Dithieno[3,2-b:2',3'-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells

Solution Sequential Deposition Pseudo-Planar Heterojunction: An Efficient Strategy for State-of-Art Organic Solar Cells

Organic solar cells (OSCs) are considered as a promising new generation of clean energy. Bulk heterojunction (BHJ) structure has been widely employed in the active layer of efficient OSCs. However, precise regulation of morphology in BHJ is still challenging due to the competitive coupling between crystallization and phase separation. Recently, a novel pseudo-planar heterojunction (PPHJ) structure, prepared through solution sequential deposition, has attracted much attention. It is an easy-to-prepare structure in which the phase separation structures, interfaces, and molecular packing can be separately controlled. Employing PPHJ structure, the properties of OSCs, such as power conversion efficiency, stability, transparency, flexibility, and so on, are usually better than its BHJ counterpart. Hence, a comprehensive understanding of the film-forming process, morphology control, and device performance of PPHJ structure should be considered. In terms of the representative works about PPHJ, this review first introduces the fabrication process of active layers based on PPHJ structure. Second, the widely applied morphology control methods in PPHJ structure are summarized. Then, the influences of PPHJ structure on device performance and other property are reviewed, which largely expand its application. Finally, a brief prospect and development tendency of PPHJ devices are discussed with the consideration of their challenges.

Combination of Transfer Learning and Chemprop Interpreter with Support of Deep Learning for the Energy Levels of Organic Photovoltaic Materials Prediction and Regulation

It is challenging to build a deep learning predictive model using traditional data mining methods due to the scarcity of available data, and the model's internal decision-making process is often nonintuitive and difficult to explain. In this work, a directed message passing neural network model with transfer learning (TL) and chemprop interpreter is proposed to improve energy levels prediction and visualization for organic photovoltaic materials. The established model shows the best performance, with coefficient of determination reaching 0.787 for HOMO and 0.822 for LUMO in a small testing set after TL, compared to the other four models. Then, the chemprop interpreter analyzes local and global effects of 12 molecular structures on the energy levels for organic materials. After a comprehensive analysis of the energy level effects of nonfullerene Y-series, IT-series, and other organic materials, 12 new IT-series derivatives are designed. 1,1-dicyano-methylene-3-indanone (IC) end group halogenation can reduce HOMO and LUMO energy levels to varying degrees, while IC end group modified by electron-withdrawing aromatic groups can increase HOMO and LUMO energy levels and obtain relatively smaller electrostatic potential (ESP) to reducing intermolecular interactions. The influence of side-chain modification on energy levels is limited. It is worth mentioning that the predicted results of IT-series derivatives match density functional theory calculations. The model also shows good generalization and transferability for predicting the energy levels of other organic electronic materials. This work not only provides a cost-effective model for predicting the energy levels of organic photovoltaic materials but also explains the potential bridge between molecular structure and electronic properties.

Precise Methylation Yields Acceptor with Hydrogen-Bonding Network for High-Efficiency and Thermally Stable Polymer Solar Cells

Utilizing intermolecular hydrogen-bonding interactions stands for an effective approach in advancing the efficiency and stability of small-molecule acceptors (SMAs) for polymer solar cells. Herein, we synthesized three SMAs (Qo1, Qo2, and Qo3) using indeno[1,2-b]quinoxalin-11-one (Qox) as the electron-deficient group, with the incorporation of a methylation strategy. Through crystallographic analysis, it is observed that two Qox-based methylated acceptors (Qo2 and Qo3) exhibit multiple hydrogen bond-assisted 3D network transport structures, in contrast to the 2D transport structure observed in gem-dichlorinated counterpart (Qo4). Notably, Qo2 exhibits multiple and stronger hydrogen-bonding interactions compared with Qo3. Consequently, PM6 : Qo2 device realizes the highest power conversion efficiency (PCE) of 18.4 %, surpassing the efficiencies of devices based on Qo1 (15.8 %), Qo3 (16.7 %), and Qo4 (2.4 %). This remarkable PCE in PM6 : Qo2 device can be primarily ascribed to the enhanced donor-acceptor miscibility, more favorable medium structure, and more efficient charge transfer and collection behavior. Moreover, the PM6 : Qo2 device demonstrates exceptional thermal stability, retaining 82.8 % of its initial PCE after undergoing annealing at 65 °C for 250 hours. Our research showcases that precise methylation, particularly targeting the formation of intermolecular hydrogen-bonding interactions to tune crystal packing patterns, represents a promising strategy in the molecular design of efficient and stable SMAs.

A computational insight into enhancement of photovoltaic properties of non-fullerene acceptors by end-group modulations in the structural framework of INPIC molecule

Improving the open circuit voltage is a major challenge for enhancing the overall efficiency of organic solar cells. Current work has concentrated on improving open-circuit voltage by designing new molecular frameworks from an INPIC molecule having a conjugated fused core. We modulated the structure by changing the terminal groups of the reference molecule (INPIC) with seven strong electron-withdrawing units. We investigated various optoelectronic attributes, charge transfer, and photovoltaic and geometrical parameters by compiling the B3LYP/6-31G(d,p) functional of the DFT approach. The optical absorption for modulated molecules ranges from 748.51 nm to 845.96 nm while showing higher oscillation strength than INPIC. At the same time, their impressive charge transport is attributed to their smaller excitation and exciton binding energy, higher electron/hole mobility, narrower band gap, and a more than 99 % intramolecular charge transfer. The larger dipole moments help in the dense interaction of acceptors with employed donor J61 which, in turn, improves charge transfer at the donor-acceptor interface. One of the triumphs that are difficult to get in organic molecules is success in achieving a higher open circuit voltage (VOC). Our conceptualized molecular frameworks of acceptors are featured with a notable VOC improvement in the range of 1.84-2.05 eV. Thus, the results of the current investigation pave the root for architecting the acceptor molecules with impressive optoelectrical properties that may be capable of providing high photovoltaic output. Thus these acceptors can be utilized for the development of advanced organic solar cells in future.

