Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells(NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However,molecular electron acceptors with 2-dimensional(2 D) configuration and conjugation are seldom reported.Herein, we designed and synthesized a series of novel 2 D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2 D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76%was achieved for photovoltaic devices with PBDB-T as the donor and these 2 D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2 D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2 D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

Non-fullerene small molecule electron acceptors for high-performance organic solar cells

Fullerenes and their derivatives are important types of electron acceptor materials and play a vital role in organic solar cell devices. However, the fullerene acceptor material has some difficulties to overcome the intrinsic shortcomings, such as weak absorption in the visible range, difficulty in modification and high cost, which limit the performance of the device and the large-scale application of this type of acceptors. In recent years, non-fullerene electron acceptor material has attracted the attention of scientists due to the advantages of adjustable energy level, wide absorption, simple synthesis, low processing cost and good solubility. Researchers can use the rich chemical means to design and synthesize organic small molecules and their oligomers with specific aggregation morphology and excellent optoelectronic prop- erties. Great advances in the field of synthesis, device engineering, and device physics of non-fullerene acceptors have been achieved in the last few years. At present, non-fullerene small molecules based photovoltaic devices achieve the highest efficiency more than 13% and the efficiency gap between fullerenetype and non-fullerene-type photovoltaic devices is gradually narrowing. In this review, we explore recent progress of non-fullerene small molecule electron acceptors that have been developed and led to highefficiency photovoltaic devices and put forward the prospect of development in the future.

Enhancing Photovoltaic Performance of Nonfused-Ring Electron Acceptors via Asymmetric End-Group Engineering and Noncovalently Conformational Locks

By employing the asymmetric end-group engineering,an asymmetric nonfused-ring electron acceptor(NFREA)was designed and synthesized.Compared with the symmetric analogs(NoCA-17 and NoCA-18),NoCA-19 possesses broader light absorption range,more coplanarπ-conjugated backbone,and appropriate crystallinity according to the experimental and theoretical results.The organic solar cells based on J52:NoCA-19 exhibited a power conversion efficiency as high as 12.26%,which is much higher than those of J52:NoCA-17(9.50%)and J52:NoCA-18(11.77%),mainly due to more efficient exciton dissociation,better and balanced charge mobility,suppressed recombination loss,shorter charge extraction time,longer charge carrier lifetimes,and more favorable blend film morphology.These findings demonstrate the great potential of asymmetric end-group engineering in exploring low-cost and high-performance NFREAs.

Near-infrared non-fused electron acceptors for efficient organic photovoltaics

Developing narrow-bandgap organic semiconductors is important to facilitate the advancement of organic photovoltaics(OPVs). Herein, two near-infrared non-fused ring acceptors(NIR NFRAs), PTBFTT-F and PTBFTT-Cl have been developed with A-π_A-π_D-D-π_D-π_A-A non-fused structures. It is revealed that the introduction of electron deficient π-bridge(π_A) and multiple intramolecular noncovalent interactions effectively retained the structural planarity and intramolecular charge transfer of NFRAs, extending strong NIR photon absorption up to 950 nm. Further, the chlorinated acceptor, with the enlarged π-surface compared to the fluorinated counterpart, promoted not only molecular stacking in solid, but also the desirable photochemical stability in ambient, which are helpful to thereby improve the exciton and charge dynamics for the corresponding OPVs. Overall, this work provides valuable insights into the design of NIR organic semiconductors.

A Simple Building Block with Noncovalently Conformational Locks towards Constructing Low-Cost and High-Performance Nonfused Ring Electron Acceptors

Nonfused ring electron acceptors(NFREAs)have attracted much attention due to their concise synthetic routes and low cost.However,developing high-performance NFREAs with simple structure remains a great challenge.In this work,a simple building block(POBT)with noncovalently conformational locks(No CLs)was designed and synthesized.Single-crystal X-ray study indicated the presence of S…O NOCLs in POBT,thus enabling it to possess a coplanar conformation comparable to that of fused-ring CPT.Two novel NFREAs based on CPT and POBT were developed,namely TT-CPT and TT-POBT,respectively.Besides,TT-POBT possessed a smaller Stokes shift and a reduced reorganization energy compared with TT-CPT,indicating the introduction of S…O No CLs can enhance the molecular rigidity even if simplifying the molecular structure.As a result,the TT-POBT-based PSC device afforded an impressive power conversion efficiency of 11.15%,much higher than that of TT-CPT counterpart(7.03%),mainly resulting from the tighterπ-πstacking,improved and balanced charge transport,and more favorable film morphology.This work demonstrates the potential of the simple building block POBT with No CLs towards constructing low-cost and highperformance NFREAs.

