Polymer donors with hydrophilic side-chains enabling efficient and thermally-stable polymer solar cells by non-halogenated solvent processing

Polymer solar cells(PSCs)with high power conversion efficiency(PCE)and environment-friendly fabrication are the main requirements enabling their production in industrial scale.While the use of non-halogenated solvent processing is inevitable for the PSC fabrication,it significantly reduces the processability of polymer donors(PDS)and small-molecule acceptors(SMAs).This often results in unoptimized blend morphology and limits the device performance.To address this issue,hydrophilic oligoethylene glycol(OEG)side-chains are introduced into a PD(2EG)to enhance the molecular compatibility between the PD and L8-BO SMA.The 2EG PD induces higher crystallinity and alleviates phase separation with the SMA compared to the reference PD(PM7)with hydrocarbon side-chains.Consequently,the 2EG-based PSCs exhibit a higher PCE(15.8%)than the PM7-based PSCs(PCE=14.4%)in the ortho-xylene based processing.Importantly,benefitted from the reduced phase separation and increased crystallinity of 2EG PDS,the 2EG-based PSCs show enhanced thermal stability(84%of initial PCE after 120 h heating)compared to that of the PM7-based PSCs(60%of initial PCE after 120 h heating).This study demonstrates the potential of OEG side-chain-incorporated materials in developing efficient,stable,and eco-friendly PSCs.

Revealing the spacing effect of neighboring side-chains in modulating molecular aggregation and orientation of M-series acceptors

Controlling the aggregation of small-molecule acceptors(SMAs)is essential to obtain an optimal morphology and to improve the photovoltaic performance of polymer solar cells(PSCs).However,reducing intermolecular aggregation of SMAs is usually accompanied by the disruption of compact molecular packing thereby leading to their decreased electron mobilities.Here,two novel M-series SMAs(MD1T and MD2T)based on ladder-type heterononacenes with neighboring side-chains separated by one or two thiophene rings are designed and synthesized.It is found that shortening the spacing of the neighboring side-chains of the SMAs can greatly alleviate the intermolecular aggregation and alter the molecular orientation from bimodal edge-on/face-on to predominant face-on while maintaining the compact molecular packing.As a result,a more favorable morphology with smaller domain sizes is formed for the MD1T-based blend films,which greatly improves the charge generation and charge transport for the corresponding PSCs.The best-performing MD1T-based device affords an efficiency of 12.43%,over seven times higher than that of the MD2T-based device.This work reveals the importance of the spacing between the neighboring side-chains in modulating the molecular aggregation and active layer morphology,and the obtained structure-performance relationships shall provide important guidance for designing highly efficient SMAs.

Semi-crystalline photovoltaic polymers with siloxane-terminated hybrid side-chains

Three types of semi-cry stalline photovoltaic polymers were synthesized by incorporating a siloxane-terminated organic/inorganic hybrid side-chain and changing the number of fluorine substituents.A branch point away from a polymer main backbone in the siloxane-containing side-chains and the intra-and/or interchain noncovalent coulombic interactions enhance a chain planarity and facile interchain organization.The resulting polymers formed strongly agglomerated films with high roughness,suggesting strong intermolecular interactions.The optical band gap of ca.1.7 eV was measured for all polymers with a pronounced shoulder peak due to tight π-π stacking.With increasing the fluorine substituents,the frontier energy levels decreased and preferential face-on orientation was observed.The siloxane-terminated side-chains and fluorine substitution promoted the intermolecular packing,showing well resolved lamellar scatterings up to(300) for this series of polymers in the grazing incidence wide angle X-ray scattering measurements.The PPsiDTBT,PPsiDTFBT and PPsiDT2 FBT devices showed a power conversion efficiency of 3.16%,4.40%and 5.65%,respectively,by blending with PC_(71)BM.Langevin-type bimolecular charge recombination was similar for three polymeric solar cells.The main loss in the photocurrent generation for PPsiDTBT:PC_(71)BM was interpreted to originate from the trap assisted charge recombination by measuring light-intensity dependent short-circuit current density(J_(SC)) and open-circuit voltage(V_(Oc)).Our results provide a new insight into the rational selection of solubilizing substituents for optimizing crystalline interchain packing with appropriate miscibility with PC71 BM for further optimizing polymer solar cells.

