Nonradiative Charge Carrier Recombination Research Articles

Interfacial engineering with a ferrocene derivative for air-stable inverted perovskite solar cells with high fill factor of 83.57 %

Perovskite solar cells have demonstrated exceptional photovoltaic performance, but stability remains a challenge due to insufficient interfaces. In this work, a UV-resistance ferrocene derivative, 1,1′-bis (diphenylphosphine) ferrocene (DPPF), is introduced between the electron transport layer and the perovskite, reducing defect density by interaction with undercoordinated Pb2+, suppressing non-radiative recombination of charge carriers, and enhancing electron extraction. Desirable interfacial properties and homogeneous perovskite grains are formed, resulting in a champion power conversion efficiency (PCE) of 20.82 % and a significantly improved fill factor of 83.57 %. Moreover, DPPF can optimize the interfaces, provide a brand-new morphology with exceptional air durability, cover the active layer to restrain water and oxygen intrusion, and suppress metal electrode corrosion. Resultant devices without encapsulation have remained above 80 % of their initial PCE for 1400 h under 25 °C air and 50–60 % relative humidity conditions.

Multifunctional surface modification of carbon-based all-inorganic CsPbI2Br solar cells by 3-(trifluoromethyl) phenyltrimethyl ammonium iodide

The preparation process of carbon-electrode perovskite solar cells (C-PSCs) without hole transport layers (HTLs) is simple, cost-effective, and the as-prepared C-PSCs exhibit excellent thermal stability. However, there are numerous defects at the interfaces of C-PSCs and grain boundaries. Due to the presence of these defect states, non-radiative recombination of charge carriers occurs, ultimately resulting in significant VOC (voltage open circuit) loss and an energy level mismatch between device structures, which seriously affect the photoelectric conversion efficiency (PCE) and stability of C-PSCs. Therefore, this work uses 3-(trifluoromethyl)-phenyltrimethyl-ammonium iodide (TFPTAI) as a surface modifier to modify the surface of CsPbI2Br. Due to the strong electronegativity of fluorine atoms (F), acting as electron-withdrawing groups, they promote the separation of positive and negative charge centers in the molecule. This, in turn, can effectively induce strong binding of ammonium cations (NH+) with negatively charged defects, consequently suppressing the non-radiative recombination of charge carriers. In addition, thanks to the ordered arrangement of F atoms, a hydrophobic protective umbrella is constructed on the perovskite surface layer, effectively improving the stability of the device. The highest PCE of the as-prepared device is 14.02%, surpassing the optimal PCE of the original device at 12.46%. Simultaneously, the surface-modified device exhibits excellent environmental stability.

Efficient Carbon‐Based Perovskite Solar Cells Passivated by Alkylammonium Chloride

Hole‐transport layer (HTL)‐free carbon‐based perovskite solar cells (C‐PSCs) have garnered significant attention in the research community due to their cost‐effectiveness and high stability. However, the absence of a traditional HTL can lead to substantial nonradiative recombination of charge carriers between the perovskite layer and the carbon electrode, significantly impacting efficiency. Herein, the performance effects of different alkylammonium chlorides, including butylammonium chloride, octylammonium chloride, and dodecylammonium chloride (DACl), on perovskite (FA0.1MA0.9PbI3) are investigated to produce high‐performance HTL‐free C‐PSCs. Following passivation, the film exhibits a more uniform grain size distribution and fewer grain boundary defects. Furthermore, with an increase in the alkyl chain length, enhanced interactions with the perovskite film result in reduced trap state density and increased carrier lifetime. After DACl passivation, the C‐PSC achieves a remarkable power conversion efficiency of 16.56% and a high open‐circuit voltage (Voc) of 1.074 V. The Voc enhancement is primarily attributed to the better energy‐level alignment, effectively improving hole transport and electron‐blocking capabilities while suppressing nonradiative recombination. Additionally, they demonstrate exceptional hydrophobic characteristics and robust stability in the presence of humidity, making them particularly promising for future practical applications in the field of photovoltaics.

