The temperature coefficient is the rate at which a material's physical properties change with temperature. Different materials have different temperature coefficients, and their performance varies under different temperature conditions. The temperature coefficient can be used to directly compare the performance of materials in high-temperature environments. Simply put, the lower the absolute value of the temperature coefficient, the better the material's high-temperature resistance.
For photovoltaic modules, a type of optoelectronic semiconductor device, silicon has a negative temperature coefficient in its bandgap. The temperature changes involved can be mainly divided into three aspects: the relationship between temperature and open-circuit voltage, the relationship between temperature and short-circuit current, and the relationship between temperature and output power.
1) The open-circuit voltage is determined by the bandgap and Fermi level of the semiconductor. Since the higher the temperature, the closer the Fermi level is to the valence band, the lower the open-circuit voltage is. In other words, the curve of temperature/open-circuit voltage is roughly a straight line with a negative slope.
From another perspective: the shared movement of electrons leads to the formation of energy bands, namely the allowed band and the band gap, in isolated atoms. As temperature increases, the shared movement of electrons intensifies, causing the allowed band to further split and widen; the widening of the allowed band results in a relative narrowing of the band gap between allowed bands. Conversely, decreasing temperature leads to a widening of the band gap.
2) The relationship between temperature and short-circuit current is that the higher the temperature, the greater the short-circuit current. However, it should be noted that the trend of increasing short-circuit current is less than the trend of decreasing open-circuit voltage in the first point above. In other words, the curve of temperature versus short-circuit current is a straight line with a slightly positive slope.
3) Because the open-circuit voltage drops significantly when the temperature rises, and the magnitude of the drop is greater than the magnitude of the increase in short-circuit current, the total output power decreases when the temperature rises. Since P=UI, the magnitude of the decrease in U is greater than the magnitude of the increase in I, so the power is inversely proportional to the temperature.
In simple terms: As temperature increases, the bandgap of photovoltaic cell materials decreases, the intrinsic carrier concentration increases, and the short-circuit current slightly increases; the built-in potential of the PN junction decreases, the recombination carrier mobility decreases, and the recombination coefficient increases, thus the open-circuit voltage and fill factor decrease.
However, the increase in short-circuit current cannot compensate for the impact of the decrease in open-circuit voltage and fill factor on conversion efficiency, and the peak power of the module will decrease.
