**3. Organic photovoltaic fibers**

In recent years, attention on fibrous and flexible optoelectronic structures is increased in both scientific and industrial areas in terms of lightweight, low-cost and large scale production possibilities. Photovoltaic fibers, cost effective and scalable way of solar energy harvesting, work with the principle of solar cell, which produces electricity by converting photons of the sun. Although solar cells made from silicon and other inorganic materials are far more efficient for powering devices than organic solar cells, they are still too expensive to be used in widespread and longterm applications. In studies of fiberbased solar cells, which are incorporated in textiles, organic semiconductors that are naturally flexible and light-weight, are ideal candidates compared to conventional inorganic semiconductors.

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

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

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.

In recent years, attention on fibrous and flexible optoelectronic structures is increased in both scientific and industrial areas in terms of lightweight, low-cost and large scale production possibilities. Photovoltaic fibers, cost effective and scalable way of solar energy harvesting, work with the principle of solar cell, which produces electricity by converting photons of the sun. Although solar cells made from silicon and other inorganic materials are far more efficient for powering devices than organic solar cells, they are still too expensive to be used in widespread and longterm applications. In studies of fiberbased solar cells, which are incorporated in textiles, organic semiconductors that are naturally flexible and light-weight, are ideal candidates compared to conventional

& Ramakrishna, S., 2369 -2372., Copyright (2010), with permission from Elsevier

**3. Organic photovoltaic fibers** 

inorganic semiconductors.

chlorobenzene and methanol under the various conditions.

approach to develop photovoltaic fabrics for smart textiles.

For developing optimum photovoltaic textile, choice of the fiber type, which determines UV resistance and maximum processing temperature for photovoltaics and textile production methods (Mather & Wilson, 2006) need to be considered.

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

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

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 used to develop a photovoltaic fiber formation, too.

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 wires (Lee et al., 2009).

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 photoactive fiber core (Shtein & Forrest, 2008).

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

### **3.1 Device structures**

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 photovoltaic fiber (Zou et al., 2010).

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

Progress in Organic Photovoltaic Fibers Research 275

PEDOT·PSS materials as electron transport layer, active layer and hole transport layer, respectively (Fig. 15). Another wire as secondary electrode was wrapped around the coated primary wire with a rotating stage similar to commercial wire winding operations. In the best cell, the short circuit current density was 11.9 mA/cm2 resulting 3.87% power

Fig. 15. Schematic of a complete fiber showing the potential for shadowing by the secondary

Gaudiana, R. A. (2009). Solar power wires based on organic photovoltaic materials. *Science*,

Many researchers considered photovoltaic fiber design for different function from an optical perspective to capture or trap more light. An optical design was investigated (Curran et al., 2006) to increase the efficiency of photovoltaic device by directing the incident light into the photoactive layer using optical fibers. Prepared fibers are worked up into bundle to confine the light in the device. Polymer based organic solar cell materials are used to develop an optical fiber-based waveguide design (Liu et al., 2007a). P3HT:PCBM is commonly used composite material to form active layer. Carroll and co-workers added top electrode (Al) to only one side of the fiber and tested the photovoltaic fibers under standard illumination at the cleaved end of the fibers. Optical loss into the fiber based solar cell increased as the fiber diameter decreased (See Fig. 16) and increasing efficiency was obtained by the smaller diameter photovoltaic fibers. In their other study (Liu et al., 2007b), performances of the photovoltaic fibers were compared as a function of incident angle of illumination (varied from 0º – 45º) on the cleaved face of the fiber. 1/3 of the circumference was coated with thick outer electrode (LiF/Al) due to fibers having small diameter. Photovoltaic performance of the devices was dependent on fiber

