Modified equivalent circuits for CuGaSe₂ thin films including space charge limited current

Current separation method has been used to identify the contribution of diode elements in the equivalent circuits of the thin film solar cells. This method is applied to analyse the current–voltage characteristics of the CuGaSe$$_2$$2/CdS thin film solar cells measured under two different temperatures and several low illumination levels. The results reveal that the conventional double or single diode models are not well fitted to the experimental data under low illumination levels. However, the introduction of the space charge limited current element into the conventional equivalent circuits improves the fitness of the diode theory and experimental data. The voltage dependency of the space charge limited current is different under each operation condition. However, this voltage-dependency is not linear as was also reported in different literatures. For each single and double diode models, and for each illumination and operation temperature, the voltage dependency $$(\hbox {V}^{\mathrm{m}})$$(Vm) varies as $${\text {m}}=0.5, 1, 1.5, 2$$m=0.5,1,1.5,2.