Mesoporous TiO2 Layer Research Articles

Performance improvement of inverted perovskite solar cells using TiO2 nanorod array and mesoporous structure

Abstract In view of the low carrier mobility of organic materials, the carrier collection ability was suffered from the short transport length before carriers were recombined. To improve performances by enhancing carrier collection ability, the optimal period was 1.5 μm which was obtained by changing the period of titanium dioxide (TiO2) nanorod array in the inverted perovskite solar cells (IPSCs). The power conversion efficiency was improved to 11.96% from the 7.66% of the standard planar IPSCs. Besides, due to the inherent properties of high absorption surface area and high light scattering ability, the 150-nm-thick TiO2 mesoporous layer was embedded in the TiO2 electron transport layer. By changing the annealing temperature, the optimal crystallinity of anatase phase and the optimal porous distribution were obtained in the TiO2 mesoporous layers annealed at 500 °C for 30 min. Using the optimal annealed TiO2 mesoporous layers in the IPSCs, the power conversion efficiency was improved to 12.73%. The power conversion efficiency of 14.47% was obtained for the IPSCs embedded with the optimal 1.5-μm-periodic TiO2 nanorod array and the optimal 500 °C-annealed TiO2 mesoporous layer in the electron transport layer, simultaneously.

Multistep spin–spray deposition of large-grain-size CH3NH3PbI3 with bilayer structure for conductive-carbon-based perovskite solar cells

Large-grain-size and void-free CH3NH3PbI3 films with bilayer structure are fabricated by spin-coating a PbI2 layer onto a mesoporous TiO2 layer and sequentially spraying CH3NH3I (methylammonium iodide, MAI) multilayers. The sprayer is controlled by a homemade three-axis computer numerical control machine; thus, the substrates are coated by successive parallel passes achieved by moving the nozzle. Spray deposition at the optimal spray rate and substrate temperature produces a large-grain-size and void-free methylammonium lead iodide (MAPbI3) bilayer structure. The mesoporous TiO2 layer plays an important role in electron transport by preventing the return of electrons to the perovskite layer and decreasing the contact resistance at the perovskite/compact TiO2/fluorine tin oxide interface. When the films are incorporated into a solar cell device with a conductive carbon counter electrode, a maximum power conversion efficiency of 10.58% is realised.

Photocurrent enhancement by incorporation of air-stable Cs2SnI6 Perovskite in dye-sensitized solar cell

Cs2SnI6 is an inorganic lead-free perovskite variant which is air and thermal stable and its application in the third generation of photovoltaic devices is undergoing rapid expansion over the past few years. This paper presents a fabrication of dye-sensitized solar cell (DSSC) with the incorporation of Cs2SnI6 perovskite in the active layer of N-719 dye. The absorbance spectra of N-719 dye on TiO2 mesoporous layer is found to increase by the addition of 2 wt% and 3 wt% of Cs2SnI6 in N719 dye in comparison to the reference of N-719 dye leading the role of Cs2SnI6 as photon absorber in the devices. Our fabricated DSSC device reveals a photocurrent enhancement from 6.36 mA/cm2 to 12.87 mA/cm2 by the addition of 3 wt% of Cs2SnI6, along with the enhancement of power conversion efficiency from 1.71% to 3.94%.

Response Enhancement of Self‐Powered Visible‐Blind UV Photodetectors by Nanostructured Heterointerface Engineering

AbstractThe development of efficient photodetectors (PDs) for ultraviolet (UV) light is of great importance for many applications. In this paper, a novel approach is proposed for boosting the performances of self‐powered PDs. Visible‐blind UV‐A PDs are built by combining a mesoporous TiO2 layer with a Spiro‐OMeTAD layer. The nanostructured heterointerface is engineered by inserting a self‐assembled layer of organic modifiers. By choosing 4‐nitrobenzoic acid (NBA), the responsivity is boosted by 70% compared to the pristine devices. It achieves 64 mA W−1 at 0 V bias, 380 nm, and 1 mW cm−2. The PD displays a very high sensitivity (>104), a fast response time (<3 ms), a high stability, and repeatability. 4‐chlorobenzoic acid, 4‐methoxy benzoic acid, 4‐nitro benzoic acid, and β‐alanine surface modifiers are studied by a combined experimental and theoretical approach. Their dipole moment is calculated. Their presence induces a step in the vacuum energy and the formed dipole field dramatically affects the charge transfer and then the photocurrent/photoresponse of the device. The higher responsivity of the NBA‐modified PD is thus explained by the better and faster electron charge transfer toward the electrical contact on TiO2.

