Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Enhanced performances of dye-sensitized solar cells based on graphite-TiO2 composites

The graphite-incorporated TiO₂ composites for the photoanodes of the Dye-sensitized solar cells were prepared by ultrasound-assisted mixing method. The performances of these solar cells with different graphite additions were investigated by the photocurrent-voltage characteristics, open-circuit voltage decay measurement and electrochemical impedance spectroscopy. The results showed that the addition of graphite had a significant impact on the electron transport and recombination. The photocurrent-voltage results indicated that short-circuit current density (J_sc) and open-circuit voltage (V_oc) enhanced by 40% and 2%, respectively. A 30% improvement in conversion efficiency of dye-sensitized solar cell from 4.44% to 5.76% was achieved using 0.01 wt% graphite-TiO₂ composite electrodes compared to the pure TiO₂ electrode.     

TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO₂-coated SnO₂ nanosheet (TiO₂-SnO₂ NS) films about 300 nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl₄ aqueous solution. The as-prepared TiO₂-SnO₂ NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO₂-SnO₂ NSs were analyzed by current-voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO₂-SnO₂ NSs can provide an efficient electron transition channel along the SnO₂ nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.     

Preparation and characterization of P3HT:CuInSe2:TiO2 thin film for hybrid solar cell applications

Improved efficiency of hybrid Al/Ca/P3HT:CISe:TiO₂/PEDOT:PSS/ITO thin film solar cells was obtained by optimizing P3HT/CISe ratio. This study also investigated the effects of TiO₂ content in the P3HT:CISe active layer, and altering annealing temperature conditions. The optimum TiO₂ content and annealing temperature for solar cell efficiency is 25 wt.% and 150 °C, respectively. The optimal results for the open circuit voltages (V_OC), short-circuit current density (J_SC), fill factor (FF), and efficiency (η) of the prepared hybrid thin film solar cell were V_OC = 0.335 V, J_SC = 8.07 mA/cm², FF = 52.75, and η = 1.425.     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Influence of gas flow during vacuum cold spraying of nano-porous TiO2 film by using strengthened nanostructured powder on performance of dye-sensitized solar cell

Nano-porous TiO₂ films, which can be applied to the flexible dye-sensitized solar cell (DSC), were deposited by vacuum cold spraying at room temperature with the strengthened nanostructured TiO₂ powder as feedstock. The spraying was conducted under different accelerating gas flows resulting in various particle velocities. Results show that the short-circuit photocurrent density of the cell (N719 dye) increases from 8.3 to 9.8 mA/cm² with the increase in gas flow from 3 to 7.5 L/min. A maximum overall energy conversion efficiency of 4.2% was obtained for the DSC with the TiO₂ film deposited at the gas flow of 7.5 L/min. The influence of particle velocity on the electron transport parameters and cell performance was discussed to reveal the important role of particle velocity in the formation of particle connection through high impact pressure during vacuum cold spraying.     

The effect of Li intercalation on different sized TiO2 nanoparticles and the performance of dye-sensitized solar cells

The effect of Li⁺ insertion into different sized TiO₂ nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO₂ nanoparticles with different particle sizes (22 nm, 14 nm and 6 nm) doped with Li⁺ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li⁺ ions could be inserted into both the surface and bulk of the TiO₂ nanoparticles. In the larger particle size, the Li⁺ ions are inserted into the bulk anatase where as Li⁺ ions bounded on the TiO₂ surface for the smaller crystallite size. The photovoltaic properties were measured by a current-voltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li⁺ was doped into the TiO₂ nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li⁺ ions into 22 nm sized TiO₂ nanoparticles (P25). The 14 nm sized TiO₂ nanoparticles (P90) showed slightly less efficiency (η) upon Li⁺ doping than that of the undoped sample. However, the smallest sized TiO₂ nanoparticles (6 nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.     

Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles

Titanium dioxide nanotubes (TiNTs) were fabricated from commercial P25 TiO₂ powders via alkali hydrothermal transformation. Dye-sensitized solar cells (DSCs) were constructed by application of TiNTs and P25 nanoparticles with various weight percentages. The influence of the TiNT concentration on the performance of DSCs was investigated systematically. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the recombination resistance, electron lifetime and time constant in DSCs both under illumination and in the dark. The DSC based on TiNT/P25 hybrids showed a better photovoltaic performance than the cell purely made of TiO₂ nanoparticles. The open-voltage (V_oc), fill factor (FF) and efficiency (η) continuously increased with the TiO₂ nanotube concentration from 0 to 50 wt%, which was correlated with the suppression of the electron recombination as found out from EIS studies. Respectable photovoltaic performance of ca. 7.41% under the light intensity of 100 mW cm⁻² (AM 1.5G) was achieved for DSCs using 90 wt% TiO₂ nanotubes incorporated in TiO₂ electrodes.     

