**1. Introduction**

254 Solar Cells – New Aspects and Solutions

Yamamoto, K., Nakashima, A., Suzuki, T., Yoshimi, M., Nishio, H., & Izumina, M. (1994).

Zhou, X., Ishida, M., Imanishi, A., & Nakato, Y. (2001). *J. Phys. Chem. B*, Vol. 105, 156.

*Jpn. J. Appl. Phys.* Vol. 33, L1751.

Energy management including production, distribution and usage of energy is an important issue, which determines internal and external policy and economical situation of countries. For generating electrical energy, use of traditional energy sources in particular fossil based fuels through long ages, caused environmental problems, in recent years. Renewable energy technologies using power of wind, sun, water, etc. can be remedies to hinder negative effects of pollution, emissions of carbon dioxide and irreversible climate change problem, which it caused. Photovoltaic technology, which converts photons of the sun into electrical energy by using semiconductors, is one of the most environmental friendly sources of renewable energy (Dennler et al., 2006a). Solar cells are used in many different fields such as in solar lambs and calculators, on roofs and windows of buildings, satellites and space craft, textile structures (fibers, fabrics and garments) and accessories (bags and suitcases).

In addition, there is an increasing interest in organic electronics from a wide range of science disciplines in which researchers search for novel, efficient and functional materials and structures. Organic materials based optoelectronic devices such as organic photovoltaics (organic solar cells), organic light emitting diodes and organic photo detectors (Curran et al., 2009) are desirable in many applications due to interesting features of organic materials such as cost advantage and flexibility. Production of electrical energy, which is necessary in both industrial and human daily life by converting sunlight using organic solar cells (organic photovoltaic technology) via easy and inexpensive techniques is also very interesting (Günes et al., 2007).

A photovoltaic textile, which is formed by combining a textile structure with a solar cell, and on which carries physical properties of textile and working principle of solar cell together, can generate electricity for powering different electrical devices. Photovoltaic fiber providing more compatibility to textiles in terms of flexibility and lightness owing to its thin and polymer-based structure may be used in a wide variety of applications such as tents, jackets, soldier uniforms and marine fabrics. This review is organized as follows: In the first section, an overview of photovoltaic technology, smart textiles and photovoltaic textiles will be presented. In the second section, a general introduction to organic solar cells and organic semi conductors, features, the working principle, manufacturing techniques, and characterization of organic solar cells as well as polymer based organic solar cells and studies about nanofibers and flexible solar cells will be given. In the third part, recent studies about photovoltaic fiber researches, production methods, and materials used and

Progress in Organic Photovoltaic Fibers Research 257

1998). Intelligent materials and structures can sense and react and more, adapt it and perform a function of changes (Takagi, 1990; Tao, 2001).Intelligent material systems consist of three parts: a sensor, a processor and an actuator. Intelligent materials can provide advancements in many fields of science for energy generation, medical treatments, and

There are also many application areas for interactive textiles, which use intelligent materials such as shape memory alloys or polymers, phase change materials, conductive materials and etc. Intelligent textiles are defined as structures that are capable of sensing external and internal stimuli and respond or adapt to them in a pre-specified way. Knowledge from different scientific fields (biotechnology, microelectronics, nanotechnology and so on) is required for intelligent textile research (Mattila, 2006). Intelligent textiles find uses in many

Power supply by using discrete batteries is an important obstacle towards functionality of intelligent textiles. Besides, flexibility, comfort and durability are other parameters concerned to manufacture consistent products (Coyle & Diamond, 2010). Flexible solar cells (Schubert & Werner, 2006), micro fuel cells (Gunter et al., 2007; enfucell, 2011), power generation by body motion and body heat (Beeby, 2010; Starner, 1996) can be some alternatives to the traditional batteries*.* Photovoltaic fibers and textiles can overcome this

Small electronic devices such as personal digital assistants, mobile phones, mp3 players, and notebook computers, usually use traditional batteries of which energy is used up in a short time. Integration of flexible solar cells into apparels and fabrics, which cells are positioned in/on the textile, can provide required electrical energy for these portable electronic devices (Schubert & Werner, 2006). Photovoltaic textiles can be formed by integrating solar cells into textile structure or making textile structure itself from photovoltaic materials. Photovoltaic textile research needs cooperation of different sciences consisting of textile, electronics, physics and chemistry. Incorporation of solar cells with fibers and textiles that are flexible can extend the applications of photovoltaics from military and space applications to lighting

Textile based solar cells are also named as photovoltaic textiles, solar textiles, energy harvesting textiles, solar powered textiles in the literature. Photovoltaic textiles, which are high value added intelligent products, and, which have a large application area, can benefit

Power conversion efficiency and price properties beside the flexibility, weight, comfort, durability and washability properties of the products are also important from a customer point of view. Position of the flexible solar cells on fabric is also important to take efficient irradiation from the light source. Places of needed wires, controllers and batteries, which have to be lightweight under the cloth, are needed to be concerned to develop viable

In recent years, there has been an increase in studies about developing photovoltaic fibers which can take charge in different textile and clothing applications. An active photovoltaic fiber, which is produced by using advanced design and suitable materials, and, which consist of adequate smooth layers, efficiency and stability, is capable of forming a flexible fabric by suitable knitting or weaving techniques, or integrating as a yarn into a cloth to

applications ranging from space to healthcare and entertainment.

power supply problem since they use the working principle of solar cells.

and providing power for consumer electronics of humankind in daily usage.

textile industry by increasing its competitiveness with long term development.

generate power for electronic devices by converting sunlight (DeCristofano, 2009)

photovoltaic textiles (Schubert & Werner, 2006).

engineering applications and so on.

