Critical Role of Removing Impurities in Nickel Oxide on High-Efficiency and Long-Term Stability of Inverted Perovskite Solar Cells

Highly conductive and homogeneous NiOx nanoparticles for stable and efficient flexible perovskite solar cells

We present a facile strategy to improve the conductivity and homogeneousness of nickel oxide nanoparticles (NiOx NPs). The inverted flexible perovskite solar cells (F-PSCs) prepared with NiOx achieved impressive efficiencies of 22.68% under AM 1.5G and 35.59% under 1000 lux, respectively.

Modification at ITO/NiOx Interface with MoS2 Enables Hole Transport for Efficient and Stable Inverted Perovskite Solar Cells

Inverted perovskite solar cells (IPSCs) utilizing nickel oxide (NiOx) as hole transport material have made great progress, driven by improvements in materials and interface engineering. However, challenges remain due to the low intrinsic conductivity of NiOx and inefficient hole transport. In this study, we introduced MoS2 nanoparticles at the indium tin oxide (ITO) /NiOx interface to enhance the ITO surface and optimize the deposition of NiOx, resulting in increased conductivity linked to a ratio of Ni3+:Ni2+. This interface modification not only optimized energy level but also promoted hole transport and reduced defects. Consequently, IPSCs with MoS2 modified at ITO/NiOx interface achieved a champion power conversion efficiency (PCE) of 21.42 %, compared to 20.25 % without modification. Additionally, unencapsulated IPSCs with this interface modification displayed improved stability under thermal, light, humidity and ambient conditions. This innovative strategy for ITO/NiOx interface modification efficiently promotes hole transportation and can be integrated with other interface engineering approaches, offering valuable insights for the development of highly efficient and stable IPSCs.

Bilateral Chemical Linking at NiOx Buried Interface Enables Efficient and Stable Inverted Perovskite Solar Cells and Modules

Inverted NiOx-based perovskite solar cells (PSCs) exhibit considerable potential because of their low-temperature processing and outstanding excellent stability, while is challenged by the carriers transfer at buried interface owing to the inherent low carrier mobility and abundant surface defects that directly deteriorates the overall device fill factor. Present work demonstrates a chemical linker with the capability of simultaneously grasping NiOx and perovskite crystals by forming a Ni-S-Pb bridge at buried interface to significantly boost the carriers transfer, based on a rationally selected molecule of 1,3-dimethyl-benzoimidazol-2-thione (NCS). The constructed buried interface not only reduces the pinholes and needle-like residual PbI2 at the buried interface, but also deepens the work function and valence band maximum positions of NiOx, resulting in a smaller VBM offset between NiOx and perovskite film. Consequently, the modulated PSCs achieved a high fill factor up to 86.24 %, which is as far as we know the highest value in records of NiOx-based inverted PSCs. The NCS custom-tailored PSCs and minimodules (active area of 18 cm2) exhibited a champion efficiency of 25.05 % and 21.16 %, respectively. The unencapsulated devices remains over 90 % of their initial efficiency at maximum power point under continuous illumination for 1700 hours.

The Huge Role of Tiny Impurities in Nanoscale Synthesis

Nanotechnology is vital to many current industries, including electronics, energy, textiles, agriculture, and theranostics. Understanding the chemical mechanisms of nanomaterial synthesis has contributed to the tunability of their unique properties, although studies frequently overlook the potential impact of impurities. Impurities can show adverse effects, clouding the interpretation of results or limiting the practical utility of the nanomaterial. On the other hand, as successful doping has demonstrated, the intentional introduction of impurities can be a powerful tool for enhancing the properties of a nanomaterial. This Review examines the complex role of impurities, unintentionally or intentionally added, during nanoscale synthesis and their effects on the performance and usefulness of the most common classes of nanomaterials: nanocarbons, noble metal and metal oxide nanoparticles, semiconductor quantum dots, thermoelectrics, and perovskites.

