Current-to-photon conversion

Fig.l. Circuit for current-to-photon conversion. Working electrode is connected to the inverting input of the amplifier. Circle at far left indicates function generator. 

A Measurement Strategy Based on Current-to-Photon Conversion... 

An issue that requires attention, in any scheme based on current-to-photon conversion. 

Research in our laboratory and by Osa and Fujihira showed that it is possible to covalently attach monolayers of chromo-phores to metal-oxide semiconductor surfaces — with no compromise in quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed in the dye layer, ne/np) is quite low, in the range of 10 5 to 10 4 and argues against device applications of these simple modified electrodes without further improvements, such as linear, multielectrode stacks of dye-modified, semi-transparent electrodes (10). 

There have been a few reports of II-VI and III-V heterojunction devices which have approached or even surpassed the performance of silicon cells. p-InP/n-CdS cells with a solar conversion efficiency of 12.5% have been fabricated.84 This good efficiency arises firstly because of the nature of InP the band gap is at 1.34 eV, which is optimal for the solar spectrum, and the transition is direct, so the absorption edge is steep. Secondly, there is an excellent crystal lattice match between InP and CdS, which means that almost fault-free junctions can be grown. p-CuInSe2/n-CdS cells which display current efficiencies (defined as electrons flowing in the short-circuit current per photon absorbed) of up to 70% between 550 and 1250 nm, and solar conversion efficiencies of ca. 5% have been made.85 p-CdTe/n-CdS cells of rather similar performance (current efficiency 85%, solar conversion efficiency 4.0%) have been produced without detailed attention to optimization of cell design, and it has been calculated that p-CdTe/n-ZiiaCdi-aS cells should be capable of a 41 44... 

For solar cells, the efficiency is given under irradiation with a solar spectrum under air mass 1.5 (AM 1.5) [65] in terms of power conversion efficiency or internal photon-to-electron conversion efficiency (ICPE) [66]. Characteristic parameters that are often used are the open circuit voltage Voc, the short circuit current density /sc, and the fill factor, given by... 

Thus, only about 1% of the photoelectrons is missed, which is inconsequential where the quantum efficiency or photon to electron conversion is at most 25%. If the photomultiplier is used in the analog mode, that is, at count rates much higher than the bandwidth, then the mean square shot noise at the output is increased by a noise factor, T =(5/( — 1) or 1.25 for our example. The expression for the output noise current is = IqVG iB with i = rjqP hv.ihQ photocathode current. 

Evaluation of an SC photovoltaic performance is usually accomplished with a set of tests. Those involve measurements of the current-voltage (I-V) curves for the cell in the dark and under light illumination, fluorescent decay upon photoexcitation, determination of the incident photon-to-electron-conversion efficiency (TPCE), etc. 

Electron multiplier. A device to multiply current in an electron beam (or in a photon or particle beam after conversion to electrons) by incidence of accelerated electrons upon the surface of an electrode. This collision yields a number of secondary electrons greater than the number of incident electrons. These electrons are then accelerated to another electrode (or another part of the same electrode), which in turn emits secondary electrons, continuing the process. 

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

With the D S SCs containing Hibiscus surattensis extract, the best performances were probably obtained because of the large amount of dye adsorbed on Ti02 (lxl cm2 active surface) in comparison with the other extracts from tropical flowers. Incident photon to current conversion efficiency (IPCE) values of 76% were calculated (2 — 590 nm). Jsc was 5.45 mAcm-2, Voc = 392mV, FF — 54%, and efficiency = 1.14%. Also, the stability of the photovoltaic devices was the best in the case of Hibiscus surattensis, even though it needs to be improved to achieve real long-term stability, especially as far as the sealing quality is concerned. 

Ideally, all photons with a wavelength of about 900 nm or shorter should be harvested and converted to electric current. This limit is derived from thermodynamic considerations showing that the conversion efficiency of any single-junction photovoltaic solar converter peaks at approximately 33% near the threshold energy of lAeV.1 2 There are numerous ways to convert the solar radiation directly into electrical power or chemical fuel. The silicon solar cell is the most efficient in this respect. Nevertheless, the capital cost of such devices is not attractive for large-scale applications. 

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