Combination of S…N and S…Cl Noncovalently Conformational Locks for Constructing High-Planarity and Low-Cost Nonfused-Ring Electron Acceptors

Comprehensive Summary By employing thiazole and 4-chlorothiazole as the A′units,two A-D-A′-D-A type nonfused-ring electron acceptors(NFREAs)Tz-H and Tz-Cl were designed and synthesized.Replacing thiazole in Tz-H with 4-chlorothiazole can not only remarkably shorten the synthetic route through C—H direct arylation but also enhance molecular planarity with the simultaneous incorporation of S…N and S…Cl noncovalently conformational locks(NoCLs).The photovoltaic devices based on PM6:Tz-Cl exhibited a power conversion efficiency as high as 11.10%,much higher than that of PM6:Tz-H(6.41%),mainly due to more efficient exciton dissociation,better and more balanced carrier mobility,less charge recombination,and more favorable morphology.These findings demonstrate the great potential of NoCLs in achieving low-cost and high-performance NFREAs.

Latest progress on fully non-fused electron acceptors for high-performance organic solar cells

Benefitting from the development of non-fullerene acceptors(NFAs),remarkable advances have been achieved with the power conversion efficiency(PCE)exceeding 19%over the last few years.However,the major achievement comes from fused ring electron acceptors(FREAs)with complex structures,leading to high cost.Hence,it is urgent to design new materials to resolve the cost issues concerning basic commercial requirements of organic solar cells.Recently,great progress has been made in fully non-fused ring electron acceptors(NFREAs)with only single-aromatic ring in the electron-donating core,which might achieve a fine balance between the efficiency and cost,thus accelerating the commercial application of organic solar cells.Therefore,this article summarizes the recent advances of fully NFREAs with efficiency over 10%,which may provide a guidance for developing the cost-effective solar cells.

Carbazolebis(thiadiazole)-core based non-fused ring electron acceptors for efficient organic solar cells

Non-fused ring electron acceptors(NFREAs)have a broad application prospect in the commercialization of organic solar cells(OSCs)due to the advantages of simple synthesis and low cost.The selection of intermediate block cores of non-fused frameworks and the establishment of the relationship between molecular structure and device performance are crucial for the realization of high-performance OSCs.Herein,two A-D-A’-D-A type NFREAs namely CBTBO-4F and CBTBO-4Cl,constructed with a novel electron-deficient block unit N-(2-butyloctyl)-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole(CBT)and bridging unit 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene(DTC)coupling with different terminals(IC-2F/2Cl),were designed and synthesized.The two NFREAs feature broad and strong photoresponse from 500 nm to 900 nm due to the strong intramolecular charge transfer characteristics.Compared with CBTBO-4F,CBTBO-4Cl shows better molecular planarity,stronger crystallinity,more ordered molecular stacking,larger van der Waals surface,lower energy level and better active layer morphology,contributing to much better charge separation and transport behaviors in its based devices.As a result,the CBTBO-4Cl based device obtains a higher power conversion efficiency of 10.18%with an open-circuit voltage of 0.80 V and a short-circuit current density of 21.20 mA/cm^(2).These results not only demonstrate the great potential of CBT,a new building block of the benzothiazole family,in the construction of high-performance organic conjugated semiconductors,but also suggest that the terminal chlorination is an effective strategy to improve device performance.

