Role of Urbach Energy in Controlling Voltage Output of Solar Cells

The Urbach energy (EU) strongly influences voltage output and efficiency, which is observed upon close analysis of performance limiting factors in various thin film solar cell systems. We simulated the one-to-one correlation between the sub-bandgap defect position and the magnitude of Urbach energy. The higher the extent of the band tail into the forbidden gap higher the Urbach energy. The simulation results show that the Urbach energy increases and efficiency decreases with increasing sub-bandgap defect energy levels. The magnitude of the EU depends upon the extent of impurities and defects in material systems, limits the achievable VOC in the device and increases the VOC deficit. The capacitance of the p–n junction diode of thin film solar cells falls for high EU > kT. The magnitude of EU larger than thermal energy kT (EU > kT) have significant and observable deviation on the device's efficiency. This result shows the applicability of the EU in selecting the new system for photovoltaic applications.