**4.5 Solar cells**

Literature reveals that the increased application of the PPy material has been made, during recent years, in solar cells, particularly in dye–sensitized solar cells (DSSCs) to improve the overall energy conversion efficiency and also to reduce the cost of the cell. Recently, PPy was synthesized by vapour phase polymerization and traditional electrochemical polymerization and employed as counter electrode in a DSSC. Both PPy and Pt electrodes showed good catalytic behavour in DSSCs (Xia et al., 2011). The composite, PPy/graphite on ITO glass also showed favourable catalytic activity for I2/I¯ redox reaction (Xiaoming et al., 2011).The overall energy conversion efficiency of the DSSCs based on the ruthenium dye (N719) with PPy/graphite composite counter electrode reached 6.01% under simulated AM 1.5 irradiation (100 mW/cm2), which was 92% of the energy conversion efficiency of the DSSCs with Pt electrode.

Photoelectrochemical and electrochemical behavior of gold electrode modified with bilayers of PPy and PANI have been investigated in acid solutions (Upadhyay et al., 1995). Both PPy and PANI films on gold exhibit photo electrochemical activity, with the former showing a considerably high activity than the latter. PEC solar cells based on nanostructured ZnO/dye/PPy/ film electrode display excellent properties as anode in the conversion process of light to electricity (Hao et al., 2000). PPy was prepared on Ru-dye sensitized TiO2 nanoporous film and solar cell was constructed using gold as the counter electrode with PPy acting as the hole conductor (Cervini et al., 2004). Photodevices comprising covalently grafted PPy films on surface modified mesoporous TiO2 substrates via 3-(trimethoxysilyl) propyl methacrylate were fabricated and tested their performances with a counter electrode having a thin layer of gold (Senadeera et al., 2006). Significant enhancements in photoresponses were observed with the above additives in PPy than the reported devices comprising TiO2/PPy. Hybrid Cu-In disulphide/PPy photovoltaic structures prepared by electrodeposition exhibited (Bereznev et al., 2005) significant photovoltage and photocurrent under standard white light illumination. Electrodeposited Cu-In-Se/PPy PV structures exhibited the formation of a n-p barrier between the n-CuInSe and p-PPy layers (Bereznev et al., 2006). The PPy encapsulated TiO2 nanotube array (PPy/TiO2 NTs) electrode was also synthesized (Zhang et al., 2008) by electropolymerization to encapsulate PPy inside the TiO2 nanotube channels and walls in order to enhance the photocurrent density.