Preparation of two-dimensional perovskite layer by solution method for improving stability of FAPbI<sub>3</sub> perovskite solar cells

Organic-inorganic metal halide perovskite solar cells (PSCs) have been widely studied due to their excellent photoelectric conversion performance, but the inherent chemical instability of CH(NH<sub>2</sub>)<sub>2</sub>PbI<sub>3</sub> (FAPbI<sub>3</sub>) hinders its sustainable development. In particular, the surface interface of the membrane has prominent humidity sensitivity due to lower activation energy, the defect of the surface interface has a strong correlation with the film stability, and the treatment result of the defect is one of the key factors to improve the long-term stability. The FAPbI<sub>3</sub> suffers phase transition from black perovskite phase to yellow non-perovskite phase at room temperature, and the moisture will accelerate this phase transition. Interface engineering is one of the common methods to improve the stability of perovskite solar cells. In addition to interface engineering, there is a strategy of stacking a two-dimensional (2D) perovskite layer on the surface for interface passivation. However, most of the preparation methods of 2D perovskite layer have limitations. In this work, the full solution method and post-treatment mode of annealing are adopted, the hybrid perovskite solar cells of vitamin perovskite are successfully fabricated. The FAPbI<sub>3</sub> perovskite surface is uniformly spin-coated with butylamine iodide (BAI) solution, and the formation of 2D perovskite is driven on the surface of FAPbI<sub>3</sub> perovskite. Due to the passivation of surface interface defects by the 2D perovskite layer, the non-radiative recombination of charge carriers is reduced, greatly improving the carrier lifetime. Because of the hydrophobicity of long chain molecules in 2D perovskite, the long-term stability of the device is significantly improved. Consequently, the unencapsulated device containing 2D perovskite layer remains above 80% after operating at room temperature in ambient air with a relative humidity (RH) of 60% for nearly 1000 hours. The 2D perovskite layer can significantly improve the long-term stability of the film without affecting the charge carrier transport performance. This method of improving the stability of the device by constructing 2D perovskite layer is in line with the requirements and development trend of high-quality perovskite solar cells, and is a strategy with great development potential.

Role of cation-mediated recombination on the hysteresis behavior of perovskite light-emitting diodes

Despite the significant advancement in the efficiency of perovskite light emitting diodes (PeLEDs), the issue of recombination-induced hysteresis during the ions migration has not been adequately addressed. This hysteresis behavior primarily arises from the slow redistribution of ions and their corresponding vacancies during the voltage sweeps. Here, our focus is on the trapping characteristics of ionic defects, which leads to the non-radiative recombination of charge carriers. We investigate the impact of various parameters, such as trapping cation energy, capturing coefficient, ion mobility, and scan rate on the hysteresis behavior observed in the J-V characteristics of device. Moreover, our results provide a framework for examining the relationship between mobile ions and charge carrier, implying that for better understanding of ions migration-induced field screening and carrier redistribution, it is crucial to consider the interplay of ions with charge carriers as a potential factor contributing to J-V hysteresis in the modeling of PeLEDs.

The Role of Alpha‐Methylbenzyl Ammonium Iodide to Reduce Defect Densities in Perovskite Devices

Hybrid organic–inorganic perovskite photovoltaic has achieved unmatched power conversion efficiency (PCE) improvement in the last decade. Nevertheless, nonradiative recombination of charge carriers due to bulk and interface defects reduces the open‐circuit voltage (V oc) and PCE of perovskite solar cells. Incorporating additives, process optimization, and interface engineering are among the effective approaches employed to reduce such issues. Herein, quasi‐2D p–i–n perovskite solar cells incorporating alpha‐methylbenzyl ammonium iodide (MBAI) cation with outstanding photovoltaic performance and stability are developed. MBAI incorporation results in films with excellent optical and electrical properties, leading to higher V OC of ≈1.15 V, fill factor of above 77%, and stability of the device. A high open‐circuit voltage and fill factor and corrected power conversion efficiencies in the range of 15% are obtained for the prepared devices. The encapsulated solar cells show excellent operational stability under white light illumination in ambient air for >500 h. Due to the simple and robust preparation process, the investigated inverted perovskite solar cell can easily be combined with other solution‐processed thin film solar cells to form multijunction devices and can easily be integrated into different lightweight and flexible products.

