Regenerative solar cells efficiency

The state of the art regenerative solar cell consists of the sensitizer cis-Ru(dcb)2(NCS)2 anchored to colloidal anatase Ti02 films with an Lil-l2 organic electrolyte. An i-V curve recorded with 1000 W m of air mass 1.5 illumination at 25 °C is shown in Figure 25 [154]. From these data, a Eqc of 0.79 V, an 4c of 4.83 mA, a fill factor of 0.71 and an of 0.11 were obtained. The active area of the electrode was kept low, 0.249 cm, to minimize resisitive losses associated with the tin oxide glass and the nanocrystalline semiconductor film. The power efficiency was not significantly changed when the temperature was raised from 20 to 60 °C. 

Dye sensitization of electrodes is an old area of science with a rich history. The field has experienced renewed interest owing to the development of high surface area colloidal semiconductor electrodes. These materials yield impressive solar conversion efficiencies when employed in regenerative solar cells that have already found niche applications and have the real possibility of replacing traditional solid-state photovoltaics. Thus for the first time in history a solar cell designed to operate on a molecular level is useful from a practical point of view. It is also likely that other applications in the growing areas of molecular photonic materials will arise. 

Table 1.1 Maximum-power conversion efficiency of selected regenerative solar cells...

Shown in Figure 21-17 is a regenerative solar cell based on colloidal Ti02, deposited on an optically transparent FTO support. With a suitable sensitizer and redox couple, these materials convert light into electricity with remarkable efficiency (O Regan, 1991). A maximum efficiency of 10.96% has been obtained under air-mass lA (AM 1.5) simulated sunlight with c -Ru(dcb)2(NCS)2, where deb is 4,4 -(COOH)2-2,2 -bipyridine, as a sensitizer and iodide as the electron donor (Nazeeruddin, 1993). 

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

Table 1. Photovoltage Up, photocurrent ip fill factor FF and efficiency p of regenerative electrochemical solar cells... 

Fig. 5.65 Dependence of the solar conversion efficiency (CE) on the threshold wavelength (Ag) for a quantum converter at AM 1.2. Curve 1 Fraction of the total solar power convertible by an ideal equilibrium converter with no thermodynamic and kinetic losses. Curve 2 As 1 but the inherent thermodynamic losses (detailed balance and entropy production) are considered. Continuous line Efficiency of a regenerative photovoltaic cell, where the thermodynamic and kinetic losses are considered. The values of Ag for some semiconductors are also shown (according to J. R. Bolton et al.)... 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

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