Enhanced Photoconversion Efficiency Research Articles

Enhancing the performance of dye-sensitized solar cells based on organic dye sensitized TiO2 nanoparticles/nanowires composite photoanodes with ionic liquid electrolyte

Dye-sensitized solar cells (DSSCs) are attractive solar cell option because of their relatively low manufacturing cost and simple process technology. In the present investigation, photoanodes were prepared using TiO2 nanoparticles (TNPs), TiO2 nanowires (TNWs) and its various composites (TNPWs). TNPs and TNWs were prepared by sol–gel/hydrothermal methods and mixed at various weight ratios (100wt.% TNPs without TNWs), (90wt.% TNPs with 10wt.% TNWs), (50wt.% TNPs with 50wt.% TNWs), (10wt.% TNPs with 90wt.% TNWs); (total weight 5g). DSSCs were fabricated based on Indigo dye sensitized photoanodes with ionic liquid electrolyte PMII (1-propyl-3-methylimmidazolium iodide) and cobalt sulfide coated counter electrode. The crystalline structure and morphology of TNPs and TNWs were characterized by X-ray diffraction (XRD) and electron microscopes. Photovoltaic study revealed that DSSC-4 made with composite photoanode (10wt.% TNWs+90wt.% TNPs) had highest photo conversion efficiency (PCE) of 2.27% as compared to DSSC-2 (made with pure TNPs photoanode; η=1.00%) and DSSC-3 (pure TNWs photoanode; η=0.77%). Electrochemical impedance spectra (EIS) demonstrated that composite TNPWs photoanodes are beneficial to improve the electron transport property as well as possessed high charge collection efficiency and light harvesting, which steer to the enhancement of PCE of DSSCs.

Titanium wire engineering and its effect on the performance of coil type cylindrical dye sensitized solar cells

This paper reports a comparative study of the photovoltaic characteristics of titanium (Ti) metal wires with different diameter and surface treatments towards their utilization as photoanodes for coil-based dye-sensitized solar cells. The surface property of the Ti wire, especially oxidized overlayer and surface treatments have been found to greatly influence the adhesion as well as optimum electrical contact between coated nanoporous titanium oxide (TiO2) and Ti-wires. Implication of adhesion of nanoporous TiO2 on the titanium wire and its influence on photovoltaic performance was analyzed using electrochemical impedance spectroscopy. Results of X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy reveals the formation of anatase TiO2 nanosheets after the H2O2 surface treatment on the titanium wire resulting into enhancement in the extent of dye loading leading to enhanced photoconversion efficiency of 4.71% under simulated solar irradiation.

Plasmon-Enhanced Photocatalysis on TiO2-Passivated Gallium Phosphide

Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for highly efficient photocatalytic processes1-4. However, the electrochemical instability of most III-V semiconductors severely limits their applicability in photocatlaysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO2 using atomic layer deposition. The TiO2 passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO2-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO2 layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO2 thickness of 0.5nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO2/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.1 Qiu, J., G. Zeng, P. Pavaskar, Z. Li and S.B. Cronin, Plasmon-Enhanced Water Splitting on TiO2-Passivated GaP Photocatalysts. Physical Chemistry Chemical Physics, 16, 3115 (2014).2 Hung, W.H., M. Aykol, D. Valley, W.B. Hou and S.B. Cronin, Plasmon Resonant Enhancement of Carbon Monoxide Catalysis. Nano Letters, 10, 1314-1318 (2010).3 Hou, W., Z. Liu, P. Pavaskar, W.H. Hung and S.B. Cronin, Plasmonic Enhancement of Photocatalytic Decomposition of Methyl Orange under Visible Light. J. Catal., 277, 149 (2011).4 Liu, Z., W. Hou, P. Pavaskar, M. Aykol and S. Cronin, Plasmon Resonant Enhancement of Photocatalytic Water Splitting Under Visible Illumination. Nano Letters, 11, 1111 (2011).