Compatibility between compact and mesoporous TiO2 layers on the optimization of photocurrent density in photoelectrochemical cells

Titanium dioxide (TiO2) is a well-known efficient photocatalyst and electron transport layer for third generation solar cells. Spin coating is commonly used to fabricate TiO2 thin films because of its facile processing protocols and cost-effectiveness. However, the varying of TiO2 layer by using this method get less attention to attain the optimal thickness though the operation is simple and less cost. In this work, TiO2 thin films were deposited by spin coating on fluorine-doped tin oxide (FTO) surfaces. The number of layers of blocking TiO2 (bl-TiO2) and mesoporous TiO2 (mp-TiO2) was varied. The optimal electrical properties of TiO2 were verified by photoelectrochemical (PEC) analysis, and the credible performance of photocurrent density had been obtained. Then, we comprehensively studied the morphological and optical properties of the TiO2 layers to determine the factors that improve PEC performance. The double layers of 38.7 ± 1.96 nm-thick bl-TiO2 and 245.2 ± 3.39 nm-thick mp-TiO2 films were the optimal layers. At optimal conditions, the optical bandgap energy was 3.26 eV, and the photocurrent density was 82.30 μA/cm2 at 0.24 V.

Engineering the mesoporous TiO2 layer by a facile method to improve the performance of perovskite solar cells

Inorganic-organic perovskites, such as CH3NH3PbX3 (X = I, Br, Cl), is significant improvements in power conversion efficiencies, demonstrating the enormous potential of perovskite materials. Rapid extraction of photo-generated charge carriers from high-quality perovskite films is essential to achieve efficient perovskite solar cells (PSCs). Here, we devote to improving the charge-extraction ability of the electron transport layers (ETLs) and the crystallization of the perovskite layers by optimizing the ETLs' fabrication process. Insight into the impacts of ETLs architecture on the ETLs/perovskite hetero-interface properties and the quality of perovskite films are originally elucidated. The optimized ETLs increase the open-circuit voltage (Voc) of the MAPbI3-based PSCs by up to 100 mV, and the optimal device achieves Voc ∼ 1.13 V. Under the AM 1.5 simulated sunlight illumination, the optimized PSCs achieves an optimum PCE of 19.7% and maintains good stability after a storage time of 30 days with exposed to the ambient air environment. One of the key limitations of current PSCs performance is effectively solved by this simple optimization method and paves the way for further improve the efficiency through interface engineering.

Preparation by electrophoretic deposition of molybdenum iodide cluster-based functional nanostructured photoelectrodes for solar cells

This work explores the potentiality of Mo6 clusters as new inorganic sensitizers with amphoteric properties for photoelectronic applications being non-toxic and stable. It reports on the design of photoelectrodes by electrophoretic deposition (EPD) of molybdenum octahedral metal cluster iodide (CMI) onto mesoporous TiO2 and NiO layers before being deposited on FTO (i. e. CMI@TSO@FTO, TSO = TiO2 or NiO). Indeed, the low-cost, low-waste and industrially scalable EPD method has allowed for the achievement of transparent, homogeneous orange/red [Mo6Ii8]4+ cluster core-based films, and appears suitable for the sensitization of TiO2- and NiO-based electrodes of n- and p-type solar cells inspired from dye-sensitized solar cells (DSSCs). First, the EPD process was optimized by the direct deposition of CMI onto an FTO substrate (CMI@FTO) leading to the realization of highly transparent colored films. Second, strongly colored CMI@TSO@FTO photoelectrodes were achieved and integrated into solar cell devices. Hence, compared to the classical soaking method, the EPD process significantly improves the quality (i.e., its homogeneity and absorption properties) of the photoelectrodes, leading consequently to much better photovoltaic performances.