Enhancing Photovoltaic Performance of Ladder-Type Heteroarene-Based Electron Acceptors by Modulating Molecular Packing

Comprehensive Summary,The development of novel building blocks with sp3-hybridized-carbon-free conjugated skeletons is important to further advance and enrich nonfullerene acceptors(NFAs),but this remains a challenge due to the lack of strategies to effectively modulate the aggregation behavior of resulting NFAs.Herein,two novel nitrogen-bridged octacyclic ladder-type heteroarenes end-capped with thiophene rings(BTPS)or selenophene rings(BTPSe)are designed and synthesized as the donor cores for constructing NFAs(MQX-2 and MQX-4).It is found that replacing the sulfur atoms(MQX-2)at the outer positions of the heteroarene core with selenium atoms(MQX-4)can effectively modulate the molecular packing mode of the NFAs.The incorporation of selenium atoms induces stronger O···Se noncovalent interaction than O···S,thus promoting the formation of mixed H/J-type aggregates in MQX-4.Benefiting from more electron hopping channels,MQX-4 exhibits higher electron transport(more than 1-fold enhancement)and photovoltaic properties compared to MQX-2,which forms only H-type aggregates.

Design and Synthesis of N-Alkylaniline-Substituted Low Band-Gap Electron Acceptors for Photovoltaic Application

A novel electron donating unit,namely N-octyl-N-phenyl-thiophene(OPT),was designed in preparing electron acceptors with non-fused ring chemical structures.By introducing different functional atoms/groups into the para-position of phenyl in the OPT units,three non-fused ring acceptors(NFREAs),C8-2F,FC8-2F and MeC8-2F,were synthesized.The absorption spectrum of the three acceptors can be extended to about 950 nm with band-gaps of 1.28—1.32 eV due to the strong electron donating ability of OPT.The frontier molecular orbital distribution of OPT based molecules obtained by quantum chemistry calculation results reveals that their energy alignment can be finely tuned to meet different requirements.Moreover,by changing the substituents on the OPT units,their Flory-Huggins interaction parameter(χ)with the donor will be greatly influenced and different phase separation behavior can be accomplished.After blended with PBDB-TF,the FC8-2F-based cell yields short circuit current density(J_(sc))of 23.21 mA·cm^(-2),fill factor(FF)of 72.11%and the highest power conversion efficiency(PCE)of 12.42%.This work provides a new pathway for molecular design of new NFREAs,and demonstrates the application potential of OPT unit in realizing low band-gap photovoltaic materials.

Realizing compact three-dimensional charge transport networks of asymmetric electron acceptors for efficient organic solar cells

Asymmetry has been demonstrated an effective approach in recent years to tune the structural and energetic orders of nonfullerene electron acceptors(NFAs)to prepare efficient organic solar cells(OSCs).In this article,five asymmetric NFAs,namely C9BTP-BO-Th Cl-2F,C9BTP-BO-Cl-2F,C9BTP-BO-2Cl-2F,C7BTP-BO-2Cl-2F and C5BTP-BO-2Cl-2F possessing varied asymmetric end-groups and alkyl chains are synthesized to tune the charge transport networks formed within these NFAs.We found that the enhanced planarity in the asymmetric NFA can facilitate closerπ-πstacking distance in either the A-to-A or A-toD type NFA dimers,whilst the larger dipole moment can promote the formation of three-dimensional(3D)charge transport networks among NFAs.Taking those advantages,C7BTP-BO-2Cl-2F exhibit a compact 3D honeycomb network with a high packing coefficient of 72.1%and molecular packing density of 0.48 g/cm^(3),contributing to a superior power conversion efficiency of 18.0%when employing PM6 as the donor,with an open-circuit voltage of 0.85 V,short-circuit current of26.7 m A cm^(-2)and fill factor of 79.3%.Our work provides guidelines in engineering the end group and side chains of asymmetric NFAs to achieve compact charge transport networks for high efficiency OSCs.

From Perylene Dimide Polymers to Fused-Ring Electron Acceptors:A 15-Year Exploration Journey of Nonfullerene Acceptors

Fullerene derivatives are classic electron acceptor materials for organic solar cells (Oscs) but possess some intrinsic drawbacks such as weak visiblelightabsorption,limitedoptoelectronic property tunability,dificult purification and photochemical/morphological instability.Fullereneacceptors area bottleneck restricting further development of this field. Ourgroup pioneered the exploration of novel nonfulerene acceptors in China in 2006,andinitiated the research of two representative acceptor systems, rylene dimide polymer and fused-ring electron acceptor (FREA).FREA breaks the theoreticalefficiencylimit of fullerene-based OsCs (-13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heraldinga nonfullerene era for OsCs.In this review, we revisit 15-year nonfullerene exploration journey,summarize the design principles,molecular engineeringstrategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible researchtopics in the nearfuture.