Designing Electron-Deficient Diketone Unit Based Non-Fused Ring Acceptors with Amplified Optoelectronic Features for Highly Efficient Organic Solar Cells: A DFT Study

Organic solar cells (OSCs) made of electron-acceptor and electron-donor materials have significantly developed in the last decade, demonstrating their enormous potential in cutting-edge optoelectronic applications. Consequently, we designed seven novel non-fused ring electron acceptors (NFREAs) (BTIC-U1 to BTIC-U7) using synthesized electron-deficient diketone units and reported end-capped acceptors, a viable route for augmented optoelectronic properties. The DFT and TDDFT approaches were used to measure the power conversion efficiency (PCE), open circuit voltage (Voc), reorganization energies (λ_h, λ_e), fill factor (FF), light harvesting efficiency (LHE) and to evaluate the potential usage of proposed compounds in solar cell applications. The findings confirmed that the photovoltaic, photophysical, and electronic properties of the designed molecules BTIC-U1 to BTIC-U7 are superior to those of reference BTIC-R. The TDM analysis demonstrates a smooth flow of charge from the core to the acceptor groups. Charge transfer analysis of the BTIC-U1:PTB7-Th blend revealed orbital superposition and successful charge transfer from HOMO (PTB7-Th) to LUMO (BTIC-U1). The BTIC-U5 and BTIC-U7 outperformed the reference BTIC-R and other developed molecules in terms of PCE (23.29% and 21.18%), FF (0.901 and 0.894), normalized Voc (48.674 and 44.597), and Voc (1.261 eV and 1.155 eV). The proposed compounds enclose high electron and hole transfer mobilities, making them the ideal candidate for use with PTB7-Th film. As a result, future SM-OSC design should prioritize using these constructed molecules, which exhibit excellent optoelectronic properties, as superior scaffolds.

Efficient Semitransparent Organic Solar Cells Enabled by Ag Grid Electrodes and Optical Coupling Layers

Semitransparent organic solar cells (ST-OSCs) show great promise for building integrated photovoltaic systems. The balance between power conversion efficiency (PCE) and average visible transmittance (AVT) is a key point of ST-OSCs. We developed a novel semitransparent organic solar cell (ST-OSC) with high PCE and AVT for building integrated renewable energy applications. We used photolithography to fabricate Ag grid bottom electrodes with high figures of merit of 292.46. We also used an optimized active layer of PM6 and Y6, achieving a PCE of 10.65% and an AVT of 22.78% for our ST-OSCs. By adding optical coupling layers of CBP and LiF alternately, we further increased the AVT to 27.61% and the PCE to 10.87%. Importantly, the balance of PCE and AVT can be achieved by the integrated optimization of the active and optical coupling layers, which leads to a significant increase in light utilization efficiency (LUE). These results are of great importance for particle applications of ST-OSCs.

Sensitive photodetection below silicon bandgap using quinoid-capped organic semiconductors

High-sensitivity organic photodetectors (OPDs) with strong near-infrared (NIR) photoresponse have attracted enormous attention due to potential applications in emerging technologies. However, few organic semiconductors have been reported with photoelectric response beyond ~1.1 μm, the detection limit of silicon detectors. Here, we extend the absorption of organic small-molecule semiconductors to below silicon bandgap, and even to 0.77 eV, through introducing the newly designed quinoid-terminals with high Mulliken-electronegativity (5.62 eV). The fabricated photodiode-type NIR OPDs exhibit detectivity (D^*) over 10¹² Jones in 0.41 to 1.2 μm under zero bias with a maximum of 2.9 × 10¹² Jones at 1.02 μm, which is the highest D^* for reported OPDs in photovoltaic-mode with response spectra beyond 1.1 μm. The high D^* in 0.9 to 1.2 μm is comparable to those of commercial InGaAs photodetectors, despite the detection limit of our OPDs is shorter than InGaAs (~1.7 μm). A spectrometer prototype with a wide measurable region (0.4 to 1.25 μm) and NIR imaging under 1.2-μm illumination are demonstrated successfully in OPDs.

Intramolecular charge transfer effect for highly efficient deep red and near infrared thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) materials with emission in the deep red and near infrared (DR/NIR) region are underresearched due to the limited choice of strong donor/acceptor units. The current mainstream strategy for the design of DR/NIR TADFs is to increase the acceptor strength via the introduction of multiple sub-acceptor units, thereby narrowing the bandgap. In this work, the intramolecular charge transfer (ICT) effect was applied for the development of acceptor units to achieve efficient DR/NIR TADFs. The ICT effect within the acceptor unit enhanced the π-electron delocalization, lowered the LUMO and redshifted the emission wavelength. In addition, the fusion of the donor unit into the planar acceptor skeleton rigidified the molecular structure and reduced the non-radiative decay. This proof-of-concept study demonstrated that ICT is an undoubtedly effective strategy for the rational design of efficient DR/NIR TADFs.

