An investigation of the influence of different transparent conducting oxide substrates/front contacts on the performance of CdS/CdTe thin-film solar cells

CdS/CdTe/Au thin film solar cells have been fabricated on different transparent conducting oxide (TCO) substrates/front contacts to study the influence of these different TCOs on the performance of the devices. The TCOs used were ZnO, ZnO:Al and SnO₂:F. Under dark condition, all three device structures of the type glass/TCO/n-CdS/n-CdTe/Au n–n heterojunction + Schottky barrier, show interesting rectifying behaviors with rectification factors (RF) in the range (10^2.5—10^5.0), Schottky barrier heights (Φ _B) greater than (0.69–0.81) eV, diode ideality factors (n) in the range (1.85–2.12), reverse saturation current densities (J ₀) in the range (3.18 × 10⁻⁶–3.18 × 10⁻⁸) A cm⁻², series resistances (R _s) in the range (507–1114) Ω and shunt resistances (R _sh) in the range (0.84–271) MΩ. The device structures glass/SnO₂:F/n-CdS/n-CdTe/Au and glass/FTO/ZnO:Al/n-CdS/n-CdTe/Au show the best performance with equal J ₀ of 3.18 × 10⁻⁸ A cm⁻², equal Φ _B > 0.81 eV, RF of 10^4.9 and 10^5.0, n value of 2.01 and 2.12, R _s of 615 Ω and 507 Ω and R _sh of 197 and 271 MΩ respectively. The device structure with ZnO shows the least performance. Under AM1.5 illumination, the device structure glass/SnO₂:F/n-CdS/n-CdTe/Au shows the best solar cell performance with open-circuit voltage of 630 mV, short-circuit current density of 23.5 mAcm⁻², fill factor of 0.44 and conversion efficiency of 6.5%, and is followed by the device structure with ZnO:Al showing a conversion efficiency of 6.0%. Suggested energy band diagrams of the devices as well as possible reasons for the observed trends in performance are presented and discussed.