**4.1 Roll-to-roll coating technique**

A continuous roll-to-roll nanoimprint lithography (R2RNIL) technique can provide a solution for high-speed large-area nanoscale patterning with greatly improved throughput. In a typical

Flexible Photovoltaic Textiles for Smart Applications 47

Thin metal electrode are exhibited 0.5% efficiency of solar power conversion to electricity which is lower than 0.76% that of the planner control device of fibre shape organic PV cells. Results are encouraged to the researchers to explore the possibility of weaving these fibres

An exhaustive research on photovoltaic fibres based on dye-sensitized TiO2-coated Ti fibers has opened up various gateways for novel PV applications of textiles. The cohesion and adhesion of the TiO2 layer are identified as crucial factors in maintaining PV efficiency after weaving operation. By proper control of tension on warp and weft fibres, high PV efficiency

The deposition of thin porous films of ZnO on metalized textiles or textile-compatible metal wires by template assisted electro-deposition technique is possible. A sensitizer was adsorbed and the performance as photoelectrodes in dye-sensitized photovoltaic cells was investigated. The thermal instability of textiles restricts its use as photovoltaic material because process temperatures are needed to keep below 150°C. Therefore, the electrodeposition of semiconductor films from low-temperature aqueous solutions has become a most reliable technique to develop textile based photovoltaics. Among low-temperature solution based photovoltaic technologies; dye sensitized solar cell technology appears most feasible. If textile materials are behaved as active textiles, the maximum electrode distance in the range of 100 µm has to be considered. Loewenstein et al., (2008) and Lincot et al., (1998) have used Ag coated polyamide threads and fibers to deposit porous ZnO as semiconductor material . The crystalline ZnO films were prepared in a cathodic electrodeposition reaction induced by oxygen reduction in an aqueous electrolyte in

Bedeloglu et al., (2009)21 were used nontransparent non-conductive flexible polypropylene (PP) tapes as substrate without use of ITO layer. PP tapes were gently cleaned in methanol, isopropanol, and distilled water respectively and then dried in presence of nitrogen. 100nm thick Ag layer was deposited by thermal evaporation technique. In next step, a thin layer of poly(3,4-ethylenedioxythiophene) doped : poly(styrene sulfonate) PEDOT: PSS mixture solution was dip coated on PP tapes. Subsequently, poly [2-methoxy-5-(3, 7 dimethyloctyloxy)-1-4-phenylene vinylene] and 1-(3-methoxycarbonyl)-propyl-1 phenyl(6,6)C61, MDMO: PPV: PCBM or poly(3-hexylthiophene) and 1-(3-methoxycarbonyl) propyl-1-phenyl(6,6)C61, P3HT: PCBM blend were dip coated onto PP tapes. Finally, a thin

layer of LiF (7nm) and Al (10nm) were deposited by thermal evaporation technique.

The enhanced conductivity will always useful to improve the photovoltaic potential of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Photovoltaic scientific community found that the conductivity of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), film is enhanced by over 100-folds if a liquid or solid organic compound, such as methyl sulfoxide (DMSO), N,Ndimethylformamide (DMF), glycerol, or sorbitol, is added to the PEDOT:PSS aqueous solution. The conductivity enhancement is strongly dependent on the chemical structure of the organic compounds. The aqueous PEDOT: PSS can be easily converted into film form on various substrates by conventional solution processing techniques and these films have excellent thermal stability

Some organic solvents such as ethylene glycol (EG), 2-nitroethanol, methyl sulfoxide or 1 methyl-2-pyrrolidinone are tried to enhance the conductivity of PEDOT: PSS. The PEDOT:

into fabric form.

**4.2.1 Dye-sensitized photovoltaics** 

presence of Zn2+ and eosinY as structure-directing agent19-20.

and high transparency in the visible range22-25.

of woven fabrics is feasible.

process, four inch wide area was printed by continuous imprinting of nanogratings by using a newly developed apparatus capable of roll-to- roll imprinting (R2RNIL) on flexible web base. The 300 nm line width grating patterns are continuously transferred on flexible plastic substrate with greatly enhanced throughput by roll-to- roll coating technique.

European Union has launched an European research project "HIFLEX" under the collaboration with Energy research Centre of the Netherland (ECN) to commercialize the roll to roll technique. Highly flexible Organic Photovoltaics (OPV) modules will allow the cost-effective production of large-area optical photovoltaic (OPV) modules with commercially viable Roll-to-Roll compatible printing and coating techniques.

