Effective Passivation with Self-Organized Molecules for Perovskite Photovoltaics

Perovskite solar cells (PSCs) have achieved power conversion efficiencies (PCEs) exceeding 25% over the past decade and effective passivation for the interface with high trap density plays a significant role in this process. Here, two organic molecules are studied as passivators, and it is demonstrated that an advantageous molecular geometry and intermolecular ordering, aside from the functional moieties, are of great significance for effective and extensive passivation. Besides, the passivation molecules spontaneously form a uniform passivation network adjacent to the bottom surface of perovskite films during a top-down crystallization via liquid medium annealing, which greatly reduces defect-assisted recombination throughout the whole perovskite/SnO₂ interface. The champion device yields an in-lab PCE of 25.05% (certified 24.39%). The investigation provides a more comprehensive understanding of passivation and a new avenue to achieve effective bottom-interface engineering for perovskite photovoltaics.

Designing Multifunctional Donor-Acceptor-Type Molecules to Passivate Surface Defects Efficiently and Enhance Charge Transfer of CsPbI2Br Perovskite for High Power Conversion Efficiency

High-density and multitype surface defects of CsPbI₂Br perovskite induce charge recombination and accumulation, hindering its device efficiency and stability. However, the surface defect types of CsPbI₂Br perovskite are still unclear, and conventional organic molecules only passivate one specific defect and cannot achieve good overall passivation. Here, density functional theory is used to explore surface defect types and properties of CsPbI₂Br with calculating the defect formation energy and electronic structure. Results show that the dominant deep-level defects are cationic defects (Pb_Br) under Br-poor conditions and anionic defects (I_i and Br_i) under moderate and Br-rich conditions, originating from Pb-Pb bonding and I-I bonding. Multifunctional organic molecules containing donor and acceptor groups are used to passivate both cationic and anionic defects simultaneously. It turns out that the deep-level defects are effectively decreased by forming strong interaction of N-Pb, O-Pb, and halide-Pb bonds. Moreover, the electron and hole transfers from perovskite to molecules increase dramatically to -9.06 × 10¹² and 2.60 × 10¹² e/cm² and maybe improve the efficiency of power conversion. Our findings not only reveal the surface defect properties of CsPbI₂Br, but also offer an approach for designing new multifunctional passivators for perovskite solar cells with high conversion efficiency.

Defect engineering in two-dimensional perovskite nanowire arrays by europium(III) doping towards high-performance photodetection

We demonstrate high-performance photodetectors based on Eu-doped 2D perovskite nanowire arrays. The pure crystallographic orientation enables efficient carrier transport and the doped Eu ions effectively suppress the trap density in the nanowire arrays. As a result, the optimized Eu-doped photodetectors show an excellent responsivity of 6.24 A W^-1, an outstanding specific detectivity of 5.83 × 10¹³ Jones and stable photo-switching behavior with a current on/off ratio of 10³.

Simultaneous Bulk and Surface Defect Passivation for Efficient Inverted Perovskite Solar Cells

Structural defects in the bulk and on the surface of the perovskite layer serving as trap sites induce nonradiative recombination losses, limiting the performance improvement of perovskite solar cells (PSCs). Herein, we report a trometamol-induced dual passivation (TIDP) strategy to fix both bulk and surface defects of perovskites, where the trometamol molecule can simultaneously act as chemical additive and surface-modification agent. Studies show that trometamol as an additive can effectively reduce ionic defects and enhance the grain size of perovskites through Pb2+/-NH2 coordination bonds and I-/-OH hydrogen bonds. As a surface-modification agent, trometamol further passivates ionic defects at the upper surface of the perovskite layer. As a result of the TIDP approach, a remarkable efficiency augmentation from 17.25% to 19.17% and an optimized thermal stability under inert conditions have been realized. These results highlight the importance of the TIDP strategy in perovskite defect management for excellent photovoltaic properties, facilitating the fabrication of high-performance PSCs.

Computational Design of α-AsP/γ-AsP Vertical Two-Dimensional Homojunction for Photovoltaic Applications

Based on first-principles calculations, we design a α-AsP/γ-AsP homojunction with minimum lattice distortion. It is found that the α-AsP/γ-AsP homojunction has an indirect bandgap with an intrinsic type-II band alignment. The proposed α-AsP/γ-AsP homojunction exhibits high optical absorption of 1.6×106 cm−1 along the zigzag direction. A high power conversion efficiency (PCE) of 21.08% is achieved in the designed α-AsP/γ-AsP homojunction, which implies it has potential applications in solar cells. Under 4% in-plane axial strain along the zigzag direction, a transition from indirect band gap to direct band gap is found in the α-AsP/γ-AsP homojunction. Moreover, the intrinsic type-II band alignment can be tuned to type-I band alignment under in-plane strain, which is crucial for its potential application in optical devices.

