Cyclopentadithiophene Research Articles

Organic donor materials based on Bis(arylene ethynylene)s for bulk heterojunction organic solar cells with high V(oc) values.

A series of purely organic small molecules 1-4 based on bis(arylene ethynylene)s with various electron-rich aromatic bridges, such as benzene, cyclopentadithiophene (CDT), and diphenyl(p-tolyl)amine, were synthesized by a Sonogashira coupling reaction. Their optical, electronic, and electrochemical properties were fully characterized. The photovoltaic properties of these molecules as donor materials and [6,6]-phenyl-C71 -butyric acid methyl ester as an acceptor material in solution-processed bulk heterojunction devices were studied. Among them, CDT-bridged molecule 2 exhibited the best photovoltaic performance and achieved a power conversion efficiency of 3.28 %. In addition, the photovoltaic efficiencies of these molecules are higher than their corresponding platinum-containing counterparts, probably owing to their stronger light absorption and improved open-circuit voltage.

The lowest triplet states of bridged cis-2,2'-bithiophenes - theory vs. experiment.

Theoretical methods that were previously used to give a good quantitative description of the 3(1)Bu state of trans-2,2'-bithiophene are applied to characterize the lowest triplet states of three bridged cis-2,2'-bithiophenes: 3,3'-cyclopentadithiophene (CPDT), 3,3'-dithienylpyrrole (DTP), and 3,3'-dithienylthiophene (DTT). By obtaining highly accurate reproductions of the phosphorescence spectra of all three compounds, we rationalize the observed vibronic activity, further explore the performance of the applied theoretical methods, and address the quality of the reported experimental spectra. Over the course of this study we have, first, characterized the changes in the electronic structures between the ground state and the lowest triplet state and, second, expressed the related geometrical differences in terms of the Huang-Rhys factors. The Huang-Rhys factors have then been used to generate theoretical emission spectra with vibronic resolution. The applied procedure has yielded quantitative reproductions of the previously reported experimental phosphorescence spectra of DTT and DTP. The experimental spectrum of CPDT, on the other hand, turned out to be considerably narrower and intensity-deficient in its low energy region when compared with the theoretical results. Our experimental reinvestigation of the CPDT phosphorescence has given a refined spectrum that is significantly wider than the previously reported one, and is in nearly quantitative agreement with the theoretical prediction. This enabled us to attribute the observed discrepancy to an experimental artifact associated with the sensitivity characteristics of the commonly used photomultipliers.

A comparison of n-type copolymers based on cyclopentadithiophene and naphthalene diimide/perylene diimides for all-polymer solar cell applications

Two cyclopentadithiophene (CPDT)-based n-type copolymers, PCPDT-NDI and PCPDT-PDI, were synthesized and used in all-polymer solar cells with PCE of 1.12% and 2.13%, respectively.

Photodegradation of C-PCPDTBT and Si-PCPDTBT: influence of the bridging atom on the stability of a low-band-gap polymer for solar cell application.

The kinetics of photodegradation and the reactivity of different sites of the low-band-gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and poly[2,6-(4,4-bis-(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT) are investigated as thin films and are compared to those of poly(3-hexylthiophene) (P3HT). The decay kinetics are monitored with UV/Vis spectroscopy and the reactivity and product evolution are investigated with X-ray photoelectron spectroscopy (XPS). Both polymers exhibit higher stability than P3HT. The bridging atom in the cyclopentadithiophene (CPDT) subunit has a significant influence on the stability. Varying oxidation rates for the different elements were observed. In the case of Si-PCPDTBT, the silicon atom is oxidized primarily, whereas the photooxidation rates of the other elements are reduced relative to C-PCPDTBT. Additionally, XPS experiments with varying excitation energies reveal a significant reaction gradient within a few nanometers of the surface of degraded thin films of C-PCPDTBT.

