Isomeric Fluorene-based Heteroundecenes with Different Side Chains Anchoring Positions for Small Molecule Acceptors

Two isomeric fluorene-based heteroundecenes of bis(thienocyclopenthieno[3,2-b]thieno)fluorene(BT2T-F)and bis(dithieno[3,2-b:2’,3’-d]thiophene)cyclopentafluorene(B3T-F)have been designed and synthesized.The side chains of 4-hexylphenyl anchor on the 5th and 8th positions in B3T-F while on the 4th and 9th positions in BT2T-F,in which the former is closer to the center of the fused ring.The corresponding acceptor-donor-acceptor(A-D-A)type small molecule acceptors(SMAs)of BT2T-FIC and B3T-FIC were prepared by linking BT2T-F and B3T-F as fused ring donor units with the acceptor unit of 2-(5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile(IC-2F),respectively.B3T-FIC presents a superior crystallinity with intense face-on π-π stacking in its neat film while BT2T-FIC is more disordered.When blended with PBDB-T-2Cl as a polymer donor,the optimized PBDB-T-2Cl:BT2T-FIC device exhibits an averaged power conversion efficiency(PCE)of 10.56%while only 7.53%in the PBDB-T-2Cl:B3T-FIC device.The improved short-circuit current(J_(sc))and fill factor(FF)of the PBDB-T-2Cl:BT2T-FIC device are the main contribution of its higher performance,which is attributed to its more efficient and balanced charge transport and better carrier recombination suppression.Given that BT2T-FIC blend and B3T-FIC blend films both take a preferential face-on orientatedπ-πstacking with comparable distances,the suitable SMA domain size obtained in the BT2T-FIC blend could account for its more efficient photovoltaic performance.These results highlight the importance of side-chain strategy in developing efficient SMAs with huge fused ring cores.

Diflurobenzothiadiazole core-based noncovalently fused small molecule acceptor exhibiting over 12% efficiency and high fill factor

The versatility and flexibility of organic photoelectric materials endow organic photovoltaic cells fine function modulation and huge commercial potential. In this work, a new noncovalent fused-ring small molecule acceptor(SMA) BID-4 F has been synthesized for high-efficient organic solar cells(OSCs). BID-4 F consists of a diflurobenzothiadiazole(DFBT) core, ladder-like indacenodithiophene(IDT) spacers, and dicyanoindanone electron-withdrawing end groups, which are supposed to be conformationally interlocked by noncovalent interactions, leading to good molecular planarity. In addition, compact solid state stacking was revealed by UV–vis–NIR absorption spectrum. The optimized PM6:BID-4 F based device delivers an eminent power conversion efficiency(PCE) of 12.30% with a high open-circuit voltage(Voc) of 0.92 V and a high fill factor(FF) of 74.3%. Most importantly, the PCE and FF are among one of the highest values reported for the OSCs based on the unfused-ring SMAs. Overall, our work demonstrates that the unfused ring central framework with high molecular planarity through noncovalent interactions provides a good strategy to construct highly efficient SMAs.

Synergetic Alkoxy Side-Chain and Chlorine-Contained End Group Strategy toward High Performance Ultra-Narrow Bandgap Small Molecule Acceptors

Ultra-narrow bandgap(ultra-NBG)small molecule acceptors(SMAs)show great potential in organic solar cells(OSCs)due to the extended near-infrared(NIR)absorption.In this work,a synergetic alkoxy side-chain and chlorine-contained end group strategy is employed to achieve A-DA'D-A type ultra-NBG SMAs by introducing alkoxy chains with oxygen atom at the second position into the thiopheneβposition as well as replacing the F atoms with Cl atoms in the end group.As a result,the heptacyclic BZO-4F shows a redshifted absorption onset(960 nm)compared with Y11(932 nm)without oxygen atoms in the side chains.Then,the fluorinated end groups are substituted with the chlorinated ones to synthesize BZO-4Cl.The absorption onset of BZO-4Cl is further redshifted to 990 nm,corresponding to an optical ultra-NBG of 1.25 eV.When blending with the polymer donor PBDB-T,the binary devices based on PBDB-T:BZO-4F and PBDB-T:BZO-4Cl deliver power conversion efficiencies(PCEs)over 12%.Furthermore,ternary devices with the addition of BZ4F-O-1 into PBDB-T:BZO-4Cl system achieve the optimal PCE of 15.51%.This work proposes a synergetic alkoxy side-chain and chlorine-contained end group strategy to achieve A-DA'D-A type ultra-NBG SMAs,which is important for future molecular design.