Role of synthesis medium of TiO2 nanoparticles in enhancing the open circuit voltage and efficiency in dye-sensitized solar cell

Dye-sensitized solar cell (DSSC) performances of anatase TiO2 nanoparticles synthesized by hydrothermal process in acidic and basic media are compared to understand the role of synthesis conditions. TiO2 nanoparticles obtained in basic medium exhibited a better DSSC performance with higher open circuit voltage (V oc) and enhanced photoconversion efficiency (5.46 %) compared to the sample synthesized in acidic medium (3.29 %). A more negative Fermi energy level (E F) and slower electron recombination kinetics are responsible for the higher efficiency of TiO2 synthesized in basic medium.

Effect of gold nanoparticles on the efficiency of poly(3-hexylthiophene): phenyl-C61-butyric-acid-methylester solar cells.

Different surface densities of gold nanoparticles (AuNPs) were deposited on (3-aminopropyl)-trimethoxysilane (APTMS)-modified indium tin oxide (ITO) electrode. The electrodes were then used in poly(3-hexylthiophene): phenyl-C61-butyric-acid-methylester (P3HT:PCBM) solar cells. Enhanced photo-conversion efficiency was observed from solar cells containing adsorbed AuNPs with surface density equals to 10 +/- 3 NPs/microm2. For higher surface densities (215 +/- 10 NPs/microm2), the presence of the plasmonic material significantly reduced the efficiency of the solar cell. Impedance spectroscopy (IS) indicates changes of the electrical characteristics, evident by a drastic reduction of the impedance relative to the reference cells, from electrodes modified with high densities of AuNPs.

Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts

Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III-V semiconductors severely limits their applicability in photocatalysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO2 using atomic layer deposition. The TiO2 passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO2-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO2 layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO2 thickness of 0.5 nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO2/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.

Photoelectrochemical activity of NiWO4/WO3 heterojunction photoanode under visible light irradiation

NiWO4/WO3 heterojunction photoanode was fabricated for the first time to enhance the photoelectrochemical (PEC) performance under visible light irradiation. Such a photoanode was prepared using a simple deposition-annealing method of NiWO4 nanoparticles onto the surface of porous WO3 films. It was found that the NiWO4/WO3 film synthesized with 25mM Ni(NO3)2 in ethanol exhibited the best PEC performance, which achieved a 70% increase compared with the pure WO3 film under similar conditions. The promising composite photoanode is desired to be employed for water splitting in the PEC cells. The heterojunction structure offers enhanced photoconversion efficiency and increased the density of carriers. This study reveals that introducing NiWO4 into WO3 films could facilitate the separation and restrain the recombination of photo-generated electron–hole pairs and accelerate the electrons transfer. Synthesis details are discussed, with film morphologies and structures characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy.

Nanostructured CuO/SrTiO3 bilayered thin films for photoelectrochemical water splitting

Bilayered thin films of CuO/SrTiO3 with varying thickness of CuO were deposited by sol–gel spin-coating technique on indium tin oxide substrate and used as photoelectrode in the photoelectrochemical cell for water splitting reaction. Maximum photocurrent density of 1.85 mA/cm2 at −0.9 V/saturated calomel electrode was exhibited by 590-nm-thick CuO/SrTiO3 bilayered photoelectrode, which is approximately eight times higher than that for CuO and 30 times higher than that for SrTiO3. The bilayered system offered increased photocurrent density and enhanced photoconversion efficiency, attributed to improved conductivity, which ameliorate separation of the photo-generated carriers at the CuO/SrTiO3 interface and higher value of flatband potential. Details about synthesis and various characterisations involving X-ray diffraction and scanning electron microscopy have been discussed. An energy band diagram has been proposed to dwell upon the mechanism of charge carrier transfer across the interface.

Bi2WO6 Quantum Dots Decorated Reduced Graphene Oxide: Improved Charge Separation and Enhanced Photoconversion Efficiency

In this study, Bi2WO6 quantum dots (QDs) decorating reduced graphene oxide (RGO) sheets were produced by a facile one-step hydrothermal synthesis process, which converts the reactant precursors to Bi2WO6 QDs and RGO simultaneously. The Bi2WO6 QDs with a size of 3–5 nm anchored uniformly on the RGO sheets to form RGO-Bi2WO6 QDs composites. The RGO sheets not only acted as a supporter and stabilizer for the Bi2WO6 QDs to prevent them from being aggregation but also largely improved the charge separation in the composite material. For this preponderance, the electron lifetime in the RGO-Bi2WO6 QDs composites was increased 8-fold compared with that of the pure Bi2WO6 QDs. The much enhanced lifetime improved its performance in environmental purification and photovoltaic conversion under the irradiation of simulated sun light. This work not only provides a principle method to produce graphene-composited multiple metal oxide QDs by one-step process but also a route to obtain efficient functional material for environmental purification and optoelectronic applications.