Novel tandem cell structure of dye-sensitized solar cell for improvement in photocurrent

A novel tandem cell structure is proposed to improve photocurrent of dye-sensitized solar cells (DSCs). Front and back parallel photoelectrodes are placed face-to-face; a common Pt-mesh counter electrode with transmittance is inserted between the electrodes. The short-circuit current density (J_sc) for the tandem cell is equivalent to the sum of the J_sc for the front and back photoelectrodes. A model using light energy absorbed by the photoelectrode is used to evaluate appropriate TiO₂ film thickness of the front photoelectrode. The J_sc for the tandem cell was improved to 13.3 mA/cm² for a cell with a 7.8-μm-thick front photoelectrode. The novel tandem cell has a great potential to improve DSC photocurrent and performance.     

Ionic liquid-based electrolyte solidified with SiO2 nanoparticles for dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSC) based on the propyl-methyl-imidazolium iodide (PMII) ionic liquid (IL) with and without the addition of SiO₂ nanoparticles is studied. Results confirm that the presence of SiO₂ nanoparticles in PMII electrolyte improves the charge transport of iodide/tri-iodide redox couple in the electrolyte and consequently increases the efficiency of DSSC up to 20%, relatively. Short circuit current density (J_SC) of the DSSC under illumination may be limited by the charge transport of the redox couple in the IL-based electrolytes and a theoretical maximum of J_SC can be evaluated from the cyclic voltammetry measurements of simple symmetric cells (TCO-PtelectrolytePt-TCO). The results show a strong temperature dependence of the DSSC performance if the PMII/I₂-based electrolytes are used.     

Growth mode transition of atomic layer deposited Al2O3 on porous TiO2 electrodes of dye-sensitized solar cells

This study investigates the growth behavior of atomic-layer-deposited (ALD) Al₂O₃ overlayers on porous TiO₂ electrodes, which comprise an anatase nanoparticle layer and a rutile particle layer, for optimizing dye-sensitized solar cells. The growth mode of the ALD Al₂O₃ overlayers changes from island growth to layer-by-layer growth during the first few ALD reaction cycles, and the growth mode transition is much more pronounced for the anatase electrode layer. The transition is likely a result of the reduction in the contractive lattice strain of the TiO₂ nanoparticles. The lattice strain in the hydroxylated TiO₂ nanoparticles is progressively reduced during the ALD Al₂O₃ deposition, resulting in the growth mode transition.     

Analysis on the interfacial properties of transparent conducting oxide and hydrogenated p-type amorphous silicon carbide layers in p-i-n amorphous silicon thin film solar cell structure

Quantitative estimation of the specific contact resistivity and energy barrier at the interface between transparent conducting oxide (TCO) and hydrogenated p-type amorphous silicon carbide (a-Si_1 − xC_x:H(p)) was carried out by inserting an interfacial buffer layer of hydrogenated p-type microcrystalline silicon (μc-Si:H(p)) or hydrogenated p-type amorphous silicon (a-Si:H(p)). In addition, superstrate configuration p-i-n hydrogenated amorphous silicon (a-Si:H) solar cells were fabricated by plasma enhanced chemical vapor deposition to investigate the effect of the inserted buffer layer on the solar cell device. Ultraviolet photoelectron spectroscopy was employed to measure the work functions of the TCO and a-Si_1 − xC_x:H(p) layers and to allow direct calculations of the energy barriers at the interfaces. Especially interface structures were compared with/without a buffer which is either highly doped μc-Si:H(p) layer or low doped a-Si:H(p) layer, to improve the contact properties of aluminum-doped zinc oxide and a-Si_1 − xC_x:H(p). Out of the two buffers, the superior contact properties of μc-Si:H(p) buffer could be expected due to its higher conductivity and slightly lower specific contact resistivity. However, the overall solar cell conversion efficiencies were almost the same for both of the buffered structures and the resultant similar efficiencies were attributed to the difference between the fill factors of the solar cells. The effects of the energy barrier heights of the two buffered structures and their influence on solar cell device performances were intensively investigated and discussed with comparisons.     

One-step electrodeposition of TiO2/dye hybrid films

Crystalline TiO₂ films can be formed by anodic electrodeposition from TiCl₃ solution. In this study we investigated the co-deposition of TiO₂ with different organic dye molecules as structure-directing agents. The best results were obtained with the pH-indicator bromothymol blue, which leads to the formation of thick, crystalline and porous films. The films were tested in dye-sensitized solar cells and gave a five times higher efficiency compared to the film deposited with sodium dodecyl sulfate in our earlier study.     

Double-layered TiO2 cavity/nanoparticle photoelectrodes for efficient dye-sensitized solar cells

TiO₂ nanoparticles (NPs) in the size of ~25 nm, namely P25, are very common material as the electron collecting layer in dye-sensitized solar cells (DSSCs). However, the light-scattering improvement of TiO₂ NP photoelectrodes is still a challenge. Here, we built TiO₂ cavities on the top of the TiO₂ NP layer by using carbonaceous microspheres as the template, forming the TiO₂ cavity/nanoparticle (C/NP) photoelectrode for the application in DSSCs. The cavity amount in the TiO₂ C/NP photoelectrode was controlled by adjusting the weight ratio of carbonaceous microspheres. SEM results confirm the successful formation of the double-layered TiO₂ C/NP electrode. J‒V tests show that the optimized TiO₂ C/NP electrode prepared with 25 wt.% carbonaceous microspheres contributes to remarkable improvement of the short-circuit current density (J_sc) and the power conversion efficiency (PCE). The best photovoltaic performance solar cell with the PCE of 9.08% is achieved with the optimized TiO₂ C/NP photoelectrode, which is over 98% higher than that of the TiO₂ NP photoelectrode. Further investigations of UV-vis DRS, IPCE, OCVD, and EIS demonstrate that the competition between light scattering effect and charges recombination in this TiO₂ C/NP photoelectrode is responsible for the PCE enhancement.     

