State open-circuit voltage

Fig. 17 Detailed balance approach for determining open circuit voltage based on the energy of the D/A charge-transfer state and its coupling to recombination loss modes. Reproduced with permission from [195]. Copyright 2009 Macmillan Publishers Limited...

There have been many investigations of photoinduced effects in -Si H films linked to material parameters. Changes have been observed in the carrier diffusion length, unpaired spin density, density of states in the gap, and infrared transmission. The transition from state A to B seems to be induced by any process that creates free carriers, including x-ray radiation and injection (double) from the electrodes. Because degradation in a solar cell is accentuated at the open-circuit voltage conditions, the A to B transition occurs upon recombination of excess free carriers in which the eneigy involved is less than the band gap. It has been pointed out that this transition is a relatively inefficient one and the increase in spin density takes place at a rate of 10-8 spins per absorbed photon. 

Photoelectrochemical cells with polythiophene film as an active electrode and a lead plate as a counter electrode in Pb (C104) acetonitrile electrolyte has an open circuit voltage of 0.8 V, short circuit current of 2 x 10 4 A cm-2, efficiency coefficient of 0.03%, fill factor of 15% [194]. The absorption and photosensitivity spectra of such a cell are shown in Fig. 24. The small bathochromic shift in the longwave region for photosensitivity may be related to the photogeneration of the charge carriers via surface states. The photosensitivity maximum is close to the maximum solar intensity. The parameters exceed the ones obtained with polyacetylene. 

The presence of a step introduces two significant changes. First, in contrast to a smooth surface which does not exhibit unsaturated bonds in the direction perpendicular to the surface, strong covalent bonds are exposed to the electrolyte at the edge of a step. These are likely to chemisorb species from the environment and introduce surface states within the band gap (29). These are recognized recombination sites and shunts in solar cells, known to reduce fill factors, current collection efficiencies and open circuit voltages (30, 31). 

Suboptimum photovoltage output. Current state of art devices yield open circuit voltages of 750 mV, less than one-half of the band gap energy of incident photons absorbed by the dye sensitizer. 

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