Strategy to Modulate the Electron-Rich Units in Donor–Acceptor Copolymers for Improvements of Organic Photovoltaics

We present an effective strategy to modulate the electron-rich capability in donor–acceptor (D-A) polymers for improving the performances of organic solar cell (OSC) devices. In order to confirm this strategy, based on a series of the reported D–A polymers ((PCPDTBT(Pa1), PCPDTFBT (Pa2), and PCPDTDFBT (Pa3)) which contain the electron-donating cyclopentadithiophene (CPDT) and differently electron-withdrawing units of benzo[c][1,2,5,]thiadiazole (BT), 5-fluorobenzo[c][1,2,5]thiadiazole (FBT), and 5,6-difluorobenzo[c] [1,2,5]thiadiazole (DFBT), we replace CPDT with electron-donating dithienogermolodithiophene (DTTG) in polymers Pa1–Pa3, respectively, and design a series of new D–A polymers Pb1–Pb3. Compared with the polymers Pa1–Pa3, the new designed polymers Pb1–Pb3 not only yield a greater red-shift of the absorption spectrum of the donor polymer and result in a larger absorption region within the solar emission spectrum and an improved light-absorbing efficiency but also exhibit much better electron transfer efficiency in active layer, larger hole transport rates and higher open circuit voltage. Moreover, the estimated power conversion efficiency of the designed polymers in OSC applications reaches up to ∼8.4%. Conclusively, the approach based on modulating the electron-donating capability in D–A polymer chain is a feasible way to enhance their intrinsic properties of donor polymers and thereby achieving the purpose that improves the performances of the OSC devices.

Small Molecules of Cyclopentadithiophene Derivatives: Effect of Sulfur Atoms Position and Substituted Groups on Their UV-abs Properties.

Thiophene-based organic semiconductors used as the active components have received much attention. Their photoelectric properties can be easily tuned with various substitutions at different positions on molecular structures. Here, we synthesized series cyclopentadithiophene (CDT) derivatives with sulfur atoms at ortho- (o-CDT), meta- (m-CDT) and para-position (p-CDT) of the bridge carbon. These CDT derivatives were substituted by carbonyl/dicyanomethylene at the bridge position and/or by phenyl groups at α position, respectively. Due to the different conjugation extent and the variation of donor-acceptor (D-A) interaction originating from the change of sulfur atom position, diverse absorption spectra were observed. Especially for dicyanomethylene substituted o-CDT with phenyl as substitution group (DPCN-o-CDT), its absorption spectrum covers the whole region of visible light. Combining with the electrochemical behaviors and theoretical calculations, it was found that the sulfur atoms mainly contribute to the molecular conjugation in these CDT derivatives, especially for o-CDT derivatives. For phenyl groups, they primarily act as electron donor in m-CDT derivatives, and chiefly contribute to molecular conjugation in p-CDT derivatives, and simultaneously work as electron donor and conjugation component in o-CDT derivatives, respectively.

Cyclopentadithiophene–naphthalenediimide polymers; synthesis, characterisation, and n-type semiconducting properties in field-effect transistors and photovoltaic devices

The synthesis, characterisation, and device performance of a series of cyclopentadithiophene (CPDT)-naphthalenediimide (NDI) donor-acceptor-donor (D-A-D) polymers is reported. The monomers with various alkyl chains are synthesised via direct arylation using palladium complex catalyst. The monomers are then polymerised by oxidative polymerisation using FeCl3 to provide high molecular weight polymers (Mn = 21,800–76,000). The polymer films show deep-red absorption including near-infrared region up to 1100 nm to give optical bandgap of approximately 1.16 eV. The polymers exhibit only n-type semiconducting properties giving the highest electron mobility of 9 × 10-3 cm2 V−1 s−1 in organic field-effect transistors (OFETs). Organic photovoltaic (OPV) devices are fabricated from solutions of the polymers as acceptors and poly(3-hexylthiophene) (P3HT) as a donor.

Adapting Ruthenium Sensitizers to Cobalt Electrolyte Systems.

In this work, we report the use of bulky substitutions in a new heteroleptic ruthenium(II) bipyridine complex, Ru(NCS)2LL', coded TT-230 to obtain high open-circuit potential in a dye-sensitized solar cell (where L is a bipyridine ligand appended with two cyclopenta(2,1-b;3,4-bA)dithiophene moieties, and L' = 4,4,'-dicarboxylic acid 2,2'-bipyridine). The electrolytes based on cobalt complexes have shown significant advantages in terms of attainable open-circuit potential compared to the standard iodide/tri-iodide redox mediators. These merits of the cobalt complexes were previously realized with a porphyrin sensitizer, achieving a VOC greater than 1 V in DSC. However, with conventional Ru(II)-polypyridyl complexes such as the C101 dye, similar increase in the VOC could not be attained due to the enhanced recombination. In this work, we have shown that the use of bulky substituents can prevent the back reaction of photogenerated electron and subsequently increase the open-circuit potential of the device. The recombination processes were investigated by transient photovoltage decay measurements.

