**2.2 Organic solar cells**

As a promising renewable energy source, organic photovoltaics have attracted attention during the last decades resulting in significant progress in cell efficiency exceeded 5% (AM1.5, 1000 W/m2) (Green et al., 2010) in the conventional bulk heterojunction solar cell architecture consisting of a polymer donor and fullerene acceptor blend. Organic solar cells achieving photovoltaic energy conversion by organic semiconductor or conductor are compatible with flexible substrates like textiles for use in novel application areas.

Photovoltaic effect, production of electricity by converting photons of the sunlight, occurs in an organic solar cell by the following steps (Nunzi*,* 2002): Absorption of photons of the light in the solar cell and exciton (electron-hole pair) creation; separation of charges and carriers generation from exciton dissociation; transport and then collection of charges by respective electrodes (Günes et al., 2007; Nunzi*,* 2002)

There are some approaches such as using conjugated polymers (Antonradis et al., 1994) and their blends (Granström et al., 1998; Halls et al., 1995; Yu & Heeger, 1995), small molecules (Tang, 1986; Wöhrle & Meissner, 1991) polymer-small molecule bilayers (Jenekhe & Yi, 2000;

Fiber based photovoltaics take the advantage of being flexible and lightweight. Integration of photovoltaic fibers into fabrics and clothes is easy to manufacture wearable technology products. Small surface of a fiber also provide large area photoactive surfaces in the case of

Traditional solar cells using silicon based semiconductors are generally rigid and are not suitable to be used with textiles. The thin film solar cells based on inorganic semiconductors can be made flexible and however they are more suitable for patching onto fabrics (Schubert

Inexpensive electricity production can be achieved, when both low-cost and high efficient manufacturing of photovoltaic cells are achieved. A potential alternative approach to conventional rigid solar cells is organic solar cells, which can be coated on both rigid and flexible substrates using easy processing techniques. In addition, the polymer based organic solar cells can be used to produce fully flexible photovoltaic textiles easily, in any scale, from

Organic semiconductors, which are generally considered as intrinsic wide band gap semiconductors (band gap>1.4 eV), have many advantages to be used in solar cells. For example, organic semiconductors of which electronic band gap can be engineered by chemical synthesis with low-cost (Günes et al., 2007) have generally high absorption

Organic semiconductors consist of different chemical structures (Nunzi*,* 2002) including polymers, oligomers, dendrimers, dyes, pigments, liquid crystals (Yilmaz Canli et al., 2010) etc. In carbon-based semiconductors, conductivity is obtained by conjugation, which single

Conjugated organics are challenging materials for solar cells owing to their semiconducting and light absorbing features. As a compound of organic solar cells, organic semiconductors can be processed by thermal evaporation techniques or solution based coating or printing

As a promising renewable energy source, organic photovoltaics have attracted attention during the last decades resulting in significant progress in cell efficiency exceeded 5% (AM1.5, 1000 W/m2) (Green et al., 2010) in the conventional bulk heterojunction solar cell architecture consisting of a polymer donor and fullerene acceptor blend. Organic solar cells achieving photovoltaic energy conversion by organic semiconductor or conductor are

Photovoltaic effect, production of electricity by converting photons of the sunlight, occurs in an organic solar cell by the following steps (Nunzi*,* 2002): Absorption of photons of the light in the solar cell and exciton (electron-hole pair) creation; separation of charges and carriers generation from exciton dissociation; transport and then collection of charges by respective

There are some approaches such as using conjugated polymers (Antonradis et al., 1994) and their blends (Granström et al., 1998; Halls et al., 1995; Yu & Heeger, 1995), small molecules (Tang, 1986; Wöhrle & Meissner, 1991) polymer-small molecule bilayers (Jenekhe & Yi, 2000;

and double bonds between the carbon atoms alternate (Pope & Swenberg, 1999).

compatible with flexible substrates like textiles for use in novel application areas.

techniques at low temperatures (Deibel & Dyakonov, 2010).

fabric, so higher power conversion efficiency can be obtained.

fibers to fabrics and using low-cost methods.

