Mesoporous Electron Transport Layer Research Articles

Improved UV stability of perovskite devices based on the synergistic effect of electron layer passivation engineering and component engineering

The electron transport layer is one of the key factors in constructing high-performance perovskite solar cells (PSCs). Pb2+ reduction induced by ultraviolet light can cause the generation of vacancies and defects, leading to the degradation of perovskite films and the reduction of PSCs performances. Ethylenediamine tetraacetic acid disodium salt (EDTA) is a chelating agent that can bind to Sn2+ divalent metal ions and hence change material properties. Herein, EDTA-SnO2/CeO2 composite electron transport layers were constructed to improve the ultraviolet light stability of PSCs. The oxygen defect in electron layers of tin dioxide could be passivated by the surface of EDTA; whilst rod-like CeO2 was used as a mesoporous electron transport layer and UV shielding agent, which can reflect UV light and prevent the absorption layer from being damaged. CeO2 has also a suitable band gap; this accelerates the charge extraction and transfer, suppresses the recombination of interfacial carriers. The experimental results showed that the ultraviolet light stability of PSCs was significantly improved. After continuous irradiation of ultraviolet light with energy of 80 mW/cm2 for 12 h, the unencapsulated device with CeO2 maintained nearly 90% the initial efficiency.

Hybrid Mesoporous TiO2/ZnO Electron Transport Layer for Efficient Perovskite Solar Cell.

In recent years, perovskite solar cells (PSCs) have gained major attention as potentially useful photovoltaic technology due to their ever-increasing power-conversion efficiency (PCE). The efficiency of PSCs depends strongly on the type of materials selected as the electron transport layer (ETL). TiO2 is the most widely used electron transport material for the n-i-p structure of PSCs. Nevertheless, ZnO is a promising candidate owing to its high transparency, suitable energy band structure, and high electron mobility. In this investigation, hybrid mesoporous TiO2/ZnO ETL was fabricated for a perovskite solar cell composed of FTO-coated glass/compact TiO2/mesoporous ETL/FAPbI3/2D perovskite/Spiro-OMeTAD/Au. The influence of ZnO nanostructures with different percentage weight contents on the photovoltaic performance was investigated. It was found that the addition of ZnO had no significant effect on the surface topography, structure, and optical properties of the hybrid mesoporous electron-transport layer but strongly affected the electrical properties of PSCs. The best efficiency rate of 18.24% has been obtained for PSCs with 2 wt.% ZnO.

A Semitransparent Silver–Bismuth Iodide Solar Cell with Voc above 0.8 V for Indoor Photovoltaics

Because the toxicity of lead (Pb) remains a potential threat to the commercialization of perovskite solar cells, silver–bismuth halides such as AgBiI4, AgBi2I7, Ag2BiI5, and Ag3BiI6 with band gaps of 1.8–1.9 eV and strong absorption are considered to be promising alternatives to Pb-based perovskites, not only for outdoor (1 Sun) but also for indoor photovoltaics because their band gap matches best with the indoor artificial light spectrum. To date, almost all studies have focused on improving the power conversion efficiency (PCE) under 1 Sun using thick absorber films made on mesoporous electron-transport layers. However, in this work, we aimed at developing optically semitransparent Ag2BiI5 solar cells that can be used indoor or on windows. We studied the effect of the precursor composition on the phase evolution, energy band levels, film morphology, and, eventually, solar cell performance. Through compositional engineering and thickness optimization, we accomplished the fabrication of a thin (∼100 nm Ag2BiI5) and semitransparent Ag2BiI5 solar cell that works with a power conversion efficiency of ∼2%, which is greater than many reported with thick and opaque films and slightly lower than the best recorded for Ag2BiI5. The cell shows an open-circuit voltage (Voc) of over 800 mV, the highest reported for Ag2BI5. More interestingly, while a thicker/opaque cell was found to work with a PCE of 5.0% under 1000 lx, the semitransparent cell shows a reasonable performance of 3.3% under the same light intensity.

SnO2 modified mesoporous ZrO2 as efficient electron-transport layer for carbon-electrode based, low-temperature mesoscopic perovskite solar cells

SnO2 modified mesoporous ZrO2 is used to replace the mesoporous TiO2 layer and serves as a kind of mesoporous electron-transport layer during the low-temperature fabrication of mesoscopic perovskite solar cells that are based on carbon electrode. X-ray/ultraviolet photoelectron spectroscopy studies and electrical test observe that SnO2 modification brought down the work function while increasing the conductivity of the mesoporous ZrO2. Transient photovoltage/photocurrent decay curves, impedance spectroscopy, and photoluminescence mapping show that after the bottom layer of ZrO2 is modified by SnO2, the charge extraction process is accelerated while recombination is retarded. This modification helps to increase the power conversion efficiency from 4.70 (±0.85)% to 10.15 (±0.35)%, along with the optimized efficiency at 13.37% (AM1.5G, 100 mW/cm2) for the low-temperature devices. In addition, the effects of modification layers of SnO2 on the power conversion properties are carefully studied. This study shows that SnO2 modified mesoporous ZrO2 could serve as an efficient electron-transport layer for the low-temperature mesoscopic devices.