Simultaneous Passivation of Bulk and Interface Defects with Gradient 2D/3D Heterojunction Engineering for Efficient and Stable Perovskite Solar Cells

Minimizing bulk and interfacial nonradiative recombination losses is key to further improving the photovoltaic performance of perovskite solar cells (PSC) but very challenging. Herein, we report a gradient dimensionality engineering to simultaneously passivate the bulk and interface defects of perovskite films. The 2D/3D heterojunction is skillfully constructed by the diffusion of an amphiphilic spacer cation from the interface to the bulk. The 2D/3D heterojunction engineering strategy has achieved multiple functions, including defect passivation, hole extraction improvement, and moisture stability enhancement. The introduction of tertiary butyl at the spacer cation should be responsible for increased film and device moisture stability. The device with 2D/3D heterojunction engineering delivers a promising efficiency of 22.54% with a high voltage of 1.186 V and high fill factor of 0.841, which benefits from significantly suppressed bulk and interfacial nonradiative recombination losses. Moreover, the modified devices demonstrate excellent light, thermal, and moisture stability over 1000 h. This work paves the way for the commercial application of perovskite photovoltaics.

Ion Migration in Perovskite Light-Emitting Diodes: Mechanism, Characterizations, and Material and Device Engineering

In recent years, perovskite light-emitting diodes (PeLEDs) have emerged as a promising new lighting technology with high external quantum efficiency, color purity, and wavelength tunability, as well as, low-temperature processability. However, the operational stability of PeLEDs is still insufficient for their commercialization. The generation and migration of ionic species in metal halide perovskites has been widely acknowledged as the primary factor causing the performance degradation of PeLEDs. Herein, this topic is systematically discussed by considering the fundamental and engineering aspects of ion-related issues in PeLEDs, including the material and processing origins of ion generation, the mechanisms driving ion migration, characterization approaches for probing ion distributions, the effects of ion migration on device performance and stability, and strategies for ion management in PeLEDs. Finally, perspectives on remaining challenges and future opportunities are highlighted.

Suppressing the Cation Exchange at the Core/Shell Interface of InP Quantum Dots by a Selenium Shielding Layer Enables Efficient Green Light-Emitting Diodes

Indium phosphide (InP) quantum dots (QDs) have demonstrated great potential for light-emitting diode (LED) application because of their excellent optical properties and nontoxicity. However, the over performance of InP QDs still lags behind that of CdSe QDs, and one of main reasons is that the Zn traps in InP lattices can be formed through the cation exchange in the ZnSe shell growth process. Herein, we realized highly luminescent InP/ZnSe/ZnS QDs by constructing Se-rich shielding layers on the surfaces of InP cores, which simultaneously protect the InP cores from the invasion of Zn²⁺ into InP lattices and facilitate the ZnSe shell growth via the reaction between Zn²⁺ precursors and Se^2- shielding layers. The as-synthesized green InP/ZnSe/ZnS QDs had a high photoluminescence quantum yield (PLQY) of up to 87%. The fabricated QLEDs present a peak external quantum efficiency of 6.2% with an improved efficiency roll-off at high luminance, which is 2 times higher than that of control devices.

Ionic migration induced loss analysis of perovskite solar cells: a poling study

Understanding the interplay between ionic migration and defect trapping in photovoltaic perovskites is critical to develop targeted passivation techniques for performance enhancement. In this study, systematic poling experiments on Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perovskite solar cells (PSCs) were conducted to resolve the principal effects of bias dependent pretreatment effects due to dynamic ionic migration. We find that under negative polarizations, iodine ion accumulation at perovskite/electron transport layer (ETL) interfaces causes enhanced global non-radiative recombination in PSCs and significant open-circuit voltage (Voc) losses. On the other hand, dramatic short-circuit current (Jsc) reduction occurs in positively polarized devices, which is ascribed to ineffective charge collection due to modified band-bending towards both charge transport materials. Spatiotemporally scanning probe microscopy on the surface of polarized perovskites provides an in situ estimation of iodine diffusion mobility and visualization of reorganizations under an external bias. Moreover, our findings suggest that the precondition effect of PSCs under operation due to defect ions is recoverable, therefore achieving a respectable lifetime of PSCs for commercialization is promising.

Lewis Base Plays a Double-Edged-Sword Role in Trap State Engineering of Perovskite Polycrystals

We report the double-edged-sword effect of the thiourea (a typical Lewis base) additive for tailoring the trap state distribution of perovskite polycrystalline films. Through the thiourea treatment, the polycrystal grain size is greatly increased because of the reduced crystallization activation energy, which, together with the surface defect passivation, alters the density of the energetically "deep" and "shallow" trap states in a trade-off manner. Based on this finding and further photoelectric and spectral studies, the nonmonotonic dependence of the photoluminescence intensity on the thiourea concentration and the complicated time-resolved photoluminescence behavior are excellently clarified. As a proof of concept, the photophysical performance of perovskite polycrystals is optimized via a modified Lewis base treatment by taking the proposed double-edged-sword effect into account.

Cation-Doping in Organic-Inorganic Perovskites to Improve the Structural Stability from Theoretical Prediction

With outstanding photoelectric properties, organic-inorganic perovskites have become promising materials in the application of solar cells. However, their low stability limits their high conversion efficiency. On the basis of first-principles calculations, we screened out the optimal dopant into MAPbI₃ from a variety of organic cations, and further revealed the mechanism underneath for the improved stability of cations doping. Our results have demonstrated that the doping of large-size cations (i.e., IPA⁺, TriMA⁺, and GA⁺) could efficiently inhibit the formation and diffusion of structural defects with high defect formation energies and large migration barriers, which is associated with the lattice expansion and greater hydrogen-bond formation. Our theoretical findings address crucial guidelines to design and synthesize the organic-inorganic perovskite materials with high stability, and provide valuable insights in understanding the stability mechanism, which may enhance the photovoltaic efficiency of perovskite materials and extend their wide applications.

