However, the solution-derived spin-coating method is time consuming, (28,29) relatively complicated for preparing the NiO x precursor, and needs very high temperature (>400 ☌) for annealing after spin-coating. (23) fabricated NiO x-based PSCs and achieved uncertified maximum PCE value of 16.1% by synthesizing NiO x precursor solution and then spin-coating and postannealing. Spin-coating is the most commonly used method because of the simple operation approach.
(24−26) To date, research on PSCs using NiO x as HTL have made great progress, and NiO x can be prepared by various methods ( Table S1). Besides, NiO x is a low-cost material with superior thermal and chemical stabilities due to its inorganic nature, which brings about a longer device lifetime. (21,22) Among them, nickel oxide (NiO x) exhibits great potential due to its wide band gap for light penetration, appropriate valence band edge exactly aligning with that of perovskite to allow for efficient hole transport, and high carrier mobility for rapid extraction and transport of carriers, (21,23) thus leading to high open circuit voltage ( V OC). Recently, inorganic metal oxides performing p-type semiconductor property, such as Cu 2O, (17) CoO x, (18) NiO x, (19) and MoO 3, (20) have been studied as a hole transport layer (HTL) to replace poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and its modifications because these organic materials are always expensive and show high instability to air, moisture, and temperature. (12) The inverted planar heterojunction PSCs with a p–i–n configuration have been demonstrated to show excellent performances, such as simple fabrication process, low hysteresis, and high stability. (9,10) Although its initial power conversion efficiency (PCE) was very low (4%) in 2009, (11) the rapid increase astonished researchers after only a decade’s development, with the highest PCE incredibly reaching 23.3% in 2018. (8) Besides, the simple fabrication process and low cost make it suitable for commercial applications competing with silicon-based solar cells. The organometal halide perovskite solar cells (PSCs) have attracted much attention due to their optimal band gap (≈1.5 eV), (1) large charge carrier mobility (33 and 115 cm 2 V –1 s –1 for microcrystalline thin films and single crystal, respectively), (2−4) long electron–hole diffusion length (>1 μm for microcrystalline films and >175 μm for single crystal), (5,6) long carrier lifetimes (>250 ns), (7) and low exciton-binding energy (≈2 meV). This simple fabrication process paves a novel way to the evolution of PSCs based on NiO x and rapid commercialization.
The prepared device shows negligible hysteresis, high reproducibility, and high uniformity with a PCE difference of 2% for measuring the different sites from edge to center. An optimal power conversion efficiency (PCE) of 17.77% with an active area of 0.25 cm 2 is achieved. The finite-difference time-domain simulation confirms the optimal thickness of the NiO x layer and coincides with our experiment results.
The optimal condition for preparing NiO x films is achieved by adjusting the deposition time at a certain applied current density, which exhibits excellent optical transmittance and suitable thickness and band gap, thus reducing optical loss and enhancing hole extraction at the interface between HTL and the perovskite layer and therefore improving photovoltaic performances. It is demonstrated that the increasing thickness and decreasing surface roughness of the NiO x film are beneficial for light transmission. Here, we develop a simple electrochemical deposition method for quickly and evenly preparing a mesoporous NiO x film. Perovskite solar cells (PSCs) based on a NiO x hole transport layer (HTL) with an inverted p–i–n configuration have yielded highly efficient and relatively stable devices.
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