β-Alanine-Anchored SnO2 Inducing Facet Orientation for High-Efficiency Perovskite Solar Cells.

SnO2 films as a promising electron transport layer (ETL) have been widely used in planar-type perovskite solar cells to achieve an impressive improvement in the conversion efficiency. However, compared with a mesoporous ETL, the interfacial charge carrier transfer of the SnO2 ETL is severely limited due to the issues of oxygen vacancy defects and crystal lattice mismatch between SnO2 and the perovskite, which generally leads to the growth of randomly stacked and porous perovskite layers and subsequently impacts the charge transport and transfer properties. In this work, we developed a facile approach by inducing a bifunctional molecule, β-alanine, into the SnO2 ETL, which can serve as a bridge to modulate the interfacial charge transfer and the perovskite crystallization kinetics. Benefited by the interfacial β-alanine, we grew a highly orientational perovskite layer that exhibited superior charge transport properties. Meanwhile, the β-alanine caused an intimate connection between the perovskite and SnO2 to enhance the interfacial charge transfer. As a result, the power conversion efficiency (PCE) of the β-alanine-modified device achieved a much-improved value of 19.67% and showed high reproducibility. This work provides a way for developing a high-performance ETL toward the scalable fabrication of highly efficient PSCs.

Mie-resonant mesoporous electron transport layer for highly efficient perovskite solar cells

All-dielectric nanophotonics is a powerful tool for improvement of thin-film optoelectronic devices because of low optical losses, strong light localization, and chemical robustness against such materials as halide perovskites. However, large-scale and low-cost approaches to create functional nanostructures are still not developed. In our work, we show a novel method to create mesoporous electron transport layer based on optically resonant silicon nanoparticles incorporated into TiO2 paste to be applied for perovskite (MAPbI3) solar cell. The inclusion of Mie-resonant silicon nanoparticles helps to improve light absorption by a perovskite layer without reduction of the active material. The management of Si nanoantennas concentration provides to reach a power conversion efficiency higher than 21% by increasing all main device parameters. Our multi-physical theoretical simulations of the solar cells with the resonant silicon nanoparticles provide physical understanding on the mechanisms of the device improvement as well as help to optimize the silicon nanoparticles concentration.

Characteristics of Perovskite Solar Cells with ZnGa2O4:Mn Phosphor Mixed Polyvinylidene Fluoride Down-Conversion Layer

To reduce the manufacturing cost of perovskite solar cells, soda-lime glass and transparent conducting oxides such as indium tin oxide and fluorine-doped tin oxide are the most widely used substrates and lighttransmitting electrodes. However, the transmittance spectra of soda-lime glass, indium tin oxide, and fluorinedoped tin oxide show that all light near and below 330 nm is absorbed; thus, with the use of these substrates, light energy near and below 330 nm cannot reach the perovskite light-absorbing layer. It is expected that the overall solar cell can be improved if the wavelength can be adjusted to reach the perovskite solar cell absorbing layer through down-conversion of energy in the optical wavelength band. In this study, a polyvinylidene fluoride transparent film mixed with a ZnGa2O4:Mn phosphor was applied to the incident side of the perovskite solar cell with the intent to increase the light conversion efficiency without changing the internal bandgap energy and structure. By adding a phosphor layer to the external surface of PSC exposed to incident light, the efficiency of the cell was increased by the down-conversion of ultraviolet light (290 nm) to the visible region (509 nm) while maintaining the transmittance. To manufacture the perovskite solar cell, a TiO2-based mesoporous electron transport layer was spin-coated onto the substrate. The perovskite layer used in this experiment was CH3NH3PbI3 and was fabricated on a TiO2 layer. Spiro-OMeTAD solution was spin-coated as a hole-transport layer.

In situ growth of an opal-like TiO2 electron transport layer by atomic layer deposition for perovskite solar cells

Here, we use an atomic layer deposition system and different sizes of polystyrene colloidal spheres to in situ prepare an opal-like TiO2 mesoporous electron transport layer for perovskite solar cells.

