**7.5 Mechanical characterization**

Textile substrates are subjected to different stresses under various situations. Hence usual tensile characterization is essential for photovoltaic textiles. For tensile testing of PV fibres, the constant rate of extension (CRE) based tensile testing machines are used at 1 mm per minute deformation rate using Linear Variable Differential Transformers (LVDT) displacement sensor. Fracture phenomenon is recorded by means of high resolution video camera integrated with tensile testers.

To study about the adhesion and crack formation in coating on textile structures, generally 30 mm gauge length is used in case of photovoltaic fibres. Fibre strength measuring tensile tester, integrated with an appropriate optical microscope to record the images of specimen at an acquisition rate of about one frame per second is used to record the dynamic fracture of PV fibres. Different softwares are available to analyse the image data like PAXit, Clemex, and Digimizer etc.

### **7.6 Absorption spectra of solid films**

Various spectrophotometers like Varian Carry 3G UV-Visible were used to observe the ultraviolet visible absorption spectra of photovoltaic films. The thin films are prepared to study the absorption spectrum of solid films. In a typical study, a thin film was prepared by spin coating of solution containing 10 mg of P3HT and 8mg of PCBM and 4.5 mg of MDMO-PPV and 18 mg of PCBM (in case of 1:4)/ml with chlorobenzene as solvent. A typical absorption spectra of MDMO-PPV:PCBM and P3HT: PCBM is illustrated in Fig 10.

### **7.7 X-ray diffraction of photovoltaic structures**

Crystallization process is very common phenomenon that takes place during photovoltaic structure development. The content of crystalline and amorphous regions in photovoltaic structures influences the photoactivity of photovoltaic structures. X-ray diffraction technique is capable to characterize the amount of total crystallinity, crystal size and crystalline orientation in photovoltaic structures.

Presently, thin film photovoltaics are highly efficient devices being developed in different crystallographic forms: epitaxial, microcrystalline, polycrystalline, or amorphous. Critical structural and microstructural parameters of these thin film photovoltaics are directly

Flexible Photovoltaic Textiles for Smart Applications 61

 To achieve a highly efficient photovoltaic device, solar radiation needs to be efficiently absorbed. In case of solar cell the absorption of light causes electron hole pairs which are split into free carriers at the interface between the donor and the acceptor material.

The power conversion efficiency of the MDMO-PPV:PCBM based photovoltaic fibre

Due to circular cross-sectional shape of photovoltaic fibres, the light is absorbed at

 Generally the photoactive layer thickness remain approximately between 280-350nm. A thick film can absorb more light compared to a thin film. By the increase of film thickness, the electrical field and the number of charge carriers decrease and consequently a decrease in the external quantum efficiency of the devices is observed. Although, the film thickness is restricted in presence of low-charge carrier. The optimum thickness is required to provide both maximum light absorption and maximum charge collection at the same fraction of moment. Optimization of thickness of various layers of photovoltaic fibres provides the possibility to increase the power

The thickness of the layers for optimal photovoltaic fibre can be controlled by solution

Photovoltaic fibre based organic solar cells can be curled and crimped without losing

 Low power conversion efficiency of photovoltaic textiles is the real challenge in this field and can be improved by significant improvement in existing photovoltaic material and techniques. In case of organic solar cells, the optical band gap is very critical and it must be as narrow as possible because the polymers with narrow band gap are able to absorb more light at longer wavelengths, such as infrared and near-infrared. Hence low band gap polymers (<1.8 eV) can be used as better alternative for higher power

The incorporation of C60 barrier layer can improve the performance of photovoltaic

 Generally the performance of freshly made photovoltaic textiles was found best because cell degradation happens fast when sun illumination takes place in absence of O2

The incorporation of polymer photovoltaics into textiles was demonstrated by Krebs et a., (2006) by two different strategies. Simple incorporation of a polyethyleneterphthalate (PET) substrate carrying the polymer photovoltaic device prepared by a doctor blade technique

The total area of the device on PET was typically much smaller than the active area due to decorative design of aluminium electrode. Elaborate integration of the photovoltaic device into the textile material involved the lamination of a polyethylene (PE) film onto a suitably

The self life of polymer based photovoltaics is short under ambient conditions70.

Active areas for photovoltaic fibres are generally found between 4 and 10mm2.

was higher than the P3HT:PCBM based photovoltaic fibres

conversion efficiency of polymer-based solar cells.

any photovoltaic performance from their structure.

harvesting efficiency in future if they are sufficiently flexible68,69.

**9. Photovoltaic textile, developments at international level** 

necessitated the use of the photovoltaic device as a structural element71.

concentration and dipping time.

**8. Some facts about the photovoltaic textiles** 

different angles

textiles.

barriers.

related to the photovoltaic performance. Various X-ray techniques like x-ray diffraction for phase identification, texture analysis, high-resolution x-ray diffraction, diffuse scattering, xray reflectivity are used to study the fine structure of photovoltaic devices65.

Fig. 10. Absorption spectra57 for solutions of P3HTPCBM and MDMO-PPV-PCBM (with permission)

### **7.8 Raman spectroscopy**

The Raman Effect takes place when light rays incidents upon a molecule and interact with the electron cloud and the bonds of that molecule. Spontaneous Raman effect is a form of scattering when a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state.

Raman spectroscopy is majorly used to confirm the chemical bonds and symmetry of molecules. It provides a fingerprint to identify the molecules. The fingerprint region for organic molecules remains in the (wavenumber) range of 500–2000 cm−1. Spontaneous Raman spectroscopy is used to characterize superconducting gap excitations and low frequency excitations of the solids. Raman scattering by anisotropic crystal offers information related to crystal orientation. The polarization of the Raman scattered light with respect to the crystal and the polarization of the laser light can be used to explore the degree of orientation of crystals66.

The in situ morphological and optoelectronic changes in various photovoltaic materials can be observed by observing the changes in the Raman and photoluminescence (PL) feature with the help of a spectrometer. Various spectrums can be recorded at a definite integration time after avoiding any possibility of laser soaking of the sample67.

related to the photovoltaic performance. Various X-ray techniques like x-ray diffraction for phase identification, texture analysis, high-resolution x-ray diffraction, diffuse scattering, x-

Fig. 10. Absorption spectra57 for solutions of P3HTPCBM and MDMO-PPV-PCBM (with

The Raman Effect takes place when light rays incidents upon a molecule and interact with the electron cloud and the bonds of that molecule. Spontaneous Raman effect is a form of scattering when a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or

Raman spectroscopy is majorly used to confirm the chemical bonds and symmetry of molecules. It provides a fingerprint to identify the molecules. The fingerprint region for organic molecules remains in the (wavenumber) range of 500–2000 cm−1. Spontaneous Raman spectroscopy is used to characterize superconducting gap excitations and low frequency excitations of the solids. Raman scattering by anisotropic crystal offers information related to crystal orientation. The polarization of the Raman scattered light with respect to the crystal and the polarization of the laser light can be used to explore the degree

The in situ morphological and optoelectronic changes in various photovoltaic materials can be observed by observing the changes in the Raman and photoluminescence (PL) feature with the help of a spectrometer. Various spectrums can be recorded at a definite integration

time after avoiding any possibility of laser soaking of the sample67.

permission)

vibrational state.

**7.8 Raman spectroscopy** 

of orientation of crystals66.

ray reflectivity are used to study the fine structure of photovoltaic devices65.
