**2.3 Characterization of organic solar cells**

Characterization of organic solar cells performed by measuring efficiencies in the dark and under an illumination intensity of 1000 W/m2 (global AM1.5 spectrum) at 25ºC (IEC 60904- 3: 2008, ASTM G-173-03 global) (Green et al., 2010). Generally a solar simulator is used as illumination source for simulating AM1.5 conditions. Air Mass (AM) is a measure of how much atmosphere sunlight must travel through to reach the earth's surface. AM1.5 means that the sun is at an angle about 48º (Benanti & Venkataraman*,* 2006)*.* A graph (Fig. 4) on which shows the current-voltage characteristics in the dark and under an illumination, gives significant information about photovoltaic performance and photoelectrical behavior of the cells (Nunzi*,* 2002).

(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

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.

Characterization of organic solar cells performed by measuring efficiencies in the dark and under an illumination intensity of 1000 W/m2 (global AM1.5 spectrum) at 25ºC (IEC 60904- 3: 2008, ASTM G-173-03 global) (Green et al., 2010). Generally a solar simulator is used as illumination source for simulating AM1.5 conditions. Air Mass (AM) is a measure of how much atmosphere sunlight must travel through to reach the earth's surface. AM1.5 means that the sun is at an angle about 48º (Benanti & Venkataraman*,* 2006)*.* A graph (Fig. 4) on which shows the current-voltage characteristics in the dark and under an illumination, gives significant information about photovoltaic performance and photoelectrical behavior of the

techniques.

Vol.2, pp.287- 289, copyright (2008).

cells (Nunzi*,* 2002).

**2.3 Characterization of organic solar cells** 

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) and the short circuit current (*I*sc), respectively. The largest power output (*P*max) is determined by the point where the product of voltage and current is maximized. Division of *P*max by the product of *I*sc and *V*oc yields the fill factor *FF* (Günes et al., 2007).

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

$$\eta = \frac{P\_{\text{max}}}{P\_{\text{in}}} = \frac{I\_{\text{sc}} \ V\_{\text{oc}} \ FF}{P\_{\text{in}}} \tag{1}$$

Here, maximum power point is the point on the I-V curve where maximum power (Pmax) is produced. The photovoltaic power conversion efficiency ( ) of a solar cell is defined as the 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 shortcircuit current and open-circuit voltage values. FF is given by following formula:

$$FF = \frac{P\_{\text{max}}}{I\_{\text{sc}}} = \frac{I\_{mpp}\ V\_{mpp}}{I\_{\text{sc}}\ V\_{\text{oc}}} \tag{2}$$

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 photons (Dennler et al., 2006a):

$$IPCE = \frac{1240 \ I\_{sc}}{\mathcal{A} \ P\_{in}} \tag{3}$$

Progress in Organic Photovoltaic Fibers Research 265

Fig. 5. Current density-voltage characteristics of P3HT:PCBM devices under AM1.5G conditions using ITO on glass (open circles) and flexible SWNTs on PET (solid squares) as the anodes, respectively. Inset: Schematic of device and photograph of the highly flexible cell using SWNTs on PET. Reprinted with permission from Rowell*,* M.W.; Topinka*,* M.A.; McGehee*,* M.D.; Prall, H.J.; Dennler, G.; Sariciftci, N.S.; Hu, L. & Gruner, G. (*2006*). Organic Solar Cells with Carbon Nanotube Network Electrodes. *Applied Physics Letters,* Vol.88*,* pp.

An inverted layer sequence (see Fig. 6) was used (Zimmermann et al., 2007) in an ITO-free wrap through approach of which device configuration included PET/ AL-Ti/ Absorber (P3HT:PCBM)/ PEDOT:PSS/Au layer sequence. Thermal evaporation, e-beam evaporation and spin coating techniques were used for device fabrication on flexible substrate. Researchers obtained a power conversion efficiency of 1.1% (under 1000W/m2 AM 1.5) from the device with additional serial circuitry, which employed top illumination by avoiding the use of ITO.

Fig. 6. Comparison of the widely used layer sequence on ITO/PEDOT:PSS electrode (left) and inverted layer sequence (right), where the ITO is replaced by a metal grid for small area devices. Reprinted from Sol. Energy Mater. Sol. Cells*,* 91, Zimmermann, B.; Glatthaar, M.; Niggemann, M.; Riede, M. K.; Hinsch, A. & Gombert, A., ITO-free wrap through organic solar cells–A module concept for cost- efficient reel-to-reel production., 374- 378, Copyright

The commercially available ITO-coated PET foils are used mostly in studies about flexible organic solar cells. PET layer is as polymeric substrate and ITO is the transparent

233506. Copyright 2006, American Institute of Physics.

(2007), with permission from Elsevier.

conducting electrode of photovoltaic device.