Regulating molecular stacking to construct a superior interfacial contact for highly efficient and stable perovskite solar cells

The non-radiative charge carrier recombination loss at the surface and interface of the perovskite layers in perovskite solar cells (PSCs) is the main cause of their limited power conversion efficiency (PCE). Here we propose an efficient strategy to minimize the non-radiative recombination by introducing symmetric molecules with regular arrangement at the interface between the perovskite layer and hole transport layer (HTL). One end of the symmetric molecule gets in a close contact with the terminal groups of the molecules in the HTL, while the other end promotes connection with the perovskite, resulting in an enhanced interfacial electrical contact and a better valence band energy-level alignment for efficient hole extraction. In contrast, introducing similar but asymmetrical molecules forms a dimer structure and mismatched energy levels at the interface region, which is detrimental to the carrier transport. The best-performing planar PSC with the symmetric molecular modification exhibited an increase in the PCE from 21.05% to 24.02% with a substantially enhanced fill factor of 84.0%, which can be ascribed to the synergistic effects of surface defect passivation and reduction in the interfacial energy barrier. Furthermore, the corresponding unencapsulated PSC retained 80% of the original PCE after 4,000 h of aging in dry air.

Mixed Organic Cations Promote Ambient Light-Induced Formation of Metallic Lead in Lead Halide Perovskite Crystals.

One major concern toward the performance and stability of halide perovskite-based optoelectronic devices is the formation of metallic lead that promotes nonradiative recombination of charge carriers. The origin of metallic lead formation is being disputed whether it occurs during the perovskite synthesis or only after light, electron, or X-ray beam irradiation or thermal annealing. Here, we show that the quantity of metallic lead detected in perovskite crystals depends on the concentration and composition of the precursor solution. Through a controlled crystallization process, we grew black-colored mixed dimethylammonium (DMA)/methylammonium (MA) lead tribromide crystals. The black color is suggested to be due to the presence of small lead clusters. Despite the unexpected black coloring, the crystals show higher crystallinity and less defect density with respect to the standard yellow-colored DMA/MAPbBr3 crystals, as indicated by X-ray rocking curve and dark current measurements, respectively. While the formation of metallic lead could still be induced by external factors, the precursor solution composition and concentration can facilitate the formation of metallic lead during the crystallization process. Our results indicate that additional research is required to fully understand the perovskite precursor solution chemistry.

Molecularly Tailored Surface Defect Modifier for Efficient and Stable Perovskite Solar Cells

AbstractSurface defects cause non‐radiative charge recombination and reduce the photovoltaic performance of perovskite solar cells (PSCs), thus effective passivation of defects has become a crucial method for achieving efficient and stable devices. Organic ammonium halides have been widely used for perovskite surface passivation, due to their simple preparation, lattice matching with perovskite, and high defects passivation ability. Herein, a surface passivator 2,4,6‐trimethylbenzenaminium iodide (TMBAI) is employed as the interfacial layer between the spiro‐OMeTAD and perovskite layer to modify the surface defect states. It is found that TMBAI treatment suppresses the nonradiative charge carrier recombination, resulting in a 60 mV increase of the open‐circuit voltage (Voc) (from 1.11 to 1.17 V) and raises the fill factor from 76.3% to 80.3%. As a result, the TMBAI‐based PSCs device demonstrates a power conversion efficiency (PCE) of 23.7%. Remarkably, PSCs with an aperture area of 1 square centimeter produce a PCE of 21.7% under standard AM1.5 G sunlight. The unencapsulated TMBAI‐modified device retains 92.6% and 90.1% of the initial values after 1000 and 550 h under ambient conditions (humidity 55%–65%) and one‐sun continuous illumination, respectively.

Interface modification of perovskite solar cell for synergistic effect of surface defect passivation and excited state property enhancement

The efficiency of perovskite solar cell (PSC) is significantly affected by the charge transport and recombination behaviors at the interfaces between the perovskite photoactive layer and charge transport intermediate layers. Especially, the defects, existing in those interfaces, induce non-radiative charge carrier recombination, reducing the stability of the device along with its performance. In this work, we newly design an imidazole- and methoxy-functionalized triphenylamine based hole-transport material (HTM), which has defect-passivating functionality together with proper carrier-transportation property, for the p-i-n PSC devices. Additionally, it is also designed to preserve its enhanced molecular dipole moment at the excited-state with longer lifetime through excited-state intramolecular proton transfer (ESIPT) process, consequently beneficial to improve the Vbi of the device. The designed HTM molecule, having a synergistic effect of defect-passivation and photo-enhanced dipole characteristic, is confirmed to be advantageous to produce PSCs with higher performance and stability.

