Non-fullerene Acceptor Materials Research Articles

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Disorder-Induced Transition from Transient Quantum Delocalization to Charge Carrier Hopping Conduction in a Nonfullerene Acceptor Material

Disorder-Induced Transition from Transient Quantum Delocalization to Charge Carrier Hopping Conduction in a Nonfullerene Acceptor Material

Dissecting the nature and dynamics of electronic excitations in a solid-state aggregate of a representative non-fullerene acceptor

Understanding electronic excitations and their dynamics in non-fullerene acceptors is crucial for enhancing opto-electronic properties. Using a Frenkel-exciton Hamiltonian and non-adiabatic dynamics, we reveal design strategies to achieve this goal.

End-capped engineering of Quinoxaline core-based non-fullerene acceptor materials with improved power conversion efficiency

Improving the light-harvesting efficiency and boosting open circuit voltage are crucial challenges for enhancing the efficiency of organic solar cells. This work introduces seven new molecules (SA1-SA7) to upgrade the optoelectronic and photovoltaic properties of Q-C-F molecule-based solar cells. All recently designed molecules have the same alkyl-substituted Quinoxaline core and CPDT donor but vary in the end-capped acceptor subunits. All the investigated molecules have revealed superior properties than the model (R) by having absorbance ranging from 681 nm to 782 nm in the gaseous medium while 726 nm–861 nm in chloroform solvent, with the lowest band gap ranging from 1.91 to 2.19 eV SA1 molecule demonstrated the highest λmax (861 nm) in chloroform solvent and the lowest band gap (1.91 eV). SA2 molecule has manifested highest dipole moment (4.5089 D), lower exciton binding energy in gaseous (0.33 eV) and chloroform solvent (0.47 eV), and lower charge mobility of hole (0.0077693) and electron (0.0042470). At the same time, SA7 showed the highest open circuit voltage (1.56 eV) and fill factor (0.9166) due to solid electron-pulling acceptor moieties. From these supportive outcomes, it is inferred that our computationally investigated molecules may be promising candidates to be used in advanced versions of OSCs in the upcoming period.

Improved Short-Circuit Current and Fill Factor in PM6:Y6 Organic Solar Cells through D18-Cl Doping

Based on the PM6:Y6 binary system, a novel non-fullerene acceptor material, D18-Cl, was doped into the PM6:Y6 blend to fabricate the active layer. The effects of different doping ratios of D18-Cl on organic solar cells were investigated. The best-performing organic solar cell was achieved when the doping ratio of D18-Cl reached 20 wt%. It exhibited a short-circuit current of 28.13 mA/cm2, a fill factor of 70.25%, an open-circuit voltage (Voc) of 0.81 V, and a power conversion efficiency of 16.08%. The introduction of an appropriate amount of D18-Cl expanded the absorption spectrum of the active layer, improved the morphology of the active layer, reduced large molecular aggregation and defects, minimized bimolecular recombination, and optimized the collection efficiency of charge carriers. These results indicate the critical importance of selecting an appropriate third component in binary systems and optimizing the doping ratio to enhance the performance of ternary organic solar cells.

Spontaneous-Symmetry-Breaking Charge Separation Induced by Pseudo-Jahn-Teller Distortion in Organic Photovoltaic Material.

The driving force of charge separation in the initial photovoltaic conversion process is theoretically investigated using ITIC, a nonfullerene acceptor material for organic photovoltaic devices. The density functional theory calculations show that the pseudo-Jahn-Teller (PJT) distortion of the S1 excimer state induces spontaneous symmetry-breaking charge separation between the identical ITIC molecules even without the asymmetry of the surrounding environment. The strong PJT effect arises from the vibronic coupling between the pseudodegenerate S1 and S2 excited states with different irreducible representations (irreps), i.e., Au for S1 and Ag for S2, via the asymmetric vibrational mode with the Au irrep. The vibrational mode responsible for the spontaneous polarization, which is opposite in one ITIC monomer and the other, is the intramolecular C-C stretching vibration between the core IT and terminal IC units. These results suggest that controlling the PJT effect can improve the charge separation efficiency of the initial photovoltaic conversion process.

