Low-lying HOMO Levels Research Articles

Benzo[d]thiazole Based Wide Bandgap Donor Polymers Enable 19.54% Efficiency Organic Solar Cells Along with Desirable Batch-to-Batch Reproducibility and General Applicability.

The limited selection pool of high-performance wide bandgap (WBG) polymer donors is a bottleneck problem of the nonfullerene acceptor (NFA) based organic solar cells (OSCs) that impedes the further improvement of their photovoltaic performances. Herein, a series of new WBG polymers, namely PH-BTz, PS-BTz, PF-BTz, and PCl-BTz, are developed by using the bicyclic difluoro-benzo[d]thiazole (BTz) as the acceptor block and benzo[1,2-b:4,5-b']dithiophene (BDT) derivatives as the donor units. By introducing S, F, and Cl atoms to the alkylthienyl sidechains on BDT, the resulting polymers exhibit lowered energy levels and enhanced aggregation properties. The fluorinated PBTz-F not only exhibits a low-lying HOMO level, but also has stronger face-on packing order and results in more uniform fibril-like interpenetrating networks in the related PF-BTz:L8-BO blend. A high-power conversion efficiency (PCE) of 18.57% is achieved. Moreover, PBTz-F also exhibits a good batch-to-batch reproducibility and general applicability. In addition, ternary blend OSCs based on the host PBTz-F:L8-BO blend and PM6 guest donor exhibits a further enhanced PCE of 19.54%, which is among the highest values of OSCs.

Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Based on Co-Dopant and Co-Host System.

The low-lying HOMO level of the blue emitter and the interfacial miscibility of organic materials result in inferior hole injection, and long exciton lifetime leads to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), so the efficiencies of blue phosphorescent organic light-emitting diodes (PhOLEDs) are still unsatisfactory. Herein, we design co-host and co-dopant structures to improve the efficiency of blue PhOLEDs by means of solution processing. TcTa acts as hole transport ladder due to its high-lying HOMO level, and bipolar mCPPO1 helps to balance carriers’ distribution and weaken TPA. Besides the efficient FIr6, which acts as the dominant blue dopant, FCNIrPic was introduced as the second dopant, whose higher HOMO level accelerates hole injection and high triplet energy facilitates energy transfer. An interesting phenomenon caused by microcavity effect between anode and cathode was observed. With increasing thickness of ETL, peak position of electroluminescence (EL) spectrum red shifts gradually. Once the thickness of ETL exceeded 140 nm, emission peak blue-shifts went back to its original position. Finally, the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of blue phosphorescent organic light-emitting diode (PhOLED) went up to 20.47 cd/A, 11.96 lm/W, and 11.62%, respectively.

Influence of Fluorine Substitution on the Photovoltaic Performance of Wide Band Gap Polymer Donors for Polymer Solar Cells.

The design and development of wide band gap (WBG) polymer donors are critical for achieving high power conversion efficiencies (PCEs) in polymer solar cells. In this work, four WBG polymer donors, Q4, Q5, Q6, and Q7, with different numbers and positions of fluorine substitution (n = 0, 2, 2, and 4, respectively) were prepared, and the effect of fluorination on their photovoltaic performance was systematically investigated. When blended with a small-molecule electron acceptor MeIC, the devices based on Q4, Q5, Q6, and Q7 showed PCEs of 10.34, 11.06, 5.26, and 0.48%, respectively. When coupled with a low band gap polymer acceptor PYIT to fabricate all-polymer solar cells (all-PSCs), while the other three polymers (Q5-Q7) exhibited much lower PCEs in the range of 0.12-6.71%, the Q4 polymer-based all-PSCs showed the highest PCE of 15.06%, comparable to that of the devices fabricated with the star polymer donor PM6 (PCE = 15.00%). Detailed physicochemical and morphological studies revealed that an over-substitution of F in Q7 results in undesired low-lying HOMO levels and phase separation with the acceptors, thus resulting in its inferior PCEs. Moreover, the less F-substitution and controlling of the positions of F-substitution position in Q4 and Q5 can improve the HOMO energy level matching as well as morphologies between these two polymers with the acceptors, which in turn gives rise to higher performances. Clearly, our results indicate that Q4 is a promising donor candidate for high-performance all-PSCs, and the fine-tuning of both the number and positions of F-substitution in the polymer backbone is essential in developing high-performance WBG polymer donors.

