Influence of Graphene–Carbon Nanotube Loading on the Morphological and Electrical Transport Properties of NiO Hole Transport Layers

Nickel oxide (NiO) has attracted considerable attention as an inorganic hole transport layer (HTL) because of its good chemical stability, wide optical bandgap, and compatibility with solution-based fabrication. However, the relatively low electrical conductivity of NiO can limit carrier transport through the layer. In this study, graphene carbon nanotubes (g-CNTs) were incorporated into NiO at different loadings to investigate their influence on the film morphology and electrical transport properties. NiO:g-CNT composites containing 3, 7, and 11 wt% g-CNT were prepared and compared with pristine NiO. The surface morphology of the films was examined using field-emission scanning electron microscopy, while Hall-effect measurements were used to evaluate carrier concentration, mobility, and resistivity. The addition of g-CNT clearly changed the electrical behaviour of the NiO films. The carrier concentration increased from 2.780 × 10¹⁴ cm⁻³ for pristine NiO to 7.481 × 10¹⁴ cm⁻³ at 3 wt% g-CNT, before showing some variation at higher g-CNT loadings. Meanwhile, carrier mobility increased steadily from 19.07 cm² V⁻¹ s⁻¹ for pristine NiO to 170.50 cm² V⁻¹ s⁻¹ at 11 wt% g-CNT. This increase was accompanied by a reduction in resistivity from 1177 to 60.95 Ω·cm and an increase in conductivity from 8.49 × 10⁻⁴ to 1.641 × 10⁻² S cm⁻¹. The findings suggest that the improvement in conductivity at higher g-CNT loadings is mainly related to enhanced carrier mobility and the presence of additional conductive pathways within the NiO matrix. Overall, the results show that incorporating g-CNT can improve the electrical transport properties of NiO.

Nickel oxide, graphene–carbon nanotube, hole transport layer, carrier mobility

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