Incident Photon Conversion Efficiency Research Articles

Size effects on the photoelectrochemical activities of single wall carbon nanotubes

This paper reveals an improved photoelectrochemical activity of single wall carbon nanotube (SWNT) with respect to enhanced photo-induced currents as they are shortened at a few nanometer sizes. Raman spectroscopy, photoelectrochemistry and scanning electronic microscopy (SEM) characteristics indicate a uniform finite-sized SWNT film with distinct physical and photochemical properties. Unlike p-type semiconductive pristine SWNTs, both cathodic photocurrent and anodic photocurrent are observed on shortened SWNT formed thin films. The incident photon conversion efficiency is eight-fold higher compared to the longer SWNTs, suggesting the electronic structure and photoelectrochemical properties of SWNTs are significantly altered by shortening the length of SWNT. The improved photoelectrochemical activities open a new perspective to use finite-sized SWNTs in combination with other semiconducting materials for fabrication of efficient optoelectronic devices, nanotube optical detectors or emitters that can be operated across a wide range of optical wavelengths.

Stable response to visible light of InGaN photoelectrodes

The photoelectrochemical properties of InxGa1−xN∕GaN (0⩽x⩽0.20) epitaxial films on sapphire (0001) substrates have been investigated. The flatband potential of InxGa1−xN is shifted to more positive voltages with increasing indium incorporation. In aqueous HBr solution, the turnover number of the In0.20Ga0.80N electrode reaches 847 after 4000s illumination, which suggests that In0.20Ga0.80N has good photostability. Moreover, In0.20Ga0.80N shows highest visible-light response among InxGa1−xN (0⩽x⩽0.20) and the incident photon conversion efficiency is about 9% at 400–430nm in the HBr solution.

Efficient bulk heterojunction solar cells from regio-regular- poly(3,3‴-didodecyl quaterthiophene)/PC 70BM blends

Blends of regio-regular poly(3,3‴-didodecyl quaterthiophene) (PQT-12) with (6,6)-phenyl-C 70-butyric acid methyl ester (PC 70BM) were investigated as active layers for application in organic photovoltaics (OPV). By optimizing the PQT-12:PC 70BM composition ratio and annealing conditions, power conversion efficiencies ( η) of 1.4% could be obtained. Effects of different preparation parameters on the incident photon conversion efficiency (IPCE), short-circuit ( J sc), fill factor (FF), open-circuit voltage ( V oc) and η are discussed.

Does carbon doping of TiO 2 allow water splitting in visible light? Comments on “Nanotube enhanced photoresponse of carbon modified (CM)- n-TiO 2 for efficient water splitting”

Reports of water splitting by carbon-doped titanium dioxide (TiO 2) photoelectrodes under visible illumination are critically examined. Xu et al. [Sol. Energy Mater. Sol. Cells 91 (2007) 938] recently reported significant incident photon conversion efficiencies (IPCEs) at visible wavelengths for carbon-doped TiO 2 in thin film and nanotube form. Evidence is given here that these results were due to an artefact in the measurements. Further, it is pointed out that the mechanism proposed for water splitting under visible illumination is unphysical, and the photocurrents presented are shown to be grossly inconsistent with the IPCE data. Other workers have also measured non-zero IPCEs at visible wavelengths for carbon-doped TiO 2, but have not presented this as evidence of water splitting. In other cases, carbon doping was performed in a reducing atmosphere, and measured visible activity is most likely a result of oxygen vacancies. It is concluded that there is no convincing evidence in the literature of water splitting under visible light in carbon-doped TiO 2.

Photoelectrochemical and Structural Properties of TiO2 and N-Doped TiO2 Thin Films Synthesized Using Pulsed Direct Current Plasma-Activated Chemical Vapor Deposition

Amorphous TiO2 thin films were synthesized using a pulsed direct current plasma deposition technique. The films were prepared in the discharges of Ar, Ar + O2, and Ar + N2. The original and annealed samples were characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, ultraviolet and visible spectroscopy (UV−vis), and electrochemistry. The presence of oxygen was found to be essential in the annealing medium for the formation of polycrystalline films. By optimizing the experimental conditions, we obtained TiO2 films that showed 80% incident photon conversion efficiency for splitting of water near 300 nm. Films containing up to 12 atom % N were obtained when mixtures of Ar and N2 were used as the plasma source gas. Although there is evidence for the presence of Ti−N bonds in the amorphous film, the annealing in the presence of oxygen to obtain polycrystalline films led to decomposition of these bonds. The resulting polycrystalline films contained 1−2 atom ...

Effect of the Combination of Spray Pyrolysis and Flame Oxidation Synthesis of Carbon Modified (CM)-n-TiO2 Thin Film Electrodes on Solar to Hydrogen Production Efficiency From Water

Carbon modified n-type titanium oxide (CM-n-TiO2) photoelectrodes were synthesized by natural gas flame oxidation (sample 1) and also by spray pyrolytic deposition on Ti metal sheets (Alfa, 0.25 mm thick) followed by their thermal flame oxidation (Sample 2). Sample 1 and sample 2 generated photocurrent density of 9.17 mA cm-2 and 13.83 mA cm-2 for water splitting at the same measured potential of - 0.7 V/SCE. at illumination intensity of 100 mW cm-2 from a 150 Watt Xenon Lamp respectively. The total photocurrent density of 12.15 mA cm-2 obtained by integration of wavelength, λ dependent photocurrent densities for sample 2 generated a total photoconversion efficiency of 12.33 % under illumination intensity of 100 mW cm-2 from a solar simulator. Importantly, under global Air Mass of 1.5 sunlight (1 sun) illumination, total photoconversion efficiency of 10.48 % for water splitting was observed for sample 2. Light concentration by oxygen gas bubbles temporarily trapped on highly porous and thicker CM-n-TiO2 surface of sample 2 acting as tiny lens helped to have observed quantum efficiency or incident photon conversion efficiency (IPCE) higher by a factor of ten in the UV region compared to theoretical maximum of unity and consequently generated such high photoconversion efficiency under global sunlight illumination.