Highly electrochemically and thermally stable donor–π–acceptor triphenylamine-based hole-transporting homopolymersviaoxidative polymerization

Polymers combining high electrochemical and thermal stability, good solubility, highTgand high coke residue with low-lying HOMO levels and reasonable hole mobilities in thin films are reported in this study.

Truxene Functionalized Star-Shaped Non-fullerene Acceptor With Selenium-Annulated Perylene Diimides for Efficient Organic Solar Cells.

An electron acceptor with a truxene core and ring-fusion perylene diimide (PDI) tripolymer annulated by selenium (Se) branch, named as FTr-3PDI-Se, is designed and synthesized. FTr-3PDI-Se exhibits large conjugated planar conformation, strong absorption spectra in the regions of 300–400 and 450–550 nm, the deep HOMO energy level of 6.10 eV, and high decomposition temperature above 400°C. The FTr-3PDI-Se: PBDB-T-2Cl based device achieved a disappointing power conversion efficiency (PCE) of 1.6% together with a high Voc of 1.12 V. The low PCE was due to the large aggregates of blend film, the imbalanced hole/electron transport and low PL quenching efficiencies. The high Voc can be attributed to the high-lying LUMO level of FTr-3PDI-Se and the low-lying HOMO level of PBDB-T-2Cl. Our research presents an interesting and effective molecule-designing method to develop non-fullerene acceptor.

A Simple Approach to Prepare Chlorinated Polymer Donors with Low-Lying HOMO Level for High Performance Polymer Solar Cells

Both photovoltaic performance and cost of materials are the key factors for commercial application of polymer solar cells (PSCs). In this study, we designed and synthesized two new conjugated polym...

Multichloro-Substitution Strategy: Facing Low Photon Energy Loss in Nonfullerene Solar Cells

The large photon energy loss (Eloss) in the conversion from photons to electrons is still one of the critical factors in limiting the performance of polymer solar cells (PSCs). In order to simultaneously obtain the decreased Eloss and expanded spectral response, the reasonable design of wide band gap conjugated polymer with a low-lying HOMO level is still a great challenge in nonfullerene PSCs. In this work, four D–A type wide band gap conjugated polymers with introduced chlorine atoms on the conjugated thienyl-side chains of the BDT units, namely, PBT1Cl-Bz, PBT2Clas-Bz, PBT2Cls-Bz, and PBT4Cl-Bz, were synthesized and characterized. Because of the large dipole moment of carbon–chlorine (C–Cl) and the strong noncovalent interaction of Cl···S and Cl···π, the tetrachloro-substituted PBT4Cl-Bz exhibits a much deeper HOMO level (−5.64 eV) than the other three polymers. Therefore, a high open-circuit voltage (Voc) of 0.96 V was achieved while keeping a decent Jsc of 16.04 mA cm–2 when matched with nonfullerene...

Conjugated polymer donor with alkylthio-thiophene π-bridge for efficient polymer solar cells

Abstract A new conjugated polymer donor with alkylthio-substituted thiophene as π-bridge (namely PBZ-TS) is developed to further exploit the capability of alkylthio chains in designing high-performance donor materials. PBZ-TS possesses a broad absorption feature, a low-lying HOMO level, and high crystallinity. The polymer solar cell devices based on PBZ-TS:m-ITIC demonstrate high open-circuit voltage (Voc) of 0.93 V. Especially, relative to the PBZ-TS:m-ITIC (1:1, w/w) based binary device, an enhanced power conversion efficiency (PCE) of 8.66% is achieved from the PBZ-TS:m-ITIC:PC71BM (1:0.6:0.4, w/w) based ternary device resulting from the suppressed carrier recombination of binary blends. The results demonstrate that using alkylthio-substituted thiophene as π-bridge is an effective strategy to down-shift HOMO level and increase Voc of the polymer donor materials for PSCs.

New Quinoxaline-Containing Monomers for Narrow-Bandgap Polymers

Two new fused quinoxaline-containing monomers—2,3-bis(9-(2-decyltetradecyl)-9H-carbazol-3-yl)dithieno[3,2-f:2'3'-h]quinoxaline (М1) and 2,5-di(nonadecan-3-yl)bis[1,3]thiazolo[4,5-a:5',4'-c]bisthieno[3,2-h:2',3'-j]phenazine (М2)—have been synthesized in high yields of 88 and 83% as promising building blocks of D-A polymers for photovoltaic applications. The optical bandgaps, found from the absorption edge, are 2.79 and 2.88 eV, respectively. The HOMO/LUMO energies of М1 and М2 are–5.83/–2.96 and–5.83/–2.98 eV, respectively. Both monomers have low-lying HOMO levels, which is favorable for a high open-circuit voltage and a high stability in air in the development of PSCs. The E g ec values of monomers М1 and М2 are 2.87 and 2.85 eV and are consistent well with the optical bandgap (2.79 and 2.88 eV, respectively).