Theoretical investigations on fluorinated and cyano copolymers for improvements of photovoltaic performances

An effective way to improve the efficiency of organic solar cells is to adjust the electron-withdrawing strength in donor-acceptor (D-A) copolymers. To achieve this goal, starting from previously reported polymers (PCPDT-BT and PDTPr-FBT) which are based on benzothiadiazole (BT) electron-deficient unit connected to each of two electron-rich units (cyclopentadithiophene (CPDT) and dithienopyrrole (DTPr)), we introduced two strong electron-withdrawing fluorine atoms or a cyano group on the BT unit to replace BT with fluorinated BT (FBT) and cyano BT (CNBT) in PCPDT-BT and PDTPr-FBT, respectively, and designed two types of D-A copolymers with different electron-withdrawing strengths. From the calculated results, the introduction of strong electron-withdrawing groups onto the copolymer can not only obviously reduce the HOMO and LUMO level of molecules, which results in increasing the open circuit voltage (Voc) in solar cells, but can also enhance light-absorbing efficiency and charge transport ability of polymers. In the meantime, the cyano copolymers of PCPDT-1CNBT and PDTPr-1CNBT show the best performances with the smallest band gaps, lowest HOMO energy levels, the highest Voc, and the largest hole mobility (3.67 × 10(-3) cm(2) V(-1) s(-1) and 8.05 × 10(-4) cm(2) V(-1) s(-1), respectively) among all the considered systems. The power conversion efficiencies (PCEs) of ~7.2% and ~6.8% for organic solar cell made of designed polymers (PCPDT-1CNBT and PDTPr-1CNBT) are predicted by Scharber models. We presented several polymer donors for comparison of how the strong electron-withdrawing group influences the electronic properties and optical absorption of the polymers and the performances of organic solar cells made of the polymers, thereby obtaining promising organic solar cells with high power conversion efficiencies.

The effect of heteroatom conformation on optoelectronic properties of cyclopentadithiophene derivatives

Cyclopentadithiophene (CPDT) derivatives with different heteroatom conformations have been synthesized. The optical, electrochemical and charge transport properties of these molecules are reported. The CPDT-anti-ketone not only exhibits the lowest optical and electronic bandgaps, but also exhibits reasonable hole mobility, 3 × 10(-3) cm(2) (V s)(-1). Changing the carbonyl conformation to the syn position or incorporating the imine functionality results in a blue-shift in the lower energy band of the absorption spectrum indicative of the increased bandgaps.

Synthesis of 5H-Dithieno[3,2-b:2′,3′-d]pyran as an Electron-Rich Building Block for Donor–Acceptor Type Low-Bandgap Polymers

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in low-bandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymerized with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regiorandom polymer (PDTP–DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers containing alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT–DFBT and PCPDT–DFBT). We found that the DTP based polymer PDTP–DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers. Consequently, very high molecular weight and soluble PDTP–DFBT can be obtained with less bulky side chains. Interestingly, PDTP–DF...

Synthesis of 5H-Dithieno[3,2-b:2′,3′-d]pyran as an Electron-Rich Building Block for Donor–Acceptor Type Low-Bandgap Polymers

We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in low-bandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymerized with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regiorandom polymer (PDTP–DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers containing alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT–DFBT and PCPDT–DFBT). We found that the DTP based polymer PDTP–DFBT shows significantly improved solubility and processability compared to the BDT or CPDT based polymers. Consequently, very high molecular weight and soluble PDTP–DFBT can be obtained with less bulky side chains. Interestingly, PDTP–DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficiencies reaching 8.0%, which is significantly higher than PBDT–DFBT and PCPDT–DFBT based devices. This study demonstrates that DTP is a promising building block for high-performance solar cell materials.

Reaction of 4H-cyclopenta[2,1-b:3,4-b′]dithiophenes with NBS—a route toward 2H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6(4H)-diones

In this paper we present a detailed study of the bromination reaction of cyclopentadithiophene (CPDT) derivatives, by means of NBS, giving access to either 2,6-dibromo-4H-cyclopenta[2,1-b:3,4-b′]dithiophenes or 2H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6(4H)-diones (CPDT-2,6-diones). The CPDT-2,6-diones are fully characterized, including an X-ray single crystal structure of one of the representative materials. A mechanism leading to the formation of the latter, which arise as a new class of compounds within the CPDT family, is proposed and is supported by Gibbs free energy calculations. The influence of the solvent, reaction time, and the number of equivalents of NBS on the selectivity of the reaction is discussed.