Suppressing Redox Reactions at the Perovskite-Nickel Oxide Interface with Zinc Nitride to Improve the Performance of Perovskite Solar Cells

For p-i-n perovskite solar cells (PSCs), nickel oxide (NiOx) hole transport layers (HTLs) are the preferred interfacial layer due to their low cost, high mobility, high transmittance, and stability. However, the redox reaction between the Ni≥3+ and hydroxyl groups in the NiOx and perovskite layer leads to oxidized CH3NH3 + and reacts with PbI in the perovskite, resulting in a large number of non-radiative recombination sites. Among various transition metals, an ultra-thin zinc nitride (Zn3N2) layer on the NiOx surface is chosen to prevent these redox reactions and interfacial issues using a simple solution process at low temperatures. The redox reaction and non-radiative recombination at the interface of the perovskite and NiOx reduce chemically by using interface modifier Zn3N2 to reduce hydroxyl group and defects on the surface of NiOx. A thin layer of Zn3N2 at the NiOx/perovskite interface results in a high Ni3+/Ni2+ ratio and a significant work function (WF), which inhibits the redox reaction and provides a highly aligned energy level with perovskite crystal and rigorous trap-passivation ability. Consequently, Zn3N2-modified NiOx-based PSCs achieve a champion PCE of 21.61%, over the NiOx-based PSCs. After Zn3N2 modification, the PSC can improve stability under several conditions.

Synergistic Redox Modulation for High-Performance Nickel Oxide-Based Inverted Perovskite Solar Modules

Nickel oxide (NiOx)-based inverted perovskite solar cells stand as promising candidates for advancing perovskite photovoltaics towards commercialization, leveraging their remarkable stability, scalability, and cost-effectiveness. However, the interfacial redox reaction between high-valence Ni4+ and perovskite, alongside the facile conversion of iodide in perovskite into I2, significantly deteriorates the performance and reproducibility of NiOx-based perovskite photovoltaics. Here, potassium borohydride (KBH4) is introduced as a dual-action reductant, which effectively avoids the Ni4+/perovskite interface reaction and mitigates the iodide-to-I2 oxidation within perovskite film. This synergistic redox modulation significantly suppresses nonradiative recombination and increases the carrier lifetime. As a result, an impressive power conversion efficiency of 24.17% for NiOx-based perovskite solar cells is achieved, and a record efficiency of 20.2% for NiOx-based perovskite solar modules fabricated under ambient conditions. Notably, when evaluated using the ISOS-L-2 standard protocol, the module retains 94% of its initial efficiency after 2000 h of continuous illumination under maximum power point at 65 °C in ambient air.

Efficient and Stable Inverted Perovskite Solar Modules Enabled by Solid-Liquid Two-Step Film Formation

A considerable efficiency gap exists between large-area perovskite solar modules and small-area perovskite solar cells. The control of forming uniform and large-area film and perovskite crystallization is still the main obstacle restricting the efficiency of PSMs. In this work, we adopted a solid-liquid two-step film formation technique, which involved the evaporation of a lead iodide film and blade coating of an organic ammonium halide solution to prepare perovskite films. This method possesses the advantages of integrating vapor deposition and solution methods, which could apply to substrates with different roughness and avoid using toxic solvents to achieve a more uniform, large-area perovskite film. Furthermore, modification of the NiOx/perovskite buried interface and introduction of Urea additives were utilized to reduce interface recombination and regulate perovskite crystallization. As a result, a large-area perovskite film possessing larger grains, fewer pinholes, and reduced defects could be achieved. The inverted PSM with an active area of 61.56 cm2 (10 × 10 cm2 substrate) achieved a champion power conversion efficiency of 20.56% and significantly improved stability. This method suggests an innovative approach to resolving the uniformity issue associated with large-area film fabrication.