D-A-D-Typed Hole Transport Materials for Efficient Perovskite Solar Cells: Tuning Photovoltaic Properties via the Acceptor Group.

Two D-A-D-structured hole-transport materials (YN1 and YN2) have been synthesized and used in perovskite solar cells. The two HTMs have low-lying HOMO levels and impressive mobility. Perovskite-based solar cells (PSCs) fabricated with YN2 showed a power conversion efficiency (PCE) value of 19.27% in ambient air, which is significantly higher than that of Spiro-OMeTAD (17.80%). PSCs based on YN1 showed an inferior PCE of 16.03%. We found that the incorporation of the stronger electron-withdrawing group in the HTM YN2 improves the PCE of PSCs. Furthermore, the YN2-based PSCs exhibit good long-term stability retaining 91.3% of its initial efficiency, whereas PSCs based on Spiro-OMeTAD retained only 42.2% after 1000 h lifetime (dark conditions). These promising results can provide a new strategy for the design of D-A-D HTMs for PSC applications in future.

Anthradithiophene-based liquid crystal molecules: High carrier mobilities enhanced by rubbed polyimides for the application in organic field-effect transistors

Abstract We synthesize three liquid crystal molecules with angular anthradithiophene (a-ADT) as the core for the application in organic film-effect transistors (OFETs). Thiophene-octyl group and thiophene-dodecyl group are introduced on both sides of the a-ADT core to yield compounds Bis-C8T-a-ADT and Bis-C12T-a-ADT. Moreover, thiophene-dodecyl group is attached to the a-ADT core on one side to yield compound C12T-a-ADT. The synthesized compounds are measured by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) to determine their mesomorphic properties. Compound Bis-C8T-a-ADT shows nematic and smectic A phases while both compounds Bis-C12T-a-ADT and C12T-a-ADT reveal smectic A phases. The optical and electrochemical properties of the compounds are characterized by UV–vis spectroscopy and cyclic voltammetry (CV). The HOMO levels for Bis-C8T-a-ADT and Bis-C12T-a-ADT are estimated to be −5.27 eV while the HOMO level of C12T-a-ADT is estimated to be −5.34 eV. The low-lying HOMO levels of the molecules indicate their great oxidation stability in OFET devices. The carrier mobilities are observed to increase 10–1000 times using rubbed polyimide (PI), resulting from the better arrangements of the liquid crystal molecules, and excellent carrier mobilities of 1.06 and 1.02 cm2/Vs are obtained from compounds Bis-C12T-a-ADT and C12T-a-ADT, respectively.

A Ladder-type Heteroheptacene 12H-Dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene Based D-A Copolymer with Strong Intermolecular Interactions toward Efficient Polymer Solar Cells.

Ladder-type electron-donating units for D-A copolymers applied in polymer solar cells usually comprise multiple tetrahedral carbon bridges bonded with out-of-plane alkyl chains for desirable solubility for device processing. However, molecular packing of resultant copolymers in the solid state and charge transport within devices are also impeded in spite of with multiple fused aromatic backbones. To mitigate this issue, a structurally well-defined ladder-type electron-donating heteroheptacene, 12H-dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene (DTTF) with an extended conjugated backbone and a single tetrahedral carbon bridge attached with two bulky alkyl chains was designed and synthesized. The copolymerization of DTTF with 4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) afforded a soluble D-A copolymer (PDTTF-DTBT) with a medium optical band gap of 1.72 eV and low-lying HOMO level at -5.36 eV. PDTTF-DTBT unprecedentedly exhibits strong intermolecular stacking ability and presents preferential face-on orientation on both ZnO and PEDOT:PSS layers. The improved packing order and appropriate phase separation of both the copolymer and PC71BM in the bulk heterojunction blend on the ZnO layer over on the PEDOT:PSS layer lead to much improved power conversion efficiency of ∼8.2% in the inverted solar cell device, among the highest for reported ladder-type D-A copolymers. The research demonstrates that it is an effective method to incorporate a single tetrahedral carbon bridge to the molecular center of a ladder-type heteroacene with heavily extended π-conjugation to prepare D-A copolymers toward highly efficient PSCs.

