What We have Learnt from PM6:Y6

Control of Chemical Doping-Mediated Space Charge for Energetic Band-Switching Modulation in Low-Noise Shortwave-Infrared Organic Photodetector

This study investigates the influence of chemical doping on the spatial-charge distributions and carrier-tunneling mechanisms in single-polymer shortwave-infrared (SWIR) photomultiplication (PM)-organic photodetectors (OPDs). By systematically analyzing the optical and photoelectric properties influenced by chemical doping, it is identified that dopant-induced defects as space charges significantly contribute to Fowler-Nordheim (FN) tunneling, thereby impacting the performance of SWIR OPDs. At a doping concentration of 0.5 mm, the formation of positively charged carriers (polarons and/or bipolarons) within the polymer matrix initiates, thereby facilitating SWIR absorption and contributing to the balance between photocurrent and noise by mitigating FN tunneling through the reduction of defect density (ND). However, as the doping concentration exceeds 5 mm, the increased ND accumulates more space charge, accelerating FN tunneling. This enhances photocurrent generation and amplifies noise disproportionately, ultimately limiting OPD performance. Under ND-minimized optimum doping concentration (at 0.5 mm), the OPD exhibited a noise equivalent power of 9.85 pW (at -8 V, bandwidth = 1 Hz, and wavelength = 1490 nm), and a linear dynamic range of 42 dB. These findings demonstrate the role of chemical doping in enhancing the performance of SWIR PM-OPDs, paving the way for advanced photonic sensors.

Multiple-Asymmetric Molecular Engineering Enables Regioregular Selenium-Substituted Acceptor with High Efficiency and Ultra-low Energy Loss in Binary Organic Solar Cells

Asymmetric molecular engineering is utilized for developing efficient small molecular acceptors (SMAs), whereas adopting multiple asymmetric strategies at the terminals, side chains, and cores of efficient SMAs remains a challenge, and effects on reducing energy loss (Eloss) have been rarely investigation. Herein, four regioregular multiple-asymmetric SMAs (DASe-4F, DASe-4Cl, TASe-2Cl2F, and TASe-2F2Cl) are constructed by delicately manipulating the number and position of F and Cl on end groups. Triple-asymmetric TASe-2F2Cl not only exhibits a unique and most compact 3D network crystal stacking structure but also possesses excellent crystallinity and electron mobility in neat film. Surprisingly, the PM1:TASe-2F2Cl-based binary organic solar cells (OSCs) yield a champion power conversion efficiencies (PCEs) of 19.32%, surpassing the PCE of 18.27%, 17.25%, and 16.30% for DASe-4F, DASe-4Cl, and TASe-2Cl2F-based devices, which attributed to the optimized blend morphology with proper phase separation and more ordered intermolecular stacking and excellent charge transport. Notably, the champion PCE of 19.32% with ultralow nonradiative recombination energy loss (ΔE3) of 0.179 eV marks a record-breaking result for selenium-containing SMAs in binary OSCs. Our innovative multiple-asymmetric molecular engineering of precisely modulating the number and position of fluorinated/chlorinated end groups is an effective strategy for obtaining highly-efficient and minimal ΔE3 of selenium-substituted SMAs-based binary OSCs simultaneously.

Utilizing hybridization effects to tune morphology and electron mobility of Y6 through asymmetric small- and large-scale modifications of terminal groups

While exploring molecular modifications of the high-performance acceptor Y6 with an A-DA'D-A framework, researchers have discovered that asymmetric modification of terminal groups (TGs) appears to be a promising approach as it frequently enhances the photovoltaic performance of organic solar cells (OSCs) effectively. However, the underlying mechanism about how asymmetric TG modifications influence morphology and charge carrier mobility remains unclear. We have conducted a systematic study in this work to investigate the morphology and electron mobility of two asymmetric Y6 derivatives with the A1-DA'D-A2 framework: Y6-asym-IM2O (A1 = IM-2F and A2 = IM2O, representing small-scale TG modification) and Y6-asym-BR (A1 = IM-2F and A2 = BR, representing large-scale TG modification), along with their symmetric counterparts (A1 = A2 = IM-2F/BR/IM2O). The results demonstrate that small-scale asymmetric TG modifications such as Y6-asym-IM2O fine-tune molecular packing, while large-scale modifications such as Y6-asym-BR drastically alter stacking patterns. In addition, hybridization effects are found in the frontier molecular orbital energy, electrostatic potential, and electron mobility of the asymmetric molecules, which fall between the values of their symmetric counterparts. In particular, the results of small-scale asymmetric modification of Y6 reveal that the introduction of promising TGs in an asymmetric manner can further improve electron mobility by tuning reorganization energy and morphology, and vice versa. While previous studies focused on symmetric modifications, this work systematically investigates asymmetric substitution patterns and further elucidates the impact of these methods on charge transfer for the first time. These discoveries underscore the potential of utilizing asymmetric modification of TGs as a quantitative means to regulate electron mobility in Y6-based OSCs.

Resonant Soft X-ray Scattering for Organic Photovoltaics

Resonant Soft X-ray Scattering (RSoXS) has emerged as a powerful technique for probing the morphology in organic bulk heterojunction (BHJ) solar cells, frequently employed as a measurement of phase purity and compositional length scales. Here we use the National Institute of Standards and Technology RSoXS Simulation Suite to systematically examine how structural features common to BHJs would contribute to RSoXS patterns in the PM6:Y6 BHJ system. Starting from experimentally determined anisotropic optical constants, we simulate scattering from controlled morphological variations including compositional heterogeneity, interfacial sharpness, surface roughness, and molecular orientation. Our results demonstrate that noncompositional features can cause increases in scattering intensity exceeding those from compositional phase separation. Surface roughness of just a few nanometers produces substantial scattering due to the high contrast between organic materials and vacuum, and molecular orientation effects─whether random, interface-aligned, or independently correlated─can dramatically influence pattern intensity and shape. However, each structural feature exhibits a distinct energy-dependent scattering signature across the carbon K-edge, suggesting that careful analysis of the complete spectral response could enable deconvolution of multiple contributions. These findings provide a broader interpretation of the excellent correlations between RSoXS measurements and BHJ solar cell device performance, while highlighting the potential of forward simulation approaches to leverage the full information content of energy-dependent RSoXS measurements.

A 2D hybrid perovskite ferroelectric with switchable polarization and photoelectric robustness down to monolayer

The continuous dimensional scaling of semiconductor and logic photoelectric device requires ferroelectrics to possess robust photoelectric activity and switchable polarization at the nanoscale. However, traditional ferroelectrics such as oxide perovskites generally suffer from relatively large bandgap and deteriorated ferroelectricity in ultrathin forms, while the polarization in many transition metal dichalcogenides is related to inter-layer effects, leading to ferroelectricity that only exists in flakes with a certain layer number and particular stacking forms. The associated challenging fabrication and high-cost synthesis of inorganic ferroelectrics currently render mass industrial production of ultrathin ferroelectric semiconductors impossible. Here with (isopentylammonium)2(ethylammonium)2Pb3I10, we report an organic-inorganic hybrid perovskite nanoflake with cheap solution synthesis, switchable polarization, a narrow bandgap (1.86 eV to 2.21 eV form bulk to monolayer), and robust photoelectric properties down to the monolayer. The present work reveals the great potential of 2D hybrid perovskite ferroelectrics as low-cost ferroelectric semiconductors at the nanoscale.

