Transition of the NiO_x Buffer Layer from a p-Type Semiconductor to an Insulator for Operation of Perovskite Solar Cells

Abstract

To overcome the fundamental limitation of the low power conversion efficiency (PCE) of perovskite solar cells (PSCs) incorporated with a magnetron sputtered p-type NiO x as a hole transport layer (HTL), the effects of various sputtering conditions of the NiO x layer on MAPbI3 PSC performance are comprehensively investigated. Based on the stoichiometry control of the bulk and surface of the NiO x layer by controlling oxygen partial pressure during the sputtering process, it is found that PSCs with a sputtered NiO x HTL prepared in an Ar atmosphere exhibits higher PCEs than those of devices with an NiO x HTL conventionally grown under Ar/O2 mixed conditions. To elucidate the working principle, influences of defects in the NiO x layer grown under different O2 partial pressure conditions are thoroughly investigated. The sputtered NiO x films under pure Ar conditions with a low Ni3+/Ni2+ ratio show high hole extraction ability due to moderate hole conductivity, high optical transparency, and the improved band alignment with a MAPbI3 perovskite layer. In particular, the NiO x film sputtered in pure Ar ambience results in higher photovoltaic performance with a negligible hysteresis behavior due to excellent crystallinity, the large grain size, and the improved interface morphology of the MAPbI3 layer. In contrast, PSCs incorporated with an NiO x film sputtered under Ar/O2 mixed conditions possess a noncontinuous MAPbI3 perovskite layer having voids and cracks at the interface between MAPbI3/NiO x layer. These results indicate that a sputtered p-type NiO x HTL in Ar ambience can be an alternative route to the conventional solution-processed NiO x HTL for mass production of large-area PSCs.

NiOx Nanocrystals with Tunable Size and Energy Levels for Efficient and UV Stable Perovskite Solar Cells

“…Figure 2i–l shows the cutoff and near E f region of the UPS spectra, from which the corresponding valence band maximum (VBM) of NiO x ‐M, NiO x ‐E, NiO x ‐P, and NiO x ‐B films was determined to be −5.04, −5.11, −5.18, and −5.30 eV, respectively. Such a downward shift of the VBM value with the increase of the Ni 3+ content of NiO x film is in good agreement with the previous study, [ 37 ] which could be caused by the hybridization of the O 2p and the Ni 3p orbitals. [ 43,44 ] Considering the HOMO of the typical MAPbI 3 or FAPbI 3 ‐based perovskite is around −5.5 eV, all of the different NiO x films are suitable for HTL applications in PSCs.…”

“…The Ni 3+ states are actually Ni 2+ vacancies in the NiO lattice, which were intentionally generated by under oxygen‐rich condition in previous study. [ 37 ] In our case, the variety of Ni 3+ content is assumed to be associated with the different size of the NiO x NCs. Since the smaller crystal size means the larger surface area (Figure S3, Supporting Information), thus more adequate exposure to oxygen during the sintering process and this explains why the highest Ni 3+ /Ni 2+ ratio is observed in the NiO x ‐B sample.…”

“…On one hand, the Ni 3+ doping is essential for enhancing the hole conductivity of the NiO x film. [ 37 ] On the other hand, the Ni 3+ species on the surface of the NiO x film are considered to be defects, [ 29 ] which may cause the degradation of the perovskite and the deterioration of the hole transfer kinetic at the NiO x /perovskite interface. Furthermore, the content of Ni 3+ has a significant impact on the electronic structure and thus hole extraction efficiency of the NiO x film, but it is less controllable in the current film preparation methods.…”

Facile fabrication of polypyrrole/NiOx core‐shell nanocomposites for hydrogen production from wastewater

“…The benefits of NiO nanoscale hybridization include controlled morphology, film‐forming capability, dimensional diversity, and activated functionalities 27 . Furthermore, NiO is classified as a p‐type semiconductor due to its wide bandgap energy range of 3.6–4.0 eV 28 . As far as we can tell, only a few articles have examined polypyrrole/NiO x nanocomposites for the generation of hydrogen.…”

“…27 Furthermore, NiO is classified as a p-type semiconductor due to its wide bandgap energy range of 3.6-4.0 eV. 28 As far as we can tell, only a few articles have examined polypyrrole/NiO x nanocomposites for the generation of hydrogen. This study demonstrates how the addition of NiO x metallic nanoparticles improves polypyrrole's ability to produce a renewable energy source (hydrogen gas fuel) by addressing many of the problems previously mentioned and offering a chance to valorize wastewater.…”