Applications of carbon nanotubes and graphene for third-generation solar cells and fuel cells

Abstract Carbon nanotubes (CNTs) and graphene have attracted great attention since decades ago because of their interesting structure and properties and important application in many areas. They can have high conductivity, high specific surface area, high transparency in the visible range and high mechanical flexibility. They have important application in energy conversion systems including solar cells and fuel cells. They have been extensively studied as the transparent electrode and interfacial materials of organic solar cells (OSCs) and perovskite solar cells (PSCs). They are also used as the catalytic counter electrode of dye-sensitized solar cells (DSSCs). In addition, graphene oxide (GO) is exploited as an auxiliary binder of TiO2 paste for the mesoporous TiO2 layer of DSSCs, and GO and functionalized CNTs are adopted as gelators of gel electrolyte for quasi-solid state DSSCs. CNTs and graphene also have important application in fuel cells. They can be used as catalyst support for the oxidation of fuels or oxygen reduction reaction (ORR). CNTs and graphene, particularly when doped with nitrogen, can be directly used metal-free catalysts. This article provides a brief review on the application of CNTs and graphene in OSCs, PSCs, DSSCs and fuel cells.

Fabrication and characterization of perovskite solar cells with ZnGa2O4 mixed TiO2 photoelectrode

Perovskite solar cells are easily decomposed by ultraviolet rays. In order to limit the function of TiO2 as a photocatalyst, ZnGa2O4 nanoparticles were mixed into a mesoporous TiO2 layer. The bandgap of the mixed layer increased by 0.1 eV and the efficiency of perovskite solar cell increased by 0.5%. However, it acts as a resistive element and lowers the current. Since ZnGa2O4 emits absorbed ultraviolet rays in blue, a thin film is additionally formed on the glass substrate to convert ultraviolet rays into electric current in a perovskite solar cell. When an ultraviolet ray is irradiated, an additional current of 100 μA cm−2 can be obtained.

Dye-Sensitized Solar Cells (DSSCs) Based on Extracted Natural Dyes

Here, three natural dyes were extracted from different fruits and leaves and used as sensitizers for dye-sensitized solar cells (DSSCs). Chlorophyll was extracted from spinach leaves using acetone as a solvent. Anthocyanin was extracted from red cabbage and onion peels using water. Different characterizations for the prepared natural dyes were conducted including UV-vis absorption, FTIR, and steady-state/time-resolved photoluminescence spectroscopy. Various DSSCs based on the extracted dyes were fabricated. The degradation in the power conversion efficiencies was monitored over a week. The effect of the TiO2 mesoporous layers on the efficiency was also studied. The interfaces between the natural dyes and the TiO2 layers were investigated using electrochemical impedance spectroscopy.

High performance perovskite solar cells based on β-NaYF4:Yb3+/Er3+/Sc3+@NaYF4 core-shell upconversion nanoparticles

The introduction of upconversion materials as spectral converter into photovoltaic devices to harvest and convert near infrared photons to visible photons that can thus be absorbed by perovskite layer is a promising strategy for enhancing the performance of perovskite solar cells. Hence, β-NaYF4:Yb3+/Er3+/Sc3+@NaYF4 rare-earth doped upconversion nanoparticles with core-shell structure are synthesized. The doped rare-earth ions Yb3+ and Er3+ convert near infrared light to red and green light, and the Sc3+ ions enhance the upconversion luminescence efficiency, which broaden the absorption range of solar irradiation and generate extra photocurrent in the solar cells. The surface defects of β-NaYF4:Yb3+/Er3+/Sc3+ is passivated by the even NaYF4 shell, which results in the obvious enhancement in upconversion emission intensity. Consequently, the perovskite solar cell based on the rare-earth doped upconversion nanoparticle mesoporous layer achieves a high power conversion efficiency of 20.19%, while the device based on TiO2 mesoporous layer gets an efficiency of 17.44%.

Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor

The ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used as a means to identify the dominant recombination mechanism in solar cells. However, applying this Suns-VOC technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), the light intensity, and the device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the temporal evolution of VOC at different light intensities. The initial values of the transient ideality were consistent with corresponding photoluminescence vs intensity as well as electroluminescence vs current density measurements. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, show that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both devices. Using transient photovoltage measurements superimposed on the evolving VOC of bifacial devices we further show that the charge collection interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This information could not be inferred from an ideality factor determined from only steady-state VOC values. The method we have developed will be useful for identifying performance bottlenecks in new variants of perovskite devices by comparison with the transient ideality signatures we have predicted for a range of possible recombination schemes.