Several Triazine-Based Small Molecules Assisted in the Preparation of High-Performance and Stable Perovskite Solar Cells by Trap Passivation and Heterojunction Engineering

The functional group is the main body in modifying the perovskite film, and different functional groups lead to different modification effects. Here, several conjugated triazine-based small molecules such as melamine (Cy-NH₂), cyanuric acid (Cy-OH), cyanuric fluoride (Cy-F), cyanuric chloride (Cy-Cl), and thiocyanuric acid (Cy-SH) are used to modify perovskite films by mixing in antisolvent. The crystallizations of perovskites are optimized by these molecules, and the perovskite films with low trap density are obtained by forming Lewis adducts with these molecules (Pb²⁺ and electron-donating groups including -NH₂, C═N-, and C═O; I^- and electron-withdrawing groups including F, Cl, N-H, and O-H). Especially for the Cy-F and Cy-Cl, the heterojunction structure is formed in the perovskite layer by p-type modification, which is conducive to charge transfer and collection in PSCs. Compared with that of control devices, the performance of devices with trap passivation and heterojunction engineering is obviously improved from 18.49 to 20.71% for MAPbI₃ and 19.27 to 21.11% for FA_0.85Cs_0.15PbI₃. Notably, the excellent moisture (retaining 67%, RH: 50% for 20 days) and thermal (retaining 64%, 85 °C for 72 h) stability of PSCs are obtained by a kind of second modification (Cy-F/Cy-SH)─spin-coating a few Cy-SH on the Cy-F-modified perovskite film surface. It also reduces Pb pollution because Cy-SH is a highly potent chelating agent. Therefore, this work also provides an effective method to obtain high-performance, stable, and low-lead pollution PSCs, combining trap passivation, heterojunction engineering, and surface treatment.

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed.

The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells

Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.

Surface Defect Formation and Passivation in Formamidinium Lead Triiodide (FAPbI3) Perovskite Solar Cell Absorbers

Formamidinium lead iodide based hybrid perovskite materials with improved efficiency and stability still lack well-understood surface defect formation mechanisms. Controlling the surface termination and defects has the potential to improve the performance of both conventional 3D and latterly reduced-dimensional perovskites photovoltaics. Here, we characterized the termination and all possible defect formations in FAPbI₃ surface by the first-principles calculations. We found that, among the surfaces we considered, FAI-termination exhibits the most stable surface with a high defect tolerance. The PbI₂-terminated surface is also found to be relatively stable; however, certain defects, such as electron-donating FA-interstitial and Pb-interstitial defects, can create deep-level stable charge-traps, potentially limiting the optoelectronic performance. We further investigate the surface treatment on these deep defects by model small molecule additives. We found that benzene additive with delocalized electron distribution can effectively passivate the deep FA-interstitial and Pb-interstitial defects by electron donating to the surface defect through charge-transfer.

Low Dark Current and Performance Enhanced Perovskite Photodetector by Graphene Oxide as an Interfacial Layer

Organic-inorganic hybrid perovskite photodetectors are gaining much interest recently for their high performance in photodetection, due to excellent light absorption, low cost, and ease of fabrication. Lower defect density and large grain size are always favorable for efficient and stable devices. Herein, we applied the interface engineering technique for hybrid trilayer (TiO2/graphene oxide/perovskite) photodetector to attain better crystallinity and defect passivation. The graphene oxide (GO) sandwich layer has been introduced in the perovskite photodetector for improved crystallization, better charge extraction, low dark current, and enhanced carrier lifetime. Moreover, the trilayer photodetector exhibits improved device performance with a high on/off ratio of 1.3 × 104, high responsivity of 3.38 AW-1, and low dark current of 1.55 × 10-11 A. The insertion of the GO layer also suppressed the perovskite degradation process and consequently improved the device stability. The current study focuses on the significance of interface engineering to boost device performance by improving interfacial defect passivation and better carrier transport.

Control of the Surface Disorder by Ion-Exchange to Achieve High Open-Circuit Voltage in HC(NH2 )2 PbI3 Perovskite Solar Cell

The ionic nature of organic trihalide perovskite leads to structural irregularity and energy disorder at the perovskite surface, which seriously affects the photovoltaic performance of perovskite solar cells. Here, the origin of the perovskite surface disorder is analyzed, and a facial ion-exchange strategy is designed to regulate the surface chemical environment. By the reconstruction of terminal irregular Pb-I bonds and random cations, the repaired surface is characteristic of the reduced band tail states, consequent to the suppression of the uplift of quasi-Fermi level splitting and photocarrier scattering. The optimized device gets a high open-circuit voltage and operational stability. These findings fully elaborate the underlying mechanism concerning perovskite surface problem, giving guidance on tailoring the energy disorder.