**2. Organic photovoltaic technology** 

**2.1 Organic semiconductors** 

**2.2 Organic solar cells** 

electrodes (Günes et al., 2007; Nunzi*,* 2002)

& Werner, 2006).

coefficients.

Breeze et al., 2002) and their blends (Tang, 1986; Shaheen et al., 2001; Dittmer et al., 2000) or combinations of inorganic-organic materials (O`Reagan & Graetzel, 1991; Greenham et al., 1996; Günes et al., 2008; to develop organic solar cells (Güneş & Sariçiftçi, 2007). Mostly, two concepts are considered in organic solar cell researches: first one, (Krebs, 2009a) which is the most successful is using conjugated polymers (Fig. 1) with fullerene derivatives by solution based techniques and second one is cooperating small molecular materials (as donor and acceptor) by thermal evaporation techniques (Deibel & Dyakonov, 2010*).* 

A conventional organic solar cell (Fig. 2) device is based on the following layer sequence: a semi-transparent conductive bottom electrode (indium tin oxide (ITO)) or a thin metal layer), a poly(3,4-ethylenedioxythiophene:poly(styrene sulfonic acid) (PEDOT:PSS) layer facilitating the hole injection and surface smoothness, an organic photoactive layer (most common poly(3- hexylthiophene):[*6*,*6*]-*phenyl*-*C61*-*butyric acid* methyl ester (P3HT:PCBM)) to absorb the light and a metal electrode (Aluminum, Al and Calcium, Ca) with a low work function to collect charges on the top of the device (Brabec et al., 2001a; Brabec et al., 2001b; Padinger et al., 2003). To form a good contact between the active layer and metal layer, an electron transporting layer (i.e. Lithium Fluoride, LiF) is also used (Brabec et al., 2002).

Fig. 1. Example of organic semiconductors used in polymer solar cells. Reprinted from Solar Energy Materials and Solar Cells, 94, Cai, W.; Gong, X. & Cao, Y. Polymer solar cells: Recent development and possible routes for improvement in the performance, 114–127, Copyright (2010), with permission from Elsevier.

Progress in Organic Photovoltaic Fibers Research 261

accepting materials. Also, phtalocyanines, porphyrins, poly(3- hexylthiophene) (P3HT) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) are good donors (Nunzi*,2002).* Most of the time, evaporation step is indispensable in the manufacturing of conventional organic photovoltaic devices, but this process tends to increase the cost of the cell. Besides, ITO-PEDOT:PSS interface (de Jong et al., 2000) and Al top electrode (Do et al., 1994) is known to be quite unstable, which limits the lifetime of organic solar cells. Organic solar cells have low charge transport and a mismatch between exciton diffusion length and organic layer thickness. While the efficient absorption of light is provided by organic film based on P3HT:PCBM having a thickness over 250 nm (Liu et al., 2007a), exciton diffusion length of which surpassing causes exciton recombination*,* is about 10-20 nm in polymer based and in organic semiconductors (Nunzi*,* 2002). Although, organic solar cells have lower power conversion efficiency (~5%) than inorganic traditional solar cells (for crystalline silicon based solar cells ~25% in laboratory conditions); their cost and processing parameters are favorable (Deibel & Dyakonov,

In order to avoid the limitations of organic semiconductor and to improve the power conversion efficiency of organic solar cells, several approaches such as optical concepts, different device configurations such as inverted layer architecture, multijunction solar cells, novel materials with lower band gap (Park et al., 2009; Chen et al., 2009; Huo et al., 2009; Coffin et al., 2009), wider absorption ranges, higher dielectric constants and higher charge carrier mobility are some approaches are studied in last few years (Deibel & Dyakonov, 2010). Reversing the nature of charge collection in organic solar cells using a less air sensitive high work function metal (Ag, Au) (de Jong et al., 2000; Do et al., 1994; Liu et al., 2007a; Park et al., 2009; Chen et al., 2009; Huo et al., 2009; Coffin et al., 2009; Wong et al., 2006) as hole collecting electrode at the back contact and a metal oxide (TiOx, ZnO) as hole blocking barrier and electron selective contact at the ITO interface to block the oxidation (Hau et al., 2008) is a beneficial approach to avoid from low power conversion efficiency, which limited absorption in solar spectrum causes. In particular, non-clorinated solvents are more appropriate for high volume manufacturing with low cost. Besides, active layer can be protected by use of metal oxides (i.e. vanadium oxide and cesium carbonate), which are used as buffer layer of inverted polymer solar cells (Li et al., 1997). However, there is still a trade-off between stability and photovoltaic