### **2.4 Flexible organic solar cells**

Solar cells generally developed on rigid substrates like glass and suffer from heavy, fragile and inflexible devices. However, stiff substrates limit usage, storage and transport of photovoltaic devices. Therefore, a big interest from both industrial and academic sides has been observed for research and development of flexible (foldable or rollable) solar cells, recently. Organic solar cells, easy scalable and suitable to roll-to-roll production with low-cost have potential to be used with flexible substrates such as textiles and fibers. Materials used in organic solar cells are also capable of producing lightweight photovoltaics. Polymer based substrates, which are used to replace rigid substrates and which have adequate flexibility are required to have mechanically and chemically stable, while organic solar cell manufacturing processes continue. Optimum substrate should have some features such as resistance to effects of chemical materials, water and air and also, mechanical robustness, low coefficients of thermal expansion, anti-permeability and smooth surface properties and so on.

Polyethylene terephtalate (PET), ITO coated PET, Poly(ethylene naphthalate) (PEN), Polyimide (PI), Kapton and Polyethersulphone (PES) are used as substrates to develop flexible solar cells.

Polyethylene terephtalate (PET) based fibers, which melt about 260ºC show good stability to UV light and most of the chemicals and exhibit good mechanical properties including flexibility and comfort ability both in fiber and fabric form (Mather *&* Wilson*, 2006*). However, PET foils are often used as substrate of the flexible photovoltaics (Breeze, et al., 2002; Aernouts et al., 2004; Winther-Jensen & Krebs, 2006; Krebs, 2009b). Manufacturing of photovoltaics using PET substrates is suitable for reel to reel production, which reduces material and production costs. PET foils are preferable materials for solar cells due to their price, mechanical flexibility and easy availability comparing to other substrates. However, use of PET foils is limited because it melts about 140 ºC (Zimmermann et al., 2007; Krebs, 2009a; Krebs, 2009b). Thermocleavable materials are used for the preparation of very stable solar cells (Liu et al., 2004; Krebs & Spanggaard 2005; Krebs & Norrman, 2007; Krebs, 2008; Bjerring et al., 2008; Krebs et al., 200*8*) but, these materials are required to heat to a temperature of around 200ºC to achieve insolubility, which is a limitation to use of PET foils in conventional methods. However, PET substrates can be used with thermocleavable materials thanks to longer processing time (Krebs, 2009b).

Spin coating and screen printing techniques are used to coat highly conductive PEDOT:PSS dispersions onto flexible PET substrates as anode, which also improves an application of a metallic silver(Ag)-grid deposited between foil and electrode (Aernouts et al., 2004). A silk screen printing procedure (Winther-Jensen & Krebs, 2006) can be applied to develop PEDOT electrode with surface resistances down to 20 -1 on flexible PET substrates. Researchers obtained 0.7 V open circuit voltage, 1 mA/cm2 short circuit current and 0.2%, efficiency under simulated sun light (AM1.5 at 1000 W m-2) with an active area of 4.2 cm2 based on MEH-PPV:PCBM mixture and Al counter electrode.

In recent years, carbon nanotubes have found a wide variety of applications in photovoltaics. Films of SWNT networks can be printed on PET foils to get flexible transparent conducting electrodes. The well dispersed and stable solutions of SWNTs can be produced as electrode of flexible polymer based solar cells with various methods, which are inexpensive, scalable to large areas, and allows for the transfer of the film to a variety of surfaces. Such a flexible photovoltaic device configuration (Rowell et al., 2006) (see Fig. 5) (PET/SWNTs/PEDOT:PSS/P3HT:PCBM/Al) gave 2.5% efficiency of which efficiency is very close to conventional ITO coated glass based rigid solar cells.

Solar cells generally developed on rigid substrates like glass and suffer from heavy, fragile and inflexible devices. However, stiff substrates limit usage, storage and transport of photovoltaic devices. Therefore, a big interest from both industrial and academic sides has been observed for research and development of flexible (foldable or rollable) solar cells, recently. Organic solar cells, easy scalable and suitable to roll-to-roll production with low-cost have potential to be used with flexible substrates such as textiles and fibers. Materials used in organic solar cells are also capable of producing lightweight photovoltaics. Polymer based substrates, which are used to replace rigid substrates and which have adequate flexibility are required to have mechanically and chemically stable, while organic solar cell manufacturing processes continue. Optimum substrate should have some features such as resistance to effects of chemical materials, water and air and also, mechanical robustness, low coefficients of thermal

Polyethylene terephtalate (PET), ITO coated PET, Poly(ethylene naphthalate) (PEN), Polyimide (PI), Kapton and Polyethersulphone (PES) are used as substrates to develop

