High-performance Nonfullerene Acceptors Research Articles

Computational method on highly efficient D-π-A-π-D-based different molecular acceptors for organic solar cells applications and non-linear optical behaviour

Eight molecular structures (BT-A1 to BT-A8) with high-performance non-fullerene acceptor (NFA) were selected for organic solar cells (OSCs) and non-linear optical (NLO) applications. Their electronic, photovoltaic (PV) and optoelectronic properties were tuned by adding powerful electron-withdrawing groups to the acceptor (A) of the D-π-A-π-D structure. Using time-dependent density functional theory (TD-DFT) techniques, based on the laws of quantum chemical calculations, the absorption spectra, stability of the highest and lowest-energy molecular orbitals (HOMO/LUMOs), electron density, intramolecular charge transfer (ICT), transition density matrix (TDM), were examined. The binding energy (Eb) and density of states (DOS) were probed to realize the optoelectronic analysis of the structures BT-A1 to BT-A8. Noncovalent interactions (NCIs) based on a reduced density gradient (RDG) were used to describe the nature and strength of D-A interactions in the molecules BT-A1 to BT-A8. The new refined molecules BT-A1 to BT-A8 exhibited strong absorbance bands between 408–721 nm and high electron transfer contribution (ETC) ranges between 87–96 %, along with the smallest excitation energies (Ex) between 1.71–3.55 eV in the solvent dichloromethane. Dipolar moment strengths ranging from 0.38 to 4.72 Debye in both the excited and ground states have determined with good solubility properties of BT-A1 to BT-A8 in polar solvent. Highly effective charge mobilities and prevention of charge recombination have been demonstrated by the electron (0.18–0.41 eV) and hole RE values (0.13–0.89 eV) for the new compounds. Power conversion efficiencies (PCE) of BT-A1 to BT-A8 were nearly the same because of better outcomes compared to the molecules in the BT. Compared to poly[4.8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b']dithiophene-2,6- diyl-alt-(4–2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2–6-diyl)] (PTB7-Th), the open circuit voltages (Voc) of compounds BT-A1 to BT-A8 were ranged from 1.52 to 2.13 eV. The polarizability (α) and hyperpolarizability (β) of the molecules BT-A1 to BT-A8 were used to determine the non-linear optical (NLO) properties. The results showed that BT-A2, BT-A6 and BT-A7 have good NLO activity. This computational analysis demonstrates the superiority of the molecules with NFA. Hence the compounds are advised for the use in production of high-performance OSCs and NLO activity.

The effects of side chain engineering on the morphology and charge transport of the A-DA1D-A type of non-fullerene acceptor: a multiscale study.

The appearance of the A-DA1D-A type of non-fullerene acceptor Y6 and its derivatives significantly improves the power conversion efficiency of organic solar cells. However, the effects of the modulation of the side chains of Y6 on its morphology and charge transport in organic thin films are still not well understood. In this work, we have systematically studied the effects of symmetric modifications of the length of alkyl side chains and the types, such as branched or straight alkyl chains, and the introduction of heteroatoms to side chains on these properties. A multiscale study, including density functional theory and classical molecular dynamics simulations, has been used to answer this open question. We find that face-on configurations are generally dominant for the AA, A1A1, and DD stacking of molecular pairs. With respect to prototype Y6, the introduction of oxygen atoms to outer alkyl side chains could enhance AA stacking but worsen the electrical network and enlarge the reorganization energy during electron transfer, and changing outer side straight alkyl chains to branched chains ruins π-π stacking of all units significantly. Finally, we discover that shortening outer alkyl side chains appropriately or changing inner branched chains to straight chains with the same number of carbon atoms is a good strategy to improve the molecular π-π stacking and electron mobility of Y6 while changing outer straight side chains to branched chains or introducing oxygen atoms to outer straight chains is the opposite. This study provides a new insight into the relationship between morphology and electron mobility and will be helpful for the design of future high-performance non-fullerene acceptors.