Exploring novel naphthalene-fused octacyclic core-based non-fullerene acceptor materials with augmented optoelectronic attributes for stable and efficient solar cells

To meet the growing need for high-efficiency photovoltaic materials in today's high-tech applications, ten acceptor molecules (SH01-SH10) with naphthalene-fused octacyclic electron-donating central core NITT are proposed. End-capped structural-alterations on reference R (NITT-2F) with strong electron-drawing groups (A01-A10) are performed for attaining improved optoelectronic characteristics and possible applications in organic solar cells (OSCs). In comparison to the reference R (NITT-2F), key parameters such as FMO analysis, energy gap (Egap), open-circuit voltage (VOC), absorption spectra (λmax), excitation energy (Ex), transition density matrix (TDM) with binding energy (Eb), the density of states (DOS), reorganization energy of the electron (λe) and hole (λh), and charge transfer with PM6 donor substance are perceived. It is found that all proposed molecules (SH01-SH10) have a reduced energy gap (2.104–1.901 eV), a broader absorption spectrum (707–820 nm), and smaller excitation energy (1.75–1.51 eV) as compared to reference molecule NITT-2F values 2.132 eV, 675 nm and 1.79 eV respectively. The significant charge transfers and 0.23 eV decrease in the Egap is noted for the designed molecule SH05. Results confirmed that all proposed molecules, especially SH05 could be an excellent choice for OSCs candidate due to the combination of strong electron-withdrawing effect in end-capped acceptor unit and extended conjugation, as well as its promising photovoltaic properties, which include the lowest Egap (1.901 eV), highest λmax (820 nm (in chloroform) and 729 nm (in gas phase)) with Ex values of 1.51 and 1.70 eV respectively, slightest electron mobility (λe = 0.0090 eV) and hole mobility (λh = 0.0073 eV), highest ionization potential (6.826 eV) and electron affinity (3.384 eV), and fine value of VOC=1.02 V with respect to (ǀEPM6HOMOǀ - ǀESH05LUMOǀ). End-capped acceptor alterations are an acceptable way to attain the desired optoelectronic characteristics demonstrated in this research work. This study can be essential in achieving efficient novel non-fullerene OSCs with higher performance, low cost, and higher optical absorption coefficient. Thus, proposed molecules (SH01-SH10) with proficient electron and hole transfer mobilities and optoelectronic features are recommended to develop higher-efficiency solar cells.

Effect of Diffusion on Charge Transport in Organic Photovoltaic Blends

Charge transport is one of the key factors in the operation of organic photovoltaic devices. Non-fullerene acceptor materials have recently attracted significant attention in organic photovoltaics due to their great potential to realize high power conversion efficiencies. In this paper, we investigate the effect of diffusion on charge transport in a variety of binary and ternary organic photovoltaic materials, including both polymer: fullerene and polymer:non-fullerene blends. It is shown that the temperature dependent current density-voltage characteristics from the drift-diffusion simulations incorporating the extended Gaussian disorder model (EGDM) are more consistent with experimental data in comparison with those obtained from the only drift model for all these material systems. Furthermore, it is found that the effect of diffusion on charge transport is more pronounced at low voltages and seems to be negligible when the applied voltage exceeds 1 V. The deviation of calculated curves from experimental measurements gradually increases with increasing temperature. It is of great importance to the influence of diffusion effect on charge transport in both polymer:fullerene and polymer:non-fullerene blends.

Modeling of Conjugated Core-Based Non-Fullerene Acceptor Materials for Organic Photovoltaic Applications

Non-Fullerene acceptors (NFAs) are attracting much attention from scientists worldwide for their potential use in organic solar cells (OSC). Herein, we developed C-shaped NFAs acceptors (FA1–FA10) by performing end-capped alteration on GL1. The structural-property relation and optoelectronic characteristics were examined theoretically. Furthermore, the impact of structural changes on the optoelectronic characteristics of FA1–FA10 molecules was investigated by using density functional theory (DFT), and time-dependent DFT. The photo-physical and optoelectronic features such as frontier molecular orbitals, electron–hole overlapping, excitation and binding energy, molecular electrostatic potential, the density of states, transition density matrix, open circuit voltage, and reorganization energies of hole and electron, of the designed materials have been simulated. Compared to GL1 (R), the developed materials displayed a red-shifted absorption (ranges 769.84–856.36[Formula: see text]nm), better electric-charge movement, lower binding (0.30[Formula: see text]eV), and excitation energies (1.28[Formula: see text]eV), and narrower bandgaps (ranges 1.59–1.79[Formula: see text]eV), respectively. Therefore, we suggest these to synthetic researchers for the future development of efficient OSCs.

