Like all other semiconductor devices, solar cells are sensitive to temperature. Increases in temperature reduce the band gap of a semiconductor, thereby effecting most of the semiconductor material parameters. The decrease in the band gap of a semiconductor with increasing temperature can be viewed as increasing the energy of the electrons in the material. Lower energy is therefore needed to break the bond. In the bond model of a semiconductor band gap, reduction in the bond energy also reduces the band gap. Therefore increasing the temperature reduces the band gap.
In a solar cell, the parameter most affected by an increase in temperature is the open-circuit voltage. The impact of increasing temperature is shown in the figure below.
The open-circuit voltage decreases with temperature because of the temperature dependence of I0. The equation for I0 from one side of a p-n junction is given by;
q is the electronic charge given in the constants page;
A is the area;
D is the diffusivity of the minority carrier given for silicon as a function of doping in the Silicon Material Parameters page;
L is the minority carrier diffusion length;
ND is the doping; and
ni is the intrinsic carrier concentration given for silicon in the Silicon Material Parameters page.
In the above equation, many of the parameters have some temperature dependence, but the most significant effect is due to the intrinsic carrier concentration, ni. The intrinsic carrier concentration depends on the band gap energy (with lower band gaps giving a higher intrinsic carrier concentration), and on the energy which the carriers have (with higher temperatures giving higher intrinsic carrier concentrations). The equation for the intrinsic carrier concentration is;
T is the temperature;
h and k are constants given in the constants page;
me and mh are the effective masses of electrons and holes respectively;
EGO is the band gap linearly extrapolated to absolute zero; and
B is a constant which is essentially independent of temperature.
Substituting these equations back into the expression for I0, and assuming that the temperature dependencies of the other parameters can be neglected, gives;
where B' is a temperature independent constant. A constant, γ, is used instead of the number 3 to incorporate the possible temperature dependencies of the other material parameters. For silicon solar cells near room temperature, I0 approximately doubles for every 10 °C increase in temperature.
The impact of I0 on the open-circuit voltage can be calculated by substituting the equation for I0 into the equation for Voc as shown below;
where EG0 = qVG0. Assuming that dVoc/dT does not depend on dIsc/dT, dVoc/dT can be found as;
The above equation shows that the temperature sensitivity of a solar cell depends on the open circuit voltage of the solar cell, with higher voltage solar cells being less affected by temperature. For silicon, EG0 is 1.2, and using γ as 3 gives a reduction in the open-circuit voltage of about 2.2 mV/°C;
The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG, decreases and more photons have enough energy to create e-h pairs. However, this is a small effect and the temperature dependence of the short-circuit current from a silicon solar cell is;
or 0.06% per °C for silicon.
The temperature dependency FF for silicon is approximated by the following equation;
The effect of temperature on the maximum power output, Pm, is;
or 0.4% to 0.5% per °C for silicon.
300 K or 25 °C ?
Most semiconductor modeling is done at 300 K since it is close to room temperature and a convenient number. However, solar cells are typically measured almost 2 degrees lower at 25 °C (298.15 K). In most cases the difference is insignificant (only 4 mV of Voc) and both are referred to as room temperature. Occasionally, the modeled results need to be adjusted to correlate with the measured results.
At 300 K, ni = 1.01 x 1010 cm-3 and kT/q = 25.852 mV
At 25 °C (298.15 K), ni = 8.6 x 109 cm-3 and kT/q = 25.693 mV