Spatiotemporal Exciton Tracking with a SPAD Camera

Spatiotemporal microscopy plays an important role in the quest for highly efficient light harvesting materials as it allows direct tracking of the nanoscale transport of excitons, the carriers of the photon energy. Unfortunately, achieving high resolution in both space and time often requires scanning beam spots or delay lines, limiting these techniques to specialized research groups. To overcome this problem, we introduce a novel implementation of photoluminescence-detected exciton tracking using a camera composed of an array of single-photon avalanche diodes (SPADs), gated with ∼150 ps temporal accuracy. The use of such a SPAD camera drastically simplifies the experiment, is free of moving parts, and provides at least 1 order of magnitude increase in photon collection efficiency due to the parallel multipixel acquisition. Moreover, the camera allows one to implement different super-resolution excitation strategies. Here we show both point and structured excitation in the same device by simply changing the optical element. The structured illumination allows direct retrieval of the diffusion from a single time-resolved imaging without fitting, even at fluences far below exciton-exciton annihilation conditions. We tested the SPAD camera effectiveness by studying the exciton diffusion properties of the organic photovoltaic material PM6, where we measured an exciton diffusion length of 45 nm. Certainly our new implementation, boosted by rapid advances in SPAD technology, will extend the range of both users and applications of spatiotemporal microscopy.

Spontaneous formation of potential cascade enhances charge separation in PM6-Y6 organic photovoltaics

Mechanisms that enhance charge separation at donor-acceptor interfaces are the key to material design of non-fullerene electron acceptors for high-efficiency organic photovoltaics (OPV). Here, the energetics of charge separation at the PM6-Y6 donor-acceptor interface in the state-of-the-art OPV is analyzed on the basis of quantum mechanics/molecular mechanics calculations. The electron energy level in Y6 becomes lower with increasing distance from the interface with PM6 at which the crystallinity is lower than in the bulk region. Electrostatic interactions from the multipoles of Y6 stabilize the electron in the crystalline region. The PM6-ITIC donor-acceptor interface also exhibits a similar potential cascade owing to the quadruple of ITIC. The potential cascade destabilizes charge transfer states at the PM6-Y6 interface, thereby decreasing the potential barrier for charge separation. Charge delocalization on several molecules via transfer integral further decreases the barrier for charge separation.

Fluoroalkylated Non-fullerene Acceptor as Surface Segregated Monolayer for Controlling Molecular Orientation of Acceptor Layer in Organic Photovoltaics

A fluoroalkyl-containing electron acceptor (Y-SSM) is designed and synthesized to control the orientation of the benchmark non-fullerene acceptor Y6 in thin films. Due to the low surface energy of the two fluoroalkyl chains at the terminal part of Y-SSM, it spontaneously segregates to the film surface during spin coating, forming a monolayer of edge-on oriented Y-SSM. The Y-SSM monolayer leads to crystallization of the underlying Y6 to induce a standing-up orientation in the bulk of the films, which is strikingly different from pure Y6 films that tend to be a face-on orientation. Solid evidence for standing-up Y6 in the film is provided by two-dimensional grazing incidence wide-angle X-ray scattering and optical anisotropy measurements based on variable angle spectroscopic ellipsometry. The surface Y-SSM can be partially removed by washing with hexane without disrupting the orientation of Y6, resulting in exposure of the standing-up Y6 on the surface. Organic photovoltaics based on a planar heterojunction structure with standing-up Y6 show a significant increase in short-circuit current density, reaching 2.5 times the value compared to face-on oriented Y6, due to the improved charge generation efficiency resulting from the different relative orientation with respect to PM6.

Fullerene-Hybridized Fused-Ring Electron Acceptor with High Dielectric Constant and Isotropic Charge Transport for Organic Solar Cells

