, the band gap of our SnO<sub>2</sub>-NC particles is larger than that of bulk SnO<sub>2</sub> causing their conduction band edge to shift also to a higher energy thereby increasing the V <sub>oc</sub>. However, for SnO<sub>2</sub>-NC there remains a barrier for electron injection into the TiO<sub>2</sub> scaffold decreasing the fill factor of the device and lowering the PCE. Introducing the a-SnO<sub>2</sub> coated mp-TiO<sub>2</sub> scaffold as electron extraction layer not only increases the V <sub>oc</sub> 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.