**5. Significance of low bandgap polymers**

The part of visible sunlight is lost by absorption in specific regions of the spectrum when passes through the atmosphere. The amount of loss depends on the air mass. Under ideal conditions, the available photons for the conversion to the electrons can be represented by the solar spectrum in photon flux as a function of wavelength. It is evident that photons must be harvest at longer wavelength but at longer wavelengths the energy of the charge carriers remains lower which restricts the voltage difference that the device can produce. Hence designing of optimum bandgap is essential. However, the practical efficiencies differ from theoretically predicted values. These above considerations are based on the fact that the low band gap polymers have the possibility to improve the efficiency of OPVs due to a better overlap with the solar spectrum. Hence to achieve maximum power generation in photovoltaic device low band gap materials are required. Majority of Photovoltaic devices are unable to convert light energy below 350–400 nm wavelengths efficiently into electrical energy because of poor absorption in the substrate and front electrodes. Although, this part

Flexible Photovoltaic Textiles for Smart Applications 53

Ramier et al., (2008), concluded that the feasibility of producing textile structure from DSC-PV fibre is quite good. The deposition of TiO2 on flexible fibre is expected to be quite fruiteful in order to maintain the structural integrity without comparing with PV

Fibre based organic PV devices inroads their applications in electronics, lighting, sensing and thermoelectric harvesting. By successful patch up between commodity fibre and photovoltaic

Coner et al.,53 have developed a photovoltaic fibre by deposition of small Molecular weight organic compound in the form of concentric layer on long fibres. They manufactured the OPV fibre by vacuum thermal evaporation (VTE) of concentric thin films upto 0.48 mm thickness on polyamide coated silica fibre. Different control devices are based on OPV cells containing identical layer structures deposited on polyimide substrates. The OPV based fibre cells were defined by the shape of the substrate and 1 mm long cathodes. All fibre surfaces were cleaned well prior to deposition. Lastly, they concluded that performance of OPV fibre cells from ITO is inferior in terms of changes in illumination angle, enabling the optical photovoltaic (OPV) fibre containing devices to outperform its planar analog under favourable operating conditions. Light emitting devices are designed in such a way that becomes friendly to weave it. The light trapping on fibre surface can be improved by using external dielectric coating which is coupled with protective coating to enhance its service time. Successful PV fibre can be manufactured by opting appropriate material with more

Dye sensitized solar cells (DSC) are low cost, applicable in wide range of application and simple to manufacture. These merits of dye-sensitized PV fibre makes it a potential

DSC works on the principle of optoelectronically active cladding on an optical fibre. This group was manufactured two type of PV fibre using polymethylmethacrylate (PMMA) baltronic quality diameter 1.3 to 2.0mm and photonium quality glass fibre with diameter 1.0 to 1.5 mm. Both virgin fibre were made electronically conductive by deposition of 130nm thick layer of ZnO:Al by atomic layer deposition technique with the help of P400 equipment. The high surface area photoelectric film for DSC was prepared in two steps. In first step TiO2 in the form of solution or paste having TiO2 nanoparticle is deposited on electronically conductive surface. In the second step dry layer of TiO2 is sintered at 450- 500°C for 30 minute to ensure proper adhesion to the fibre surface. PMMA fibre is suitable to survive upto 85°C. Hence mechanical compression is alternate technique to ensure the

concept, a very useful and cost effective way of power harvesting is matured50-52.

Fig. 6. A model DSC Photovoltaic fibre by surface deposition

alternative to the conventional silicon and thin film PV devices55.

improved fabrication potential54.

fixation.

performance49.

of light spectrum contains very little intensity and consequently do not have a major contribution and i.e only 1.4% to the total possible current. It is evident from above discussion that to increase the current realization λmax have to increase from 650 to 1000nm, in turn decreasing the band gap. Poly (3-hexylthiophene) is a typical example of low badgap polymeric material has a band gap of 650nm (1.9 eV) which can harvest up to 22.4% of the available photons. Hence, it is necessary to fabricate the polymers having broad absorption to achieve increase in the efficiency of the solar cell33-34.