Redox-Sensitive NiOx Stabilizing Perovskite Films for High-Performance Photovoltaics

Metal halide perovskite materials inherently possess imperfections, particularly under nonequilibrium conditions, such as exposure to light or heat. To tackle this challenge, we introduced stearate ligand-capped nickel oxide (NiOx), a redox-sensitive metal oxide with variable valence, into perovskite intermediate films. The integration of NiOx improved the efficiency and stability of perovskite solar cells (PSCs) by offering multifunctional roles: (1) chemical passivation for ongoing defect repair, (2) energetic passivation to bolster defect tolerance, and (3) field-effect passivation to mitigate charge accumulation. Employing a synergistic approach that tailored these three passivation mechanisms led to a substantial increase in the devices' efficiencies. The target cell (0.12 cm2) and module (18 cm2) exhibited efficiencies of 24.0 and 22.9%, respectively. Notably, the encapsulated modules maintained almost 100 and 87% of the initial efficiencies after operating for 1100 h at the maximum power point (60 °C, 50% RH) and 2000 h of damp-heat testing (85 °C, 85% RH), respectively. Outdoor real-time tests further validated the commercial viability of the NiOx-assisted PSMs. The proposed passivation strategy provides a practical and uncomplicated approach for fabricating high-efficiency and stable photovoltaics.

Multiarmed Aromatic Ammonium Salts Boost the Efficiency and Stability of Inverted Organic Solar Cells

Inverted organic solar cells (OSCs) have attracted much attention because of their outstanding stability, with zinc oxide (ZnO) being commonly used as the electron transport layer (ETL). However, both surface defects and the photocatalytic effect of ZnO could lead to serious photodegradation of acceptor materials. This, in turn, hampers the improvement of the efficiency and stability in OSCs. Herein, we developed a multiarmed aromatic ammonium salt, namely, benzene-1,3,5-triyltrimethanaminium bromide (PhTMABr), for modifying ZnO. This compound possesses mild weak acidity aimed at removing the residual amines present within ZnO film. In addition, the PhTMABr could also passivate surface defects of ZnO through multiple hydrogen-bonding interactions between its terminal amino groups and the oxygen anion of ZnO, leading to a better interface contact, which effectively enhances charge transport. As a result, an efficiency of 18.75% was achieved based on the modified ETL compared to the bare ZnO (PCE = 17.34%). The devices utilizing the modified ZnO retained 87% and 90% of their initial PCE after thermal stress aging at 65 °C for 1500 h and continuous 1-sun illumination with maximum power point (MPP) tracking for 1780 h, respectively. Importantly, the extrapolated T80 lifetime with MPP tracking exceeds 10 000 h. The new class of materials employed in this work to modify the ZnO ETL should pave the way for enhancing the efficiency and stability of OSCs, potentially advancing their commercialization process.

Understanding the Heterointerfaces in Perovskite Solar Cells via Hole Selective Layer Surface Functionalization

Interfaces in perovskite solar cells (PSCs) play a pivotal role in determining device performance by influencing charge transport and recombination. Understanding the physical processes at these interfaces is essential for achieving high-power conversion efficiency in PSCs. Particularly, the interfaces involving oxide-based transport layers are susceptible to defects like dangling bonds, excess oxygen, or oxygen deficiency. To address this issue, the surface of NiOx is passivated using octadecylphosphonic acid (ODPA), resulting in improved charge transport across the perovskite hole transport layer (HTL) interface. This surface treatment has led to the development of hysteresis-free devices with an impressive ≈13% increase in power conversion efficiency. Computational studies have explored the halide perovskite architecture of ODPA-treated HTL/Perovskite, aiming to unlock superior photovoltaic performance. The ODPA surface functionalization has demonstrated enhanced device performance, characterized by superior charge exchange capacity. Moreover, higher band-to-band recombination in photoluminescence and electroluminescence indicates presence of lower mid-gap energy states, thereby increasing the effective photogenerated carrier density. These findings are expected to promote the utilization of various phosphonic acid-based self-assembly monolayers for surface passivation of oxide-based transport layers in perovskite solar cells. Ultimately, this research contributes to the realization of efficient halide PSCs by harnessing the favorable architecture of NiOx interfaces.