Competition of Lamellar Crystal and Smectic Liquid Crystal in Precise Polyethylene Derivative Bearing Mesogenic Side-Chains

For side-chain liquid-crystalline polymers(SCLCPs)bearing calamitic mesogens,nematic and typical smectic(Sm)phases are expected.Here,using a precise polyethylene derivative with the biphenyl moiety tethered on every ninth carbon(P9,b),we demonstrated that an SCLCP could form a crystal(Cr)phase in addition to an Sm phase.Two opposing interrelations of main-and side-chains determined the molecular packings.While the side-chain inclined to be perpendicular to the main-chain in the Sm phase,they paralleled and coassembled in the Cr phase with an orthorhombic structure.The Sm phase of P9,b was new in SCLCPs.Within the mesogen sublayers,the biphenyls were tilted and had a type of two-dimensional positional order,making this Sm phase ordering higher than that of smectic C.The Sm phase was metastable and kinetically grew faster below its isotropic temperature.However,it could not convert to the stable Cr phase unless it transitioned into the melting state.Fantastically,P9,b rendered the lamellar crystal in the Cr phase.The lenticular lamellae resembled that found in linear polyethylene crystallized at low undercooling.Importantly,albeit having the large anisotropic pendants situated regularly in the backbone,P9,b could follow the principle of chain-folding crystallization.

Role of ball milling during Cs/X catalyst preparation and effects on catalytic performance in side-chain alkylation of toluene with methanol

Ball milling modification was performed on Cs/X catalysts before or after cesium ion exchange.Multiple characterization results(such as pyridine-FTIR,XPS,and solid-state NMR)demonstrated that ball milling played a distinct role in these two different preparation procedures of the catalyst.Ball milling performed after the cesium modification has a strong influence on the Cs/X structure and acid-base properties,which results in the enhancement of the catalytic performance for side-chain methylation of toluene with methanol.Detailed studies revealed that ball milling intensified the interactions between oxides and molecular sieves,which not only increased the dispersion of the Cs species but also generated some weaker basic centers.It is proposed that the new basic centers could be Si-O-Cs and Al-O-Cs,which are produced by breaking of the Si-O-Al bonds of the zeolite framework under the synergetic effect of ball milling and alkali treatment.These new active sites may help to promote the side-chain methylation reaction.However,excessive ball milling will lead to the vanishing of zeolite micropores,thus deactivating side-chain methylation activity,which indicates that microporosity plays a key role in side-chain methylation and individual basic centers cannot catalyze this reaction.

Enhancing the side-chain alkylation of toluene with methanol to styrene over the Cs-modified X zeolite by the assistance of basic picoline as a co-catalyst

Side-chain alkylation of toluene with methanol is a green pathway to realize the one-step production of styrene under mild conditions,but the low selectivity of styrene is difficult to be improved with by-products of ethylbenzene and xylene.In this study,a new way is introduced to improve the catalytic performance by means of assisting basic compounds as co-catalysts during the toluene side-chain alkylation with methanol to styrene.As a result,high activity of side-chain alkylation appears over the basic Cs-modified zeolite catalysts prepared by ion exchange and impregnation methods.This high performance should be mainly attributed to two co-catalysis actions:(1)the promotion of basic compounds for methanol dehydrogenation to formaldehyde as the intermediate for side-chain alkylation;(2)the suppression of the styrene transfer hydrogenation on basic Cs-modified zeolites to avoid the formation of ethylbenzene.Especially for Cs_(2)O/CsX-ex catalyst,the addition of 2%mol/mol 2-picoline in reaction mixture could achieve both 12.3%toluene conversion and 84.1%styrene selectivity.Whereas the higher concentration of 2-picoline(>6%mol/mol)caused an inhibition to the catalytic activity because the excessive basic compound poisoned the combined acid-base pathway required for the side-chain alkylation process.In addition,two possible side-chain alkylation reaction routes on Cs-modified zeolite under the different 2-picoline absorption were described.