Coatema, Germany with Renewable Technologies and Konarka Technologies has started a joint project to manufacture commercial coating machine. Coatema, Germany alongwith US Company Solar Integrated Technologies (SIT) has developed a process of hot-melt lamination of flexible photovoltaic films by continuous roll-to-roll technique14. Roll-to-roll (R2R) processing technology is still in neonatal stage. The novel innovative aspect of R2R technology is related to the roll to roll deposition of thin films on textile surfaces at very high speed to make photovoltaic process cost effective. This technique is able to produce direct pattern of the materials15, 16.

### **4.2 Thin -film deposition techniques**

Various companies of the world have claimed the manufacturing of various photovoltaic thin films of amorphous silicon (a-Si), copper indium selenide (CIGS), cadmium telluride (CdTe) and dye-sensitized solar cell (DSSC) successfully. Thin film photovoltaics became cost effective after the invention of highly efficient deposition techniques. These deposition techniques offer more engineering flexibilities to increase cell efficiencies, reflectance and dielectric strength, as well as act as a barrier to ensure a long life of the thin film photovoltaics and create high vapour barrier to save the chemistry of these types of photovoltaics17-18.

A fibre shaped organic photovoltaic cell was produced by utilizing concentric thin layer of small molecular organic compounds as shown in Fig 3.

Fig. 3. Photovoltaic fibre

Thin metal electrode are exhibited 0.5% efficiency of solar power conversion to electricity which is lower than 0.76% that of the planner control device of fibre shape organic PV cells. Results are encouraged to the researchers to explore the possibility of weaving these fibres into fabric form.

### **4.2.1 Dye-sensitized photovoltaics**

46 Solar Cells – New Aspects and Solutions

process, four inch wide area was printed by continuous imprinting of nanogratings by using a newly developed apparatus capable of roll-to- roll imprinting (R2RNIL) on flexible web base. The 300 nm line width grating patterns are continuously transferred on flexible plastic

European Union has launched an European research project "HIFLEX" under the collaboration with Energy research Centre of the Netherland (ECN) to commercialize the roll to roll technique. Highly flexible Organic Photovoltaics (OPV) modules will allow the cost-effective production of large-area optical photovoltaic (OPV) modules with

Coatema, Germany with Renewable Technologies and Konarka Technologies has started a joint project to manufacture commercial coating machine. Coatema, Germany alongwith US Company Solar Integrated Technologies (SIT) has developed a process of hot-melt lamination of flexible photovoltaic films by continuous roll-to-roll technique14. Roll-to-roll (R2R) processing technology is still in neonatal stage. The novel innovative aspect of R2R technology is related to the roll to roll deposition of thin films on textile surfaces at very high speed to make photovoltaic process cost effective. This technique is able to produce

Various companies of the world have claimed the manufacturing of various photovoltaic thin films of amorphous silicon (a-Si), copper indium selenide (CIGS), cadmium telluride (CdTe) and dye-sensitized solar cell (DSSC) successfully. Thin film photovoltaics became cost effective after the invention of highly efficient deposition techniques. These deposition techniques offer more engineering flexibilities to increase cell efficiencies, reflectance and dielectric strength, as well as act as a barrier to ensure a long life of the thin film photovoltaics and create

A fibre shaped organic photovoltaic cell was produced by utilizing concentric thin layer of

substrate with greatly enhanced throughput by roll-to- roll coating technique.

commercially viable Roll-to-Roll compatible printing and coating techniques.

high vapour barrier to save the chemistry of these types of photovoltaics17-18.

small molecular organic compounds as shown in Fig 3.

direct pattern of the materials15, 16.

Fig. 3. Photovoltaic fibre

**4.2 Thin -film deposition techniques** 

An exhaustive research on photovoltaic fibres based on dye-sensitized TiO2-coated Ti fibers has opened up various gateways for novel PV applications of textiles. The cohesion and adhesion of the TiO2 layer are identified as crucial factors in maintaining PV efficiency after weaving operation. By proper control of tension on warp and weft fibres, high PV efficiency of woven fabrics is feasible.

The deposition of thin porous films of ZnO on metalized textiles or textile-compatible metal wires by template assisted electro-deposition technique is possible. A sensitizer was adsorbed and the performance as photoelectrodes in dye-sensitized photovoltaic cells was investigated. The thermal instability of textiles restricts its use as photovoltaic material because process temperatures are needed to keep below 150°C. Therefore, the electrodeposition of semiconductor films from low-temperature aqueous solutions has become a most reliable technique to develop textile based photovoltaics. Among low-temperature solution based photovoltaic technologies; dye sensitized solar cell technology appears most feasible. If textile materials are behaved as active textiles, the maximum electrode distance in the range of 100 µm has to be considered. Loewenstein et al., (2008) and Lincot et al., (1998) have used Ag coated polyamide threads and fibers to deposit porous ZnO as semiconductor material . The crystalline ZnO films were prepared in a cathodic electrodeposition reaction induced by oxygen reduction in an aqueous electrolyte in presence of Zn2+ and eosinY as structure-directing agent19-20.

