Feiyan Wu

Triple Dipole Effect from Self‐Assembled Small‐Molecules for High Performance Organic Photovoltaics

A novel triple dipole effect has been observed for Cl-assisted self-assembled small-molecules on ITO substrate, and a highest polymer solar cell performance of 9.2% is obtained.

Enhanced Power-Conversion Efficiency in Inverted Bulk Heterojunction Solar Cells using Liquid-Crystal-Conjugated Polyelectrolyte Interlayer

Two novel liquid-crystal-conjugated polyelectrolytes (LCCPEs) poly[9,9-bis[6-(4-cyanobiphenyloxy)-hexyl]-fluorene-alt-9,9-bis(6-(N,N-diethylamino)-hexyl)-fluorene] (PF6Ncbp) and poly[9,9-bis[6-(4-cyanobiphenyloxy)-hexyl]-fluorene-alt-9,9-bis(6-(N-methylimidazole)-hexyl]-fluorene] (PF6lmicbp) are obtained by covalent linkage of the cyanobiphenyl mesogen polar groups onto conjugated polyelectrolytes. After deposition a layer of LCCPEs on ZnO interlayer, the spontaneous orientation of liquid-crystal groups can induce a rearrangement of dipole moments at the interface, subsequently leading to the better energy-level alignment. Moreover, LCCPEs favors intimate interfacial contact between ZnO and the photon harvesting layer and induce active layer to form the nanofibers morphology for the enhancement of charge extraction, transportation and collection. The water/alcohol solubility of the LCCPEs also enables them to be environment-accepted solvent processability. On the basis of these advantages, the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C60-butyric acid methyl ester (PC60BM)-based inverted polymer solar cells (PSCs) combined with ZnO/PF6Ncbp and ZnO/PF6lmicbp bilayers boost the power conversion efficiency (PCE) to 3.9% and 4.2%, respectively. Incorporation of the ZnO/PF6lmicbp into the devices based on a blend of a narrow band gap polymer thieno[3,4-b]thiophene/benzodithiophene (PTB7) with [6,6]-phenyl C70-butyric acid methyl ester (PC71BM) affords a notable efficiency of 7.6%.

Interface-induced face-on orientation of the active layer by self-assembled diblock conjugated polyelectrolytes for efficient organic photovoltaic cells

Systematic investigation of the instinctive self-assembly of diblock conjugated polyelectrolytes (CPEs) as electron transport layers (ETLs) on the arrangement and morphology of the upper active layer in polymer solar cells (PSCs) is usually ignored. The two water/alcohol-soluble diblock CPEs with different terminal ionic groups, PFEO-b-PTNBr and PFEO-b-PTImBr, offer an ohmic contact between the ITO electrode and the active layer by substantially reducing the work function of ITO via modulation of the interfacial dipoles. More intriguingly, the spontaneous self-assembly of the diblock polymers causes an ordered scolopendra-like conformation to develop on the ITO cathode. These self-assembled diblock CPEs can act as a template to further partially induce a preferable face-on orientation of the donor component. Surprisingly, n-type self-doping is observed in the two CPEs based on the p-type conjugated backbone, particularly in PFEO-b-PTImBr with imidazolium salt. As a result, the power conversion efficiencies (PCEs) of the devices based on PTB7-Th:PC71BM are remarkably enhanced to 9.0% for PFEO-b-PTNBr and 9.4% for PFEO-b-PTImBr. Intriguingly, both ZnO/PFEO-b-PTNBr and ZnO/PFEO-b-PTImBr based PTB7-Th:PC71BM devices exhibit superior PCEs compared to that of the classical published and commonly used PFN (8.42%) and PFNBr (8.38%) ETLs based devices. These results indicate that the development of self-assembled diblock CPEs opens a new point of view and provides a straightforward approach to achieve high performance polymer solar cells by simultaneously modulating the morphology of the interlayer, active layer and interfacial work function.

