Transparent Conducting Oxide Substrates Research Articles

Planar versus mesoscopic perovskite microstructures: The influence of CH3NH3PbI3 morphology on charge transport and recombination dynamics

Perovskite solar cells (PSCs) employing planar and mesoscopic architectures have both resulted in high efficiency devices. However, there is presently a limited understanding of the inherent advantages of both systems, particularly in terms of the charge transport and recombination dynamics. In the present study we characterize the relative benefits of the two most prominent CH3NH3PbI3 morphologies, primarily through time-resolved microwave conductivity (TRMC) and time-resolved photoluminescence (TRPL) measurements. The comparatively large perovskite grains, typical of planar assemblies, exhibited higher charge mobilities and slower trap-mediated recombination compared to the mesoscopic architectures. These findings reveal the injurious influence of grain boundaries on both charge transport and recombination kinetics, and suggest an innate advantage of planar morphologies. However, through impedance spectroscopy (IS) measurements, mesoscopic architectures were found to limit the interfacial recombination at the transparent conductive oxide (TCO) substrate. The lessons learnt through the characterization measurements were subsequently utilized to produce an optimized cell morphology, resulting in a maximum conversion efficiency of 16%.

Photoelectrochemical studies on BiVO4 membranes deposition on transparent conductive substrates by a facile electrophoresis route

BiVO4 membranes were deposited on transparent conducting oxide substrates by a facile electrophoretic deposition route, followed by a thermal sintering process. As the sintering temperature (T a) raise, the BiVO4 grains got larger due to the interaction and agglomeration. As a result, the 600 °C sintered BiVO4 membranes exhibited a photocurrent density of ~23 μA/cm2 at a potential 0.5 V versus the saturated calomel electrode. Besides, a rapid decay behavior was also observed in the curves of photocurrent versus time, which might be attributed to a high carrier recombination. Through applying a positive potential, that instability behavior got suppressed effectively, while the actual photocurrent was enhanced significantly.

Solvent‐Mediated Dimension Tuning of Semiconducting Oxide Nanostructures as Efficient Charge Extraction Thin Films for Perovskite Solar Cells with Efficiency Exceeding 16%

The recent surge in efficiency and progress of organohalide perovskite solar cells (PSCs) has been significant. The PSC performance is significantly influenced by nanostructuring as this varies the intrinsic optical, electrical, and electrochemical properties. Diverse TiO2 electron transport layers (ETLs) are solvothermally grown on the transparent conducting oxide substrate with different dimensionalities, 0D nanoparticles (TNP), 1D nanowires (TNW) to 2D nanosheets (TNS), by varying the organic solvent used. These layers feature enhanced optical transparency (≈2%–5% transmittance improvement compared to pristine fluorine doped tin oxide, FTO, glass) minimizing light absorption losses. PSCs constructed using 1D TNW or 2D TNS yield enhanced photovoltaic performance compared to the 0D TNP counterparts. This is a result of i) improved infiltration of the perovskite in the porous TNW or TNS network and ii) facilitated electron transport and charge extraction at the TNW/perovskite or TNS/perovskite interfaces, thus reduced interfacial recombination loss. Employing a bilayered ETL film consisting of a self‐assembled TiO2 blocking layer and a subsequent TNW active layer, produces PSC devices with an efficiency exceeding 16%. This bilayered ETL film can simultaneously block the photogenerated holes and enhance electron ­extraction, therefore improving PSC performance.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting.

Tantalum nitride, Ta3N5, is one of the most promising materials for solar energy driven water oxidation. One significant challenge of this material is the high temperature and long duration of ammonolysis previously required to synthesize it, which has so far prevented the use of transparent conductive oxide (TCO) substrates to be used which would allow sub-bandgap light to be transmitted to a photocathode. Here, we overcome this challenge by utilizing atomic layer deposition (ALD) to directly deposit tantalum oxynitride thin films, which can be fully converted to Ta3N5via ammonolysis at 750 °C for 30 minutes. This synthesis employs far more moderate conditions than previous reports of efficient Ta3N5 photoanodes. Further, we report the first ALD of Ta-doped TiO2 which we show is a viable TCO material that is stable under the relatively mild ammonolysis conditions employed. As a result, we report the first example of a Ta3N5 electrode deposited on a TCO substrate, and the photoelectrochemical behavior. These results open the door to achieve efficient overall water splitting using a Ta3N5 photoanode.