Wide band tri-receptor quaternary organic photodetector based on PC61BM, ITIC, and IICO-4F

In this paper, we report a wide band spectrum tri-receptor quaternary organic photodetector (OPD) based on the additional two non-fullerene acceptor materials, ITIC and IEICO-4F, with the polymer material P3HT as donor and the fullerene PC61BM as the first acceptor. The second acceptor ITIC and the third accptor IEICO-4F, with different molecular structures and absorption, were introduced into the conventional system P3HT:PC61BM to improve narrow response spectral range and the uneven response of OPDs. The experiments results show, that proposed non-fullerene acceptor materials of the narrow optical band gap were successful to broaden the absorption spectral range to near infrared. Respectively, the responses (R) and external quantum efficiency at 630, 530, and 460 nm, are up to 0.293 A W−1, 0.288 A W−1, 0. 243 A W−1, and 58%, 67%, and 66%. The quaternary OPDs achieve significant response spectrum across the 400–950 nm range with the specific detection rate (D*) exceeding 1012 Jones. It demonstrates that expanding spectrum non-fullerene materials ITIC and IEICO-4F can improve the mixing degree of the mixed materials, decrease agglomeration brought on by single widening excess, and increase spectrum absorption and moderate non-uniform response. The exciton dissociation efficiency is considerably increased by simultaneous charge transfer between the receptors and the receptors in the active layer. Moreover, the carrier mobility is improved, bimolecular recombination is suppressed.

Critical Progress of Polymer Solar Cells with a Power Conversion Efficiency over 18%

The power conversion efficiencies (PCEs) of organic photovoltaics (OPVs) have reached more than 19%, along with the prosperous development of materials and device engineering. It is meaningful to make a comprehensive review of the research of OPVs for further performance improvement. In this review, some typical materials of high-performance OPVs are summarized, including representative polymer donor materials, non-fullerene acceptor materials, and interfacial modification materials, as well as their design rules for molecular engineering. From the point of view of device engineering, active layer treatment and deposition technology are introduced, which can play a critical role in adjusting the degree of molecular aggregation and vertical distribution. Meanwhile, a ternary strategy has been confirmed as an efficient method for improving the performance of OPVs, and the multiple roles of the appropriate third component in the photo-electronic conversion process are emphasized and analyzed. The challenges and perspectives concerning this region are also put forward for further developing high-performance OPVs.

Designed and synthesized de novo ANTPABA-PDI nanomaterial as an acceptor in inverted solar cell at ambient atmosphere

In this work, a novel soluble and air-stable electron acceptor containing perylenediimide moiety named ANTPABA-PDI was designed and synthesized with band gap 1.78eV and that was used as non-fullerene acceptor material. ANTPABA-PDI possess not only good solubility but also much lower LUMO (lowest unoccupied molecular orbital) energy level. Furthermore, its excellent electron acceptor capability also supported by density functional theory calculation which validates the experimental observations. Inverted organic solar cell has been fabricated using ANTPABA-PDI along with P3HT as standard donor material in ambient atmosphere. The device, after characterization in open air, exhibited a power conversion efficiency of 1.70%. This is the first ever PDI based organic solar cell that has been fabricated completely in ambient atmosphere. The characterizations of the device have also been performed in ambient atmosphere. This kind of stable organic material can easily be used in fabricating organic solar cell and therefore it can be used as the best alternative as non-fullerene acceptor materials.

Latest Updates of Single-Junction Organic Solar Cells up to 20% Efficiency

Single-junction organic solar cells have reached a power conversion efficiency of 20% with narrow bandgap non-fullerene electron acceptor materials such as Y6, as well as with large band gap electron donor materials and their derivatives. The power conversion efficiency improvement of single-junction organic solar cells is a result of highly efficient light harvesting in the near-infrared light range and reduced energy losses with the most promising active layer layout currently available, Bulk-Heterojunction. Ternary blending is known to be the most advanced strategy to construct Bulk-Heterojunction structures in organic solar cells at present. In this review, we examine different devices based on Bulk-Heterojunction structures with efficient electron donors and acceptors. Then, we review the performance of binary and ternary organic solar cells with high power conversion efficiency, in conjunction with different anode and cathode interfaces used in recent studies of high-power conversion efficiency. Finally, we present perspectives on the future development of single-junction organic solar cells.

