Perovskite Grain Boundaries Research Articles

Thermal-triggered healing strategy for efficient and stable perovskite solar cells with efficiency over 25 %

The power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PSCs) is hindered by the abundant defects within the grain boundaries (GBs) of perovskite films. Herein, a directed thermal-triggered healing strategy based on phthalide-3-acetic acid (P3AC) is proposed, aiming to mitigate GBs defects and optimize energy level arrangement by utilizing the strong polarity effect of P3AC and its thermal-triggered interaction with perovskite. The introduced dual-sites molecular (P3AC) anchors at the GBs of perovskite, which can passivate the uncoordinated Pb2+ and form hydrogen bonds with FA+, yielding dense, uniform and low-defects and large-grain perovskite film. Compared to the pristine devices, the P3AC-modified devices achieved a champion PCE of 25.28 % (certified PCE of 24.46 %) and impressive operational stability. This work provides an effective and direct solution for enhancing the performance of PSCs.

Grain boundary defect passivation and iodine migration inhibition for efficient and stable perovskite solar cells

To inhibit the I−migration and passivate the uncoordinated Pb2+ on perovskite grain boundary (GB), 3-methyl-(1,1′-biphenyl)-4,4′-diformaldehyde (BPDA-Me) and 3-chlorine-(1,1′-biphenyl)-4,4′-diformaldehyde (BPDA-Cl) were synthesized and incorporated into perovskite films, respectively. The C = O groups in both additives can strongly interact with the uncoordinated Pb2+, allowing them to be anchored in perovskite GB. At the same time, the benzene ring skeleton in these two molecular structures can interact with the migrating I− in GB. The Cl atom in the BPDA-Cl molecule delocalize electrons into the C = O group due to the conjugation effect of the benzene ring, so that the C = O group can provide stronger electronegativity, thus enhancing the interaction with the uncoordinated Pb2+. Meanwhile, Cl atom can coordinate with Pb2+ to synergically passivate the I vacancy defect on GB and enhance the lattice strength of PbI6. This cooperative passivation further effectively inhibited the I−migration occurring at the perovskite GB. The functional group electron density regulation and cooperative passivation of Cl and C = O in BPDA-Cl make the passivation effect better than BPDA-Me. Consequently, the BPDA-Cl based perovskite solar cell achieved the highest power conversion efficiency of 24.96 % and stability with 92.1 % of the initial performance retained after 500 h of operation under continuous lighting and maximum power point tracking conditions.

2D Phase Formation on 3D Perovskite: Insights from Molecular Stiffness.

Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.

2-(Perfluorooctyl) ethyl acrylate functionalized pervoskite: A selective and stable photoelectrochemical platform for ultrasensitive detection of cholesterol

Herein, a novel ultrasensitive cholesterol photoelectrochemical (PEC) sensor was designed based on fluorinated molecularly imprinted polymers (FMIPs) functionalized CH3NH3PbI3@carbon nanodots (CH3NH3PbI3@CNDs) platform. 2-(Perfluorooctyl) ethyl acrylate (PFEA), a functional monomer, was chosen not only to endow CH3NH3PbI3 good selectivity but also improve its aqueous stability because of the fluorine containing long hydrophobic chain. Furthermore, the FMIPs can inhibit the recombination of electron holes and prolonged the carrier life time, and dramatically enhance the charge transfer efficiency by the reduction of defect density through the polymer passivation of grain boundaries of perovskite. The morphology, the enhanced hydrophobicity and photoelectric properties of FMIPs@CH3NH3PbI3@CNDs interface was characterized by transmission electron microscopy, Fourier transform infrared spectrum, scanning electron microscope, contact angle, photoluminescence spectrum and intensity modulated photovoltage spectrum, respectively. Under the optimized conditions, the linear range of the designed PEC sensor for cholesterol detection was 1.0 × 10−13–1.0 × 10−9 mol/L, and the detection limit was down to 8.9 × 10−14 mol/L. The new PEC sensor exhibited specific selectivity, good stability and hypersensitivity, and was successfully applied to cholesterol detection in human serum samples. This work extended the development of the MIPs based sensor and the high-performance perovskite.

