Characteristics of NiO_x as Hole Transport layer using Atomic Layer Deposition

Abstract

This research evaluates the fabrication of inorganic nickel oxide NiO_x as a hole transport layer in inverted perovskite solar cells using Atomic Layer Deposition (ALD). Inorganic hole transport layers are desired for their long term stability and degradation resistance. Systematic optimization established a self-limiting growth rate of 0.0457 nm per cycle using a 3.0 s nickel precursor pulse and a 0.2 s H₂O co-reactant pulse for the ALD system. The resulting NiO_x films exhibited highly conformal growth on bare silicon and indium tin oxide substrates with less than a nanometer of roughness and sample-to-sample uniformity, maintaining over 90% visible transparency and a stable optical band gap of approximately 3.84 eV.

Post-deposition annealing in ambient air and nitrogen at 300°C for 15 minutes increased the relative fraction of Ni³⁺ surface states and lowered the absolute work function to an operative range of 4.35 to 4.55 eV, whereas ozone treatments proved detrimental by forming surface hydroxides. When integrated into photovoltaic devices, a 7.5 nm air-annealed NiO_x layer emerged as the optimum configuration compared to thinner 5 nm and thicker 10 nm samples, yielding a peak power conversion efficiency 3.90\%, alongside a V_oc of 0.874 V and a J_sc of 12.33 mAcm^-2 Ultimately, this performance lagged behind the efficiency of organic PTAA which by comparison achieved a power conversion efficiency of 8.99%, a V_oc of 0.992 V, a J_sc of  16.84mAcm^-2, and a fill factor of 53.82% reference cells. This is due to significant interfacial charge extraction barriers that caused S-shaped current-voltage deformations. This study shows that further research is needed to better characterize the interactions/reactions between the NiO_x and the perovskite layer.