**7.1 Thickness and morphology of photovoltaic textiles**

Scanning electron microscope is used to investigate the thickness and morphology of various donor, acceptor layers. Scanning electron microscopes from LEO Supra 35 and others can be used to measure the existence and thickness of various coated layers on various textile surfaces at nanometer level. Various layers on photovoltaic fibres become clearly visible with 50000X magnification. The thickness of the layers can be seen from SEM photographs by bright interface line between the polymer anode and the photoactive layer.

### **7.2 Current and voltage**

In order to characterize the Photovoltaic fibres open circuit voltage, short circuit current density, current and voltage at the maximum power point under an illumination of 100 mW/cm2 are carried out.

In order to calculate the Photovoltaic efficiency of Photovoltaic textiles, current verses voltage study is essential. To achieve this target a computer controlled sourcemeter equipped with a solar simulator under a range of illumination power is required with proper calibration. All photoelectrical characterizations are advised to conduct under nitrogen or argon atmosphere

Flexible Photovoltaic Textiles for Smart Applications 59

camera is used to capture the images of intensity distribution of the luminescence radiation. Generally, actual solar cells offer inhomogeneous electroluminescence images but for an ideal solar cell it must be homogeneous. A cooled 12 CCD camera is used to capture electroluminescence images. The flexibility to adjust the distance between the camera and

The quality of solar cells is measured in terms of fill factor. The fill factor for a ideal solar cell is one but as internal resistance of solar cell becomes large or bad contact becomes between layers, fill factor reduces. The fill factor of textile based photovoltaics remains low due to

It can be improved towards unit by selection of appropriate textile substrate and further

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

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,

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

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

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

bad quality of electrodes and/or poor contact between different layers of materials64.

the solar cells offers the potential to analyse wide variety of solar cells.

optimization process parameters and processes.

**7.5 Mechanical characterization** 

camera integrated with tensile testers.

**7.6 Absorption spectra of solid films** 

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

crystalline orientation in photovoltaic structures.

and Digimizer etc.

Fig 10.

**7.4 Fill factor** 

inside a glove box to maintain the preciseness of observations. The overall efficiency of the PV devices can be representing by following equation.

$$\eta = \frac{V\_{\alpha c} \times I\_{sc} \times FF}{P\_{in}} = \frac{P\_{out}}{P\_{in}} \tag{1}$$

Where

*Voc* is the open circuit voltage (for l=0) typically measured in volt (V)

*sc I* is the short circuit current density (for V=0) in ampere /square meter (A/m2)

*Pout* is the output electrical power of the device under illumination

*Pin* the incident solar radiation in (watt/meter2) W/m2

*FF* is the fill factor and can be explained by the following relationship:

$$FF = \frac{I\_{mpp} \times V\_{mpp}}{I\_{sc} \times V\_{oc}} \tag{2}$$

where,

*Vmpp* voltage at the maximum power point (MPP)

*mpp I* is the current at the maximum power point (MPP)

Where the product of the voltage and current is maximized

To assure an objective measurement for precise comparison of various photovoltaic devices, characterization has to be performed under identical conditions.

An European research group has used Keithley 236 source measure unit in dark simulated AM 1.5 global solar conditions at an intensity of 100mW cm-2. The solar simulator unit made by K.H. Steuernagel Lichttechnik GmbH was calibrated with the help of standard crystalline silicon diode. PV fibres were illuminated through the cathode side and I-V characteristics were measured. The semi-logarithmic I-V curves demonstrate the current density versus voltage behaviour of photovoltaic fibres under various conditions. It gives a comparative picture of voltage Vs current density as a function of various light intensities.

Durisch et al., (1996) has developed a computer based testing instrument to measure the performance of solar cells under actual outdoor conditions. This testing system consist a suntracked specimen holder, digital multimeters, devices to apply different electronic loads and a computer based laser printer. Pyranometers, pyrheliometers and a reference cell is used to measure and record the insulation. This instrument is able to test wide dimensions of photovoltaic articles ranging from 3mm X 3mm to 1 meter X 1.5meter. The major part of world's energy scientist community predicts that photovoltaic energy will play a decisive role in any sustainable energy future63.

#### **7.3 Electroluminescence**

The institute for Solar Energy Research Hameln (ISFH) Emmerthal Germany introduced a new technique to characterize the solar cells based on electroluminescence. Electroluminescence can be defined as the emission of light resulting from a forward bias voltage application to the solar cells. The electrons recombine radioactively which are injected into the solar cells by transferring their extra energy to an emitted photon with available holes. The consequence of the electron and hole concentration is able to represent the intensity of the luminescence radiation. A powerful charge coupled device (CCD) camera is used to capture the images of intensity distribution of the luminescence radiation. Generally, actual solar cells offer inhomogeneous electroluminescence images but for an ideal solar cell it must be homogeneous. A cooled 12 CCD camera is used to capture electroluminescence images. The flexibility to adjust the distance between the camera and the solar cells offers the potential to analyse wide variety of solar cells.
