High Efficiency and Stability of Inverted Perovskite Solar Cells Using Phenethyl Ammonium Iodide-Modified Interface of NiOx and Perovskite Layers

Defect Engineering at Buried Interface of Perovskite Solar Cells

Perovskite solar cells (PSC) have developed rapidly since the past decade with the aim to produce highly efficient photovoltaic technology at a low cost. Recently, physical and chemical defects at the buried interface of PSC including vacancies, impurities, lattice strain, and voids are identified as the next formidable hurdle to the further advancement of the performance of devices. The presence of these defects has unfavorably impacted many optoelectronic properties in the PSC, such as band alignment, charge extraction/recombination dynamics, ion migration behavior, and hydrophobicity. Herein, a broad but critical discussion on various essential aspects related to defects at the buried interface is provided. In particular, the defects existing at the surface of the underlying charge transporting layer (CTL) and the bottom surface of the perovskite film are initially elaborated. In situ and ex situ characterization approaches adopted to unveil hidden defects are elucidated to determine their influence on the efficiency, operational stability, and photocurrent-voltage hysteresis of PSC. A myriad of innovative strategies including defect management in CTL, the introduction of passivation materials, strain engineering, and morphological control used to address defects are also systematically elucidated to catalyze the further development of more efficient, reliable, and commercially viable photovoltaic devices.

Improvement of Photovoltaic Performance of Perovskite Solar Cells by Synergistic Modulation of SnO2 and Perovskite via Interfacial Modification

In the past decade, perovskite solar cell (PSC) photoelectric conversion efficiency has advanced significantly, and tin dioxide (SnO2) has been extensively used as the electron transport layer (ETL). Due to its high electron mobility, strong chemical stability, energy level matching with perovskite, and easy low-temperature fabrication, SnO2 is one of the most effective ETL materials. However, the SnO2 material as an ETL has its limitations. For example, SnO2 films prepared by low-temperature spin-coating contain a large number of oxygen vacancies, resulting in energy loss and high open-circuit voltage (VOC) loss. In addition, the crystal quality of perovskites is closely related to the substrate, and the disordered crystal orientation will lead to ion migration, resulting in a large number of uncoordinated Pb2+ defects. Therefore, interface optimization is essential to improve the efficiency and stability of the PSC. In this work, 2-(5-chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol (CBTBC) was introduced for ETL modification. On the one hand, the hydroxyl group of CBTBC forms a Lewis mixture with the Sn atom, which reduces the oxygen vacancy defect and prevents nonradiative recombination. On the other hand, the SnO2/CBTBC interface can effectively improve the crystal orientation of perovskite by influencing the crystallization kinetics of perovskite, and the nitrogen element in CBTBC can effectively passivate the uncoordinated Pb2+ defects at the SnO2/perovskite interface. Finally, the prevailing PCE of PSC (1.68 eV) modified by CBTBC was 20.34% (VOC = 1.214 V, JSC = 20.49 mA/cm2, FF = 82.49%).

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.

Synergistic Passivation With Phenylpropylammonium Bromide for Efficient Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) are a promising technology for commercialization due to their reliable operation and scalable fabrication. However, in inverted PSCs, depositing a high-quality perovskite layer comparable to those realized in normal structures still presents some challenges. Defects at grain boundaries and interfaces between the active layer and carrier extraction layer seriously hinder the power conversion efficiency (PCE) and stability of these cells. In this work, it is shown that synergistic bulk doping and surface treatment of triple-cation mixed-halide perovskites with phenylpropylammonium bromine (PPABr) can improve the efficiency and stability of inverted PSCs. The PPABr ligand is effective in eliminating halide vacancy defects and uncoordinated Pb2+ ions at both grain boundaries and interfaces. In addition, a 2D Ruddlesden-Popper (2D-RP) perovskite capping layer is formed on the surface of 3D perovskite by using PPABr post-treatment. This 2D-RP perovskite capping layer possesses a concentrated phase distribution ≈n = 2. This capping layer not only reduces interfacial non-radiative recombination loss and improves carrier extraction ability but also promotes stability and efficiency. As a result, the inverted PSCs achieve a champion PCE of over 23%, with an open-circuit voltage as high as 1.15 V and a fill factor of over 83%.