The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells

The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells

Effect of Steric Hindrance at the Anthracene Core on the Photovoltaic Performance of Simple Nonfused Ring Electron Acceptors

Solving the contradiction between good solubility and dense packing is a challenge in designing high-performance nonfullerene acceptors. Herein, two simple nonfused ring electron acceptors (o-AT-2Cl and m-AT-2Cl) carrying ortho- or meta-substituted hexyloxy side chains can be facilely synthesized in only three steps. The two ortho-substituted phenyl side chains in o-AT-2Cl cannot freely rotate due to a big steric hindrance, which endows the acceptor with good solubility. Moreover, o-AT-2Cl displays a more ordered packing than m-AT-2Cl as revealed by the absorption measurement. When blended with polymer donor D18 for the fabrication of organic solar cells (OSCs), o-AT-2Cl-based devices exhibit a favorable morphology, more efficient exciton dissociation, and better charge transport. Consequently, the optimal OSCs based on D18:o-AT-2Cl exhibit a power conversion efficiency (PCE) of 12.8%, which is significantly higher than the moderate PCE (7.66%) for D18:m-AT-2Cl-based devices. Remarkably, o-AT-2Cl shows a higher figure-of-merit value compared with classic high-efficiency fused ring electron acceptors. As a result, our research succeeds in obtaining nonfused ring acceptors with cost-effective photovoltaic performance and provides a valuable experience for simultaneously improving solubility as well as ensuring ordered packing of acceptors through regulating the steric hindrance via changing the position of substituents.

Y-Type Non-Fullerene Acceptors with Outer Branched Side Chains and Inner Cyclohexane Side Chains for 19.36% Efficiency Polymer Solar Cells.

Raising the lowest unoccupied molecular orbital (LUMO) energy level of Y-type non-fullerene acceptors can increase the open-circuit voltage (Voc ) and thus the photovoltaic performance of the current top performing polymer solar cells (PSCs). One of the viable routes is demonstrated by the successful Y6 derivative of L8-BO with the branched alkyl chains at the outer side. This will introduce steric hindrance and reduce intermolecular aggregation, thus open up the bandgap and raise the LUMO energy level. To take further advantages of the steric hindrance influence on optoelectronic properties of Y6 derivatives, two Y-type non-fullerene acceptors of BTP-Cy-4F and BTP-Cy-4Cl are designed and synthesized by adopting outer branched side chains and inner cyclohexane side chains. An outstanding Voc of 0.937V is achieved in the D18:BTP-Cy-4F binary blend devices along with a power conversion efficiency (PCE) of 18.52%. With the addition of BTP-eC9 to extend the absorption spectral coverage, a remarkable PCE of 19.36% is realized finally in the related ternary blend devices, which is one of the highest values for single-junction PSCs at present. The results illustrate the great potential of cyclohexane side chains in constructing high-performance non-fullerene acceptors and their PSCs.

The impact of linker on the photovoltaic performance of helical perylene diimide based non-fullerene acceptors

Linking two or more perylene diimide (PDI) directly together and/or with congested spacers at the N-position, α- or bay-region is a robust strategy for constructing high-performance non-fullerene acceptors (NFAs). On the other hand, ethylene fused PDI dimer (PDI2) has emerged as alternative building blocks to the parent PDI in constructing NFAs owing to the built-in twisted configuration, outstanding optoelectronic properties, concise gram-scale synthesis and so forth in recent years. In line with the parent PDI, tailoring PDI2 by dimerization, trimerization as well as by introduction of heteroatoms can also result in enhanced photovoltaic performance. However, to the best of our knowledge, most of the modifications occurred at the bay region of PDI2. In this contribution, two PDI2 dimers namely P2–P2, P2–C6–P2 are constructed by using hydrazine and 1,6-hexanediamine as linkers at imide positions. The photovoltaic performance of these two NFAs as well as the parent PDI2 were evaluated using commercially available PM6 as the electron donor. The two PDI2 dimers show superior photovoltaic performances compared with PDI2 and P2–C6–P2 gives the best power conversion efficiency (PCE) of 8.03% among these three NFAs. These results argue that modification at the imide groups is also a robust strategy for constructing high-performance PDI2-based NFAs and the choice of linkers plays a vital role in device performance.

Influence of twisted or planar linkers on the performance of quasi-two-dimensional fused perylene diimide acceptors for organic solar cells

Quasi-two-dimensional (quasi-2D) fused perylene diimide building block is effective for constructing high-performance non-fullerene acceptors, while the exploration of their structure-property relationship remains critical, yet challenging. Herein, two quasi-2D non-fullerene acceptors (FPDI-V and FPDI-E) featuring vinylene or ethynylene linker were developed to reveal the effects of molecular geometry on photophysics, morphology and device performances. We observed that such minor chemical variations can lead to distinct difference on absorption spectra, energy levels, and the photovoltaic properties, but only have slight effects on crystallinity, molecular packing and film morphology. The resulting device based on PTB7-Th:FPDI-V delivered a power conversion efficiency of 7.53%, which is higher than those of FPDI-E based device (6.37%). The improved photovoltaic performance of FPDI-V with a twisted vinylene linker should come from the enhanced absorption coefficient, upshifted molecular energy levels, higher exciton dissociation rate, and weaker charge recombination. This result provides an updated idea for further designing and structural optimizing of quasi-2D non-fullerene acceptors.

PBDB-T-Based Binary-OSCs Achieving over 15.83% Efficiency via End-Group Functionalization and Alkyl-Chain Engineering of Quinoxaline-Containing Non-Fullerene Acceptors.

Molecular backbone modification, alkyl-chain engineering, and end-group functionalization are promising strategies for developing efficient high-performance non-fullerene acceptors (NFAs). Herein, two new NFAs, named TPQ-eC7-4F and TPQ-eC7-4Cl, are designed and synthesized. Both molecules have linear octyl chains on fused quinoxaline-containing heterocyclics as the central backbone and difluorinated (2F)/dichlorinated (2Cl) 1,1-dicyanomethylene-3-indanone (IC) as the end-group units. The influences of alkyl-chains on fused quinoxaline backbone and different halogenated end-groups on optical, electrochemical, and photovoltaic performances of organic solar cells (OSCs) are studied. In comparison with TPQ-eC7-4Cl, TPQ-eC7-4F exhibits blue-shifted absorptions with higher molar extinction coefficients in the film state as well as in the donor/acceptor (D/A) blend film state and up-shifting lowest unoccupied molecular orbital (LUMO) energy level. As a result, the OSC devices based on the PBDB-T:TPQ-eC7-4F display an outstanding power conversion efficiency (PCE) of 15.83% with a simultaneously increased open-circuit voltage (Voc) of 0.85 V, a short-circuit current-density (Jsc) of 25.89 mA cm-2, and a fill factor (FF) of 72.20%, whereas the PBDB-T:TPQ-eC7-4Cl-based OSC device shows a decent PCE of 14.48% with a Voc of 0.84 V, a Jsc of 24.56 mA/cm2, and an FF of 69.77%. To the best of our knowledge, this is the highest photovoltaic performance of PBDB-T-based single-junction binary-OSCs. In comparison, ascribed to the high crystallinity and low solubility of BTP-eC7-4Cl, the corresponding PBDB-T:BTP-eC7-4Cl-based OSC device shows poor photovoltaic performance (PCE of 11.87%). The experimental results demonstrate that fine-tuning the fused quinoxaline backbone with alkyl-chain and end-group functionalization are promising strategies to construct high-performance NFAs for PBDB-T-based single-junction binary-OSCs.

