Investigation of ZnO:Al/carbon nanotubes bilayers effects on the photovoltaic performance of hybrid solar cells

In this work, a study of the physical properties of commercial single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) synthesized by the microwave method and processed by spray-coating method on aluminum-doped zinc oxide (ZnO:Al, AZO) substrates and their potential application as transparent front electrode or electron transport layer in CdTe based on hybrid solar cells is discussed. The transparent conducting AZO films were deposited by a radio frequency (RF) magnetron sputtering system at room temperature in a high vacuum chamber. The synthesized AZO/carbon nanotubes bilayers were examined by the characterization techniques of scanning electron microscopy, Raman spectroscopy, Ultraviolet–Visible spectrophotometry, and four-point probe method. An improvement in the electrical properties of the AZO/SWCNTs bilayers with respect to the AZO substrates was observed, mainly attributed to the contribution of the carbon nanotubes which provide more effective conductive pathways to increase the electron transport and reduce the charge recombination. Haacke’s figure of merit values of the AZO/CNTs bilayers that was used to evaluate its potential application in photovoltaic devices as a TCO layer were also obtained. The AZO/SWCNTs bilayer with the higher figure of merit value corresponds to an AZO film deposited at 175 W, with a resistivity value of 6.8 × 10^–2 Ω·cm and transmittance average value around 87%. The AZO/MWCNTs bilayer with the higher figure of merit value corresponds to an AZO film deposited at 100 W, with a resistivity value of 2.9 × 10^–1 Ω·cm and transmittance average value around 88%. CdTe based on hybrid solar cells (HSCs) in a superstrate configuration with an AZO/CNTs bilayer as a transparent front contact were fabricated and the photovoltaic effect in each hybrid solar cell was measured. A conversion efficiency value of 2.4% was obtained when an AZO/SWCNTs bilayer was used in the structure of a HSCs. When an AZO/SWCNTs bilayer was used in the hybrid device structure, an increment in Jsc values was observed, meanwhile when an AZO/MWCNTs bilayer was added in the solar cell configuration the Voc electrical parameter increased.