Quenching Detrimental Reactions and Boosting Hole Extraction via Multifunctional NiOx /Perovskite Interface Passivation for Efficient and Stable Inverted Solar Cells

Nickel oxide (NiOx ) is one of the most promising hole transport materials for inverted perovskite solar cells (PSCs). However, its application is severely restrained due to unfavorable interfacial reactions and insufficient charge carrier extraction. Herein, a multifunctional modification at the NiOx /perovskite interface is developed via introducing fluorinated ammonium salt ligand to synthetically solve the obstacles. Specifically, the interface modification can chemically convert detrimental Ni≥3+ to lower oxidation state, resulting in the elimination of interfacial redox reactions. Meanwhile, interfacial dipole is incorporated simultaneously to tune the work function of NiOx and optimize energy level alignment, which effectively promotes the charge carrier extraction. Therefore, the modified NiOx -based inverted PSCs achieve a remarkable power conversion efficiency (PCE) of 22.93%. Moreover, the unencapsulated devices obtain a significantly enhanced long-term stability, maintaining over 85% and 80% of the initial PCEs after storage in ambient air with a high relative humidity of 50-60% for 1000 h and continuous operation at maximum power point under one-sun illumination for 700 h, respectively.

Fully Aromatic Self-Assembled Hole-Selective Layer toward Efficient Inverted Wide-Bandgap Perovskite Solar Cells with Ultraviolet Resistance

Ultraviolet-induced degradation has emerged as a critical stability concern impeding the widespread adoption of perovskite solar cells (PSCs), particularly in the context of phase-unstable wide-band gap perovskite films. This study introduces a novel approach by employing a fully aromatic carbazole-based self-assembled monolayer, denoted as (4-(3,6-dimethoxy-9H-carbazol-9-yl)phenyl)phosphonic acid (MeO-PhPACz), as a hole-selective layer (HSL) in inverted wide-band gap PSCs. Incorporating a conjugated linker plays a pivotal role in promoting the formation of a dense and highly ordered HSL on substrates, facilitating subsequent perovskite interfacial interactions, and fostering the growth of uniform perovskite films. The high-quality film could effectively suppress interfacial non-radiative recombination, improving hole extraction/transport efficiency. Through these advancements, the optimized wide-band gap PSCs, featuring a band gap of 1.68 eV, attain an impressive power conversion efficiency (PCE) of 21.10 %. Remarkably, MeO-PhPACz demonstrates inherent UV resistance and heightened UV absorption capabilities, substantially improving UV resistance for the targeted PSCs. This characteristic holds significance for the feasibility of large-scale outdoor applications.

Achieving High Fill Factor in Efficient P-i-N Perovskite Solar Cells

Lead halide perovskite solar cells (PSCs) have made unprecedented progress, exhibiting great potential for commercialization. Among them, inverted p-i-n PSCs provide outstanding compatibility with flexible substrates, more importantly, with silicon (Si) bottom devices for higher efficiency perovskite-Si tandem solar cells. However, even with recently obtained efficiency over 25%, the investigation of inverted p-i-n PSCs is still behind the n-i-p counterpart so far. Recent progress has demonstrated that the fill factor (FF) in inverted PSCs currently still underperforms relative to open-circuit voltage and short-circuit current density, which requires an in-depth understanding of the mechanism and further research. In this review article, the recent advancements in high FF inverted PSCs by adopting the approaches of interfacial optimization, precursor engineering as well as fabrication techniques to minimize undesirable recombination are summarized. Insufficient carrier extraction and transport efficiency are found to be the main factors that hinder the current FF of inverted PSCs. In addition, insights into the main factors limiting FF and strategies for minimizing series resistance in inverted PSCs are presented. The continuous efforts dedicated to the FF of high-performance inverted devices may pave the way toward commercial applications of PSCs in the near future.