Optimized vertical phase separation via systematic Y6 inner side-chain modulation for non-halogen solvent processed inverted organic solar cells

Organic solar cells (OSCs) have entered a new phase of development with the advent of a high-performance non-fullerene acceptor known as Y6. Considering that the molecular curvature shape of Y6 allows for dense intermolecular packing with a grid-like packing structure, the modification of branched alkyl chains located on the pyrrole motif of Y6 is expected to have a significant impact on morphological properties, such as molecular packing/orientation behavior and vertical phase separation. In this work, we fine-tune the branched alkyl chain attached to nitrogen atoms of the Y6 main backbone and systematically investigate the relationship between the morphological features and photovoltaic performance of OSCs with a PM6 donor polymer. The systematic alkyl chain engineering effectively reduces the aggregation of Y6-analogs and substantially improves solubility, thereby providing solution processability in a non-halogenated solvent. Additional in-depth analyses reveal that the modification of branched alkyl chains allows the formation of an optimized bulk heterojunction morphology, which is advantageous for an inverted device architecture. In particular, the PM6:Y6-HU-based inverted OSC utilizing o-xylene as a processing solvent achieves a power conversion efficiency of 17.4% with an outstanding fill factor of 77.9 % under the binary blend system and single non-halogen solvent processing.

Ultrafast Carrier Dynamics of Non-fullerene Acceptors with Different Planarity: Impact of Steric Hindrance.

Most high-performance non-fullerene acceptors are of the acceptor-donor-acceptor (A-D-A)-type structure. Under photoexcitation, the intramolecular charge transfer effect on the A-D-A framework results in a large dipole moment change, facilitating the efficient generation of charge carriers. Achieving more efficient intramolecular charge transfer by adjusting the molecular structure is one of the current research ideas. Recently, we found that the power conversion efficiency can be improved from 4.41 to 13.13% by tuning the planarity of the non-fused ring electron acceptor backbone through steric hindrance of lateral substituents. We found that the planar backbone can effectively improve the intramolecular charge transfer, which has a great influence on the power conversion efficiency of the device. Our results demonstrate that charge transfer dynamics can be controlled by optimizing steric hindrance, which plays a crucial role in the photovoltaic performance of organic solar cells.

High-performance non-fullerene acceptor inverted organic photovoltaics incorporating solution processed doped metal oxide hole selective contact

Inverted organic photovoltaics (OPVs) allow flexibility on designing a roll-to-roll production process of OPVs, providing technological opportunities. The OPV roll-to-roll production process demands thick and high-performance solution-based hole selective contacts. Here, we show that a solution processed antimony-doped tin oxide (ATO) hole selective contact produced by spray pyrolysis route exhibits exceptional optoelectronic properties and functionality within non-fullerene acceptor PM6:Y6:PC70BM inverted OPVs. The corresponding solution processed inverted OPVs provide high power conversion efficiency values when a thick hole selective contact of solution processed doped ATO is incorporated within the inverted OPV device structure and similar light stability to that achieved with the commonly used thermally evaporated MoO3 hole selective contact.

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.

Branched Alkoxy Side Chain Enables High-Performance Non-Fullerene Acceptors with High Open-Circuit Voltage and Highly Ordered Molecular Packing

Branched Alkoxy Side Chain Enables High-Performance Non-Fullerene Acceptors with High Open-Circuit Voltage and Highly Ordered Molecular Packing

Non-fused molecular photovoltaic acceptor with a planar core structure enabled by bulky and embracing-type side chains

Other than multi-ring fused or non-covalent lock strategies, the work demonstrates the use of bulky and embracing type sidechains to force non-fused rings staying in a coplanar configuration for high performance non-fullerene electron acceptors.