Optimization of Titanium Dioxide Nanoparticles in Mesoporous Electron Transport Layer Perovskite Solar Cell

Research about mesoporous TiO2 as an electron transport layer in perovskite solar cell has been done to obtain the best fabricated cell’s performance. In this research, the concentrations of opaque and transparent TiO2 nanoparticle were varied, in order to optimize the TiO2 mesoporous electron transport layer in FTO/CL-TiO2/MS-TiO2/Perovskite/P3HT/Ag perovskite-based solar cell. Morphological, optical, and electrical characteristics of TiO2 layers were investigated using scanning electron microscopy (SEM), four-point probe (FPP), and UV-Vis spectroscopy. The influences of those characteristics in solar cell performance were analyzed by using illumination of sun simulator with a light intensity of 500 W/m2. The results showed that transparent TiO2 has a higher conductivity and transmittance compared to the opaque TiO2. The concentration of TiO2 solutionin1:17 ratio resulted in higher electrical performance in both the transparent and opaque TiO2 layer. The best perovskite solar cell performance with PCE of 0.37% was achieved from the sample using TiO2 transparent layer with a concentration of 1:7 ratio.

TiO2@PbTiO3 core-shell nanoparticles as mesoporous layer to improve electron transport performance in carbon-based perovskite solar cells

The hole-transport material (HTM) free carbon-based perovskite solar cells (C–PSCs) lacks a HTM layer inside, resulting in poor charge transport performance. The power conversion efficiencies (PCE) of the C–PSCs could be effectively improved by increasing the transmission of photo-generated electrons from the perovskite to the electron transport layer. Here, the nanoparticles with TiO2@ PbTiO3 core-shell structure were prepared by in-situ reaction and used as mesoporous electron transport layer to improve electron transport performance. It is worth noting that the density of defect states on the surface of TiO2 significantly reduced due to the coating of PbTiO3, which effectively inhibits the interfacial charge recombination caused by the defect as the carrier recombination center. The electron injection from perovskite to electron transport layer enhanced ascribing to the optimized energy band arrangement. The open circuit voltage (Voc) and short circuit density (Jsc) were significantly improved, as well as the hysteresis retarded. Finally, the optimal PCE prepared in the air condition increased from 6.55% to 7.97%. This study provides a very feasible idea for obtaining high-performance C–PSCs.

Record Photocurrent Density over 26 mA cm−2 in Planar Perovskite Solar Cells Enabled by Antireflective Cascaded Electron Transport Layer

Planar perovskite solar cells (PSCs) hold promise for simple processing at low temperatures; however, they usually show lower short‐circuit current density (Jsc) than their mesoporous counterparts owing to their higher primary optical reflection losses at the front side. The antireflective nature of a mesoporous electron transport layer (ETL) enables a low optical reflection in the front surface of solar cells, which is challenging to realize in planar PSCs. Herein, an antireflective cascaded ETL structure using SnO2/TiO2–Cl bilayers is devised to reduce optical reflection losses and to improve electrical performance in planar PSCs. The antireflective cascaded ETL results in an enhanced Jsc of 25.4 mA cm−2 in formamidinium lead triiodide based planar PSCs, compared with the control Jsc of 24.6 mA cm−2 using single‐layered SnO2 ETL. A record‐high Jsc of 26.1 mA cm−2 is further achieved using an additional antireflective coating on the front glass side, leading to a power conversion efficiency of 22.9%.

High‐Efficiency Low‐Temperature‐Processed Mesoscopic Perovskite Solar Cells from SnO2 Nanorod Self‐Assembled Microspheres

Mesoporous scaffolds in perovskite solar cells (PSCs) can accelerate the formation of heterogeneous nucleation sites, leading to enhanced quality of perovskite films and uniform perovskite coverage over large areas. Nevertheless, the mesoporous electron transport layers (ETLs) can effectively compensate for the drawback of shorter electron diffusion lengths than their hole counterparts. Therefore, most mesoscopic PSCs usually show superior photovoltaic performance to their planar counterparts. However, mesoporous ETLs, particularly those prepared with metal oxide nanocrystals, often require a high‐temperature sintering process for the removal of residual organics and the improved crystallization of metal oxides. Here, a novel emulsion‐based bottom‐up self‐assembly strategy is used to prepare sizable SnO2 microspheres from oleic acid capped SnO2 nanorods. Combined with an in‐situ ligand‐stripping strategy, the low‐temperature solution‐processed mesoscopic PSCs can achieve efficiency as high as 21.35% with slight hysteresis and good reproducibility. In particular, the emulsion‐based bottom‐up self‐assembly strategy is a general way for preparing microspheres from several kinds of semiconductor nanocrystals, so it will greatly expand the material selection range for preparing efficient mesoscopic PSCs and even inverted mesoscopic devices.

