Defect Passivation Effect of Chemical Groups on Perovskite Solar Cells

Interface Passivation of a Pyridine-Based Bifunctional Molecule for Inverted Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have recently been demonstrated to be promising renewable harvesters because of their prominent photovoltaic power conversion efficiency (PCE), although their stability and efficiency still have not reached commercial criteria. Trouble-oriented analyses showcase that defect reduction among the grain boundaries and interfaces in the prepared perovskite polycrystalline films is a practical strategy, which has prompted researchers to develop functional molecules for interface passivation. Herein, the pyridine-based bifunctional molecule dimethylpyridine-3,5-dicarboxylate (DPDC) was employed as the interface between the electron-transport layer and perovskite layer, which achieved a champion PCE of 21.37% for an inverted MAPbI3-based PSC, which was greater than 18.64% for the control device. The mechanistic studies indicated that the significantly improved performance was mainly attributed to the remarkably enhanced fill factor with a value greater than 83%, which was primarily due to the nonradiative recombination suppression offered by the passivation effect of DPDC. Moreover, the promoted carrier mobility together with the enlarged crystal size contributed to a higher short-circuit current density. In addition, an increase in the open-circuit voltage was also observed in the DPDC-treated PSC, which benefited from the improved work function for reducing the energy loss during carrier transport. Furthermore, the DPDC-treated PSC showed substantially enhanced stability, with an over 80% retention rate of its initial PCE value over 300 h even at a 60% relative humidity level, which was attributed to the hydrophobic nature of the DPDC molecule and effective defect passivation. This work is expected not only to serve as an effective strategy for using a pyridine-based bifunctional molecule to passivate perovskite interfaces to enhance photovoltaic performance but also to shed light on the interface passivation mechanism.

Regulating Surface-Passivator Binding Priority for Efficient Perovskite Light-Emitting Diodes

Suppressing trap-assisted nonradiative losses through passivators is a prerequisite for efficient perovskite light-emitting diodes (PeLEDs). However, the complex bonding between passivators and perovskites severely suppresses the passivation process, which still lacks comprehensive understanding. Herein, the number, category, and degree of bonds between different functional groups and the perovskite are quantitatively assessed to study the passivation dynamics. Functional groups with high electrostatic potential and large steric hindrance prioritize strong bonding with organic cations and halides on the perfect surface, leading to suppressed coordination with bulky defects. By modulating the binding priorities and coordination capacity, hindrance from the intense interaction with perfect perovskite is significantly reduced, leading to a more direct passivation process. Consequently, the near-infrared PeLED without external light out-coupling demonstrates a record external quantum efficiency of 24.3% at a current density of 42 mA cm-2. In addition, the device exhibits a record-level-cycle ON/OFF switching of 20 000 and ultralong half-lifetime of 1126.3 h under 5 mA cm-2. An in-depth understanding of the passivators can offer new insights into the development of high-performance PeLEDs.

Defects and Defect Passivation in Perovskite Solar Cells

Perovskite solar cells have made significant strides in recent years. However, there are still challenges in terms of photoelectric conversion efficiency and long-term stability associated with perovskite solar cells. The presence of defects in perovskite materials is one of the important influencing factors leading to subpar film quality. Adopting additives to passivate defects within perovskite materials is an effective approach. Therefore, we first discuss the types of defects that occur in perovskite materials and the mechanisms of their effect on performance. Then, several types of additives used in perovskite solar cells are discussed, including ionic compounds, organic molecules, polymers, etc. This review provides guidance for the future development of more sustainable and effective additives to improve the performance of solar cells.

Scintillation Properties of CsPbBr3 Nanocrystals Prepared by Ligand-Assisted Reprecipitation and Dual Effect of Polyacrylate Encapsulation toward Scalable Ultrafast Radiation Detectors

Lead halide perovskite nanocrystals (LHP-NCs) embedded in polymeric hosts are gaining attention as scalable and low-cost scintillation detectors for technologically relevant applications. Despite rapid progress, little is currently known about the scintillation properties and stability of LHP-NCs prepared by the ligand assisted reprecipitation (LARP) method, which allows mass scalability at room temperature unmatched by any other type of nanostructure, and the implications of incorporating LHP-NCs into polyacrylate hosts are still largely debated. Here, we show that LARP-synthesized CsPbBr3 NCs are comparable to particles from hot-injection routes and unravel the dual effect of polyacrylate incorporation, where the partial degradation of LHP-NCs luminescence is counterbalanced by the passivation of electron-poor defects by the host acrylic groups. Experiments on NCs with tailored surface defects show that the balance between such antithetical effects of polymer embedding is determined by the surface defect density of the NCs and provide guidelines for further material optimization.

Enhancing the Efficiency and Stability of Perovskite Solar Cells through Defect Passivation and Controlled Crystal Growth Using Allantoin

In this study, we present a robust approach that concurrently manages crystal growth and defect passivation within the perovskite layer through the introduction of a small molecule additive─allantoin. The precise regulation of crystal growth in the presence of allantoin yields perovskite films characterized by enhanced morphology, larger grain size, and improved grain orientation. Notably, the carbonyl and amino groups present in allantoin passivate under-coordinated Pb2+ and I- defects, respectively, through molecular interactions. Trap density in the perovskite layer is measured, and it is 0.39 × 1016 cm-3 for the allantoin-incorporated device and 0.83 × 1016 cm-3 for the pristine device. This reduction in defects leads to reduced trap-assisted nonradiative recombination, as confirmed by the photoluminescence, transient photo voltage, and impedance measurements. As a result, when these allantoin-incorporated perovskite films are implemented as the active layer in solar cells, a noteworthy efficiency enhancement to 20.63% is attained, surpassing the 18.04% of their pristine counterparts. Furthermore, devices with allantoin exhibit remarkable operational stability, maintaining 80% of their efficiency even after 500 h of continuous illumination, whereas the pristine device degraded to 65% of its initial efficiency in 400 h. Also, allantoin-incorporated devices exhibited exceptional stability against high humidity and elevated temperatures.