Modification of Mesoporous TiO2 Films by Electrochemical Doping: Impact on Photoelectrocatalytic and Photovoltaic Performance

The effect of reductive electrochemical doping of nanocrystalline TiO2 electrodes on their photoelectrocatalytic properties and their performance in dye-sensitized solar cells is studied. It is observed that accumulation of negative charge (compensated by proton insertion from the supporting electrolyte) leads to a photocurrent increase for water oxidation. Furthermore, an enhanced photoconversion efficiency in dye-sensitized solar cells is observed for the doped electrodes with respect to the unmodified ones. The effect has been analyzed using small-perturbation electrochemical techniques (impedance spectroscopy, intensity-modulated photovoltage, and photocurrent spectroscopy). The results indicate that the better photoelectrocatalytic and photovoltaic efficiency is due to a more rapid electron transport combined with reduced recombination, contributing to improved electron collection. Furthermore, an enhancement in electron injection is also inferred from the analysis of the present results.

Plasmonic organic photovoltaic devices with graphene based buffer layers for stability and efficiency enhancement

Enhancement of photoconversion efficiency (PCE) and stability in bulk heterojunction (BHJ) plasmonic organic photovoltaic devices (OPVs) incorporating graphene oxide (GO) thin films as the hole transport layer (HTL) and surfactant free Au nanoparticles (NPs) between the GO HTL and the photoactive layers is demonstrated. In particular the plasmonic GO-based devices exhibited a performance enhancement by 30% compared to the devices using the traditional PEDOT:PSS layer. Likewise, they preserved 50% of their initial PCE after 45 h of continuous illumination, contrary to the PEDOT:PSS-based ones that die after 20 h. The performance increase is attributed to the improved photocurrent and fill factor owing to the enhanced exciton generation rate due to NP-induced plasmon absorption enhancement. Besides this, the stability enhancement can be attributed to limited oxygen and/or indium diffusion from the indium tin oxide (ITO) electrode into the active layer. The industrial exploitation of composite GO/NPs as efficient buffer layers in OPVs is envisaged.

Simultaneous improvements in power conversion efficiency and operational stability of polymer solar cells by interfacial engineering

This article addresses simultaneous improvements in the photovoltaic performance and operational stability of organic photovoltaic devices (OPVs) in the inverted configuration when nanostructured ZnO characterized by a lower density of localized surface atomic energy states is employed as an electron transport layer. Two sets of devices with the configuration ITO/ZnO/P3HT:PCBM/MoO3/Ag are employed in the present study. A difference in the density of localized energy states in the band gap of ZnO was produced by altering the crystallinity by annealing the ZnO at two temperatures, viz. 160 and 240 °C. The devices are characterized by scanning electron microscopy, X-ray diffractometry, current-voltage (I-V) measurements as functions of temperature and illumination intensity, incident photon to current conversion efficiency (IPCE) spectroscopy, and charge extraction by linearly increasing photovoltage (CELIV) spectroscopy. The devices fabricated using the ZnO nanostructures annealed at 240 °C have shown remarkably higher power conversion efficiency (PCE) and IPCE values than the other device. From I-V measured as a function of photon flux and temperature we show that the device with higher PCE is characterized by a lower depth of localized energy states by a factor of two than the other device. The implications of the lower trap depth was also evaluated using CELIV and the corresponding charge mobility obtained differed by a factor of three between the two sets of devices. The device with lower equilibrium concentration at the interface has three fold higher charge mobility and 40% enhanced photoconversion efficiency. The stability of the devices was evaluated with and without encapsulation under simulated sunlight (AM 1.5) following the ISOS-D-1 (shelf) and the ISOS-L-1 protocols; the device with higher PCE also showed higher operational stability. The findings in this study are expected to provide new directions in fabricating organic-inorganic heterojunction devices with high performance and stability.

