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Asymmetrical side-chain engineering of small-molecule acceptors enable high-performance nonfullerene organic solar cells
摘要： Three new small molecules based on the benzo[1,2-b:4,5-b’]dithiophene (BDT) fused central core with different side-chains, namely DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl, are designed and synthesized to investigate the side-chain effect on the properties of nonfullerene acceptors. DPBDT-4Cl has symmetrical phenylalkyl side-chains on the central BDT unit. In order to narrow the bandgap and reduce the steric hindrance, the phenylalkyl chains are systematically replaced with the flexible electron-donating alkoxy side-chain (POBDT-4Cl) and alkyl side-chain (COBDT-4Cl). As a result, POBDT-4Cl and COBDT-4Cl are characterized with asymmetry-featured side-chains. From DPBDT-4Cl to POBDT-4Cl to COBDT-4Cl, their light absorption abilities, molecular packing behaviors and crystallinity are gradually enhanced. The devices based on these three acceptors all show power conversion efficiencies (PCEs) over 11% with energy loss below 0.55 eV. Compared to DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl obviously exhibit enhanced device performance with improved short-circuit current densities (Jsc) and fill factors (FFs), which mainly ascribe to their reduced charge recombination and enhanced charge transport. In addition, the COBDT-4Cl achieved a high efficiency of 13.5% with a Jsc of 21.8 mA cm-2 and an FF of 0.71. This result is among the best performance obtained from asymmetry-featured small molecules.
关键词： side-chains,benzo[1,2-b:4,5-b’]dithiophene,small-molecule acceptors,asymmetrical
更新于2025-09-19 17:13:59
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Molecular designing of naphthalene diimide based fullerene-free small organic solar cell - Acceptors with high photovoltaic performance by density functional theory
摘要： With the help of computational chemistry tools, three non-fullerene acceptors, which are 2-methylene-malononitrile (M-1), 2-(3-methyl-5-methylene-2-thioxothiazolidin-4-ylidene) malononitrile (M-2) and 1-methyl-5-methylene-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (M-3), are designed with naphthalene diimide (NDI) central unit. Their different photovoltaic and optoelectronic properties like absorption spectrum, electrons density, solubility strength, reorganization energies, % ETC from donor to acceptor part, excitation energies, oscillating strength, morphology and crystallinity of device for constructing the thin film bulk hetro junction devices were computed at the WB97XD/6-31 G (d, p) level of density functional theory (DFT). Expected open circuit voltages of designed molecules are high as 4.05 eV to 4.49 eV, which are significantly larger than that of the previously reported 3-methyl-5-methylene-2-thioxothiazolidin-4-one (R) with the value of 3.60 eV at the zero current level. Charge carrier mobilities of designed molecules are high due to having low re-organization energies varying from 0.0163 eV to 0.0280 eV for electron and 0.0160 eV to 0.0190 eV for hole, strong absorption properties between the 420 nm to 550 nm in chloroform and 400 nm to 540 nm in gas phase conditions, respectively.
关键词： opto-electronic properties,non-fullerene acceptors,theoretical calculations,dipole moment,organic solar cells,naphthalene di-imide
更新于2025-09-19 17:13:59
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Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells
摘要： Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels–Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor, or can even work as a new class of non-fullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.
关键词： Tetrabenzoporphyrin,organic solar cells,thermal precursor approach,frontier-orbital energies,non-fullerene acceptors
更新于2025-09-16 10:30:52
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Assessing the energy offset at the electron donor/acceptor interface in organic solar cells through radiative efficiency measurements
摘要： Energy offsets at the electron donor/acceptor interface play an important role in the operation of organic solar cells (OSCs), because their magnitude strongly affects the efficiency of photo-induced charge separation and hence the short-circuit current of a device under illumination. However, quantitative assessment of energy offsets in operating devices remains an open challenge that is still out of the reach of present techniques. Here we show that the ratio of the radiative efficiency (EQEEL) for a blend device to that for the lower bandgap component device (usually a non-fullerene acceptor), namely the EQEEL ratio, can be a strong indicator of the energy offsets in the blend device. In photovoltaic devices based on donors and acceptors with similar backbone structure but varied energy levels, lowering the highest occupied molecular orbital (HOMO) offset increases the open-circuit voltage (VOC) from 0.95 V to 1.05 V, which is consistent with the EQEEL variation trends in the devices. The blend EQEEL approaches that of the emissive low bandgap acceptor when the HOMO of the donor is sufficiently deepened, which at the same time corresponds to a reduction in VOC loss and inefficient photoinduced charge separation. The results suggest that the intrinsic energy loss associated with charge separation can be minimized in practice by minimizing the energy offsets but at the expense of lowering the charge separation efficiency. Statistics from several state-of-the-art material systems reveal that efficient charge generation occurs when the EQEEL ratio is less than 0.1, corresponding to an additional non-radiative voltage loss due to the energy offset of 60 mV. Based on this finding and a modified Shockley–Queisser theory, we estimate an upper thermodynamic efficiency limit for single-junction organic solar cells of about 31%, which is slightly below the Shockley–Queisser limit.