Surface Reconstruction and In Situ Formation of 2D Layer for Efficient and Stable 2D/3D Perovskite Solar Cells

The 2D/3D composite structure possesses both the excellent stability of 2D perovskite and the excellent performance of 3D perovskite, which recently have attracted special attention. Different from the popular isopropanol, a novel additive solvent-polypropylene glycol bis (2-aminopropyl ether) (A-PPG) is introduced here to dissolve excess PbI₂ and perovskite, and then reconstruct and in situ form the quasi-2D perovskite layer on 3D perovskite bulk. The lone electron pairs of the ether-oxygen and amino in A-PPG can form coordination bonds with Pb²⁺ . The introduction of A-PPG tunes the energy array of functional layers, passivates defects, and mitigates carrier nonradiative recombination. Consequently, the 2D/3D perovskite device exhibits a championship efficiency of 22.24% with a distinguished open-circuit voltage of 1.21 V (the thermodynamic limit of 1.30 V). Moreover, the 2D/3D device still maintains 90% of the original efficiency in the ambient atmosphere with a relative humidity of 30 ± 10% after 50 days.

Defect Passivation for Perovskite Solar Cells: from Molecule Design to Device Performance

Perovskite solar cells (PSCs) are a promising third-generation photovoltaic (PV) technology developed rapidly in recent years. Further improvement of their power conversion efficiency is focusing on reducing the non-radiative charge recombination induced by the defects in metal halide perovskites. So far, defect passivation by the organic small molecule has been considered as a promising approach for boosting the PSC performance owing to their large structure flexibility adapting to passivating variable kinds of defect states and perovskite compositions. Here, the recent progress of defect passivation toward efficient and stable PSCs was reviewed from the viewpoint of molecular structure design and device performance. To comprehensively reveal the structure-performance correlation of passivation molecules, it was separately discussed how the functional groups, organic frameworks, and side chains affect the corresponding PV parameters of PSCs. Finally, a guideline was provided for researchers to select more suitable passivation agents, and a perspective was given on future trends in development of passivation strategies.

Effect of vacancy concentration on the lattice thermal conductivity of CH3NH3PbI3: a molecular dynamics study

Hybrid halide perovskites are drawing great interest for photovoltaic and thermoelectric applications, but the relationship of thermal conductivities with vacancy defects remains unresolved. Here, we present a systematic investigation of the thermal conductivity of perfect and defective CH3NH3PbI3, performed using classical molecular dynamics with an ab initio-derived force field. We calculate the lattice thermal conductivity of perfect CH3NH3PbI3 as the temperature increases from 300 K to 420 K, confirming a good agreement with the previous theoretical and experimental data. Our calculations reveal that the thermal conductivities of defective systems at 330 K, containing vacancy defects such as VMA, VPb and VI, decrease overall with some slight rises, as the vacancy concentration increases from 0 to 1%. We show that such vacancies act as phonon scattering centers, thereby reducing the thermal conductivity. Moreover, we determine the elastic moduli and sound velocities of the defective systems, revealing that their slower sound speed is responsible for the lower thermal conductivity. These results could be useful for developing hybrid halide perovskite-based solar cells and thermoelectric devices with high performance.

Facile Removal of Bulk Oxygen Vacancy Defects in Metal Oxides Driven by Hydrogen-Dopant Evaporation

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (V_O) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing V_O bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of V_O but also push V_O away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of V_O as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.

Synergistic Effect of Defect Passivation and Crystallization Control Enabled by Bifunctional Additives for Carbon-Based Mesoscopic Perovskite Solar Cells

The emerging carbon-based mesoscopic perovskite solar cells (MPSCs) are known as one of the most promising candidates for photovoltaic applications thanks to their screen-printing process and excellent stability. Unfortunately, they usually suffer from serious defects because it is challenging to realize sufficient mesopore filling of the perovskite precursor solution throughout the triple-mesoporous scaffold. Herein, a bifunctional additive, biuret, endowed with both carbonyl and amino groups, was designed to realize a convenient fabrication approach for controllable crystallization of the precursor solution. Owing to the strong coordination ability with perovskite components, the incorporation of biuret can not only regulate crystallization kinetics allowing for the growth of high-quality perovskite crystals but also associate with uncoordinated ions for defect passivation to enhance the overall photovoltaic performance of MPSCs. A champion power conversion efficiency (PCE) of 13.42% with an enhanced short-circuit current density of 19.49 mA cm^-2 and a much higher open-circuit voltage of 0.96 V was achieved for the device doped with 3 mol % biuret, which is 26% higher than that of the control device (10.66%). Moreover, the unencapsulated devices with biuret incorporation demonstrated superior stability, maintaining over 90% of the original PCE after 50 days of storage under ambient conditions. This work helps exploit bifunctional additive strategies for simultaneous defect passivation and crystallization control toward high-efficiency and long-term stability of carbon-based MPSCs.

Defect Passivation in Lead-Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

Interfacial Embedding of Laser-Manufactured Fluorinated Gold Clusters Enabling Stable Perovskite Solar Cells with Efficiency Over 24

Tackling the interfacial loss in emerged perovskite-based solar cells (PSCs) to address synchronously the carrier dynamics and the environmental stability, has been of fundamental and viable importance, while technological hurdles remain in not only creating such interfacial mediator, but the subsequent interfacial embedding in the active layer. This article reports a strategy of interfacial embedding of hydrophobic fluorinated-gold-clusters (FGCs) for highly efficient and stable PSCs. The p-type semiconducting feature enables the FGC efficient interfacial mediator to improve the carrier dynamics by reducing the interfacial carrier transfer barrier and boosting the charge extraction at grain boundaries. The hydrophobic tails of the gold clusters and the hydrogen bonding between fluorine groups and perovskite favor the enhancement of environmental stability. Benefiting from these merits, highly efficient formamidinium lead iodide PSCs (champion efficiency up to 24.02%) with enhanced phase stability under varied relative humidity (RH) from 40% to 95%, as well as highly efficient mixed-cation PSCs with moisture stability (RH of 75%) over 10 000 h are achieved. It is thus inspiring to advance the development of highly efficient and stable PSCs via interfacial embedding laser-generated additives for improved charge transfer/extraction and environmental stability.

Marked Passivation Effect of Naphthalene-1,8-Dicarboximides in High-Performance Perovskite Solar Cells

As game-changers in the photovoltaic community, perovskite solar cells are making unprecedented progress while still facing grand challenges such as improving lifetime without impairing efficiency. Herein, two structurally alike polyaromatic molecules based on naphthalene-1,8-dicarboximide (NMI) and perylene-3,4-dicarboximide (PMI) with different molecular dipoles are applied to tackle this issue. Contrasting the electronically pull-pull cyanide-substituted PMI (9CN-PMI) with only Lewis-base groups, the push-pull 4-hydroxybiphenyl-substituted NMI (4OH-NMI) with both protonic and Lewis-base groups can provide better chemical passivation for both shallow- and deep-level defects. Moreover, combined theoretical and experimental studies show that the 4OH-NMI can bind more firmly with perovskite and the polyaromatic backbones create benign midgap states in the excited perovskite to suppress the damage by superoxide anions (energetic passivation). The polar and protonic nature of 4OH-NMI facilitates band alignment and regulates the viscosity of the precursor solution for thicker perovskite films with better morphology. Consequently, the 4OH-NMI-passivated perovskite films exhibit reduced grain boundaries and nearly three-times lower defect density, boosting the device efficiency to 23.7%. A more effective design of the passivator for perovskites with multi-passivation mechanisms is provided in this study.

An in-situ defect passivation through a green anti-solvent approach for high-efficiency and stable perovskite solar cells

Surface and grain boundary defects in halide perovskite solar cells are highly detrimental, reducing efficiencies and stabilities. Widespread halide anion and organic cation defects usually aggravate ion diffusion and material degradation on the surfaces and at the grain boundaries of perovskite films. In this study, we employ an in-situ green method utilizing nontoxic cetyltrimethylammonium chloride (CTAC) and isopropanol (IPA) as anti-solvents to effectively passivate both surface and grain boundary defects in hybrid perovskites. Anion vacancies can be readily passivated by the chloride group due to its high electronegativity, and cation defects can be synchronously passivated by the more stable cetyltrimethylammonium group. The results show that the charge trap density was significantly reduced, while the carrier recombination lifetime was markedly extended. As a result, the power conversion efficiency of the cell can reach 23.4% with this in-situ green method. In addition, the device retains 85% of its original power conversion efficiency after 600 h of operation under illumination, showing that the stability of perovskite solar cells is improved with this in-situ passivation strategy. This work may provide a green and effective route to improve both the stability and efficiency of perovskite solar cells.

Crystal Orientation Modulation and Defect Passivation for Efficient and Stable Methylammonium-Free Dion-Jacobson Quasi-2D Perovskite Solar Cells

Dion-Jacobson (DJ) quasi-2D perovskite solar cells (PSCs) have received increasing attention due to their greater potentials in realizing efficient and stable quasi-2D PSCs relative to their Ruddlesden-Popper counterpart. The substitution of methylammonium (MA⁺) with formamidinium is expected to be able to further increase the stability and power conversion efficiency (PCE) of DJ quasi-2D PSCs. Herein, we report a multifunctional additive strategy for preparing high-quality MA-free DJ quasi-2D perovskite films, where 1,1'-carbonyldi(1,2,4-triazole) (CDTA) molecules are incorporated into the perovskite precursor solution. CDTA modification can control phase distribution, enlarge grain size, modulate crystallinity and crystal orientation, and passivate defects. After CDTA modification, more favorable gradient phase distribution and accordingly gradient band alignment are formed, which is conducive to carrier transport and extraction. The improved crystal orientation can facilitate carrier transport and collection. The enlarged grain size and effective defect passivation contribute to reduced defect density. As a result, the CDTA-modified device delivers a PCE of 16.07%, which is one of the highest PCEs ever reported for MA-free DJ quasi-2D PSCs. The unencapsulated device with CDTA maintains 92% of its initial PCE after aging under one sun illumination for 360 h and 86% after aging at 60 °C for 360 h.

Reducing Defects in Organic-Lead Halide Perovskite Film by Delayed Thermal Annealing Combined with KI/I2 for Efficient Perovskite Solar Cells

This study improved quality of CH₃NH₃PbI₃ (MAPbI₃) perovskite films by delaying thermal annealing in the spin coating process and introducing KI and I₂ to prepare MAPbI₃ films that were low in defects for high-efficiency perovskite solar cells. The influences of delayed thermal annealing time after coating the MAPbI₃ perovskite layer on the crystallized perovskite, the morphology control of MAPbI₃ films, and the photoelectric conversion efficiency of solar cells were investigated. The optimal delayed thermal annealing time was found to be 60 min at room temperature. The effect of KI/I₂ additives on the growth of MAPbI₃ films and the corresponding optimal delayed thermal annealing time were further investigated. The addition of KI/I₂ can improve perovskite crystallinity, and the conductivity and carrier mobility of MAPbI₃ films. Under optimized conditions, the photoelectric conversion efficiency of MAPbI₃ perovskite solar cells can reach 19.36% under standard AM1.5G solar illumination of 100 mW/cm².

Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation thin-film photovoltaic technology owing to their excellent optoelectronic properties and cost-effectiveness. To gain the full potential of device performance, an in-depth understanding of the surface/interface science is an urgent need. Here, we present a review of molecularly engineered studies on interface modifications of PSCs. We elaborate a systematic classification of the existing optimization techniques employed in molecularly engineered perovskite and interface materials and analyze the insights underlying the reliability issues and functional behaviors. The achievements allow us to highlight the crucial strengths of molecular design for further tailoring of the interfacial properties, mitigating the nonradiative losses, optimizing the device performance, and retarding the degradation process of PSCs. Finally, the remaining challenges and potential development directions of molecularly engineered interfaces for high-performance and stable PSCs are also proposed.

cPCN-Regulated SnO2 Composites Enables Perovskite Solar Cell with Efficiency Beyond 23

Efficient electron transport layers (ETLs) not only play a crucial role in promoting carrier separation and electron extraction in perovskite solar cells (PSCs) but also significantly affect the process of nucleation and growth of the perovskite layer. Herein, crystalline polymeric carbon nitrides (cPCN) are introduced to regulate the electronic properties of SnO₂ nanocrystals, resulting in cPCN-composited SnO₂ (SnO₂-cPCN) ETLs with enhanced charge transport and perovskite layers with decreased grain boundaries. Firstly, SnO₂-cPCN ETLs show three times higher electron mobility than pristine SnO₂ while offering better energy level alignment with the perovskite layer. The SnO₂-cPCN ETLs with decreased wettability endow the perovskite films with higher crystallinity by retarding the crystallization rate. In the end, the power conversion efficiency (PCE) of planar PSCs can be boosted to 23.17% with negligible hysteresis and a steady-state efficiency output of 21.98%, which is one of the highest PCEs for PSCs with modified SnO₂ ETLs. SnO₂-cPCN based devices also showed higher stability than pristine SnO₂, maintaining 88% of the initial PCE after 2000 h of storage in the ambient environment (with controlled RH of 30% ± 5%) without encapsulation.

Review of Interface Passivation of Perovskite Layer

Perovskite solar cells (PSCs) are the most promising substitute for silicon-based solar cells. However, their power conversion efficiency and stability must be improved. The recombination probability of the photogenerated carriers at each interface in a PSC is much greater than that of the bulk phase. The interface of a perovskite polycrystalline film is considered to be a defect-rich area, which is the main factor limiting the efficiency of a PSC. This review introduces and summarizes practical interface engineering techniques for improving the efficiency and stability of organic-inorganic lead halide PSCs. First, the effect of defects at the interface of the PSCs, the energy level alignment, and the chemical reactions on the efficiency of a PSC are summarized. Subsequently, the latest developments pertaining to a modification of the perovskite layers with different materials are discussed. Finally, the prospect of achieving an efficient PSC with long-term stability through the use of interface engineering is presented.

Reducing Open-Circuit Voltage Deficit in Perovskite Solar Cells via Surface Passivation with Phenylhydroxylammonium Halide Salts

Suppressing non-radiative recombination via passivating surface defects of perovskite films has demonstrated an excellent strategy for high-performance perovskite solar cells (PSCs). However, it is still hard to realize both high open-circuit voltage (V_oc ) of >1.2 V and high power conversion efficiency (PCE) of >22%, because the optimized bandgap of perovskite films is less than 1.60 eV for efficient light harvesting and V_oc deficit is generally unavoidable due to carriers recombination. Here, the surface of the perovskite film is treated with a series of phenylhydroxylammonium halide salts and it is found that all of them can remarkably prolong the carrier lifetime owing to their excellent capability of surface defects passivation. The best PSC with phenylbutylammonium bromide treatment realizes a PCE of 22.67% with a V_oc of 1.216 V, corresponding to a small V_oc deficit of ≈344 mV.

High-temperature inverted annealing for efficient perovskite photovoltaics

High-quality perovskite films with large grains and reduced surface defects were obtained via an inverted annealing process. Corresponding photovoltaic devices achieved a highest efficiency of 20.4% with a stabilized power conversion efficiency (PCE) of 19.8%.