Developments of c-Si1-xGex:H thin films as near-infrared absorber for thin film silicon solar cells

Hydrogenated microcrystalline silicon germanium (c-Si1-xGex:H) thin films have been developed as alternative bottom sub-cell absorbers for multi-junction thin film silicon solar cells due to their narrower band-gaps and higher absorption coefficients than conventional hydrogenated microcrystalline silicon (c-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of c-Si1-xGex:H film properties on Ge composition have not been understood yet. In this work, c-Si1-xGex:H thin films with various Ge content and similar crystalline volume fraction are fabricated by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The evolutions of c-Si1-xGex:H material properties by Ge incorporation are characterized by X-ray fluorescence spectrometry, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, absorption coefficient spectrum, and conductivity measurement. The results show that the properties of c-Si1-xGex:H thin films are strongly determined by Ge content. With the increase of Ge content, the absorption coefficient, (111) grain size, microstructure factor, and dark conductivity of c-Si1-xGex:H thin films increase, while the H content, (220) grain size, and photosensitivity of c-Si1-xGex:H thin film decrease. Then, c-Si1-xGex:H is used as the intrinsic layer in the single junction solar cells. The performances of c-Si1-xGex:H solar cells with different Ge content and two types of transparent conductive oxide (SnO2 and ZnO) substrates are systematically studied. The results indicate that although c-Si1-xGex:H thin films become more defective and less compact with Ge incorporation, c-Si1-xGex:H solar cells exhibit a significant improvement in near-infrared response, especially under the circumstances of thin cell thickness and inefficient light trapping structure. Meanwhile, by using ZnO substrates, initial efficiencies of 7.15% (Jsc=22.6 mA/cm2, Voc=0.494 V, FF=64.0%) and 7.01% (Jsc=23.3 mA/cm2, Voc=0.482 V, FF=62.4%) are achieved by c-Si0.9Ge0.1:H solar cell and c-Si0.73Ge0.27:H solar cell, respectively. Furthermore, the c-Si0.73Ge0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a Jsc.bottom of 12.30 mA/cm2 can be obtained with the bottom sub-cell thickness as thin as 800 nm, which is even higher than that of c-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/c-Si0.73Ge0.27:H double junction cell structure. It is demonstrated that by using the c-Si1-xGex:H solar cell as the bottom sub-cell in multi-junction thin film silicon solar cells, a higher tandem cell performance can be achieved with a thin thickness, which has a great potential for cost-effective photovoltaics.

Li4Ti5O12 and LiMn2O4 thin-film electrodes on transparent conducting oxides for all-solid-state and electrochromic applications

Transparent Li4Ti5O12 and LiMn2O4 thin films were prepared on transparent conducting oxide substrates via sol–gel dip coating technique. The resulting films were highly uniform and transparent after annealing at 600 °C and 400 °C for Li4Ti5O12 and LiMn2O4, respectively. Different film thicknesses ranging from 50 nm to about 800 nm were attained and the films were characterized by scanning electron microscopy (SEM) for surface morphology and X-ray diffraction (XRD) for evaluating the crystallization of the active materials. Ellipsometric porosimetry was used to measure the porosity of the anode material. Furthermore, electrochemical techniques such as cyclovoltametry and galvanostatic charging/discharging confirm the successful synthesis of electrochemically active LiMn2O4 and Li4Ti5O12 films. Single-layer electrodes show stable capacities at discharge rates up to 100C indicating the high rate capability and therefore and therefore are classified as materials for power. Furthermore, the electrochromic effects were observed for both thin-film systems and evaluated in preliminary tests.