Polyethylene terephtalate (PET) based fibers, which melt about 260ºC show good stability to UV light and most of the chemicals and exhibit good mechanical properties including flexibility and comfort ability both in fiber and fabric form (Mather *&* Wilson*, 2006*). However, PET foils are often used as substrate of the flexible photovoltaics (Breeze, et al., 2002; Aernouts et al., 2004; Winther-Jensen & Krebs, 2006; Krebs, 2009b). Manufacturing of photovoltaics using PET substrates is suitable for reel to reel production, which reduces material and production costs. PET foils are preferable materials for solar cells due to their price, mechanical flexibility and easy availability comparing to other substrates. However, use of PET foils is limited because it melts about 140 ºC (Zimmermann et al., 2007; Krebs, 2009a; Krebs, 2009b). Thermocleavable materials are used for the preparation of very stable solar cells (Liu et al., 2004; Krebs & Spanggaard 2005; Krebs & Norrman, 2007; Krebs, 2008; Bjerring et al., 2008; Krebs et al., 200*8*) but, these materials are required to heat to a temperature of around 200ºC to achieve insolubility, which is a limitation to use of PET foils in conventional methods. However, PET substrates can be used with thermocleavable

Spin coating and screen printing techniques are used to coat highly conductive PEDOT:PSS dispersions onto flexible PET substrates as anode, which also improves an application of a metallic silver(Ag)-grid deposited between foil and electrode (Aernouts et al., 2004). A silk screen printing procedure (Winther-Jensen & Krebs, 2006) can be applied to develop PEDOT electrode with surface resistances down to 20 -1 on flexible PET substrates. Researchers obtained 0.7 V open circuit voltage, 1 mA/cm2 short circuit current and 0.2%, efficiency under simulated sun light (AM1.5 at 1000 W m-2) with an active area of 4.2 cm2 based on

In recent years, carbon nanotubes have found a wide variety of applications in photovoltaics. Films of SWNT networks can be printed on PET foils to get flexible transparent conducting electrodes. The well dispersed and stable solutions of SWNTs can be produced as electrode of flexible polymer based solar cells with various methods, which are inexpensive, scalable to large areas, and allows for the transfer of the film to a variety of surfaces. Such a flexible photovoltaic device configuration (Rowell et al., 2006) (see Fig. 5) (PET/SWNTs/PEDOT:PSS/P3HT:PCBM/Al) gave 2.5% efficiency of which efficiency is

expansion, anti-permeability and smooth surface properties and so on.

materials thanks to longer processing time (Krebs, 2009b).

MEH-PPV:PCBM mixture and Al counter electrode.

very close to conventional ITO coated glass based rigid solar cells.

**2.4 Flexible organic solar cells** 

flexible solar cells.

Fig. 5. Current density-voltage characteristics of P3HT:PCBM devices under AM1.5G conditions using ITO on glass (open circles) and flexible SWNTs on PET (solid squares) as the anodes, respectively. Inset: Schematic of device and photograph of the highly flexible cell using SWNTs on PET. Reprinted with permission from Rowell*,* M.W.; Topinka*,* M.A.; McGehee*,* M.D.; Prall, H.J.; Dennler, G.; Sariciftci, N.S.; Hu, L. & Gruner, G. (*2006*). Organic Solar Cells with Carbon Nanotube Network Electrodes. *Applied Physics Letters,* Vol.88*,* pp. 233506. Copyright 2006, American Institute of Physics.

An inverted layer sequence (see Fig. 6) was used (Zimmermann et al., 2007) in an ITO-free wrap through approach of which device configuration included PET/ AL-Ti/ Absorber (P3HT:PCBM)/ PEDOT:PSS/Au layer sequence. Thermal evaporation, e-beam evaporation and spin coating techniques were used for device fabrication on flexible substrate. Researchers obtained a power conversion efficiency of 1.1% (under 1000W/m2 AM 1.5) from the device with additional serial circuitry, which employed top illumination by avoiding the use of ITO.

Fig. 6. Comparison of the widely used layer sequence on ITO/PEDOT:PSS electrode (left) and inverted layer sequence (right), where the ITO is replaced by a metal grid for small area devices. Reprinted from Sol. Energy Mater. Sol. Cells*,* 91, Zimmermann, B.; Glatthaar, M.; Niggemann, M.; Riede, M. K.; Hinsch, A. & Gombert, A., ITO-free wrap through organic solar cells–A module concept for cost- efficient reel-to-reel production., 374- 378, Copyright (2007), with permission from Elsevier.

The commercially available ITO-coated PET foils are used mostly in studies about flexible organic solar cells. PET layer is as polymeric substrate and ITO is the transparent conducting electrode of photovoltaic device.