A novel isotropic fullerene-hybridized fused-ring electron acceptor, designated C60-Y, has been synthesized via a mild [4+2] Diels-Alder cycloaddition reaction with fullerene C60 to enhance the performance of organic solar cells (OSCs). Comparative analysis shows that C60-Y significantly outperforms the control acceptor Me-Y, with a notable increase in the relative dielectric constant from 2.79 to 3.95. This improvement enhances exciton dissociation and reduces non-radiative energy losses. Additionally, the isotropic molecular packing of C60-Y, similar to fullerene, facilitates efficient interface formation with donor polymers and improves charge mobility. As a result, incorporating C60-Y as an electron acceptor increases the power conversion efficiency (PCE) of binary OSCs to 15.02 %, surpassing the 13.31 % achieved with Me-Y. Moreover, when integrated into a ternary blend system, an impressive PCE of 19.22 % is achieved, top-performing among reported ternary OSCs utilizing fullerene derivatives as the third component. These results suggest that fullerene-hybridized acceptors like C60-Y hold great potential for advancing high-efficiency OSCs by enhancing exciton dissociation, reducing energy losses, and improving charge mobility.

Optimally Miscible Polymer Bulk-Heterojunction-Particles for Nonsurfactant Photocatalytic Hydrogen Evolution

Mini-emulsion and nanoprecipitation techniques relied on large amounts of surfactants, and unresolved miscibility issues of heterojunction materials limited their efficiency and applicability in the past. Through our molecular design and developed surfactant-free precipitation method, we successfully fabricated the best miscible bulk-heterojunction-particles (BHJP) ever achieved, using donor (PS) and acceptor (PSOS) polymers. The structural similarity ensures optimal miscibility, as supported by the interaction parameter of the PS/PSOS blend is positioned very close to the binodal curve. Experimental studies and molecular dynamics simulations further revealed that surfactants hinder electron output sites and reduce the concentration of sacrificial agents at the interface, slowing polaron formation. Multiscale experiments verified that these BHJP, approximately 12 nm in diameter, further form cross-linked fractal networks of several hundred nanometers. Transient absorption spectroscopy showed that BHJP facilitates polaron formation and electron transfer. Our BHJP demonstrated a superior hydrogen evolution rate (HER) compared to traditional methods. The most active BHJP achieved an HER of 251.2 mmol h-1 g-1 and an apparent quantum yield of 26.2% at 500 nm. This work not only introduces a practical method for preparing BHJP but also offers a new direction for the development of heterojunction materials.

Dipole Moment Modulation of Terminal Groups Enables Asymmetric Acceptors Featuring Medium Bandgap for Efficient and Stable Ternary Organic Solar Cells

This study puts forth a novel terminal group design to develop medium-band gap Y-series acceptors beyond conventional side-chain engineering. We focused on the strategical integration of an electron-donating methoxy group and an electron-withdrawing halogen atom at benzene-fused terminal groups. This combination precisely modulated the dipole moment and electron density of terminal groups, effectively attenuating intramolecular charge transfer effect, and widening the band gap of acceptors. The incorporation of these terminal groups yielded two asymmetric acceptors, named BTP-2FClO and BTP-2FBrO, both of which exhibited open-circuit voltage (Voc) as high as 0.96 V in binary devices, representing the highest VOCs among the asymmetric Y-series small molecule acceptors. More importantly, both BTP-2FClO and BTP-2FBrO exhibit modest aggregation behaviors and molecular crystallinity, making them suitable as a third component to mitigate excess aggregation of the PM6 : BTP-eC9 blend and optimize the devices' morphology. As a result, the optimized BTP-2FClO-based ternary organic solar cells (OSCs) achieved a remarkable power conversion efficiency (PCE) of 19.34 %, positioning it among the highest-performing OSCs. Our study highlights the molecular design importance on manipulating dipole moments and electron density in developing medium-band gap acceptors, and offers a highly efficient third component for high-performance ternary OSCs.