Bedeloglu et al., (2009)21 were used nontransparent non-conductive flexible polypropylene (PP) tapes as substrate without use of ITO layer. PP tapes were gently cleaned in methanol, isopropanol, and distilled water respectively and then dried in presence of nitrogen. 100nm thick Ag layer was deposited by thermal evaporation technique. In next step, a thin layer of poly(3,4-ethylenedioxythiophene) doped : poly(styrene sulfonate) PEDOT: PSS mixture solution was dip coated on PP tapes. Subsequently, poly [2-methoxy-5-(3, 7 dimethyloctyloxy)-1-4-phenylene vinylene] and 1-(3-methoxycarbonyl)-propyl-1 phenyl(6,6)C61, MDMO: PPV: PCBM or poly(3-hexylthiophene) and 1-(3-methoxycarbonyl) propyl-1-phenyl(6,6)C61, P3HT: PCBM blend were dip coated onto PP tapes. Finally, a thin layer of LiF (7nm) and Al (10nm) were deposited by thermal evaporation technique.

The enhanced conductivity will always useful to improve the photovoltaic potential of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Photovoltaic scientific community found that the conductivity of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), film is enhanced by over 100-folds if a liquid or solid organic compound, such as methyl sulfoxide (DMSO), N,Ndimethylformamide (DMF), glycerol, or sorbitol, is added to the PEDOT:PSS aqueous solution. The conductivity enhancement is strongly dependent on the chemical structure of the organic compounds. The aqueous PEDOT: PSS can be easily converted into film form on various substrates by conventional solution processing techniques and these films have excellent thermal stability and high transparency in the visible range22-25.

Some organic solvents such as ethylene glycol (EG), 2-nitroethanol, methyl sulfoxide or 1 methyl-2-pyrrolidinone are tried to enhance the conductivity of PEDOT: PSS. The PEDOT:

Flexible Photovoltaic Textiles for Smart Applications 49

capacities from 2cc to 400cc. e-Vap® 100 miniature evaporation systems is a precise wire-fed electron beam source designed specifically for depositing monolayer thin films in ultrahigh vacuum environments capable to deposit metals at atomic level. e-Vap® 3000 and Caburn-MDC e-Vap® are other electron beam evaporation system of different capacity for a wide range of applications30. Various companies are working in the field of thin film

Major companies Technology Status of manufacturing

Organic semiconductor based solar cells can be integrated fast with textile substrates and molecular heterojunction cells can be printed using inkjet printing efficiently. This technology has opened new routes to produce organic solar cells. Credit of invention of printed solar cells goes to Konarka Technologies31 for successful demonstration of

others Amorphous Silicon Commercial Production under 10 MW

Initial Small Quantity Manufacture under 100 kW at SSI

First Solar Production under 1 MW, Others Lower

at Several Plants

Copper Indium Diselenide

First Solar, BP Solar, Matsushita

photovoltaics as shown in Table 1.

Siemens Solar Industries (SSI), Global Solar

First Solar, BP Solar, Matsushita

Solarex, United Solar, Canon,

**4.3 Printing of plastic solar cells** 

Table 1. Photovoltaic thin film manufacturing

manufacturing of solar cells by inkjet printing as shown in Fig.4 .

Fig. 4. Konarka's plastic photovoltaic cells by printing technology

PSS film which is soluble in water becomes insoluble after treatment with EG. Raman spectroscopy indicates that interchain interaction increases in EG treated PEDOT: PSS by conformational changes of the PEDOT chains, which change from a coil to linear or expanded-coil structure. The electron spin resonance (ESR) was also used to confirm the increased interchain interaction and conformation changes as a function of temperature. It was found that EG treatment of PEDOT: PSS lowers the energy barrier for charge among the PEDOT chains, lowers the polaron concentration in the PEDOT: PSS film by w 50%, and increases the electrochemical activity of the PEDOT: PSS film in NaCl aqueous solution by w100%. Atomic force microscopy (AFM) and contact angle measurements were used to confirm the change in surface morphology of the PEDOT: PSS film. The presence of organic compounds was helpful to increase the conductivity which was strongly dependent on the chemical structure of the organic compounds, and observed only with organic compound with two or more polar groups. Experimental data were enough to make a statement that the conductivity enhancement is due to the conformational change of the PEDOT chains and the driving force is the interaction between the dipoles of the organic compound and dipoles on the PEDOT chains26.

Thin film PV structure offers following advantages 27-29:

