Alkoxy substituted benzodithiophene-alt-fluorobenzotriazole copolymer as donor in non-fullerene polymer solar cells

A new benzodithiophene(BDT)-alt-fluorobenzotriazole(FBTA) D-A copolymer J40 was designed and synthesized by introducing 2-octyldodecyloxy side chains on its BDT units, for expanding the family of the BDT- alt-FBTA-based copolymers and investigating the side chain effect on the photovoltaic performance of the polymer in non-fullerene polymer solar cells(PSCs).J40 exhibits complementary absorption spectra and matched electronic energy levels with the n-type organic semiconductor(n-OS)(3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-sindaceno[1,2-b:5,6-b′]dithiophene)(ITIC) acceptor, and was used as polymer donor in the non-fullerene PSCs with ITIC as acceptor. The power conversion efficiency(PCE) of the PSCs based on J40:ITIC(1:1, w/w) with thermal annealing at 120 °C for 10 min reached 6.48% with a higher open-circuit voltage(Voc) of 0.89 V. The high Voc of the PSCs is benefitted from the lower-lying highest occupied molecular orbital(HOMO) energy level of J40. Although the photovoltaic performance of the polymer J40 with alkoxy side chain is lower than that of J60 and J61 with alkylthio-thienyl conjugated side chains, the PCE of6.48% for the J40-based device is still a relatively higher photovoltaic efficiency in the non-fullerene PSCs reported so far. The results indicate that the family of the BDT-alt-FBTA-based D-A copolymers are high performance polymer donor materials for non-fullerene PSCs and the side chain engineering plays an important role in the design of high performance polymer donors in the non-fullerene PSCs.     

Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency

A high performance polymer solar cells(PSCs) based on polymer donor PM6 containing fluorinated thienyl benzodithiophene unit and n-type organic semiconductor acceptor IT-4 F containing fluorinated end-groups were developed. In addition to complementary absorption spectra(300–830 nm) with IT-4 F, the PM6 also has a deep HOMO(the highest occupied molecular) level(-5.50 e V), which will lower the open-circuit voltage(V_(oc)) sacrifice and reduce the E_(loss) of the IT-4 F-based PSCs. Moreover, the strong crystallinity of PM6 is beneficial to form favorable blend morphology and hence to suppress recombination. As a result, in comparison with the PSCs based on a non-fluorinated D/A pair of PBDB-T:ITIC with a medium PCE of 11.2%, the PM6:IT-4 Fbased PSCs yielded an impressive PCE of 13.5% due to the synergistic effect of fluorination on both donor and acceptor, which is among the highest values recorded in the literatures for PSCs to date. Furthermore, a PCE of 12.2% was remained with the active layer thickness of up to 285 nm and a high PCE of 11.4% was also obtained with a large device area of 1 cm~2. In addition, the devices also showed good storage, thermal and illumination stabilities with respect to the efficiency. These results indicate that fluorination is an effective strategy to improve the photovoltaic performance of materials, as well as the both fluorinated donor and acceptor pair-PM6:IT-4 F is an ideal candidate for the large scale roll-to-roll production of efficient PSCs in the future.     

The first application of isoindigo-based polymers in non-fullerene organic solar cells

Although isoindigo(IID)-based polymers can realize high charge mobility, these materials are currently confined to fullerenebased organic solar cells(OSCs). Herein, we designed a pair of alternative D-π-A type copolymers, PE71 and PE72, to expand the application in non-fullerene OSCs, where benzo[1,2-b:4,5-b′]thiophene(BDT), thieno[3,2-b]thiophene(TT) and IID units were used as D, A and π-bridge, respectively. The aim of optimizing the length of alkyl chains on TT bridge is to ensure polymer solubility, crystallinity as well as miscibility with acceptor molecules. We find that PE71 and PE72 exhibit similar optical and electronic properties, but PE71 with shorter hexyl chain tends to aggregate into fiber-like structure. In the end, Y6 is selected as the electron acceptor because of suitable energy levels and complementary absorption spectrum. Finally, PE71:Y6 device realizes a power conversion efficiency(PCE) of 12.03%, which is obviously higher than that of PE72:Y6 device(9.74%) and is also the highest value for IID-based photovoltaic polymers. The charge transport, molecular aggregation, film morphology and energy loss analysis were systematically investigated. The improved photovoltaic performance of PE71:Y6 mainly originates from the better interpenetrating network structure toward facilitating exciton seperation and free charge carrier transportation.Our results indicate that IID-based D-π-A polymers can also be utilized in non-fullerene OSCs and the alkyl chains on the thieno[3,2-b]thiophene π-bridge have a vital effect on the photovoltaic performance.     