Quantum modeling of dimethoxyl-indaceno dithiophene based acceptors for the development of semiconducting acceptors with outstanding photovoltaic potential

In the current DFT study, seven dimethoxyl-indaceno dithiophene based semiconducting acceptor molecules (ID1-ID7) are designed computationally by modifying the parent molecule (IDR). Here, based on a DFT exploration at a carefully selected level of theory, we have compiled a list of the optoelectronic properties of ID1-ID7 and IDR. In light of these results, all newly designed molecules, except ID5 have shown a bathochromic shift in their highest absorbance (λ _max). ID1-ID4, ID6 and ID7 molecules have smaller band gap (E _gap) and excitation energy (E _x). IP of ID5 is the smallest and EA of ID1 is the largest among all others. Compared to the parent molecule, ID1-ID3 have increased electron mobility, with ID1 being the most improved in hole mobility. ID4 had the best light harvesting efficiency in this investigation, due to its strongest oscillator. The acceptor molecules' open-circuit voltages (V _OC) were computed after being linked to the PTB7-Th donor molecule. Fill factor (FF) and normalized V _OC of ID1-ID7 were calculated and compared to the parent molecule. Based on the outcomes of this study, the modified acceptors may be further scrutinised for empirical usage in the production of organic solar cells with enhanced photovoltaic capabilities.

New Non-Fullerene Acceptor with Extended Conjugation of Cyclopenta [2,1-b:3,4-b’] Dithiophene for Organic Solar Cells

Herein, we design and characterize 9-heterocyclic ring non-fullerene acceptors (NFAs) with the extended backbone of indacenodithiophene by cyclopenta [2,1-b:3,4-b’] dithiophene (CPDT). The planar conjugated CPDT donor enhances absorption by reducing vibronic transition and charge transport. Developed NFAs with different end groups shows maximum absorption at approximately 790–850 nm in film. Because of the electronegative nature of the end-group, the corresponding acceptors showed deeper LUMO energy levels and red-shifted ultraviolet absorption. We investigate the crystallinity, film morphology, surface energy, and electronic as well as photovoltaic performance. The organic photovoltaic cells using novel NFAs with the halogen end groups fluorine or chlorine demonstrate better charge collection and faster exciton dissociation than photovoltaic cells using NFAs with methyl or lacking a substituent. Photovoltaic devices constructed from m-Me-ITIC with various end groups deliver power conversion efficiencies of 3.6–11.8%.

Effect of Terminal Electron-Withdrawing Group on the Photovoltaic Performance of Asymmetric Fused-Ring Electron Acceptors

The terminal electron-withdrawing group is thought to strongly influence π-π stacking interactions and thereby the charge transport properties of fused-ring electron acceptors (FREAs). In this work, we designed and synthesized three asymmetric/symmetric small molecule FREAs (LC301, LC302, and LC303), in which three electron-withdrawing functional groups with different polarities (phenyl-fused indanone < thienyl-fused indanone < F-modified phenyl-fused indanone) were selected as the terminal groups. Photophysical properties, electrochemistry, charge transport, and crystalline properties of the materials were studied to investigate the effect of electron-withdrawing abilities of the terminal groups on the properties of FREAs. Starting from the symmetric LC302 (two thienyl-fused indanone terminal groups), we have found that by simply replacing only one thienyl-fused indanone terminal group in symmetric LC302 with one difluorinated phenyl-fused indanone terminal group, the asymmetric LC301-based organic solar cells (OSCs) can yield a high power conversion efficiency (PCE) of 17.21% and a promising fill factor (FF) of 78.1%, which are higher than those of symmetric LC302-based OSCs (PCE = 15.18%, FF = 73.3%) and the asymmetric LC303-based OSCs bearing one thienyl-fused indanone terminal group and one nonfluorinated phenyl-fused indanone terminal group (PCE = 14.28%, FF = 70.3%). The results indicate that the rational selection of terminal groups with different electron-withdrawing capabilities is highly desirable for designing high-performance asymmetric FREAs.

The principles, design and applications of fused-ring electron acceptors

Fused-ring electron acceptors (FREAs) have a donor-acceptor-donor structure comprising an electron-donating fused-ring core, electron-accepting end groups, π-bridges and side chains. FREAs possess beneficial features, such as feasibility to tailor their structures, high property tunability, strong visible and near-infrared light absorption and excellent n-type semiconducting characteristics. FREAs have initiated a revolution to the field of organic solar cells in recent years. FREA-based organic solar cells have achieved unprecedented efficiencies, over 20%, which breaks the theoretical efficiency limit of traditional fullerene acceptors (~13%), and boast potential operational lifetimes approaching 10 years. Based on the original studies of FREAs, a variety of new structures, mechanisms and applications have flourished. In this Review, we introduce the fundamental principles of FREAs, including their structures and inherent electronic and physical properties. Next, we discuss the way in which the properties of FREAs can be modulated through variations to the electronic structure or molecular packing. We then present the current applications and consider the future areas that may benefit from developments in FREAs. Finally, we conclude with the position of FREA chemistry, reflecting on the challenges and opportunities that may arise in the future of this burgeoning field.

