Progress, highlights and perspectives on NiO in perovskite photovoltaics

Sacrificial layer concept interface engineering for robust, lossless monolithic integration of perovskite/Si tandem solar cells yielding high fill factor of 0.813

Efficient monolithic perovskite/Si tandem solar cells require a robust recombination junction (RJ) with excellent electrical and optical properties. This study introduces an interface engineering method using an organic sacrificial layer to enable effective monolithic integration. An ultrathin layer of poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS) is inserted between the transparent conductive oxide recombination layer and the hole transport layer (HTL) of a methylammonium lead iodide (MAPbI3)-based perovskite top cell. This layer restores junction functionality and enables charge transfer between sub-cells via efficient carrier recombination at the RJ, which electrically connects the two cells. Acting as a sacrificial layer, PEDOT:PSS temporarily prevents resistive SiOx formation and improves interface quality. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy confirm suppression of SiOx growth during HTL annealing. Moreover, the Cu-doped NiOx HTL fabrication method proves critical, where process optimization improves electrical contact. Combined with PEDOT:PSS interface engineering, these enhancements promote efficient recombination by tuning interfacial energy levels and increasing band bending at the RJ. As a result, tandem devices comprising an aluminum back-surface field p-type homojunction Si bottom cell and a p-i-n perovskite top cell achieve 21.95% power conversion efficiency and an 81.3% fill factor -among the highest reported for monolithic perovskite/Si tandem solar cells.

The response to shock loads of Ni-MOF and NiO NPs on aerobic granular sludge and algal-bacterial aerobic granular sludge

Currently, the increasing use of nickel metal-organic frameworks (Ni-MOF) and nickel oxide nanoparticles (NiO NPs) has raised concerns regarding their potential environmental impact on wastewater treatment systems. Herein, the responses of aerobic granular sludge (AGS) and algal-bacterial aerobic granular sludge (AB-AGS) to Ni-MOF and NiO NPs were investigated. The results showed that Ni-MOF concentrations of 50, 100, and 200 mg/L significantly reduced nutrient removal in both systems, particularly affecting ammonia, nitrite, and phosphorus removal, while denitrification processes remained stable. AB-AGS exhibited greater tolerance to nickel than AGS, likely due to its higher content of extracellular polymeric substances (EPSs), in which the algae were embedded, indicating a robust bacterial-algal symbiotic system. Conversely, NiO NPs had no adverse effects on bioreactor performance, likely due to their insolubility and integration into the sludge matrix. This research provides valuable insights into the potential future applications of AGS and AB-AGS technologies for treating wastewater contaminated with nickel and other heavy metals, highlighting the superior resilience of AB-AGS to nickel exposure.

Efficient and Stable Inverted Perovskite Solar Cells Enabled by a Fullerene-Based Hole Transport Molecule

Designing an efficient modification molecule to mitigate non-radiative recombination at the NiOx/perovskite interface and improve perovskite quality represents a challenging yet crucial endeavor for achieving high-performance inverted perovskite solar cells (PSCs). Herein, we synthesized a novel fullerene-based hole transport molecule, designated as FHTM, by integrating C60 with 12 carbazole-based moieties, and applied it as a modification molecule at the NiOx/perovskite interface. The in situ self-doping effect, triggered by electron transfer between carbazole-based moiety and C60 within the FHTM molecule, along with the extended π conjugated moiety of carbazole groups, significantly enhances FHTM's hole mobility. Coupled with optimized energy level alignment and enhanced interface interactions, the FHTM significantly enhances hole extraction and transport in corresponding devices. Additionally, the introduced FHTM efficiently promotes homogeneous nucleation of perovskite, resulting in high-quality perovskite films. These combined improvements led to the FHTM-based PSCs yielding a champion efficiency of 25.58 % (Certified: 25.04 %), notably surpassing that of the control device (20.91 %). Furthermore, the unencapsulated device maintained 93 % of its initial efficiency after 1000 hours of maximum power point tracking under continuous one-sun illumination. This study highlights the potential of functionalized fullerenes as hole transport materials, opening up new avenues for their application in the field of PSCs.

Interfacial Engineering of Nickel Oxide-Perovskite Interface with Amino Acid Complexed NiO to Improve Perovskite Solar Cell Performance

The interface between NiO and perovskite in inverted perovskite solar cells (PSCs) is a major factor that can limit device performance due to defects and inappropriate redox reactions, which cause nonradiative recombination and decrease in open-circuit voltage (VOC). In the present study, a novel approach is used for the first time, where an amino acid (glycine (Gly), alanine (Ala), and aminobutyric acid (ABA))-complexed NiO are used as interface modifiers to eliminate defect sites and hydroxyl groups from the surface of NiO. The Ala-complexed NiO suppresses interfacial non-radiative recombination, improves the perovskite layer quality and better energy band alignment with the perovskite, resulting in improved charge transfer and reduced recombination. The incorporation of the Ala-complexed NiO leads to a PCE of 20.27% with enhanced stability under the conditions of ambient air, light soaking, and heating to 85 °C, as it retains over 82%, 85%, and 61% of its initial PCE after 1000, 500, and 350 h, respectively. The low-temperature technique also leads to the fabrication of a NiO thin film that is suitable for flexible PSCs. The Ala-complexed NiO is fabricated on the flexible substrate and achieved 17.12% efficiency while retaining 71% of initial PCE after 5,000 bending.