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

The development of A-DA′D-A type small molecule acceptors(SMAs)has promoted the rapid progress of polymer solar cells(PSCs)in recent years.The outer side chains on the terminal thiophene ring and inner side chains on nitrogen atoms of the pyrrole ring of the DA′D fused ring play important roles in the photovoltaic performance of the SMAs.Here,we synthesized two new SMAs,2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(4-(2-ethylhexyl)thiophen-2-yl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(T2EH)and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(3-(2-ethylhexyl)phenyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)(P2EH),with 2-ethylhexylβ-substituted thienyl or phenyl as the outer side chains,respectively,to improve the photovoltaic properties of the SMAs.Compared with P2EH,T2EH exhibits closerπ−πstacking,slightly red-shifted absorption,and higher electron mobility.Moreover,the active layer of T2EH blended with the low-cost polymer donor poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)](PTQ10)possesses higher mobilities,a longer lifetime,and less recombination of the charge carriers in comparison with that of the PTQ10:P2EH active layer.Eventually,the PTQ10:T2EH-based PSCs showed an outstanding power conversion efficiency(PCE)of 18.55%,while the PSC based on PTQ10:P2EH displayed a PCE of 17.50%.Importantly,18.55%is the highest PCE in the PTQ10-based binary PSCs so far.The results indicate that T2EH is one of the best SMAs for the PTQ10-based PSCs and is a promising SMA for the application of PSCs.

Introducing alkoxy groups as outer side chains and substituents ofπ-bridges obtains high-performance medium-bandgap polymerized small molecule acceptors

The medium-bandgap polymerized small molecule acceptors(PSMAs)have broad application scenarios.However,the effort in the molecular design of the high-performance medium-bandgap PSMAs is limited.In this article,we introduce alkoxy groups as outer side chains and as substituents of the thiopheneπ-bridges of the high-performance PSMA PY-IT to synthesize a mediumbandgap PSMA PO-TO.Due to the fact that the non-covalent interaction between the alkoxy groups and the terminal groups of the small molecule acceptor(SMA)unit can weaken the intramolecular charge transfer(ICT)effect,the bandgap of PO-TO is enlarged and its absorption is blue-shifted compared with PY-IT,while the absorbance of PO-TO solution and film is enhanced significantly compared with that of PY-IT.When blended PO-TO with the polymer donor PBQx-TF,the corresponding all-polymer solar cells(all-PSCs)exhibit an open-circuit voltage(V_(oc))exceeding 1.04 V with a power conversion efficiency(PCE)of 13.75%.Furthermore,PO-TO was used as the third component to fabricate ternary all-PSCs with PBQx-TF as the polymer donor and PY-IT as the main polymer acceptor,and the ternary all-PSCs based on PBQx-TF:PY-IT:PO-TO(1:1:0.2,w/w/w)demonstrated a high PCE of 17.71%with simultaneously improved V_(oc)of 0.940 V,short-circuit current density(J_(sc))of 24.60 m A cm^(-2)and fill factor(FF)of76.81%.In comparison,the binary all-PSCs based on PBQx-TF:PY-IT showed a PCE of 16.77%.This result indicates that introducing alkoxy groups is a promising strategy for synthesizing high-performance medium-bandgap PSMAs.

