Chen Weijie

Publication date: August 2024

Source: Journal of Energy Chemistry, Volume 95

Author(s): Huanhuan Yao, Chang Shi, Tai Wu, Shurong Wang, Mingyu Yin, Liming Ding, Yong Hua, Feng Hao

In this study, the design and synthesis of a novel wide-bandgap small molecule guest, ITOA is presented. Utilizing ITOA as a morphology modulator, ternary organic solar cells achieved a remarkable power conversion efficiency (PCE) exceeding 19.3% along with exceptional stability. Impressively, without any postprocessing, the PCE of ternary devices reached an impressive 18.04%.

Abstract

In this study, a novel wide-bandgap small molecule guest material, ITOA, designed and synthesized for fabricating efficient ternary organic solar cells (OSCs) ITOA complements the absorbance of the PM6:Y6 binary system, exhibiting strong crystallinity and modest miscibility. ITOA optimizes the morphology by promoting intensive molecular packing, reducing domain size, and establishing a preferred vertical phase distribution. These features contribute to improved and well-balanced charge transport, suppressed carrier recombination, and efficient exciton dissociation. Consequently, a significantly enhanced efficiency of 18.62% for the ternary device is achieved, accompanied by increased short-circuit current density (J_SC ), fill factor (FF), and open-circuit voltage (V_OC ). Building on this success, replacing Y6 with BTP-eC9 leads to an outstanding PCE of 19.33% for the ternary OSCs. Notably, the introduction of ITOA expedites the formation of the optimized morphology, resulting in an impressive PCE of 18.04% for the ternary device without any postprocessing. Moreover, the ternary device exhibits enhanced operational stability under maximum power point (MPP) tracking. This comprehensive study demonstrates that a rationally designed guest molecule can optimize morphology, reduce energy loss, and streamline the fabrication process, essential for achieving high efficiency and stability in OSCs, paving the way for practical commercial applications.

Random-terpolymerized donors containing hydrogen-bonding sites are strategically designed and convergently synthesized for highly efficient and stable PSCs processed from halogen-free solvent. By this strategy, the PSC based on PM6-ThEG:PM6-ThOH:Y6-BO ternary system delivers a remarkable PCE of 17.2% with superior photo, thermal, and mechanical stability, which is ascribed to molecular lock via hydrogen-bonding interactions between photoactive materials.

Abstract

Although polymer solar cells (PSCs) have shown considerable power conversion efficiency (PCE) potential, their poor operational stability is a major obstacle for their future commercialization. In this study, the ternary-blend strategy based on D1–A–D1–D2-type conjugated random terpolymers containing hydrogen–bonding sites is employed to simultaneously improve device efficiency and long-term stability. Notably, the PM6-ThEG:PM6-ThOH:Y6-BO ternary-blend system exhibits a remarkable PCE of 17.2% with superior photo, thermal, and mechanical stability, outperforming those of binary devices based on PM6, PM6-ThEG, and PM6-ThOH polymer donors. These outstanding results are likely attributed to the robust molecular lock via hydrogen bonds between PM6-ThEG and PM6-ThOH terpolymers, which can induce strong intermolecular packing, a dense 3D terpolymer network, and optimized morphology. These results also correlate well with the computational study. A comprehensive analysis of optoelectronic and morphological properties as well as the exploration of underlying physical mechanisms collectively verifies the effectiveness of this approach based on mixed random terpolymers with hydrogen-bonding moiety to achieve the non-halogenated solvent-processed PSCs with exceptional efficiency and operational stability.

A key strategy for predicting efficient guest components to minimize voltage losses in ternary organic solar cells (OSCs) is demonstrated. The larger electrostatic potential (ESP) difference between the guest acceptor and the host acceptor, the smaller non-radiative voltage losses.

