mV Open Circuit Voltage Research Articles

Ultrathin TaOx film based photovoltaic device

Application of the economical metal oxide thin-film photovoltaic devices is hindered by the poor energy efficiency. This paper investigates the photovoltaic effect with an ultrathin tantalum oxide (TaOx) tunnel barrier, formed by the plasma oxidation of a pre-deposited tantalum (Ta) film. These ~ 3 nm TaOx tunnel barriers showed approximately 160 mV open circuit voltage and 3–5% energy efficiency, for varying light intensity. The ultrathin TaOx (~ 3 nm) could absorb approximately 12% of the incident light radiation in 400–1000 nm wavelength range; this strong light absorbing capability was found to be associated with the dramatically large extinction coefficient. Spectroscopic ellipsometry revealed that the extinction coefficient of 3 nm TaOx was ~ 0.2, two orders higher than that of tantalum penta oxide (Ta 2O 5). Interestingly, refractive index of this 3 nm thick TaOx was comparable with that of stochiometeric Ta 2O 5. However, heating and prolonged high-intensity light exposure deteriorated the photovoltaic effect in TaOx junctions. This study provides the basis to explore the photovoltaic effect in a highly economical and easily processable ultrathin metal oxide tunnel barrier or analogous systems.

Fabrication and electrochemical performance of thin-film solid oxide fuel cells with large area nanostructured membranes

Thin-film solid oxide fuel cells (SOFCs) with large (5-mm square) membranes and ultra-thin La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ (LSCF) cathodes have been fabricated and their electrochemical performance was measured up to 500 °C. A grid of plated nickel on the cathode with 5–10 μm linewidth and 25–50 μm pitch successfully supported a roughly 200-nm-thick LSCF/yttria-stabilized zirconia/platinum membrane while covering less than 20% of the membrane area. This geometry yielded a maximum performance of 1 mW cm −2 and 200 mV open-circuit voltage at 500 °C. Another approach toward realizing large area fuel cell junctions consists of depositing the membrane on a smooth substrate, covering it with a high-porosity material formed in situ, then removing the substrate. We have used a composite of silica aerogel and carbon fiber as the support, and show that this material can be created in flow channels etched into the underside of a silicon chip bonded to the top of the SOFC membrane. We anticipate these integrated fuel cell devices and structures to be of relevance to advancing low-temperature SOFCs for portable applications.

Hybrid solar cells using nanorod zinc oxide electrodes and perylene monoimide–monoanhydride dyes

Hybrid solar cells have been fabricated using perylene monoimide–monoanhydride dyes with nanorod zinc oxide electrodes as electron transporting layers. We have investigated the influence of the spacer alkyl chain length of perylene monoimide–monoanhydride (PMIMA) dyes on the device performance in hybrid solar cells using nanorod zinc oxide electrodes. Nanorod zinc oxide electrodes with 50–150 nm of diameter were synthesized in the presence of PEG400 by using microwave heating method. We observed that the dyes with longer and brunched alkyl chains exhibit higher efficiencies in hybrid solar cells. We report the highest efficiency obtained with zinc oxide nanorods under standard conditions for perylene monoimide–monoanhydride derivative with PMIMA_1 that performs 400 mV open circuit voltage, 2.81 mA/cm 2 short-circuit current and 0.59% overall conversion efficiency.

Development of a semitransparent CdMgTe/CdS top cell for applications in tandem solar cells

The band gap of Cd1−xMgxTe can be easily tuned from 1.48 to 3.5 eV (x = 0 to 1), and hence this material is a potential candidate for developing the wide band gap top cell in tandem solar cells. In this paper, we present an all-sputtered Cd1−xMgxTe/CdS top cell developed on commercial SnO2:F-coated (Tec-7) glass substrates with sheet resistance of ∼7Ω/□. The Cd1−xMgxTe (x = 0.05, 0.21, 0.26) films were deposited by RF sputtering at deposition temperatures in the range 250–300 °C. The films were characterized for structure, morphology and optical properties. Raman spectroscopic studies showed that the lattice perturbations increase with Mg content in the film. We have explored cadmium chloride vapor annealing of Cd1−xMgxTe thin films, and found that the film composition has a significant influence on stability. Films with a lesser amount of Mg show better stability during vapor chloride treatments at 387 °C. Longer annealing durations resulted in a decrease in band gap of the films. Recrystallization due to the vapor chloride treatments were studied using transmittance, XRD, AFM and Raman spectroscopy. Prototype devices of the configuration Tec-7/CdS/Cd0.95Mg0.05Te/metal show promising photovoltaic characteristics such as 630 mV open circuit voltage and 17 mAcm−2 short circuit current.

Sony's Biofuel Cell

We have developed the passive-type biofuel cell having 3.0mW/cm2 power at 500 mV and 800 mV open circuit voltage. This performance has been achieved by introducing 2-amino-1,4-naphthoquinone (ANQ) as an anode electron mediator. Although the redox potential of ANQ, -0.36V (vs Ag/AgCl) is much more negative than that of vitamin K3 (VK3), -0.22V, the kcat value of diaphorase (DI) against ANQ, 620 sec-1 is not so small in comparison with that of VK3, 1300 sec-1. This character has been maintained even in the immobilized electrode modified by DI and ANQ, so that the biofuel cell power has increased from 1.5mW/cm2 of VK3 to 3.0mW/cm2 of ANQ.