Critical Role of Functional Groups in Defect Passivation and Energy Band Modulation in Efficient and Stable Inverted Perovskite Solar Cells Exceeding 21% Efficiency

Interfaces in perovskite solar cells (PSCs) are closely related to their power conversion efficiency (PCE) and stability. It is highly desirable to minimize the interfacial nonradiative recombination losses through rational interfacial engineering. Herein we develop an effective and easily reproducible interface engineering strategy where three mercaptobenzimidazole (MBI)-based molecules are employed to modify the perovskite/electron transport layer (ETL) interface. MBI and MBI-OCH₃ can not only passivate defects at surface and grain boundaries (GBs) of perovskite films but can also improve energy level alignment (ELA), which leads to enhanced PCE and stability. Consequently, the PCE is improved from 19.5% for the control device to 21.2% for MBI-modified device, which is among the best reported inverted MAPbI₃-based PSCs. In contrast, incorporation of MBI-NO₂ increases defect density and negligibly influences the energy level alignment. This work indicates that defect passivation and ELA modulation can be achieved simultaneously through modulating functional groups in interface modification molecules.

All-inorganic Sn-based Perovskite Solar Cells: Status, Challenges, and Perspectives

Recently, the power conversion efficiency (PCE) of perovskite solar cells (PSC) based on organic-inorganic hybrid Pb halide perovskites has reached 25.2 %. However, the toxicity of Pb has still been a main concern for the large-scale commercialization of Pb-based PSCs. Efforts have been made during the past few years to seek eco-friendly Pb-free perovskites, and it is a growing consensus that Sn is the best choice as Pb alternative over any other Pb-free metal elements. Among Sn-based perovskites, all-inorganic cells are promising candidates for PSCs owing to their more suitable bandgap, better stability, and higher charge mobility compared to the organic-inorganic hybrid counterparts. However, the poor phase stability of all-inorganic Sn-based perovskites (AISPs) and low PCE of their PSCs are most challenging in the field at present. Herein, recent developments on PSCs based on AISPs, including CsSnX₃ and Cs₂ SnX₆ (X=Br, I), are comprehensively reviewed. Primarily, the intrinsic characteristics of the two AISPs are overviewed, including crystallographic property, band structure, charge carrier property, and defect property. Sequentially, state-of-the-art progress, regarding the photovoltaic application of AISPs as light absorber, is summarized. At last, current challenges and future opportunities of AISP-based PSCs are also discussed.

Traps in metal halide perovskites: characterization and passivation

Metal halide perovskites (MHPs) have become a research focus in the field of optoelectronics due to their excellent optoelectronic properties and simple and cost-effective thin film manufacturing processes. In particular, the power conversion efficiency (PCE) of solar cells (SCs) and external quantum efficiency (EQE) of light-emitting diodes (LEDs) based on perovskite materials have reached 25.2% and 21.6%, respectively, in a short period, making perovskites especially promising for fabricating next-generation optoelectronic devices. Despite these inspiring results, obtaining high-performance, high-stability MHP-based devices still faces many challenges, among which the defects and the consequent traps in MHPs are key factors. Defect-induced traps can trap charge carriers or even act as non-radiative recombination centers, seriously degrading the device performance, causing hysteresis and deteriorating the stability of MHP-based devices. Thus, understanding the chemical/physical nature of traps and adopting appropriate strategies to passivate traps are important to enhance the device performance and stability. Herein we present a review in which the knowledge and understanding of traps in MHPs are considered and discussed. Moreover, the latest efforts on passivating traps in MHPs for improving device performance are summarized, with the hope of providing guidance to future development of high-performance and high-stability MHP-based devices.

Revealing Energy Loss and Nonradiative Recombination Pathway in Mixed-Ion Perovskite Solar Cells

Multiple-cation lead mixed-halide perovskites (MLMPs) with tunable band gaps have been demonstrated as ideal candidates to achieve perovskite solar cells with high efficiencies. It is well-known that a large open-circuit voltage (V_OC) loss caused by nonradiative recombination still limits the approach to the Shockley-Queisser limit. However, there are few comprehensive contributions regarding the origin and pathway of nonradiative recombination in n-i-p structured MLMPs. Here, we compare the performance of MLMPs containing different halides and analyze the energy loss and interface trap-assisted nonradiative recombination characterizations. It is found that Br-containing devices with a lower interface trap density of 3.2 × 10¹³ cm^-2 obtain a high V_OC of 1.12 V, a small energy loss of 0.02 eV, radiative recombination current density of 8.05 × 10^-21 A m^-2, and total recombination current density of 22.16 mA cm^-2. This work provides an opportunity to understand the device physics and reveals the nature of nonradiative recombination based on experiment and simulation.

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05 (MA_0.17 FA_0.83 )_0.95 PbBr₃ (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Establishing Multifunctional Interface Layer of Perovskite Ligand Modified Lead Sulfide Quantum Dots for Improving the Performance and Stability of Perovskite Solar Cells

While organic-inorganic halide perovskite solar cells (PSCs) show great potential for realizing low-cost and easily fabricated photovoltaics, the unexpected defects and long-term stability against moisture are the main issues hindering their practical applications. Herein, a strategy is demonstrated to address the main issues by introducing lead sulfide quantum dots (QDs) on the perovskite surface as the multifunctional interface layer on perovskite film through establishing perovskite as the ligand on PbS QDs. Meanwhile, the multifunctions are featured in three aspects including the strong interactions of PbS QDs with perovskites particularly at the grain boundaries favoring good QDs coverage on perovskites for ultimate smooth morphology; an inhibition of iodide ions mobilization by the strong interaction between iodide and the incorporated QDs; and the reduction of the dangling bonds of Pb²⁺ by the sulfur atoms of PbS QDs. Finally, the device performances are highly improved due to the reduced defects and non-radiative recombination. The results show that both open-circuit voltage and fill factor are significantly improved to the high values of 1.13 V and 80%, respectively in CH₃ NH₃ PbI₃ -based PSCs, offering a high efficiency of 20.64%. The QDs incorporation also enhances PSCs' stability benefitting from the induced hydrophobic surface and suppressed iodide mobilization.