关键词： energy offsets,charge separation,organic solar cells,non-fullerene acceptors,radiative efficiency
更新于2025-09-16 10:30:52
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Introducing an identical benzodithiophene donor unit for polymer donors and small-molecule acceptors to unveil the relationship between the molecular structure and photovoltaic performance of non-fullerene organic solar cells
摘要： The side-chain conjugation strategy is adopted in the synthesis of heptacyclic non-fullerene electron acceptors, ITIC2, ITIC-S, and ITIC-SF, for application in organic solar cells (OSCs). The new ITIC-SF molecule features a BDT-SF building block which is the electron-donating component in the widely used donor PBDB-T-SF. Compared with ITIC-S without fluorine substituents, fluorination weakens the crystallinity of ITIC-SF, while the intermolecular interaction between the PBDB-T-SF donor and ITIC-SF acceptor, containing the same BDT-SF building block, results in the increased crystallinity of the corresponding blend films. The OSC based on PBDB-T-SF:ITIC-SF exhibits a champion power conversion efficiency (PCE) of 12.1%, higher than the PCE of 10.1% for the device based on PBDB-T-SF:ITIC2 and the PCE of 11.6% for the device based on PBDB-T-SF:ITIC-S. The better photovoltaic performance of the OSC based on PBDB-T-SF:ITIC-SF is benefitted from the weaker bimolecular recombination and more efficient charge transfer and extraction of the device. The structure–property relationship of the non-fullerene acceptors revealed in this work will play an important role in instructing the molecular structure design of high performance photovoltaic materials for the development of OSCs.
关键词： organic solar cells,non-fullerene acceptors,side-chain conjugation,photovoltaic performance,fluorination
更新于2025-09-16 10:30:52
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Suppressing photo-oxidation of non-fullerene acceptors and their blends in organic solar cells by exploring material design and employing friendly stabilizers
摘要： In addition to a high power conversion efficiency, ambient stability is another impact factor for the successful commercialization of organic solar cells (OSCs). Understanding the role of photovoltaic materials is the key to address this challenge, but no such studies have been systematically performed on non-fullerene acceptors (NFAs). In this work, we firstly investigate the role of NFA photo-oxidation in device degradation. Relevant investigation of physical dynamics underlines the effects on the device performance for NFA photo-oxidation acting as trap states in exposed blends. In addition, taking ITIC as an example, we shed some light on the possible mechanisms of NFA photo-oxidation, which cannot be eliminated by relevant strategies and principles of material design. These results drive us to further investigate the photobleaching rates of thirty-three NFAs, including fused-ring electron acceptors and perylene diimide acceptor derivatives. Surprisingly, most of them show a higher optical density loss as compared to their fullerene-based counterparts. In view of relevant comparative analysis in the Discussion section, we further propose some design strategies to improve the photo-oxidation stability of NFAs. More importantly, we also find a stabilizer (namely nickel chelate S6) that can effectively suppress the photo-oxidation of NFAs and their blends and thus improve the ambient stability of OSCs.
关键词： stability,organic solar cells,non-fullerene acceptors,stabilizers,photo-oxidation
更新于2025-09-16 10:30:52
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Significantly improving the performance of polymer solar cells by the isomeric ending-group based small molecular acceptors: Insight into the isomerization
摘要： Compared to benzene-fused end-capping groups (EGs), thiophene-fused EGs have some unique characteristics due to the non-centrosymmetric structure of the thiophene ring, which make them easy to form different types of isomers. Here, we develop three isomeric brominated thiophene-fused EGs, which are linked to the IDTT core to acquire three novel isomeric small-molecule acceptors (SMAs) named ITC-2Br, ITC-2Br1, and ITC-2Br2. From ITC-2Br to ITC-2Br1, the change of the bromine substituent group on the thiophene ring has only a minor impact on the physicochemical properties and photovoltaic performance. However, from ITC-2Br to ITC-2Br2, the change in the fused sites on the thiophene leads to dramatically modified absorption, energy levels, and photovoltaic performance. Theoretical simulations provide an in-depth understanding of the absorption and electrochemical differences among the three acceptors. Thanks to the favorable properties, the ITC-2Br2-based polymer solar cells (PSCs) yield a significantly higher power conversion efficiency (PCE) (13.1%) than the devices based on ITC-2Br (10.9%) and ITC-2Br1 (11.9%). From the ITC-2Br-, ITC-2Br1- to the ITC-2Br2-based devices, the JSC and FF exhibit a monotonic increase similar to the trend of PCE, which demonstrates the success of the isomerization strategy, highlighting its future prospects for the development of high-performance SMAs.