Revealing Interaction of Fluorinated Propylamine Hydrochloride with Precursor and Defect States of Perovskite Films Toward Efficient Flexible Solar Cells

AbstractThe trap state at the surfaces and grain boundaries of perovskite is one of the major obstacles to the further commercialization of flexible perovskite solar cells (FPSCs). Herein, two innovative multifunctional fluorinated propylamine salt 2,2,3,3,3‐pentafluoropropylamine hydrochloride (PFPACl) and 3,3,3‐triflupropylamine hydrochloride (TFPACl) are in situ introduced onto the photo absorbing layer to improve the performance of the FPSCs. The nuclear magnetic resonance (NMR) spectroscopy indicates strong interactions of both PFPACl and TFPACl with the perovskite precursor components. For the first time, the structures of the supramolecular complexes formed by two additives with FAI are deduced from NOESY NMR data, thus pointing to the importance of the preorganization of the perovskite components in solution before film casting. The experiments and density functional theory（DFT) calculations reveal that PFPACl is likely dissociated more into the form of R‐NH3+‐Cl− due to the higher electronegativity of the fluoroalkyl tail. Therefore, PFPA+ binds more strongly to VFA defects than TFPA+, and anion Cl− has strong enough interaction with VFAI and uncoordinated Pb2+, leading to homogeneous coverage of PFPACl on the entire surface of the perovskite films and better energy alignment with the hole transport layer. Consequently, PFPACl‐treated FPSCs achieved a relatively high PCE of 23.59% with excellent mechanical robustness and operational stability.

Dimethylamine oxalate manipulating CsPbI3 perovskite film crystallization process for high efficiency carbon electrode based perovskite solar cells

Crystallization process determines the quality of perovskite films and the performances of resultant perovskite solar cells (PSCs). Dimethylamine oxalate has been proven as a multifunctional modulator, and is explored as an efficient additive in manipulating the crystallization process of CsPbI3 perovskite films. On one hand, oxalate serves as the precipitator that facilitates the nucleation process of intermediate. The larger size of intermediate is conductive to the larger size and smaller grain boundaries of resultant perovskite. On the other hand, in subsequent annealing process, the phase conversion and growth process of transient perovskite can be decelerated due to the strong interactions of oxalate with both dimethylamine cation (DMA+) and Pb2+. Due to the optimized crystallization kinetics, the morphology and quality of CsPbI3 perovskite films are comprehensively improved with lower defect concentrations, and charge recombination loss is effectively suppressed. Benefiting from the optimized crystal quality of perovskite films, the carbon electrode-based CsPbI3 PSCs exhibit a champion efficiency of 18.48%. This represents one of the highest levels among all hole transport layer-free inorganic perovskite solar cells.

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AbstractMorphologically noticeable mixed‐dimensional perovskite films induced by ionic liquids (ILs) engineering have been rarely reported. This work (DOI: 10.1002/inf2.12459) innovatively introduced IL 1‐(3‐cyanopropyl) ‐3‐methylimidazolium chloride (CPMIMCl) into the 3D perovskite precursor system to form 1D‐3D mixed perovskite via two‐step method. The generated 1D CPMIMPbI3 with in‐plane orientation resides at the grain boundaries of 3D perovskite can be appreciably observed from the morphology level, leading to creation of high‐quality films with enlarged grain size. image

Simultaneously Achieved Defect Passivation and Crystallization Modulation by a Multifunctional Pseudohalogen Salt for Efficient and Stable Perovskite Solar Cells

The accumulation of defects and ion migration at the surfaces and grain boundaries of perovskite impedes the improvement of performance and stability in perovskite solar cells. Therefore, developing strategies to reduce trap‐assisted nonradiative recombination and suppress ion migration in perovskite films is urgently needed. Herein, a multifunctional pseudohalogen salt additive, (benzotriazol‐1‐yloxy) dipyrrolidinocarbenium hexafluorophosphate (denoted as BDPF6), composed of a benzotriazole derivative cation and hexafluorophosphate anion, is incorporated into the perovskite precursor solution. The anion vacancies of perovskite films are filled by , whereas the cation and anion in BDPF6 form ionic and coordination bonds with the perovskites. The BDPF6 additive promotes the crystallization of perovskite films with large grain size, reducing defect densities, prolonging carrier lifetimes, and inhibiting ion migration. Thus, the power conversion efficiency (PCE) of the BDPF6‐modified device remarkably improves from 20.36% to 22.68%. The unencapsulated BDPF6‐modified device maintains 97% of its initial PCE after 1,400 h of exposure to an ambient environment with a relative humidity of 10–20%, whereas the control device maintains only 85% of its initial PCE. Similarly, the BDPF6‐modified device maintains 78% of its original PCE after aging at 60 °C for 1,400 h, whereas the control device only maintains 55%.