A-DA’D-A Type Pentacyclic Small Molecule Acceptors to Exceed 16.5%Efficiency by Heteroatom Effect at the Outer Side Chain

Comprehensive Summary In this work,we adopt a“heteroatom side-chains”modification strategy to modify the thiophene units in A-DA'D-A(acceptor-donor-acceptor’-donor-acceptor)type pentacyclic SMAs(small molecule acceptors),that is,introducing branched alkyl chain at theβ-position of thiophene instead of straight alkyl chain,and then introducing oxygen atom at the third-position on the basis of branched chain.Two new pentacyclic SMAs(BZ4F-EH and BZ4F-OEH)were synthesized,and the influence of the heteroatom side-chains on photoelectric properties of A-DA'D-A type pentacyclic SMAs was systematically studied.Compared with our previously reported BZ4F(Y26),BZ4F-EH shows slightly blue-shifted absorption,while BZ4F-OEH has obvious red-shifted absorption.As a result,BZ4F-OEH-based binary device achieved a high power conversion efficiency(PCE)of 16.56%with a fill factor(FF)of 79.3%,which is the highest efficiency of pentacyclic SMAs to date.

Organic small molecule acceptor materials for organic solar cells

The active layer of organic solar cells(OSCs)is composed of a p-type conjugated polymer as the donor and an ntype organic semiconductor as the acceptor.Since the report of bulk-heterojunction OSCs with soluble C60 derivative PCBM as the acceptor in 1995,fullerene derivatives,including PCBM and the C70 derivative PC71BM,have been the dominant acceptors in OSCs for 20 years.In 2015,the A–D–A structured small molecule acceptor(SMA)was developed,which possesses the advantages of a narrow bandgap,strong absorption in the long wavelength region,and suitable electronic energy levels,in contrast to the fullerene derivative acceptors.A–D–A SMAs boost the power conversion efficiency(PCE)of OSCs to the 10–14%level.Recently,benefiting from the innovation of A–DA0D–A structured SMAs,the PCE of OSCs has rapidly increased from 15%to 19%.In this review,the development history of n-type organic semiconductor acceptor materials is briefly introduced.The molecular structures and the physicochemical and photovoltaic properties of acceptors,including fullerene derivatives and narrow bandgap SMAs,are described.In particular,the effect of regulating the molecular packing and miscibility of SMAs on their photovoltaic performance is discussed.Finally,current challenges and prospects for n-type organic semiconductor acceptors are analyzed and discussed.

Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Recently, the fused-ring based low band gap （LBG） small molecule acceptors （SMAs） have emerged as efficient nonfullerene acceptors. So far, these LBG SMAs are mainly designed with IC （2-methylene-（3- （1,1 -dicyanomethylene）indanone）） or its analogs, the benzo-type electron-accepting （A） units. Compared to benzene, thiophene is less aromatic and thus the thiophene-involving semiconducting molecule has more quinoidal character, which effectively reduces the energy gap between the highest occupied molecular orbit （HOMO） and the lowest unoccupied molecular orbit （LUMO）. Herein, we show that replacing the IC units in ITIC with the CT （cyclopenta[c]thiophen-4-one-5-methylene-6-（1,1-dicyano- methylene））, a thiophene-fused A unit, the quinoidal character is enhanced from 0.0353 on ITIC to 0.0349 on ITCT, the CT-ended SMA. The increase in the quinoidal character reduces the optical band gap and enhances the near IR absorptivity. When blended with the wide band gap （WBG） polymer donor, PBDB-T, an average power conversion efficiency of 10.99% is obtained with a short-circuit current-density （Jso） of 17.88 mA/cm2 and a fill-factor （FF） of 0.723. For comparisons, theJsc is of 16.92 mA/cm2, FF is of 0.655 and PCE is of 9.94% obtained from the ITIC：PBDB-T device. This case indicates that the replacement of the benzene ring on the IC unit with a more polarizable five-member ring such as thiophene is an effective way to enhance the absorption of the near IR solar photons towards designing high-performance nonfullerene polymer solar cells.

