Influence of Inorganic NiO_x Hole Transport Layer on the Growth of CsBi₃I₁₀ Perovskite Films for Photovoltaic Applications

Abstract

Lead (Pb)‐free bismuth (Bi) halide perovskites are promising alternatives to Pb‐based ones for the fabrication of perovskite solar cells (PSCs). However, the energy‐level mismatch at the interface between Bi perovskite (CsBi3I10;CBI) and the charge carrier transport layer limits the performance of the PSCs. Here, spray pyrolysis processed nickel oxide (sp‐NiOx) is reported as a hole transport layer (HTL) for CBI‐based inverted planar PSCs. Influence of inorganic NiOx HTL is systematically studied on the structural and morphological properties of the CBI perovskite layer growth. The CBI perovskite deposited on top of the sp‐NiOx exhibits improved crystallinity. The fabricated sp‐NiOx layer also exhibits favorable optical and electrical properties. The deep valence band (−5.4 eV) of the sp‐NiOx HTL is able to reduce the energy‐level mismatch up to 0.3 eV at the interface with the CBI layer. The PSCs fabricated with sp‐NiOx as HTL also exhibits a power conversion efficiency (PCE) of 0.72%, with a short‐circuit current density of 2.89 mA cm−2. The sp‐NiOx HTL based device maintains 85% of its initial PCE value even after 100 h of light soaking. This work highlights the importance of having a suitable HTL along with appropriate interface engineering for the Bi halide photovoltaic devices.

Serendipitous Doping in Nickel Oxide upon Microwave‐Induced Low‐Temperature Crystallization Enhances Efficiency of Perovskite Solar Cells

“…[ 17,18 ] However, strong interfacial recombination and shrinkage of quasi‐Fermi‐level splitting by a large mismatch in energy levels limit the open‐circuit voltage ( V oc ). [ 19–21 ] Moreover, the electrical conductivity of stoichiometric NiO, which is a charge‐transfer insulator with partial Mott−Hubbard character, is intrinsically limited because the valence band maxima consist of localized O 2 p orbitals. [ 22–24 ] The Ni deficiency from NiO is a key factor in enhancing the electrical conductivity by the release of holes from the nickel vacancies or the occupation of substitutional metal cations into nickel sites in the NiO host lattice.…”

“…[17,18] However, strong interfacial recombination and shrinkage of quasi-Fermi-level splitting by a large mismatch in energy levels limit the open-circuit voltage (V oc ). [19][20][21]…”

Alloying Sb into all inorganic lead-free CsBi3I10 for improving the crystal growth and photovoltaic performance

“…15 Later, several groups have focused their studies on enhancing the PCEs of CsBi 3 I 10 solar cells via improving the lm morphologies by adopting various strategies such as composition tuning, 18 solvent engineering, [19][20][21][22][23] and interface engineering. 24 For example, very recently, Zhang's group reported a gas quenching assisted antisolvent method combined with introducing a thiourea Lewis base additive to engineer the lm microstructure. 22 However, the most reported PCEs of CsBi 3 I 10 solar cells developed using the above-mentioned strategies are around 1% due to suboptimal morphology control.…”

“…Achieving a high-quality thin lm with large crystal grains, uniform coverage and smooth surface remains a big challenge for the CsBi 3 I 10 perovskite, due to its rapid crystallization rate. 16,[18][19][20][21][22][24][25][26] It is therefore necessary to explore new strategies capable of modulating the crystallization kinetics (retarding the crystallization rate), to improve the lm quality and photovoltaic performance of CsBi 3 I 10 .…”

“…However, PEDOT possesses a low-lying highest occupied molecular orbital (HOMO) of À5.10 eV compared with that (À5.90 eV) of CsBi 3 I 10 . 24 This large band offset is not favourable for holes transfer from HOMO of CsBi 3 I 10 to HOMO of PEDOT, causing charge recombination at the interfaces. The PTAA with a deeper-lying HOMO (À5.42 eV) was then employed as the HTL instead of PEDOT to reduce the energy barrier of holes transfer.…”