On the V OC loss in NiO-based inverted metal halide perovskite solar cells

Recent reports have shown that nickel oxide (NiO) when adopted as a hole transport layer (HTL) in combination with organic layers, such as PTAA or self-assembled monolayers (SAMs), leads to a higher device yield for both single junction as well as tandem devices. Nevertheless, implementing NiO in devices without PTAA or SAM is seldom reported to lead to high-performance devices. In this work, we assess the effect of key NiO properties deemed relevant in literature, namely- resistivity and surface energy, on the device performance and systematically compare the NiO-based devices with those based on PTAA. To this purpose, (thermal) atomic layer deposited (ALD) NiO (NiOBu-MeAMD), Al-doped NiO (Al:NiOBu-MeAMD), and plasma-assisted ALD NiO (NiOMeCp) films, characterized by a wide range of resistivity, are investigated. Although Al:NiOBu-MeAMD (∼400 Ω cm) and NiOMeCp(∼80 Ωcm) films have a lower resistivity than NiOBu-MeAMD (∼10 kΩ cm), the Al:NiOBu-MeAMD and NiOMeCp-based devices are found to have a modest open circuit voltage (V OC) gain of ∼30 mV compared to NiOBu-MeAMD-based devices. Overall, the best-performing NiO-based devices (∼14.8% power conversion efficiency (PCE)) still lag behind the PTAA-based devices (∼17.5%), primarily due to a V OC loss of ∼100 mV. Further investigation based on light intensity analysis of the V OC and FF and the decrease in V OC compared to the quasi-Fermi level splitting (QFLS) indicates that the V OC is limited by trap-assisted recombination at the NiO/perovskite interface. Additionally, SCAPS simulations show that the presence of a high interfacial trap density leads to a V OC loss in NiO-based devices. Upon passivation of the NiO/perovskite interface with Me-4PACz, the V OC increases by 170-200 mV and is similar for NiOBu-MeAMD and Al:NiOBu-MeAMD, leading to the conclusion that there is no influence of the NiO resistivity on the V OC once interface passivation is realized. Finally, our work highlights the necessity of comparing NiO-based devices with state-of-the-art HTL-based devices to draw conclusion about the influence of specific material properties on device performance.

Self-Assembled Monolayer Suppresses Interfacial Reaction between NiOx and Perovskite for Efficient and Stable Inverted Inorganic Perovskite Solar Cells

Inorganic NiOx has attracted tremendous attention in organic-inorganic hybrid perovskite solar cells (PSCs) in recent years but is relatively less used in all-inorganic PSCs. In this study, we have discovered and confirmed the detrimental interfacial reaction between NiOx and DMAI-containing CsPbI3 inorganic perovskites. Thus, a self-assembled monolayer, Br-2PACz, is employed to modify the NiOx surface to obstruct the adverse interfacial reaction and further improve the device performance. The results demonstrate that Br-2PACz modification on NiOx can also improve interface contact, perovskite film morphology, and energy level alignments. Consequently, a champion power conversion efficiency (PCE) of 19.34% with a high open-circuit voltage (VOC) of 1.15 V is obtained for inverted NiOx/Br-2PACz-based CsPbI3 PSCs compared to the reference NiOx-based PSC with a moderate PCE of 15.16% (VOC 1.05 V). Moreover, the stabilities of both CsPbI3 films and devices exhibited significant enhancement after Br-2PACz modification. The unpacked PSCs could maintain 80, 73, and 89% of the initial efficiency after aging in 30-35% RH for 960 h, heating at 60 °C for 48 h, and continuous illumination for 284 h, respectively, highly superior to the reference devices. Our work offers a facile and effective approach for developing high-performance inverted NiOx-based CsPbI3 PSCs.

Nickel Oxide Hole Injection Layers for Balanced Charge Injection in Quantum Dot Light-Emitting Diodes

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation displays, but suffer from carrier imbalance arising from lower hole injection compared to electron injection. A defect engineering strategy is reported to tackle transport limitations in nickel oxide-based inorganic hole-injection layers (HILs) and find that hole injection is able to enhance in high-performance InP QLEDs using the newly designed material. Through optoelectronic simulations, how the electronic properties of NiOx affect hole injection efficiency into an InP QD layer, finding that efficient hole injection depends on lowering the hole injection barrier and enhancing the acceptor density of NiOx is explored. Li doping and oxygen enriching are identified as effective strategies to control intrinsic and extrinsic defects in NiOx, thereby increasing acceptor density, as evidenced by density functional theory calculations and experimental validation. With fine-tuned inorganic HIL, InP QLEDs exhibit a luminance of 45 200 cd m-2 and an external quantum efficiency of 19.9%, surpassing previous inorganic HIL-based QLEDs. This study provides a path to designing inorganic materials for more efficient and sustainable lighting and display technologies.