Efficient small-molecule donor with improved structural order and molecular aggregation enabled by side-chain modification

Side-chain modification is a proven effective approach for morphology manipulation in organic solar cells(OSCs).However,in-depth analysis and investigation involving side-chain modification towards morphology improvement,including molecular microstructure,orientating packing and aggregation are urgent for all-small-molecule(ASM)systems.Herein,employing a fluorine-modified two-dimension benzodithiophene(BDT)as central unit,we contrastively synthesized two small-molecule donors,namely BDT-F-SR and BDT-F-R,each welding alkylthio side-chains on thienyl of central BDT unit and the other grafted non-sulfuric alkyl side-chains.As predicted,the synergetic side-chain modification of fluorination and alkyl changeover triggers diverse molecular dipole moments and orientations,resulting in different molecular energy levels,thermal stabilities,molecular planarity and order.Eventually,together with the preeminent small-molecule acceptor Y6,BDT-F-R-based ASM OSCs obtain enhanced power conversion efficiency(PCE)of 13.88%compared to BDT-F-SR-based devices(PCE of 12.75%)with more suitable phase-separation and balanced carrier mobilities.The contrast results reveal that alkyl sidechains seem to be a more satisfactory partner for fluorine-modified 2D BDT-based small-molecule donors compared to alkylthio pendants,and highlight the significance of subtle side-chain modification for molecular structural order fun-tuning and morphology control,laying the foundation for efficient ASM OSCs.

A Novel Diamine,4'-Phenyiphenyi 4-(3",5"-Diaminobenzoyioxy)-benzoate and Longer Side-chain Polyimide

This is a part of our systematic research work on polyimides with mesogenic unit side chain. In this study, a new 4＇-phenylpbenyl 4-（3＂,5＂-diaminobenzoyloxy）benzoate and polyimide were synthesized, and characterized by FTIR, ^1H-NMR, inherent viscosity, mechanical properties, and solubility measurements. The diamine composed with mesogenic unit aryl ester groups and bipbenyl group with longer L/D ratio, was synthesized by two key steps. Firstly, the hydroxy group of 4-hydroxybenzoic acid was protected by acetoxy group for avoiding self- polymerization of 4-hydroxybenzoic acid, and then selectively hydrolyzed after esterification of carboxyl. Secondly, a selective catalysis hydrogenation was adopted to prevent the aryl ester from deoxidation. Based on this diamine, a novel polyimide was prepared by polycondensation of 4＇-phenylphenyl 4-（3＂, 5＂-diaminobenzoyloxy）benzoate and 4-aminophenyl ether（ODA） with 4, 4＇-oxydiphthalic anhydride（ODPA） in N-methyl-2-pyrrolidone （NMP）. The resulting polyimide with longer side chain showed better solubility and more regular structure. Its inherent viscosity is lower than that without side chains, but its modulus and strength not only maintained, even improved.

MORPHOLOGICAL AND KINETIC STUDIES OF PHASE TRANSITIONS OF A SIDE-CHAIN LIQUID CRYSTALLINE POLYMER

The morphological changes of a side-chain liquid-crystalline polymethacrylate during isotropization and liquid-crystallization transitions were studied by means of polarizing microscopy. These transitions were found to be composed of the initiation of a new phase at local places of the old phase matrix and the growth of the new phase: domains. The kinetics of the liquid-crystallization of the polymer from an isotropic melt to a smectic mesophase was also investigated. The isothermal process of the transition can be described by the Avrami equation. The values of the Avrami exponent were found to be around 2.6. which is lower than the value usually obtained for crystallization transition of polymers, but larger than that reported for liquid-crystallization transition of main-chain polymers. These results may indicate the difference in growth geometry of new phase during transition between crystallization and liquid-crystallization in general and between liquid-crystallization of main-chain and side-chain polymers. It was found that the liquid-crystallization of the used side-chain polymethacrylate may occur at small undercoolings with high transformation rate similar to that of main-chain polymers and small-molecule liquid crystals, while the crystallization of polymers can only proceed at large undercoolings. These phenomena can be explained by the idea that the surface free energy of nucleus during liquid-crystallization transition is less than that for crystallization, and evidence was obtained from analysis of the temperature dependence of the transformation rate.