Phase-Engineering of Layered Nickel Hydroxide for Synthesizing High-Quality NiOx Nanocrystals for Efficient Inverted Flexible Perovskite Solar Cells

Nickel oxide (NiOx) nanocrystals have been widely used in inverted (p-i-n) flexible perovskite solar cells (fPSCs) due to their remarkable advantages of low cost and outstanding stability. However, anion and cation impurities such as NO3- widely exist in the NiOx nanocrystals obtained from calcinated nickel hydroxide (Ni(OH)2). The impurities impair the photovoltaic performance of fPSCs. In this work, we report a facile but effective way to reduce the impurities within the NiOx nanocrystals by regulating the Ni(OH)2 crystal phase. We add different alkalis, such as organic ammonium hydroxide and alkali metal hydroxides, to nickel nitrate solutions to precipitate layered Ni(OH)2 with different crystalline phase compositions (α and β mixtures). Especially, Ni(OH)2 with a high β-phase content (such as from KOH) has a narrower crystal plane spacing, resulting in fewer residual impurity ions. Thus, the NiOx nanocrystals, by calcinating the Ni(OH)x with excess β phase from KOH, show improved performance in inverted fPSCs. A champion power conversion efficiency (PCE) of 20.42% has been achieved, which is among the state-of-art inverted fPSCs based on the NiOx hole transport material. Moreover, the reduced impurities are beneficial for enhancing the fPSCs' stability. This work provides an essential but facile strategy for developing high-performance inverted fPSCs.

Neglected acidity pitfall: boric acid-anchoring hole-selective contact for perovskite solar cells

The spontaneous formation of self-assembly monolayer (SAM) on various substrates represents an effective strategy for interfacial engineering of optoelectronic devices. Hole-selective SAM is becoming popular among high-performance inverted perovskite solar cells (PSCs), but the presence of strong acidic anchors (such as -PO3H2) in state-of-the-art SAM is detrimental to device stability. Herein, we report for the first time that acidity-weakened boric acid can function as an alternative anchor to construct efficient SAM-based hole-selective contact (HSC) for PSCs. Theoretical calculations reveal that boric acid spontaneously chemisorbs onto indium tin oxide (ITO) surface with oxygen vacancies facilitating the adsorption progress. Spectroscopy and electrical measurements indicate that boric acid anchor significantly mitigates ITO corrosion. The excess boric acid containing molecules improves perovskite deposition and results in a coherent and well-passivated bottom interface, which boosts the fill factor (FF) performance for a variety of perovskite compositions. The optimal boric acid-anchoring HSC (MTPA-BA) can achieve power conversion efficiency close to 23% with a high FF of 85.2%. More importantly, the devices show improved stability: 90% of their initial efficiency is retained after 2400 h of storage (ISOS-D-1) or 400 h of operation (ISOS-L-1), which are 5-fold higher than those of phosphonic acid SAM-based devices. Acidity-weakened boric acid SAMs, which are friendly to ITO, exhibits well the great potential to improve the stability of the interface as well as the device.

Bottom-Up Templated and Oriented Crystallization for Inverted Triple-Cation Perovskite Solar Cells with Stabilized Nickel-Oxide Interface

Inverted-structure perovskite solar cells (PSCs) are known for their superior device stability. However, based on nickel-oxide (NiO_x ) substrate, disordered crystallization and bottom interface instability of perovskite film are still the main factors that compromise the power conversion efficiency (PCE) of PSCs. Here, 2D perovskite of thiomorpholine 1,1-dioxide lead iodide (Td₂ PbI₄ ) is introduced as a template to prepare 3D perovskite thin film with high crystal orientation and large grain size via a bottom-up growth method. By adding TdCl to the precursor solution, pre-crystallized 2D Td₂ PbI₄ seeds can accumulate at the bottom interface, lowering the barrier of nucleation, and templating the growth of 3D perovskite films with improved (100) orientation and reduced defects during crystallization. In addition, 2D Td₂ PbI₄ at the bottom interface also hinders the interfacial redox reaction and reduces the hole extraction barrier on the buried interface. Based on this, the Td-0.5 PSC achieves a PCE of 22.09% and an open-circuit voltage of 1.16 V. Moreover, Td-0.5 PSCs show extremely high stability, which retains 84% of its initial PCE after 500 h of continuous illumination under maximum power point operating conditions in N₂ atmosphere. This work paves the way for performance improvement of inverted PSCs on NiO_x substrate.