Organic Sensitizers Containing Julolidine Moiety for Dye-Sensitized Solar Cells

We prepared organic sensitizers (S1 and S2) containing julolidine moiety as a donor, phenyl or phenylene thiophene units as a conjugation bridge, and cyano acetic acid as an acceptor for dye sensitized solar cells. S1 exhibited two absorption maxima at 441 nm (epsilon = 26,200) and 317 nm (epsilon = 15,500) due to the pi-pi transition of the dye molecule. S2 dyes with an additional thiophene unit showed the absorption maximum extended by 18 nm. DSSCs based on S1 dye achieved 2.66% of power conversion efficiency with 8.3 mA cm(-2) of short circuit current, 576 mV of open circuit voltage, and 0.56 of fill factor. DSSCs using S2 dye with a longer conjugation attained only 1.48% of power conversion efficiency. The 0.21 V lower driving force for regeneration of the S2 dye compared to the Si dye is one of the reasons for low conversion efficiency of the S2 dye.

New perylene derivative dyes for dye-sensitized solar cells

We have studied the influence of the spacer alkyl chain length of perylenemonoimide (PMI) dyes on the device performance in dye-sensitized solar cells (DSSCs). We observed that the dyes with longer and brunched alkyl chains exhibit higher efficiencies in DSSCs. In line with these statements we now report the highest efficiency obtained under standard conditions for a perylene imide derivative with PMI-DA1 that performs 300 mV open circuit voltage, 9.79 mA/cm 2 short-circuit current and 1.61% overall conversion efficiency.

Photoelectrochemical behaviour of a dye-grafted nanocrystalline SnO 2 powder

A perylene-based dye was synthesized and covalently bound to undoped SnO 2 nanosized powder. Because of the grafted dye, film sintering is not a straightforward process. In a preliminary step, it is proposed to check the photoactivity of this new powdered material directly by means of the cavity microelectrode technique under green or blue laser illumination through an optical microscope. Taking advantage of the chemical stability of the Sn–C bond, aqueous redox electrolytes were considered. Two types of microcavities were used, a laser-ablated with a residual Pt-coating on the overall cavity inner side, and a chemically etched with the rear Pt contact at the bottom only. Voltammetry and electrochemical impedance spectroscopy (EIS) were used to characterize the response of the SnO 2 powder inserted in the cavity. From the results presented here, two contributions have been identified: (i) that of the light insensitive Pt|electrolyte interface due to the Pt-coated inner side when present; (ii) that of the dye-grafted SnO 2|electrolyte photoactive interface. It is shown that direct information on available photopotentials for a given redox electrolyte can be readily obtained. From this point of view, a 600 mV open-circuit voltage has been obtained with the new dye-sensitized SnO 2 powder in contact with an aqueous bromide-containing electrolyte.

Guiding Principle to Develop Intrinsic Microcrystalline Silicon Absorber Layer For Solar Cell By Hot-Wire Cvd

ABSTRACTWe report on ways to develop device quality microcrystalline silicon (μc-Si:H) intrinsic layer with high growth rate by hot-wire chemical vapor deposition (HWCVD). With combine approach of controlling impurities and moderate H-dilution [H2/SiH4 ͌ 2.5], we developed, for the first time, highly photosensitive (103 μc-Si:Hfilms with high growth rate (>1 nm/s); the microstructure of the film is found to be close to amorphous phase (fc ͌ 46 ̻± 5%). The photosensitivity systematically decreases with fc and saturates to 10 for fc> 70%. On application of these materials in non-optimized pin [.proportional]c-Si:H solar cell structure yields 700 mV open-circuit voltage however, surprisingly low fill factor and short circuit current. The importance of reduction of oxygen impurities [O], adequate passivation of grain boundary (GB) as well as presence of inactive GB of (220) orientation to achieve efficient [.proportional]c-Si:H solar cells are discussed.

Series resistance caused by the localized rear contact in high efficiency silicon solar cells

Abstract High efficiency PERL (passivated emitter and rear locally-diffused) cells have demonstrated over 700 mV open-circuit voltage and 23% efficiency under standard test conditions. However, the relatively low fill factor is the main parameter limiting performance. This paper analyses the series resistance caused by the localized rear contact design. This resistance is found not to limit the performance of one-sun PERL cells, even though it does become significant for concentrator PERL cells.

Back-surface field design for n +p GaAs cells

Back-surface field (BSF) design issues for n +p GaAs solar cells are addressed. An expression for the effective back-surface recombination velocity, S pp+ , valid for both homojunction and heterojunction BSFs, is derived. Computations for a typical n +p shallow junction cell demonstrate: (1) the importance of so-called bandgap narrowing effects in p + GaAs and (2) that pp + homojunction BSFs cannot attain the low (less than about 10 3 cm s −1) effective surface recombination velocities required for very high efficiency. AlGaAs/GaAs heterojunction BSFs are shown to be capable of nearly zero effective surface recombination velocity. Comparison of n +p shallow junction cells with homojunction BSFs to similar cells with heterojunction BSFs reveals an advantage of approximately 20 mV in open-circuit voltage for the heterojunction BSF. Theoretical estimates are in approximate agreement with the increase of about 30 mV that has been observed experimentally, and suggest that heterojunction BSFs may be required in order to reach the efficiency limits of n +p GaAs cells.