The Effect of Annealing Temperature of ZnO Compact Layer and TiO2 Mesoporous on Photo-Supercapacitor Performance

Photo-supercapacitor is a combination of solar cells and supercapacitor which intensively being developed. Photo-supercapacitor performance is influenced by the efficiency of solar cells and storing and releasing capacity of supercapacitors. Any types of either solar section or supercapacitor sections could be used. In the DSSC solar cell, one of the influential variables is the performance of the photoanode. The photoanode with semiconducting metal oxides play a role in charges mobility and light absorption process, which are influenced by crystal morphology and structures. The common metal oxides used are ZnO and TiO2 which show high electron mobility, wide band gap, and good optical properties. This work is designed to investigate the effect of annealing temperature of the composite layer of ZnO and mesoporous TiO2 on structure, morphology, optical absorption, and photo-supercapacitor performance. The ZnO compact layer was deposited onto the FTO substrate by a spin coating method with various annealing temperature. The mesoporous TiO2 layer was deposited on top of the ZnO compact layer by means of screen printing method. The construction of photo-supercapacitor model comprises DSSC and BaTiO3-PVDF symmetric supercapacitor which integrated by using aluminum substrate. Characterization was done using XRD, SEM, UV-Vis, and I-V solar simulators for the performance of photo-supercapacitor.

Performance enhancement of perovskite solar cells via Nb/Ta-doped TiO2 mesoporous layers

The power conversion efficiency of mesoporous perovskite solar cells (PSCs) is closely related with TiO2 mesoporous layer. As the most universal mesoporous layer material, however, TiO2 does weakly in electrical properties. As a result, structure modifications of TiO2 mesoporous layer are required. In this work, niobium/tantalum doped TiO2 mesoporous layer were prepared via a facile one-pot solution process, and applied successfully as high quality mesoporous layer for mesoporous PSCs. Performance of perovskite solar cells prepared with Nb/Ta-doped TiO2 mesoporous layer enhanced apparently compared with the control device, especially in the short circuit current density (Jsc) which was improved from 18.4 to 21.3 mA cm−2.

Applying BaTiO3-coated TiO2 core–shell nanoparticles films as scaffold layers to optimize interfaces for better-performing perovskite solar cells

In this paper, we replaced mesoporous TiO2 nanoparticles scaffold layers by BaTiO3-coated TiO2 core–shell nanoparticles films which obtained by treating pure mesoporous TiO2 layers with 1.0 wt% barium nitrate solution, successfully realized the aim of optimizing interfaces bonding at TiO2/CH3NH3PbI3. Ultrathin BaTiO3 shell layer can combine better with CH3NH3PbI3 layer so as to reduce the existence of carrier recombination centers. Moreover, better optical absorption and larger fill factor were obtained in this manner by the reason of larger CH3NH3PbI3 grain size and fewer crystal boundaries. Furthermore, photoluminescence spectra and electrochemical impedance spectroscopy verified that our core–shell scaffold material contributes to accelerate carrier separation and retard carrier recombination. As a result, average power conversion efficiency enhanced from 11.20 to 13.76% under ambient conditions, which realized almost a quarter improvement than the devices based on pure mesoporous TiO2 layers. Such results have a certain guiding effect on solving interface defects and carrier recombination.

Enhancing Photostability of Perovskite Solar Cells by Eu(TTA)2(Phen)MAA Interfacial Modification.

Organic-inorganic lead halide perovskite solar cells (PSCs) exhibit spectacular changes in the photovoltaic area, but they still face the challenges of full spectral utilization and photostability under continuous light irradiation. The ultraviolet (UV) part in sunlight could induce oxygen vacancy in the mesoporous TiO2 (m-TiO2) layer, resulting in the degradation of perovskite photoactive films and the rapidly decreased device performance. In this work, we demonstrate that an effective luminescent downconversion material, Eu(TTA)2(Phen)MAA (ETPM), can be used as an interfacial modifier between the m-TiO2 layer and the perovskite photoactive layer to improve the power conversion efficiency (PCE) from 17.00 to 19.07%. The improved device performance can be ascribed to the effective utilization of incident UV light and reduced carrier recombination. Meanwhile, the conversion of the UV light by ETPM could inhibit the stability loss of the device under irradiation. As a result, the modified PSCs can maintain 86% of their initial value under continuous light soaking for 100 h, higher than that of 40% for the control device. This work indicates that the introduction of the luminescent downconversion material ETPM can successfully improve the PCE and photostability of PSCs.

