**7.4 Fill factor**

58 Solar Cells – New Aspects and Solutions

inside a glove box to maintain the preciseness of observations. The overall efficiency of the PV

*oc sc out in in*

> *mpp mpp sc oc*

*I V*

*I V*

To assure an objective measurement for precise comparison of various photovoltaic devices,

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

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

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)

(1)

(2)

*V I FF P P P*

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

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

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

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

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

role in any sustainable energy future63.

**7.3 Electroluminescence** 

*mpp I* is the current at the maximum power point (MPP) Where the product of the voltage and current is maximized

characterization has to be performed under identical conditions.

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

*FF*

picture of voltage Vs current density as a function of various light intensities.

devices can be representing by following equation.

Where

where,

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

It can be improved towards unit by selection of appropriate textile substrate and further optimization process parameters and processes.