Recent progress in low-cost noncovalently fused-ring electron acceptors for organic solar cells

The power conversion efficiencies(PCEs)of organic solar cells(OSCs)have improved considerably in recent years with the development of fused-ring electron acceptors(FREAs).Currently,FREAs-based OSCs have achieved high PCEs of over 19%in single-junction OSCs.Whereas the relatively high synthetic complexity and the low yield of FREAs typically result in high production costs,hindering the commercial application of OSCs.In contrast,noncovalently fused-ring electron acceptors(NFREAs)can compensate for the shortcomings of FREAs and facilitate large-scale industrial production by virtue of the simple structure,facile synthesis,high yield,low cost,and reasonable efficiency.At present,OSCs based on NFREAs have exceeded the PCEs of 15%and are expected to reach comparable efficiency as FREAs-based OSCs.Here,recent advances in NFREAs in this review provide insight into improving the performance of OSCs.In particular,this paper focuses on the effect of the chemical structures of NFREAs on the molecule conformation,aggregation,and packing mode.Various molecular design strategies,such as core,side-chain,and terminal group engineering,are presented.In addition,some novel polymer acceptors based on NFREAs for all-polymer OSCs are also introduced.In the end,the paper provides an outlook on developing efficient,stable,and low-cost NFREAs for achieving commercial applications.

Ethynyl-linked perylene bisimide based electron acceptors for non-fullerene organic solar cells

In this work, star-shaped perylene bisimide（PBI） derivatives with spiro-aromatic cores linked with ethynyl units were developed as electron acceptors for non-fullerene organic solar cells. The ethynyl linkers were found to enhance the planarity of the conjugated backbone, resulting in high electron mobilities and near-infrared absorption. The ethynyl-linked PBI acceptors showed high power conversion efficiencies（PCEs） up to 4.27% due to the high short-circuit current density（Jsc） of 8.52 mA/cm^2 and fill factor（FF） of 0.59, while the PBI acceptor without ethynyl units provided a low PCE of 3.57% in nonfullerene solar cells. The results demonstrate that ethynyl units can be applied into designing new PBI electron acceptors with improved charge transport properties and photovoltaic performance.

Effects of Thieno[3,2-b]thiophene Number on Narrow-Bandgap Fused-Ring Electron Acceptors

We synthesize and compare four near-infrared absorbing fused-ring electron acceptors named as nTTIC(n=2,3,4,and 5),based on different number of thieno[3,2-b]thiophene(TT)unit as the electron-donating core.With increasing the TT unit,absorption spectrum of the TTIC series red shifts,and the highest occupied molecular orbital(HOMO)upshifts notably.It is worth noting that 4TTIC and 5TTIC exhibit absorption edges approaching 1100 nm,which is the photoresponse limit of solar cells based on crystal silicon.When the TTIC series acceptors are blended with polymer donor PM6,the binary-blend organic solar cells based on 3TTIC show the best power conversion efficiency(PCE)of 13.1%.In contrast,2TTIC-based devices exhibit relatively lower PCE of 8.32%,mainly caused by the larger energy loss and blue-shifted absorption.Due to insufficient driving force of charge separation caused by very high HOMO,4TTIC and 5TTIC show poor PCEs lower than 3%.

Polymer Electron Acceptors Based on Fluorinated Isoindigo Unit for Polymer Solar Cells

Most of efficient polymer electron acceptors for polymer solar cells （PSCs） are based on naphthalene diimide or perylene diimide as the electron deficient building block. In this paper, for the first time, we report polymer electron acceptors based on fluorinated isoindigo IF-lID） as the electron deficient building block. We synthesized two polymer electron acceptors consisting of alternating F-lID unit and thiophene/selenophen unit, They show low-lying LUMo/HOMO energy levels of-3.69/-5.69 eV, high electron mobilities of 1.31 ×10-5 cm2.V-1s-1 and broad absorption spectra with the optical bandgap of 1.61 eV. PSC devices using the two F-liD-based polymers as polymer electron acceptors show encouraging power conversion efficiencies （PCEs） of up to 1.50% with an open-circuit voltage （Voc） of 0.97V, a short-circuit current density （Jsc） of 2.91 mA.cm2, and a fill factor （FF） of 53.2%. This work suggests a new kind of polymer electron acceptors based on F-IID unit.