Efficient Air-Processed MA-Free Perovskite Solar Cells by SH-Based Silane Interface Modification

Air-processed perovskite solar cells (PSCs) with high photoelectric conversion efficiency (PCE) can not only further reduce the production cost but also promote its industrialization. During the preparation of the PSCs in ambient air, the contact of the buried interface not only affects the crystallization of the perovskite film but also affects the interface carrier transport, which is directly related to the performance of the device. Here, we optimize the buried interface by introducing 3-mercaptopropyltrimethoxysilane (MPTMS, (CH3O)3Si(CH2)3SH) on the nickel oxide (NiOx) surface. The crystallization of the perovskite film is improved by enhancing surface hydrophobicity; besides, the SH-based functional group of MPTMS passivates the uncoordinated lead at the interface, which effectively reduces the defects at the bottom interface of perovskite and inhibits the nonradiative recombination at the interface. Moreover, the energy level between the NiOx layer and the perovskite layer is better matched. Based on multiple functions of MPTMS modification, the open circuit voltage of the device is obviously improved, and efficient air-processed methylamine-free (MA-free) PSCs are realized with PCE reaching 21.0%. The device still maintains the initial PCE of 85% after 1000 h aging in the glovebox. This work highlights interface modification in air-processed MA-free PSCs to promote the industrialization of PSCs.

Enhanced photocatalytic and electrochemical performance of hydrothermally prepared NiO-doped Co nanocomposites

In this study, we synthesize nanostructured nickel oxide (NiO) and doped cobalt (Co) by combining nickel(II) chloride hexahydrate (NiCl2.6H2O) and sodium hydroxide (NaOH) as initial substances. We analyzed the characteristics of the product nanostructures, including their structure, optical properties, and magnetic properties, using various techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet absorption spectroscopy (UV-Vis), Fourier transform infrared (FTIR) spectroscopy, and vibrating sample magnetometers (VSM). The NiO nanoparticles doped with Co showed photocatalytic activity in degrading methylene blue (MB) dye in aqueous solutions. We calculated the degradation efficiencies by analyzing the UV-Vis absorption spectra at the dye's absorption wavelength of 664 nm. It was observed that the NiO-doped Co nanoparticles facilitated enhanced recombination and migration of active elements, which led to more effective degradation of organic dyes during photocatalysis. We also assessed the electrochemical properties of the materials using cyclic voltammetry (CV) and impedance spectroscopy in a 1 mol% NaOH solution. The NiO-modified electrode exhibited poor voltammogram performance due to insufficient contact between nanoparticles and the electrolyte solution. In contrast, the uncapped NiO's oxidation and reduction cyclic voltammograms displayed redox peaks at 0.36 and 0.30 V, respectively.

Improved stability and efficiency of inverted triple-cation mixed-halide perovskite solar cells with CsI-modified NiOx hole transporting layer

Addressing the critical challenge of mitigating defect generation and enhancing the extended durability of perovskite solar cells (PeSCs) requires effective passivation materials. In our study, we investigated the impact of varying concentrations of cesium iodide (CsI), an alkali halide, on the interface layer among the hole transporting layer (HTL) and the perovskite film in a triple-cation lead hybrid halide Cs0.15FA0.81MA0.04Pb(I2.86Br0.14)3 perovskite layer. Our findings revealed that the introduction of CsI into the NiOx HTL led to improved crystallinity and a reduction in defects within the perovskite film. Consequently, the photovoltaic performance of the CsI-modified PeSC exhibited a notable enhancement. Specifically, the photoelectric conversion efficiency (PCE) increased from 18.7 % in the original PeSC, which lacked CsI modification, to 20.5 %. Moreover, this improvement in PCE was accompanied by excellent stability, with the CsI-modified PeSC retaining 80 % of its opening PCE even afterward 144 h of testing.

Heterojunction Devices Fabricated from Sprayed n-Type Ga2O3, Combined with Sputtered p-Type NiO and Cu2O

This work reports on the properties of heterojunctions consisting of n-type Ga2O3 layers, deposited using ultrasonic spray pyrolysis at high temperature from water-based solution, combined with p-type NiO and Cu2O counterparts, deposited by radio frequency and reactive, direct-current magnetron sputtering, respectively. After a comprehensive investigation of the properties of the single layers, the fabricated junctions on indium tin oxide (ITO)-coated glass showed high rectification, with an open circuit voltage of 940 mV for Ga2O3/Cu2O and 220 mV for Ga2O3/NiO under simulated solar illumination. This demonstrates in praxis the favorable band alignment between the sprayed Ga2O3 and Cu2O, with small conduction band offset, and the large offsets anticipated for both energy bands in the case of Ga2O3/NiO. Large differences in the ideality factors between the two types of heterojunctions were observed, suggestive of distinctive properties of the heterointerface. Further, it is shown that the interface between the high-temperature-deposited Ga2O3 and the ITO contact does not impede electron transport, opening new possibilities for the design of solar cell and optoelectronic device architectures.

