# Solar Energy - Cell Efficiency

Efficiency refers to the ratio of power input to power output. In the case of a photovoltaic, efficiency is the ratio of power output in terms of electricity to the solar energy incident on the cell.

Now, Output power, pm = Voltage (v) * current (I) in the circuit (max value).

And, Input power Pi = Incident energy G (Wm-2) * Surface area of cell, A (m2).

Thus, Efficiency is calculated as −

$$\eta\:=\:\frac{P_{m}}{G\: \times\:A_{c}}$$

Where, Pm must be the maximum power of the circuit. It is obtained by using the voltage across open circuit (Voc) and current across the short circuit (Isc) and the fill factor (FF).

$$P_{max}\:=\:V_{OC}I_{SC}FF$$

These measurements must be measured under standard conditions i.e. 25°C, Air mass of 1.5gm-3, and incident energy, G of 1000Wm-2.

The factors that affect output of a photovoltaic cell include −

• The wavelength of the incident light
• Recombination of electrons and holes
• Electrical resistance
• Temperature
• Fill factor
• Reflection factor of the material

Therefore, to maximize power, the cell should be constructed to have a greater fill i.e. surface area utilized. Positioning of a solar cell also determines its output for two reasons. First, the angle determines the level of reflection on the cell, and secondly the positioning determines the amount of sunshine captured from 9 am to 3 pm. For maximum efficiency, it is important to avoid any shadowing on cells.