Lower HOMO Energy Levels Research Articles

Donor–acceptor copolymers based on benzo[1,2-b:4,5-b′]dithiophene and pyrene-fused phenazine for high-performance polymer solar cells

Two novel donor–acceptor (D–A)-type conjugated polymers of PTTPPz-BDT and PTTPPz-BDTT were successfully synthesized by Stille coupling polymerization, in which 7,8-dialkoxy benzo[1,2-b:4,5-b′]dithiophene (BDT) and 7,8-bithienyl benzo[1,2-b:4,5-b′]dithiophene (BDTT) were used as donor units, thiophene and pyrene-fused phenazine (PPz) were employed as π-bridges and acceptor units, respectively. High carrier mobilities, broad absorption spectra, narrow optical band gaps, and low HOMO energy levels were observed for both polymers. Furthermore, high-efficiency photovoltaic performance with power conversion efficiency (PCE) over 4.25% was exhibited in their polymer solar cells (PSCs) using [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC71BM) as acceptor. The maximum PCE of 4.86% with short-circuit current density of 11.10mAcm−2 and fill factor of 62.5% was obtained in the PTTPPz-BDTT based cell. These results indicate that incorporating large planar PPz moiety into D–A-type copolymer is an efficient approach to improve photovoltaic performance for PSCs.

Shortcut Approach to 1,4-Diazepine from 3-Pyridylnitrene Intermedietes under Mild Condition

The reaction of nitropyridine derivatives and tributylphosphine (Bu 3 P) with the existence of nucleophilic solvent gives ring expansion product as diazepines in medium yield. Reaction mechanism subjected the formation of phenylnitrene, followed by intramolecular electrophilic insertion reaction to pyridine ring and subsequent ring enlargement. The intermediate in the reaction confirmed by computational calculation using B3LYP/6-31G* level. The intramolecular insertion reaction of pyridylnitrene is considered suppressed by the low HOMO (-9.932 eV) energy level of pyridine ring compared to that of benzene (-9.653 eV), hence 1,4-diazepine is obtained when employed 3-nitro-2,6-lutidine as starting material. The formation of diazepines was confirmed by the analysis of 1 H NMR data. Separation of the product mixture using column chromatography on SiO 2 was carried out and found to give expected diazepine along with the reduction product.

Triphenylamine modified bis-diketopyrrolopyrrole molecular donor materials with extended conjugation for bulk heterojunction solar cells

Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800nm with an optical band at approximately 1.50eV and relatively low HOMO energy levels from −5.12 to approximately −5.16eV. Expectedly, the UV–Vis absorption onset (810nm) of TPA–DPP–BDT is largely red-shifted (60nm) relative to that (750nm) of previously reported BDT(TDPP)2, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC61BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (Jsc) of 11.40mAcm−2 and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130°C for 10min.

Efficient diketopyrrolopyrrole-based small-molecule bulk-heterojunction solar cells with different electron-donating end-groups.

A series of small molecules that contained identical π-spacers (ethyne), a central diketopyrrolopyrrole (DPP) unit, and different aromatic electron-donating end-groups were synthesized and used in organic solar cells (OSCs) to study the effect of electron-donating groups on the device performance. The three compounds, DPP-A-Ph, DPP-A-Na, and DPP-A-An, possessed intense absorption bands that covered a wide range, from 350 to 750 nm, and relatively low HOMO energy levels, from -5.50 to -5.55 eV. DPP-A-An, which contained anthracene end-groups, demonstrated a stronger absorbance and a higher hole mobility than DPP-A-Ph, which contained phenyl groups, and DPP-A-Na, which contained naphthalene units. The power-conversion efficiencies (PCEs) of OSCs based on organic:PC71BM blends (1:1, w/w) with a processed DIO additive were 3.93% for DPP-A-An, 3.02% for DPP-Na, and 2.26% for DPP-A-Ph. These findings suggest that a DPP core that is functionalized with electron-donating capping groups constitutes a promising new class of solution-processable small molecules for OSC applications.

Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells

Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,3′-hexyl-terthiophene (IDT-3Th-ID(I)) or 4,4′-hexyl-terthiophene (IDT-3Th-ID(II)) as π-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good film-forming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52cm2V−1s−1 for IDT-3Th-ID(I) and 0.61cm2V−1s−1 for IDT-3Th-ID(II) were achieved, with corresponding high ION/IOFF of ca. 107 and ∼109 respectively. The OSCs based on IDT-3Th-ID(I)/PC70BM (2:1, w/w) and IDT-3Th-ID(II)/PC70BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100mW/cm2. The results demonstrate that the small molecules constructed with the highly π-conjugated IDT as donor unit, 3Th as π-bridges and ID as acceptor units, could be promising organic semiconductors for high-performance OFETs and OSCs applications.

A comparative study of diketopyrrolopyrrole and isoindigo based polymers for organic photovoltaic applications

Two new donor/acceptor π-conjugated polymers with functional dyes, either diketopyrrolopyrrole or isoindigo as the electron accepting units and 5,10-bis(2,3-didecylthiophen-5-yl)-naphtho[1,2-b:5,6-b′]difuran as an electron donating units have been synthesized. The photo-physical, electrochemical and photovoltaic properties of the new polymers have been studied and compared. With the same donor unit, the diketopyrrolopyrrole based polymer showed low bandgap and broad absorption extended into near IR region. The isoindigo based polymer showed little absorption in near IR region. However, the isoindigo based polymer showed more efficient absorption than diketopyrrolopyrrole based polymer in the most of visible region (from 300 to 690 nm) in solution. The isoindigo based polymer showed lower HOMO and LUMO energy levels than diketopyrrolopyrrole based polymer. The isoindigo polymer based solar cell devices consistently showed higher open circuit voltages than the diketopyrrolopyrrole polymer based devices.

Indacenodithiophene core-based small molecules with tunable side chains for solution-processed bulk heterojunction solar cells

Two indacenodithiophene (IDT) core-based small molecules with different side chains of bulky 4-hexylphenyl and flexible n-dodecyl with the same number of carbon atom, namely SM1 and SM2, respectively, were designed and synthesized as the donor materials in organic solar cells (OSCs). The impacts of the different side chains combined with the IDT core on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 450–700 nm and relatively low HOMO energy levels of −5.46 and −5.52 eV. The power conversion efficiency (PCE) of the OSCs based on SM2 as the donor material and PC61BM as the acceptor material (1 : 2, w/w) is 2.33%. In contrast, a PCE of 4.72% was achieved for the device based on SM1 as the donor and PC71BM as the acceptor (1 : 2, w/w) without any treatment such as thermal annealing or the utilization of a solvent additive.

Theoretical design of donor-acceptor conjugated copolymers based on furo-, thieno-, and selenopheno[3,4-c] thiophene-4,6-dione and benzodithiophene units for organic solar cells

In this work, a series of donor-acceptor (D-A) copolymers (PBDTFPD(Pa1), PBDTTPD (Pa2) and PBDTSePD(Pa3)) were selected and theoretically investigated using O3LYP/6-31G(d), PBE0/6-31G(d), TD-O3LYP/6-31G(d)//O3LYP/6-31G(d) and periodic boundary conditions methods. The calculated results go well with the available experimental data of highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) energy levels and band gaps. A series of conjugated polymers (Pb1 ~ Pb3) comprised of electron-deficient benzodithiophene and electron-rich furo-, thieno-, and selenopheno[3,4-c]thiophene-4,6-dione were further designed and studied. Compared with Pa1-Pa3, the designed polymers of Pb1 ~ Pb3 show better performances with smaller band gaps, lower HOMO energy levels, red shift of absorption spectra, and larger open circuit voltage (Voc). For investigated polymers (Pa1, Pa2, Pa3, Pb1, Pb2, Pb3), the power conversion efficiencies (PCEs) of ~6.1 %, ~7.2 %, ~7.9 %, ~8.0 %, ~9.5 % and ~9.0 % are predicted by Scharber diagrams when they are used in combination with PC60BM as an acceptor. The results illustrate that these designed polymers which turn the electron-withdrawing capability in D-A conjugated polymers are expected to turn into highly efficient donor materials for organic solar cells.