Novel regular D–A-conjugated polymers based on 2,6-bis(6-fluoro-2-hexyl-2H-benzotriazol-4-yl)-4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene derivatives: Synthesis, optoelectronic, and electrochemical properties

Regioregular (P1) and random (P2) polymers were prepared by the reaction of equimolar amounts of dibromides M2 or M3 with bis(stannyl) monomer M1. The regioregularity of the conjugated chain in P1 was shown to affect considerably its optical properties. The HOMO/LUMO values and electrochemical band gap for polymers P1 or P2 found by cyclic voltammetry are–5.40/–3.40,–5.31/–3.33, and 2.00/1.96 eV, respectively. The LUMO levels for P1 and P2 are located higher than for PC71BM (–4.3 eV) by 0.84 and 0.97 eV, respectively. The low-lying HOMO levels of P1 and P2 are favorable for the high value of open-circuit voltage and make these polymers attractable candidates for polymer solar cells.

D-A conjugated polymers containing substituted thiophene, 1,3,4-oxadiazole and non-conjugation linkers: Synthesis and study of optical and electrochemical properties

In this communication, we report synthesis and characterization of new D-A conjugated polymers (P1-P3) consisting of electron-donating (D) 3,4-didodecyloxythiophene, electron-accepting (A) 1,3,4-oxadiazole unit and non-conjugation linkers. The conjugated segment in P1-P3 contains only five aromatic rings resulting in short conjugation length, but has an alternate D-A arrangement which significantly enhances the intramolecular charge transfer (ICT) interaction within the segment. As a result, these polymers exhibited low optical band gap in the range 2.51–2.76 eV. Fluorescence emission studies revealed that the polymer thin films emit intense blue light with emission maxima in the wavelength rage 430–480 nm. All three polymers undergo both oxidation and reduction processes under electrochemical conditions. Further, these polymers (P1–P3) exhibit low-lying HOMO and LUMO levels as a result of D-A structure of the conjugated segment. Polymer light-emitting devices were fabricated using these polymers as emissive layer with a device configuration of ITO/MoO 3/polymer/LiF/Al. The test device based on P2 emitted blue light with a low threshold voltage of 5 V. Z-scan studies reveal that the polymers exhibit a strong optical limiting behavior. The value of the nonlinear absorption coefficient (β) of polymers is of the order 10 −11m/W which indicates that these materials may be accomplished for fabricating optical limiters.

Synthesis of TPD–thiophene-based small molecule donor for organic photovoltaic cells

ABSTRACTThe donor–acceptor–donor (D–A–D) type small molecule, TPD2T, was synthesized by the Stille coupling reaction and consisted of the electron-withdrawing thiene[3,4-c]pyrrole-4,6-dione (TPD) unit as an electron-accepting (A) core and electron-rich bithiophene units as electron-donating (D) groups. The absorption maximum of the TPD2T film was observed at 460 nm, and was broader and red-shifted compared to that in solution (λmax = 450 nm). Organic photovoltaic cells fabricated with the conventional and inverted structures both exhibited similar power conversion efficiencies of 0.14–0.15%. Despite the low short-circuit current values, the TPD2T devices showed high open-circuit voltages of over 1.0 V owing to the relatively low-lying HOMO level of TPD2T (−5.4 eV).

10.20% Efficiency polymer solar cells via employing bilaterally hole-cascade diazaphenanthrobisthiadiazole polymer donors and electron-cascade indene-C70 bisadduct acceptor