Organic solar cells have many advantages such as potential to be semi-transparent, manufacture on both large or small areas compatible with mass production and low-cost, production possibility with continuous coating and printing processes on lightweight and flexible substrates (for example textiles), ecological and low-temperature production

Polymer heterojunction organic solar cells have attracted much attention because of their potential applications in large area, flexible and low-cost devices (Park et al., 2009; Yu et al., 1995; Chang et al., 2009; Dridi et al., 2008; Peet et al., 2009; Oey et al., 2006; Yu et al., 2008). Polymer based thin films on flexible and non-flexible substrates can be achieved by various printing techniques (see Fig. 3) (screen printing, inkjet printing, offset printing, flexo printing and so on), solution based coating techniques (dipping, spin coating, doctor blading, spray coating, slot-die coating and so on), and electrospinning (Krebs, 2009a). Roll to roll, reel to reel, process is suitable for solar cells, which are on long flexible substrates (polymeric substrates and thin metal foils), and, which can be wound on a roll

performance in inverted solar cells (Hsieh et al., 2010).

possibilities (Dennler et al., 2006a; Dennler & Sariciftci, *2005*).

2010*).* 

Fig. 2. Bulk heterojunction configuration in organic solar cells (Günes et al., 2007)

ITO is the most commonly used transparent electrode due to its good transparency in the visible range and good electrical conductivity (Zou et al., 2010). However, ITO, which exhibits poor mechanical properties on polymer based substrates, has limited conductivity for fabricating large area solar cells and needs complicated techniques, which tend to increase the cost of the solar cells (Zou et al., 2010). Indium availability is also limited. To alleviate limitations arised from ITO, alternative materials are needed to replace transparent conducting electrode. There are some approaches such as using carbon nanotubes (CNTs) (Rowell et al., 2006; Glatthaar et al., 2005; (Celik) Bedeloglu et al., 2011; Dresselhaus et al., 2001), graphenes (Eda et al., 2008), different conductive polymers (i.e. PEDOT:PSS and its mixtures (Ouyang et al., 2005; Kushto et al., 2005; Huang et al., 2006; Ahlswede et al., 2008; Zhou et al., 2008), metallic grids (Tvingstedt & Inganäs, 2007; Kang et al., 2008), nanowires (Lee et al., 2008) for potential candidates to substitute ITO layer and to perform as hole collecting electrode. In particular, CNTs have a wide variety of application area due to their unique features in terms of thermal, mechanical and electrical properties (Ajayan, 1999; Baughman et al., 2002). A nanotube has a diameter of a few nanometers and from a few nanometers to centimeters in length. Carbon nanotubes can be classified into two groups according to the number of combinations that form their walls: Single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) (Wang et al., 2009) Recently, CNTs are used in solar cells and can substitute ITO as a transparent electrode in organic solar cells (Rowell et al., 2006; Glatthaar et al., 2005; (Celik) Bedeloglu et al., 2011; Dresselhaus et al., 2001).

In the organic solar cell*,* the photoactive layer, light absorbing layer, is formed by combination of electron donor (p) and an electron accepting (n) materials (Deibel & Dyakonov, 2010*)* C60, its derivatives and Perylen pigments are mostly used as electron

Fig. 2. Bulk heterojunction configuration in organic solar cells (Günes et al., 2007)

Glatthaar et al., 2005; (Celik) Bedeloglu et al., 2011; Dresselhaus et al., 2001).