Molecular design of donor–acceptor conjugated copolymers based on C-, Si- and N-bridged dithiophene and thienopyrroledione derivatives units for organic solar cells

The aim of this work is to adjust the electron-withdrawing strength in donor–acceptor (D–A) conjugated polymers for the improvements of photovoltaic performances. To achieve this goal, starting from the reported a series of D–A copolymers(PCPDTTPD(Pa1), PDTSTPD (Pa2) and PDTPTPD (Pa3)) based on electron-rich C-, Si-, N-bridged bithiophene and electron-deficient thienopyrroledione (TPD), we replace nitrogen atoms with sulfur or oxygen atoms on TPD unit to form two types of new D–A copolymers (Pb1–Pb3 and Pc1–Pc3). Compared with Pa1–Pa3, the designed copolymers of Pb1–Pb3 and Pc1–Pc3 show better performances with smaller band gaps, lower HOMO energy levels, wider optical absorption, larger open circuit voltage (Voc), and larger hole mobility. The power conversion efficiencies (PCEs) of ∼8.8%, ∼10.0%, ∼8.4%, ∼8.5%, ∼9.9% and ∼7.9% for organic solar cell made from designed polymers (Pb1, Pb2, Pb3, Pc1, Pc2, Pc3) are predicted by Scharber models. Conclusively, we conclude that the introduction of the strong electron-withdrawing groups into copolymer can effectively improve the electronic properties of the polymer in order to better the performance of organic solar cells.

Synthesis and Characterization of Copolymers Consisting of 3-Hexylthiophene and Biphenyl Derivatives

Two novel copolymers consisting of 3-hexylthiophene and 4,4-di (thiophen-2-yl) biphenyl (DTBHT) or 4,4-di (thiophen-2-yl)-2-nitrobiphenyl (DTNHT) were synthesized. Both of the copolymers show lower HOMO energy level (-5.62, -5.51 eV) than that of the P3HT (-5.1 eV). Compared to DTBHT, adding nitro group in DTNHT improves effective conjugation of the main chain and successfully lowers the electrochemical bandgap of the copolymers from 2.39 to 2.28 eV and optical band-gap from 2.17 to 2.07 eV with significant increase in fluorescence quantum yield from 0.140 to 0.252. Keywords: biphenyl, 3-Hexylthiophene, conjugated polymer

Synthesis and photovoltaic properties of conjugated copolymers with benzo[1,2-b:4,5-b′]dithiophene and thiadiazolo[3,4-c]pyridine moieties

A series of main-chain and side-chain type D–A copolymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and thienylbenzo[1,2-b:4,5-b′]dithiophene (BDTT) electron-donating units and an electron-withdrawing unit based on thiadiazolo[3,4-c]pyridine, PBDT-DTPyT, PBDT-T-DTPyT, and PBDTT-T-DTPyT were synthesized for polymer solar cells (PSCs). The copolymers exhibit good thermal stability, strong absorption in the visible region, and relatively lower HOMO energy levels from −5.22 to −5.38eV. The results indicate thiadiazolo[3,4-c]pyridine led to enhanced π–π stacking of the polymer film owing to its stronger polarity. PBDTT-T-DTPyT exhibited a high hole mobility of 1.2×10−2cm2V−1S−1. The interaction of the thiadiazolo[3,4-c]pyridine in the polymers and the hole extraction layer led to a non-ohmic contact at the interface. The conventional and inverted bulk heterojunction polymer solar cells based on PBDTT-T-DTPyT as the electron donor and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor exhibited power conversion efficiencies of 1.68% and 2.08%, respectively.

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM) as an acceptor exhibited PCEs as high as 3.15%.