Electrocatalysis of the hydrogen evolution reaction by rhenium oxides electrodeposited by pulsed-current

Rhenium oxides are materials of interest for applications in the catalysis of reactions such as those occurring in fuel cells and photoelectrochemical cells. This research work was devoted to the production of rhenium oxide by means of pulsed current electrodeposition for the electrocatalysis of the hydrogen evolution reaction (HER). Rhenium oxides were electrodeposited over a transparent conductive oxide substrate (Indium Tin-doped Oxide – ITO) in an alkaline aqueous electrolyte. The electrodeposition process allowed the production of rhenium oxides islands (200–600nm) with the presence of three oxidized rhenium species: ReIV associated to ReO2, ReVI associated to ReO3 and ReVII associated to H(ReO4)H2O. Electrodeposited rhenium oxides showed electrocatalytic behavior over the HER and an increase of one order of magnitude of the exchange current density was observed compared to the reaction taking place on the bare substrate. The electrocatalytic behavior varied with the morphology and relative abundance of oxidized rhenium species in the electrodeposits. Finally, two mechanisms of electrocatalysis were proposed to explain experimental results.

Highly Efficient Solar Water Splitting from Transferred TiO2 Nanotube Arrays

We report a synergistic effect of flame and chemical reduction methods to maximize the efficiency of solar water splitting in transferred TiO2 nanotube (TNT) arrays on a transparent conducting oxide (TCO) substrate. The flame reduction method (>1000 °C) leads to few oxygen vacancies in the anatase TNT arrays, but it exhibits unique advantages for excellent interfacial characteristics between transferred TNT arrays and TCO substrates, which subsequently induce a cathodic on-set potential shift and sharp photocurrent evolution. By contrast, the employed chemical reduction method for TNT arrays/TCO gives rise to an abrupt increase in photocurrent density, which results from the efficient formation of oxygen vacancies in the anatase TiO2 phase, but a decrease in charge transport efficiency with increasing chemical reduction time. We show that flame reduction followed by chemical reduction could significantly improve the saturation photocurrent density and interfacial property of TNT arrays/TCO photoanodes simultaneously without mechanical fracture via the synergistic effects of coreducing methods.

Preparation of WO3 Thin Film by Successive Dip Coating for Electrochromic and Photoelectrochromic Devices

Here we report the superiority of WO3 thin films prepared by successive dip coating in electrochromic devices. Dip coatings were done on transparent conducting oxide substrates in the WO3 precursor solution a number of times without drying in between the coating steps. This process enabled the fabrication of WO3 films with optimum thickness and large roughness, which led to enhanced performance when they were employed as the electrode in electrochromic devices. We made a further observation by applying these electrodes in photoelectrochromic devices and verified the excellent properties of the successively dip‐coated WO3 films.

Transparent conducting oxide free backside illuminated perovskite solar cells

Recently, hybrid perovskites have attracted great attention because of their promising applications in solar cells. However, perovskite solar devices reported till now are mostly based on transparent conducting oxide (TCO) substrates which account for a large proportion in the total cost. Herein, TCO-free perovskite solar cells are fabricated. A photo-electricity conversion efficiency of 5.27% is obtained with short circuit current density (Jsc) of 10.7 mA/cm2, open circuit voltage (Voc) of 0.837 V, and fill factor of 0.588. This study points a feasible way of replacing TCO substrate by low cost substrates, indicating promising potentials in solar energy conversion applications.

Thin Films of Dendritic Anatase Titania Nanowires Enable Effective Hole‐Blocking and Efficient Light‐Harvesting for High‐Performance Mesoscopic Perovskite Solar Cells

To achieve high‐performance perovskite solar cells, especially with mesoscopic cell structure, the design of the electron transport layer (ETL) is of paramount importance. Highly branched anatase TiO2 nanowires (ATNWs) with varied orientation are grown via a facile one‐step hydrothermal process on a transparent conducting oxide substrate. These films show good coverage with optimization obtained by controlling the hydrothermal reaction time. A homogeneous methyl­ammonium lead iodide (CH3NH3PbI3) perovskite thin film is deposited onto these ATNW films forming a bilayer architecture comprising of a CH3NH3PbI3 sensitized ATNW bottom layer and a CH3NH3PbI3 capping layer. The formation, grain size, and uniformity of the perovskite crystals strongly depend on the degree of surface coverage and the thickness of the ATNW film. Solar cells constructed using the optimized ATNW thin films (220 nm in thickness) yield power conversion efficiencies up to 14.2% with a short‐circuit photocurrent density of 20.32 mA cm−2, an open‐circuit photovoltage of 993 mV, and a fill factor of 0.70. The dendritic ETL and additional perovskite capping layer efficiently capture light and thus exhibit a superior light harvesting efficiency. The ATNW film is an effective hole‐blocking layer and efficient electron transport medium for excellent charge separation and collection within the cells.