Tuning charge carrier dynamics through spacer cation functionalization in layered halide perovskites: an ab initio quantum dynamics study

Fluorination of spacer cations compacts the layered halide perovskites, weakens the electron–phonon interactions, and suppresses the non-radiative charge carrier recombination processes.

Synergistic enhancement of efficiency and stability of perovskite solar cells via dual interface modification

In perovskite solar cells (PSCs), rapid charge transfer at the interfaces between photoactive layer and charge transport layers is very important. Various defects and energy barriers at the interfaces will lead to non-radiative recombination of charge carriers in the charge transfer process, which is not conducive to charge collection and eventually hinder the improvement of power conversion efficiency (PCE) and stability of PSCs. Herein, we report a dual interface modification strategy by using different phosphonic acid molecules to modify the interface between SnO2 layer and perovskite layer and the interface between perovskite layer and hole transport layer, respectively. Acidic diphenylphosphinate chloride (DPC) with UV resistance is employed to modify SnO2 surface before depositing perovskite layer to alleviate the corrosion of OH− in SnO2 to perovskite film and regulate the crystal growth process of perovskite. Methyldiphenylphosphine oxide (MPO) that can coordinate with Pb2+ is used to post-treat the surface of perovskite film to passivate defects at the film surface and grain boundary. The effects of organic interface modifiers on charge transport and charge recombination are comprehensively studied by photoluminescence spectra and electrical measurement. The optimal FA0.85Cs0.15PbI3 device with active area of 0.255 cm2 obtained a PCE of 23.37 %. In addition, the device with working area of ​​1 cm2 achieved the best PCEs of 20.29 %. More interestingly, owing to the strong ultraviolet light absorption ability of DPC, the final device with the dual interface modification can still maintain 91.4 % of the initial efficiency after 1000 h in air and 87.8 % when placed in AM1.5G light for 500 h.

Vacancy-Regulated Charge Carrier Dynamics and Suppressed Nonradiative Recombination in Two-Dimensional ReX2 (X = S, Se).

Point defects in semiconductors usually act as nonradiative charge carrier recombination centers, which severely limit the performance of optoelectronic devices. In this work, by combining time-domain density functional theory with nonadiabatic molecular dynamics simulations, we demonstrate suppressed nonradiative charge carrier recombination and prolonged carrier lifetime in two-dimensional (2D) ReX2 (X = S, Se) with S/Se vacancies. In particular, a S vacancy introduces a shallow hole trap state in ReS2, while a Se vacancy introduces both hole and electron trap states in ReSe2. Photoexcited electrons and holes can be rapidly captured by these defect states, while the release process is slow, which contributes to an elongated photocarrier lifetime. The suppressed charge carrier recombination lies in the vacancy-induced low-frequency phonon modes that weaken electron-phonon coupling, as well as the reduced overlap between electron and hole wave functions that decreases nonadiabatic coupling. This work provides physical insights into the charge carrier dynamics of 2D ReX2, which may stimulate considerable interest in using defect engineering for future optoelectronic nanodevices.

Synergistic modification of benzimidazole and bromohexyl for highly efficient and stable perovskite solar cells