**1.3 Photovoltaic textiles** 

application areas will be recounted. Finally, suggestions on future studies and the conclusions will be given.

### **1.1 Photovoltaic technology**

"Photovoltaic" is a marriage of two words: "photo", which means light, and "voltaic", which means electricity. Electrical energy produced by solar cells is one of the most promising sustainable alternative energy and can provide energy demand of the world, in the future (Green, 2005). Today, silicon based solar cells having the highest power conversion efficiency are dominated in the market; however they have still high production costs. Electricity generation by solar cells is still more expensive than that of fossil fuels due to materials and manufacturing processes used in solar cells and installation costs. However, photovoltaic technology, compared to traditional energy production technologies, have interesting features such as using endless and abundant source of sun's energy, direct, environmental friendly and noiseless energy generation without the need of additional generators, customization according to requirements, having low maintenance costs and portable modules producing power ranging between milliwatt to megawatts even in remote areas, which make it unique (Dennler et al., 2006a). A photovoltaic system can convert sun light into electricity on both sunny and cloudy days (European PhotoVoltaic Industry Association (EPIA), 2009). The worldwide cumulative photovoltaic power installed reached about 23GW, in the beginning of 2010 and produces about 25TWh of electricity on a yearly basis (European PhotoVoltaic Industry Association (EPIA), 2010).

The electricity produced by solar cells can be utilized in many applications such as cooling, heating, lighting, charging of batteries and providing power for different electrical devices (Curran et al., 2009). Solar cells using silicon wafers are classified as first generation technology having high areal production costs and moderate efficiency. Thin-film solar cells using Amorphous silicon (a-Si), Cadmium telluride (CdTe) and Copper indium gallium (di)selenide (CIGS) as second generation technology have advantages such as increased size of the unit of manufacturing and reduction in material costs. However, this technology has modest efficiency beside these advantages compared to first generation technology. Therefore, third generation technology concept has been developed to eliminate disadvantages of earlier photovoltaic technologies (Green, 2005). There are two approaches in third generation photovoltaic technology. The first one aims to achieve very high efficiencies and second one tries to achieve cost per watt balance via moderate efficiency at low cost. Therefore, this uses inexpensive semiconductor materials and solutions at low temperature manufacturing processes. The third generation photovoltaics use various technologies and grouped under organic solar cells (Dennler et al., 2006a).

### **1.2 Smart textiles**

Humankind has always been inspired to mimic intelligence of nature to create novel materials and structures with fascinating functions. Over the last decades, in industrial and daily life, paralleling to growth in world population and advancements in science and technology, human requirements have changed and begun to diverge from each other. Therefore, different functional products have emerged according to expectations and requirements of human kind. One of these, intelligent materials, can coordinate their characteristic behavior according to changes of external or internal stimulus (chemical, mechanical, thermal, magnetic, electrical and so on) as in biological systems and have different functions owing to their unique molecular structure (Mattila, 2006; Tani et al.,

application areas will be recounted. Finally, suggestions on future studies and the

"Photovoltaic" is a marriage of two words: "photo", which means light, and "voltaic", which means electricity. Electrical energy produced by solar cells is one of the most promising sustainable alternative energy and can provide energy demand of the world, in the future (Green, 2005). Today, silicon based solar cells having the highest power conversion efficiency are dominated in the market; however they have still high production costs. Electricity generation by solar cells is still more expensive than that of fossil fuels due to materials and manufacturing processes used in solar cells and installation costs. However, photovoltaic technology, compared to traditional energy production technologies, have interesting features such as using endless and abundant source of sun's energy, direct, environmental friendly and noiseless energy generation without the need of additional generators, customization according to requirements, having low maintenance costs and portable modules producing power ranging between milliwatt to megawatts even in remote areas, which make it unique (Dennler et al., 2006a). A photovoltaic system can convert sun light into electricity on both sunny and cloudy days (European PhotoVoltaic Industry Association (EPIA), 2009). The worldwide cumulative photovoltaic power installed reached about 23GW, in the beginning of 2010 and produces about 25TWh of electricity on a yearly

