Well-Defined Nanostructured, Single-Crystalline TiO₂ Electron Transport Layer for Efficient Planar Perovskite Solar Cells

Abstract

An electron transporting layer (ETL) plays an important role in extracting electrons from a perovskite layer and blocking recombination between electrons in the fluorine-doped tin oxide (FTO) and holes in the perovskite layers, especially in planar perovskite solar cells. Dense TiO2 ETLs prepared by a solution-processed spin-coating method (S-TiO2) are mainly used in devices due to their ease of fabrication. Herein, we found that fatal morphological defects at the S-TiO2 interface due to a rough FTO surface, including an irregular film thickness, discontinuous areas, and poor physical contact between the S-TiO2 and the FTO layers, were inevitable and lowered the charge transport properties through the planar perovskite solar cells. The effects of the morphological defects were mitigated in this work using a TiO2 ETL produced from sputtering and anodization. This method produced a well-defined nanostructured TiO2 ETL with an excellent transmittance, single-crystalline properties, a uniform film thickness, a large effective area, and defect-free physical contact with a rough substrate that provided outstanding electron extraction and hole blocking in a planar perovskite solar cell. In planar perovskite devices, anodized TiO2 ETL (A-TiO2) increased the power conversion efficiency by 22% (from 12.5 to 15.2%), and the stabilized maximum power output efficiency increased by 44% (from 8.9 to 12.8%) compared with S-TiO2. This work highlights the importance of the ETL geometry for maximizing device performance and provides insights into achieving ideal ETL morphologies that remedy the drawbacks observed in conventional spin-coated ETLs.

One-Dimensional Electron Transport Layers for Perovskite Solar Cells

“…Improved perovskite deposition methods and optimized nanotube dimensions could ameliorate these problems and lead to considerably higher efficiencies. The highest PCE of perovskite solar cells with TiO 2 NTs as the ETL at present is 15.2%, reported by Choi et al [107]. They found that HPSCs using 40 nm-thick highly ordered single crystal-like TiO 2 nanopores obtained by anodizing sputtered Ti gave a better photovoltaic performance compared to HPSCs using spin-coated compact TiO 2 films because of the high contact area between anodic TiO 2 and perovskite as well as the superior electron transport properties of anodic TiO 2 NTs.…”

Progression towards high efficiency perovskite solar cells via optimisation of the front electrode and blocking layer

“…The increased conformal behaviour of ALD over spin coating would reduce the number of pin holes introduced by large FTO particles or debris which could not be covered by spin coating. In addition the non-conformal spin coating process, as seen previously, 17 can lead to a smoother top surface and hence a smaller contact area between BL and perovskite. These processes in spin coating gave increased shunting pathways within the cells and hence resulting in lower device performance.…”

“…A thicker BL would decrease the charge recombination between the perovskite holes and TCO electrons, but would also reduce the electron flow to the TCO due to a higher series resistance in the cell, so a balance of conditions is required. A detailed studied by Choi et al 17 The role of the TCO characteristics has had very limited discussion, with researchers commonly using a commercially supplied standard material. The most utilised TCO is F-doped SnO 2 (FTO) such as TEC 7, TEC 8 (NSG), or TCO22-15 (Solaronix).…”

Gradient Energy Band Driven High‐Performance Self‐Powered Perovskite/CdS Photodetector

“…Tailoring the gradient bandgap from 0.35 to 1.42 eV, Pan and co-workers synthesized InAs x P 1-x nanosheets for band-selective infrared photodetectors. The commonly used electron extraction layers in perovskite photodiodes are TiO 2 , [8,9] SnO 2 , [10,11] C 60 , [12,13] and phenyl-C61 butyric acid methyl ester (PCBM). [24] In p-i-n structured devices, [25][26][27][28] graded composition (bandgap) is basically used in the main photosensitive layer, for which the narrow bandgap region increases the absorption wavelength range, the large bandgap results in a high voltage output, and graded bandgap structure promotes the carriers transport.…”