Abstract

The ternary strategy, in which one guest component is introduced into one host binary system, is considered to be one of the most effective ways to realize high-efficiency organic solar cells (OSCs). To date, there is no efficient method to predict the effectiveness of guest components in ternary OSCs. Herein, three guest compositions (i.e., ANF-1, ANF-2 and ANF-3) with different electrostatic potential (ESP) are designed and synthesized by modulating the electron-withdrawing ability of the terminal groups through density functional theory simulations. The effects of the introduction of guest component into the host system (D18:N3) on the photovoltaic properties are investigated. The theoretical and experimental studies provide a key rule for guest acceptor in ternary OSCs to improve the open-circuit voltage, that is, the larger ESP difference between the guest and host acceptor, the stronger the intermolecular interactions and the higher the miscibility, which improves the luminescent efficiency of the blend film and the electroluminescence quantum yield (EQE_EL) of the device by reducing the aggregation-caused-quenching, thereby effectively decreasing the non-radiative voltage loss of ternary OSCs. This work will greatly contribute to the development of highly efficient guest components, thereby promoting the rapid breakthrough of the 20% efficiency bottleneck for single-junction OSCs.

2-(naphthalen-2-yl)ethylamine hydriodide (NEAI) is employed as the surface passivator for perovskite films without the use of an anti-solvent, and plays a critical role in constructing the ideal interface for PSCs with suppressing interface nonradiative carrier recombination, improving hole-extraction ability and enhancing water-resistance ability. Finally, the devices deliver a PCE of 24.19% and demonstrate increased stability.

Abstract

The anti-solvent-free fabrication of high-efficiency perovskite solar cells (PSCs) holds immense significance for the transition from laboratory-scale to large-scale commercial applications. However, the device performance is severely hindered by the increased occurrence of surface defects resulting from the lack of control over nucleation and crystallization of perovskite using anti-solvent methods. In this study, 2-(naphthalen-2-yl)ethylamine hydriodide (NEAI) is employed as the surface passivator for perovskite films without using any anti-solvent. Naphthalene demonstrates strong π-π conjugation, which aids in the efficient extraction of charge carriers. Additionally, the naphthalene-ring moieties form a tight attachment to the perovskite surface. After NEAI treatment, FA and I vacancies are selectively occupied by NEA⁺ and I⁻ in NEAI respectively, thus effectively passivating the surface defects and isolating the surface from moisture. Ultimately, the optimized NEAI-treated device achieves a promising power conversion efficiency (PCE) of 24.19% (with a certified efficiency of 23.94%), featuring a high fill factor of 83.53%. It stands out as one of the reported high PCEs achieved for PSCs using the spin-coating technique without the need for any anti-solvent so far. Furthermore, the NEAI-treated device can maintain ≈87% of its initial PCE after 2000 h in ambient air with a relative humidity of 30% ± 5%.

Perovskite films based on the concept of ordered structures with functional units (OSFU) are constructed. With this structure, the ion migration within the active layer is successfully suppressed. Solar cells show negligible hysteresis and improved stability, including continuous operational and light/dark cycling tests.

Abstract

Ion migration is one of the most critical challenges that affects the stability of metal-halide perovskite solar cells (PSCs). However, the current arsenal of available strategies for solving this issue is limited. Here, novel perovskite active layers following the concept of ordered structures with functional units (OSFU) to intrinsically suppress ion migration, in which a three-dimensional (3D) perovskite layer is deposited by vapor deposition for light absorption and a 2D layer is deposited by solution process for ion inhibition, are constructed. As a promising result, the activation energy of ion migration increases from 0.36 eV for the conventional perovskite to 0.54 eV for the OSFU perovskite. These devices exhibit substantially enhanced operational stability in comparison with the conventional ones, retaining >85% of their initial efficiencies after 1200 h under ISOS-L-1. Moreover, the OSFU devices show negligible fatigue behavior with a robust performance under light/dark cycling aging test (ISOS-LC-1 protocol), which demonstrates the promising application of functional motif theory in this field.

The pyrene-based HTMs exhibit excellent performance in PSCs with a short-circuit current density (J _SC) of 24.71 mA cm⁻², open-circuit voltage (V _oc) of 1.18 V, and a fill factor (FF) of 80.41%, with the highest PCE of 23.5%, an average PCE of 23.44%, which is the highest PCE reported to-date for PSCs with a pyrene-core based HTM.