Ionic Effect Enhances Light Emission and the Photovoltage of Methylammonium Lead Bromide Perovskite Solar Cells by Reduced Surface Recombination

The achievement of optimal power conversion efficiencies (PCEs) in wide-band-gap perovskite solar cells (PSCs) is delayed by photovoltage losses associated with poor understanding of recombination dynamics. In this work, we use high-quality methylammonium lead bromide perovskite solar cells with selective contacts treated with lithium-containing additives to investigate recombination mechanisms. By comparison of the photovoltaic performance of devices we confirm that the presence of this additive in the electron selective layer (ESL) significantly increases the open-circuit potential to values of 1.58 V. Impedance spectroscopy coupled with electroluminescence and photoluminescence analysis reveals that lithium ions present at the mesoporous TiO2 layer dramatically enhances the radiative recombination in the perovskite by reduction of undesired nonradiative and surface recombination pathways. This work highlights that the employ of additives helps to modify the electronic charge distribution at the metal o...

Low-Temperature In Situ Amino Functionalization of TiO2 Nanoparticles Sharpens Electron Management Achieving over 21% Efficient Planar Perovskite Solar Cells.

Titanium oxide (TiO2 ) has been commonly used as an electron transport layer (ETL) of regular-structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO2 layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low-cost fabrication and flexible devices. Herein, a facile one-step, low-temperature, nonhydrolytic approach to in situ synthesizing amino-functionalized TiO2 nanoparticles (abbreviated as NH2 -TiO2 NPs) is developed by chemical bonding of amino (-NH2 ) groups, via TiN bonds, onto the surface of TiO2 NPs. NH2 -TiO2 NPs are then incorporated as an efficient ETL in n-i-p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs0.05 FA0.83 MA0.12 PbI2.55 Br0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH2 -TiO2 ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO2 ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current-voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Trilayer Chlorophyll-Based Cascade Biosolar Cells

Inspired by the mimic of dual charge separation in natural photosynthesis systems for biosolar cells (BSCs) applications where the charge extraction at the interface was insufficient, we report chlorophyll (Chl)-based trilayer cascade BSCs composed of Chl-C1 and/or a Chl-C2 (Chl-C)-sensitized mesoporous TiO2 layer (TiO2–Chl-C) acting as electron extraction layers as well as active layers to simulate the primary electron acceptor and two additional Chl derivatives layers (Chl-A/Chl-D) mimicking the reaction centers of photosystems I and II in the Z-scheme process of natural oxygenic photosynthesis. All of the Chls could be excited to generate photocurrent, which is similar to the natural photosynthesis of multipigment synergy. The charge extraction has a significant enhancement compared to bilayer BSCs. Co-sensitization of Chl-C1 with Chl-C2 could further enhance the electron injection efficiency at the TiO2–Chl-C interface, leading to a power conversion efficiency of 4.14%, which is more than 3 times larg...

A robust ALD-protected silicon-based hybrid photoelectrode for hydrogen evolution under aqueous conditions.

Hydrogen production through direct sunlight-driven water splitting in photo-electrochemical cells (PECs) is a promising solution for energy sourcing. PECs need to fulfill three criteria: sustainability, cost-effectiveness and stability. Here we report an efficient and stable photocathode platform for H2 evolution based on Earth-abundant elements. A p-type silicon surface was protected by atomic layer deposition (ALD) with a 15 nm TiO2 layer, on top of which a 300 nm mesoporous TiO2 layer was spin-coated. The cobalt diimine-dioxime molecular catalyst was covalently grafted onto TiO2 through phosphonate anchors and an additional 0.2 nm ALD-TiO2 layer was applied for stabilization. This assembly catalyzes water reduction into H2 in phosphate buffer (pH 7) with an onset potential of +0.47 V vs. RHE. The resulting current density is -1.3 ± 0.1 mA cm-2 at 0 V vs. RHE under AM 1.5 solar irradiation, corresponding to a turnover number of 260 per hour of operation and a turnover frequency of 0.071 s-1.