High efficiency and long-term durability are two crucial factors for commercialization application of perovskite solar cells (PSCs). In this paper, we designed and fabricated two organic molecules to modify perovskite films. As nitrogen-rich adducts, both CN and CN groups in 1H-benzimidazole (BIE-H) and 1-(6-bromohexyl)-1H-benzimidazole (BIE-Br) possess the tremendous potential to passivate unsaturated Pb2+ defect of perovskites via Lewis acid-base coordination. The Br atoms of BIE-Br could also passivate uncoordinated Pb2+, improving perovskite film quality and inhibiting non-radiative recombination of charge carriers. Moreover, Br− ions in BIE-Br could participate in the lattice restructuring of perovskite, which enlarges the band gap of perovskite and improves the energy level match of device, contributing to the decrease of energy losses during charge migration. Furthermore, the presence of hydrophobic alkyl chains could protect the active layer from moisture erosion, which enhances the durability of PSCs. As a result, the PCE of BIE-Br modified device reaches to 20.47 %, much higher than those of the control and BIE-H treated PSCs, with efficiencies of 18.98 % and 19.74 %, respectively. The BIE-Br modified device also exhibits the significant long-term stability without any encapsulation, which maintains 78 % of its initial PCE after 600 h storage under ambient conditions.

Strong room temperature exciton photoluminescence in electrochemically deposited Cu2O films

Strong room temperature exciton photoluminescence (PL) has been observed in copper (I) oxide films electrochemically deposited in a tartrate electrolyte. The PL intensity of these films is two orders of magnitude higher than that of films deposited from the classical lactate electrolyte. X-ray diffraction (XRD) and Raman spectroscopy analyses demonstrate that the films prepared using tartrate electrolyte are characterized by higher grain size, which reduces a non-radiative recombination of charge carriers. Better optical quality of the Cu2O films prepared using tartrate electrolyte is explained taking into account stronger tartrate-copper complexes, which results in lower density of grain boundaries in such films. Moreover, higher buffering capacity of the tartrate complex provides stability of pH value in the diffusion layer of the near-electrode space preventing defect formation. Our study demonstrates the promise of using Cu2O films deposited from tartrate solution for solar energy applications like photoelectric energy conversion, hydrogen production, and photocatalysis.

Superhalogen Passivation for Efficient and Stable Perovskite Solar Cells

Metal‐halide perovskites are optoelectronic materials applied to solar cells as a light absorber due to their excellent optoelectronic properties. The power conversion efficiency of perovskite solar cells (PSCs) reaches 25.7% certified, which stands in comparison with Si solar cells. Importantly, compositional engineering of perovskites has been one of the keys to the breakthrough. However, the presence of defects within perovskites is a matter of importance as it can cause nonradiative recombination of charge carriers. In addition, defect migration can degrade the photovoltaic performance and stability of PSCs. Previous studies have commonly addressed that iodide‐related defects such as interstitial iodide and iodide vacancy are problematic due to their low formation energy. Thus, halide engineering is imperative to mitigate the defect‐related dynamics and improve the materials quality of perovskites. In this sense, superhalogen is a promising candidate for defect passivation and stabilization of perovskites based on its higher electronegativity and electron affinity than halides, which are beneficial to the formation of a more robust interaction with adjacent elements in perovskites. This perspective gives an overview of studies regarding the use of superhalogen to develop efficient and stable PSCs and concludes with an outlook of further research directions.

Enhancing performance and stability of perovskite solar cells through defect passivation with a polyamide derivative obtained from benzoxazine-isocyanide chemistry

• A phenolic hydroxyl-substituted polyamide derivative PAB serves as an passivating agent. • Hole transport polymer PM6-T10 is used in perovskite solar cells for the first time. • Three kinds of polar groups linked by flexible backbone provide effective passivation. • Devices PAB-0.15 display better device performances than the reference devices PAB-0. Owing to the presence of amounts of defects in organic/inorganic hybrid perovskite, defect passivation is considered as a significant way to improve the efficiency and stability of perovskite solar cells (PVSCs). Herein, for the first time, a phenolic hydroxyl-substituted polyamide derivative (PAB), which was prepared by benzoxazine-isocyanide chemistry (BIC) and simultaneously contained hydroxyl, secondary amine, and carboxyl functional segments, was selected as the precursor additive to passivate perovskite active layer. Different passivation groups attached in the chain of a polymer are vital for the defect passivation of perovskite containing various defects. Hydroxyl and carboxyl can act as Lewis bases to passivate the undercoordinated lead defects, and secondary amine can coordinate with iodide ion. These passivation groups linked by the flexible polymer chains provide effective interaction with trap states on the surfaces and grain boundaries of perovskite, suppressing the trap-assisted non-radiative charge carrier recombination thus. As a result, these positive effects increased the efficiency of PVSCs from 19.45% to 21.13%, and maintained 95% initial efficiency after 32 days in ambient air (humidity around 50%, room temperature). This work opens a viable avenue to develop efficient passivating polymers via benzoxazine-isocyanide chemistry.