MECHANICAL PERTURBATION INDUCED MOLECULAR ALIGNMENTS IN A SIDE-CHAIN LIQUID CRYSTALLINE POLYACETYLENE, POLY{10-[4-(4'-METHOXYPHENOXY-CARBOML)PHENOXYCARBONYL]-1-DECYNE}

A new liquid crystalline polyacetylene containing a phenyl benzoate mesogen (5) is synthesized,whose mesomorphic properties are found to be easily 'tunab1e' by simple mechanical perturbation. Thepolymerization of 10- [ 4 - (4' -methoxyphenoxycarbonyl )phenoxycarbonyl] - 1 -decyne (4 ) in itiated by theWCl_6-Ph_4Sn/dioxane complex yields polymer 5 with a M_w of 28400. The molecular structure of 5 ischaracterized by NMR, IR, and UV spectroscopy and its liquid crystalline behavior is investigated by DSC,POM, and XRD analysis. Upon mechanical perturbation, 5 exhibits unusual agitation-induced high-strengthdisclinations, shear-induced inversion walls, and solidification-induced banded textures. Such phenomenahave been observed in the main-chain liquid crystalline polymers with rigid backbones, but have seldom beenreported for the side-chain liquid crystalline polymers with flexible backbones, suggesting that the rigidpolyacetylene backbone of 5 plays a constructive role in inducing the novel molecular alignments.

The effect of intermolecular interactions on photoluminescence of a porphyrin side-chain polymer

Photoluminescence properties and exciton decay dynamics in a porphyrin side-chain polymer, poly[porphyrin acrylate- acrylonitrile （abbreviated p[（por）A-AN]）, have been investigated by femtosecond time-resolved photoluminescence spectroscopy. All the luminescences of p[（por）A-AN] films are due to the emissive decay of the photoexcited singlet excitons in the porphyrins. The luminescence efficiencies and lifetimes are increased for samples from pure films to dilute blend films. However, they are increased as the intrachain concentration of the porphyrin sidechain groups is decreased. The intrachain rotation motions of porphyrin sidechain groups result in the initial ultrafast luminescence decays, which are much faster than those due to the interchain interactions. All the samples show no significant red-shift and broadening of the transient luminescence spectra. The interchain and intrachain nonradiative exciton relaxation processes may play an important role in the luminescence dynamics in the p[（por）A-AN] films. The possible origin of different intrachain and interchain dynamic behaviours in p[（por）A-AN] films is discussed.

MORPHOLOGICAL STUDIES OF A THERMOTROPIC SIDE-CHAIN LIQUID CRYSTALLINE POLYMER DURING MESOPHASE TRANSITIONS

The morphological features of a side-chain liquid crystalline polymer during the mesophase transitions were investigated by using the DSC technique. The polymer used was polyacrylate with mesogens of three benzene rings attached to the main chain through a flexible spacer. A special two-phase texture was observed in the transition temperature range. Similar to main-chain liquid crystalline polymers the transition process of the side-chain liquid crystalline polymer was composed of an initiation of the new phase at local places of the old phase matrix and a growth process of the new phase domains.

Methylthio side-chain modified quinoidal benzo-[1,2-b:4,5-b']dithiophene derivatives for high-performance ambipolar organic field-effect transistors

Quinoidal small molecule semiconductors hold huge potential in ambipolar organic field-effect transistors(OFETs)and organic spintronic devices.Here,two quinoidal molecules with methylthio side chains were synthesized to develop molecular semiconductors with high ambipolar mobility,designated QBDTS and QTBDTS.The theoretical calculation results reveal that QBDTS has a closed-shell structure while QTBDTS showed an open-shell structure with a biradical character(y0)of 0.46 and its magnetic properties were further investigated using electron paramagnetic resonance(EPR)and superconducting quantum interference device(SQUID)methods.The methyl side chains showed a large impact on the molecular orbital levels.The HOMO/LUMO levels of QBDTS and QTBDTS were measured to be-5.66/-4.56 and-5.27/-4.48 eV,respectively,which are favorable for ambipolar charge transport in OFETs.Importantly,the spin-coated QBDTS displayed hole and electron mobilities of 0.01 and 0.5 cm^(2)V^(-1)s^(-1)while QTBDTS showed a record high hole mobility of 1.8 cm^(2)V^(-1)s^(-1)and electron mobility of 0.3 cm^(2)V^(-1)s^(-1).Moreover,comparative studies of the thin film morphologies also manifested the beneficial influence of methyl side chains on film crystallinity and molecule orientation.These results strongly proved that methyl side chain engineering can be a simple but efficient strategy for modulating electronic properties and molecular stacking behaviors.This work also represents a big advancement for quinoidal molecular semiconductors in ambipolar OFET applications.