Enhancing Surface Modification and Carrier Extraction in Inverted Perovskite Solar Cells via Self-Assembled Monolayers

Perovskite solar cells (PSCs) have been significantly improved by utilizing an inorganic hole-transporting layer (HTL), such as nickel oxide. Despite the promising properties, there are still limitations due to defects. Recently, research on self-assembled monolayers (SAMs) is being actively conducted, which shows promise in reducing defects and enhancing device performance. In this study, we successfully engineered a p-i-n perovskite solar cell structure utilizing HC-A1 and HC-A4 molecules. These SAM molecules were found to enhance the grain morphology and uniformity of the perovskite film, which are critical factors in determining optical properties and device performance. Notably, HC-A4 demonstrated superior performance due to its distinct hydrophilic properties with a contact angle of 50.3°, attributable to its unique functional groups. Overall, the HC-A4-applied film exhibited efficient carrier extraction properties, attaining a carrier lifetime of 117.33 ns. Furthermore, HC-A4 contributed to superior device performance, achieving the highest device efficiency of 20% and demonstrating outstanding thermal stability over 300 h.

Critical role of dopant in NiOx hole transport layer for mitigating redox reactivity at NiOx/absorber interface in mixed cation perovskite solar cells

Redox chemistry transpiring at the interface of NiOx hole transport layer (HTL) and perovskite absorber is a critical phenomenon leading to relatively low values of open circuit voltage (VOC) and fill factor (FF), in turn hampering the overall device performance and stability. In this work, for the first time, the hard acid electronic nature of vanadium (V) dopant in nickel oxide HTL is opportunely exploited to mitigate the undesirable Lewis acid-base reactions occurring at the HTL/mixed-cation perovskite interface. The findings of the study show that vanadium doping results in improved interfacial energetics along with decreased VOC loss, confirming that despite the increase in Ni3+/Ni2+ ratio with the vanadium dopant, the redox reaction catalyzed by Ni3+ ions is kept under check. Vanadium doping also aided in the realization of superior perovskite films with lower Urbach energy, which translated into one order increase in maximum photoinduced carrier generation rate per unit volume. Carrier dynamics investigations show fewer defect states (lower VTFL) and trap-assisted recombination (lower diode ideality factor), which optimize the devices' photovoltaic performance. These benefits collectively contribute to low-loss charge transfer across the NiOx/mixed-cation perovskite interface, which increases the relative efficiency by ∼30% for 5 wt% V-doped NiOx devices compared to pristine NiOx devices, augmented by an increase in device J-V parameters like open circuit voltage (VOC), short circuit current density (JSC), and fill factor (FF).

Key Strategies on Cu2O Photocathodes toward Practical Photoelectrochemical Water Splitting

Cuprous oxide (Cu2O) has been intensively in the limelight as a promising photocathode material for photoelectrochemical (PEC) water splitting. The state-of-the-art Cu2O photocathode consists of a back contact layer for transporting the holes, an overlayer for accelerating charge separation, a protection layer for prohibiting the photocorrosion, and a hydrogen evolution reaction (HER) catalyst for reducing the overpotential of HER, as well as a Cu2O layer for absorbing sunlight. In this review, the fundamentals and recent research progress on these components of efficient and durable Cu2O photocathodes are analyzed in detail. Furthermore, key strategies on the development of Cu2O photocathodes for the practical PEC water-splitting system are suggested. It provides the specific guidelines on the future research direction for the practical application of a PEC water-splitting system based on Cu2O photocathodes.

Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells

P-i-n geometry perovskite solar cells (PSCs) offer simplified fabrication, greater amenability to charge extraction layers, and low-temperature processing over n-i-p counterparts. Self-assembled monolayers (SAMs) can enhance the performance of p-i-n PSCs but ultrathin SAMs can be thermally unstable. We report a thermally robust hole-selective layer comprised of nickel oxide (NiOx) nanoparticle film with a surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC) SAM that can improve and stabilize the NiOx/perovskite interface. The energetic alignment and favorable contact and binding between NiOx/MeO-4PADBC and perovskite reduced the voltage deficit of PSCs with various perovskite compositions and led to strong interface toughening effects under thermal stress. The resulting 1.53-electron-volt devices achieved 25.6% certified power conversion efficiency and maintained >90% of their initial efficiency after continuously operating at 65 degrees Celsius for 1200 hours under 1-sun illumination.

Synthesizing and formulating metal oxide nanoparticle inks for perovskite solar cells

The perovskite solar cell has commercial potential due to the low-cost of materials and manufacturing processes with cell efficiencies on par with traditional technologies. Nanomaterials have many properties that make them attractive for the perovskite devices, including low-cost inks, low temperature processing, stable material properties and good charge transport. In this feature article, the use of nanomaterials in the hole transport and electron transport layers are reviewed. Specifically, SnO2 and NiOx are the leading materials with the most promise for translation to large scale applications. The review includes a discussion of the synthesis, formulation, and processing of these nanoparticles and provides insights for their further deployment towards commercially viable perovskite solar cells.

Scanning Electrochemical Microscope Studies of Charge Transfer Kinetics at the Interface of the Perovskite/Hole Transport Layer

Interfacial carrier transfer kinetics is critical to the efficiency and stability of perovskite solar cells. Herein, we measure the regeneration rate constant, absorption cross-section, reduction rate constant, and conductivity of hole transport layered perovskites using scanning electrochemical microscopy (SECM). The SECM feedback revealed that the regeneration rate constant, absorption cross-section, and reduction rate constant of the nickel oxide (NiO) layer perovskite layer are higher than those of the poly (3,4-ethyenedioxythiophene)-poly (styrenesulfonate) layered perovskite. Also, at a specific flux density ( $J_{hv}$ ), the value of the regeneration rate constant (keff) in both blue and red illuminations for the NiO/CH3NH3PbI3 film is significantly higher than in both PEDOT: PSS/CH3NH3PbI3 and FTO/CH3NH3PbI3 films. The difference in keff between layered and nonlayered perovskite conforms to the impact of the hole conducting layer on the charge transfer kinetics. According to the findings, SECM is a powerful approach for screening an appropriate hole transport layer for stable perovskite solar cells.