Solution-processed small molecules based on benzodithiophene and difluorobenzothiadiazole for inverted organic solar cells

Monofluorinated small molecules have been proved to be promising donors for high performance small molecular solar cells (SMSCs). Herein, two solution-processable acceptor–donor–acceptor (A–D–A) type small molecules (SMs) with 4,8-bis(5-hexylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) as the donor core and benzothiadiazole (BT) or difluoro-2,1,3-benzothiadiazole (DFBT) as the acceptor unit, namely BDT(TBTTT6)2 and BDT(TffBTTT6)2 respectively, have been designed and synthesized to extensively investigate the effect of fluorination on the optoelectronic properties, molecular organization, and photovoltaic performance of SMSCs. The fluorinated BDT(TffBTTT6)2 shows a relatively broader and stronger absorbance, more favorable molecular packing and deeper highest occupied molecular orbital (HOMO) energy level than nonfluorinated BDT(TBTTT6)2, leading to a higher open-circuit voltage (Voc), circuit current (Jsc), fill factor (FF), and hole mobility in SMSCs. Additionally, the power conversion efficiency (PCE) is significantly improved from 1.76% to 4.17% for BDT(TffBTTT6)2 and from 1.3% to 3.17% for BDT(TBTTT6)2 based inverted devices after optimization by 1,8-diiodooctane (DIO) and thermal annealing (TA). It should be noted that the PCE of 4.17% is the highest reported value for the solution-processed inverted SMSCs based on BDT and BT units. Grazing incident X-ray diffraction (GIXRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) demonstrate that the fluorinated small molecule optimization by DIO and thermal annealing promote a more well-intermixed microphase morphology as well as a better donor/acceptor interpenetrating network in an active film for more efficient charge transfer and transportation than the nonfluorinated one, leading to a dramatic improvement in the device performance. These results unambiguously demonstrate that the introduction of F atoms into the molecular backbone as well as optimization by the solvent additive process can be an effective strategy for the development of electron-donating materials for stabilized inverted-based SMSCs.

Novel photovoltaic donor 1–acceptor–donor 2–acceptor terpolymers with tunable energy levels based on a difluorinated benzothiadiazole acceptor

A novel donor 1–acceptor–donor 2–acceptor (D1–A–D2–A) type terpolymer PBDT-DTffBT-F-DTffBT was prepared to further tune the energy levels of donor–acceptor (D–A) type copolymer PBDT-DTffBT and PF-DTffBT, and the corresponding optoelectronic properties have been investigated. By incorporating the weak donor fluorene unit into the backbone of PBDT-DTffBT, the PBDT-DTffBT-F-DTffBT exhibits a lower HOMO level and higher LUMO level compared with PBDT-DTffBT as expected. The polymer solar cell (PSC) based on PBDT-DTffBT-F-DTffBT affords the improved PCE with a higher Voc of 0.853 V compared to the D–A copolymer PBDT-DTffBT and PF-DTffBT. Therefore, through carefully choosing the suitable donor group, the class of D1–A–D2–A type copolymers would be a promising organic semiconducting material.

High charge mobility polymers based on a new di(thiophen-2-yl)thieno[3,2-b]thiophene for transistors and solar cells

A novel donor–acceptor (D–A) donor di(thiophen-2-yl)thieno[3,2-b]thiophene (DTTT) was designed and synthesized according to the structure of the diketopyrrolopyrrole (DPP) acceptor. The random polymers with different ratios of DTTT/DPP, PDTTT-T-DPP_3/7 and PDTTT-T-DPP_4/6, and the alternating polymer PDTTT-DPP were synthesized through random copolymerization and direct arylation schemes respectively. Although the planarity of the DTTT structure is not as good as the DPP molecule, incorporation of DTTT and DPP molecules stimulates the polymers to achieve a high hole mobility, as determined by polymer thin film transistor (PTFT) devices, as well as yield fine power conversion efficiencies in polymer solar cells. Especially for PDTTT-T-DPP_3/7, the PTFT devices obtain a high hole mobility of 0.627 cm2 V−1 s−1. The results demonstrate that DTTT-T-DPP based random polymers are promising candidates for various high-performance organic electronic devices. Moreover, the strategy of designing new D/A molecules on the basis of high-performance structures is proved to be effective.