High-Efficiency and Low-Energy-Loss Organic Solar Cells Enabled by Tuning Conformations of Dimeric Electron Acceptors

Dimeric fused-ring electron acceptors(DFREAs)have attracted much attention due to the combined advantages of their monomeric and polymeric acceptors,including a well-defined molecular structure,excellent repeatability,and stable morphology.However,the additionally introduced single-bonds during dimerization may result in a twisted backbone of DFREAs,which is detrimental to intermolecular packing and charge transport.Herein,three DFREAs are designed and synthesized,in which DFREA conformations were systematically tuned via adjusting the intensities of intramolecular noncovalent interactions(INIs)to achieve high-performance organic solar cells(OSCs).Theoretical and experimental results show that the gradual introduction of S…F INIs can continuously improve molecular planarity and rigidity,resulting in reduced reorganization energies,ordered packing mode,and enhanced crystallization of DFREAs.Benefiting from the incorporation of fourfold S…F INIs,DYF-TF-based binary OSCs show a record high efficiency of 18.26%with an extremely low energy loss(0.493 eV)for DFREAbased OSCs.In addition,DYF-TF-based OSCs exhibited good long-term stability with a T_(80%)lifetime of 2681 h,and the power conversion efficiency of the DYF-TF-based ternary device is further enhanced to 18.73%.This contribution demonstrates the great potential of the INIs strategy in achieving excellent DFREAs materials.

Pomelo biochar as an electron acceptor to modify graphitic carbon nitride for boosting visible-light-driven photocatalytic degradation of tetracycline

In this study,biochar(BC)derived from pomelo was prepared via a high-temperature calcination method to modify the graphitic carbon nitride(g-C_(3)N_(4))to synthesize the BC/g-C_(3)N_(4)composite for the degradation of the tetracycline(TC)antibiotic under visible light irradiation.The experimental results exhibit that the optimal feeding weight ratio of biochar/urea is 0.03:1 in BC/g-C_(3)N_(4)composite could show the best photocatalytic activity with the degradation rate of tetracycline is 83%in 100 min irradiation.The improvement of photocatalytic activity is mainly attributed to the following two points:(i)the strong bonding with π-π stacking between BC and g-C_(3)N_(4)make the photogenerated electrons of light-excited g-C_(3)N_(4)transfer to BC,quickly and improve the separation efficiency of carriers;(ii)the introduction of BC reduces the distance for photogenerated electrons to migrate to the surface and increases the specific surface area for providing more active sites.This study provides a sustainable,economical and promising method for the synthesis of photocatalytic materials their application to wastewater treatment.

PHOTOINDUCED CHARGE TRANSFER POLYMERIZATION OF STYRENE INITIATED BY ELECTRON ACCEPTOR

Photoinduced charge transfer polymerization of styrene(St) with electron acceptor as initiator was investigated. In case of fumaronitrile (FN) or maleic anhydride (MA) as initiator the polymerization takes place regularly, whereas the tetrachloro-1,4-benzenequinone (TCQ), 2,3-dichloro-5, 6-dicyano-1, 4-benzenequinone (DDQ). or tetracyano ethylene (TCNE) as initiator the polymerization proceeds reluctantly only after the photoaddition reaction. A mechanism was proposed that free radicals would be formed following the charge and proton transfer in the exciplex formed between St and electron accepters.

Theoretical Studies on the Intramolecular Electron Transfer in Some Aromatic Systems

In this work, the excited states of the title compounds are calculated by using time-dependent density functional theory (TDDFT). Firstly, the variation of the sequences of intramolecular charge local excitation (LE) and intramolecular charge transfer (CT) in the gas phase is studied by changing electron donors or acceptors. Then, the effects of Polarized Continuum Model (PCM) and Self-consistent Isodensity Polarized Continuum Model (SCI-PCM) in solvents on the calculated results are discussed. It is shown that the calculated results obtained by the PCM are in more agreement with the experimental data. Finally, the excited states of compounds in different solvents were calculated by PCM. Compared with the results obtained in gas phase, the absorption spectra of the CT state in the solvents have a larger red shift; however, the spectra for the LE state are hardly changed.