Transient charge carrier dynamics in organic solar cell devices studied by simultaneous optical and electric detection

For the clarification of dynamics of photogenerated carriers in practical organic solar cell devices, we have developed a methodology to simultaneously acquire reflection-mode transient optical absorption (ΔA) and transient electric current (Δi) signals. For a typical polythiophene:fullerene bulk heterojunction solar cell device, both the ΔA and Δi signals due to the photogenerated carriers are characterized by the power-law decays of ∝t-α, which are interpreted by detrapping-limited recombination at earlier times than ∼1 μs and trap-free diffusion/drift at later times. Furthermore, we have succeeded in observing switching of the power index α for ΔA signals as well as for Δi signals; the time at which switching occurs indicates the extraction of carriers by electrodes (transit times). From the transit times for ΔA and Δi, transit mobilities μtrΔA and μtrΔi are obtained, which are on the same order. It has been found from the comparison of the cell parameters among several devices fabricated under similar conditions that the device-to-device variation of photon energy conversion efficiency (0.5%-2%) is strongly correlated with the ratio μtrΔA/μtrΔi. It is considered that the charge accumulation at the active layer/electrode interfaces induces a delay between the carrier transport and electrode collection, which significantly lowers the power conversion efficiency. Our simultaneous optical and electrical detection thus allows us to diagnose carrier dynamics in individual devices that affect the solar cell performance.

Giant molecule acceptors prepared by metal-free catalyzed reactions towards efficient organic solar cells

Due to their defined structure and large molecular size, giant molecular acceptors (GMAs) have achieved significant progress in device efficiency and stability for organic solar cells. Unlike the classical synthesis of GMAs by Stille coupling, which still suffers from high cost and lack of environmental sustainability, this work develops three GMAs via metal-free catalyzed reactions. By introducing varying quantities of malononitrile groups into the linker, the optoelectric properties of GMAs can be significantly regulated. A GMA containing only one malononitrile in the central linker unit exhibits a favorable absorption range, energy levels, and molecular packing, consequently achieving high efficiency and stability. Therefore this work provides a feasible approach to developing low-cost and scalable GMAs.

Accurate Determination of the Uniaxial Complex Refractive Index and the Optical Band Gap of Polymer Thin Films to Correlate Their Absorption Strength and Onset of Absorption

The advanced development of optoelectronic devices requires a methodical knowledge of the fundamental material properties of the key active components. Systematic investigations and correlations of such basic optical properties can lead to new insights for the design of more potent materials. In this perspective, we provide a systematic overview of the uniaxial anisotropic complex refractive indices and the absorption coefficients obtained by ellipsometry as well as the optical band gap energies derived from Tauc plots of six selected solution-processed polymer thin films. While the optical band gap energies are intentionally distributed over the visible spectral range, we found that the absorption strength of all polymer samples are grouped in a random distribution within a rather uniform range of values.

Constraining the Excessive Aggregation of Non-Fullerene Acceptor Molecules Enables Organic Solar Modules with the Efficiency >16

Translating high-performance organic solar cell (OSC) materials from spin-coating to scalable processing is imperative for advancing organic photovoltaics. For bridging the gap between laboratory research and industrialization, it is essential to understand the structural formation dynamics within the photoactive layer during printing processes. In this study, two typical printing-compatible solvents in the doctor-blading process are employed to explore the intricate mechanisms governing the thin-film formation in the state-of-the-art photovoltaic system PM6:L8-BO. Our findings highlight the synergistic influence of both the donor polymer PM6 and the solvent with a high boiling point on the structural dynamics of L8-BO within the photoactive layer, significantly influencing its morphological properties. The optimized processing strategy effectively suppresses the excessive aggregation of L8-BO during the slow drying process in doctor-blading, enhancing thin-film crystallization with preferential molecular orientation. These improvements facilitate more efficient charge transport, suppress thin-film defects and charge recombination, and finally enhance the upscaling potential. Consequently, the optimized PM6:L8-BO OSCs demonstrate power conversion efficiencies of 18.42% in small-area devices (0.064 cm2) and 16.02% in modules (11.70 cm2), respectively. Overall, this research provides valuable insights into the interplay among thin-film formation kinetics, structure dynamics, and device performance in scalable processing.