Synthesis, characterization and photovoltaic properties of poly(cyclopentadithiophene-alt-isoindigo)

Isoindigo based conducting polymers have attracted extensive interest for polymer solar cell application since isoindigo is a green material and renewable from plants. We have synthesized four soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as the donor unit and isoindigo (I) as the acceptor unit, decorated with two kinds of alkyl side chains, octyl (8) and 2-ethylhexyl (e), via the Stille cross-coupling reaction denoted as PC8I8, PC8Ie, PCeI8 and PCeIe. By changing the side chain of copolymers from linear (PC8I8) to branched (PCeIe), the λmax of absorption is blue shifted from 1.37 to 1.48 eV and the HOMO level is lowered from −5.24 to −5.45 eV. The changes are due to the twist coplanarity of the polymer backbone. The density functional theory calculation revealed that the dihedral angle of copolymers has been increased from 14° to 20°. The properties of PC8Ie and PCeI8 lie between those of PC8I8 and PCeIe. The type of the side chain plays a major role in determining the photovoltaic performance of copolymers. The branched side chain improves the solubility of the polymer and increases the effective phase separation between the copolymer and PCBM. This results in favorable nanomorphology of the active layer. Thus, PCeIe with branched side chains on both donor and acceptor units exhibits the best photovoltaic properties with a Voc of 0.80 eV, Jsc of 11.6 mA cm−2 and fill factor of 43.0% and power conversion efficiency of 4.0%. The power conversion efficiency of this type of polymer could be further improved by optimizing the fabrication conditions and interlayer modification. This study offers a useful guideline for the molecular design of high efficiency isoindigo-based polymer solar cells.

Synthesis and characterization of cyclopentadithiophene-based low bandgap copolymers for all-polymer solar cells

Three conjugated copolymers based on cyclopentadithiophene (CPDT) units, namely, poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl} (P1), Poly{4,4-bis(2-ethylhexyl)cyclopeanta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl} (P2) and Poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-N,N′-bis(2-ethylhexyl)-3,4,9,10-perylene diimide-1,7-diyl} (P3) have been synthesized via Stille coupling reaction. The polymers were characterized by 1H NMR spectroscopy, gel permeation chromatography (GPC), UV–vis absorption spectroscopy, and cyclic voltammetry. These new polymers exhibit broad and strong absorption between 500 and 800 nm. The highest occupied molecular orbital energy levels of polymers vary between −4.98 and −5.27 eV and the lowest unoccupied molecular orbital energy levels range from −3.43 to −3.70 eV. By employing P1 and P2 as electron donors (D) and P3 as electron acceptor (A), all-polymer solar cells with bulk heterojunction structure have been fabricated. Preliminary results indicate that these devices show higher open circuit voltage (VOC) in comparison with the traditional polymer/fullerene systems of P1 and P2 blended with the acceptor (6,6)-phenyl C61-butyric acid methyl ester (PCBM).

Cyclopentadithiophene–benzothiadiazole oligomers: Synthesis via direct arylation, X-ray crystallography, optical properties, solution casted field-effect transistor and photovoltaic characteristics

This article reports the synthesis, crystallographic structure and OFET and OPV performance of the conjugated oligomer of cyclopentadithiophene (CPDT) with benzothiadiazole (BT). Synthesis of the oligomer composed of the CPDT-BT-CPDT sequence is accomplished using direct arylation reactions. Theoretical and experimental X-ray single crystallography confirms that two CPDT-BT-CPDT molecules are not entirely disordered, but are actually stacking directly across each other at the central BT units with an intermolecular distance of 3.61Å, providing valuable insight into the polymer bulk structure. The performance of the oligomer in OFET devices is investigated by fabricating bottom gate top contact devices and demonstrates a hole mobility of 5.0×10−3cm2V−1s−1. OPV devices of the oligomer blended with PC61BM and PC71BM show power conversion efficiency (PCE) of 1.61%. One potential use for the oligomer could be as a sensitiser in a ternary blend with P3HT–PC61BM or PCPDTBT–PC61BM OPVs; the PCE can be relatively increased by 3–9% depending on concentration, primarily as a result of increased short circuit current density.

