**4.3 Printing of plastic solar cells**

Organic semiconductor based solar cells can be integrated fast with textile substrates and molecular heterojunction cells can be printed using inkjet printing efficiently. This technology has opened new routes to produce organic solar cells. Credit of invention of printed solar cells goes to Konarka Technologies31 for successful demonstration of manufacturing of solar cells by inkjet printing as shown in Fig.4 .

Fig. 4. Konarka's plastic photovoltaic cells by printing technology

Flexible Photovoltaic Textiles for Smart Applications 51

Fig. 5. Inkjet printing of photovoltaic cells

deposition of polymer films over a large area.

does not require additional patterning.

**5. Significance of low bandgap polymers** 

Inkjet printing of organic photovoltaic materials offers following advantages:

 Inkjet printing is a commonly used technique for controlled deposition of solutions of functional materials in specific locations on a substrate and can provide easy and fast

Organic solar cells can be processed with printing technologies with little or no loss

 Inkjet printing could become a smart tool to manufacturer solar cells with multiple colors and patterns for lower power requirement products, like indoor or sensor

 Inkjet printing technique is considered very promising because the polymer devices can be fabricated very easily because of the compatibility with various substrates and it

The part of visible sunlight is lost by absorption in specific regions of the spectrum when passes through the atmosphere. The amount of loss depends on the air mass. Under ideal conditions, the available photons for the conversion to the electrons can be represented by the solar spectrum in photon flux as a function of wavelength. It is evident that photons must be harvest at longer wavelength but at longer wavelengths the energy of the charge carriers remains lower which restricts the voltage difference that the device can produce. Hence designing of optimum bandgap is essential. However, the practical efficiencies differ from theoretically predicted values. These above considerations are based on the fact that the low band gap polymers have the possibility to improve the efficiency of OPVs due to a better overlap with the solar spectrum. Hence to achieve maximum power generation in photovoltaic device low band gap materials are required. Majority of Photovoltaic devices are unable to convert light energy below 350–400 nm wavelengths efficiently into electrical energy because of poor absorption in the substrate and front electrodes. Although, this part

compared to clean room, semiconductor technologies such as spin coating.

**4.3.1 Advantages of inkjet printing** 

applications.

The inkjet printing technology enables manufacturing of solar cells with multiple colors and patterns for lower power requirement products, like indoor or sensor applications. A mixture of high and low boiling solvents, (68% orthodichlorobenzene and 32% 1,3,5 trimethylbenzene), is found suitable for the production of inkjet printed organic solar cells with power conversion efficiency upto 3%. During the drying process and subsequent annealing, the suggested oDCB–mesitylene solvent mixture leads to an optimum phase separation network of the polymer donor and fullerene acceptor and therefore strongly enhances the performance. During drying and subsequent heat-setting process, the recommended ortho-dichlorobenzene (oDCB)-mesitylene solvent mixture leads to an optimum phase separation of polymer donor and fullerene acceptor as suggested by Pagliaro et al., (2008)32. Solvents formulation and temperature of printing table are two prime parameters to control the spreading and wetting of liquid on substrate surface. Fig.4 shows a schematic representation of organic film formation by inkjet printing.

In a typical case, the photoactive formulation is formed by blending poly(3-hexylthiophene) (P3HT) with fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in a tetralene and oDCB–mesitylene solvent mixture. A uniform film and reliable printing with respect to the spreading and film formation was performed by keeping the inkjet platen temperature 40°C. The combination of higher/lower boiling solvent mixture, oDCB–mesitylene, offers following advantages:


According to Hoth et al., (2007) for an efficient bulk heterojunction solar cell, precise control of the morphology is essential. The active layer deposition tool strategy decides the morphology. It was evident from AFM study of the inkjet printed active layers that the P3HT–PCBM blend films show significant difference in the grain size and surface roughness. The roughness of active layer surface affects the performance of the inkjet printed photovoltaic device. The credit of commercialization of power plastic cells (PPC) goes to Konarka alongwith a German firm Leonhard Kurz by opting simple, energy efficient, environmentally friendly, replicable and scalable process. The semiconducting conjugated polymers to make the photosensitive layers of the cell are created in batches of several liters each. Finally fluffy powder is formed and manufacturers combine it with standard industrial solvents to create an ink or coatable liquid. This coatable liquid is fed in reservoir of inkjet print head. Specific types of pumps are used to exert continuous pressure to maintain constant through put rate from orifice inkjet printhead throughout the printing process. Inkjet head has facility to move in different directions which helps to create various printing patterns of semiconducting polymer liquid on textile substrate layer by layer as shown in Fig.5. These layers are considerably thin. During deposition of semiconducting polymer cleanliness is very important and whole printing process is carried out in a clean room31.

The inkjet printing technology enables manufacturing of solar cells with multiple colors and patterns for lower power requirement products, like indoor or sensor applications. A mixture of high and low boiling solvents, (68% orthodichlorobenzene and 32% 1,3,5 trimethylbenzene), is found suitable for the production of inkjet printed organic solar cells with power conversion efficiency upto 3%. During the drying process and subsequent annealing, the suggested oDCB–mesitylene solvent mixture leads to an optimum phase separation network of the polymer donor and fullerene acceptor and therefore strongly enhances the performance. During drying and subsequent heat-setting process, the recommended ortho-dichlorobenzene (oDCB)-mesitylene solvent mixture leads to an optimum phase separation of polymer donor and fullerene acceptor as suggested by Pagliaro et al., (2008)32. Solvents formulation and temperature of printing table are two prime parameters to control the spreading and wetting of liquid on substrate surface. Fig.4 shows a schematic representation of organic film formation by

In a typical case, the photoactive formulation is formed by blending poly(3-hexylthiophene) (P3HT) with fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in a tetralene and oDCB–mesitylene solvent mixture. A uniform film and reliable printing with respect to the spreading and film formation was performed by keeping the inkjet platen temperature 40°C. The combination of higher/lower boiling solvent mixture, oDCB–mesitylene, offers

a. oDCB with b.p.¼180°C can be used to prevent nozzle clogging and provide a reliable

b. the second component, mesitylene, with lower boiling point of 165°C of the solvent mixture, with a lower surface tension, is used to achieve optimum wetting and spreading of the solution on the substrate. It has a higher vapor pressure of 1.86mm Hg at 20°C and a lower boiling point of 165°C compared to oDCB and tetralene. It increases the drying rate of the solvent mixture, which is a critical parameter to decide the

According to Hoth et al., (2007) for an efficient bulk heterojunction solar cell, precise control of the morphology is essential. The active layer deposition tool strategy decides the morphology. It was evident from AFM study of the inkjet printed active layers that the P3HT–PCBM blend films show significant difference in the grain size and surface roughness. The roughness of active layer surface affects the performance of the inkjet printed photovoltaic device. The credit of commercialization of power plastic cells (PPC) goes to Konarka alongwith a German firm Leonhard Kurz by opting simple, energy efficient, environmentally friendly, replicable and scalable process. The semiconducting conjugated polymers to make the photosensitive layers of the cell are created in batches of several liters each. Finally fluffy powder is formed and manufacturers combine it with standard industrial solvents to create an ink or coatable liquid. This coatable liquid is fed in reservoir of inkjet print head. Specific types of pumps are used to exert continuous pressure to maintain constant through put rate from orifice inkjet printhead throughout the printing process. Inkjet head has facility to move in different directions which helps to create various printing patterns of semiconducting polymer liquid on textile substrate layer by layer as shown in Fig.5. These layers are considerably thin. During deposition of semiconducting polymer cleanliness is very important and whole printing process is carried out in a clean

inkjet printing.

following advantages:

room31.

jetting of the printhead

morphology of PV prints.

Fig. 5. Inkjet printing of photovoltaic cells