Abstract

The high-performance hole transporting material (HTM) is one of the most important components for the perovskite solar cells (PSCs) in promoting power conversion efficiency (PCE). However, the low conductivity of HTMs and their additional requirements for doping and post-oxidation greatly limits the device performance. In this work, three novel pyrene-based derivatives containing methoxy-substituted triphenylamines units (PyTPA, PyTPA-OH and PyTPA-2OH) are designed and synthesized, where different numbers of hydroxyl groups are connected at the 2- or 2,7-positions of the pyrene core. These hydroxyl groups at the 2- or 2,7-positions of pyrene play a significantly role to enhance the intermolecular interactions that are able to generate in situ radicals with the assistance of visible light irradiation, resulting in enhanced hole transferring ability, as well as an enhanced conductivity and suppressed recombination. These pyrene-core based HTMs exhibit excellent performance in PSCs, which possess a higher PCE than those control devices using the traditional spiro-OMeTAD as the HTM. The best performance can be found in the devices with PyTPA-2OH. It has an average PCE of 23.44% (PCE_max = 23.50%), which is the highest PCE among the reported PSCs with the pyrene-core based HTMs up to date. This research offers a novel avenue to design a dopant-free HTM by the combination of the pyrene core, methoxy triphenylamines, and hydroxy groups.

A tailored strategy is proposed to reduce the energy offset at both hetero-interface within perovskite solar cells (PSCs) for decreasing the V _OC losses. The resulting PSCs achieved a PCE of 25.13%, and the V _OC is increased from 1.134 to 1.174 V. In addition, the PSCs with synergistical reduced both hetero-interface energy offset also exhibit excellent stability.

Abstract

The open circuit voltage (V _OC) losses at multiple interfaces within perovskite solar cells (PSCs) limit the improvements in power conversion efficiency (PCE). Herein, a tailored strategy is proposed to reduce the energy offset at both hetero-interfaces within PSCs to decrease the V _OC losses. For the interface of perovskite and electron transport layer where exists a mass of defects, it uses the pyromellitic acid to serve as a molecular bridge, which reduces non-radiative recombination and energy level offset. For the interface of perovskite and hole transport layer, which includes a passivator of PEAI, the detrimental effect (negative shift of work function) of PEAI passivation and optimizing the interface energy level alignment are neutralized by incorporating (2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid. Owing to synergistically reduced hetero-interface energy offset, the PSCs achieve a PCE of 25.13%, and the V _OC is increased from 1.134 to 1.174 V. In addition, the resulting PSCs possess enhanced stability, the unencapsulated PSCs can maintain ≈96% and ≈97% of their initial PCE after 2000 h of aging under ambient conditions and 210 h under operation conditions.

Semitransparent organic solar cells (ST-OSCs) are constructed with spectrally selective transparent electrodes for high transparency in visual regions and strong light harvesting beyond the human eye's response spectrum. The optical admittance matching induces the ST-OSCs to achieve a power conversion efficiency of 12.6%, an average visible transmittance of 44.3%, and a record high light utilization efficiency of 5.6%.

Abstract

Converting non-visual light into photocurrent while maintaining high visual transparency is vital for semitransparent organic solar cells (ST-OSCs) application, yet often challenging over insufficient invisible light-harvesting. Herein, spectrally selective optical manipulation for ST-OSCs with high visual light transparency and full-spectral non-visual light reflection is proposed by matching the optical admittance of ultrathin Ag films using ZnS and MgF₂. The reflection of optically enhanced ST-OSCs at the spectral region beyond the human eye's response spectrum is improved and the transmission in the visual region is simultaneously enhanced. By further integrating an anti-reflective structure, the optimal structure boosts the average visible transmittance and power conversion efficiency of ST-OSCs to 44.3% and 12.6%, respectively, yielding a record light utilization efficiency of 5.6%. Corresponding flexible ST-OSCs with high mechanical stability implies that this work provides a facile and universal strategy for ST-OSCs aiming at building integrated photovoltaics.

This work introduces a method for extracting high-quality lifetime maps from rapidly acquired and noisy time-resolved photoluminescence images through the application of total variation regularization techniques. Furthermore, this approach offers novel perspectives for accelerated experiments critical in characterizing the degradation processes of metal halide perovskites.

Abstract

Halide perovskite materials offer significant promise for solar energy and optoelectronics yet understanding and enhancing their efficiency and stability require addressing lateral inhomogeneity challenges. While photoluminescence imaging techniques are employed for the measurement of their opto-electronic and transport properties, going further in terms of precision requires longer acquisition times. Prolonged exposure of perovskites to light, given their high reactivity, can substantially alter these layers, rendering the acquired data less meaningful for analysis. In this paper, a method to extract high-quality lifetime images from rapidly acquired, noisy time-resolved photoluminescence images is proposed. This method leverages concepts of the field of constrained reconstruction and includes the Huber loss function and a specific form of total variation regularization. Through both simulations and experiments, it is demonstrated that the approach outperforms conventional pointwise methods. Optimal acceleration and optimization parameters tailored for decay time imaging of perovskite materials, offering new perspectives for accelerated experiments crucial in degradation process characterization are identified. Importantly, this methodology holds the potential for broader applications: it can be extended to explore additional beam-sensitive materials, and other imaging characterization techniques and employed with more complex physical models to treat time-resolved decays.