Progress in Organic Photovoltaic Fibers Research 267

coated PET substrates. zinc oxide nanoparticles (ZnO-nps) were applied using either knifeover- edge coating or slot-die coating. A mixture of the thermocleavable poly-(3-(2 methylhexan-2-yl)- oxy-carbonyldithiophene)(P3MHOCT) and ZnO-nps was applied by a modified slot-die coating procedure as second layer. The third layer was patterned into stripes and juxtaposed with the ITO layer. The fourth layer comprised screen-printed or slot-diecoated PEDOT:PSS and the fifth and the final layer comprised a screen-printed or slot-diecoated silver electrode. Coating ITO onto the PET substrate by sputtering process in a vacuum, cost of ITO and thermal disadvantage of PET foils (temperatures only up to 140°C) were some implications of the research. Also, efficient inverted polymer solar cell fabricated by roll-to-roll (R2R) process could be obtained in terms of both power conversion efficiency and operational stability. Maximum 1.7% efficiency for the active area of the full module was obtained from eight serially connected cells (Krebs et al., 2009a). They (Krebs et al., 2009b) showed the versatility of the polymer solar cell technology with abstract forms for the active area, a flexible substrate, processing entirely from solution, complete processing in air using commonly available screen printing, and finally, simple mechanical encapsulation using a flexible packaging material and electrical contacting post-production using crimped contacts.

PET/ITO/ZnO/P3CT/ZnO/PEDOT:PSS/Ag paste/Cold laminated PET with acrylic resin

PET/ITO/ZnO/P3CT/PCBM/ZnO/PEDOT:PSS/Ag paste/Cold laminated PET with

Poly(ethylene naphthalate) (PEN), has higher glass transition temperature than PET and this provides potential post-treatment of devices (Dennler et al., 2006b). However, shrinkage is seen in the material and so, subsequent processes will be problematic (Krebs, 2009b). PEN substrates (Dennler et al., 2006b) were used to develop flexible solar cells and were coated with ultra-high barrier multilayer coatings (Fig. 7). Shelf lifetime of conjugated polymer:fullerene

Fig. 7. (a) Cross-sectional view of the conjugated polymer:fullerene solar cells investigated here; (b) picture of a bent device. Reprinted from Thin Solid Films, 511–512, Dennler*,* G*.;*  Lungenschmied*,* C*.;* Neugebauer*,* H*.;* Sariciftci*,* N*. S.;* Latreche*,* M.; Czeremuszkin, *G*. & Wertheimer, M. R., A new encapsulation solution for flexible organic solar cells, *349*–353,

Following two different devices were developed:

Copyright (2006), with permission from Elsevier.

and

acrylic resin

Researchers (Brabec et al., 1999) performed efficiency and stability studies on large area (6 cm x 6 cm) flexible solar cells based on MDMO-PPV and PCBM materials and compared them with small area devices. Thin films were produced on two different substrates including ITO coated glass substrates and ITO coated PET foils with different active areas. The overall conversion efficiency of the flexible plastic solar cell is calculated with app. 1,2 % and a filling factor FF - 0.35 under monochromatic illumination (488 nm) with 10 mW/cm2. It is possible to produce organic solar cells on flexible substrates without loosing efficiency, whereas fullerene bulk heterojunctions was still limited by charge transport. Al-Ibrahim et al. (Al-Ibrahim et al., 2005) developed photovoltaic devices based on P3HT and PCBM materials on ITO coated polyester foils with an active cell area of 0.5×0.5 cm2 with the following photovoltaic device configuration: PET/ITO/PEDOT:PSS/P3HT:PCBM/Al. Device parameters without any special postproduction treatment were obtained as: VOC = 600 mV, ISC = 6.61 mA/cm2, FF=0.39 and η=1.54% under irradiation with white light (AM1.5, 100mW/cm2). These results were hopeful for device up-scaling and development of processing technologies for reel to reel production of flexible organic photovoltaic devices.

Different oligothiophene materials are used to develop (Liu et al., 2008) flexible organic photovoltaic devices on ITO-coated PET films. The organic layers (5-formyl-2,2′:5′,2″:5″,2"" quaterthiophene (4T-CHO), 5-formyl-2,2′:5′, 2″:5″,2"":5"",2""-quinquethiophene (5T-CHO) and 3,4,9,10-perylenetertracarboxylic dianhydride (PTCDA)) were deposited by vacuum deposition. While the PET-ITO/4T-CHO/PTCDA/Al device showed an open circuit voltage (Voc) of 1.56 V and a photoelectric conversion efficiency of 0.77%, the PET-ITO/5T-CHO/PTCDA/Al device exhibited a Voc of 1.70 V and photoelectric conversion efficiency of 0.84%. Stakhira et al. (2008) fabricated an organic solar cell consisting of an ITO/PEDOT:PSS/ pentacene (Pc)/Al multilayer structure on flexible PET substrate coated with conductive ITO layer. PEDOT:PSS/Pc and Al contact were formed by electron beam deposition technique. The photovoltaic effect was measured with open circuit voltage of ~0.5 V, short circuit current of 0.6 lA and fill factor 0.2. Researchers (Blankenburg et al., 2009) used continuous reel-to-reel (R2R) slot die coating process to develop polymer based solar cells on plastic foils with adjustable coating thicknesses. Transparent conducting and photoactive layers were prepared with good reproducibility and promising power conversion efficiencies (0.5–1% (1.7% as maximum value)).