Over 19% Efficiency Perovskite Solar Modules by Simultaneously Suppressing Cation Deprotonation and Iodide Oxidation

Perovskite solar cells (PSCs) based on sputtered nickel oxide (NiOx) hole transport layer have emerged as promising configuration due to their good stability, cost-effectiveness, and scalability. However, the adverse chemical redox reaction at the NiOx/perovskite interface remains an ever-present problem that has not yet been well solved. To address this issue before, the problems that cation deprotonation and iodide oxidation that occurred in precursor solution easily result in the interfacial chemical reaction should be prevented. Hence, we report an efficient strategy to simultaneously suppress the interfacial reaction and stabilize the precursor solution by incorporating a reducing and weakly acidic stabilizer, l-ascorbic acid (l-AA). l-AA can reduce I2 generated in the precursor solution and during the interfacial reaction to I-. Furthermore, the protons ionized by adjacent enol hydroxyl groups in l-AA effectively impede the deprotonation of organic cations in the precursor solution as well as at the NiOx/perovskite interface resulting from the chemical reaction. Attributing to the improved crystallization of the perovskite film and the suppression of the interfacial reaction by l-AA, the inverted PSC based on such good light absorber achieves an impressive power conversion efficiency (PCE) of 22.72% along with a high open-circuit voltage of 1.19 V. Notably, further introducing l-AA into the large-area solar modules by the slot-die coating method in air enables a remarkable PCE of 19.17%, which reaches one of the highest PCEs reported for inverted perovskite solar modules (PSMs) (active area >50 cm2) to date. l-AA located at the buried interface also forms a barrier layer that can prevent undesirable chemical reactions at the NiOx/perovskite interface, significantly enhancing the device stability of solar cells and PSMs. These findings in our work provide important guidance for improving the NiOx/perovskite interface and the fabrication of highly efficient, low-cost, and large-area PSMs.

Surface Passivation by Sulfur-Based 2D (TEA)2PbI4 for Stable and Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) with superior performance have been recognized as a potential candidate in photovoltaic technologies. However, defects in the active perovskite layer induce nonradiative recombination which restricts the performance and stability of PSCs. The construction of a thiophene-based 2D structure is one of the significant approaches for surface passivation of hybrid PSCs that may combine the benefits of the stability of 2D perovskite with the high performance of three-dimensional (3D) perovskite. Here, a sulfur-rich spacer cation 2-thiopheneethylamine iodide (TEAI) is synthesized as a passivation agent for the construction of a three-dimensional/two-dimensional (3D/2D) perovskite bilayer structure. TEAI-treated PSCs possess a much higher efficiency (20.06%) compared to the 3D perovskite (MA_0.9FA_0.1PbI₃) devices (17.42%). Time-resolved photoluminescence and femtosecond transient absorption spectroscopy are employed to investigate the effect of surface passivation on the charge carrier dynamics of the 3D perovskite. Additionally, the stability test of TEAI-treated perovskite devices reveals significant improvement in humid (RH ∼ 46%) and thermal stability as the sulfur-based 2D (TEA)₂PbI₄ material self-assembles on the 3D surface, making the perovskite surface hydrophobic. Our findings provide a reliable approach to improve device stability and performance successively, paving the way for industrialization of PSCs.

Enhanced Photovoltaic Performance of Inverted Perovskite Solar Cells through Surface Modification of a NiOx-Based Hole-Transporting Layer with Quaternary Ammonium Halide-Containing Cellulose Derivatives

In this study, we positioned three quaternary ammonium halide-containing cellulose derivatives (PQF, PQCl, PQBr) as interfacial modification layers between the nickel oxide (NiO_x) and methylammonium lead iodide (MAPbI₃) layers of inverted perovskite solar cells (PVSCs). Inserting PQCl between the NiO_x and MAPbI₃ layers improved the interfacial contact, promoted the crystal growth, and passivated the interface and crystal defects, thereby resulting in MAPbI₃ layers having larger crystal grains, better crystal quality, and lower surface roughness. Accordingly, the photovoltaic (PV) properties of PVSCs fabricated with PQCl-modified NiO_x layers were improved when compared with those of the pristine sample. Furthermore, the PV properties of the PQCl-based PVSCs were much better than those of their PQF- and PQBr-based counterparts. A PVSC fabricated with PQCl-modified NiO_x (fluorine-doped tin oxide/NiO_x/PQCl-0.05/MAPbI₃/PC₆₁BM/bathocuproine/Ag) exhibited the best PV performance, with a photoconversion efficiency (PCE) of 14.40%, an open-circuit voltage of 1.06 V, a short-circuit current density of 18.35 mA/cm³, and a fill factor of 74.0%. Moreover, the PV parameters of the PVSC incorporating the PQCl-modified NiO_x were further enhanced when blending MAPbI₃ with PQCl. We obtained a PCE of 16.53% for this MAPbI₃:PQCl-based PVSC. This PQCl-based PVSC retained 80% of its initial PCE after 900 h of storage under ambient conditions (30 °C; 60% relative humidity).