The electricity produced by solar cells can be utilized in many applications such as cooling, heating, lighting, charging of batteries and providing power for different electrical devices (Curran et al., 2009). Solar cells using silicon wafers are classified as first generation technology having high areal production costs and moderate efficiency. Thin-film solar cells using Amorphous silicon (a-Si), Cadmium telluride (CdTe) and Copper indium gallium (di)selenide (CIGS) as second generation technology have advantages such as increased size of the unit of manufacturing and reduction in material costs. However, this technology has modest efficiency beside these advantages compared to first generation technology. Therefore, third generation technology concept has been developed to eliminate disadvantages of earlier photovoltaic technologies (Green, 2005). There are two approaches in third generation photovoltaic technology. The first one aims to achieve very high efficiencies and second one tries to achieve cost per watt balance via moderate efficiency at low cost. Therefore, this uses inexpensive semiconductor materials and solutions at low temperature manufacturing processes. The third generation photovoltaics use various

Humankind has always been inspired to mimic intelligence of nature to create novel materials and structures with fascinating functions. Over the last decades, in industrial and daily life, paralleling to growth in world population and advancements in science and technology, human requirements have changed and begun to diverge from each other. Therefore, different functional products have emerged according to expectations and requirements of human kind. One of these, intelligent materials, can coordinate their characteristic behavior according to changes of external or internal stimulus (chemical, mechanical, thermal, magnetic, electrical and so on) as in biological systems and have different functions owing to their unique molecular structure (Mattila, 2006; Tani et al.,

basis (European PhotoVoltaic Industry Association (EPIA), 2010).

technologies and grouped under organic solar cells (Dennler et al., 2006a).

conclusions will be given.

**1.2 Smart textiles** 

**1.1 Photovoltaic technology** 

1998). Intelligent materials and structures can sense and react and more, adapt it and perform a function of changes (Takagi, 1990; Tao, 2001).Intelligent material systems consist of three parts: a sensor, a processor and an actuator. Intelligent materials can provide advancements in many fields of science for energy generation, medical treatments, and engineering applications and so on.

There are also many application areas for interactive textiles, which use intelligent materials such as shape memory alloys or polymers, phase change materials, conductive materials and etc. Intelligent textiles are defined as structures that are capable of sensing external and internal stimuli and respond or adapt to them in a pre-specified way. Knowledge from different scientific fields (biotechnology, microelectronics, nanotechnology and so on) is required for intelligent textile research (Mattila, 2006). Intelligent textiles find uses in many applications ranging from space to healthcare and entertainment.

Power supply by using discrete batteries is an important obstacle towards functionality of intelligent textiles. Besides, flexibility, comfort and durability are other parameters concerned to manufacture consistent products (Coyle & Diamond, 2010). Flexible solar cells (Schubert & Werner, 2006), micro fuel cells (Gunter et al., 2007; enfucell, 2011), power generation by body motion and body heat (Beeby, 2010; Starner, 1996) can be some alternatives to the traditional batteries*.* Photovoltaic fibers and textiles can overcome this power supply problem since they use the working principle of solar cells.

### **1.3 Photovoltaic textiles**

Small electronic devices such as personal digital assistants, mobile phones, mp3 players, and notebook computers, usually use traditional batteries of which energy is used up in a short time. Integration of flexible solar cells into apparels and fabrics, which cells are positioned in/on the textile, can provide required electrical energy for these portable electronic devices (Schubert & Werner, 2006). Photovoltaic textiles can be formed by integrating solar cells into textile structure or making textile structure itself from photovoltaic materials. Photovoltaic textile research needs cooperation of different sciences consisting of textile, electronics, physics and chemistry. Incorporation of solar cells with fibers and textiles that are flexible can extend the applications of photovoltaics from military and space applications to lighting and providing power for consumer electronics of humankind in daily usage.

Textile based solar cells are also named as photovoltaic textiles, solar textiles, energy harvesting textiles, solar powered textiles in the literature. Photovoltaic textiles, which are high value added intelligent products, and, which have a large application area, can benefit textile industry by increasing its competitiveness with long term development.

Power conversion efficiency and price properties beside the flexibility, weight, comfort, durability and washability properties of the products are also important from a customer point of view. Position of the flexible solar cells on fabric is also important to take efficient irradiation from the light source. Places of needed wires, controllers and batteries, which have to be lightweight under the cloth, are needed to be concerned to develop viable photovoltaic textiles (Schubert & Werner, 2006).

In recent years, there has been an increase in studies about developing photovoltaic fibers which can take charge in different textile and clothing applications. An active photovoltaic fiber, which is produced by using advanced design and suitable materials, and, which consist of adequate smooth layers, efficiency and stability, is capable of forming a flexible fabric by suitable knitting or weaving techniques, or integrating as a yarn into a cloth to generate power for electronic devices by converting sunlight (DeCristofano, 2009)

Progress in Organic Photovoltaic Fibers Research 259

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

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.

acceptor) by thermal evaporation techniques (Deibel & Dyakonov, 2010*).* 

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 fabric, so higher power conversion efficiency can be obtained.

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 & Werner, 2006).

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 fibers to fabrics and using low-cost methods.