Side-Chain Engineering of Non-Fullerene Acceptors with Trialkylsilyloxy Groups for Enhanced Photovoltaic Performancet

Side-chain engineering has emerged as a highly effective strategy for tailoring the aggregation behavior and charge transport properties of non-fullerene small molecule acceptors(SMAs).In this study,we designed and synthesized two SMAs,namely BTPSi-Bu and BTPSi-Pr,respectively incorporating tributylsilyloxy and trisopropylsilyloxy groups in their outer positions.Notably,BTPSi-Bu exhibited better planarity,crystallization,and favorable phase separation when paired with PM6 donor polymer compared to its counterpart,BTPSi-Pr.The resulting organic solar cells,utilizing the PM6:BTPSi-Bu blend,demonstrated a remarkable power conversion efficiency of 17.41%and a high open-circuit voltage of 0.859 V.These findings underscore the significance of integrating trialkylsilyloxy side chains into SMAs as a rational design approach for enhancing the performance of photovoltaic systems.

Side-chain symmetry-breaking strategy on porphyrin donors enables high-efficiency binary all-small-molecule organic solar cells

Side-chain symmetry-breaking strategy plays an important role in developing photovoltaic materials for high-efficiency all-small-molecule organic solar cells(ASM OSCs).However,the power conversion efficiencies(PCEs)of ASM OSCs still lag behind their polymer-based counterparts,which can be attributed to the difficulties in achieving favorable morphology.Herein,two asymmetric porphyrin-based donors named DAPor-DPP and DDPor-DPP were synthesized,presenting stronger intermolecular interaction and closer molecular stacking compared to the symmetric ZnP-TEH.The DAPor-DPP:6TIC blend afforded a favorablemorphologywith nanoscale phase separation and more ordered molecular packing,thus achievingmore efficient charge transportation and suppressed charge recombination.Consequently,the DAPor-DPP:6TIC-based device exhibited superior photovoltaic parameters,yielding a champion PCE of 16.62%higher than that of the DDPor-DPP-based device(14.96%).To our knowledge,16.62%can be ranked as one of the highest PCE values among the binary ASM OSC filed.Thiswork provides a prospective approach to address the challenge ofASM OSCs in improving film morphology and further achieving high efficiency via side-chain symmetry-breaking strategy,exhibiting great potential in constructing efficient ASM OSCs.

Side-chain engineering for regulating structure and properties of a novel visible-light-driven perylene diimide-based supramolecular photocatalyst

Systematic and in-depth explorations of the effects of side-chain modulation on the molecular assembly,optoelectronic properties,and photocatalytic properties of supramolecular systems,as well as the kinetics of charge separation and migration in these systems,are rare.In this study,a novel supramolecular photocatalyst with an alkoxy side chain(S-EPDI)was successfully developed through subtle design of the short and linear alkoxyl side chains,affording a phenol degradation efficiency approximately four times that of the counterpart with an alkyl side chain(S-APDI).Notably,combined density functional theory(DFT)calculations,absorption spectroscopy,and other characterizations revealed that the perylene diimide(PDI)molecular units,throughπ-πstacking,formed a unique rotationally offset stacked supramolecular structure,exhibiting a significant dipole moment.This gave rise to the formation of a larger inherent electric field within S-EPDI compared to S-APDI.Moreover,the study quantitatively demonstrated that a stronger inherent electric field and lower rate of surface charge recombination facilitate efficient separation of the photogenerated carriers.Therefore,the side-chain molecular engineering method employed in this study offers an effective approach for modulating the kinetics of charge migration.