Modulation of the molecular geometry of carbazolebis(thiadiazole)-based conjugated polymers for photovoltaic applications

Two novel conjugated copolymers PTBDTCBT and PDTSCBT are prepared by alternating copolymerzation of N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole (CBT) with alkylthienyl benzodithiophene (TBDT) and dithenosilole (DTS), respectively. The energy levels and molecular geometry of all of the CBT-based polymers are compared by theoretical calculation and experimental observation. It has been found that the band gap and energy levels of all of the CBT-based polymers are well modulated by various building blocks, and the molecular geometry of polymers varies with the block structures as well. Among these CBT-based polymers, PDTSCBT shows the lowest band gap (1.53 eV), which matches the solar flux well, but the low degree of crystallinity and absence of preferential alignment of the π–π stacking result in a relative low PCE of 1.52%. However, its 2-D structure endows PTBDTCBT with favorable molecular packing to achieve the PCE of 1.71% under illumination (AM 1.5G 100 mW cm−2) without considerable optimization, although its band gap is larger than PDTSCBT. These results indicate that a good balance between energy levels and molecular microstructure arrangement is crucial for the performance improvement of photovoltaic with CBT-based polymers.

Post‐Treatment‐Free Main Chain Donor and Side Chain Acceptor (D‐s‐A) Copolymer for Efficient Nonfullerene Solar Cells with a Small Voltage Loss

Main chain donor and side chain acceptor (D-s-A) copolymers are an important branch of the D-A copolymer family. However, the development of D-s-A copolymers significantly falls behind the alternative D-A copolymers, especially for organic solar cells, because a breakthrough in device performance is not yet obtained with a reported power conversion efficiency (PCE) of 2%-4%. Herein, a newly developed D-s-A copolymer PDRCNBDT, bearing 2-(1, 1-dicyanomethylene) rhodanine pendant group as the donor material, delivers a high PCE of 5.3% for nonfullerene solar cells. To the best of our knowledge, this is the best value reported for D-s-A copolymers to date. The improved PCE is observed to be associated with a very small energy loss (Eloss ) of 0.57 eV, accompanied by a high open-circuit voltage (Voc ) of 1.015 eV. It is important to note that this efficient D-s-A copolymer is employed in organic solar cells (OSCs), free of additive and annealing treatments.

Alternating Terpolymers Based on Tunable Bi-donors with Manipulating Energy Levels and Molecular Geometry

Three novel regular acceptor-donor1-acceptor-donor2(A-D1-A-D2) terpolymers were prepared via embedding a second donor(D2) unit into the traditional D-A backbone to manipulate the energy levels and molecular geometry with no complex synthesis or solubility loss. In these A-D1-A-D2 terpolymers, benzodithiophene(BDT, D1) and diketopyrrolopyrrole(DPP, A) were selected as the basic skeleton, and the dithienopyrrole(DTPy), carbazole(CZ) and fluorine(FL) units with different electron donating ability were chosen as the second donor unit(D2). The HOMO energy levels can be effectively modulated by only varying D2 unit because of the push-pull interaction between donor and acceptor units. Versus the D-A bipolymer PDPP-BDT, incorporation of the D2 unit into the copolymers can distinctly lower the highest occupied molecular orbital(HOMO) levels to –5.47 eV for PDDPP-BDT-DTPy, –5.38 eV for PDDPP-BDT-CZ and –5.23 eV for PDDPP-BDT-FL, which shows the strong dependence on electron-donating ability. Density functional theory(DFT) simulation and X-ray diffraction(XRD) measurements also reveal the effect of the D2 units on the molecular geometry of the terpolymers and their molecular packing. Notably, a PDDPP-BDT-DTPy combined with a thiophene ring and forked tail pendant away from the backbone had less back-bone torsion and more compact packing than the other two counterparts. These results demonstrate that embedding a second donor(D2) unit into the backbone to construct an A-D1-A-D2 structure can be an effective and direct strategy to manipulate the energy levels and molecular geometry and develop organic semiconducting materials.