Improving current and mitigating energy loss in ternary organic photovoltaics enabled by two well-compatible small molecule acceptors

Ternary organic photovoltaic(OPV)strategy is an effective but facile approach to enhance the photovoltaic performance for single-junction devices.Herein,a series of ternary OPVs were fabricated by employing a wide bandgap donor(PBDB-TF)and two acceptor-donor-acceptor(A-D-A)-type nonfullerene small molecule acceptors(NF-SMAs,called F-2 Cl and 3 TT-OCIC).As the third component,the near-infrared SMA,3 TT-OCIC,has complementary absorption spectrum,narrow bandgap and wellcompatible crystallization property to the host acceptor(F-2 Cl)for efficient ternary OPVs.With these,the optimal ternary devices yield significantly enhanced power conversion efficiency of 15.23%,one of the very few examples with PCE higher than15%other than Y6 systems.This is mainly attributed to the increased short-circuit current density of 24.92 m A cm^(-2) and dramatically decreased energy loss of 0.53 e V.This work presents a successful example for simultaneously improving current,minimizing energy loss and together with modifying the morphology of active layers in OPVs,which will contribute to the further construction of high performance ternary OPVs.

A Novel BODIPY-Based Low-Band-Gap Small-Molecule Acceptor for Efficient Non-fullerene Polymer Solar Cells

We report herein an efficient A^1-C=C-A^2-C=C-A^1 type small-molecule 4,4＇-difluoro-4-bora-3a,4a-diaza-sindacene （BODIPY） acceptor （A^1= BODIPY and A^2= diketopyrrolopyrrole （DPP）） by following the A-to-A excited electron delocalization via the BODIPY meso-position, the inherent directionality for the excited electron delocalization. The lowest unoccupied molecular orbital （LUMO） delocalizes across over whole the two flanking A^1 and the central A^2, and the highest occupied molecular orbital （HOMO） localizes dominantly on the -C=C-DPP-C=C- segment. The excited electron upon light excitation of the DPP segment delocalizes over both the BODIPY and DPP segments. The acceptor in chloroform shows an unprecedented plateau-like broad absorption between 550 and 700 nm with a large FWHM value of 195 nm. Upon transition into solid film, the acceptor shows absorption in the whole near ultraviolet-visible-near infrared wavelength region （300-830 nm） with a low band gap of 1.5 eV and a maximum absorptivity of 0.85 × 10^5 cm^-1. Introduction of the ethynyl spacer between the A^1 and A^2 and the close BODIPY-to-DPP LUMO energy levels are crucial for the excited n-electron delocalization across over whole the conjugation backbone. A power conversion efficiency of 6.60% was obtained from the ternary non-fullerene solar cell with PTB7-Th：p-DTS（FBTTh2）2 （0.5 ; 0.5） as the donor materials, which is the highest value among the non-fullerene organic solar cells with BODIPY as the electron acceptor material.

A simple small molecule as the acceptor for fullerene-free organic solar cells

A simple small molecule named DICTiF was designed,synthesized and used as the acceptor for solution processed bulk-heterojunction solar cells with polymer PBDB-T as the donor.A power conversion efficiency of 7.11%was obtained.

A-D-A-type small molecular acceptor with one hexyl-substituted thiophene as π bridge for fullerene-free organic solar cells

An A-D-A-type small molecule, DCF-2HT, was synthesized using fluorene as the central block and 2- （2,3-dihydro-3-oxo- 1H-inden- 1-ylidene）propanedinitrile as the end groups, with one hexyl-substituted thiophene as a n bridge, for use as an acceptor material in organic solar cells. Devices based on DCF-2HT and the polymer donors PBDB-T or PTB7-Th were fabricated and optimized. Power conversion efficiencies of 5.71% and 4.83% were obtained for PBDB-T： DCF-2HT- and PTB7-Th： DCF-2HT-based devices, respectively.

Recent Research Progress of n-Type Conjugated Polymer Acceptors and All-Polymer Solar Cells

The active layer of all polymer solar cells(all-PSCs)is composed of a blend of a p-type conjugated polymer(p-CP)as donor and an n-type conjugated polymer(n-CP)as acceptor.All-PSCs possess the advantages of light weight,thin active layer,mechanical flexibility,low cost solution processing and high stability,but the power conversion efficiency(PCE)of the all-PSCs was limited by the poor photovoltaic performance of the n-CP acceptors before 2016.Since the report of the strategy of polymerized small molecule acceptors(PSMAs)in 2017,the photovoltaic performance of the PSMA-based n-CPs improved rapidly,benefitted from the development of the A-DA’D-A type small molecule acceptors(SMAs).PCE of the all-PSCs based on the PSMA acceptors reached 17%-18%recently.In this review article,we will introduce the development history of the n-CPs,especially the recent research progress of the PSMAs.Particularly,the structure-property relationship of the PSMAs is introduced and discussed.Finally,current challenges and prospects of the n-CP acceptors are analyzed and discussed.