Novel Donor-Acceptor Copolymers Based on Dithienosilole and Ketone Modified Thieno[3,4-b]thiophene for Photovoltaic Application

Two novel donor-acceptor （D-A） copolymers containing dithienosilole （DTS） donor unit and ketone modified thieno[3,4-b]thiophene （TT） acceptor unit, namely, poly{4,4＇-bis（2-ethylhexyl）dithieno[3,2-b：2＇,Y-d]silole-5,5＇-diyl- alt-l-（thieno[3,4-b]thiophen-2-yl）-2-ethylhexan-l-one} （PDTSTT） and poly{4,4＇-bis（2-ethylhexyl）dithieno[3,2- b：2＇,3＇-d]silole-5,5＇-diyl-alt- 1-（4,6-bis（4-ethylhexylthien-2-yl）thieno[3,4-b]thiophen-2-yl）-2-ethylhexan- 1-one} （PD- TSDTTT）, were synthesized for the application in polymer solar cells （PSCs）, and the effects of thiophene bridge on the structural geometry and photovoiltaie performance have been investigated. The polymer PDTSTT and PDTSDTTT have the electrochemical bandgaps of 1.54 and 2.02 eV, together with low-lying HOMO energy levels of-5.47 and -5.37 eV, respectively. Molecular geometry simulation result shows that compared with PDTSTT, the insertion of thiophene bridge in PDTSDTTT can well relieve the steric hindrance between the TT and DTS unit, as confirmed by the reduced dihedral angle between DTS and DTTT unit. Under the illumination of AM 1.5G, 100 mW/cm2, the PSC based on PDTSDTTT/PC61BM （1 ： 1, w ： w） demonstrates a power conversion efficiency of 1.0%, which is significantly improved in comparison with the device based on PDTSTT/PC61BM （1 ： 2, w ： w） under the same experimental condition. This is because the enhanced planarity and increased effective conjugation of the main chain in PDTSDTTT promote the more favorable morphology for charge transportation, although PDTSTT possesses the broader absorption band than PDTSDTTT.     

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.     

Thienopyrazine or dithiadiazatrindene containing low band gap conjugated polymers for polymer solar cells

Four new low-band-gap alternating copolymers （P-1, P-2, P-3 and P-4） based on electron-rich benzodithiophene and newly developed electron-deficient units, thienopyrazine or dithiadiazatrindene derivatives, were synthesized by Stille polycondensation. All polymers exhibit good solubility in common organic solvents and a broad absorption band in the visible to near-infrared regions. The film optical band gaps of the polymers are in the range of 1.28-2.07 eV and the highest occupied molecular orbital （HOMO） energy levels are in the range of-4.99 eV to -5.28 eV. Bulk heterojunction polymer solar cells （PSCs） of the polymers were fabricated with phenyl-C61-butyric acid methyl ester （PC61BM） as acceptor material, and a power conversion efficiency of 0.80% was realized with P-1 as donor material.     

Small bandgap non-fullerene acceptor enables efficient PTB7-Th solar cell with near 0 eV HOMO offset

Three small bandgap non-fullerene(SBG NFAs) acceptors,BDTI,BDTI-2 F and BDTI-4 F,based on a carbon-oxygen bridged central core and thieno[3,4-b]thiophene linker,end-capped with varied electronwithdrawing terminal groups,were designed and synthesized.The acceptors exhibit strong absorption from 600 nm to 1000 nm.The optimal device incorporating designed NFA and PTB7-Th polymer donor achieves a power conversion efficiency of 9.11% with near 0 eV HOMO offset.The work presents a case study of efficient non-fullerene solar cells with small HOMO offsets,which is achieved by blending PTB7-Th with fine-tuned SBG acceptor.     

Dye-sensitized solar cells based on functionalized truxene structure

Three new metal-free organic dyes(TX1, TX2 and TX3) based on truxene core structure, with triphenylamine as the electron donor, thiophene as the p spacers, and cyanoacetic acid or rhodanine-3-acetic acid as the electron acceptor are designed and synthesized. Their UV–vis absorption spectra,electrochemical and photovoltaic properties were investigated. The cyanoacrylic acid is verified to be a better acceptor unit(meanwhile the anchoring group) compared to the rhodanine-3-acetic acid. And also, two anchoring groups in TX2 could provide stronger adsorption ability on the Ti O2 surface. In addition, the EIS results indicate a slower charge recombination processes for TX2. As a result, dye TX2 bearing two cyanoacetic acid outperforms the other two dyes, exhibiting the photo-conversion efficiency of 2.64%, with Jsc= 5.09 m A cm–2, Voc= 729 m V, FF = 71.1.     

A Near-infrared Non-fullerene Acceptor with Thienopyrrole-expanded Benzo[1,2-b:4,5-b′]dithiophene Core for Polymer Solar Cells

A near-infrared non-fullerene acceptor(NFA) BDTIC, based on thienopyrrole-expanded benzo[1,2-b:4,5-b′]dithiophene unit(heptacyclic S,N-heteroacene) as core, is designed and synthesized. The aromatic pyrrole ring with strong electron-donating ability in the core enhances the intramolecular charge transfer effect, finely tunes the optical bandgap and absorption profile of BDTIC, and thus results in a narrowed optical bandgap(E_g~(opt)) of 1.38 eV and a near-infrared absorption to 900 nm. When BDTIC is paired with donor polymer PBDB-T to fabricate organic solar cells, the optimized device achieves a best power conversion efficiency of 12.1% with a short-circuit current density of 20.0 mA·cm~(-2) and an open-circuit voltage of 0.88 V. The photovoltaic performance benefits from the broad absorption, weak bimolecular recombination, efficient charge separation and collection, and favorable blend morphology. This work demonstrates that thienopyrroleexpanded benzo[1,2-b:4,5-b′]dithiophene unit(heptacyclic S,N-heteroacene) is a promising building unit to construct high-performance NFAs by enhancing the intramolecular charge transfer effect, broadening absorption as well as maintaining good intermolecular stacking property.     

A new D–D–π–A dye for efficient dye-sensitized solar cells

New metal-free organic dye sensitizers containing mono-triphenylamine or bis-triphenylamine as the electron donor, a thiophene as the π-conjugated system, and a cyanoacrylic acid moiety as the electron acceptor were synthesized. The optical and electrochemical properties of the dyes were investigated,and their performance as sensitizers in solar cells was evaluated. Dye-sensitized solar cells based on dye containing bis-triphenylamine as the electron donor produced a photon-to-current conversion efficiency of 6.06%(Jsc = 14.21 m A/cm~2, Voc = 0.62 V, ff = 0.69) under 100 m W/cm~2 simulated AM 1.5 G solar irradiation(100 m W/cm~2).     

Effects of spin-coating speed on the morphology and photovoltaic performance of the diketopyrrolopyrrole-based terpolymer