Krebs et al. (2007) fabricated organic solar cells on ITO coated PET substrates. Active layer consisted of MEH-PPV was coated by screen-printing method and an optional layer of fullerene (C60) and the final Al electrode were applied by vacuum coatings. Thirteen individual solar cells with an active area of 7.2 cm2 were connected in series. In the simple geometry ITO/MEH-PPV/Al the module gave a *V*oc of 10.5 V, an *I*sc of 5 A, a FF of 13% and an efficiency (*η*) of 0.00001% under AM1.5 illumination with an incident light intensity. A geometry (ITO/MEH-PPV/C60/Al) employing a sublimed layer of C60 improved *V*oc, *I*sc, FF and *η* to 3.6V, 178 A, 19% and 0.0002%, respectively. The results of roll-to-roll coated flexible large-area polymer solar-cell modules (eight serially connected stripes), which was performed in 18 different laboratories in Northern America, Europe and Middle East, were presented in another study (Krebs et al., 2009c). In all steps, roll-to-roll processing was employed. A zinc oxide nanoparticle layer, P3HT-PCBM and PEDOT:PSS layers were coated onto ITO coated PET by a modified slot-die coating procedure, respectively. ZnO as buffer layer has high electron mobility compared to titanium oxide (Yip et al., 2008) and so, can be ideal electron selective contact layer in polymer solar cells (Hau et al., 2008). The devices were completed by screen-printing silver paste and lamination of PET protective layer on top. In another study of Krebs (2009c) they prepared polymer solar cell module using all-solution processing on ITO

Researchers (Brabec et al., 1999) performed efficiency and stability studies on large area (6 cm x 6 cm) flexible solar cells based on MDMO-PPV and PCBM materials and compared them with small area devices. Thin films were produced on two different substrates including ITO coated glass substrates and ITO coated PET foils with different active areas. The overall conversion efficiency of the flexible plastic solar cell is calculated with app. 1,2 % and a filling factor FF - 0.35 under monochromatic illumination (488 nm) with 10 mW/cm2. It is possible to produce organic solar cells on flexible substrates without loosing efficiency, whereas fullerene bulk heterojunctions was still limited by charge transport. Al-Ibrahim et al. (Al-Ibrahim et al., 2005) developed photovoltaic devices based on P3HT and PCBM materials on ITO coated polyester foils with an active cell area of 0.5×0.5 cm2 with the following photovoltaic device configuration: PET/ITO/PEDOT:PSS/P3HT:PCBM/Al. Device parameters without any special postproduction treatment were obtained as: VOC = 600 mV, ISC = 6.61 mA/cm2, FF=0.39 and η=1.54% under irradiation with white light (AM1.5, 100mW/cm2). These results were hopeful for device up-scaling and development of processing technologies for reel to reel

Different oligothiophene materials are used to develop (Liu et al., 2008) flexible organic photovoltaic devices on ITO-coated PET films. The organic layers (5-formyl-2,2′:5′,2″:5″,2"" quaterthiophene (4T-CHO), 5-formyl-2,2′:5′, 2″:5″,2"":5"",2""-quinquethiophene (5T-CHO) and 3,4,9,10-perylenetertracarboxylic dianhydride (PTCDA)) were deposited by vacuum deposition. While the PET-ITO/4T-CHO/PTCDA/Al device showed an open circuit voltage (Voc) of 1.56 V and a photoelectric conversion efficiency of 0.77%, the PET-ITO/5T-CHO/PTCDA/Al device exhibited a Voc of 1.70 V and photoelectric conversion efficiency of 0.84%. Stakhira et al. (2008) fabricated an organic solar cell consisting of an ITO/PEDOT:PSS/ pentacene (Pc)/Al multilayer structure on flexible PET substrate coated with conductive ITO layer. PEDOT:PSS/Pc and Al contact were formed by electron beam deposition technique. The photovoltaic effect was measured with open circuit voltage of ~0.5 V, short circuit current of 0.6 lA and fill factor 0.2. Researchers (Blankenburg et al., 2009) used continuous reel-to-reel (R2R) slot die coating process to develop polymer based solar cells on plastic foils with adjustable coating thicknesses. Transparent conducting and photoactive layers were prepared with good reproducibility and promising power