Efficient soluble PTCBI-type non-fullerene acceptor materials for organic solar cells

Single perylene diimide (PDI) used as a non-fullerene acceptor (NFA) in organic solar cells (OSCs) is enticing because of its low cost and excellent stability. To improve the photovoltaic performance, it is vital to narrow the bandgap and regulate the stacking behavior. To address this challenge, we synthesize soluble perylenetetracarboxylic bisbenzimidazole (PTCBI) molecules with a bulky side chain at the bay region, by replacing the widely used “swallow tail” type alkyl chains at the imide position of PDI molecules with a planar benzimidazole structure. Compared with PDI molecules, PTCBI molecules exhibit red-shifted UV–vis absorption spectra with larger extinction coefficient, and one magnitude higher electron mobility. Finally, OSCs based on one soluble PTCBI-type NFA, namely MAS-7, exhibit a champion power conversion efficiency (PCE) of 4.34%, which is significantly higher than that of the corresponding PDI-based OSCs and is the highest PCE of PTCBI-based OSCs reported. These results highlight the potential of soluble PTCBI derivatives as NFAs in OSCs.Graphical abstract

Outer sidechain engineering of selenophene and thiophene-based Y-series acceptors to produce efficient indoor organic solar cells

In recent years, efficient indoor organic photovoltaics (OPVs) have emerged as promising energy harvesters to drive Internet of Nanothings (IoNT) applications. The diverse design strategies developed for non-fullerene acceptor (NFA) materials have dramatically increased the power conversion efficiency (PCE) of indoor OPVs to 31%, and progress tends to continue. In this context, the current study investigated the indoor performance of NFA-OPVs by modifying the chalcogen-containing heterocycle of the acceptor molecule to thiophene (ThTh) and selenophene (ThSe), and changing the length of the external side chain using n-hexyl (C6) and n-nonyl (C9) components. Compared with the ThSe systems, the ThTh-containing OPVs exhibited enhanced current densities (JSC) and open-circuit voltages (VOC) owing to their enhanced crystallinity and reduced degree of bimolecular recombination. Similarly, the introduction of a short side chain (i.e., C6) led to relatively reduced monomolecular recombination and a moderated planarity compared to the system modified with the long side chain (i.e., C9). Ultimately, this resulted in an enhanced JSC and improved charge transport properties. Finally, the optimized OPV exhibited a PCE of 24.6% under a 1000 lx light-emitting diode lamp.

A Review on Fullerene Derivatives with Reduced Electron Affinity as Acceptor Materials for Organic Solar Cells

Organic solar cells (OSCs) represent a promising emerging photovoltaic technology offering such benefits as light weight, mechanical flexibility, semitransparency, environmental friendliness and aesthetic design of solar panels. Furthermore, organic solar cells can be produced using scalable and high-throughput solution-based printing and coating technologies, which are expected to lead to very low product costs. Fullerene derivatives have been used as acceptor materials in virtually all efficient organic solar cells for more than two decades, following the demonstration of the first proof-of-concept devices in the middle of 1990s. Still, the power conversion efficiencies of fullerene-based organic solar cells became stuck at around 12% due to the suboptimal optoelectronic properties of conventional fullerene acceptors. Therefore, the latest efficiency records (>18%) for organic solar cells were set using different types of non-fullerene acceptor (NFA) materials with tailorable properties. However, NFA materials appeared to be very sensitive to light, thus impairing the operational stability of OSCs. On the contrary, there is growing evidence that rationally designed fullerene-based acceptors enhance the photostability of conjugated polymers and also NFAs, when used in ternary blends. Hence, a renaissance of fullerene-based materials is currently expected in the context of their use in multicomponent organic solar cells (e.g., as stabilizers) and also lead halide perovskite solar cells, where they play an important role of electron transport materials. The success in both of these applications requires the tunability of optoelectronic characteristics of fullerene derivatives. In particular, electron affinity of the fullerene cage has to be reduced in many cases to match the energy levels of other absorber material(s). Herein, we present a systematic review of different strategies implemented to reduce the acceptor strength of the fullerene derivatives and the results of their performance evaluation in OSCs with model conjugated polymers. Particular attention is paid to correlations between the chemical structure of organic addends and their influence on the electronic properties of the fullerene core. We believe this review would be valuable to researchers working on the rational design of new fullerene-based materials with tailored properties for photovoltaic and other electronic applications.