Polymer solar cells（PSCs） were fabricated by combining a diketopyrrolopyrrole-based terpolymer（PTBT-HTID-DPP） as the electron donor, and [6,6]-phenyl C₆₁ butyric acid methyl ester(PC₆₁BM) as the electron acceptor, and the power conversion efficiency（PCE） of 4.31% has been achieved under AM 1.5 G(100 m W cm^-2) illumination condition via optimizing the polymer/PC₆₁BM ratio, the variety of solvent and the spin-coating speed. The impact of the spin-coating speed on the photovoltaic performance of the PSCs has been investigated by revealing the effects of the spin-coating speed on the morphology and the absorption spectra of the polymer/PC₆₁BM blend films. When the thickness of the blend films are adjusted by spin-coating a fixed concentration with different spin-coating speeds, the blend film prepared at a lower spin-coating speed shows a stronger absorption per unit thickness, and the correspond device shows higher IPCE value in the longer-wavelength region. Under the conditions of similar thickness, the blend film prepared at a lower spin-coating speed forms a more uniform microphase separation and smaller domain size which leads to a higher absorption intensity per unit thickness of the blend film in long wavenumber band, a larger short-circuit current density(J_sc) and a higher power conversion efficiency（PCE） of the PSC device. Noteworthily, it was found that spin-coating speed is not only a way to control the thickness of active layer but also an influencing factor on morphology and photovoltaic performance for the diketopyrrolopyrrole-based terpolymer.     

Side-chain engineering of high-efficiency conjugated polymer photovoltaic materials

In recent years,conjugated polymers have attracted great attention in the application as photovoltaic donor materials in polymer solar cells(PSCs).Broad absorption,lower-energy bandgap,higher hole mobility,relatively lower HOMO energy levels,and higher solubility are important for the conjugated polymer donor materials to achieve high photovoltaic performance.Side-chain engineering plays a very important role in optimizing the physicochemical properties of the conjugated polymers.In this article,we review recent progress on the side-chain engineering of conjugated polymer donor materials,including the optimization of flexible side-chains for balancing solubility and intermolecular packing(aggregation),electron-withdrawing substituents for lowering HOMO energy levels,and two-dimension(2D)-conjugated polymers with conjugated side-chains for broadening absorption and enhancing hole mobility.After the molecular structural optimization by side-chain engineering,the2D-conjugated polymers based on benzodithiophene units demonstrated the best photovoltaic performance,with powerconversion efficiency higher than 9%.     

Synthesis and photoelectric properties of an organic dye containing benzo[1,2-b：4,5-b’]dithiophene for dye-sensitized solar cells

A novel benzodithiophene-containing organic dye BDT was synthesized and characterized as a sensitizer for a nanocrystalline TiO₂-based dye-sensitized solar cell.The BDT dye shows two major electronic absorptions.The absorption of the BDT dye covers a broad visible range from 300 nm to 550 nm.The benzodithiophene unit was used as aπbridge with several advantages:（1） It facilitates the electron transfer from the donor to the acceptor;（2） A facile structural modification on the 4,8-positions in the benzodithiophene unit can be achieved;（3） Fusing benzene with two flanking thiophene units improves the thermal stability.Under simulated AM1.5G solar light（100 mW/cm²） illumination,the DSC based on BDT gives a power conversion efficiency of 1.78%.     

Comparison of conventional and inverted structures in fullerene-free organic solar cells

A n-type small molecule DC-IDT2 F, with 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b]dithiophene as a central building block, furan as π-bridges, and 1,1-dicyanomethylene-3-indanone as end acceptor groups,was synthesized and used as an electron acceptor in solution-processed organic solar cells(OSCs). DC-IDT2 F exhibited good thermal stability, broad and strong absorption in 500–850 nm, a narrow bandgap of 1.54 e V,LUMO of –3.88 eV, HOMO of –5.44 eV and an electron mobility of 6.5 × 10–4cm2/(V·s). DC-IDT2F-based OSCs with conventional and inverted structures exhibited power conversion efficiencies of 2.26 and 3.08%, respectively. The effect of vertical phase separation and morphology of the active layer on the device performance in the two structures was studied.     