A series of crystalline copolymers, PBDTSA-DAPT, PBDTTA-DAPT and PBDTTS-DAPT, have been designed towards the application of multi-blend polymer solar cells (PSCs). These copolymers exhibit low-lying HOMO levels by incorporating benzodithiophene (BDT) donor and 9,10-diazaphenanthro[3,4-c;5,6-c]bis[1,2,5]thiadiazole (DAPT) acceptor in the main chains. They were added as the hole-cascade polymer donors into the PTB7: PC71BM blend. When adding 10wt% of the resulting copolymers, the PCEs of 8.59%, 9.03% and 8.91% were achieved for PBDTSA-DAPT-, PBDTTA-DAPT- and PBDTTS-DAPT-based ternary devices, respectively. In order to further elevate the performance of PBDTTA-DAPT devices, indene-C70 bisadduct (IC70BA) was also incorporated into the PBDTTA-DAPT-based ternary blend. This cascade band structure would give steadier energy level alignment and afford more charge transfer channels, which could promote simultaneously both the hole and electron transfer. When adding 5wt% of IC70BA, the quaternary PSC device showed a top PCE of 10.20%. This PCE value is the best efficiency for PTB7 based PSCs reported so far. Our work not only demonstrates a design strategy for high performance hole-cascade polymer donors in multi-blend PSCs, but also provides a first quaternary strategy with bilateral cascade energy band structures to increase the Voc, Jsc and FF simultaneously for high efficient organic solar cell applications.

Large π-Conjugated Quinacridone Derivatives: Syntheses, Characterizations, Emission, and Charge Transport Properties.

Two 11-ring-fused quinacridone derivatives, TTQA and DCNTTQA, have been synthesized by ferric chloride mediated cyclization and Knoevenagel reaction. Replacement of the carbonyl groups (in TTQA) with dicyanoethylene groups (in DCNTTQA) not only red-shifted the emission to the near-infrared region but also led to a nonplanar skeleton that significantly improved the solubility of DCNTTQA. Moreover, dicyanoethylene groups rendered DCNTTQA low-lying HOMO and LUMO levels. DCNTTQA-based solution-processed field-effect transistors showed a hole mobility up to 0.217 cm(2) V(-1) s(-1).

A New D-A conjugated polymer P(PTQD-BDT) with PTQD acceptor and BDT donor units for BHJ polymer solar cells application

A novel donor–acceptor (D–A) copolymer comprising of weak electron donating BDT moiety and strong 9-(2-octyldodecyl)−8H-pyrrolo[3,4-b] bisthieno[2,3-f:3',2'-h] quinoxaline-8,10(9H)-dione (PTQD) unit denoted as P(PTQD-BDT) was synthesized as donor material for polymer solar cells. P(PTQD-BDT) shows a broad visible-near-infrared absorption band with an optical bandgap of 1.74 eV and possesses a relatively low-lying HOMO level at −5.28 eV. Bulk-heterojunction polymer solar cell with the optimized blend of 1:2 (weight ratio) P(PTQD-BDT):PC71BM (processed with chloroform) shows an open circuit voltage of 0.92 V, a short circuit current density of 7.84 mA/cm2, and a fill factor of 0.50, achieving a power conversion efficiency (PCE) of 3.61%. The PCE has been further improved to 5.55% (Jsc = 10.34 mA/cm2, Voc = 0.88V and FF = 0.61), when 3% vol 1,8-diio-dooctane (DIO) was used as solvent additive for the processing of P(PTQD-BDT):PC71BM blended film. The enhancement in Jsc is as a result of the appropriate morphology and efficient exciton dissociation into free charge carrier. The increase in PCE has been attributed to the favorable nanoscale morphology for efficient exciton dissociation and charge transport (reduction in the electron to hole mobility ratio). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2390–2398

ChemInform Abstract: Use of the Wilkinson Catalyst for the ortho‐C—H Heteroarylation of Aromatic Amines: Facile Access to Highly Extended π‐Conjugated Heteroacenes for Organic Semiconductors.

AbstractThe heteroarylation products can be further converted to give heteroacenes, e.

Diporphyrin magnesium complex with long-wavelength light absorption for organic solar cells

We synthesized a diporphyrin compound [4,7-bis[5-[arylethynyl]-10,20-bis{(triisopropylsilyl)ethynyl}porphyrin-15-yl]]-2,1,3-benzothiadiazole dimagnesium(II)], in which two porphyrin units were linked using a benzodiathiazole unit as an electron-withdrawing moiety. These diporphyrins have low-lying HOMO and LUMO levels with long-wavelength light absorption property. Power conversion efficiency of the organic solar cell using this diporphyrin and mix-PCBM (a 85:15 mixture of [6,6]-phenyl- C 61-butyric acid methyl ester ( PC 61 BM ) and [6,6]-phenyl- C 71-butyric acid methyl ester ( PC 71 BM ) was 2.75% with short-circuit current density of 8.79 mA/cm2, open-circuit voltage of 0.80 V, and fill factor of 0.39. Photocurrent conversion in the near infrared region was demonstrated by the incident photon-to-current efficiency spectrum.