Precursor engineering for efficient and stable perovskite solar cells

The perovskite film prepared by the two-step spin coating method is widely used in photovoltaic devices due to its good film morphology and great reproducibility. However, there usually exists excessive lead iodide (PbI₂) in the perovskite film for this method, which is believed to passivate the grain boundaries (GBs) to increase the efficiency of the perovskite solar cells. Nevertheless, the excessive PbI₂at the GBs of perovskite is believed to induce the decomposition of the perovskite film and undermine the long-term stability of devices. In this study, we utilize precursor engineering to realize the preparation of perovskite solar cells with high efficiency and stability. The concentration of organic salts (AX: A = MA⁺, FA⁺; X = I^-, Cl^-) in the precursor solution for the second step of the two-step spin coating method is adjusted to optimize the perovskite light-absorbing layer so that the excessive PbI₂is converted into perovskite to obtain a smooth and pinhole-free perovskite film with high performance. Our results indicate that by adjusting the concentration of AX in the precursor solution, PbI₂in the film could be completely converted into perovskite without excessive AX residue. Both the efficiency and stability of the perovskite solar cells without excessive PbI₂have been significantly improved. A planar perovskite solar cell with the highest power conversion efficiency (PCE) of 21.26% was achieved, maintaining about 90% of the initial PCE after 300 h of storage in a dry air environment and in the dark, about 76% of the initial PCE after 300 h of continuous illumination of 1 Sun.

Multifunctional Ionic Liquid as an Interfacial Modifier for High-Performance and Stable NiOx-Based Inverted Perovskite Solar Cells

Interface instability has evolved into the primary aspect that limits the durability improvement of perovskite solar cells (PSCs). Interface modification with suitable molecules is widely considered an effective path for improving the interface state. Herein, an ionic liquid modified layer, 1-ethyl-3-methylimidazolium aminoacetate (EMIMAE), is brought to modify NiO_x/perovskite interface. The EMIMAE layer can interact with the adjacent layer to regulate the perovskite growth, passivate defects in the film, and promote charge transport in PSCs. Eventually, the optimized device's efficiency rises to 18.6%, which is a substantial improvement over the control device. Particularly, after 1000 h of continuous maximum power point tracking, the device can still retain 95% of its initial efficiency. This work proposes a simple idea to ameliorate the device interface and boost the commercialization of NiO_x based devices.

Suppressing Nickel Oxide/Perovskite Interface Redox Reaction and Defects for Highly Performed and Stable Inverted Perovskite Solar Cells

The inorganic hole transport layer of nickel oxide (NiO_x ) has shown highly efficient, low-cost, and scalable in perovskite photovoltaics. However, redox reactions at the interface between NiO_x and perovskites limit their commercialization. In this study, ABABr (4-(2-Aminoethyl) benzoic acid bromide) between the NiO_x and different perovskite layers to address the issues has been introduced. How the ABABr interacts with NiO_x and perovskites is experimentally and theoretically investigated. These results show that the ABABr molecule chemically reacts with the NiO_x via electrostatic attraction on one side, whereas on the other side, it forms a strong hydrogen bond via the NH₃ ⁺ group with perovskites layers, thus directly diminishing the redox reaction between the NiO_x and perovskites layers and passivating the layer surfaces. Additionally, the ABABr interface modification leads to significant improvements in perovskite film morphology, crystallization, and band alignment. The perovskites solar cells (PSCs) based on an ABABr interface modification show power conversion efficiency (PCE) improvement by over 13% and maintain over 90% of its PCE after continuous operation at maximum power point for over 500 h. The work not only contributes to the development of novel interlayers for stable PSCs but also to the understanding of how to prevent interface redox reactions.

High-Efficiency and Stable Perovskite Photodetectors with an F4-TCNQ-Modified Interface of NiOx and Perovskite Layers

Nickel oxide (NiO_x), a typical p-type semiconductor, is emerging as the most promising hole transport layer material. However, the inferior interfacial contact of the NiO_x/perovskite interface has limited the improvement of the performance of photodetectors (PDs). In this work, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced to modify the NiO_x/perovskite interface to prepare high-performance PDs. This study shows that the F4-TCNQ layer interacts with the NiO_x/perovskite layers. It can increase the Ni³⁺/Ni²⁺ ratio and then enhance the hole extraction and charge carrier mobility; on the contrary, it can form a new Lewis adduct and passivate the undercoordinated Pb²⁺ ions. Furthermore, with the F4-TCNQ modification, the perovskite film exhibits good thermal stability and photostability. The PDs demonstrate excellent photoelectric properties and long-term stability in the atmosphere. This finding provides a simple and efficient way to further develop the NiO_x/perovskite interface.