Design a thieno[3,2-b]thiophene bridged nonfullerene acceptor to increase open-circuit voltage,short-circuit current-density and fill factor via the ternary strategy

In this study,we report a new small molecule acceptor(named TT-4 F)which uses 3,6-dimethoxylthieno[3,2-b]thiophene(TT)as theπ-bridge.Addition of 0.05 weight ratio amount of TT-4 F into the host binary blend of PTB7-Th:IEICO-4 F,resulting in a ternary blend in a weight ratio of 1:1:0.05,enables increased open-circuit voltage(Voc),short-circuit curre nt-density(Jsc),and fill-factor(FF)at the same time.Finally,12.1%efficiency is obtained.Compared to the 3-(2-ethylhexyloxylthiophene)bridge on IEICO-4 F,the additional methoxyl group on the TT-6 position is involved in the lowest unoccupied molecular orbital(LUMO)and the largerπ-system on TT increases the electron-donating nature,both of which help to raise the LUMO level,one reason of the increased Voc.Upon addition of 0.05 TT-4 F,the hole mobility is increased,the monomolecular recombination is reduced,and the charge dissociation and collection is enhanced.All of these contribute to the increased Jsc and FF.

High-efficiency organic solar cells processed from a halogen-free solvent system

The use of non-halogenated solvents for the green manufacture of high-efficiency organic solar cells(OSCs)is important for their future application.However,the power conversion efficiency(PCE)of the non-halogenated solvent processed OSCs is generally lower than their halogenated counterpart due to the poor film microstructure caused by the solubility issue.Herein,we propose a halogen-free solvent system to optimize film microstructure of the photovoltaic blend based on the polymer donor D18and small-molecule acceptor(SMA)L8-BO towards high-efficiency OSCs.The solvent system is consisted of a main solvent carbon disulfide and an additive paraxylene,where the former ensures the good solution-processability and promotes the solution aggregation of L8-BO,and the latter can finely control the phase-separation process by selectively dissolving the SMA.This solvent combination robustly produces a high-quality active layer,i.e.,the bicontinuous networks of donor and acceptor with nano-sized phase-separation and strongπ-πstacking.With the effective charge generation,transport and collection,the resulting device from the non-halogenated solvent system shows a high PCE of 17.50%,which is comparable to that of the device prepared from the halogenated solvent chloroform(ca.17.11%).This article proposes a new strategy for the green fabrication of high-efficiency OSCs to accelerate their industrialization.

Low-Cost and High-Performance Polymer Solar Cells with Efficiency Insensitive to Active-Layer Thickness

The power conversion efficiency(PCE)of polymer solar cells(PSCs)has recently increased quickly,propelling PSCs closer to large-scale commercialization.However,several critical issues,such as the cost of materials and the sensitivity of the PCE to active-layer thickness,must be addressed before industrial application can be realized on a large scale.Here,we fabricated a high-performance ternary PSC based on a low-cost polymer donor PTQ10 and an A-DA’D-A-type small molecule acceptor(SMA)2,2'-((2Z,2'Z)-((12,13-bis(2-butyloctyl)-3,9-bis(4-(2-ethylhexyl)thiophen-2-yl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(m-TEH)with a newA-DA’D-A-type SMA 2,2'-((2Z,2'Z)-((12,13-bis(2-butyloctyl)-3,9-bis(3-(2-ethylhexyl)phenyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(m-PEH)(with phenyl outer side chains)as the third component.Benefitting from the good compatibility and the unique alignment of the energy levels between PTQ10 and the two SMAs,the ternary system showed favorable phase separation and dominant face-on orientation,exhibiting suitable film morphology and enhanced charge transport.Therefore,the optimized ternary PSCs based on PTQ10∶m-TEH∶m-PEH(1.0∶0.9∶0.3,w/w/w)demonstrated an outstanding PCE of 19.34%,which is one of the highest PCEs reported for the single junction PSCs to date.More importantly,the ternary PSCs demonstrated superior tolerance to the active-layer thickness and showed a high PCE of 18.02% with a high fill factor(FF)of 76.56% for the devices with the active-layer thickness even reaching 300 nm.These results indicate that the ternary devices based on PTQ10∶m-TEH∶m-PEH are highly promising for future large-area fabrication and commercial application of PSCs.