Enabling water-free PEDOT as hole selective layer in lead-free tin perovskite solar cells

Metal halide perovskites are set to revolutionise photovoltaic energy harvesting owing to an unmatched combination of high efficiency and low fabrication costs. However, to improve the sustainability of this technology, replacing lead with less toxic tin is highly desired. Tin halide perovskites are approaching 15% in power conversion efficiency (PCE), mainly employing PEDOT:PSS as a hole-selective layer. Unfortunately, PEDOT:PSS is processed from an aqueous solution, which is hardly compatible with the strict anoxic requirements for processing tin halide perovskites due to tin's instability to oxidation. Here, we present a water-free PEDOT formulation for developing tin-based lead-free perovskite solar cells. We show that the main difference between the PCE of devices made from aqueous and water-free PEDOT is due to the marked hydrophobicity of the latter, which complicates the perovskite deposition. By modifying the surface of water-free PEDOT with a thin Al2O3 interlayer, we could achieve good perovskite morphology that enabled perovskite solar cells with a PCE of 7.5%.

Fluorinated Interlayer Modulation of NiOx-Based Inverted Perovskite Solar Cells

p-Type inorganic nickel oxide (NiOx) exhibits high transparency, tunable-optoelectronic properties, and a work function (W_F) that is potentially suitable for hole extraction in inverted perovskite solar cells (PSCs). However, NiOx films possess surface defects that lead to high interfacial recombination and an energy offset with the ionization potential of the perovskite. Herein, we show that fluorinated 3-(2,3,4,5,6-pentafluorophenyl)propan-1-aminium iodide (FPAI) can be used to modify the electronic properties of the NiOx anode interlayer. The FPAI modification led to good perovskite crystal growth and films with reduced surface defects. The FPAI modification also increased the W_F of NiOx and improved charge extraction. These improvements led to an increased V_oc value compared with control devices without FPAI modification, 1.05 V versus 1.00 V, and a higher short-circuit current and larger fill factor. As a result, the best PSCs with FPAI-modified NiOx had a power conversion efficiency of 19.3%. Finally, the PSCs with the FPAI-modified NiOx layer were found to have improved stability.

Roles of Inorganic Oxide Based HTMs towards Highly Efficient and Long-Term Stable PSC-A Review

In just a few years, the efficiency of perovskite-based solar cells (PSCs) has risen to 25.8%, making them competitive with current commercial technology. Due to the inherent advantage of perovskite thin films that can be fabricated using simple solution techniques at low temperatures, PSCs are regarded as one of the most important low-cost and mass-production prospects. The lack of stability, on the other hand, is one of the major barriers to PSC commercialization. The goal of this review is to highlight the most important aspects of recent improvements in PSCs, such as structural modification and fabrication procedures, which have resulted in increased device stability. The role of different types of hole transport layers (HTL) and the evolution of inorganic HTL including their fabrication techniques have been reviewed in detail in this review. We eloquently emphasized the variables that are critical for the successful commercialization of perovskite devices in the final section. To enhance perovskite solar cell commercialization, we also aimed to obtain insight into the operational stability of PSCs, as well as practical information on how to increase their stability through rational materials and device fabrication.

First-Principles Study of Cu-Based Inorganic Hole Transport Materials for Solar Cell Applications

Perovskite solar cells (PSCs) and dye-sensitized solar cells (DSCs) both represent promising strategies for the sustainable conversion of sunlight into electricity and fuels. However, a few flaws of current devices hinder the large-scale establishment of such technologies. On one hand, PSCs suffer from instabilities and undesired phenomena mostly linked to the perovskite/hole transport layer (HTL) interface. Most of the currently employed organic HTL (e.g., Spiro-OMeTAD) are supposed to contribute to the perovskite decomposition and to be responsible for charge recombination processes and polarization barriers. On the other hand, power conversion efficiencies (PCEs) of DSCs are still too low to compete with other conversion technologies. Tandem cells are built by assembling p-type and n-type DSCs in a cascade architecture and, since each dye absorbs on a different portion of the solar spectrum, the harvesting window is increased and the theoretical efficiency limit for a single chromophore (i.e., the Shockley-Queisser limit) is overcome. However, such a strategy is hindered by the lack of a p-type semiconductor with optimal photocathode features. Nickel oxide has been, by far, the first-choice inorganic p-type semiconductor for both PV technologies, but its toxicity and non-optimal features (e.g., too low open circuit voltage and the presence of trap states) call for alternatives. Herein, we study of three p-type semiconductors as possible alternative to NiO, namely CuI, CuSCN and Cu₂O. To this aim, we compare the structural and electronic features of the three materials by means of a unified theoretical approach based on the state-of-the art density functional theory (DFT). We focus on the calculation of their valence band edge energies and compare such values with those of two widely employed photo-absorbers, i.e., methylammonium lead iodide (MAPI) and the triple cation MAFACsPbBrI in PSCs and P1 and Y123 dyes in DSCs, given that the band alignment and the energy offset are crucial for the charge transport at the interfaces and have direct implications on the final efficiency. We dissect the effect a copper vacancy (i.e., intrinsic p-type doping) on the alignment pattern and rationalize it from both a structural and an electronic perspective. Our data show how defects can represent a crucial degree of freedom to control the driving force for hole injection in these devices.