Efficient mesoscopic perovskite solar cells from emulsion-based bottom-up self-assembled TiO2 microspheres

Mesoscopic perovskite solar cells (PSCs) usually show superior photovoltaic performance to their planar counterparts because of the mesoporous electron transport layers (ETLs) benefiting for perovskite crystal growth and electron transport. However, the preparation of high-quality mesoporous ETLs needs selecting sizable semiconductor nanocrystals commonly within 20–50 nm, smaller or larger nanocrystals will lead to either too narrow pores to effectively infiltrate perovskite materials or too small surface area to efficiently extract charges from perovskite materials. Using sizable semiconductor microspheres can solve the issue, whereas it is also not easy to synthesis sizable semiconductor microspheres. Here, a novel emulsion-based bottom-up self-assembly strategy is used to prepare small size (about 150 nm) TiO2 microspheres from ultra-small (about 3.6 nm) TiO2 nanocrystals. The as-prepared mesoscopic PSCs can yield a champion PCE as high as 19.27% with a much smaller current–voltage hysteresis compared with those of the planar one. Specially, the emulsion-based bottom-up self-assembly strategy is a general way for preparing microspheres from wide kinds of semiconductor nanocrystals, so it will greatly expand the material selection range for preparing efficient mesoscopic PSCs and even inverted mesoscopic devices.

In-situ passivation of TiO2 mesoporous scaffold with nano-sized heteropolyacid for boosting the efficiency of the perovskite solar cells

TiO2 is a widely used electron transport layer for high efficiency perovskite solar cells. However, many intrinsic shortcomings seriously deteriorate the devices performance. Herein, we demonstrate a facile and effective approach to improve the photovoltaic performance by passivating the TiO2 mesoporous layer with a nanoscale dispersed 12-tungstophosphoric heteropolyacid (H3PW12O40·nH2O). Due to the strong interaction between H3PW12O40·nH2O and TiO2, the passivated TiO2 mesoporous electron transport layer (hereafter denoted as PW12/TiO2) with an optimal weight ratio shows a more uniform coverage on the underlying substrate in comparison with the bare TiO2. Furthermore, the resulted PW12/TiO2 also shows an obviously improved conductivity and significantly increased photoluminescence quenching for the perovskite emission. Thereby, this strategy leads to an apparently improved photovoltaic performance of perovskite solar cells with power conversion efficiency enhancement from 15.05% to 18.20%. In conclusion, our present study provides an effective approach for further improving the performance of perovskite solar cells by passivating the mesoporous electron transport layer with the well-known heteropolyacids.

Eliminating Charge Accumulation via Interfacial Dipole for Efficient and Stable Perovskite Solar Cells.

Elimination of interfacial charge trapping is still a challenge for promoting both efficiency and operational stability of organic-inorganic perovskite solar cells (PSCs). Herein, an effective interface dipole, trimethylamine oxide (TMAO) regarded as a connecting bridge, is inserted between the electron transport layer (ETL) and the perovskite layer to suppress charge accumulation and fabricate highly efficient and stable PSCs. As demonstrated by energy level alignment and morphology characterization, TMAO dipoles could achieve a decreased energetic barrier of electron transport and substantial padding of perovskite in the mesoporous ETL. Thus, they facilitate the charge transfer and reduce trapped charge densities as well as recombination centers at the interface between perovskite and ETL. These desirable properties improve the device efficiency to 21.77% and weaken the hysteresis index almost to 0. More importantly, the stability of the unencapsulated PSCs is remarkably enhanced. The findings provide valuable insights into the role of a dipolar molecule in boosting the performance of PSC devices.

Optimizing the efficiency of perovskite solar cells by a sub-nanometer compact titanium oxide electron transport layer

An electron transport layer for electron collection and transport from the perovskite photoactive layer is indispensable in high-performance of mesoscopic perovskite solar cells. A well-organized electron transport layer can significantly improve the quality of the deposited perovskite film, enhance the transport of electrons and help the extraction of electron-hole pairs, but the optimized structural constitution remains elusive. In this work, we find that the optimal performance of perovskite solar cells can be achieved by introducing a sub-nanometer compact titanium dioxide layer to the mesoporous electron transport layers as an ultrathin compact layer is enough to enhance electron extraction and reduce carrier recombination. It is shown that an efficiency around 18% can be reliably achieved at the optimized structural constitution of a 230-nm mesoporous TiO2 layer as well as a compact TiO2 layer in a large equivalent thickness ranging from sub-nanometer to 30 nm. The mesoporous layer is also important as the power conversion efficiencies of all the fabricated perovskite solar cells are less than 11.8% without it. The results shed light on fabrication perovskite solar cells with thinnest compact layer with optimized efficiency.