Solution-Processable Organic Molecule Photovoltaic Materials with Bithienyl-benzodithiophene Central Unit and Indenedione End Groups

Two solution-processable acceptor–donor–acceptor (A-D-A) structured organic molecules with bithienyl-substituted benzodithiophene (BDTT) as central and donor unit, indenedione (ID) as acceptor unit and end groups, and thiophene (T) or bithiophene (bT) as π-bridges, D1 and D2, are designed and synthesized for the application as donor materials in organic solar cells (OSCs). Two corresponding molecules with alkoxy side chains on BDT, DO1, and DO2 are also synthesized for comparison. The four compounds possess broad absorption covering the wavelength range 450–740 nm and relatively lower HOMO energy levels from −5.16 to about −5.19 eV. D2 and DO2 with bithiophene π-bridges demonstrate stronger absorbance and higher hole mobilities than the compounds with thiophene π-bridges. The power conversion efficiency (PCE) values of the OSCs based on the organic compounds/PC70BM (1.5:1, w/w) are 6.75% for D2, 5.67% for D1, 5.11% for DO2, and 4.15% for DO1. The results indicate that the molecules with thienyl conjugated...

A new isoindigo-based molecule with ideal energy levels for solution-processable organic solar cells

A new easily-accessible solution-processed oligomer, with an isoindigo group as an electron acceptor and a thieno[3,2-b]thiophene flanked by thiophenes as electron donors, has been synthesized by a Stille coupling reaction. Through introducing the extended pi-conjugated groups into isoindigo, the electro-optical properties of the material can be fine-tuned. The isoindigo oligomer has a broad absorption in the region from 300 to 800 nm with a narrow bandgap (1.54 eV), which is believed to be an ideal bandgap as donor materials. The oligomer possesses low HOMO energy level (-5.39 eV). The potential of optical and electronic properties encouraged us to explore the photovoltaic performance using the oligomer as the donor material in bulk heterojunction organic solar cell along with 6,6-phenyl-C61-butyric acid methyl ester as the acceptor. The solar cell based on the oligomer with an inverted device configuration provided a power conversion efficiency of 1.41% under the AM 1.5G illumination with an intensity of 100 mW cm(-2) from a solar simulator. (C) 2013 Elsevier Ltd. All rights reserved.

New Alkylfuranyl-Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Donor–Acceptor Polymers for Highly Efficient Solar Cells

Two new alkylfuranyl-substituted conjugated donor–acceptor polymers—PBDTF-DPP and PBDTF-DPPF—were designed and synthesized. To compare the properties of the new polymers, PBDT-DPP and PBDT-DPPF with alkoxy side chains were also synthesized. The photophysical and electrochemical measurement demonstrated that the alkylfuranyl-substituted polymers had smaller optical band gaps, broader absorption range, and lower HOMO energy levels, thus leading to a larger short current density (Jsc) and higher open circuit voltage (Voc) in photovoltaic devices. Under the same fabricating conditions, the efficiency of the polymer solar cells (PSCs) based on PBDTF-DPP and PBDTF-DPPF reached 3.5% and 5.1%, respectively, whereas PSCs based on PBDT-DPP and PBDT-DPPF only showed an efficiency of 1.0% and 2.9%. After thermal annealing, the efficiency of the PSCs based on PBDTF-DPPF:PC71BM further achieved as high as 6.1%. The results indicate a great potential for largely improving the efficiency of the PSCs by replacing alkoxy with alkyfuranyl group and the building block BDTF in creating exceptional performance materials for PSCs.

Benzotrithiophene and benzodithiophene-based polymers for efficient polymer solar cells with high open-circuit voltage

Two new conjugated polymers based on benzodithiophene (BDT) and alkyl-benzotrithiophene (alkyl-BTT, P1), or acyl-benzotrithiophene (acyl-BTT, P2) were synthesized by a Stille cross-coupling reaction. The polymers were characterized by gel permeation chromatography (GPC), ultraviolet-visible (UV-vis) absorption as well as electrochemical cyclic voltammetry (CV) tests. Compared to the alkyl-BTT-based polymer (P1), the acyl-BTT-based polymer (P2) displayed a lower optical bandgap and a lower HOMO energy level. Polymer solar cells (PSCs) also confirmed that P2-based devices possessed better photovoltaic properties than those of P1-based devices. The optimized photovoltaic devices were fabricated with an active layer of a blend P2 and PC71BM using 2 vol% 1,8-diiodooctane (DIO) as a solvent additive, and a PCE of 4.20% was obtained, with a Voc as high as 0.96 V under AM 1.5G illumination at 100 mW cm−2 with a solar simulator.