Synthesis and Photovoltaic Properties of Polymers Based on Cyclopentadithiophene and Benzimidazole Units

The new semiconducting copolymers with 4,4-dialkyl--cyclopenta[2,1-:3,4-]dithiophene and 2,2-dimethyl--benzimidazole units were synthesized. The fused aromatic rings, such as cyclopentadithiophene (CPDT) unit, can make the polymer backbone more rigid and coplanar, which induces long conjugation length, narrow band gap, and strong intermolecular interaction. The stacking ability was controlled through attaching of linear or branched alkyl side chains. The spectra of PEHCPDTMBI and PHCPDTMBI in the solid films show absorption bands with maximum peaks at 401, 759 and 407, 768 nm, and the absorption onsets at 925 and 954 nm, corresponding to band gaps of 1.34 and 1.30 eV, respectively. The devices comprising PHCPDTMBI with showed a of 0.39 V, a of 1.14 , and a of 0.34, giving a power conversion efficiency of 0.15%. The PHCPDTMBI with linear alkyl chain on CPDT shows good solubility in organic solvent with higher PCE value than that of PEHCPDTMBI.

Synthesis, Photophysical and Photovoltaic Properties of Conjugated Polymers Containing Fused Donor–Acceptor Dithienopyrrolobenzothiadiazole and Dithienopyrroloquinoxaline Arenes

We have developed two nitrogen-bridged pentacyclic donor–acceptor dithienopyrrolobenzothiadiazole (DTPBT) and dithienopyrroloquinoxaline (DTPQX) arenes where the two outer electron-rich thiophene moieties are covalently fastened with the central electron-deficient benzothiadiazole and quinoxaline cores by two nitrogen bridges. These rigid and coplanar DTPBT and DTPQX building blocks were copolymerized with fluorene (F), carbazole (C) and cyclopentadithiophene (CPDT) units via Suzuki or Stille coupling polymerization to afford six new alternating copolymers PFDTPBT, PCDTPBT, PCPDTDTPBT, PFDTPQX, PCDTPQX and PCPDTDTPQX, respectively. The nitrogen bridges not only planarize the structure to induce stronger intermolecular π–π interaction but also play an important role in determining the electronic and photophysical properties of the polymers. The device based on PFDTPQX/PC71BM (1:4, w/w) exhibited a open-circuit voltage (Voc) of 0.72 V, a short-circuit current (Jsc) of 8.62 mA/cm2 and a FF of 0.55 leading to...

Cyclopentadithiophene-benzothiadiazole oligomers and polymers; synthesis, characterisation, field-effect transistor and photovoltaic characteristics

Conjugated oligomers with various ratios of cyclopentadithiophene (CPDT) to benzothiadiazole (BT) repeating units are reported. These oligomers can be polymerised to high molecular weight polymers (Mn > 100k) by oxidative polymerisation using iron(III)chloride. The electronic properties of these materials were examined by cyclic voltammetry and UV-vis spectroscopy and the results compared to those calculated using density functional theory (DFT). Polymers with optimum ratios of CPDT : BT (2 : 1) units show hole mobilities in excess of 10−2 cm2 V−1 s−1 as the active layer in organic field effect transistors (OFETs). Bulk heterojunction organic photovoltaic (OPV) devices of these polymers with PCBM as the electron acceptor show a power conversion efficiency of 2.1% when processed in the absence of any additives. The reported OFET performance is significantly higher than the parent PCPDTBT alternating copolymer (CPDT : BT = 1 : 1) and also shows a moderate improvement in OPV performance.

Impact of regioregularity on thin-film transistor and photovoltaic cell performances of pentacene-containing polymers

Regioregular pentacene-containing polymers were synthesized with alkylated bithiophene (BT) and cyclopentadithiophene (CPDT) as comonomers. Among them, 2,9-conjugated polymers PnBT-2,9 and PnCPDT-2,9 achieved the best performance in transistor and photovoltaic devices respectively. The former achieved the most highly ordered structures in thin films, yielding ambipolar transistor behavior with hole and electron mobilities up to 0.03 and 0.02 cm2 V−1 s−1 on octadecylsilane-treated substrates. The latter achieved photovoltaic power conversion efficiencies up to 0.33%. The impact of regioregularity and direction of conjugation-extension (2,9 vs. 2,10), on thin-film order and device performance has been demonstrated for the pentacene-containing polymers for the first time, providing insight towards future functional material design.