Solar cells 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. 

Solar cells that extract electrical energy from light, including visible light. Each cell consists of a photo-sensitive electrode and a conducting counter electrode immersed in an electrolyte. Some photo-electrochemical cells simply produce a direct current, whereas others liberate hydrogen in a process similar to the conventional electrolysis of water. 

F.L. Zhang, E. Perzon, X.J. Wang, W. Mammo, M.R. Andersson, and O. Inganas, Polymer solar cells based on a low-bandgap fluorene copolymer and a fullerene derivative with photo-current extended to 850 nm, Adv. Funct. Mater., 15, 745-750 (2005). 

The other important aspect in dye-sensitized solar cells is water-induced desorption of the sensitizer from the surface. Extensive efforts have been made in our laboratory to overcome this problem by introducing hydrophobic properties in the ligand. The heteroleptic complexes containing hydrophobic ligands of the type [Ru(dcbpy)(mhdbpy)(NCS)2] 1, [Ru(dcbpy)(dtdbpy)(NCS)2] 2 [Ru(dcbpy) (mddbpy)(NCS)2] 3 (dcbpy = 4,4 -dicar-boxy-2,2 -bipyridine, mhdbpy = 4-methyl-4 -hexadecyl-2,2 -bipyridine and dtdbpy = 4,4 -ditridecyl-2,2 -bipyridine, mddbpy = 4-methyl-4 -didodecyl-2,2 bipyridine) have been synthesized (Fig. 6). The photo-current action spectra of these complexes show broad features covering a large part of visible spectrum and displays a maxima around 550 run, where the incident monochromatic IPCE exceeds 80%. The performance of these hydrophobic complexes as CT photosensitizers in nanocrystaUine Ti02-based solar cell shows excellent stabdity toward water-induced desorption [55]. 

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. 

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