Defect Passivation Scheme toward High-Performance Halide Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted much attention in recent years due to their simple manufacturing process, low cost, and high efficiency. So far, all efficient organic-inorganic halide PSCs are mainly made of polycrystalline perovskite films. There are transmission barriers and high-density defects on the surface, interface, and grain boundary of the films. Among them, the deep-level traps caused by specific charged defects are the main non-radiative recombination centers, which is the most important factor in limiting the photoelectric conversion efficiency of PSCs devices to the Shockley-Queisser (S-Q) theoretical efficiency limit. Therefore, it is imperative to select appropriate passivation materials and passivation strategies to effectively eliminate defects in perovskite films to improve their photovoltaic performance and stability. There are various passivation strategies for different components of PSCs, including interface engineering, additive engineering, antisolvent engineering, dopant engineering, etc. In this review, we summarize a large number of defect passivation work to illustrate the latest progress of different types of passivators in regulating the morphology, grain boundary, grain size, charge recombination, and defect density of states of perovskite films. In addition, we discuss the inherent defects of key materials in carrier transporting layers and the corresponding passivation strategies to further optimize PSCs components. Finally, some perspectives on the opportunities and challenges of PSCs in future development are highlighted.

Strain-Driven Solid-Solid Crystal Conversion in Chiral Hybrid Pseudo-Perovskites with Paramagnetic-to-Ferromagnetic Transition

Hybrid organic-inorganic perovskites (HOIPs) are promising stimuli-responsive materials (SPMs) owing to their molecular softness and tailorable structural dimensionality. The design of mechanically responsive HOIPs requires an in-depth understanding of how lattice strain induces intermolecular rearrangement that impacts physical properties. While chirality transfer from an organic cation to an inorganic lattice is known to influence chiral-optical properties, its effect on strain-induced phase conversion has not been explored. As opposed to achiral or racemic organic cations, chiral organic cations can potentially afford a new dimension in strain-responsive structural change. Herein, we demonstrate that mechanical strain induces a solid phase crystal conversion in chiral halide pseudo-perovskite single crystals (R/S)-(FE)₂CuCl₄ (FE = (4-Fluorophenyl)ethylamine) from a 0D isolated CuCl₄ tetrahedral to 1D corner-sharing CuFCl₅ octahedral framework via the incorporation of Cu···F interaction and N-H···F hydrogen bonding. This strain-induced crystal-to-crystal conversion involves the connection of neighboring 0D CuCl₄ tetrahedra via Cu²⁺-Cl^--Cu²⁺ linkages as well as the incorporation of a F-terminated organic cation as one of the X atoms in BX₆ octahedra, leading to a reduced band gap and paramagnetic-to-ferromagnetic conversion. Control experiments using nonchiral or racemic perovskite analogs show the absence of such solid phase conversion. To demonstrate pressure-sensitive properties, the 0D phase is dispersed in water-soluble poly(vinyl alcohol) (PVA) polymer, which can be applied to a large-scale pressure-induced array display on fibrous Spandex substrates via a screen-printing method.

Cinnamate-Functionalized Cellulose Nanocrystals as Interfacial Layers for Efficient and Stable Perovskite Solar Cells

The poor interfacial contact and imperfections between the charge transport layer and perovskite film often result in carrier recombination, inefficient charge collection, and inferior stability of perovskite solar cells (PSCs). Therefore, interface engineering is quite crucial to achieve high-performance and stable PSCs. Here, we introduced a cinnamate-functionalized cellulose nanocrystals (Cin-CNCs) interfacial layer between SnO₂ and perovskite active layer for enhancing carrier transport ability and crystal growth of perovskite, meanwhile endowing additional functional of long-term device stability against ultraviolet light. The enhancement of interfacial contact between SnO₂ and perovskite layer and cascade energy alignment are realized, which is beneficial for obtaining the desirable perovskite film morphology, passivating the interfacial defects, and restraining charge recombination in the SnO₂/perovskite interface. An efficiency as high as 23.18%, with an open-circuit voltage of 1.15 V and a significantly enhanced fill factor of 81.07%, is achieved. In addition, the unencapsulated PSCs maintain 75% of the initial PCE after aging for over 1500 h under 25 °C and 30% relative humidity, with better light-soaking stability. These results exhibit the vital role for Cin-CNCs in interfacial modification and constructing high-performance perovskite solar cells.

Revisiting the Iodine Vacancy Surface Defects to Rationalize Passivation Strategies in Perovskite Solar Cells

Current knowledge on the nature of surface iodine vacancies (V_I), which are important for the photovoltaic performance and stability of perovskite solar cells, is debatable. We investigated V_I on a stable MAI-terminated CH₃NH₃PbI₃ (MAPbI₃) surface. First-principles calculations indicated the sensitivity of the atomic structure of surface V_I to the charge states and locations on the surface layer. V_I in the outermost layer are benign; however, those near the surface can be detrimental. Illumination can promote the diffusion of V_I from the outermost layer into the bulk, making them detrimental. There are two mechanisms for the surface passivation of V_I: (i) passivation in the second layer to eliminate deep-state V_I and (ii) passivation in the outermost layer to inhibit V_I diffusion upon illumination (working condition of solar cells). This work rationalizes contradictory reports on the surface properties of halide perovskites and proposes insights into their surface passivation to fabricate high-performing solar cells.