Krebs et al. (2007) fabricated organic solar cells on ITO coated PET substrates. Active layer consisted of MEH-PPV was coated by screen-printing method and an optional layer of fullerene (C60) and the final Al electrode were applied by vacuum coatings. Thirteen individual solar cells with an active area of 7.2 cm2 were connected in series. In the simple geometry ITO/MEH-PPV/Al the module gave a *V*oc of 10.5 V, an *I*sc of 5 A, a FF of 13% and an efficiency (*η*) of 0.00001% under AM1.5 illumination with an incident light intensity. A geometry (ITO/MEH-PPV/C60/Al) employing a sublimed layer of C60 improved *V*oc, *I*sc, FF and *η* to 3.6V, 178 A, 19% and 0.0002%, respectively. The results of roll-to-roll coated flexible large-area polymer solar-cell modules (eight serially connected stripes), which was performed in 18 different laboratories in Northern America, Europe and Middle East, were presented in another study (Krebs et al., 2009c). In all steps, roll-to-roll processing was employed. A zinc oxide nanoparticle layer, P3HT-PCBM and PEDOT:PSS layers were coated onto ITO coated PET by a modified slot-die coating procedure, respectively. ZnO as buffer layer has high electron mobility compared to titanium oxide (Yip et al., 2008) and so, can be ideal electron selective contact layer in polymer solar cells (Hau et al., 2008). The devices were completed by screen-printing silver paste and lamination of PET protective layer on top. In another study of Krebs (2009c) they prepared polymer solar cell module using all-solution processing on ITO

production of flexible organic photovoltaic devices.

conversion efficiencies (0.5–1% (1.7% as maximum value)).

coated PET substrates. zinc oxide nanoparticles (ZnO-nps) were applied using either knifeover- edge coating or slot-die coating. A mixture of the thermocleavable poly-(3-(2 methylhexan-2-yl)- oxy-carbonyldithiophene)(P3MHOCT) and ZnO-nps was applied by a modified slot-die coating procedure as second layer. The third layer was patterned into stripes and juxtaposed with the ITO layer. The fourth layer comprised screen-printed or slot-diecoated PEDOT:PSS and the fifth and the final layer comprised a screen-printed or slot-diecoated silver electrode. Coating ITO onto the PET substrate by sputtering process in a vacuum, cost of ITO and thermal disadvantage of PET foils (temperatures only up to 140°C) were some implications of the research. Also, efficient inverted polymer solar cell fabricated by roll-to-roll (R2R) process could be obtained in terms of both power conversion efficiency and operational stability. Maximum 1.7% efficiency for the active area of the full module was obtained from eight serially connected cells (Krebs et al., 2009a). They (Krebs et al., 2009b) showed the versatility of the polymer solar cell technology with abstract forms for the active area, a flexible substrate, processing entirely from solution, complete processing in air using commonly available screen printing, and finally, simple mechanical encapsulation using a flexible packaging material and electrical contacting post-production using crimped contacts. Following two different devices were developed:

PET/ITO/ZnO/P3CT/ZnO/PEDOT:PSS/Ag paste/Cold laminated PET with acrylic resin and

PET/ITO/ZnO/P3CT/PCBM/ZnO/PEDOT:PSS/Ag paste/Cold laminated PET with acrylic resin

Poly(ethylene naphthalate) (PEN), has higher glass transition temperature than PET and this provides potential post-treatment of devices (Dennler et al., 2006b). However, shrinkage is seen in the material and so, subsequent processes will be problematic (Krebs, 2009b). PEN substrates (Dennler et al., 2006b) were used to develop flexible solar cells and were coated with ultra-high barrier multilayer coatings (Fig. 7). Shelf lifetime of conjugated polymer:fullerene

Fig. 7. (a) Cross-sectional view of the conjugated polymer:fullerene solar cells investigated here; (b) picture of a bent device. Reprinted from Thin Solid Films, 511–512, Dennler*,* G*.;*  Lungenschmied*,* C*.;* Neugebauer*,* H*.;* Sariciftci*,* N*. S.;* Latreche*,* M.; Czeremuszkin, *G*. & Wertheimer, M. R., A new encapsulation solution for flexible organic solar cells, *349*–353, Copyright (2006), with permission from Elsevier.

Progress in Organic Photovoltaic Fibers Research 269

Fig. 9. (a) Architecture of an inverted PSC featuring an inverted sequence on NiPI as the back contact electrode. (b) Optical image of an inverted PSC on NiPI. (c) TEM cross-sectional image of an inverted PSC on NiPI. Scale bars, 500 nm. Reprinted from Org.Electron, 10, Hsiao, Y. S.; Chen, C. P. ; Chao, C. H. & Whang, W. T., All-solution-processed inverted polymer solar cells on granular surface-nickelized polyimide, 551-561, Copyright (2009), with permission from Elsevier. Kapton®, which was synthesized by polymerizing an aromatic dianhydride with an aromatic diamine, has good chemical and thermal resistance (>400 ºC). Kapton® polyimide films can be used in a variety of electrical and electronic uses such as wire and cable tapes, substrates for printed circuit boards, and magnetic and pressure-sensitive tapes (matweb, 2010). Guillen and Herrero (2003) developed both bottom and top electrodes onto polyimide sheets (Kapton KJ) to be used in applications of lightweight and flexible thin film photovoltaic devices. ITO as the frontal electrical contact and Mo, Cr and Ni layers as the back electrical connections were prepared and then compared with conventional electrodes on glass substrates. ITO deposited polyimide sheets showed similar optical transmittance and higher electrical conductivity than ITO coated glass substrates. Mo, Cr and Ni coated polyimide sheets showed similar structure and electrical conductivity to Mo, Cr and Ni