Facile phase and composition tuned synthesis of tin chalcogenide nanocrystals

We synthesized polytypic tin sulfide, SnS, Sn2S3, and SnS2 nanocrystals, by means of novel gas-phase laser photolysis of tetramethyl tin and hydrogen sulfide. A facile composition tuning through the pressure of precursors (addition of dimethyl selenium) yields a series of orthorhombic phase SnX and hexagonal phase SnX2, where X = SxSe1−x with 0 ≤ x ≤ 1. Various polytypic hybrids such as SnS–Sn2S3–SnS2, SnS–SnS2, Sn2S3–SnS2, and SnSe–SnSe2 were synthesized. This resulted in the ability to tune the band gap over a wide range (1.0–2.3 eV). Their photon energy conversion properties were investigated by fabricating photodetector devices using the nanocrystal-reduced graphene oxide nanocomposites. The enhanced photoconversion efficiency was observed from the polytypic hybrid nanostructures. This original synthesis method for tin chalcogenide nanocrystals is expected to help expand applications in high-performance energy conversion systems.

Random nanowires of nickel doped TiO2with high surface area and electron mobility for high efficiency dye-sensitized solar cells

Mesoporous TiO(2) with a large specific surface area (~150 m(2) g(-1)) is the most successful material in dye-sensitized solar cells so far; however, its inferior charge mobility is a major efficiency limiter. This paper demonstrates that random nanowires of Ni-doped TiO(2) (Ni:TiO(2)) have a dramatic influence on the particulate and charge transport properties. Nanowires (dia ~60 nm) of Ni:TiO(2) with a specific surface area of ~80 m(2) g(-1) were developed by an electrospinning technique. The band gap of the Ni:TiO(2) shifted to the visible region upon doping of 5 at% Ni atoms. The Mott-Schottky analysis shows that the flat band potential of Ni:TiO(2) shifts to a more negative value than the undoped samples. The electrochemical impedance spectroscopic measurements showed that the Ni:TiO(2) offer lower charge transport resistance, higher charge recombination resistance, and enhanced electron lifetime compared to the undoped samples. The dye-sensitized solar cells fabricated using the Ni:TiO(2) nanowires showed an enhanced photoconversion efficiency and short-circuit current density compared to the undoped analogue. The transient photocurrent measurements showed that the Ni:TiO(2) has improved charge mobility compared with TiO(2) and is several orders of magnitude higher compared to the P25 particles.

Synthesis of TiO<sub>2</sub> Film for Dye-Sensitized Solar Cells

Mesoporous solid TiO2 film (2-50nm) offering large specific surface area and well connected cavities have been researched extensively in the last decade, aiming to enhance photo-conversion efficiency in the dye-sensitised solar cells (DSSCs). The efficiency of DSSCs is highly sensitive to surface morphology, crystallinity and porosity of the TiO2 film which are in turn dependent upon processing temperature. Multilayered porous TiO2 films were synthesised under different temperature regimes using sol-gel method in combination with spin coating. The influence of temperature variation on the films morphology, crystallinity, and its interfacial adhesiveness to the substrate was studied. A modified method of pre-curing temperature was employed, in order to attain firm adhesiveness of the film to the Fluorine-doped Tin Oxide (FTO) glass substrate. The films prepared by different pre-curing temperature protocols were incorporated into DSSCs for evaluating the affect of varying temperature on photo-conversion efficiency of the cells.

Enhanced Photoconversion Efficiency of All‐Flexible Dye‐Sensitized Solar Cells Based on a Ti Substrate with TiO2 Nanoforest Underlayer

An easily prepared TiO2 nanoforest underlayer improves the photovoltaic performance of Ti substrate-based dye-sensitized solar cells (DSSCs). A high photoconversion efficiency of 8.46% is reached, which is almost 2.5 times that of the DSSCs without surface treatment of the flexible substrates. The nanoforest underlayer increases both the surface roughness of the substrate and the electrical contact between the Ti substrate and the screen-printed TiO2 nanoparticles.

