**2.5 Solar cell integrated textiles**

Among the photovoltaic technologies, organic solar cells are the most suitable ones to textile structures in terms of favorable features such as flexibility, lightness, cost-effectiveness and usage performance. Studies about photovoltaic textiles consider two main approaches: First, solar cell is formed elsewhere and then, photovoltaic structure is integrated in/onto textiles using various techniques, i.e. patching. Second, solar cell is formed in fiber or textile form. So, it can be used as fiber itself or can form textile structures, which are partly or completely photovoltaic. Shelf lifetime, cost and efficiency of organic solar cells are still important issues for also photovoltaic fibers and textiles to be overcome before commercialization.

Utilizing flexible solar cells with textiles can open many application fields for photovoltaic textiles such as electronic textiles besides powering movable electronic devices. Solar cell integrated bags, jackets and dresses are some of the recent applications of polymer based solar cells. For example, in study of Krebs et al. (2006) incorporation of polymer based organic solar cells into textile structures were performed by two ways: In first one, PET substrate was coated with ITO, MEH-PPV, C60 and Al, respectively. Then, device was laminated using PET. In second one, PE layer was laminated onto textile substrate. Then, by applying PEDOT, active material and final electrode, respectively, device was completed. Completed devices were integrated into clothes (Fig. 10).

Fig. 10. An example of patterned polymer solar cells on a PET substrate incorporated into clothing by sewing through the polymer solar cell foil using an ordinary sewing machine. Connections between cells were made with copper wire that could also be sewn into the garment. The solar cells were incorporated into a dress and a belt. Design by Tine Hertz Reprinted from Sol. Energy Mater. Sol. Cells, 90, Krebs F.C.; Biancardo M.; Jensen B.W.; Spanggard H. & Alstrup J., Strategies for incorporation of polymer photovoltaics into garments and textiles, 1058-1067, Copyright (2006), with permission from Elsevier.

glass based substrates, and, which use a hole transport material, 4,4-bis[*N*-(1-naphthyl)-*N*phenyl-amino]biphenyl (α-NPD) and C60 bilayer structure, exhibited high carrier mobilities

Among the photovoltaic technologies, organic solar cells are the most suitable ones to textile structures in terms of favorable features such as flexibility, lightness, cost-effectiveness and usage performance. Studies about photovoltaic textiles consider two main approaches: First, solar cell is formed elsewhere and then, photovoltaic structure is integrated in/onto textiles using various techniques, i.e. patching. Second, solar cell is formed in fiber or textile form. So, it can be used as fiber itself or can form textile structures, which are partly or completely photovoltaic. Shelf lifetime, cost and efficiency of organic solar cells are still important issues

Utilizing flexible solar cells with textiles can open many application fields for photovoltaic textiles such as electronic textiles besides powering movable electronic devices. Solar cell integrated bags, jackets and dresses are some of the recent applications of polymer based solar cells. For example, in study of Krebs et al. (2006) incorporation of polymer based organic solar cells into textile structures were performed by two ways: In first one, PET substrate was coated with ITO, MEH-PPV, C60 and Al, respectively. Then, device was laminated using PET. In second one, PE layer was laminated onto textile substrate. Then, by applying PEDOT, active material and final electrode, respectively, device was completed.

Fig. 10. An example of patterned polymer solar cells on a PET substrate incorporated into clothing by sewing through the polymer solar cell foil using an ordinary sewing machine. Connections between cells were made with copper wire that could also be sewn into the garment. The solar cells were incorporated into a dress and a belt. Design by Tine Hertz Reprinted from Sol. Energy Mater. Sol. Cells, 90, Krebs F.C.; Biancardo M.; Jensen B.W.; Spanggard H. & Alstrup J., Strategies for incorporation of polymer photovoltaics into garments and textiles, 1058-1067, Copyright (2006), with permission from Elsevier.

for also photovoltaic fibers and textiles to be overcome before commercialization.

and high *V*oc=0.85V (AM1.5, 97 mW/cm2) (Kushto et al., 2005).