Achieving 19% Power Conversion Efficiency in Planar-Mixed Heterojunction Organic Solar Cells Using a Pseudosymmetric Electron Acceptor

A record power conversion efficiency (PCE) of over 19% is realized in planar-mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE₂ ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe-4F achieve a PCE of 18.48%. By incorporating another mid-bandgap acceptor Y6-O into D18/BS3TSe-4F to form a ternary PMHJ, a higher open-circuit voltage (V_OC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Heteroheptacene-based acceptors with thieno[3,2-b]pyrrole yield high-performance polymer solar cells

Rationally utilizing and developing synthetic units is of particular significance for the design of high-performance non-fullerene small-molecule acceptors (SMAs). Here, a thieno[3,2-b]pyrrole synthetic unit was employed to develop a set of SMAs (ThPy1, ThPy2, ThPy3 and ThPy4) by changing the number or the position of the pyrrole ring in the central core based on a standard SMA of IT-4Cl, compared to which the four thieno[3,2-b]pyrrole-based acceptors exhibit bathochromic absorption and upshifted frontier orbital energy level due to the strong electron-donating ability of pyrrole. As a result, the polymer solar cells (PSCs) of the four thieno[3,2-b]pyrrole-based acceptors yield higher open-circuit voltage and lower energy loss relative to those of the IT-4Cl-based device. What is more, the ThPy3-based device achieves a power conversion efficiency (PCE) (15.3%) and an outstanding fill factor (FF) (0.771) that are superior to the IT-4Cl-based device (PCE = 12.6%, FF = 0.758). The ThPy4-based device realizes the lowest energy loss and the smallest optical band gap, and the ternary PSC device based on PM6:BTP-eC9:ThPy4 exhibits a PCE of 18.43% and a FF of 0.802. Overall, this work sheds light on the great potential of thieno[3,2-b]pyrrole-based SMAs in realizing low energy loss and high PCE.

D-A-D Compounds Combining Dithienopyrrole Donors and Acceptors of Increasing Electron-Withdrawing Capability: Synthesis, Spectroscopy, Electropolymerization, and Electrochromism

Five D-π-A-π-D compounds consisting of the same donor unit (dithieno[3,2-b:2',3'-d]pyrrole, DTP), the same π-linker (2,5-thienylene), and different acceptors of increasing electron-withdrawing ability (1,3,4-thiadiazole (TD), benzo[c][1,2,5]thiadiazole (BTD), 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 1,2,4,5-tetrazine (TZ), and benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)) were synthesized. DTP-TD, DTP-BTD, and DTP-DPP turned out to be interesting luminophores emitting either yellow (DTP-TD) or near-infrared (DTP-BTD and DTP-DPP) radiation in dichloromethane solutions. The emission bands were increasingly bathochromically shifted with increasing solvent polarity. Electrochemically determined electron affinities (|EA|s) were found to be strongly dependent on the nature of the acceptor changing from 2.86 to 3.84 eV for DTP-TD and DTP-NDI, respectively, while the ionization potential (IP) values varied only weakly. Experimental findings were strongly supported by theoretical calculations, which correctly predicted the observed solvent dependence of the emission spectra. Similarly, the calculated IP and EA values were in excellent agreement with the experiment. DTP-TD, DTP-BTD, DTP-TZ, and DTP-NDI could be electropolymerized to yield polymers of very narrow electrochemical band gap and characterized by redox states differing in color coordinates and lightness. Poly(DTP-NDI) and poly(DTP-TD) showed promising electrochromic behavior, not only providing a rich color palette in the visible but also exhibiting near-infrared (NIR) electrochromism.

Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers

Two new fused-ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m-BAIDIC and p-BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p-BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p-BAIDIC-based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open-circuit voltage (VOC ) and short-current current density (JSC ) of the devices. Therefore, the linear FREA, p-BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m-BAIDIC. When p-BAIDIC is incorporated into the binary PM6/BO-4Cl system to form a ternary system, synergistic enhancements in VOC , JSC , fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.

Interfacial Energy Band Alignment Enables the Reduction of Potential Loss for Hole-Conductor-Free Printable Mesoscopic Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved high efficiencies with diversified device architectures. In particular, printable mesoscopic PSC has attracted intensive research attention due to its simple fabrication process and superior stability. However, in the absence of hole conductors, the unfavorable energy band alignment between the perovskite and the carbon electrode usually leads to the reduction of device performance, especially the open-circuit voltage (V_OC). Here, a p-type molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is utilized to post-treat the perovskite/carbon interface, which benefits the charge transfer and suppresses the charge recombination within the device. As a result, the post-treated device delivers a power conversion efficiency of 18.05% with an enhanced V_OC of 1044 mV. This work provides a facile method for tuning the interfacial energy band alignment and improving performance of printable mesoscopic PSCs.