Using an optical fiber having 400 µm in diameter, microconcentrator cell (Curran et al., 2008) was fabricated to develop an efficient method of light capturing for the optical concentration by using a mathematical based model to pinpoint how to concentrate light within the microconcentrator cell. Behaviour of light between the fiber entrance and active semiconductor layer was investigated. The fiber-based photovoltaic cell, which was a solar collector that utilized internal reflector to confine light into an organic absorber, collected nearly 80% of the incoming photons as current, at ~3 kOhms.cm (Zhou et al., 2009). Li et al. (2010) developed a mathematical model that was also supported by experimental results, for light transmission, absorption and loss in fiber-based organic solar cells using ray tracing

electrode. From Lee, M. R.; Eckert, R. D. ; Forberich, K. ; Dennler, G.; Brabec, C. J. &

Vol. 324, pp. 232–235. Reprinted with permission from AAAS.

diameter and the angle of the incidence light onto the cleaved fiber face.

conversion efficiency.

organic compounds was demonstrated by Shtein and co-workers (O'Connor et al., 2008). Light was entered the cell through a semitransparent outer electrode in the fiber-based photovoltaic cell. Concentric thin films of Mg/Mg:Au/Au/CuPc/C60/Alq3/Mg:Ag/Ag were deposited onto rotated polyimide coated silica fibers having 0.48 mm diameter by thermal evaporation technique in a vacuum (see Fig. 13). The cell exhibited 0.5% power conversion efficiency, which was much less dependent on variations in illumination angle. However, coated fiber length was limited by the experimental deposition chamber geometry.

Fig. 13. A flexible polyimide coated silica fiber substrate device, with the layers deposited concentrically around the fiber workers. Reprinted with permission from O'Connor, B.; Pipe, K. P. & Shtein, M. (2008). Fiber based organic photovoltaic devices. *Appl. Phys. Lett.*, vol. 92, pp. 193306-1–193306-3. Copyright 2008, American Institute of Physics.

Bedeloglu et al. developed flexible photovoltaic devices (Bedeloglu et al., 2009, 2010a, 2010b, 2010c, 2011) to manufacture textile based photovoltaic tape and fiber by modifying planar organic solar cell sequence. The non-transparent and non-conductive polymeric materials (PP tapes and fibers) were used as substrate and dip coating and thermal evaporation technique were used to coat active layer and top electrode, respectively. Devices gave moderate efficiencies in photovoltaic tape (PP/Ag/PEDOT:PSS/P3HT:PCBM/LiF/Al) and in photovoltaic fiber (PP/PEDOT:PSS/P3HT:PCBM/LiF/Al) (see Fig. 14). Light entered the photovoltaic structure from the outer semi-transparent cathode (10 nm LiF/Al). Obtained structures that were very flexible and lightweight were hopeful for further studies using textile fibers.

Fig. 14. Schematic drawing of a photovoltaic fiber and I–V curves of P3HT:PCBM -based photovoltaic fibers, lighting through the cathode direction. The final, definitive version of this paper has been published in *<* Textile Research Journal>, 80/11/July/2010 by <<SAGE Publications Ltd.>>/<<SAGE Publications, Inc.>>, All rights reserved. ©.

Flexible photovoltaic wires based on organic materials can also be produced to be used in a broad range of applications including smart textiles (Lee et al., 2009). In the study, a stainless steel wire used as primary electrode was coated with TiOx, P3HT and PC61BM,

organic compounds was demonstrated by Shtein and co-workers (O'Connor et al., 2008). Light was entered the cell through a semitransparent outer electrode in the fiber-based photovoltaic cell. Concentric thin films of Mg/Mg:Au/Au/CuPc/C60/Alq3/Mg:Ag/Ag were deposited onto rotated polyimide coated silica fibers having 0.48 mm diameter by thermal evaporation technique in a vacuum (see Fig. 13). The cell exhibited 0.5% power conversion efficiency, which was much less dependent on variations in illumination angle. However, coated fiber

Fig. 13. A flexible polyimide coated silica fiber substrate device, with the layers deposited concentrically around the fiber workers. Reprinted with permission from O'Connor, B.; Pipe, K. P. & Shtein, M. (2008). Fiber based organic photovoltaic devices. *Appl. Phys. Lett.*,