A commercially available polyimide foil (Kapton), which was overlayed with copper, was used as the substrate of polymer solar cell in a roll-to-roll process that does not involve ITO (Krebs, 2009d). Titanium metal was sputtered onto the kapton/copper layer in the vacuum and both the monolithic substrate and back electrode for the devices were obtained. PEDOT:PSS and the active layer were slot-die coated onto the kapton (25 µm) /Cu/Ti foil, respectively. A front electrode, a protective layer and finally a silver grid was applied by screen printing technique. Vacuum coating step was the current limitation of the device. Polyethersulphone (PES), which is related to polyetheretherketone and polyetherimide, is used as thermoplastic substrate and has high glass transition temperature (Tg ~223ºC). PES was used as substrate to fabricate small molecule organic solar cells, which have single heterojunction structure, and, which use PEDOT:PSS anodes possessing low sheet resistance (~450 /). High conductivity PEDOT:PSS layers were prepatterned using photolithographic technique and spin cast onto fully flexible thermoplastic PES-based substrates having %90 optical transmission. Both organic solar cells, which have plastic and

coated glass substrates without bending or adhesion failure.

solar cells fabricated on PEN substrates and encapsulated with flexible, transparent PENbased ultra-high barrier material entirely fabricated by plasma enhanced chemical vapor deposition (PECVD) was studied. ITO bottom electrodes were sputtered through a mask onto flexible substrates and so, good adhesion and ~60 /square sheet resistance was obtained. The complete device provided a shelf lifetime of more than 3000h. Lungenschmied et al. (2007) also studied interconnected organic solar cell modules on flexible ultrahigh barrier foils (Fig. 8). Flexible solar cell modules had 11 cm2 total active area and reached 0.5% overall powerconversion efficiency under AM1.5 conditions. ITO bottom electrode was structured by deposition through a shadow mask directly onto substrate and a sheet resistance of approximately 60 /square was obtained. PEDOT:PSS and P3HT: PCBM were coated using the doctor blade technique. Al top electrode was thermally evaporated using a shadow mask.

Fig. 8. Serial connection of organic solar cells. Reprinted from Sol. Energy Mater. Sol. Cells, 91, Lungenschmied, C.; Dennler, G.; Neugebauer, H. ; Sariciftci, N. S. ; Glatthaar, M. ; Meyer, T. & Meyer, A., Flexible, long-lived, large-area, organic solar cells, 379–384, Copyright (2007), with permission from Elsevier.

A roll-to-roll process enables fabrication of polymer solar cells with many layers on flexible substrates. Inverted solar cell designs (Krebs, 2009b) can be used on both transparent and non-transparent flexible substrates. Silver nanoparticles on PEN were developed as bottom electrode. ZNO-nps from solution, P3HT-PCBM as active layer and PEDOT:PSS as hole transporting layers were coated, respectively, using slot-die coating. The last electrode was applied by screen printing of a grid structure that allowed for transmission of 80% of the light. The devices were tested under simulated sunlight (1000Wm-2, AM1.5G) and gave 0.3% of power conversion efficiency for the active layer. The illumination of the device is through the top electrode enabling the use of non-transparent substrates. The poor optical transmission in PEDOT:PSS-silver grid electrode caused a decrease in performance.

Polyimide (PI) films, which show high glass transition temperatures, low surface roughnesses, low coefficients of thermal expansion, and high chemical resistance under manufacturing conditions, are suitable for fabrication of flexible electronics. Inverted polymer solar cells were studied on PI substrates (Hsiao et al., 2009). Surface-nickelized polyimide films (NiPI films) as cathodes (back contact electrode) and high-conductivity PEDOT:PSS films as anodes were coated using solution based processes (see Fig. 9). The resulting FF of 0.43 was lower than that of standard devices. However, this ITO-free inverted polymer solar cells exhibited high performance, with the power conversion efficiency reaching 2.4% under AM 1.5 illumination (100mWcm-2).

solar cells fabricated on PEN substrates and encapsulated with flexible, transparent PENbased ultra-high barrier material entirely fabricated by plasma enhanced chemical vapor deposition (PECVD) was studied. ITO bottom electrodes were sputtered through a mask onto flexible substrates and so, good adhesion and ~60 /square sheet resistance was obtained. The complete device provided a shelf lifetime of more than 3000h. Lungenschmied et al. (2007) also studied interconnected organic solar cell modules on flexible ultrahigh barrier foils (Fig. 8). Flexible solar cell modules had 11 cm2 total active area and reached 0.5% overall powerconversion efficiency under AM1.5 conditions. ITO bottom electrode was structured by deposition through a shadow mask directly onto substrate and a sheet resistance of approximately 60 /square was obtained. PEDOT:PSS and P3HT: PCBM were coated using the doctor blade technique. Al top electrode was thermally evaporated using a shadow mask.