Recent Progress in Indacenodithiophene-Based Acceptor Materials for Non-Fullerene Organic Solar Cells

Domesticating solar energy by exploiting photovoltaic technology has become a quintessential strategy for future global energy production. Since 2015, non-fullerene organic solar cells (NF-OSC) have attracted a great deal of attention owing to the marvellous properties of non-fullerene acceptors (NFA) such as structural versability, broad absorption, suitable energy levels, tunable charge transport and morphology, leading to remarkable accomplishments in power conversion efficiency (PCE) from 1% to nearly 20%. One class of materials is provided by the fused ring aromatic indacenodithiophene (IDT) and its derivatives, which are emerging continuously as promising next-generation building blocks to construct high performance photovoltaic materials. Encouraging PCEs of more than 15% have been achieved in their binary NF-OSCs, while careful device engineering and proper amalgamation of a third component have led to PCEs of almost 18% in ternary devices. This review surveys recent developments in the area of IDT-based materials for photovoltaic applications. Different strategies to develop efficient IDT-based NFA and factors influencing the bandgaps, molecular energy levels, charge transport properties, and film morphologies, as well as the photovoltaic performance of these materials, are discussed.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Quantum chemical designing of novel fullerene-free acceptor molecules for organic solar cell applications

Organic solar cells (OSCs) with bulk heterojunction (BHJ) structures consisting of electron-donor and electron-acceptor materials have achieved impressive progress over the past decade, demonstrating their great potential in practical applications. In this study, we have designed five fullerene-free acceptor-based molecules containing indaceno-dithiophene as a central core moiety. We studied the optoelectronic features of these newly architecture molecules by using DFT and TD-DFT approaches. For the investigation of the optoelectronic characteristics of the reference and newly designed molecules, we performed different parameters including FMO's, absorption maxima, excitation energy, transition density matrix (TDM) along with binding energy, dipole moment, the partial density of states, charge mobility, and charge transfer analysis. Among all engineered molecules, SK1 has proven to be the most efficient solar cell due to its promising optoelectronic and photovoltaic properties. SK1 reveals smaller band-gap (E_gap = 1.959 eV) and lesser λ_h (0.0070 eV) and λ_e (0.0051 eV). SK1 illustrated comparable binding energy value (0.33 eV) and lowest excitation energy (1.62 eV) which will lead to improved power conversion efficiency values. The SK1 molecule demonstrated the highest λ_max value (764 nm) in the solvent phase which could lead to redshift absorption for achieving the high efficiency of OSCs. This molecular modeling approves that the best working efficiency of organic solar devices can be achieved by terminal group modifications due to their promising photovoltaic and optoelectronic properties. It is evident from the current analysis that all the theoretically fabricated molecules (SK1-SK5) are fabulous and highly suggested to experimental workers for their synthesis and advancement of these highly competent solar devices in the future.

NIR-Absorbing Electron Acceptor Based on a Selenium-Heterocyclic Core Attaching to Phenylalkyl Side Chains for Polymer Solar Cells with 17.3% Efficiency

Selenium-heterocyclic and side-chain strategies for developing near-infrared (NIR) small fused-ring acceptors (FRAs) to further obtain short-circuit current density (J_sc) have proven advantageous in the top-performing polymer solar cells (PSCs). Herein, a new electron-rich central selenium-containing heterocycle core (BTSe) attaching alkyl side chains with a terminal phenyl group was coupled with a difluorinated and dichlorinated electron-accepting terminal 1,1-dicyanomethylene-3-indanone (IC) to afford two types of new FRAs, BTSe-IC2F and BTSe-IC2Cl. Interestingly, in spite of the weaker intramolecular charge transfer, BTSe-IC2F shows a stronger NIR response because of the smaller bandgap (E_g^opt) up to 1.26 eV, benefiting from the stronger ordered molecular packing in comparison to BTSe-IC2Cl with an E_g^opt of 1.30 eV. Additionally, thermal annealing induced an evident red shift by ∼50 nm in the absorption of D18:BTSe-IC2F blend films. Such a phenomenon may be attributed to the synergistic impact of the formation of inward constriction toward the molecular backbone because of the combination of bulky side chains and fluorinated IC as well as the reduced aromaticity of the selenium heterocycle. Consequently, the thermally annealed device based on BTSe-IC2F/D18 achieves a champion power conversion efficiency (PCE) of 17.3% with a high fill factor (FF) of 77.22%, which is among the highest reported PCE values for selenium-heterocyclic FRAs in binary PSCs. The improved J_sc and FF values of the D18:BTSe-IC2F film are simultaneously achieved mainly because of the preferred face-on orientations, the well-balanced electron/hole mobility, and the favorable blend morphology compared to D18:BTSe-IC2Cl. This work suggests that the selenium-heterocyclic fused-ring core (with proper side chains) combined with fluorinated terminal groups is an effective strategy for obtaining highly efficient NIR-responsive FRAs.

Over 17% Efficiency of Ternary Organic Photovoltaics Employing Two Acceptors with an Acceptor-Donor-Acceptor Configuration

Ternary organic photovoltaics (OPVs) were constructed with one wide-band-gap donor PM6 and two A-D-A-type acceptors (M-series M36 and MQ5) with similar chemical structures. Power conversion efficiency (PCE) of the optimal ternary OPVs reaches 17.24% with 20 wt % MQ5 content, arising from a simultaneously increased short circuit current density (J_SC) of 25.36 mA cm^-2 and a fill factor (FF) of 76.02% as compared to those of two binary OPVs. The photon harvesting of ternary active layers can be maximized by adjusting the MQ5 content by reason of the complementary absorption spectra of M36 and MQ5. The molecular arrangement of PM6 and M36 can be collectively optimized by introducing an appropriate amount of MQ5 as a morphology regulator for facilitating effective charge transportation in ternary active layers. The improved photon harvesting and charge transport in active layers should be two important factors responsible for J_SC and FF improvement of optimal ternary OPVs, respectively. More than an 8.8% improvement of PCE is achieved in ternary OPVs with an appropriate amount of MQ5 as the photon-harvesting enhancer and morphology regulator. The huge potential of A-D-A-type materials in constructing highly efficient OPVs can be further exploited based on a ternary strategy.