Interface Regulation by an Ultrathin Wide-Bandgap Halide for Stable and Efficient Inverted Perovskite Solar Cells

The nonradiative recombination between hole transport layers (HTLs) and perovskites generally leads to obvious energy losses. The trap states at the HTL/perovskite interface directly influence the improvement of the power conversion efficiency (PCE) and stability. Interface regulation is a simple and commonly used method to decrease nonradiative recombination in inverted perovskite solar cells (PSCs). Here, a wide-bandgap halide was used to regulate the PTAA/MAPbI₃ interface, in which n-hexyltrimethylammonium bromide (HTAB) was used to modify the upper surface of poly[bis(4-phenyl)-(2,4,6-trimethylphenyl)amine] (PTAA). Upon introduction of the HTAB layer, the contact between PTAA and MAPbI₃ is strengthened, the defect state density in PSCs is reduced, the MAPbI₃ crystallinity is improved, and the nonradiative recombination loss is suppressed. The device with HTAB delivers the highest PCE of 21.01% with negligible hysteresis, which is significantly higher than that of the control device (17.71%), and it maintains approximately 87% of its initial PCE for 1000 h without encapsulation in air with a relative humidity of 25 ± 5%. This work reveals an effective way of using a wide-bandgap halide to regulate the PTAA/MAPbI₃ interface to simultaneously promote the PCE and stability of PSCs.

Metal oxide charge transporting layers for stable high-performance perovskite solar cells

This review summarizes the recent progress in metal oxide charge transporting layers to achieve stable high-performance perovskite solar cells.

Organic spacer engineering in 2D/3D hybrid perovskites for efficient and stable solar cells

The PMAI-based PSCs form multiple NH⋯I hydrogen bonds, which can passivate interface defects and suppress ion migration and diffusion.

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.

Water-Repellent Perovskites Induced by a Blend of Organic Halide Salts for Efficient and Stable Solar Cells

Despite tremendous progress in the power conversion efficiency (PCE) of perovskite solar cells (PeSCs), the long-term stability issue remains a significant challenge for commercialization. In this study, by blending organic halide salts, phenylethylammonium halide (PEAX, X = I, Br), with CH₃NH₃PbI₃ (MAPbI₃), we achieved remarkable enhancements in the water-repellency of perovskite films and long-term stability of PeSCs, together with enhanced PCE. The hydrophobic aromatic PEA⁺ group in PEAX protects the perovskite film from destruction by water. In addition, the smaller halide Br^- in PEABr restructures MAPbI₃ to form MAPbI_3-xBr_x during post-annealing, leading to lattice contraction with beneficial crystallization quality. The perovskite films modified by PEAX exhibited excellent water resistance. When the perovskite films were directly immersed in water, no obvious decompositions were observed, even after 60 s. The PEAX-decorated PeSCs exhibited considerable long-term stability. Under high-humidity conditions (60 ± 5%), the PEAX-decorated PeSCs held 80.5% for PEAI and 85.2% for PEABr of their original PCE after exposure for 100 h, whereas the pristine PeSC device lost more than 99% of its initial PCE after exposure for 60 h under the same conditions. Moreover, compared to the pristine device with a PCE of 13.28%, the PEAX-decorated PeSCs exhibited enhanced PCEs of 17.33% for the PEAI device and 17.18% for the PEABr device.

Scalable Production of Ambient Stable Hybrid Bismuth-Based Materials: AACVD of Phenethylammonium Bismuth Iodide Films*

Large homogeneous and adherent coatings of phenethylammonium bismuth iodide were produced using the cost-effective and scalable aerosol-assisted chemical vapor deposition (AACVD) methodology. The film morphology was found to depend on the deposition conditions and substrates, resulting in different optical properties to those reported from their spin-coated counterparts. Optoelectronic characterization revealed band bending effects occurring between the hybrid material and semiconducting substrates (TiO2 and FTO) due to heterojunction formation, and the optical bandgap of the hybrid material was calculated from UV-visible and PL spectrometry to be 2.05 eV. Maximum values for hydrophobicity and crystallographic preferential orientation were observed for films deposited on FTO/glass substrates, closely followed by values from films deposited on TiO2 /glass substrates.