In the organic solar cell*,* the photoactive layer, light absorbing layer, is formed by combination of electron donor (p) and an electron accepting (n) materials (Deibel & Dyakonov, 2010*)* C60, its derivatives and Perylen pigments are mostly used as electron

ITO is the most commonly used transparent electrode due to its good transparency in the visible range and good electrical conductivity (Zou et al., 2010). However, ITO, which exhibits poor mechanical properties on polymer based substrates, has limited conductivity for fabricating large area solar cells and needs complicated techniques, which tend to increase the cost of the solar cells (Zou et al., 2010). Indium availability is also limited. To alleviate limitations arised from ITO, alternative materials are needed to replace transparent conducting electrode. There are some approaches such as using carbon nanotubes (CNTs) (Rowell et al., 2006; Glatthaar et al., 2005; (Celik) Bedeloglu et al., 2011; Dresselhaus et al., 2001), graphenes (Eda et al., 2008), different conductive polymers (i.e. PEDOT:PSS and its mixtures (Ouyang et al., 2005; Kushto et al., 2005; Huang et al., 2006; Ahlswede et al., 2008; Zhou et al., 2008), metallic grids (Tvingstedt & Inganäs, 2007; Kang et al., 2008), nanowires (Lee et al., 2008) for potential candidates to substitute ITO layer and to perform as hole collecting electrode. In particular, CNTs have a wide variety of application area due to their unique features in terms of thermal, mechanical and electrical properties (Ajayan, 1999; Baughman et al., 2002). A nanotube has a diameter of a few nanometers and from a few nanometers to centimeters in length. Carbon nanotubes can be classified into two groups according to the number of combinations that form their walls: Single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) (Wang et al., 2009) Recently, CNTs are used in solar cells and can substitute ITO as a transparent electrode in organic solar cells (Rowell et al., 2006; accepting materials. Also, phtalocyanines, porphyrins, poly(3- hexylthiophene) (P3HT) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) are good donors (Nunzi*,2002).* Most of the time, evaporation step is indispensable in the manufacturing of conventional organic photovoltaic devices, but this process tends to increase the cost of the cell. Besides, ITO-PEDOT:PSS interface (de Jong et al., 2000) and Al top electrode (Do et al., 1994) is known to be quite unstable, which limits the lifetime of organic solar cells. Organic solar cells have low charge transport and a mismatch between exciton diffusion length and organic layer thickness. While the efficient absorption of light is provided by organic film based on P3HT:PCBM having a thickness over 250 nm (Liu et al., 2007a), exciton diffusion length of which surpassing causes exciton recombination*,* is about 10-20 nm in polymer based and in organic semiconductors (Nunzi*,* 2002). Although, organic solar cells have lower power conversion efficiency (~5%) than inorganic traditional solar cells (for crystalline silicon based solar cells ~25% in laboratory conditions); their cost and processing parameters are favorable (Deibel & Dyakonov, 2010*).* 

In order to avoid the limitations of organic semiconductor and to improve the power conversion efficiency of organic solar cells, several approaches such as optical concepts, different device configurations such as inverted layer architecture, multijunction solar cells, novel materials with lower band gap (Park et al., 2009; Chen et al., 2009; Huo et al., 2009; Coffin et al., 2009), wider absorption ranges, higher dielectric constants and higher charge carrier mobility are some approaches are studied in last few years (Deibel & Dyakonov, 2010). Reversing the nature of charge collection in organic solar cells using a less air sensitive high work function metal (Ag, Au) (de Jong et al., 2000; Do et al., 1994; Liu et al., 2007a; Park et al., 2009; Chen et al., 2009; Huo et al., 2009; Coffin et al., 2009; Wong et al., 2006) as hole collecting electrode at the back contact and a metal oxide (TiOx, ZnO) as hole blocking barrier and electron selective contact at the ITO interface to block the oxidation (Hau et al., 2008) is a beneficial approach to avoid from low power conversion efficiency, which limited absorption in solar spectrum causes. In particular, non-clorinated solvents are more appropriate for high volume manufacturing with low cost. Besides, active layer can be protected by use of metal oxides (i.e. vanadium oxide and cesium carbonate), which are used as buffer layer of inverted polymer solar cells (Li et al., 1997). However, there is still a trade-off between stability and photovoltaic performance in inverted solar cells (Hsieh et al., 2010).