Fabrication of TiO2 nanotube–nanocube array composite electrode for dye-sensitized solar cells

One dimensional TiO2 nanotube structure recently plays an important role in the application of dye sensitized solar cells (DSSCs) due to its faster electron transport. The fabrication of photoanode using the TiO2 nanotube structures mixed with the TiO2 nanoparticles was investigated to enhance the photovoltaic efficiency of DSSCs by increasing the surface area of electrode. In this work, self-organized and vertically-oriented TiO2 nanotube arrays (TNAs) covered with uniformly distributed TiO2 nanocubes (TNCs) were fabricated in a simple one-step anodization process. The X-ray diffraction patterns reveal that both TNAs and TNCs are in anatase phase. The scanning electron microscopy analysis demonstrates that the wall thickness and inner diameter of hexagonal close-packed TiO2 nanotubes from chemically polished Ti foils are 10–15 and 100–120nm, respectively, and the particle size of TNCs is 60–75nm. The DSSC fabricated by the mixed morphological TNAs with TNCs shows an enhanced photoconversion efficiency of ~63% than that of TNAs alone, due to the increase of both dye adsorption and electron transportation rate.

Enhanced Water Photolysis with Pt Metal Nanoparticles on Single Crystal TiO2 Surfaces

Two novel deposition methods were used to synthesize Pt-TiO(2) composite photoelectrodes: a tilt-target room temperature sputtering method and aerosol-chemical vapor deposition (ACVD). Pt nanoparticles (NPs) were sequentially deposited by the tilt-target room temperature sputtering method onto the as-synthesized nanostructured columnar TiO(2) films by ACVD. By varying the sputtering time of Pt deposition, the size of deposited Pt NPs on the TiO(2) film could be precisely controlled. The as-synthesized composite photoelectrodes with different sizes of Pt NPs were characterized by various methods, such as SEM, EDS, TEM, XRD, and UV-vis. The photocurrent measurements revealed that the modification of the TiO(2) surface with Pt NPs improved the photoelectrochemical properties of electrodes. Performance of the Pt-TiO(2) composite photoelectrodes with sparsely deposited 1.15 nm Pt NPs was compared to the pristine TiO(2) photoelectrode with higher saturated photocurrents (7.92 mA/cm(2) to 9.49 mA/cm(2)), enhanced photoconversion efficiency (16.2% to 21.2%), and increased fill factor (0.66 to 0.70). For larger size Pt NPs of 3.45 nm, the composite photoelectrode produced a lower photocurrent and reduced conversion efficiency compared to the pristine TiO(2) electrode. However, the surface modification by Pt NPs helped the composite electrode maintain higher fill factor values.

Effect of Nickel Oxide Thin Films on Photoconversion Efficiency in Zinc Oxide Dye-Sensitized Solar Cells

ZnO dye-sensitized solar cells (ZnO DSSCs) with different thickness of NiO thin films coated in photo-electrode and counter-electrode were investigated. NiO thin films were prepared by thermal evaporation of NiO onto FTO glass substrate. The films were characterized by FE-SEM. For the photo-electrode, NiO thin films were coated on ZnO with 0.2, 0.6, 1.1 and 2.2 mg to form a barrier layer. For the counter-electrode, NiO thin films were coated on FTO glass with 5.4, 10.8, 16.2 and 21.6 mg in order to increase a surface-to-volume ratio. The photoconversion efficiency of ZnO DSSCs was measured under illumination of stimulated sunlight obtained from solar simulator with the radiant power of 100 mW/cm2. It was found that ZnO DSSCs coated with 0.6 mg NiO in photo-electrode and 10.8 mg in counter-electrode exhibited the highest photoconversion efficiency of 1.00% and 0.92%, respectively. The enhancement of photoconversion efficiency with NiO coating maybe explained by decreasing of charge recombination in photo-electrode and increasing of active surface area in counter-electrode.

Enhancement of Photoconversion Efficiency of ZnO Nanorod-Based Dye-Sensitized Solar Cells in Presence of ZrO<SUB>2</SUB> Thin Energy Barrier

In order to enhance the power conversion efficiency of ZnO nanorods-based dye-sensitized solar cells (DSSCs), ZrO2 thin energy barriers were formed on ZnO nanorods using a sol-gel method. In DSSCs, the short-circuit current was substantially increased, and the dark current was significantly reduced in the presence of the ZrO2 layer. Due to suppressed recombination in the presence of the ZrO2 layer, 81.3% increment of power conversion efficiency is achieved compared to those without ZrO2 layer.