Disentangling Electronic Transport and Hysteresis at Individual Grain Boundaries in Hybrid Perovskites via Automated Scanning Probe Microscopy

Underlying the rapidly increasing photovoltaic efficiency and stability of metal halide perovskites (MHPs) is the advancement in the understanding of the microstructure of polycrystalline MHP thin film. Over the past decade, intense efforts have been aimed at understanding the effect of microstructures on MHP properties, including chemical heterogeneity, strain disorder, phase impurity, etc. It has been found that grain and grain boundary (GB) are tightly related to lots of microscale and nanoscale behavior in MHP thin films. Atomic force microscopy (AFM) is widely used to observe grain and boundary structures in topography and subsequently to study the correlative surface potential and conductivity of these structures. For now, most AFM measurements have been performed in imaging mode to study the static behavior; in contrast, AFM spectroscopy mode allows us to investigate the dynamic behavior of materials, e.g., conductivity under sweeping voltage. However, a major limitation of AFM spectroscopy measurements is that they require manual operation by human operators, and as such only limited data can be obtained, hindering systematic investigations of these microstructures. In this work, we designed a workflow combining the conductive AFM measurement with a machine learning (ML) algorithm to systematically investigate grain boundaries in MHPs. The trained ML model can extract GBs locations from the topography image, and the workflow drives the AFM probe to each GB location to perform a current-voltage (IV) curve automatically. Then, we are able to have IV curves at all GB locations, allowing us to systematically understand the property of GBs. Using this method, we discovered that the GB junction points are less conductive, potentially more photoactive, and can play critical roles in MHP stability, while most previous works only focused on the difference between GB and grains.

Multifunctional Conjugated Molecular Additives for Highly Efficient Perovskite Light-Emitting Diodes.

Further optimization of perovskite light-emitting diodes (PeLEDs) is impeded by crystal deformation caused by residual stress and defect formation with subsequent non-radiative recombination. Molecular additives for defect passivation are widely studied; however, the majority have insulating properties that hinder charge injection and transport. Herein, highly efficient green-emitting PeLEDs are reported by introducing semiconducting molecular additives (Fl-OEGA and Fl-C8A). Transmission electron microscopy shows that conjugated additives exist primarily at the grain boundaries of perovskite, and Kelvin probe force microscopy confirms that the variation in contact potential difference between grain boundaries and perovskite crystal domains is significantly reduced. The residual tensile stress is reduced by 13% and the activation energy for ion migration increases in the Fl-OEGA-treated perovskite film, compared to those of the film without additives. Compared to insulating 2,2'-(ethylenedioxy)diethylamine (EDEA), the introduction of semiconducting additives prevents a significant reduction in the charge-transport capability. Furthermore, the PeLEDs with Fl-OEGA show a negligible shift in the turn-on voltage and a significantly smaller decrease in the current density with increasing Fl-OEGA compared to the devices with EDEA. Finally, the 3D CsPbBr3 -PeLEDs show the highest external quantum efficiency of 21.3% by the incorporation of semiconducting Fl-OEGA as a new multifunctional additive.

Controllable extrinsic ion transport in two-dimensional perovskite films for reproducible, low-voltage resistive switching

Organic-inorganic halide perovskite has attracted significant interest in being switching medium for resistive random access memory (RRAM), yet the in-depth understanding of ion spatial distribution and transport kinetics—which is responsible for the filament formation and normally suffers from a paradox between stochasticity and dynamics—remains limited. Herein, we show evidence of space-confined, fast extrinsic ion transport within grain boundaries (GBs) of two-dimensional perovskite films through systematic ex-situ and in-situ studies. The filament growth can be dominated by the geometrical feature of GBs that act as the channel for cation transport in the dielectric film. By tailoring the structure of perovskite GBs, electroforming-free RRAM devices are fabricated with an ultralow set voltage of 0.09 V (1.8kVcm−1) and small temporal/spatial variations (<10%). The devices can also be integrated with flexible substrates for multifunctional applications, including multilevel writing and light erasing. Our work may open new perspectives for regulating filament formation kinetics in RRAMs and provides a reliable building block for future applications in electronic and photonic circuits.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Stable and environmentally friendly perovskite solar cells induced by grain boundary engineering with self-assembled hydrogen-bonded porous frameworks