A Bionic Interface to Suppress the Coffee-Ring Effect for Reliable and Flexible Perovskite Modules with a Near-90% Yield Rate

The inhomogeneity, poor interfacial contact, and pinholes caused by the coffee-ring effect severely affect the printing reliability of flexible perovskite solar cells (PSCs). Herein, inspired by the bio-glue of barnacles, a bionic interface layer (Bio-IL) of NiO_x /levodopa is introduced to suppress the coffee-ring effect during printing perovskite modules. The coordination effect of the sticky functional groups in Bio-IL can pin the three-phase contact line and restrain the transport of perovskite colloidal particles during the printing and evaporation process. Moreover, the sedimentation rate of perovskite precursor is accelerated due to the electrostatic attraction and rapid volatilization from an extraordinary wettability. The superhydrophilic Bio-IL affords an even spread over a large-area substrate, which boosts a complete and uniform liquid film for heterogeneous nucleation as well as crystallization. Perovskite films on different large-area substrates with negligible coffee-ring effect are printed. Consequently, inverted flexible PSCs and perovskite solar modules achieve a high efficiency of 21.08% and 16.87%, respectively. This strategy ensures a highly reliable reproducibility of printing PSCs with a near 90% yield rate.

High-performance Ruddlesden-Popper two-dimensional perovskite solar cells via solution processed inorganic charge transport layers

Two-dimensional (2D) layered halide perovskites have been shown to enable improved long-term stability in comparison to the well-known three-dimensional hybrid organic-inorganic halide perovskites. The optoelectronic properties of the 2D perovskites are strongly influenced by the chemical nature of the charge transport layer. In this work, we fabricated Ruddlesden-Popper 2D perovskite solar cells (PSCs) using solution processed inorganic NiO_x and a C₆₀ : C₇₀ (1 : 1) mixture as the hole and electron transport layers, which significantly improved the performance of the 2D PSCs. Time resolved photoluminescence measurements indicate the shortened lifetime of excitons, which demonstrates the excellent charge extraction properties. The PSCs based on these inorganic charge transport materials (CTMs) exhibit an average power conversion efficiency (PCE) of 14.1%, which is higher than that (12.3%) of PSCs using organic CTMs of poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PCBM). Compared with PEDOT:PSS and PCBM based cases, the PSCs using inorganic CTMs also show improved long-term stability, with the PCE degradation significantly suppressed from 20% to 12% after a measurement of 15 days. The best PSCs using NiO_x and C₆₀ : C₇₀ show a high PCE of 14.4%, with a stable power output and negligible hysteresis.

Doping Free and Amorphous NiOx Film via UV Irradiation for Efficient Inverted Perovskite Solar Cells

High crystallization and conductivity are always required for inorganic carrier transport materials for cheap and high-performance inverted perovskite solar cells (PSCs). High temperature and external doping are inevitably introduced and thus greatly hamper the applications of inorganic materials for mass production of flexible and tandem devices. Here, an amorphous and dopant-free inorganic material, Ni3+ -rich NiOx , is reported to be fabricated by a novel UV irradiation strategy, which is facile, easily scaled-up, and energy-saving because all the processing temperatures are below 82 ℃. The as-prepared NiOx film shows highly improved conductivity and hole extraction ability. The rigid and flexible PSCs present the champion efficiencies of 22.45% and 19.7%, respectively. This work fills the gap of preparing metal oxide films at the temperature below 150 °C for inverted PSCs with the high efficiency of >22%. More importantly, this work upgrades the substantial understanding about inorganic materials to function well as efficient carrier transport layers without external doping and high crystallization.

Recent Developments in Upscalable Printing Techniques for Perovskite Solar Cells

Just over a decade, perovskite solar cells (PSCs) have been emerged as a next-generation photovoltaic technology due to their skyrocketing power conversion efficiency (PCE), low cost, and easy manufacturing techniques compared to Si solar cells. Several methods and procedures have been developed to fabricate high-quality perovskite films to improve the scalability and commercialize PSCs. Recently, several printing technologies such as blade-coating, slot-die coating, spray coating, flexographic printing, gravure printing, screen printing, and inkjet printing have been found to be very effective in controlling film formation and improving the PCE of over 21%. This review summarizes the intensive research efforts given for these printing techniques to scale up the perovskite films as well as the hole transport layer (HTL), the electron transport layer (ETL), and electrodes for PSCs. In the end, this review presents a description of the future research scope to overcome the challenges being faced in the printing techniques for the commercialization of PSCs.