Efficiency of solar water splitting using semiconductor electrodes

Reliable measurement of the photoconversion efficiency for semiconductor electrodes is essential to the assessment of electrode performance. In this paper, the influence of the choice of light source on measured photoconversion efficiencies for semiconductor photoelectrodes is examined. Measurements of efficiency performed under xenon lamp and solar illumination are compared with efficiencies calculated by integrating the incident photon conversion efficiency (IPCE) over the lamp and solar spectra. It is shown that use of a xenon lamp as the light source can lead to a large overestimate of the photoconversion efficiency, relative to that obtained under standard AM1.5 solar illumination. The overestimate is greater when a water filter is fitted to the xenon lamp, and when a wide-band gap semiconductor such as TiO 2 is used as the photoelectrode. Achievable photoconversion efficiencies using rutile TiO 2 are calculated taking into account the losses due to imperfect absorption, reflection and charge-carrier recombination; these calculated efficiencies agree with the measurements to within experimental uncertainties. It is demonstrated that many photoconversion efficiencies presented in the literature are overestimated. It is concluded that reliable estimation of efficiency under standard conditions is best obtained by measuring the IPCE as a function of wavelength, and integrating over the AM1.5 solar spectrum, or by measuring under sunlight with a similar zenith angle to that of the AM1.5 spectrum.

TiO2 Photoelectrodes for Water Splitting: Carbon Doping by Flame Pyrolysis?

AbstractTitanium dioxide is often used as a photoelectrode material for water splitting, but its band gap is too wide to achieve high conversion efficiencies in sunlight. Flame oxidation (pyrolysis) of titanium sheet has been suggested as a method to produce carbon doped TiO2 with a reduced band gap. In the present study, films were prepared using this and other thermal oxidation techniques and the photoelectrochemical water splitting efficiency of the resultant materials was examined using a xenon lamp. The oxide films were characterised physically, optically and electrochemically. Incident photon conversion efficiency measurements indicated no significant reduction in the band gap offilms prepared using a hydrocarbon flame compared to rutile TiO2. Furthermore, these films were found to have an efficiency comparable to that offilms produced in a carbon free flame (hydrogen/oxygen). The results suggest that flame pyrolysis may not ‘dope’ TiO, with carbon in the manner previously proposed. Any enhancement in photocatalytic eficiency resulting from the technique is likely to be due to the formation of non‐stoichiometric TiO2 or titanium sub‐oxides.

Gel polymer electrolytes based on polyacrylonitrile and a novel quaternary ammonium salt for dye-sensitized solar cells

Gel polymer electrolytes were prepared by incorporating polyacrylonitrile (PAN) in a mixture of polysiloxane with quaternary ammonium side groups (PSQAS), ethylene carbonate (EC), propylene carbonate (PC) and iodine. The influence of PAN content on the ionic conductivity of gel polymer electrolytes and the charge-transfer kinetic performance in counterelectrode–electrolyte interface was investigated. The dye-sensitized solar cell with the gel polymer electrolyte containing 5 wt.% PAN showed the best photovoltaic performance; a maximum incident photon conversion efficiency of 63% at 520 nm was obtained, the short-circuit photocurrent density ( J sc), the open-circuit voltage ( V oc) and the fill factor (FF) were 7 mA cm −2, 0.565 V and 0.65, respectively. The corresponding overall conversion efficiency ( η) is 4.3%.

Dye-sensitised photoelectrochemical solar cells with polyacrylonitrile based solid polymer electrolytes

Novel all solid state dye-sensitised photolectrochemical solar cells of the type, FTO–TiO 2–dye–PAN, EC, PC, Pr 4N +I −, I 2–Pt–FTO, have been fabricated and characterised using current–voltage characteristics and action spectra. Liquid electrolyte generally used for such solar cells has successfully replaced by a quasi solid electrolyte comprised of polyacrylonitrile (PAN) with ethylene carbonate (EC) and propylene carbonate (PC) as plasticisers and Pr 4N +I −/I 2 redox couple with tetrapropylammoniumiodide as the complexing salt. For the polymer electrolyte, the optimum conductivity of 2.95×10 −3 S cm −1 was obtained for the electrolyte composition, PAN:EC:PC=15:35:50 (wt.%). The short circuit current density ( J SC) and the open circuit voltage ( V OC) obtained for an incident light intensity of 600 W m −2 were 3.73 mA cm −2 and 0.69 V, respectively. This corresponds to an overall quantum efficiency of 2.99%. From the action spectrum, the maximum incident photon conversion efficiency (IPCE) of 33% was obtained for incident light of wavelength 480 nm.

Photoelectrochemical properties of metal-ion-doped TiO2 nanocrystalline electrodes

Metal-ion-doped TiO2 nanoparticles were prepared with sol-gel method. The photocurrents of these metal-ion-doped TiO2 electrodes were measured at different electrode potentials and different excitation wavelengths, and different photoelectrochemical behaviors were observed. The phenomenon of p–n conversion was observed for Fe3+- doped TiO2 electrode; however, the characteristic of pure n-type semiconductor was observed for lanthanide ion (such as La3+, Nd3+, Pr3+, Sm3+ and Eu3+), Zn2+, Cd2+, Cr3+ and Co2+-doped TiO2 electrodes. The effect of the content of Zn2+ on photocurrent was investigated and the result showed that the incident photon conversion efficiency (IPCE) was the largest when the content of Zn2+ was 0.5%.