Revealing Donor–Acceptor Interaction on the Printed Active Layer Morphology and the Formation Kinetics for Nonfullerene Organic Solar Cells at Ambient Conditions

AbstractSlot‐die coating is a powerful method for upscaling the production of organic solar cells (OSCs) with low energy consumption print processes at ambient conditions. Herein, chlorobenzene (CB) and chloroform (CF) are compared as host solvents for printing films of the neat novel fused‐ring unit based wide‐bandgap donor polymer (PDTBT2T‐FTBDT), the small molecule nonfullerene acceptor based on a fused ring with a benzothiadiazole core (BTP‐4F) as well as the respective PDTBT2T‐FTBDT:BTP‐4F blend films at room temperature in air. Using CF printing of the PDTBT2T‐FTBDT:BTP‐4F active layer, OSCs with a high power conversion efficiency of up to 13.2% are reached in ambient conditions. In comparison to CB printed blend films, the active layer printed out of CF has a superior morphology, a smoother film surface and a more pronounced face‐on orientation of the crystallites, which altogether result in an enhanced exciton dissociation, a superior charge transport, and suppressed nonradiative charge carrier recombination. Based on in situ studies of the slot‐die coating process of PDTBT2T‐FTBDT, BTP‐4F, and PDTBT2T‐FTBDT:BTP‐4F films, the details of the film formation kinetics are clarified, which cause the superior behavior for CF compared to CB printing due to balancing the aggregation and crystallization of donor and acceptor.

Method for studying the photostability of solar cells based on organic-inorganic perovskites using a confocal spectrometer

A method for studying photosensitive structures using the example of studying the photodegradation and dark recovery of organic-inorganic perovskite solar cells is present. The method is based on the use of a confocal spectrometer to measure Raman spectra, photo- and electroluminescence spectra and kinetics, the kinetics of the short-circuit current and the open-circuit voltage under local exposure to monochromatic radiation. Raman spectra make it possible to establish the presence or absence of secondary phases, the formation of which is possible in photosensitive layers under the influence of light. The photoluminescence spectra enable to reveal the single-phase nature of the object under study. Mapping the intensity and position of the center of mass of the photoluminescence band in the plane of the object makes it possible to judge the spatial arrangement of the centers of non-radiative recombination of charge carriers, the distribution of the regions with the highest and lowest efficiency of the extraction of charge carriers by transport-acceptor layers and the spatial homogeneity of the chemical composition. Comparison of the electroluminescence spectra before and after light exposure allows revealing the formation of non-radiative recombination centers in the photoabsorbing layer and at interfaces with the transport layers. Analysis of the kinetics of changes in the parameters of the photoluminescence band obtained by measuring the short-circuit current and the open-circuit voltage makes it possible to establish the dominant photoinduced processes leading to a change in the intensity of the photoluminescence band.

Excited State Dynamics in Dual-Defects Modified Graphitic Carbon Nitride.

Significant efforts are focused on defect-engineering of metal-free graphitic carbon nitride (g-C3N4) to amplify its efficacy. A conceptually new multidefect-modified g-C3N4 having simultaneously two or more defects has attracted strong attention for its enhanced photocatalytic properties. We model and compare the excited state dynamics in g-C3N4 with (i) nitrogen defects (N vacancy and CN group) and (ii) dual defects (N vacancy, CN group, and O doping) and show that the nonradiative recombination of charge carriers in these systems follows the Shockley-Read-Hall mechanism. The nitrogen defects create three midgap states that trap charges and act as recombination centers. The dual-defect modified systems exhibit superior properties compared with pristine g-C3N4 because the defects facilitate rapid charge separation and extend the spectrum of absorbed light. The system doped with O shows better performance due to enhanced carrier lifetime and higher oxidation potential caused by a downshifted valence band. The study provides guidance for rational design of stable and efficient photocatalytic materials.