n-Octyl substituted quinoxaline-based polymer donor enabling all-polymer solar cell with efficiency over 17%

Recently, the power conversion efficiencies(PCEs) of all-polymer solar cells(all-PSCs) have increased rapidly. To further increase the PCE of all-PSCs, it is necessary to create new donor polymers matching the polymer acceptors. In this paper, we synthesize a new quinoxaline-based polymer donor PBQ8 with n-octyl side chain on the quinoxaline unit, which possesses the same skeleton structure to the previously reported PBQ5(with isooctyl side chain). The effects of alkyl side chains on the physicochemical properties of the polymer donor were investigated. In comparison with PBQ5, PBQ8 exhibits stronger intermolecular interactions and better molecular packing. When blending with polymer acceptor PY-IT, the PBQ8:PY-IT based devices demonstrated a higher PCE value of 17.04%, which is one of the highest PCEs occurred in the all-PSCs. And the PBQ5:PY-IT(PCE 15.56%, Voc0.907 V, FF 69.72%, and Jsc24.60 m A cm^(-2)) is much lower. The PBQ8:PY-IT blend displayed more efficient exciton dissociation, better molecular stacking properties, preferable phase separation and higher mobility. These indicate that as an effective method, side chain engineering can improve the efficiency of the all-PSCs.

Control of aggregated structure of photovoltaic polymers for high-efficiency solar cells

π-Conjugated organic/polymer materials-based solar cells have attracted tremendous research interest in the fields of chemistry,physics,materials science,and energy science.To date,the best-performance polymer solar cells(PSCs)have achieved power conversion efficiencies exceeding 18%,mostly driven by the molecular design and device structure optimization of the photovoltaic materials.This review article provides a comprehensive overview of the key advances and current status in aggregated structure research of PSCs.Here,we start by providing a brief tutorial on the aggregated structure of photovoltaic polymers.The characteristic parameters at different length scales and the associated characterization techniques are overviewed.Subsequently,a variety of effective strategies to control the aggregated structure of photovoltaic polymers are discussed for polymer:fullerene solar cells and polymer:nonfullerene small molecule solar cells.Particularly,the control strategies for achieving record efficiencies in each type of PSCs are highlighted.More importantly,the in-depth structure-performance relationships are demonstrated with selected examples.Finally,future challenges and research prospects on understanding and optimizing the aggregated structure of photovoltaic polymers and their blends are provided.

Tailoring the photophysical and photovoltaic properties of boron-difluorodipyrromethene dimers

Five boron-difluorodipyrromethene（BODIPY） dimers have been designed and synthesized successfully via acid-catalysed condensation and Pd-catalysed cross-coupling reactions.The structural modification,including verifying the structures of the π-bridges,altering the positions the bridges link（meso-or β-positions）,and regulating the molecular planarity,can modulate the photophysical properties and the aggregation behaviors of the five dimers efficiently.Solution-processed organic solar cells were fabricated to evaluate the photovoltaic properties of these molecules further either as acceptors or donors.When using as nonfullerene acceptor and blended with the polymer donor of PTB7,an opencircuit voltaic（V_（oc）） of 1.12 and 1.08 V was achieved from the thiophene and benzodithiophene bridged BODIPY dimers,respectively.This V_（oc） is among the top values achieved from the non-fullerene organic solar cells so far.