Synthesis and photovoltaic properties of low bandgap dimeric perylene diimide based non-fullerene acceptors

Non-fullerene organic acceptors have attracted increasing attention in recent years. One of the challenges in the synthesis of non-fullerene organic acceptors is to tune the absorption spectrum and molecular frontier orbitals, affording low bandgap molecules with improved absorption of the near-infrared solar photons. In this paper, we present the synthesis, optoelectronic and photovoltaic properties of a series of dimeric perylene diimide(PDI) based non-fullerene acceptors. These PDI dimers are bridged by oligothiophene(T) from 1T to 6T. With the increase of the oligothienyl size, the highest occupied molecular orbital(HOMO) energy is raised from ?5.65 to ?5.10 e V, while that of the lowest unoccupied molecular orbit(LUMO) is kept constant at ?3.84 e V, affording narrow bandgap from 1.81 to 1.26 e V. The absorption from the oligothiophene occurs between 350 and 500 nm, which is complementary to that from its bridged PDI units, leading to a wide spectral coverage from 350 to 850 nm. The optimal dihedral angle between the bridged two perylene planes is dependent on the oligothienyl size, varying from 5° to 30°. The solubility of the dimers depends on the oligothienyl size and can be tuned by the alkyl chains on the bridged thienyl units. The possible applications as the solution-processable non-fullerene organic acceptor is primarily studied using commercial P3 HT as the blend donor. The photovoltaic results indicate that 1T, 4T and 6T all yield a higher efficiency of ?1.2%, whereas 2T, 3T and 5T all give a lower efficiency of 0.5%. The difference in the cell performance is related with the tradeoff between the differences of absorption, HOMO level and film-morphology between these dimers.     

8.0% Efficient all-polymer solar cells based on novel starburst polymer acceptors

The polymer N2200, with its π-conjugated backbone composed of alternating naphthalene diimide(NDI) and bithiophene(DT)units, has been widely used as an acceptor for all-polymer solar cells(all-PSCs) owing to its high electron mobility and suitable ionization potential and electron affinity. Here, we developed two naphthalene diimide derivatives by modifying the molecular geometry of N2200 through the incorporation of a truxene unit as the core and NDI-DTas the branches. These starburst polymers exhibited absorption spectra and molecular orbital energy levels that were comparable to N2200. These copolymers were paired with the wide-bandgap polymer donor PTz BI-O to fabricate all-polymer solar cells(all-PSCs), which displayed impressive power conversion efficiencies up to 8.00%. The improved photovoltaic performances of all-PSCs based on these newly developed starburst acceptors can be ascribed to the combination of increased charge carrier mobilities, reduced bimolecular recombination, and formation of more favorable film morphology. These findings demonstrate that the construction of starburst polymer acceptors is a feasible strategy for the fabrication of high-performance all-PSCs.     

Combining chlorination and sulfuration strategies for high-performance all-small-molecule organic solar cells

Three small-molecule donors based on dithieno [2,3-d:2’,3 ’-d’]-benzo[1,2-b:4,5-b’] dithiophene(DTBDT)unit were designed and synthesized by side chain regulation with chlorinated or/and sulfurated substitutions(namely ZR1,ZR1-Cl,and ZR1-S-Cl respectively),along with a crystalline non-fullerene acceptor IDIC-4 Cl with a chlorinated 1,1-dicyanomethylene-3-indanone(IC) end group.Energy levels,molar extinction coefficients and crystallinities of three donor molecules can be effectively altered by combining chlorination and sulfuration strategies.Especially,the ZR1-S-Cl exhibited the best absorption ability,lowest higher occupied molecular orbital(HOMO) energy level and highest crystallinity among three donors,resulting in the corresponding all-small-molecule organic solar cells to produce a high power conversion efficiency(PCE) of 12.05% with IDIC-4 Cl as an acceptor.     

