Synergistic Effect of Codoped Nickel Oxide Hole–Transporting Layers for Highly Efficient Inverted Perovskite Solar Cells

Abstract

Inverted perovskite solar cells (PSCs), which feature an attractive structure for diverse applications such as tandem SCs or flexible devices, continue to be rapidly developed. Among various hole‐transporting layer (HTL) materials, nickel oxide (NiO x ) is widely used as a stable and superior HTL even though it exhibits poor conductivity. Although various methods have been proposed to overcome the low conductivity of NiO x films, codoping methods have not been extensively studied and remain poorly understood. Herein, Li:Cu:NiO x is systematically investigated to explore the synergistic effect of codoping in various NiO x HTLs and PSCs. The optical, chemical, and morphological properties of the films are characterized and the dependence of these properties on the codoping ratio are investigated. A gradual improvement of the electrical properties and a tunable Fermi energy level resulting from the Li:Cu codopants is subsequently demonstrated. Furthermore, the structure of the perovskite films on HTLs and the synergistic effect on the preferred crystal growth behavior are elucidated. An inverted PSC with high efficiency was attained as a result of the enhanced electrical properties of the NiO x HTLs and the high quality of the perovskite film, which were attributed to the synergistic effect of the Li:Cu codoping method.

Halogen doping of p-type inorganic hole transport layer: electronic nature-based dopant engineering for modulating hole selectivity in inverted planar perovskite solar cells

“…A similar observation was made by Yi et al for Li, Cu co-doped NiO x , where the Ni 3+ : Ni 2+ ratio increased from 1.6 (pristine) to 2.8 upon Li : Cu (10 : 0 vol%) doping, which further increased to 3.89 upon Li : Cu (0 : 10 vol%) doping. 26 Yin et al and Chen et al, in their work on K-doped NiO x also reported a similar trend, where the Ni 3+ : Ni 2+ ratio increased from 1.13 to 2.55 and 1.69 to 2.24, respectively. 12,38 This is a clear indication that cation dopants have a pronounced impact on improving the hole density by introducing acceptor-type defects of nickel vacancies.…”

“…S8 (ESI †), and the resulting reduction in morphology-induced scattering effect upon doping. 26 This observation was verified further by calculating the weighted average transmittance (% WAT). The WAT plot is shown in Fig.…”

NiO as Hole Transporting Layer for Inverted Perovskite Solar Cells: A Study of X‐Ray Photoelectron Spectroscopy

“…Specifically, the Ni 3+ /Ni 2+ ratio is enhanced by ≈12.5%, 19.8%, 13.13%, 5.66%, 2.31%, 32.5%, 17.8%, and 18.7% in Cs-doped, [89] Zn-doped, [90,91] Cddoped, [92] Pb-doped, [93] Al-doped, [94,95] K-doped, [96] Fe-doped, [97] and Eu-doped NiO, [98] respectively, whereas it is suppressed by 14.3% in Co-doped NiO. [99] The Ni 3+ /Ni 2+ ratio was reported to increase in Li-Mg, [87] Li-Cu, [100][101][102] Li-Ag, [103] Zn-Ce, [104] Li-Co, [105] and Li-Co-Mg [106] codoped NiO layers compared to the pristine NiO layers, suggesting an increase in the hole conductivity. Therefore, when the Ni 3+ /Ni 2+ ratio is increased, the oxidation of Ni 2+ into Ni 3+ in NiO is also reported to increase, resulting in a high Ni 2+ deficiency and an oxygen-rich nature, which directly contribute to the p-type conductivity of NiO.…”

“…[107] This type of behavior is not observed in doped-NiO when they were used as HTL in inverted perovskite solar cells. Thus, to determine the influence of the doping concentration on increasing the conductivity or hole mobility in doped NiO films, a few reports have presented conductivity measurements [83,87,92,95,97,102] /Hall effect measurements, [80,84,97] which we believe is essential for this complex system.…”