Due to the unpredictable composition and vulnerable nature of the grain boundaries (GBs), grain boundaries engineering with improving the order degree of perovskite GBs would be the breakthrough for high-performance polycrystalline perovskite solar cells (PSCs). However, there are rare works on the stacking structures of passivators and on their influence on performance. Herein, based on the easily aggregate nature, a bicarbazole molecule featuring cyanogroup was used as a GB passivator to self-assemble ordered porous framework structures in GBs. The formation of pores depends on the inversion of the carbazole plane. Besides defect passivation, humidity barrier and crystallization promotion abilities, the ordered porous stacking structure shows extra advantages on lead leakage elimination and tensile stress relief. The easily deformable quadrilateral pores, which are connected by weak forces with adjustable bond length, give the skeleton a function of “turning joint” that releases tensile stresses during the thermal process by its structural expansion, and thus, restricting perovskite expansion with ion migration/evaporation suppression, which is commendable for passivators. As a result, a champion PCE of 23.15% is realized, and PSCs show significantly enhanced thermal and operational stability, as well as diminished lead leakage. This work reveals more possibilities in grain boundary engineering and provides a new avenue for stable and environmental-friendly PSCs.

Grain boundary sliding and distortion on a nanosecond timescale induce trap states in CsPbBr3: ab initio investigation with machine learning force field.

Grain boundaries (GBs) in perovskite solar cells and optoelectronic devices are widely regarded as detrimental defects that accelerate charge and energy losses through nonradiative carrier trapping and recombination, but the mechanism is still under debate owing to the diversity of GB configurations and behaviors. We combine ab initio electronic structure and machine learning force field to investigate evolution of the geometric and electronic structure of a CsPbBr3 GB on a nanosecond timescale, which is comparable with the carrier recombination time. We demonstrate that the GB slides spontaneously within a few picoseconds increasing the band gap. Subsequent structural oscillations dynamically produce midgap trap states through Pb-Pb interactions across the GB. After several hundred picoseconds, structural distortions start to occur, increasing the occurrence of deep midgap states. We identify a distinct correlation of the average Pb-Pb distance and fluctuations in the ion coordination numbers with the appearance of the midgap states. Suppressing GB distortions through annealing and breaking up Pb-Pb dimers by passivation can efficiently alleviate the detrimental effects of GBs in perovskites. The study provides new insights into passivation of the detrimental GB defects, and demonstrates that structural and charge carrier dynamics in perovskites are intimately coupled.

Pre-annealing treatment for high-efficiency perovskite solar cells via sequential deposition

<h2>Summary</h2> Grain boundaries (GBs) have been shown to initiate metal halide perovskite degradation under light, moisture, and/or heat stresses due to the existence of various defects, making them extremely vulnerable in perovskite solar cells (PSCs). Although various post-treatments have been proven to be effective in repairing GBs, preventing the degradation from perovskite GBs remains an extremely difficult challenge. We report here a generic pre-annealing treatment for reconstructing sequentially deposited perovskite crystallites and GBs. With this strategy, robust GBs are created upon defect suppression with preferred oriented large perovskite grains. As a result, the resultant n-i-p-structured PSCs deliver an impressive power conversion efficiency of nearly 25% and an extremely high fill factor of 85.5%. The unencapsulated cells maintain 81% of their initial efficiencies after 1,000 h of continuous operation at maximum power point at about 55°C in an inert atmosphere, showing greatly improved light stability.

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 PbI2 at 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 PbI2 is 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, PbI2 in the film could be completely converted into perovskite without excessive AX residue. Both the efficiency and stability of the perovskite solar cells without excessive PbI2 have 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.