Ladder-type Diindenopyrazine Based Conjugated Copolymers for Organic Solar Cells with High Open-circuit Voltages

A ladder-type diindenopyrazine （IPY） was synthesized and used as a building block for constructing conjugated copolymers. Three copolymers based on the IPY moiety were obtained via the Suzuki coupling reaction with dif- ferent monomers, including 4,7-dithien-2-yl-2,1,3-benzothiadiazole （DBT）, 5,8-dithien-2-yl-2,3-diphenylquinoxa- line （DTQ）, and 5,8-dithien-2-yl-2,3-di（4-fluorophenyl）quinoxaline （DFTQ）. The obtained polymers were charac- terized by 1H NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry, and gel permeation chro- matography （GPC）. Owing to the four solubilizing alkyl chains on the IPY unit, all the three copolymers have good solubility in common solvents. These polymers have deep-lying HOMO energy levels in the range of-5.55-5.60 eV, and exhibit field-effect mobilities as high as 0.006 cm2.V-l.s i. Photovoltaic applications of these polymers as light-harvesting and hole-conducting materials were investigated in conjunction with [6,6]-phenyl-C6rbutyric acid methyl ester （PC61BM）. Both conventional and inverted devices were fabricated based on these three polymers. A power conversion efficiency （PCE） of 2.53% and a high open-circuit voltage of 1.00 V were obtained under simu- lated solar light AM 1.5 G （100 mW/cm2） from an inverted solar cell with an active layer containing 25 wt% lad- der-type IPY containing copolymer （PIPYDTQ） and 75 wt% PC61BM. Moreover, a high open-circuit voltage of 1.02 V and a PCE of 2.40% were achieved from a conventional solar cell based on PIPYDTQ.     

Second-order Nonlinear Optical Properties of a Series of Benzothiazole Derivatives

The second-order nonlinear optical (NLO) properties of a series of benzothiazole derivatives were studied by use of the ZINDO-SOS method.These chromophores are formed by a donor- π- bridge-acceptor system,based on a nitro group connected with benzothiazole as the acceptor and a hydroxyl-functional amino group as the donor.For the purpose of comparison,we also designed molecules in which nitrobenzene is an acceptor,The calculation results indicate that benzothiazole derivatives exhibit larger second-odrder polarizabilities than nitrobenzene derivatives.In order to clarify the origin of the NLO response of these chromophores,their electron properties were investigated as well.The benzothiazole derivatives are good candidates for application in electro-optical device due to their high optical nonlinearities,good thermal and photonic stability.     

Efficient solar cells based on cosensitizing porphyrin dyes containing a wrapped donor,a wrapped π-framework and a substituted benzothiadiazole unit

Three new porphyrin dyes XW45-XW47 have been synthesized employing a dialkoxy-wrapped phenothiazine donor, a tetraalkoxy-wrapped porphyrin π-framework, a benzothiadiazole(BTD)-based auxiliary acceptor, and an anchoring benzoic acid group. On the basis of our previously reported dye XW36, XW45 was synthesized by introducing a BTD unit to broaden the absorption spectrum, further introducing a hexyl-substituent into the BTD unit afforded XW46, and an additional fluorine atom was introduced to the carboxyphenyl acceptor to afford XW47. As expected, the BTD unit obviously broadens and red-shifts the absorption threshold of XW45-XW47 to ca.750 nm. Dye-sensitized solar cells(DSSCs) were fabricated based on a cobalt electrolyte using chenodeoxycholic acid(CDCA) as the coadsorbent. Under full sun illumination, XW45 exhibits an efficiency of 9.73%, which is slightly lower than that of 10.19% obtained for the reference dye SM315. By contrast, XW46 and XW47 show higher efficiencies than SM315 owing to the improved anti-aggregation ability associated with the hexyl group on the BTD unit and better ICT effect induced by the fluorine atom on the carboxyphenyl unit. Thus, XW47 exhibits the highest efficiency of 10.41% among the porphyrin dyes. Furthermore, PT-C6 was used as the cosensitizer to improve the light harvesting ability and efficiencies of the cells due to its broad absorption within 350–560 nm. Thus, high efficiencies of 10.32%, 11.38% and 10.90% were achieved for the cosensitized solar cells based on XW45–XW47, respectively, owing to the obviously enhanced photocurrent density(JSC). In addition, under 30% full sun illumination, XW46+PT-C6 exhibits a high efficiency of 13.08%. These results give an effective method for building high performance DSSCs through the cosensitization of porphyrin dyes containing a wrapped donor, a wrapped porphyrin framework and a properly substituted auxiliary benzothiadiazole unit.