Interfacial modification towards highly efficient and stable perovskite solar cells

Organic-inorganic perovskite solar cells (PSCs) have attracted tremendous attention due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap, and their excellent efficiency was boosted to a certified 25.2% efficiency in 2019. However, due to the presence of a high-density of charge traps in perovskite films, plenty of charge recombination occurs at grain boundaries and defects caused by precursor compositions, the process of preparation and crystal growth, thereby restricting the power conversion efficiency (PCE). At present, interfacial modifications by using additives play an important role in various breakthroughs of PSCs. Herein, the effects of various additives with the main types of functional groups, length and spatial configuration of molecules on interfacial modifications in PSCs are reviewed, and their influences on perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, stabilities of PSCs, and adjusting the interface of structures and energy levels for device performances are also described and summarized. Finally, an outlook of interfacial modifications is provided on the selection and design of efficient additives with respect to the fabrication and development of highly efficient and stable PSCs.

Improving Efficiency and Stability in Quasi-2D Perovskite Light-Emitting Diodes by a Multifunctional LiF Interlayer

Owing to the enlarged exciton binding energy and the ability to confine charge carriers compared to their three-dimensional (3D) counterparts, research on quasi-two-dimensional (quasi-2D) perovskite materials and the correlative application in light-emitting diodes (LEDs) has attracted considerable attention. However, high density of defects, exciton emission trapping, and unbalanced charge injection are still the main intractable obstacles to their further development and practical application. Herein, we report an efficient multifunctional interlayer, lithium fluoride (LiF), to boost the performance of green-emitting quasi-2D perovskite LEDs (PeLEDs) by simultaneously overcoming the aforementioned issues. The introduced LiF interlayer not only eliminates the defects at perovskite grain boundaries and the surface by reinforcing the chemical bonds with uncoordinated lead ions but also restrains the emission of perovskite from quenching triggered by the electron transport layer and reduces excess electron injections to effectively balance carriers in the device. As a result, the resulting green quasi-2D PeLED shows a maximum external quantum efficiency of 16.35%, which is the best value obtained for quasi-2D perovskite-based LEDs reported so far, with simultaneous improvement in the operating lifetime of the device.

Towards commercialization: the operational stability of perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency (25.2%) and low fabrication cost. However, the short lifetime under operation is the major obstacle for their commercialization. With efforts from the entire PSC research community, significant advances have been witnessed to improve the device operational stability, and a timely summary on the progress is urgently needed. In this review, we first clarify the definition of operational stability and its significance in the context of practical use. By analyzing the mechanisms in established approaches for operational stability improvement, we summarize several effective strategies to extend device lifetime in a layer-by-layer sequence across the entire PSC. These mechanisms are discussed in the contexts of chemical reactions, photo-physical management, technological modification, etc., which may inspire future R&D for stable PSCs. Finally, emerging operational stability standards with respect to testing and reporting device operational stability are summarized and discussed, which may help reliable device stability data circulate in the research community. The main target of this review is gaining insight into the operational stability of PSCs, as well as providing useful guidance to further improve their operational lifetime by rational materials processing and device fabrication, which would finally promote the commercialization of perovskite solar cells.

Realization of Moisture-Resistive Perovskite Films for Highly Efficient Solar Cells Using Molecule Incorporation

The development of highly crystalline perovskite films with large crystal grains and few surface defects is attractive to obtain high-performance perovskite solar cells (PSCs) with good device stability. Herein, we simultaneously improve the power conversion efficiency (PCE) and humid stability of the CH₃NH₃PbI₃ (CH₃NH₃ = MA) device by incorporating small organic molecule IT-4F into the perovskite film and using a buffer layer of PFN-Br. The presence of IT-4F in the perovskite film can successfully improve crystallinity and enhance the grain size, leading to reduced trap states and longer lifetime of the charge carrier, and make the perovskite film hydrophobic. Meanwhile, as a buffer layer, PFN-Br can accelerate the separation of excitons and promote the transfer process of electrons from the active layer to the cathode. As a consequence, the PSCs exhibit a remarkably improved PCE of 20.55% with reduced device hysteresis. Moreover, the moisture-resistive film-based devices retain about 80% of their initial efficiency after 30 days of storage in relative humidity of 10-30% without encapsulation.

New Extraction Technique of In-Gap Electronic-State Spectrum Based on Time-Resolved Charge Extraction

The in-gap electronic state (trap state) is an important factor that determines the photovoltaic performance of solar cells. In this article, we put forward a new technique for extracting the density of trap state (DOS_T) distribution based on the time-resolved charge extraction (TRCE) experiment result. Based on strict derivation, we find that when the TRCE result is linear, the extracted DOS_T distribution is exponential type and vice versa. Compared to the approach given by Wang et al., the method introduced in this paper is more accurate and reliable. Compared to the approach based on the space charge-limited current (SCLC) experiment result, our method needs less computation.