ZnO Nanorods Grown Electrochemically on Different Metal Oxide Underlays

In this study we present results on electrochemically grown ZnO nanorods on different metal oxide underlays, such as ZnO seed layers with different morphologies, ZnS and TiO2 compact thin films produced by spray pyrolysis on transparent conductive oxide (TCO) substrates. Also in this work we present results on ZnO nanorods directly deposited on some chosen TCO substrates. The relationship between nanorod formation and substrate properties were studied. All ZnO nanorod layers were grown electrochemically using ZnCl2 aqueous solutions (c=0.2 mmol/L) at the bath temperature of 80 °C during one hour. The structural properties and morphology of metal oxide underlays and ZnO nanorods grown on them were studied by scanning electron microscopy (SEM), x-ray diffraction spectroscopy (XRD). Depending on the substrate morphology, ZnO rods with different dimension, orientation, shape and density were obtained. For instance, larger rods (d~200 nm, l~700 nm) were obtained on substrates, such as ITO/glass, FTO/glass and ZnO:In/ITO/glass. Smaller rods (d~60 nm, l~350 nm) were obtained on smooth, uniform and fine-grained underlays, such as ZnS and TiO2.

Oxygen-Controlled Seed Layer in DC Sputter-Deposited ZnO:Al Substrate for Si Thin-Film Solar Cells

Oxygen-controlled seed layer in Al-doped ZnO (ZnO:Al) thin films deposited by the industrially compatible dynamic dc magnetron sputter results in both enhanced electron mobilities and appropriate etched morphologies for the Si thin-film solar cells. At the relatively low deposition temperature of 300 °C, optimized ZnO:Al film grown on the seed layer has the carrier mobility of 45 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s and proper postetching morphology with around 1-2-μm crater size. Reduced angular distribution of the (002) grains analyzed by the diffraction rocking curve is shown as the key structural feature for the improved carrier mobility. Finally, the performance of the microcrystalline Si solar cell on the developed ZnO:Al substrate shows high-efficiency potential of the tandem solar cell adapting this transparent conductive oxide substrate.

Three-dimensional TiO2/ZnO hybrid array as a heterostructured anode for efficient quantum-dot-sensitized solar cells.

The development of a novel nanoarray photoanode with a heterostructure on a transparent conducting oxide substrate provides a promising scheme to fabricate efficient energy conversion devices. Herein, we successfully synthesize the vertically aligned hierarchical TiO2 nanowire/ZnO nanorod or TiO2 nanowire/ZnO nanosheet hybrid arrays, which are proven to be excellent anode candidates for superior light utilization. Consequently, the quantum-dot-sensitized solar cells based on such hybrid arrays exhibit an impressive power conversion efficiency (PCE) under AM 1.5G one sun illumination with improved short-circuit current density (JSC) and fill factor compared to pristine TiO2 nanowire arrays. Combined with the chemical-bath-deposited Cu2S counter electrode, the eventual PCE can be further optimized to as high as 4.57% for CdS/CdSe co-sensitized quantum dot solar cells.

Rapid synthesis of Cu2ZnSnS4 nanocrystalline thin films directly on transparent conductive glass substrates by microwave irradiation

In this letter, a rapid and single-step microwave-assisted solution method was proposed to grow Cu2ZnSnS4 (CZTS) thin films directly on transparent conductive fluorine-doped tin oxide (FTO) substrates. The CZTS thin films synthesized in one pot without any post-treatment are characterized by X-ray diffraction, Raman spectroscopy, scanning electronic microscope, transmission electron microscopy, X-ray photoelectron spectrometer and UV–vis spectrophotometer. The CZTS thin films were single kesterite structure and composed of uniform sphere-like nanocrystalline particles with an average diameter of about 250nm. The optical band gap is found to be 1.58eV, which is suitable for photovoltaic applications.

A redox-flow electrochromic window.

