Efficiency photo current

The overall conversion efficiency (rj) of the dye-sensitized solar cell is determined by the photo current density (/ph), the open circuit potential (Voc), the fill factor (FF) of the cell and the intensity of the incident light (Is), (Eq.8) [19]. 

In order to obtain high overall light to electric power conversion efficiencies, optimization of the short circuit photo current (z sc) and open circuit potential (Voc) of the solar cell is essential. The conduction band of the TiC>2 is known to have a Nernstian dependence on pH [55,67]. The fully protonated sensitizer 2, upon adsorption transfers most of its protons to the TiC>2 surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic Ru complex and assists electron injection from the excited state of the sensitizer into the titania conduction band, favoring high photocurrents (18-19 mA cm-2). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction band edge induced by the surface protonation. 

As can be seen from the action spectra in Figures 7-9, of all the transition metal cyanides tested to date, the ferrocyanide derivatization of Ti02 gives the most effective sensitization at the peak of the photocurrent action spectrum at a wavelength of 420 nm, corresponding to the absorption maximum of this complex, the quantum efficiency for electron transfer is 37% (monochromatic incident light intensity 0.84 W.nT2, photo- current 10.4 pA.crn 2). 

If the thickness of the insulator is reduced below about 10 A the concept of a tunnel MIS diode apparently becomes invalid, based at least on experimental evidence, and these thin structures perform as basic Schottky barriers. Above 28-30 A the diodes behave as equilibrium tunnel diodes. From Fig 7a it can be observed that even in the minority carrier regime under forward bias the region over which ideal p-n junction diode behaviour is predicted is insulator thickness dependent. Since in the case of p-n junctions in silicon under normal AMI illumination about 0.5 - 0.7 V is developed across the junction this means that for significant conversion efficiencies in these mi MIS devices insulator thickness should not exceed about 20 A. At greater thickness there will be some suppression of the photo-current due to the shape of the I-V characteristic rather similar to that observed in p-n junction solar cells with large series resistance. 

Figure 8.26 (a) Molecular structure of compound 54. (b) External quantum efficiency versus current density in multilayer EL devices of 54. (Inset) Photo of the EL device of 54. (Reprinted with permission from Ref [39]. Copyright 2010, The Royal Society of Chemistry.)... 

As a complement, Shiga et al. reported on porphyrins that link to the Ti02 nanoparticles by using a fullerene bearing carboxylic acid groups (Fig. 18.7) [116]. Although the authors measured very low efficiencies of around 0.53 %, the success of their approach was demonstrated by the incident photo-to-current efficiency spectra, in which the porphyrin features, that is, the Soret band, are discernible. In addition, they reported extraordinary stabilities with values up to 3000 hours. 

The performance of the cell can be quantified on a macroscopic level with parameters such as incident photon to current efficiency (IPCE), open-circuit photo voltage (Voc), and the overall efficiency of the photovoltaic cell (i7ceM). 

Fig. 3. Like a photoelectrochemical cell, such a powder includes sites for photo-induced oxidation and reduction, but no external current flow accompanies these transformations. Photoactivity is also maintained as the size of the particle decreases to the colloidal range although the absorption characteristics, the quantum efficiency of charge separation, and the kinetics of interfacial electron transfer may be influenced by the particle size. On sufficiently small particles, for example, the calculated space-charge width necessary for effective band bending may exceed the dimensions of the particle.

The transient current response of photo-electrodes to stepped-illumination changes suggests itself as a method of mechanistically interpreting this quantum efficiency problem. Though such transients have been studied for p-type GaP (1) and a number of n-type transition metal compounds (2, 3, 5, 6), published... 

---