The sequential deposition (SD) doping strategy is adopted to control the dopant's location and achieve effective n-doping of non-fullerene acceptors in the SD films. Combining the advantage of vertical component distribution, the doped SD device exhibits superior charge transport properties with enhanced electron mobility, resulting in increased power conversion efficiency (19.55%).

Abstract

Charge transport in the active layer, which can be effectively modulated by molecular doping of organic semiconductors, significantly affects the photovoltaic performance of organic solar cells (OSCs). However, it is difficult to control the dopant distribution in the bulk heterojunction (BHJ) films, which hinders efficient doping in OSCs. Herein, an effective n-doping strategy is developed via sequential deposition (SD) of D18 donor and doped acceptor. The favorable vertical component distribution in SD films helps to optimize carrier transport pathways. The SD method confines the n-dopant N-DMBI to the acceptor layer, allowing positive effects of molecular doping. Consequently, the doped SD device exhibits superior charge transport with suppressed charge recombination, lower trap density, and enhanced charge extraction compared to the undoped one, resulting in a high power conversion efficiency of 19.55% for D18/L8-BO binary OSCs. In addition, the doping does not affect the thermal stability of the devices, with the doped SD device retaining over 90% of its initial efficiency after 1200 h of heating at 80 °C. The universality of the SD doping method is also verified in other non-fullerene acceptor systems. These results demonstrate the great potential of SD doping strategy for building high-performance OSCs with enhanced charge transport.

Two D–π–A–π–D materials, RC19 and RC20, with benzothiadiazole as the acceptor core and meso- or β-functionalized Ni-porphyrins, are synthesized and studied in organic solar cell devices. RC19:TOCR1 exhibits a remarkable 13.72% power conversion efficiency, the highest reported for porphyrin-based binary organic solar cells. This achievement results from enhanced photon harvesting, efficient exciton dissociation, balanced charge transport, and reduced recombination in RC19-based organic solar cell.

Porphyrin derivatives are widely used as donors in organic solar cells (OSCs) due to their excellent optical and electrochemical properties. Although porphyrins can be functionalized at the meso- and β-positions, only meso-functionalized porphyrins have been reported as OSCs. Consequently, a direct comparison of the properties of porphyrins functionalized at these two positions is needed. The synthesis of two similar D–π–A–π–D materials is described herein and these compounds contain benzothiadiazole as the acceptor core and two Ni-porphyrins as donors functionalized at the meso- and β-positions to give RC19 and RC20, respectively. The optical and electrochemical properties of these compounds are reported. All-small-molecule OSCs based on RC19:TOCR1 and RC20:TOCR1 active layers show power conversion efficiencies (PCEs) of 13.72% and 5.20%, respectively. It should be noted that the PCE of 13.72% obtained for RC19:TOCR1 devices is, to one's knowledge, the highest value reported for porphyrin-based binary OSCs. The higher PCE obtained for RC19 is due to its higher photon harvesting ability, more efficient exciton dissociation and charge transfer, balanced charge transport, and lower bimolecular and trap-assisted recombination.

Acceptor components experience an abnormal decrease in luminescence under photoaging due to a photoinduced conformational change. This phenomenon leads to an increase in nonradiative recombination at the mixed phase, resulting in higher burn-in voltage loss of organic solar cells (OSCs). This work demonstrates that a third component with high luminescence stability can effectively reduce the burn-in voltage loss of ternary OSCs.