A low-cost electrochromic (EC) window based on a redox-flow system that does not require expensive transparent conductive oxide (TCO) substrates is introduced and demonstrated for the first time. An aqueous I3–/I– redox electrolyte is used in place of a TCO to oxidize/reduce a molecular layer of an EC triphenylamine derivative that is anchored to a mesoporous TiO2 scaffold on the inner faces of a double-paned window. The redox electrolyte is electrochemically oxidized/reduced in an external two-compartment cell and circulated through the window cavity using an inexpensive peristaltic pump, resulting in coloration or decoloration of the window due to reaction of the redox solution with the triphenylamine derivative. The absorption characteristics, coloration/decoloration times, and cycling stability of the prototype EC window are evaluated, and prospects for further development are discussed.

Template-free and low temperature CVD synthesis of vertically aligned 1-D ZnO nanostructures for photovoltaic devices by precursor oxidation protection

ABSTRACTZinc oxide (ZnO) is a crystalline material with diverse morphology, large bandgap and high visible light transparency. All of these characteristics make ZnO a suitable material for applications in optical devices such as photovoltaic cells and photodiodes. Regarding photovoltaic applications, it is necessary to grow ZnO on a transparent conducting oxide (TCO) substrate. In this work, vertically aligned 1-dimensional ZnO have been synthesized on a TCO substrate through chemical vapor deposition (CVD). Although ZnO is capable of being synthesized at lower temperatures through the use of Zn powder precursor, oxidation of precursor remains a significant limiting factor to control dimensional characteristics of the synthesized product.In our work we have developed a method by which ZnO is synthesized under lower temperatures through the prevention of precursor oxidation and control of Zn vapor fluid dynamics. Partial pressure of Zn vapor—a significant factor in the morphology and quality of product—is controlled and maintained during growth. The morphology and crystal structure of the synthesized ZnO is characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD). We also demonstrate the fabrication of dye-sensitized solar cell (DSSC) with synthesized 1-dimensional ZnO, as a photoelectrode, and compare the photovoltaic characteristics of two devices fabricated under same conditions, except for the photoelectrode utilized.

Optimizing the packing density of TiO2 nanorod arrays for enhanced light harvesting by a light trapping effect and its photocatalytic decomposition of gaseous benzene

A single-crystalline TiO2 nanorod array with a preferred [002] axial orientation was grown on transparent conductive fluorine-doped tin oxide substrates by a hydrothermal process.

Epitaxial 1D electron transport layers for high-performance perovskite solar cells.

We demonstrate high-performance perovskite solar cells with excellent electron transport properties using a one-dimensional (1D) electron transport layer (ETL). The 1D array-based ETL is comprised of 1D SnO2 nanowires (NWs) array grown on a F:SnO2 transparent conducting oxide substrate and rutile TiO2 nanoshells epitaxially grown on the surface of the 1D SnO2 NWs. The optimized devices show more than 95% internal quantum yield at 750 nm, and a power conversion efficiency (PCE) of 14.2%. The high quantum yield is attributed to dramatically enhanced electron transport in the epitaxial TiO2 layer, compared to that in conventional nanoparticle-based mesoporous TiO2 (mp-TiO2) layers. In addition, the open space in the 1D array-based ETL increases the prevalence of uniform TiO2/perovskite junctions, leading to reproducible device performance with a high fill factor. This work offers a method to achieve reproducible, high-efficiency perovskite solar cells with high-speed electron transport.

Stoichiometry gradient, cation interdiffusion, and band alignment between a nanosized TiO2 blocking layer and a transparent conductive oxide in dye-sensitized solar cell front contacts.

An angle-resolved photoemission spectroscopy study allowed us to identify cation interdiffusion and stoichiometry gradients at the interface between a nanosized TiO2 blocking layer and a transparent conductive Cd-Sn oxide substrate. A stoichiometry gradient for the Sn cations is already found in the bare Cd-Sn oxide layer. When TiO2 ultrathin layers are deposited by RF sputtering on the Cd-Sn oxide layer, Ti is found to partially replace Sn, resulting in a Cd-Sn-Ti mixed oxide layer with a thickness ranging from 0.85 to 3.3 nm. The band gap profile across the junction has been reconstructed for three TiO2 layers, resulting in a valence band offset decrease (and a conduction band offset increase) with the blocking layer thickness. The results are related to the cell efficiencies in terms of charge injection and recombination processes.