Surface Passivation of Sputtered NiO x Using a SAM Interface Layer to Enhance the Performance of Perovskite Solar Cells

Sputtered NiO _x (sp-NiO _x ) is a preferred hole transporting material for perovskite solar cells because of its hole mobility, ease of manufacturability, good stability, and suitable Fermi level for hole extraction. However, uncontrolled defects in sp-NiO _x can limit the efficiency of solar cells fabricated with this hole transporting layer. An interfacial layer has been proposed to modify the sp-NiO _x /perovskite interface, which can contribute to improving the crystallinity of the perovskite film. Herein, a 2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) self-assembled monolayer was used to modify an sp-NiO _x surface. We found that the MeO-2PACz interlayer improves the quality of the perovskite film due to an enlarged domain size, reduced charge recombination at the sp-NiO _x /perovskite interface, and passivation of the defects in sp-NiO _x surfaces. In addition, the band tail states are also reduced, as indicated by photothermal deflection spectroscopy, which thus indicates a reduction in defect levels. The overall outcome is an improvement in the device efficiency from 11.9% to 17.2% due to the modified sp-NiO _x /perovskite interface, with an active area of 1 cm² (certified efficiency of 16.25%). On the basis of these results, the interfacial engineering of the electronic properties of sp-NiO _x /MeO-2PACz/perovskite is discussed in relation to the improved device performance.

Construction of Charge Transport Channels at the NiOx/Perovskite Interface through Moderate Dipoles toward Highly Efficient Inverted Solar Cells

NiO_x-based perovskite solar cells (PSCs) have attracted much attention because of their low fabrication temperature, suppressed hysteresis, and superior stability. However, the poor interfacial contacts between NiO_x and perovskite layers always limit the progress of PSCs. Here, we applied 2-thiophenemethylamine (TPMA) as charge transport channels at the interface between NiO_x and perovskite layers. The introduction of TPMA provides moderate dipole moment pointing to the perovskite side and effectively promotes the charge transportation. Meanwhile, TPMA anchorage also passivates the defect states at the surfaces of both NiO_x and MAPbI₃, which compensates the voltage loss due to the change in NiO_x work function induced by the dipole. Thus, the device performance has been significantly enhanced in both electrochemical properties and power conversion efficiency. Our work has demonstrated a new way of improving current and voltage in the NiO_x-based PSCs simultaneously through a moderate interfacial dipole moment toward highly efficient PSCs.

NiO-Based Electronic Flexible Devices

Personal, portable, and wearable electronics have become items of extensive use in daily life. Their fabrication requires flexible electronic components with high storage capability or with continuous power supplies (such as solar cells). In addition, formerly rigid tools such as electrochromic windows find new utilizations if they are fabricated with flexible characteristics. Flexibility and performances are determined by the material composition and fabrication procedures. In this regard, low-cost, easy-to-handle materials and processes are an asset in the overall production processes and items fruition. In the present mini-review, the most recent approaches are described in the production of flexible electronic devices based on NiO as low-cost material enhancing the overall performances. In particular, flexible NiO-based all-solid-state supercapacitors, electrodes electrochromic devices, temperature devices, and ReRAM are discussed, thus showing the potential of NiO as material for future developments in opto-electronic devices.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

Synergistic Approach toward Erbium-Passivated Triple-Anion Organic-Free Perovskite Solar Cells with Excellent Performance for Agrivoltaics Application

All-inorganic perovskite solar cells (IPSCs) have gained massive attention due to their less instability against common degradation factors (light, heat, and moisture) than their organic-inorganic hybrid counterparts. Inorganic perovskites bear a general formula of CsPbX3 (X = Cl, I, Br). The mixed halide CsPbIBr2 perovskite possesses an intermediate band gap of 2.03 eV with enhanced stability, which is still available for photovoltaic applications and the research focus of this work. We present a synergistic approach of pre-heated solution dropping with inorganic additive inclusion to deposit the organic-free triple anion CsPbIBr2 PSC. Erbium (Er)-passivated triple-anion CsI(PbBr2)0.97(ErCl3)0.03 IPSCs with inorganic carrier selective layers (CTLs), that is, organic-free, are fabricated with enhanced carrier diffusion length and crystalline grain size while lessening the grain boundaries near perovskite active layer (PAL)-bulk/carrier selective interfaces. As a result, the trap-state densities within the perovskite bulk were suppressed with stabilized CTL/PAL interfaces for smooth and enhanced carrier transportation. Therefore, for the first time, we contradict the common belief of VOC loss due to halide segregation, as a nice VOC of about 1.34 V is achieved for an organic-free IPSC through enriching initial radiative efficiency, even when halide segregation is present. The optimized organic-free IPSC yielded a power conversion efficiency of 11.61% and a stabilized power output of 10.72%, which provides the potential opportunity to integrate into agrivoltaics (AgV) projects.