Although organic solar cells (OSCs) have received increasing attention because of their high device efficiencies, the fundamental mechanism governing their photostability remains elusive. Herein, the effect of the luminescence stability of the acceptor components on the burn-in voltage loss of binary and ternary OSCs is demonstrated. A systematic characterization reveals that the acceptor component experiences an abnormal decrease in luminescence under photoaging—specifically, photoinduced luminescence quenching—because of a photoinduced conformational change. This phenomenon increases nonradiative recombination in blends and thus causes a substantial nonradiative voltage loss of OSCs. Moreover, an introduction of a third component with high luminescence stability can effectively reduce the burn-in voltage loss of OSCs. A composition-dependent photostability study of the resultant ternary OSCs reveals that the reduction in the burn-in voltage loss of OSCs is mainly driven by the third component distributed in the mixed phase; high luminescence stability of this component effectively prevents the increase in nonradiative voltage loss by photoaging. The results suggest that improving the luminescence stability of the acceptor components can be an effective method for highly stable photovoltaics with greatly reduced burn-in voltage loss.

A scalable buried interface that is formed by ASP-Na-SnO₂/perovskite is realized with continuous interface contact, uniformly passivated buried defects, and matched energy levels. The devices with this new buried interface achieved a PCE of 25.47% for small cell (certified 25.02%, 0.0797 cm²) and 20.11% for mini-module (18.30 cm²), reducing the efficiency loss with increase in the area of perovskite photovoltaic devices.

Abstract

The quality of the buried interface plays a key role in achieving high-performance perovskite solar cells (PSCs). However, it is challenging to guarantee its quality on a larger area, which is pivotal for the commercialization of PSCs. Here, a facile strategy is developed to modify the SnO₂/perovskite buried interface by incorporating L-Aspartic acid monosodium salt (ASP-Na) into SnO₂ colloidal dispersion. ASP-Na with multidentate ligands can coordinate with Sn to form stable dispersion, inhibiting the agglomeration of nanoparticles at the buried interface. In addition, the coordination between ASP-Na and SnO₂ nanoparticles in turn promotes the uniform distribution of ASP-Na, which facilitates the uniform and effective passivation of the buried defects. Consequently, the ASP-Na treatment improves the device efficiency from 23.44% to 25.47% (certified 25.02%) with an aperture area of 0.0797 cm² without hysteresis and enhances the operation stability. The perovskite mini-module achieves an efficiency of 20.11% with an aperture area of 18.30 cm², demonstrating the potential of the strategy for scalability.

A heteronuclear clusters-based donor-acceptor polymer is prepared by bridging two [WS₄Cu₄Br]⁺ and [WS₄Cu₅Br₂]⁺ cluster units with triphenylamine ligands. In such a polymer, Cu-deficient unit can be as donor unit and another is the acceptor, resulting in hole mobility of this polymer film up to 3.81 × 10⁻⁵ cm² V⁻¹ s⁻¹. This polymer with highly exposed S and Cu surface sites from cluster centers can be efficiently bound on the surface of lead halide perovskite film, contributing to the improved efficiency and stability of perovskite solar cells.

Abstract

Designing the donor–acceptor polymers-based modifiers with good charge mobility and abundant surface functional groups to bind on perovskite material is highly demanded to boost interfacial charge extraction and transport while yet realized. Here, two [WS₄Cu₄Br]⁺ and [WS₄Cu₅Br₂]⁺ cluster units are bridged by triphenylamine ligands to yield an unprecedented a heteronuclear W/Cu/S clusters-based donor–acceptor polymer. Due to the effect of Rydberg orbital components in different clusters mainly contributed by Cu ions, Cu-rich units have negative potential, whereas Cu-deficient groups display positive potential. This facilitates the formation of a circuit network with ligands acting as “wires” to realize efficient charge transport. Mobility tests reveal that the hole mobility of polymer film is 3.81 × 10⁻⁵ cm² V⁻¹ s⁻¹. Such a polymer can efficiently extract and transport the holes from perovskite film, and thus improving the cell performance and stability. This work opens the opportunities for designing donor–acceptor polymers based on heteronuclear W/Cu/S clusters.

The additive 5-ammonium acid (5-AVA) has the ability to bond with perovskite, effectively inhibiting the appearance of defects during film production and limiting losses due to non-radiative recombination. The enhancement of crystal quality and reduction in defects has considerably improved the photovoltaic performance and long-term stability of perovskite solar cells.