Fig. 8. Serial connection of organic solar cells. Reprinted from Sol. Energy Mater. Sol. Cells, 91, Lungenschmied, C.; Dennler, G.; Neugebauer, H. ; Sariciftci, N. S. ; Glatthaar, M. ; Meyer, T. & Meyer, A., Flexible, long-lived, large-area, organic solar cells, 379–384, Copyright

A roll-to-roll process enables fabrication of polymer solar cells with many layers on flexible substrates. Inverted solar cell designs (Krebs, 2009b) can be used on both transparent and non-transparent flexible substrates. Silver nanoparticles on PEN were developed as bottom electrode. ZNO-nps from solution, P3HT-PCBM as active layer and PEDOT:PSS as hole transporting layers were coated, respectively, using slot-die coating. The last electrode was applied by screen printing of a grid structure that allowed for transmission of 80% of the light. The devices were tested under simulated sunlight (1000Wm-2, AM1.5G) and gave 0.3% of power conversion efficiency for the active layer. The illumination of the device is through the top electrode enabling the use of non-transparent substrates. The poor optical

transmission in PEDOT:PSS-silver grid electrode caused a decrease in performance.

efficiency reaching 2.4% under AM 1.5 illumination (100mWcm-2).

Polyimide (PI) films, which show high glass transition temperatures, low surface roughnesses, low coefficients of thermal expansion, and high chemical resistance under manufacturing conditions, are suitable for fabrication of flexible electronics. Inverted polymer solar cells were studied on PI substrates (Hsiao et al., 2009). Surface-nickelized polyimide films (NiPI films) as cathodes (back contact electrode) and high-conductivity PEDOT:PSS films as anodes were coated using solution based processes (see Fig. 9). The resulting FF of 0.43 was lower than that of standard devices. However, this ITO-free inverted polymer solar cells exhibited high performance, with the power conversion

(2007), with permission from Elsevier.

Fig. 9. (a) Architecture of an inverted PSC featuring an inverted sequence on NiPI as the back contact electrode. (b) Optical image of an inverted PSC on NiPI. (c) TEM cross-sectional image of an inverted PSC on NiPI. Scale bars, 500 nm. Reprinted from Org.Electron, 10, Hsiao, Y. S.; Chen, C. P. ; Chao, C. H. & Whang, W. T., All-solution-processed inverted polymer solar cells on granular surface-nickelized polyimide, 551-561, Copyright (2009), with permission from Elsevier.

Kapton®, which was synthesized by polymerizing an aromatic dianhydride with an aromatic diamine, has good chemical and thermal resistance (>400 ºC). Kapton® polyimide films can be used in a variety of electrical and electronic uses such as wire and cable tapes, substrates for printed circuit boards, and magnetic and pressure-sensitive tapes (matweb, 2010). Guillen and Herrero (2003) developed both bottom and top electrodes onto polyimide sheets (Kapton KJ) to be used in applications of lightweight and flexible thin film photovoltaic devices. ITO as the frontal electrical contact and Mo, Cr and Ni layers as the back electrical connections were prepared and then compared with conventional electrodes on glass substrates. ITO deposited polyimide sheets showed similar optical transmittance and higher electrical conductivity than ITO coated glass substrates. Mo, Cr and Ni coated polyimide sheets showed similar structure and electrical conductivity to Mo, Cr and Ni coated glass substrates without bending or adhesion failure.

A commercially available polyimide foil (Kapton), which was overlayed with copper, was used as the substrate of polymer solar cell in a roll-to-roll process that does not involve ITO (Krebs, 2009d). Titanium metal was sputtered onto the kapton/copper layer in the vacuum and both the monolithic substrate and back electrode for the devices were obtained. PEDOT:PSS and the active layer were slot-die coated onto the kapton (25 µm) /Cu/Ti foil, respectively. A front electrode, a protective layer and finally a silver grid was applied by screen printing technique. Vacuum coating step was the current limitation of the device.

Polyethersulphone (PES), which is related to polyetheretherketone and polyetherimide, is used as thermoplastic substrate and has high glass transition temperature (Tg ~223ºC). PES was used as substrate to fabricate small molecule organic solar cells, which have single heterojunction structure, and, which use PEDOT:PSS anodes possessing low sheet resistance (~450 /). High conductivity PEDOT:PSS layers were prepatterned using photolithographic technique and spin cast onto fully flexible thermoplastic PES-based substrates having %90 optical transmission. Both organic solar cells, which have plastic and

Progress in Organic Photovoltaic Fibers Research 271

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

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

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

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.,

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

based substrates, which have low glass transition temperature (Bertho et al., 2009).

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

solution effected the photovoltaic performances of cells.

Copyright (2010), with permission from Elsevier.

(Chuangchote et al., 2008a).

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