Efficient and Stable Perovskite Solar Cells Using Bathocuproine Bilateral-Modified Perovskite Layers

Surface modification engineering is an effective method to improve the crystallinity and passivate the perovskite interface and grain boundary, which can improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). The typical interface modification method is usually introduced at the interface of the perovskite/hole transport layer (HTL) or perovskite/electron transport layer (ETL) through coordination of the groups in the material with the perovskite. In this work, the n-type semiconductor bathocuproine (BCP) including the pyridine nitrogen bond was modified at the interfaces of perovskite/HTL or perovskite/ETL to improve perovskite crystallinity and interface contact properties. The better crystallinity and superior interface contact properties are obtained using BCP unilateral modification, which obviously increases the PCEs of PSCs. The BCP bilateral modification at both perovskite/ETL and perovskite/HTL interfaces can further improve the crystallinity with fewer defects and superior contact properties, which show the largest V_oc (1.14 V) and fill factors (FF 77.1%) compared to PSCs with BCP unilateral modification. PSCs with BCP bilateral modification obtained 20.6% PCEs, which is greatly higher than that (17.5%) of the original PSCs. The stability of PSCs with BCP bilateral modification can be greatly improved due to the better crystal quality and hydrophobic property of the interfaces. The results demonstrated that the n-type BCP material can efficiently modify both perovskite/HTL and perovskite/ETL interfaces beyond its semiconductor type, which can greatly improve the PCEs and stability of PSCs because BCP modification can passivate interfaces, improve interface contact and hydrophobic properties, promote crystallinity of the perovskite layer with fewer defects, and block carrier recombination at both interfaces.

CH3NH3PbBr3-xIx Quantum Dots Enhance Bulk Crystallization and Interface Charge Transfer for Efficient and Stable Perovskite Solar Cells

Obtaining a perovskite light-absorbing layer with good crystallization, low defect concentration, good stability, and well-matched energy levels is critical to obtaining high-efficiency perovskite solar cells (PSCs). Here, a hybrid PSC with a graded band gap is explored using MAPbBr₃ (MA = CH₃NH₃) and MAPbBr_0.9I_2.1 quantum dots (QDs) as component cells. We have creatively designed a solar cell device with a double-QD structure [indium tin oxide (ITO)/SnO₂/perovskite:MAPbBr₃ QDs/MAPbBr_0.9I_2.1 QDs/Spiro-OMeTAD/Au]. A better crystal film of the perovskite absorption layer can be obtained because the MAPbBr₃ QDs are doped in an antisolvent, which induces nucleation and growth in the polycrystalline perovskite. In addition, we expect that digestive ripening occurred in the crystallization, and the oleic acid ligands on the surface of the QDs disintegrate during the doping process and transfer to the surface of the perovskite absorption layer finally; it follows that the hydrophobicity and stability of the perovskite film are greatly enhanced. Moreover, a thin film of MAPbBr_0.9I_2.1 QDs is introduced between the perovskite absorption layer and the hole layer, acting as an energy-level ladder, which leads to well-matched energy levels, an increase in fill factor (FF), and an enhanced hole transport capability. In particular, the mechanism of the crystallization process involving the effect of oleic acid ligands on the interior and surface of the perovskite film is fully discussed here. The final research results from the PSCs show that both high efficiency and long-term stability are achieved successfully by this design strategy.

Polymer Modification on the NiOx Hole Transport Layer Boosts Open-Circuit Voltage to 1.19 V for Perovskite Solar Cells

Inverted-structure perovskite solar cells (PVSCs) applying NiO_x as the hole transport layer (HTL) have attracted increasing attention. It is still a challenge to optimize the contact between NiO_x and the perovskite layer and to suppress energy loss at the interface. In this study, interface engineering was carried out by modifying the NiO_x layer with different polymers such as polystyrene, poly(methyl methacrylate) (PMMA), or poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to improve the surface contact between NiO_x and the perovskite, to decrease the defect states, and to make the energy level alignment better. The NiO_x/PMMA-based device presents a V_oc as high as 1.19 V because of the improved interfacial contact and the interaction of the carbonyl and methoxy group with Pb²⁺. The NiO_x/PTAA-based device with the structure ITO/NiO_x/PTAA/(MAPbI₃)_0.95(MAPbBr₂Cl)_0.05/PCBM/BCP/Ag exhibits the highest power conversion efficiency of 21.56% with a high V_oc of 1.19 V. The enhanced performance can be attributed to the deepened highest occupied molecular orbital level of NiO_x/PTAA, which matched well with that of the perovskite and suppressed interface energy loss as well. This work provides a facile approach for efficiently improving the V_oc of NiO_x-based PVSCs.