Organoboron molecules and polymers for organic solar cell applications

Organic solar cells (OSCs) are emerging as a new photovoltaic technology with the great advantages of low cost, light-weight, flexibility and semi-transparency. They are promising for portable energy-conversion products and building-integrated photovoltaics. Organoboron chemistry offers an important toolbox to design novel organic/polymer optoelectronic materials and to tune their optoelectronic properties for OSC applications. At present, organoboron small molecules and polymers have become an important class of organic photovoltaic materials. Power conversion efficiencies (PCEs) of 16% and 14% have been realized with organoboron polymer electron donors and electron acceptors, respectively. In this review, we summarize the research progress in various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. This review also discusses how to tune their opto-electronic properties and active layer morphology for enhancing OSC device performance. We also offer our insight into the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

Asymmetric Non-Fullerene Small-Molecule Acceptors toward High-Performance Organic Solar Cells

Applying an asymmetric strategy to construct non-fullerene small-molecule acceptors (NFSMAs) in organic solar cells (OSCs) plays a vital role in the development of organic photovoltaic materials. In the past several years, taking advantage of the larger dipole moment and stronger intermolecular interactions, asymmetric NFSMAs have witnessed tremendous progress in OSCs with a power conversion efficiency of over 18%. From a structural point of view, besides the possible changes in the conformation effect on molecular packing, asymmetric acceptors can also achieve a balance between the solubility and the crystallinity. Herein, we systematically investigate the structure-property-performance relationships of asymmetric NFSMAs that have recently emerged and try to clarify the feasibility and practicality of an asymmetric strategy for the design of higher-performance NFSMAs. Finally, we put forward our views and a concise outlook on the asymmetric strategy.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (E_CT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an E_CT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Modulating Crystal Packing, Film Morphology, and Photovoltaic Performance of Selenophene-Containing Acceptors through a Combination of Skeleton Isomeric and Regioisomeric Strategies

Here, we report a series of acceptor-donor-acceptor (A-D-A) architecture isomeric acceptors (SeCT-IC, CSeT-IC, and CTSe-IC), which have an identical electron-deficient terminal A-group and three different central D-cores with the selenophene at the innermost, relatively outer, and outermost positions of the central core, respectively. From CSeT-IC to the atom regioisomer of CTSe-IC and to the conjugated skeleton isomer of SeCT-IC, the optical band gap of neat films continuously reduced and highest occupied molecular orbitals (HOMO) gradually upshifted with changing the selenophene from relatively outer position to the outermost position and to the innermost position of the central core. More importantly, the single-crystal structure and the GIWAXS measurements revealed that CTSe-IC presents the closest π-π stacking distance, the largest CCL, and the best molecular order and crystallinity, which led to the highest electron mobility in neat films. Furthermore, the J71:CTSe-IC blend film presents a more ordered film morphology with more proper phase separation domain size, more dominant face-on orientation, and relatively higher and more balanced electron-hole mobilities in comparison with that of J71:SeCT-IC and J71:CSeT-IC. Consequently, the J71:CTSe-IC-based organic solar cell gave a superior power conversion efficiency (PCE) of 11.59%, which was obviously higher than those for J71:SeCT-IC (10.89%) and J71:CSeT-IC (8.52%). Our results demonstrate that the acceptor with selenophene in the outermost position led to significantly enhance the PCE. More importantly, rational modulation of the central fused core in combination with the conjugated skeleton isomeric method and the atom regioisomeric method provides an effective way to understand the structure-crystallinity-photovoltaic property relationship of selenophene-based regioisomers.

Molecular engineering of indenoindene-3-ethylrodanine acceptors with A2-A1-D-A1-A2 architecture for promising fullerene-free organic solar cells

Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1-IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528-2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λmax at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (Voc), PBDBT donor polymer was blended with acceptor chromophores, and a significant Voc (0.696-1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices.

Dodecacyclic-Fused Electron Acceptors with Multiple Electron-Deficient Units for Efficient Organic Solar Cells

Fused aromatic cores in non-fullerene electron acceptors (NFEAs) play a significant role in determining their optoelectronic properties and photovoltaic performance. In this work, a dodecacyclic-fused core with three electron-deficient units is synthesized through a double intramolecular Cadogan reduction cyclization. Terminal groups with different halogen substitution (F or Cl) are grafted onto the dodecacyclic-fused core to afford MS-4F and MS-4Cl, both of which showed strong and broad absorption, narrow bandgaps around 1.40 eV, and variable molecular packing model in pristine and blend films. Photovoltaic performance of solar cells containing MS-4F and MS-4Cl as NFEAs were investigated with resultant power conversion efficiencies (PCEs) of 11.75 % and 11.79 %, respectively. The mechanism study indicates that both of PBDB-T : MS-4F- and PBDB-T : MS-4Cl-based devices displayed high hole and electron mobility values, efficient charge transfer, and low charge recombination etc. These results indicate that designing multiple-fused aromatic cores with multiple electron-deficient units is a promising strategy to obtain high-performance NFEAs.