Suppressing the crystallographic disorders induced by excess PbI2 to achieve trade-off between efficiency and stability for PbI2-rich perovskite solar cells

Excess PbI2 can improve the efficiency of perovskite solar cells (PSCs). However, it usually leads to poor cell stability due to the chemically active nature of PbI2. Improving the stability of PbI2-rich perovskite solar cells without scarifying the efficiency remains challenging. In this work, it is found that the non-uniform and multiple nucleation of perovskite induces crystallographic disorders at grain boundaries (GBs) of PbI2-rich perovskite. These disorders working as the n-type shallow dopant reduces the energy barrier for photogenerated electrons to reach GBs, triggering the decomposition of excess PbI2 there. Therefore, the instability of PbI2-rich PSCs is related with the remote coupling of excess PbI2 with the disorders at GBs. In light of this finding, we introduced a small amount (0.5 mol%) of CdI2 into precursor solution to optimize the crystallization of PbI2-rich perovskite, and finally achieved a trade-off between efficiency and stability in PbI2-rich PSCs. The optimized PSCs with 10 mol% excess PbI2 (PC-Device) achieved a champion efficiency of 24.26% at the bandgap 1.55 eV with robust stability. PC-Devices maintain more than 90% of the initial efficiency after being stressed under UV light in ambient with a relative humidity of 50% for 500 h, and 70% under heat at 85˚C for 400 h, both of which are far superior to the pristine PbI2-rich solar cells.

Dimethylammonium Cation-Induced 1D/3D Heterostructure for Efficient and Stable Perovskite Solar Cells.

Mixed-dimensional perovskite engineering has been demonstrated as a simple and useful approach to achieving highly efficient and more-durable perovskite solar cells (PSCs), which have attracted increasing research interests worldwide. In this work, 1D/3D mixed-dimensional perovskite has been successfully obtained by introducing DMAI via a two-step deposition method. The additive DMA+ can facilitate the crystalline growth and form 1D DMAPbI3 at grain boundaries of 3D perovskite, leading to improved morphology, longer charge carrier lifetime, and remarkably reduced bulk trap density for perovskite films. Meanwhile, the presence of low-dimension perovskite is able to prevent the intrusion of moisture, resulting in enhanced long-term stability. As a result, the PSCs incorporated with 1D DMAPbI3 exhibited a first-class power conversion efficiency (PCE) of 21.43% and maintained 85% of their initial efficiency after storage under ambient conditions with ~45% RH for 1000 h.

Suppression of PbI2 decomposition with Lewis base semicarbazide hydrochloride for photostable and efficient perovskite solar cells

Introducing excessive PbI2 in perovskite is widely used to fabricate high performance perovskite solar cells (PSCs) via two-step spin coating, whereas the photoinduced decomposition of excessive PbI2 may induce perovskite photodegradation for long-term illumination. In this work, a Lewis base semicarbazide hydrochloride (SH) is introduced in the first-step PbI2 film, which can anchor strongly PbI2 at the grain boundaries of perovskite, suppressing its photoinduced decomposition to Pb0 and I2, and consequently enhancing the long-term photostability of PSCs. In addition, it is verified that SH can also coordinate with the uncoordinated Pb2+ in perovskite, and thus stabilize [PbX6]4- octahedral to inhibit photoinduced ion migration. Therefore, the optimized PSC exhibits not only an improved power conversion efficiency (PCE) of 21.88%, but also an enhanced photostability with 82.8% of its initial PCE under 500 h continuous AM 1.5G illumination. This work provides a new strategy to suppress PbI2 photoinduced decomposition and perovskite photodegradation for the realization of photostable and efficient PSCs.

Improved p-i-n MAPbI3 perovskite solar cells via the interface defect density suppression by PEABr passivation.

Organic-inorganic hybrid perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics due to their excellent optoelectronic properties and process compatibility. In this report, numerical simulations show the effect of perovskite surface defect density on the inverted MAPbI3 perovskite device. The Phenethylammonium bromide (PEABr) is introduced to passivate the MAPbI3 layer surface of the perovskite solar cell devices, PEA+ diffuses into the grain boundaries of the 3D perovskite to form 2D/3D hybrid structure during the thermal annealing process, thus improve the surface morphology and decrease the interface defects between MAPbI3 layer and PCBM layer. The power conversion efficiency (PCE) of the PSCs increased from 17.95% to 19.24% after PEABr treatment. In addition, the 2D/3D hybrid structure can also hinder the intrusion of water and oxygen, the stability of perovskite devices has been greatly improved.