2D π-conjugated benzo[1,2-b:4,5-b′]dithiophene- and quinoxaline-based copolymers for photovoltaic applications

Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (<−5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-FQ-BDT-4TR:PC61BM blend, showing a JSC of −10.2 mA cm−2, VOC of 0.90 V, FF of 58% and PCE of 5.3%, without the need for any additional thermal treatment.

Synthesis and photovoltaic properties of copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thiazole with different conjugated side groups

Two new copolymers (PT-Tz-DTBT and PT-Tz-DTffBT) of benzo[1,2-b:4,5-b′]dithiophene and thiazole (Tz) with different conjugated side groups 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT) and 5,6-difluoro-4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTffBT) were synthesized via the Stille coupling polymerization and developed for polymer solar cell applications. The thermal, photophysical, electrochemical and photovoltaic properties of the copolymers were investigated. The two copolymers exhibited relatively low HOMO energy levels of −5.58 eV for PT-Tz-DTBT and −5.59 eV for PT-Tz-DTffBT, respectively. Bulk heterojunction (BHJ) solar cells with two as-synthesized copolymers as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor exhibit power conversion efficiencies (PCEs) of 2.63% and 3.75% for PT-Tz-DTBT and PT-Tz-DTffBT, respectively. When PC71BM was used as an electron acceptor, a promising PCE of 3.65% and 4.42% for PT-Tz-DTffBT was achieved by using a conventional and inverted device structure, respectively.

A benzo[1,2-b:4,5-b′]difuran- and thieno-[3,4-b]thiophene-based low bandgap copolymer for photovoltaic applications

A new low bandgap conjugated polymer—PBDFTT-C was synthesized from thieno-[3,4-b]thiophene and benzo[1,2-b:4,5-b′]difuran units by Stille coupling reactions. The structure was verified by 1H NMR and elemental analysis, the molecular weight was determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). PBDFTT-C showed a low HOMO energy level of −5.27 eV. The polymer film displayed broad absorption in the wavelength region from 300 nm to 840 nm with a low bandgap of 1.48 eV. The field effect hole mobility of PBDFTT-C reached 5.4 × 10−3 cm2 V−1 s−1. By using 1,8-diiodooctane (DIO) as the solvent additive, photovoltaic cells with the structure of ITO/PEDOT:PSS/PBDFTT-C:PC71BM (1 : 1.5, w/w)/Ca/Al demonstrated a power conversion efficiency of 4.4% with a short circuit current of 10.45 mA cm−2, open circuit voltage of 0.66 V and a fill factor of 0.64, under the illumination of AM 1.5G, 100 mW cm−2.

Theoretical Investigations on Donor–Acceptor Conjugated Copolymers Based on Naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole for Organic Solar Cell Applications

Conjugated polymers with donor–acceptor architectures have been successfully applied in bulk heterojunction solar cell devices. Tuning the electron-withdrawing capability in donor–acceptor (D–A) conjugated polymers allows for design of new polymers with enhanced electrical and optical properties. In this paper, a series of D–A copolymers, PBDFDTBT (P1a), PBDTDTBT (P2a), PNDTDTBT (P3a), and PQDTDTBT (P4a), were selected and theoretically investigated using PBE0/6-311G** and TD-PBE0/6-311G**//PBE0/6-311G** methods. The calculated results agree well with the available experimental data of HOMO energy levels and band gaps. We further designed and studied four novel copolymers, P1b, P2b, P3b, and P4b, by substituting the 2,1,3-benzothiadiazole (BT) unit in P1a–P4a with a stronger unit of naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), respectively. Compared with P1a–P4a, the newly designed polymers of P1b–P4b show better performance with the smaller band gaps and lower HOMO energy levels. The PCEs of ∼5%, ∼7%, ∼7%, and ∼7% for P1b–P4b, predicted by Scharber diagrams, are much higher than those of P1a–P4a when used in combination with PCBM. These results clearly reveal that tuning the electron-withdrawing capability in D–A conjugated polymers is an effective way to improve the electrical and optical properties and the efficiency of the photovoltaic device.