Abstract

The quality of perovskite films plays a fundamental role in determining the performance of perovskite solar cells (PSCs). It is widely recognized that achieving high crystalline quality and minimizing defect density in perovskite films are essential. In this study, the utilization of the multifunctional additive 5-ammonium acid (5-AVA) to improve the morphology of perovskite films is proposed. The -NH₂ and -COOH groups in 5-AVA enable effective coordination with Pb ions and organic ions in perovskite, resulting in the suppression of defect formation and the reduction of non-radiative recombination losses. Furthermore, the addition of 5-AVA facilitates a more favorable energy level alignment, thereby enhancing the transfer of carriers between the perovskite layer and transport layers. Consequently, this process contributes to the improvement of the open circuit voltage (V _OC) in PSCs. Attributing to the addition of 5-AVA, the optimized PSCs achieve significantly enhanced performance with a maximum photoelectric conversion efficiency (PCE) of 24.74% and excellent long-term stability. This work presents a straightforward and effective approach to enhance the performance and long-term stability of PSCs.

The innovative strategy introduced in this study involves pre-bending the flexible perovskite film before applying the passivation agent, allowing the bidentate-coordinated passivation agent 2-mercaptopyridine to more effectively penetrate into the flexible film. The optimized device achieves a power conversion efficiency of 14.74%. Undergoing 70 000 bending cycles at a curvature radius of 5 mm, f-PSC retains over 93% of its initial efficiency.

Abstract

The mechanical durability and efficiency of all-inorganic flexible perovskite solar cells (f-PSCs) still require enhancement for practical applications. In this study, a creative debridement strategy to improve the mechanical durability and photovoltaic performance of all-inorganic f-PSCs by pre-bending the flexible perovskite film and then depositing the passivation agent 2-mercaptopyridine is proposed. The pre-bending process induced the generation of microcracks in the perovskite film surface, and 2-mercaptopyridine can more effectively penetrate the interior of the film through the microcracks, thereby further passivating deep surface defects. These microcracks and defects can be perfectly repaired by 2-mercaptopyridine. Bidentate coordination sites of S and N in 2-mercaptopyridine show stronger binding energy with surface defects. The debridement strategy effectively enhanced the crystallization of the film surface and markedly inhibited crack propagation during the film's bending process. The optimized device achieves a champion power conversion efficiency (PCE) of 14.74%. The pre-bent and passivated all-inorganic f-PSC shows 104% of its initial PCE after 15 000 bending cycles at a curvature radius of 3 mm. Remarkably, even after undergoing 70 000 bending cycles at a curvature radius of 5 mm, pre-bent, and passivated f-PSC can retain over 93% of its initial PCE, exhibiting excellent mechanical durability.

In this work, four distinct ionic liquids (ILs) with distinct cations, were introduced on the surface of 3D perovskite to induce the formation of 1D perovskite. We focused on ILs induced formation mechanism of 1D perovskite capping layers atop 3D perovskites and analyzed the effects of 1D perovskite capping layers on enhancing the photovoltaic performance of the resulted devices.

Abstract

The synergistic utilization of low-dimensional perovskites with 3D perovskite architectures represents a pervasive approach for fabrication of high-performance and enduring perovskite solar cells (PSCs). In this work, four distinct ionic liquids (ILs) with distinct cations, were introduced on the surface of 3D perovskite to induce the formation of 1D perovskite.Starting with the analysis of 1D/3D heterojunction structures, the assessment foucsed on the binding energies in four ILs-induced 1D/3D heterojunctions, comparing electron cloud density within 1D/3D structures, and calculating the formation energies associated with iodine and lead defects within these four 1D/3D perovskite structures via DFT calculations. Furthermore, the time-resolved grazing-incidence wide-angle X-ray scattering technique, as employed in this study, offers real-time insights into the phase-transition occurring during the process of ILs coating and the formation of 1D/3D heterojunctions. The well-designed and optimized 1D perovskite layer significantly reduces the residual lead iodide (PbI₂), modulates the work function of the perovskite, and passivates defects in the 3D perovskite, thereby reducing non-radiative recombination and enhancing charge transport. With the assistance of 1D/3D hybrid films, we achieved an exceptional power conversion efficiency (PCE) of 24.75% in the generated PSCs with remarkable stability.

The heterogeneous nucleating agent (BTO-BO) is developed to suppress the excessive aggregation of L8-BO in high-boiling-point nonhalogenated solvents processing, achieving the active layer with high crystallinity and nano-scaled phase separation morphology. The resultant OSCs achieve record power conversion efficiencies of 19.42% (0.062-cm²) and 16.35% (15. 03-cm²) with excellent operational stabilities.