Organic solar cells have many advantages such as potential to be semi-transparent, manufacture on both large or small areas compatible with mass production and low-cost, production possibility with continuous coating and printing processes on lightweight and flexible substrates (for example textiles), ecological and low-temperature production possibilities (Dennler et al., 2006a; Dennler & Sariciftci, *2005*).

Polymer heterojunction organic solar cells have attracted much attention because of their potential applications in large area, flexible and low-cost devices (Park et al., 2009; Yu et al., 1995; Chang et al., 2009; Dridi et al., 2008; Peet et al., 2009; Oey et al., 2006; Yu et al., 2008). Polymer based thin films on flexible and non-flexible substrates can be achieved by various printing techniques (see Fig. 3) (screen printing, inkjet printing, offset printing, flexo printing and so on), solution based coating techniques (dipping, spin coating, doctor blading, spray coating, slot-die coating and so on), and electrospinning (Krebs, 2009a).

Roll to roll, reel to reel, process is suitable for solar cells, which are on long flexible substrates (polymeric substrates and thin metal foils), and, which can be wound on a roll

Progress in Organic Photovoltaic Fibers Research 263

Fig. 4. Current-voltage (I-V) curves of an organic solar cell (dark, - - -; illuminated, -). The characteristic intersections with the abscissa and ordinate are the open circuit voltage (*V*oc)

determined by the point where the product of voltage and current is maximized. Division of

max *sc oc in in P I V FF P P*

Here, maximum power point is the point on the I-V curve where maximum power (Pmax) is

ratio of power output to power input. Isc is the short-circuit current, which flows through the cell when applied voltage is zero, under illumination. Under an external load, current will always be less than max. current value. Voc, the open-circuit voltage is the voltage when no current is flowing, under illumination. When current flows, voltage will be less than max. voltage value. FF, fill factor is the ratio of max. power output to the external the short-

> max *mpp mpp sc oc sc oc P I V*

> > 1240 *sc in I*

where Impp and Vmpp represent the current and the voltage at the maximum power point (Pmax) in the four quadrant, respectively (Nunzi*,2002;* Benanti & Venkataraman*,* 2006). The incident photon to collected electron (IPCE) or external quantum efficiency (EQE) under monochromatic lightning at a wavelength *λ* includes losses by reflection and transmission (Benanti & Venkataraman*,* 2006) and gives the ratio of collected charge carriers per incident

(1)

*IV IV* (2)

*<sup>P</sup>* (3)

) of a solar cell is defined as the

and the short circuit current (*I*sc), respectively. The largest power output (*P*max) is

*P*max by the product of *I*sc and *V*oc yields the fill factor *FF* (Günes et al., 2007).

circuit current and open-circuit voltage values. FF is given by following formula:

*IPCE*

*FF*

The overall efficiency of a solar cell can be expressed as follows :

produced. The photovoltaic power conversion efficiency (

photons (Dennler et al., 2006a):

(Krebs, 2009a). Various coating and printing techniques including knife-over-edge coating and slot die coating can be used for manufacturing flexible solar cells. The most appropriate processes for flexible photovoltaics should be free from Indium, toxic solvents and chemicals, and should have solution based manufacturing steps (coating and printing techniques), which results an environmentally recyclable product (Hoppe & Sariciftci, 2006). The studies about improving polymer solar cells (Günes et al., 2007; Hoppe & Sariciftci, 2006; Bundgaard & Krebs, 2007; Jørgensen et al., 2008; Thompson & Frechet, 2008; Tromholt et al., 2009) include developments in material properties and manufacturing techniques.

Fig. 3. Schematic description of printing process used for manufacturing of polymer-based photovoltaic cells. Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics, *Gaudiana,* R. & *Brabec,* C. (2008). Organic materials: Fantastic plastic. Vol.2, pp.287- 289, copyright (2008).