Bedeloglu et al. developed flexible photovoltaic devices (Bedeloglu et al., 2009, 2010a, 2010b, 2010c, 2011) to manufacture textile based photovoltaic tape and fiber by modifying planar organic solar cell sequence. The non-transparent and non-conductive polymeric materials (PP tapes and fibers) were used as substrate and dip coating and thermal evaporation technique were used to coat active layer and top electrode, respectively. Devices gave moderate efficiencies in photovoltaic tape (PP/Ag/PEDOT:PSS/P3HT:PCBM/LiF/Al) and in photovoltaic fiber (PP/PEDOT:PSS/P3HT:PCBM/LiF/Al) (see Fig. 14). Light entered the photovoltaic structure from the outer semi-transparent cathode (10 nm LiF/Al). Obtained structures that were very

 Fig. 14. Schematic drawing of a photovoltaic fiber and I–V curves of P3HT:PCBM -based photovoltaic fibers, lighting through the cathode direction. The final, definitive version of this paper has been published in *<* Textile Research Journal>, 80/11/July/2010 by <<SAGE

Flexible photovoltaic wires based on organic materials can also be produced to be used in a broad range of applications including smart textiles (Lee et al., 2009). In the study, a stainless steel wire used as primary electrode was coated with TiOx, P3HT and PC61BM,

vol. 92, pp. 193306-1–193306-3. Copyright 2008, American Institute of Physics.

flexible and lightweight were hopeful for further studies using textile fibers.

Publications Ltd.>>/<<SAGE Publications, Inc.>>, All rights reserved. ©.

length was limited by the experimental deposition chamber geometry.

PEDOT·PSS materials as electron transport layer, active layer and hole transport layer, respectively (Fig. 15). Another wire as secondary electrode was wrapped around the coated primary wire with a rotating stage similar to commercial wire winding operations. In the best cell, the short circuit current density was 11.9 mA/cm2 resulting 3.87% power conversion efficiency.

Fig. 15. Schematic of a complete fiber showing the potential for shadowing by the secondary electrode. From Lee, M. R.; Eckert, R. D. ; Forberich, K. ; Dennler, G.; Brabec, C. J. & Gaudiana, R. A. (2009). Solar power wires based on organic photovoltaic materials. *Science*, Vol. 324, pp. 232–235. Reprinted with permission from AAAS.

Many researchers considered photovoltaic fiber design for different function from an optical perspective to capture or trap more light. An optical design was investigated (Curran et al., 2006) to increase the efficiency of photovoltaic device by directing the incident light into the photoactive layer using optical fibers. Prepared fibers are worked up into bundle to confine the light in the device. Polymer based organic solar cell materials are used to develop an optical fiber-based waveguide design (Liu et al., 2007a). P3HT:PCBM is commonly used composite material to form active layer. Carroll and co-workers added top electrode (Al) to only one side of the fiber and tested the photovoltaic fibers under standard illumination at the cleaved end of the fibers. Optical loss into the fiber based solar cell increased as the fiber diameter decreased (See Fig. 16) and increasing efficiency was obtained by the smaller diameter photovoltaic fibers. In their other study (Liu et al., 2007b), performances of the photovoltaic fibers were compared as a function of incident angle of illumination (varied from 0º – 45º) on the cleaved face of the fiber. 1/3 of the circumference was coated with thick outer electrode (LiF/Al) due to fibers having small diameter. Photovoltaic performance of the devices was dependent on fiber diameter and the angle of the incidence light onto the cleaved fiber face.

Using an optical fiber having 400 µm in diameter, microconcentrator cell (Curran et al., 2008) was fabricated to develop an efficient method of light capturing for the optical concentration by using a mathematical based model to pinpoint how to concentrate light within the microconcentrator cell. Behaviour of light between the fiber entrance and active semiconductor layer was investigated. The fiber-based photovoltaic cell, which was a solar collector that utilized internal reflector to confine light into an organic absorber, collected nearly 80% of the incoming photons as current, at ~3 kOhms.cm (Zhou et al., 2009). Li et al. (2010) developed a mathematical model that was also supported by experimental results, for light transmission, absorption and loss in fiber-based organic solar cells using ray tracing

Progress in Organic Photovoltaic Fibers Research 277

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