Chlorination strategy on polymer donors toward efficient solar conversions

Bulk heterojunction(BHJ)polymer solar cells(PSCs)are promising candidates for next-generation solar cells.Benefitting from the persistent efforts in material design and synthesis,systematic device engineering and fundamental understanding of the device physics,the power conversion efficiency(PCE)of single PSC has been pushed to surpass 15%,and that of the tandem PSCs is over 17%.Recently,chlorination has drawn much interest and the chlorinated PSCs have been frequently reported in donor-acceptor(D-A)type conjugated polymers.This review summarizes the recent progress of the chlorinated strategy for highly efficient photovoltaic applications.We firstly discuss the chlorination on the acceptor units in D-A type donor polymers,emphasizing the 4 widely used acceptor units with their improved PCE.secondly,the chlorination on the donor units will be discussed,mainly focusing on the chlorination of benzo[1,2-b:4,b]dithiophene(BDT)unit and 2,2-bithiophene unit.Remarkably,the PCE of the chlorinated BDT-based device has been improved to over 14%.Overall,this review discusses the structure-property correlations of these chlorinated polymers in photovoltaic study,which could further provide guidance on the chlorinated strategy and the molecular design for high-performance photovoltaic devices.

N-alkyl chain modification in dithienobenzotriazole unit enabled efficient polymer donor for high-performance non-fullerene solar cells

Molecular design of either polymer donors or acceptors is a promising strategy to tune the morphology of the active layer of organic solar cells,enabling a high-performance device.Thereinto,developing novel polymer donors is an alternative method to obtain high photovoltaic performance.Herein,we present a facile side-chain engineering on the dithiophenobenzotriazole(DTBTz)unit of newly-designed polymer donors(named p BDT-DTBTz-EH and p BDT-DTBTz-Me)to boost the performance of non-fullerene solar cells.Compared with p BDT-DTBTz-EH with long N-alkyl side chains,p BDT-DTBTz-Me with a short methyl exhibits stronger molecular aggregation,higher absorption coefficient,and preferred face-on orientation packing.As a consequence,p BDT-DTBTz-Me based devices achieve an optimal power conversion efficiency of 15.31%when donors are paired with the narrow bandgap acceptor Y6,which is superior to that of p BDT-DTBTz-EH based devices(9.17%).Additionally,the p BDT-DTBTz-Me based devices manifest more effective charge separation and transfer than p BDT-DTBTz-EH based devices.These results indicate that fine-tuning side chains of polymer donors provide new insights for the design of high-performance polymer donors in non-fullerene solar cells.

POLYMER SCAFFOLDS BEARING AZOBENZENE-POTENTIAL FOR OPTICAL INFORMATION STORAGE

The fundamental optical storage mechanism of the laser light eddressable azobenzene moiety is briefly introduced.A modular and flexible synthesis design furnishes polyester matrices covalently integrating cyanoazobenzene in regularlyspaced side chains. Thin films of these materials are particularly well suited for holographic storape. Notable figures of meritsof liquid crystalline polyesters are response time to blue-green laser light of the order of nanoseconds, storage capacityexpressed as 5000 lines/mm, and high, permanent (almost nine years) diffraction efficiency of the order of 50% or greater,and erasability, The implications of the main chain nature for polyester morphology and for the permanency of the inducedanisotropy are discussed, The design and methods of preparation of other significantly different polymer scaffolds supportingcyanoazobenzene are elaborated. Oligopeptides always result in amorphous materials, whereas copolymethacrylates anddendritic or hyperbranched polyesters provide some materials that exhibit liquid crystallinity. However, none of these scaffolds affords materials that result in permanent anisotropy when exposed to interfering laser light.

Gradually modulating the three parts of D-π-A type polymers for high-performance organic solar cells

Organic solar cells(OSCs)have received great attention for the prominent advantage of low-cost,light-weight and potential for fabricating flexible and semi-transparent device via roll-to-roll printing toward making better use of inexhaustible renewable clean energy during the past years[1-4].