Cross-Linking-Integrated Sequential Deposition: A Method for Efficient and Reproducible Bulk Heterojunctions in Organic Solar Cells

The formation of bulk heterojunctions (BHJs) through sequential deposition (SqD) of polymer donor and nonfullerene acceptor (NFA) solutions offers advantages over the widely used single-step deposition of polymer:NFA blend solutions (BSD). To enhance the application of SqD in organic solar cell production, it is crucial to improve reproducibility and stability while maintaining a high efficiency. This study introduces a novel method termed cross-linking-integrated sequential deposition (XSqD) for fabricating efficient and reproducible BHJs. In this method, polymers are cross-linked using efficient 2Bx-4EO or 2Bx-8EO cross-linkers, which enhance the solvent resistance of the polymer donor layer against the solvents used for NFAs. This approach addresses the challenge of selecting a suitable solvent for NFAs, a major obstacle in SqD-processed OSCs. The utilization of 2Bx-4EO in XSqD leads to a significant increase in reproducibility compared to that of conventional SqD, coupled with a high-power conversion efficiency (PCE) of 14.1%. Furthermore, XSqD devices exhibit superior stability, showing only 1% and 6% reductions in their initial PCE after thermal stress at 80 and 120 °C for 50 h, respectively.

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.

Polyfluoride Acceptor with Limited Molecular Diffusion Enables Efficient and Stable Ternary Organic Solar Cells

Due to the slow diffusion of photovoltaic molecules, in particular, small-molecule acceptors (SMAs), under light and heating, the morphology of the active layer in organic solar cells (OSCs) prefers to deviate from the favorably metastable status, leading to the challenge of stability during long-term operation. Employing materials with a high glass transition temperature (Tg) as the third component to suppress molecular diffusion is an efficient method to achieve the balance of efficiency and stability of OSCs. Herein, a dimerized small-molecule acceptor denoted as F6D is synthesized by introducing a polyfluoride moiety as the linker to enhance the Tg. Benefitting from a rational molecular design, F6D not only exhibits a higher Tg, complementary absorption, and cascade energy levels with the host materials of the polymer donor PM6 and the SMA Y6 but also has excellent miscibility and multiple intermolecular interactions with Y6. As a result, a champion power conversion efficiency of 17.52% is achieved in the optimal PM6:Y6:F6D-based device. More importantly, the ternary device exhibits superior stability under continuous heating and lighting compared with the binary device.

Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force.

The Development of Quinoxaline-Based Electron Acceptors for High Performance Organic Solar Cells

In the recent advances of organic solar cells (OSCs), quinoxaline (Qx)-based nonfullerene acceptors (QxNFAs) have attracted lots of attention and enabled the recorded power conversion efficiency approaching 20%. As an excellent electron-withdrawing unit, Qx possesses advantages of many modifiable sites, wide absorption range, low reorganization energy, and so on. To develop promising QxNFAs to further enhance the photovoltaic performance of OSCs, it is necessary to systematically summarize the QxNFAs reported so far. In this review, all the focused QxNFAs are classified into five categories as following: SM-Qx, YQx, fused-YQx, giant-YQx, and polymer-Qx according to the molecular skeletons. The molecular design concepts, relationships between the molecular structure and optoelectronic properties, intrinsic mechanisms of device performance are discussed in detail. At the end, the advantages of this kind of materials are summed up, the molecular develop direction is prospected, the challenges faced by QxNFAs are given, and constructive solutions to the existing problems are advised. Overall, this review presents unique viewpoints to conquer the challenge of QxNFAs and thus boost OSCs development further toward commercial applications.