Effects of functionalization of Y6 end-groups with electron-withdrawing groups on the photovoltaic properties at the donor-acceptor interfaces of PM6/Y6 OSCs: A theoretical insight

Since the high performance non-fullerene acceptor Y6 appears, many organic solar cells (OSCs) based on it and its derivatives have been fabricated and studied. These OSCs show delicate changes of open circuit voltage (V OC ), short circuit density (J SC ), and fill factor (FF) with respect to prototype PM6/Y6 OSC. So far, the effects of functionalization of Y6 end-groups with electron-withdrawing groups on the photovoltaic properties at the donor-acceptor interfaces of PM6/Y6 OSC are still not well understood. In the present work, by modelling four OSCs based on symmetrical (Y6, Y6-4Cl, and Y6-4CN) and asymmetrical (Y6-2Cl) molecules, we find that for symmetrical Y6 and Y6-4Cl based OSCs, the computed V OC s and FFs obtained with reliable computational method show the same trend as experimental data, but the computational method used does not apply for asymmetrical Y6-2Cl based OSC. In addition, with respect to fluorination, chlorination generally would bring stronger simulated interfacial absorption spectrum, the smaller exciton binding energy of local excited state of monomers and charge transfer state at the interface, smaller singlet−triplet energy gap, the higher total oscillator strength of all CT states, and larger interfacial electrostatic potential difference. In contrast, PM6/Y6-4CN has relative much lower total oscillator strength of all CT states and predicted V OC and FF than prototype Y6/PM6 though the end-groups with CN substitutions have much stronger electron withdrawing ability. Our results gain insights into the donor-acceptor interfacial properties of OSCs based on Y6 and its derivatives. • The computed V OC s and FFs of PM6/Y6 and PM6/Y6-4Cl OSCs show exactly the same trends as experiments. • PM6/Y6-4CN seems not qualified as a promising acceptor due to low predicted FF and V OC . • For asymmetrical Y6-2Cl based OSC, new computational modelling and method should be developed for V OC and FF.

Increasing Photostability of Inverted Nonfullerene Organic Solar Cells by Using Fullerene Derivative Additives.

Organic solar cells (OSCs) recently achieved efficiencies of over 18% and are well on their way to practical applications, but still considerable stability issues need to be overcome. One major problem emerges from the electron transport material zinc oxide (ZnO), which is mainly used in the inverted device architecture and decomposes many high-performance nonfullerene acceptors due to its photocatalytic activity. In this work, we add three different fullerene derivatives-PC71BM, ICMA, and BisPCBM-to an inverted binary PBDB-TF:IT-4F system in order to suppress the photocatalytic degradation of IT-4F on ZnO via the radical scavenging abilities of the fullerenes. We demonstrate that the addition of 5% fullerene not only increases the performance of the binary PBDB-TF:IT-4F system but also significantly improves the device lifetime under UV illumination in an inert atmosphere. While the binary devices lose 20% of their initial efficiency after only 3 h, this time is increased fivefold for the most promising ternary devices with ICMA. We attribute this improvement to a reduced photocatalytic decomposition of IT-4F in the ternary system, which results in a decreased recombination. We propose that the added fullerenes protect the IT-4F by acting as a sacrificial reagent, thereby suppressing the trap state formation. Furthermore, we show that the protective effect of the most promising fullerene ICMA is transferable to two other binary systems PBDB-TF:BTP-4F and PTB7-Th:IT-4F. Importantly, this effect can also increase the air stability of PBDB-TF:IT-4F. This work demonstrates that the addition of fullerene derivatives is a transferable and straightforward strategy to improve the stability of OSCs.

Effects of Halogenated End Groups on the Performance of Nonfullerene Acceptors.