Abstract

High-boiling-point nonhalogenated solvents are superior solvents to produce large-area organic solar cells (OSCs) in industry because of their wide processing window and low toxicity; while, these solvents with slow evaporation kinetics will lead excessive aggregation of state-of-the-art small molecule acceptors (e.g. L8-BO), delivering serious efficiency losses. Here, a heterogeneous nucleating agent strategy is developed by grafting oligo (ethylene glycol) side-chains on L8-BO (BTO-BO). The formation energy of the obtained BTO-BO; while, changing from liquid in a solvent to a crystalline phase, is lower than that of L8-BO irrespective of the solvent type. When BTO-BO is added as the third component into the active layer (e.g. PM6:L8-BO), it easily assembles to form numerous seed crystals, which serve as nucleation sites to trigger heterogeneous nucleation and increase nucleation density of L8-BO through strong hydrogen bonding interactions even in high-boiling-point nonhalogenated solvents. Therefore, it can effectively suppress excessive aggregation during growth, achieving ideal phase-separation active layer with small domain sizes and high crystallinity. The resultant toluene-processed OSCs exhibit a record power conversion efficiency (PCE) of 19.42% (certificated 19.12%) with excellent operational stability. The strategy also has superior advantages in large-scale devices, showing a 15.03-cm² module with a record PCE of 16.35% (certificated 15.97%).

Publication date: 17 April 2024

Source: Joule, Volume 8, Issue 4

Author(s): Mingwei Hao, Yuanyuan Zhou

We assess the theoretical limit of local contact structures in perovskite solar cells . We identify that optimal efficiency is achieved with nanometer-scale contacts. Micrometer-scale contacts can enhance efficiency in some cases, and are easier to manufacture, however, performance is lower. These results motivate self-forming techniques which create locally contacted nanoscale structures as the optimum path for locally contacted PSCs.

In perovskite solar cells (PSCs), a common characteristic of highly effective interface passivation materials is low conductivity. Gains in voltage are thus often disproportionately offset by resistive losses. Local contact approaches can minimize this trade-off and have a proven track record in conventional silicon photovoltaics. Indeed, recent record efficiencies for centimeter-scale PSCs exploit architectures where the passivation layer partially covers the perovskite-transport layer interface. Herein, a three-dimensional numerical device model is used to determine practical performance limits to local contact geometries and consider both the optimum contact dimensions and the trade-offs involved in relaxing these dimensions for ease of fabrication. It is observed that the potential for substantial power conversion efficiency (PCE) increases with local contacts. In devices where power loss occurs solely through recombination at the contacted interface, PCE can be enhanced by up to 10% absolute compared to a full-area contact. However, optimum PCEs depend on contacts on the order of nanometers. It is shown that more fabrication-friendly micrometer-scale contacts still boost PCE, but the absolute enhancement falls short due to the relatively low bulk perovskite charge carrier diffusion length. This may ultimately motivate methods of interface engineering that produce “effective” local contact geometries at nanometer scales, such as via self-forming layers.

A series of nonfullerene acceptors, CH─6F─Cn, with gradient substituent lengths of side chains are synthesized. The different chain lengths dramatically modulated intermolecular packing modes, crystallinity, and nanoscale morphology of the active layer. The highest PCE of 18.73% is achieved by OSCs employing D18:PM6:CH-6F-C8 as light-harvesting materials.

Abstract

Balancing the rigid backbones and flexible side chains of light-harvesting materials is crucially important to reach optimized intermolecular packing, micromorphology, and thus photovoltaic performance of organic solar cells (OSCs). Herein, based on a distinctive CH-series acceptor platform with 2D conjugation extended backbones, a series of nonfullerene acceptors (CH-6F-Cn) are synthesized by delicately tuning the lengths of flexible side chains from n-octyl to n-amyl. A systemic investigation has revealed that the variation of the side chain's length can not only modulate intermolecular packing modes and crystallinity but also dramatically improve the micromorphology of the active layer and eventual photovoltaic parameters of OSCs. Consequently, the highest PCE of 18.73% can be achieved by OSCs employing D18:PM6:CH-6F-C8 as light-harvesting materials.

Nature Energy, Published online: 29 March 2024; doi:10.1038/s41560-024-01491-0