关键词： polymer solar cells,power conversion efficiency,isomerization,small-molecule acceptors,end-capping groups
更新于2025-09-16 10:30:52
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How Does Polymorphism Affect the Interfacial Charge-Transfer States in Organic Photovoltaics?
摘要： The bulk heterojunction in organic photovoltaic (OPV) devices is a mixture of polymer (electron donor) and an electron acceptor material (typically functionalized fullerenes), and it is crucial for the device operation, as this is where excitons are split into electrons and holes to produce current. Non-fullerene acceptors (NFAs) are promising new materials for improving the device efficiency, and their solid-state arrangement with respect to the electron donor polymer is critical for the charge mobility and the performance of OPV devices. Although there have been numerous studies on NFAs, most of the current understanding comes from empirical considerations, with little atomistic-level interpretation of why and how the packing influences the charge transport properties of these materials. In this work we describe large-scale (with up to 3462 atoms) DFT simulations for ground and excited states on a number of polymer-NFA interfaces of realistic size, whose NFA domains consist of polymorphs of the same materials. Hence, we bridged the gap between experimental evidence and the intuitive expectation on the importance of intermolecular π-π stacking interactions in the NFA phase. We show that low connectivity leads to highly localized excitons, whereas in phases with a higher connectivity excitons are able to delocalize over multiple directions. Remarkably, excitons with a three-dimensional delocalization were also observed, leading to isotropic mobilities, similarly to fullerenes. Furthermore, a lower charge-transfer exciton binding energy and a lower energy loss between the lowest excitation of the polymer and the first charge-transfer state in the interface were both observed in systems characterized by a highly interconnected NFA phase. This suggests a higher probability of exciton splitting for these interfaces, which could potentially lead to higher device efficiencies.
关键词： organic photovoltaics,non-fullerene acceptors,DFT simulations,polymorphism,charge-transfer states
更新于2025-09-16 10:30:52
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A bromine and chlorine concurrently functionalized end group for benzo[1,2-b:4,5-b']diselenophene-based nonfluorinated acceptors: new hybrid strategy to balance the crystallinity and miscibility of blend films enabling highly efficient polymer solar cells
摘要： A bromine and chlorine concurrently functionalized end group for nonfluorinated benzo[1,2-b:4,5-b']diselenophene-based acceptors: new hybrid strategy to balance the crystallinity and miscibility of blend films enabling highly efficient polymer solar cells. Dihalogenated 1,1-dicyanomethylene-3-indanone (IC) plays a key role in top-performing fused-ring electron acceptors (FREAs)-based polymer solar cells (PSCs). Here, we firstly synthesized a hybrid dihalogenated IC (IC-BrCl), which simultaneously grafted one Br atom and one Cl atom onto the same IC skeleton. Three nonfluorinated FREAs (BDSe-4Cl, BDSe-2(BrCl) and BDSe-4Br) are synthesized by employing benzo[1,2-b:4,5-b′]diselenophene-based core unit and dichlorinated IC, hybrid dihalogenated IC (IC-BrCl), and dibrominated IC for highly efficient PSCs, respectively. These three acceptors exhibit very similar absorption spectra with 1.39 eV of optical band gap, but slightly different in the HOMO/LUMO energy levels in thin films. The crystallinity of acceptors was progressively enhanced and miscibility with PM7 was gradually reduced with the increase of Br atoms. The BDSe-2(BrCl):PM7 blend films exhibited the strongest face-on crystallization orientation, the most proper phase separation feature, the highest and most balanced carrier mobility and the weakest charge recombination owing to the excellent balance of miscibility and crystallinity of blend film. Notably, BDSe-2(BrCl):PM7-based PSCs demonstrated an outstanding PCE of 14.5% with an impressive FF of 76.5%, which substantially outperformed its counterparts (13.8% for BDSe-4Cl, 13.2% for BDSe-4Br, respectively) and is the highest value in hybrid IC-based FREAs for binary PSCs. Our results demonstrated that hybrid dihalogenated IC with one Br atom and one Cl atom provide a promising strategy to tune crystallinity and miscibility of FREAs for boosting the FF and PCE of PSCs.
关键词： crystallinity,nonfluorinated acceptors,polymer solar cells,miscibility,hybrid dihalogenated IC
更新于2025-09-16 10:30:52
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Critical Role of Polymer Aggregation and Miscibility in Nonfullerenea??Based Organic Photovoltaics
摘要： Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature-dependent aggregation, namely PffBT4T-2OD [poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3″′-di(2-octyldodecyl)-2,2′;5′,2″;5″,2″′-quaterthiophen-5,5-diyl)], also known as PCE-11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O-IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O-IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.
关键词： nonfullerene acceptors,charge transport,morphology,charge generation,polymer aggregation
更新于2025-09-16 10:30:52