Completed devices were integrated into clothes (Fig. 10).

**2.5 Solar cell integrated textiles** 

### **2.6 Studies about polymer nanofibers for solar cells**

There are several studies about developing conductive polymer nanofibers used to fabricate solar cells. Various methods such as self-assembly (Merlo & Frisbie, 2003), polymerization in nanoporous templates (Martin, 1999), dip-pen nano-lithography (Noy et al., 2002), and electrospinning (Babel et al., 2005; Wutticharoenmongkol et al., 2005; Madhugiri; 2003) techniques are used to produce conductive polymer nanowires and nanofibers. Nanofibers having ultrafine diameters provide some advantages including mechanical performance, very large surface area to volume ration and flexibility to be used in solar cells (Chuangchote et al., 2008a).

Since morphology of the active layer in organic solar cells plays an important role to obtain high power conversion efficiencies, many researchers focus on developing P3HT nanofibers for optimized morphologies (Berson et al., 2007; Li et al., 2008; Moulé & Meerholz, 2008). Nanofibers can be deposited onto both conventional glass-based substrates flexible polymer based substrates, which have low glass transition temperature (Bertho et al., 2009).

A fabrication method (Berson et al., 2007) was presented to produce highly concentrated solutions of P3HT nanofibers and to form highly efficient active layers after mixing these with a molecular acceptor (PCBM), easily. A maximum PCE of 3.6% (AM1.5, 100 mWcm–2) has been achieved without any thermal post-treatment with the optimum composition:75 wt% nanofibers and 25 wt% disorganized P3HT. Manufacturing processes were appropriate to be used with flexible substrates at room temperatures. Bertho et al. (Bertho et al., 2009) demonstrated that the fiber content of the P3HT-fiber:PCBM casting solution can be easily controlled by changing the solution temperature. Optimal solar cell efficiency was obtained when the solution temperature was 45 ºC and the fiber content was 42%. Fiber content in the solution effected the photovoltaic performances of cells.

Fig. 11. Jsc–V graph of the P3HT/PCBM based solar cloth measured under 1 Sun conditions. Inset shows a picture of the solar cloth fabricated using electrospinning. Reprinted from *Materials Letters*, 64, Sundarrajan, S.; Murugan, R.; Nair, A. S. & Ramakrishna, S., 2369 -2372., Copyright (2010), with permission from Elsevier.

Electrospinning technique (Chuangchote et al., 2008b) is also used to prepare photoactive layers of polymer-based organic solar cells without thermal post-treatment step. Electrospun MEH-PPV nanofibers were obtained after polyvinylpyrrolidone (PVP) was removed from

Progress in Organic Photovoltaic Fibers Research 273

For developing optimum photovoltaic textile, choice of the fiber type, which determines UV resistance and maximum processing temperature for photovoltaics and textile production

In recent years, there are several studies about photovoltaic fibers based on polycrystalline silicon (Kuraseko et al., 2006), dye sensitized solar cells (Fan et al., 2008; Ramier et al., 2008; Toivola et al., 2009) and organic solar cells (Bedeloglu et al., 2009, 2010a, 2010b, 2010c, 2011; Curran et al., 2006; Curran et al., 2008; Curran et al., 2009; Lee et al., 2009; Liu et al., 2007a; Liu et al., 2007b; O'Connor et al., 2008; Zhou et al., 2009; Zou et al., 2010). Protection of liquid electrolyte in DSSCs is problematic causing leakage and loss of performance*.* However, solid type DSSCs suffer from cracking due to low elongation and bending properties. The organic solar cells based fibers still suffer from low power conversion efficiency and stability*.*  However, organic materials are very suitable to develop flexible photovoltaic fibers with low-