Emerging polymer electrodes for aqueous energy storage

New generation energy storage devices call for electrodes with high capacity, high cycling performance and environmental benignity. Polymer electrode materials (PEMs) are attractive for their abundant structural diversity and tunability as well as engineered conductivity, desirable processability and electrochemical properties for aqueous batteries. We herein overview the state-of-the-art development of PEMs for aqueous batteries, including conventional doped, redox-backbone, redox-pendant and hydrophilic conducting polymers. The merits and demerits of PEMs, and their structural modification and energy storage performance are discussed in detail. To provide a comprehensive understanding of polymer-based aqueous batteries, we correlate the molecular structures of PEMs with their conductivity, morphology and electrochemical behaviors. The review offers an insight into the rational design of conducting polymer electrodes for safe and cost-effective aqueous batteries.

High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions

Nonfullerene acceptors (MQ3, MQ5, MQ6) are synthesized using asymmetric and symmetric ladder-type heteroheptacene cores with selenophene heterocycles. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end-capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O⋅⋅⋅Se compared to that of O⋅⋅⋅S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport, leading to a higher power conversion efficiency (PCE) of 15.64 % compared to MQ3, which shows a PCE of 13.51 %. Based on the asymmetric heteroheptacene core, MQ6 shows an improved carrier transport induced by the reduced π-π stacking distance, related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. MQ6 exhibits a PCE of 16.39 % with a V_OC of 0.88 V, a FF of 75.66 %, and a J_SC of 24.62 mA cm^-2 .

A Wide Bandgap Polymer Donor Composed of Benzodithiophene and Oxime-Substituted Thiophene for High-Performance Organic Solar Cells

Oxime-substituted thiophene (TO) is used as an acceptor (A) unit to copolymerize with the benzodithiophene (BDT) donor (D) unit to form a novel D-A polymer donor, PBDTTO, which has a low-lying highest occupied molecular orbital energy level (E_HOMO) of -5.60 eV and a wide bandgap of 2.03 eV, forming complementary absorption and matching energy levels with the narrow bandgap nonfullerene acceptors. Organic solar cells using PBDTTO and Y6 as the donor and acceptor, respectively, exhibited a J_SC of 27.03 mA cm^-2, a V_OC of 0.83 V, and a fill factor of 0.59, reaching a high power conversion efficiency of 13.29%. The unencapsulated devices show good long-term stability in ambient air. Compared with the acceptor monomers used in other high-performance BDT-based D-A polymer donors, which are synthesized tediously in low yields, the TO acceptor monomer can be conveniently synthesized in only two steps with a high overall yield of 70%. These results demonstrate that TO unit can be used as a promising acceptor unit for developing BDT-based D-A polymer donors at low cost while maintaining high photovoltaic performance.

A Simple Dithieno[3,2-b:2',3'-d]pyrrol-Rhodanine Molecular Third Component Enables Over 16.7% Efficiency and Stable Organic Solar Cells

Organic solar cells (OSCs) can achieve greatly improved power conversion efficiency (PCE) by incorporating suitable additives in active layers. Their structure design often faces the challenge of operation generality for more binary blends. Herein, a simple dithieno[3,2-b:2',3'-d]pyrrole-rhodanine molecule (DR8) featuring high compatibility with polymer donor PM6 is developed as a cost-effective third component. By employing classic ITIC-like ITC6-4Cl and Y6 as model nonfullerene acceptors (NFAs) in PM6-based binary blends, DR8 added PM6:ITC6-4Cl blends exhibit significantly promoted energy transfer and exciton dissociation. The PM6:ITC6-4Cl:DR8 (1:1:0.1, weight ratio) OSCs contribute an exciting PCE of 14.94% in comparison to host binary devices (13.52%), while PM6:Y6:DR8 (1:1.2:0.1) blends enable 16.73% PCE with all simultaneously improved photovoltaic parameters. To the best of the knowledge, this performance is among the best for ternary OSCs with simple small molecular third components in the literature. More importantly, DR8-added ternary OSCs exhibit much improved device stability against thermal aging and light soaking over binary ones. This work provides new insight on the design of efficient third components for OSCs.

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells

Side-chain engineering on nonfullerene acceptors (NFAs) is crucial for modulating their solubility and crystallinity as well as packing behaviours in active layers to pursue high-performance organic solar cells (OSCs). High weight ratios of side chains are generally used by NFAs for the desired device efficiencies. Side-chain economy has seldom been discussed despite increased cost and difficulties in synthesis when optimizing the molecular design. Herein, we introduce 7H-dibenzo[c,g]carbazole (DCB) as an electron-donating core to design unfused-ring acceptors (UFAs) with a dramatically low weight ratio of side chains. DCB-4F has thus been designed and compared with the carbazole cored analogue (CB-4F). The unique conformation of the DCB core endows DCB-4F with higher solubility (8.2 mg mL^-1 in chloroform) compared to CB-4F (2.2 mg mL^-1) when using the same side chains. Featuring a lowest unoccupied molecular orbital (LUMO) level of -3.86 eV and an optical bandgap of 1.55 eV, the DCB-4F film exhibits an absorption profile (maximum 667 nm) complementary to polymer donor PM6. The PM6:DCB-4F as-cast OSCs deliver a power conversion efficiency (PCE) of 9.56% with a high open-circuit voltage (V_OC) of 1.00 V. By adding 10 wt% PC₇₁BM into the casting solutions, a greatly improved PCE of 11.17% is readily achieved, which is one of the highest PCEs for as-cast single-junction UFA-based devices. The PM6:DCB-4F based blends show homogeneous nano-fiberous morphology and higher hydrophobicity. The design of conformation-tuned NFAs using sterically hindered DCB-like cores is promising to achieve highly efficient as-cast OSCs.