Stabilizing Perovskite Precursor by Synergy of Functional Groups for NiOx -Based Inverted Solar Cells with 23.5 % Efficiency

Perovskite solar cells suffer from poor reproducibility due to the degradation of perovskite precursor solution. Herein, we report an effective precursor stabilization strategy via incorporating 3-hydrazinobenzoic acid (3-HBA) containing carboxyl (-COOH) and hydrazine (-NHNH₂ ) functional groups as stabilizer. The oxidation of I^- , deprotonation of organic cations and amine-cation reaction are the main causes of the degradation of mixed organic cation perovskite precursor solution. The -NHNH₂ can reduce I₂ defects back to I^- and thus suppress the oxidation of I^- , while the H⁺ generated by -COOH can inhibit the deprotonation of organic cations and subsequent amine-cation reaction. The above degradation reactions are simultaneously inhibited by the synergy of functional groups. The inverted device achieves an efficiency of 23.5 % (certified efficiency of 23.3 %) with an excellent operational stability, retaining 94 % of the initial efficiency after maximum power point tracking for 601 hours.

Passivating Lead Halide Perovskites Using Pyridinium Salts with Superhalogen Atoms

Passivating lead halide perovskites using pyridinium salts has attracted enormous attention, but the excellent surface passivation of the halide perovskites has not been achieved by using only a pyridinium salt until now. Herein, passivating the (001) planes of the cubic CsPbI₃, CH₃NH₃PbI₃, and NH₂CHNH₂PbI₃ perovskites using the pyridinium salts of C₅NH₆X (X = Cl, Br, I, PF₆, ClO₄, or BF₄) is systematically studied by high-throughput first-principle calculations and ab initio molecular dynamics simulations. The results show that the excellent surface passivation of the three perovskites is achieved by the pyridinium salt of C₅NH₆BF₄ (i.e., shallow level, negative formation energy, unchanged band-edge construction, and stable dynamics property are obtained for the three passivated perovskites), which strongly imply that their devices can show excellent performances, such as long-term stability, low ion migration, and high efficiency. However, the C₅NH₆ClO₄ and C₅NH₆PF₆ pyridinium salts are only profitable for passivating the (001) PbI₂ plane of the three perovskites, and other C₅NH₆X pyridinium salts have adverse effects.

CsPbCl3 -Cluster-Widened Bandgap and Inhibited Phase Segregation in a Wide-Bandgap Perovskite and its Application to NiOx -Based Perovskite/Silicon Tandem Solar Cells

Nickel oxide (NiO_x ) is an attractive hole-transport material for efficient and stable p-i-n metal-halide perovskite solar cells (PSCs). However, an undesirable redox reaction occurs at the NiO_x /perovskite interface, which results in a low open-circuit voltage (V_OC ), instability, and phase separation of the NiO_x -based wide-bandgap perovskite (Br > 20%). In order to simultaneously address the abovementioned phase separation problem and redox chemistry at the perovskite/NiO_x interface, the bandgap is widened from 1.64 to 1.67 eV by adding inorganic CsPbCl₃ -clusters (3 mol%) to the Cs₂₂ Br₁₅ perovskite precursor solution. Moreover, adding extra 2 mol% CsCl enriches the NiO_x /perovskite interface with Cl, thereby preventing the redox reaction at the interface, while controlling the Br content to within 15% improves the photostability of the wide-bandgap perovskite. Consequently, the power conversion efficiency (PCE) of a single-junction p-i-n PSC increases from 17.82% to 19.76%, which leads to the fabrication of highly efficient monolithic p-i-n-type NiO_x -based perovskite/silicon tandem solar cells with PCEs of up to 27.26% (certified PCE: 27.15%). The perovskite to an n-i-p-type perovskite/silicon tandem solar cell is also applied to deliver a V_OC of 1.93 V and a final efficiency of 25.5%. These findings provide critical insight into the fabrication of highly efficient and stable wide-bandgap perovskites.