Sputter deposition and surface treatment of TiO2 films for dye-sensitized solar cells using reactive RF plasma

Sputter deposition followed by surface treatment was studied using reactive RF plasma as a method for preparing titanium oxide (TiO₂) films on indium tin oxide (ITO) coated glass substrate for dye-sensitized solar cells (DSCs). Anatase structure TiO₂ films deposited by reactive RF magnetron sputtering under the conditions of Ar/O₂(5%) mixtures, RF power of 600 W and substrate temperature of 400 °C were surface-treated by inductive coupled plasma (ICP) with Ar/O₂ mixtures at substrate temperature of 400 °C, and thus the films were applied to the DSCs. The TiO₂ films made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼ 25 nm. The DSCs made of this TiO₂ material exhibited an energy conversion efficiency of about 2.25% at 100 mW/cm² light intensity.     

Optimization of titanium dioxide film prepared by electrophoretic deposition for dye-sensitized solar cell application

Nanocrystalline TiO₂ films were deposited on a conducting glass substrate by the electrophoretic deposition technique. It was found that the thickness of TiO₂ film increased proportionally with an increase in deposition time and deposition voltage. However, as the deposition duration or deposition voltage increased, the film surface was more discontinuous, and microcracks became more evident. The characteristic of the dye-sensitized solar cell using TiO₂ film as a working electrode was analyzed. The results of the energy conversion efficiency and the photocurrent density exhibited a relationship dependent on the TiO₂ thickness. Curve fitting of energy conversion efficiency vs. TiO₂ thickness revealed the optimum solar cell efficiency ~ 2.8% at the film thickness of ~ 14 μm.     

Fabrication of dye-sensitized solar cells using TiO2-nanotube arrays on Ti-grid substrates

Self-aligned TiO₂ nanotube arrays (20 μm in length) were fabricated by anodic oxidation of Ti-grid with a thickness of 100 μm in an ethylene glycol electrolyte with an addition of H₂O (1.5 vol.%) and NH₄F (0.2 wt.%). Voltage applied between Ti and Pt cathodes is 60 V at ~ 22 °C. Dye-sensitized solar cell utilizing photoanode structure of TiO₂-nanotube/Ti-grid was fabricated with no transparent conducting oxide (TCO) layer, in which Ti-grid replaces TCO. Overall photoconversion efficiency is very low (< 0.5%) due to the large pore size (100 nm in diameter) of the nanotubes, which may cause insufficient dye molecules to be attached, thus limiting light harvesting.     

DNA-like dye-sensitized solar cells based on TiO2 nanowire-covered nanotube bilayer film electrodes

This paper reports a two-step formation of a TiO₂ nanowire-covered nanotube bilayer film technique and its application in DNA-like dye-sensitized solar cells. The bilayer film was prepared by the electrochemical anodization first and then the hydrothermal method. From the reflectivity spectrum and scanning electron microscopy it is observed that the nanowire layer on the top cannot only decrease the reflectivity of the film, but also play a role to modify the film cracks. Compared with the dye-sensitized solar cells based on a single layer electrode, the cell with the bilayer film showed higher photovoltaic parameters and a lower dark current, which is due to its higher light harvesting efficiency and lower charge recombination between the electrolyte and the substrates.     

Influences of deposition and post-annealing temperatures on properties of TiO2 blocking layer prepared by spray pyrolysis for solid-state dye-sensitized solar cells

Influences of the temperature (T_s) for spray pyrolysis deposition of TiO₂ blocking layer (BL) using titanium diisopropoxide bis(acetylacetonate) (TAA) as a precursor and the temperature (T_p) for post-annealing of the BL films on the resulting BL film morphology and photovoltaic performance of solid-state dye-sensitized solar cells (SDSC) are investigated. A T_s ranging from 300 to 400 °C is found preferable for the formation of BL films with smooth surface and dense grain packing. A T_s lower than 300 °C results in insufficient decomposition of the TAA precursor and is unable to form smooth BL films, while a T_s over 400 °C leads to loosely packed grain in BL films. Power conversion efficiency (PCE) of ~ 4.0% is obtained for SDSC devices with BL films prepared at a T_s in the range of 300-450 °C. High temperature post-annealing (T_p = 500-550 °C) of the BL films prepared at a low T_s, such as 300 °C, can improve the PCE up to 4.6%. The improvements are considered due to the higher purity, increased crystallinity, and retained high grain packing density of the post-annealed BL films, which facilitate charge transport and suppress charge recombination.