Study on Ternary Blend Organic Solar Cells Based on Multiple Non-fullerene Acceptors

In recent years, non-fullerene acceptor materials for organic solar cells ( OSCs ) have attracted much attention. Among them, ITIC, Y6 and perylene diimide ( PDI ) have been widely studied because of their easy structure regulation. The application and development of ITIC and its derivatives, Y6, PDI and its derivatives in ternary blend organic solar cells are reviewed in detail. This work focuses on the molecular structure optimization method and the selection of third component materials. At present, the efficiency of organic solar cells has exceeded 18%, but the relationship between molecular structure and photovoltaic performance needs further exploration. Optimizing the synthesis route is still an important research direction in this field.

Machine learning framework for the analysis and prediction of energy loss for non-fullerene organic solar cells

The efficiency of organic solar cells (OSCs) has been improved more than 19% recently with the development of non-fullerene acceptor materials. Further improvement is still attainable with the optimal combinations of donors and acceptors that provide minimal energy losses. In this work, a data-enabled machine-learning (ML) framework was employed to predict the energy losses in the polymer:non-fullerene acceptor based devices. Based on the collected experimental dataset, the prediction accuracies of various machine learning models were systematically compared by estimating mean absolute percentage errors (MAPE), root mean squared errors (RMSE), and person’s r coefficient. The Random Forest regression model showed the best performance in predicting the energy losses with a correlation coefficient of 0.83 and relative error in the range of 0 – 20%. The predictive ability of this model was further validated using the different parameters of devices with power conversion efficiency range of 6 – 18%. Three different donor–acceptor combinations were chosen for fabricating the photovoltaic devices to fit this model into practical devices and experimentally obtained energy loss values were compared with the predicted values. In addition, the device parameters with the molecular descriptors to understand the correlation and energy loss is highly correlated with the HOMO offset. This study demonstrates that the ML approach provide an effective method to predict and virtual screen of promising donor–acceptor pairs with minimal energy loss and would be useful for developing next-generation high performance solar cell materials.

Highly efficient flexible organic photovoltaic modules for sustainable energy harvesting under low-light condition via suppressing voltage-drop by metal-mediated cross-linkable polymer interfacial layer

This study aims to expand the practical applications of large-area flexible organic photovoltaics (OPV), such that they retain their high stability and efficiency even under various weather conditions and external stresses. By developing polyethyleneimine derivative-based cathode interfacial layer (CIL) and non-fullerene acceptor (NFA) material in OPVs, the efficiency and stability decrease rapidly due to an undesirable interaction when these two materials are concomitantly applied. In this study, we designed a novel low-cost metal-mediated cross-linked non-conjugated polymer interfacial layer (c-PEIE) and improved the device performance and stability of flexible OPVs from the cell-to-module scale. c-PEIE CIL based flexible OPV cell achieves one of the highest power conversion efficiencies (PCE) of 16.45% and remarkable photostability, retaining 77.58% of its initial PCE for 110 h under continuous light illumination, while the conventional PEIE based cells realize a PCE of 12.58% and poor device stability. Accordingly, a 50 cm2, large-area, flexible OPV module was also fabricated with an excellent PCE of 13.12% which is ∼ 80% as efficient as small-area flexible cells. Notably, c-PEIE based OPV devices maintain high output power on cloudy days, which is a particular point of interest to efficiently harvest sunlight energy constantly. The tunable energy barrier and significant low-leakage current of CIL play a critical role in reducing open-circuit voltage and fill factor losses under low-light environments. Such energy sources can provide power efficiently all day long in low-light or cloudy environments to the Internet of Things wireless networks integrated with battery-independent photovoltaics.

Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells.

Nonfullerene acceptor based organic solar cells (NF-OSCs) have witnessed rapid progress over the past few years owing to the intensive research efforts on novel electron donor and nonfullerene acceptor (NFA) materials, interfacial engineering, and device processing techniques. Interfacial layers including electron transporting layers (ETL) and hole transporting layers (HTLs) are crucially important in the OSCs for facilitating electron and hole extraction from the photoactive blend to the respective electrodes. In this review, the lates progress in both ETLs and HTLs for the currently prevailing NF-OSCs are discussed, in which the ETLs are summarized from the categories of metal oxides, metal chelates, non-conjugated electrolytes and conjugated electrolytes, and the HTLs are summarized from the categories of inorganic and organic materials. In addition, some bifunctional interlayer materials served as both ETLs and HTLs are also introduced. Finally, the prospects of ETL/HTL materials for NF-OSCs are provided.