Synergistic Interplay between Asymmetric Backbone Conformation, Molecular Aggregation, and Charge-Carrier Dynamics in Fused-Ring Electron Acceptor-Based Bulk Heterojunction Solar Cells

Asymmetric fused-ring electron acceptors (a-FREAs) have proved to be a promising type of electron acceptor for high-performance organic solar cells (OSCs). However, the relationship among molecular structures of a-FREAs and their nanoscale morphology, charge-carrier dynamics, and device performance remains unclear. In this contribution, two FREAs differing in conjugated backbone geometry with an asymmetric conformation (IPT-2F) or symmetric one (INPIC-2F) are selected to systematically explore the superiorities of the asymmetric conformation. Despite the frailer extinction coefficient and weaker crystallinity, IPT-2F shows stronger dipole interactions in the asymmetrical backbone, which would induce a closer lamellar packing than that of the symmetrical counterpart. Using PBDB-T as the electron donor, the IPT-2F-based OSCs achieve the best power conversion efficiency of 14.0%, which is ca. 67% improvement compared to the INPIC-2F-based ones (8.37%), resulting from a simultaneously increased short-circuited current density (J_sc) and fill factor. Systematical investigations on optoelectronic and morphological properties show that the asymmetric conformation-structured IPT-2F exhibits better miscibility with the polymer donor to induce a favorable blend ordering with small domain sizes and suitable phase separation compared to the INPIC-2F symmetric molecule. This facilitates an efficient charge generation and transport, inhibits charge-carrier recombination, and promotes valid charge extraction in IPT-2F-based devices.

Evaluating the nature of the vertical excited states of fused-ring electron acceptors using TD-DFT and density-based charge transfer

Acceptor-donor-acceptor structured fused-ring electron acceptors (FREAs) are the most efficient electron acceptors used in organic solar cells. We use density functional theory (DFT), its time-dependent version (TD-DFT), and an intra-molecular charge transfer index to evaluate the nature of the excited states of FREAs. Typically, several efficient electronic transitions contribute to the absorption spectra of FREAs. An investigation of every efficient electronic transition of each FREA is performed based on the electronic density variation in the donor and acceptor moieties of the molecules upon absorbing solar photons. Not all these transitions are equivalent for light-to-electricity conversion. The first transition contributes the most to the absorption spectra. This transition is intense and extremely efficient for light-to-electricity conversion, giving a higher value of intra-molecular charge transfer. For certain effective transitions of FREAs, the phenyl rings in the donor unit behave as the electron-donating units, such as IDT-NTI-2EH, BTCN-M, and MeIC. The foremost finding of the present research work is that the furthermost strong electronic transitions are not essentially the most effective ones for the conversion of sunlight into electricity.

The Role of Demixing and Crystallization Kinetics on the Stability of Non-Fullerene Organic Solar Cells

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10^-23 and 2.0 × 10^-22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

Control over π-π stacking of heteroheptacene-based nonfullerene acceptors for 16% efficiency polymer solar cells

Nonfullerene acceptors are being investigated for use in polymer solar cells (PSCs), with their advantages of extending the absorption range, reducing the energy loss and therefore enhancing the power conversion efficiency (PCE). However, to further boost the PCE, mobilities of these nonfullerene acceptors should be improved. For nonfullerene acceptors, the π-π stacking distance between cofacially stacked molecules significantly affects their mobility. Here, we demonstrate a strategy to increase the mobility of heteroheptacene-based nonfullerene acceptors by reducing their π-π stacking distances via control over the bulkiness of lateral side chains. Incorporation of 2-butyloctyl substituents into the nonfullerene acceptor (M36) leads to an increased mobility with a reduced π-π stacking distance of 3.45 Å. Consequently, M36 affords an enhanced PCE of 16%, which is the highest among all acceptor-donor-acceptor-type nonfullerene acceptors to date. This strategy of control over the bulkiness of side chains on nonfullerene acceptors should aid the development of more efficient PSCs.

Enhancing Open-Circuit Voltage of High-Efficiency Nonfullerene Ternary Solar Cells with a Star-Shaped Acceptor

The ternary strategy has been widely used in high-efficiency organic solar cells (OSCs). Herein, we successfully incorporated a mid-band-gap star-shaped acceptor, FBTIC, as the third component into the PM6/Y6 binary blend film, which not only achieved a panchromatic absorption but also significantly improved the open-circuit voltage (V_OC) of the devices due to the high-lying lowest unoccupied molecular orbital (LUMO) of the FBTIC. Morphology characterizations show that star-shaped FBTIC molecules are amorphously distributed in the ternary system, and the finely tuned ternary film morphology facilitates the exciton dissociation and charge collection in ternary devices. As a result, the best PM6/Y6/FBTIC-based ternary OSCs achieved a power conversion efficiency (PCE) of 16.7% at a weight ratio of 1.0:1.0:0.2.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.