Photovoltaic cells and panels operate under varying weather conditions. These conditions affect the cell parameters. A factor which has a significant influence on the change of cell parameters is its temperature.

Changes in incident light intensity and ambient temperature affect the temperature of the cells, as does humidity and wind speed. Wind is a natural cooling factor. Better cooling (ventilation) can be influenced by mounting the panels (placing the panels on racks increases air circulation).
In Poland, the range of considered temperatures in which photovoltaic cells work is from \( -20_{}^{o}\textrm{C} \) to \( 70_{}^{o}\textrm{C} \) (standard photovoltaic panels operate in a temperature range from \( -40_{}^{o}\textrm{C} \) to \( +85_{}^{o}\textrm{C} \)).

Higher light intensity means more energy is absorbed, so increasing light intensity has a positive effect on cell efficiency. In the case of commonly used silicon cells, as the temperature increases, the efficiency of the cell decreases.

The material parameters change with temperature:

• absorption coefficient,
• energy gap,
• charge mobility,
• concentration of charge carriers.

The absorption coefficient of silicon [1] and energy gap [2] decrease with decreasing temperature. Concentration of carriers increases with increasing temperature [3]. The mobility of carriers in semiconductors depends exponentially on temperature. At low temperatures, the temperature dependence of mobility is a dependency factor ( 1 ). In the formula as \( \mu \) is marked as the carrier mobility and \( T \) as absolute temperature.

For high temperatures the mobility is described by the dependency factor ( 2 ).

An increase in temperature is also associated with a decrease in the separability of holes and electrons and an increase in the dispersion of charge carriers on the vibrations of the crystal lattice. These changes cause a decrease in the voltage of the p-n junction and changes in the mobility of the charge carriers. Consequently, the short circuit current increases with increasing temperature. In the case of a temperature increase from \( 25_{}^{o}\textrm{C} \) to \( 60_{}^{o}\textrm{C} \) the change in the open circuit voltage value will be \( 1.2\% \), the output power intensity about \( 1.3\% \), and the coefficient about \( 1.0\% \) [4].

In the case of a commercial photovoltaic panel, the susceptibility to temperature changes is indicated by parameters specified on the data sheet for each cell, the so-called temperature coefficient. The temperature coefficient is given for maximum power, open circuit voltage and short circuit current of the panel. The panel should exhibit a cell temperature during NOTC (Normal Operating Cell Temperature) conditions equal to at most \( 45_{}^{o}\textrm{C} \) (the lower the value, the higher the quality of the panel).

In the Fig. 1 temperature parameters for four commercially available photovoltaic panels are presented:
I - monocrystalline module BEM 355W White (Extreme plus series) 66 cells (company: BrukBet),
II - double active module with PERT type cells, BEM 335W, III - Opti series Nivo Extreme (company: BrukBet),
III - polycrystalline module series SV60P 280Wp (company: Selfa),
IV- monocrystalline module series SV60M of power up to 315Wp (company: Selfa).

The performance variation as a function of temperature for the above photovoltaic panels is shown in the graph ( Fig. 2 ).

As can be read from the graph Fig. 2, at 60 degrees the performance drops by \( 1.5\% \) relative to the value at 40 degrees Celsius. Continuous operation of photovoltaic panels at high temperatures also carries the risk of shortening their service life. Protection and longer life can be ensured by using additional protective layers (e.g. reinforced electro-insulating film or additional glass sheet) and by ensuring good ventilation.