The fiber geometry due to circular cross-section and cylindrical structure brings advantages in real usage conditions. Contrast to planar solar cells, absorption and current generation results in a greater power generation, which can be kept constant during illumination owing to its symmetric structure. A photovoltaic fiber has very thin coatings (about a few hundred nanometers). Therefore, a photovoltaic fabric made from this fiber will be much lighter than

Organic photovoltaic fibers have been produced in different thicknesses and lengths, using different techniques and materials in previous studies. In order to develop fiber based solar cells, mainly solution based coating techniques were applied to develop polymer based electrodes and light absorbing layers. However, deposition techniques in a vacuum were

Current studies about fiber shaped organic photovoltaics used different substrate materials such as optical fibers (Do et al., 1994), polyimide coated silica fibers (O'Connor et al., 2008), PP fibers and tapes (Bedeloglu et al., 2009, 2010a, 2010b, 2010c, 2011) and stainless steel

In order to fabricate photovoltaic fiber with low-cost and high production rate, an approach is using a drawing a metal or metalized polymer based fiber core through a melt containing a blend of photosensitive polymer. A conductor can also be applied parallel to the axis of the

In optical fiber concept, photovoltaic fiber takes the light and transmitted down the fiber by working as an optical can. The fiber shaped photovoltaics approach can reduce the disadvantage of organic solar cells, which is trade-off between exciton diffusion length and the photoactive film thickness in conjugated polymers based solar cells, by forming the solar

Organic solar cell materials are generally coated around the fibers concentrically in an order in photovoltaic fibers, as in planar solar cells. The Substrate, active layer and conductive electrodes do their own duties. Recent studies about photovoltaic fibers can be classified in two groups: First one is interested with photovoltaic fibers that were illuminated from outside as in photovoltaic textiles, second one is the study of illuminated from inside the

For the outside illuminated photovoltaic fibers, different device sequences and manufacturing techniques were used. A fiber-shaped, ITO-free organic solar cell using small molecular

methods (Mather & Wilson, 2006) need to be considered.

cost and in large scale (Bedeloglu et al., 2009; DeCristofano, 2008).

used to develop a photovoltaic fiber formation, too.

photoactive fiber core (Shtein & Forrest, 2008).

cell around the fiber (Li et al., 2010b).

photovoltaic fiber (Zou et al., 2010).

wires (Lee et al., 2009).

**3.1 Device structures** 

that of other thin film technologies or laminated fabric (Li et al., 2010a).

as-spun MEH-PPV/PVP fibers. A ribbon-like structure aligned with wrinkled surface in fiber direction was gained. Bulk heterojunction organic solar cells were manufactured by using the electrospun MEH-PPV nanofibers with a suitable acceptor. Chuangchote et al. produced ultrafine MEH-PPV/PVP composite fibers (average diameters ranged from 43 nm to 1.7 mm) by electrospinning of blended polymer solutions in mixed solvent of chlorobenzene and methanol under the various conditions.

Recently, a photovoltaic fabric (Sundarrajan et al., 2010) based on P3HT and PCBM materials were developed. The non-woven organic solar cloth was formed by coelectrospinning of two materials: the core-shell nanofibers as the core and PVP as the shell. The efficiency of the fiber-based solar cloth was obtained as 8.7×10−8 due to processing conditions and thickness of structure (Fig. 11-12). However, this is an novel and improvable approach to develop photovoltaic fabrics for smart textiles.

Fig. 12. Schematic diagram of core-shell electrospinning set-up used in this study: direct current voltage at 18 KV, the flow rate of P3HT/PCBM in chloroform/toluene (3:1 ratio, as core) and PVP in chloroform/ethanol (1:1 ratio, shell) was set at 1.3 mL/h and 0.8 mL/h, Respectively. Reprinted from *Materials Letters*, 64, Sundarrajan, S.; Murugan, R.; Nair, A. S. & Ramakrishna, S., 2369 -2372., Copyright (2010), with permission from Elsevier