Advancing Integration of Direct C-H Arylation-Derived Star-Shaped Oligomers as Second Acceptors for Ternary Organic Solar Cells

Organic solar cells (OSCs) could benefit from the ternary bulk heterojunction (BHJ), a method that allows for fine-tuning of light capture, cascade energy levels, and film shape, in order to increase their power conversion efficiency (PCE). In this work, the third components of PM6:Y6 and PM6:BTP-eC9 BHJs are a set of four star-shaped unfused ring electron acceptors (SSUFREAs), i.e., BD-IC, BFD-IC, BD-2FIC, and BFD-2FIC, that are facilely synthesized by direct C-H arylation. The four SSUFREAs all show complete complementary absorption with PM6, Y6, and BTP-eC9, which facilitates light harvesting and exciton collection. When BFD-2FIC is added as a third component, the PCEs of PM6:Y6 and PM6:BTP-eC9 binary BHJs are able to be improved from 15.31% to 16.85%, and from 16.23% to 17.23%, respectively, showing that BFD-2FIC is useful for most effective ternary OSCs in general, and increasing short circuit current (JSC) and better film morphology are two additional benefits. The ternary PM6:Y6:BFD-2FIC exhibits a 9.7% percentage of increase in PCE compared to the PM6:Y6 binary BHJ, which is one of the highest percentage increases among the reported ternary BHJs, showing the huge potential of BFD-2FIC for ternary BHJ OSCs.

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87 V, higher JSC of 28.2 mA cm-2, greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Low-Cost Nonfused-Ring Electron Acceptors Enabled by Noncovalent Conformational Locks

ConspectusSince the first bilayer-structured organic solar cells (OSCs) in 1986, fullerenes and their derivatives have dominated the landscape for two decades due to their unique properties. In recent years, the breakthrough in nonfullerene acceptors (NFAs) was mainly attributed to the development of fused-ring electron acceptors (FREAs), whose photovoltaic performance surpassed that of fullerene derivatives. Through the unremitting efforts of the whole community, the power conversion efficiencies (PCEs) have surpassed 19% in FREA-based OSCs. However, FREAs generally suffered from complex synthetic approaches and high product costs, which hindered large-scale production. Therefore, many researchers are seeking a new type of NFA to achieve cost-effective, highly efficient OSCs.In collaboration with Marks and Facchetti in 2012, Huang et al. (Huang, H. J. Am. Chem. Soc. 2012, 134, 10966-10973, 10.1021/ja303401s) proposed the concept of "noncovalent conformational locks" (NoCLs). In the following years, our group has been focusing on the theoretical and experimental exploration of NoCLs, revealing their fundamental nature, formulating a simple descriptor for quantifying their strength, and employing this approach to achieve high-performance organic/polymeric semiconductors for optoelectronics, such as OSCs, thin-film transistors, room-temperature phosphorescence, and photodetectors. The NoCLs strategy has been proven to be a simple and effective approach for enhancing molecular rigidity and planarity, thus improving the charge transport mobilities of organic/polymeric semiconductors, attributed to reduced reorganization energy and suppressed nonradiative decay.In 2018, Chen et al. (Li, S. Adv. Mater. 2018, 30, 1705208, 10.1002/adma.201705208) reported the first example of nonfused-ring electron acceptors (NFREAs) with intramolecular noncovalent F···H interactions. The NoCLs strategy is essential in NFREAs, as it simplifies the conjugated structures while maintaining high coplanarity comparable to that of FREAs. Due to their simple structures and concise synthesis routes, NFREAs show great potential for achieving cost-effective and highly efficient OSCs. In this Account, we provide an overview of our efforts in developing NFREAs with the NoCLs strategy. We begin with a discussion on the distinct features of NFREAs compared with FREAs, and the structural simplification from FREAs to NFREAs to completely NFREAs. Next, we examine several selected typical examples of NFREAs with remarkable photovoltaic performance, aiming to provide an in-depth exploration of the molecular design principle and structure-property-performance relationships. Then, we discuss how to achieve a balance among efficiency, stability, and cost through a two-in-one strategy of polymerized NFREAs (PNFREAs). Finally, we offer our views on the current challenges and future prospects of NFREAs. We hope this Account will trigger intensive research interest in this field, thus propelling OSCs into a new stage.