A TiO2 embedded structure for perovskite solar cells with anomalous grain growth and effective electron extraction (original) (raw)
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The electron transfer layer (ETL) plays a vital role in achieving high-performance perovskite solar cells (PSCs). Titanium dioxide (TiO2) is primarily utilised as the ETL since it is low-cost, chemically stable, and has the simplest thin-film preparation methods. However, TiO2 is not an ideal ETL because it leads to low conductivity, conduction band mismatch, and unfavourable electron mobility. In addition, the exposure of TiO2 to ultraviolet light induces the formation of oxygen vacancies at the surface. To overcome these issues, doping TiO2 with various metal ions is favourable to improve the surface structure properties and electronic properties. This review focuses on the bulk modification of TiO2 via doping with various metal ions concentrations to improve electrical and optical properties, charge carrier density, and interfacial electron–hole recombination, thus contributing to enhancing the power conversion efficiency (PCE) of the PSCs.
Nanostructured TiO2 Films with a Mixed Phase for Perovskite Solar Cells
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A series of thin films made with TiO 2 nanoparticles with a varied anatase/rutile phase ratio is prepared on conducting glass substrates using a spin-coating method. The structure, morphology, and optical properties of TiO 2 nanopowders and thin films fabricated are studied using powder X-ray diffraction, scanning electron microscopy, and optical spectroscopy. The TiO 2 nanostructured films created are used as photoelectrodes for the fabrication of perovskite solar cells (PSCs). The photovoltaic characteristics of PSCs under AM1.5 light illumination (1000 W/m 2) under ambient conditions are studied. It is shown that the best efficiency of solar-to-electrical energy conversion, namely, 9.3%, is obtained for the PSC with a photoelectrode based on a TiO 2 film with an anatase/rutile mixed phase ratio of 86/14%.
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Japanese Journal of Applied Physics, 2017
In perovskite solar cells, the morphology of the porous TiO 2 electron transport layer (ETL) largely determines the quality of the perovskites. Here, we chose micro-scale TiO 2 (0.2 µm) and compared it with the conventional nanoscale TiO 2 (20 nm) in relation to the crystallinity of perovskites. The results show that the micro-scale TiO 2 is favorable for increasing the grain size of the perovskites and enhancing the light scattering. However, the oversized TiO 2 results in an uneven surface. The evenness of the perovskites can be improved by nanoscale TiO 2 , while the crystallinity and compactness are not as good as those of the films based on micro-scale TiO 2. To combine the advantages of both micro-scale and nanoscale TiO 2 , by mixing 0.2 µm/20 nm TiO 2 with a ratio of 1 : 2 as the composite ETL, the device average power conversion efficiency was increased to 11.2% from 9.9% in the case of only 20 nm TiO 2 .
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In this work, a titanium oxide buffer layer was explored as a possible buffer electron transporting layer (ETL) with iodine-tin-based perovskite material for enhancement of a thin-film lead-free perovskite solar cell. The open-circuit voltage of the device was used as an indicator for the interface energy barrier’s change with the thickness of the TiO2. The buffer and photoabsorbing layers were deposited by vacuum reactive sputtering and a low-temperature ion-assisted process from a confocal sintered source, respectively, allowing precise tuning of the film properties and reproducibility of the solar cell behavior. The surface roughness of the buffer layers was investigated by atomic force microscopy and together with the measured absorbance spectra conclusions about the optical losses in the device were made. It was found that the highest voltage was generated from the structure with 75 nm-thick ETL. The electrical behavior of the cell with this buffer layer was additionally studie...
Advanced Functional Materials, 2020
The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single-crystalline TiO 2 nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl 4 treatment combined with a subsequent PC 61 BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show J SC , V OC , and fill factor as high as 23.2 mA cm −2 , 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO 2 NR-based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.
Journal of Power Sources, 2019
While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the perovskite and TiO2, coupled with strong Mie scattering in the nanorod geometry results in broadband near-zero backscattering and high forward scattering, upon coating of HATNRs with perovskite. The maximum suppression of backscattering is found at ∼600 nm. The HATNRs ETL also improves the extraction of electrons from the perovskite layer and results in superior blocking of carrier recombination at the perovskite layer/FTO interface.
Optically uniform thin films of mesoporous TiO2 for perovskite solar cell applications
Optical Materials, 2019
Mesoporous titanium oxide (mp-TiO 2) thin films are effective electron transport layers in hybrid perovskite solar cells. In this work the mp-TiO 2 are obtained via spin coating by mixing titanium isopropoxide and poly (vinylpyrrolidone) (PVP). Optically uniform anatase TiO 2 thin films are formed after annealing at temperatures ≥500°C. The surface morphology shows porous structure with pore sizes of ≅15-30 nm. The volume porosity of a mp-TiO 2 thin film is estimated for the first time by using the Volume Average Theory with the experimentally measured effective refractive index of the film, and confirmed by a statistical and 3D interactive method for void volume determination in the cross sectional image of the same film. It is found that this volume porosity varies slightly, from 40.3% to 43.6%, when the spin speed is between 1000 and 2500 rpm. However, the volume porosity is significantly reduced to 33% for a very slow spin speed like 500 rpm. Using these mp-TiO 2 thin films as electron transport layers in perovskite solar cells, it is confirmed that the power conversion efficiency (PCE) of the cells is a function of both the mp-TiO 2 layer thickness and volume porosity, giving the maximum PCE in a cell sample that combines a larger thickness and higher volume porosity.