The end groups' halogenations among the nonfullerene acceptors (NFAs) were a very useful method to fabricate high-performance NFAs-based organic solar cells (OSCs). We report three high-performance NFAs, BTIC-4EO-4F, BTIC-4EO-4Cl, and BTIC-4EO-4Br. They all have a fused benzothiadiazole as the core unit and different dihalogenated end groups (IC-2F, IC-2Cl, and IC-2Br) as the terminal unit. Thanks to the improved intramolecular charge-transfer ability of the brominated NFAs, bromination is more effective than fluorination and chlorination in lowering the energy levels and red-shifting the absorption spectra of the resulting NFAs. When compared with the chlorinated and fluorinated counterparts, the BTIC-4EO-4Br blend films exhibit lower roughness, better phase separation size, and stronger face-on stacking. When blended with poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b']-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]]} (PBDB-TF) as the polymer donor material, the BTIC-4EO-4Br-based OSCs exhibit the highest power conversion efficiency (12.41%), with a higher current density and a higher open-circuit voltage than the BTIC-4EO-4Cl-based OSCs (11.29%) and BTIC-4EO-4F-based OSCs (10.64%). These results show that the bromination of the NFAs' electron-withdrawing end groups can also be very effective in constructing high-performance photovoltaic materials.

Efficient Organic Solar Cells from Molecular Orientation Control of M-Series Acceptors

<h2>Summary</h2> A "face-on" molecular orientation is essential for photovoltaic materials with efficient vertical carrier transport, but understanding how the molecular structures control their orientations remains challenging. Based on a ladder-type fused-ring core without <i>sp</i>³-hybridized bridging atoms, novel acceptor-donor-acceptor (A-D-A or ADA)-type nonfullerene acceptors (M3 and M32) are developed, and their molecular orientation behaviors are successfully controlled by the neighboring side chains. With linear n-cetyl chains on the nitrogen atoms, the acceptor (M32) tends to adopt an "edge-on" molecular orientation, whereas with bulky branched 2-hexyldecyl chains, the acceptor (M3) has a preferred "face-on" orientation. Blended with a donor polymer of PM6, M3 shows optimal phase separation and dramatically improved electron transport, consequently leading to a much higher device performance than M32. Further optimization of the M3-based devices yields an outstanding efficiency of 16.66%. The strategy of molecular orientation control proposed here will inspire many other innovative designs and syntheses of high-performance nonfullerene acceptors.

Silicon and oxygen synergistic effects for the discovery of new high-performance nonfullerene acceptors

In organic electronics, an aromatic fused ring is a basic unit that provides π-electrons to construct semiconductors and governs the device performance. The main challenge in developing new π-skeletons for tuning the material properties is the limitation of the available chemical approach. Herein, we successfully synthesize two pentacyclic siloxy-bridged π-conjugated isomers to investigate the synergistic effects of Si and O atoms on the geometric and electronic influence of π-units in organic electronics. Notably, the synthesis routes for both isomers possess several advantages over the previous approaches for delivering conventional aromatic fused-rings, such as environmentally benign tin-free synthesis and few synthetic steps. To explore their potential application as photovoltaic materials, two isomeric acceptor–donor–acceptor type acceptors based on these two isomers were developed, showing a decent device efficiency of 10%, which indicates the great potential of this SiO-bridged ladder-type unit for the development of new high-performance semiconductor materials.

Ladder-Type Heteroheptacenes with Different Heterocycles for Nonfullerene Acceptors.

The design, synthesis, and characterization of two novel nonfullerene acceptors (M8 and M34) based on ladder-type heteroheptacenes with different heterocycles are reported. Replacing the furan heterocycles with the thiophene heterocycles in the heteroheptacene backbone leads to a hypsochromically shifted absorption band and greatly improved carrier transport for the resulting nonfullerene acceptor (M34) although the π-π-stacking distances are barely affected. Bulk-heterojunction polymer solar cells fabricated from M34 and a wide band gap polymer (PM6) as the donor showed a best power conversion efficiency (PCE) of 15.24 % with an open circuit voltage (VOC ) of 0.91 V, much higher than a PCE of 4.21 % and a VOC of 0.83 V for the counterparts based on M8:PM6. These results highlight the importance of key atoms in the construction of high-performance nonfullerene acceptors.