Enhanced Self-Assembled Monolayer Surface Coverage by ALD NiO in p-i-n Perovskite Solar Cells

Metal halide perovskites have attracted tremendous attention due to their excellent electronic properties. Recent advancements in device performance and stability of perovskite solar cells (PSCs) have been achieved with the application of self-assembled monolayers (SAMs), serving as stand-alone hole transport layers in the p-i-n architecture. Specifically, phosphonic acid SAMs, directly functionalizing indium-tin oxide (ITO), are presently adopted for highly efficient devices. Despite their successes, so far, little is known about the surface coverage of SAMs on ITO used in PSCs application, which can affect the device performance, as non-covered areas can result in shunting or low open-circuit voltage. In this study, we investigate the surface coverage of SAMs on ITO and observe that the SAM of MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) inhomogeneously covers the ITO substrate. Instead, when adopting an intermediate layer of NiO between ITO and the SAM, the homogeneity, and hence the surface coverage of the SAM, improve. In this work, NiO is processed by plasma-assisted atomic layer deposition (ALD) with Ni(MeCp)₂ as the precursor and O₂ plasma as the co-reactant. Specifically, the presence of ALD NiO leads to a homogeneous distribution of SAM molecules on the metal oxide area, accompanied by a high shunt resistance in the devices with respect to those with SAM directly processed on ITO. At the same time, the SAM is key to the improvement of the open-circuit voltage of NiO + MeO-2PACz devices compared to those with NiO alone. Thus, the combination of NiO and SAM results in a narrower distribution of device performance reaching a more than 20% efficient champion device. The enhancement of SAM coverage in the presence of NiO is corroborated by several characterization techniques including advanced imaging by transmission electron microscopy (TEM), elemental composition quantification by Rutherford backscattering spectrometry (RBS), and conductive atomic force microscopy (c-AFM) mapping. We believe this finding will further promote the usage of phosphonic acid based SAM molecules in perovskite PV.

Metal Oxide-Induced Instability and Its Mitigation in Halide Perovskite Solar Cells

Halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology for sustainable energy solutions because of their impressive power conversion efficiency and a path to be manufactured by low-cost, high-throughput methods. To reach PSCs' full potential for practical implementation, it is crucial to solve the issues related to long-term operational stability. Given that PSCs consist of many layers of dissimilar materials which form multiple internal interfaces, it is prudent to examine whether there exist interfacial interactions, most importantly between transport layers and perovskite absorbers, that can trigger instability and affect device performance. In this Perspective, we bring to the attention of the PSC research community the lesser-known interfacial degradation of halide perovskites promoted by contact with metal oxide transport layers and highlight the deleterious effects on the PSCs' performance and stability. We also discuss various mitigation strategies that have shown promise for achieving high-performing and stable PSCs.

Study on the Ozone Gas Sensing Properties of rf-Sputtered Al-Doped NiO Films

Al-doped NiO (NiO:Al) has attracted the interest of researchers due to its excellent optical and electrical properties. In this work, NiO:Al films were deposited on glass substrates by the radio frequencies (rf) sputtering technique at room temperature and they were tested against ozone gas. The Oxygen content in (Ar-O2) plasma was varied from 2% to 4% in order to examine its effect on the gas sensing performance of the films. The thickness of the films was between 160.3 nm and 167.5 nm, while the Al content was found to be between 5.3 at% and 6.7 at%, depending on the oxygen content in plasma. It was found that NiO:Al films grown with 4% O2 in plasma were able to detect 60 ppb of ozone with a sensitivity of 3.18% at room temperature, while the detection limit was further decreased to 10 ppb, with a sensitivity of 2.54%, at 80 °C, which was the optimum operating temperature for these films. In addition, the films prepared in 4% O2 in plasma had lower response and recovery time compared to those grown with lower O2 content in plasma. Finally, the role of the operating temperature on the gas sensing properties of the NiO:Al films was investigated.

Enhanced Photovoltaic Properties of Perovskite Solar Cells by Employing Bathocuproine/Hydrophobic Polymer Films as Hole-Blocking/Electron-Transporting Interfacial Layers

In this study, we improved the photovoltaic (PV) properties and storage stabilities of inverted perovskite solar cells (PVSCs) based on methylammonium lead iodide (MAPbI3) by employing bathocuproine (BCP)/poly(methyl methacrylate) (PMMA) and BCP/polyvinylpyrrolidone (PVP) as hole-blocking and electron-transporting interfacial layers. The architecture of the PVSCs was indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester/BCP based interfacial layer/Ag. The presence of PMMA and PVP affected the morphological stability of the BCP and MAPbI3 layers. The storage-stability of the BCP/PMMA-based PVSCs was enhanced significantly relative to that of the corresponding unmodified BCP-based PVSC. Moreover, the PV performance of the BCP/PVP-based PVSCs was enhanced when compared with that of the unmodified BCP-based PVSC. Thus, incorporating hydrophobic polymers into BCP-based hole-blocking/electron-transporting interfacial layers can improve the PV performance and storage stability of PVSCs.

Upscaling Inverted Perovskite Solar Cells: Optimization of Laser Scribing for Highly Efficient Mini-Modules

The upscaling of perovskite solar cells is one of the challenges that must be addressed to pave the way toward the commercial development of this technology. As for other thin-film photovoltaic technologies, upscaling requires the fabrication of modules composed of series-connected cells. In this work we demonstrate for the first time the interconnection of inverted modules with NiOx using a UV ns laser, obtaining a 10.2 cm2 minimodule with a 15.9% efficiency on the active area, the highest for a NiOx based perovskite module. We use optical microscopy, energy-dispersive X-ray spectroscopy, and transfer length measurement to optimize the interconnection. The results are implemented in a complete electrical simulation of the cell-to-module losses to evaluate the experimental results and to provide an outlook on further development of single junction and multijunction perovskite modules.