Thienopyrazine Research Articles

Screening novel candidates of ZL003-based organic dyes for dye-sensitized solar cells by modifying auxiliary electron acceptors: A theoretical study

In this work, a series of ZL003-based free-metal sensitizers with the donor-acceptor-π- conjugated spacer-acceptor (D-A-π-A) structure were designed by modifying auxiliary electron acceptors for the potential application in dye-sensitized solar cells. The energy levels of frontier molecular orbitals, absorption spectra, electronic transition, and photovoltaic parameters for all studied dyes were systematically evaluated using density functional theory (DFT)/time-dependent DFT calculations. Results illustrated that thienopyrazine (TPZ), selenadiazolopyridine (SDP), and thiadiazolopyridine (TDP) are excellent electron acceptors, and dye sensitizers functionalized by these acceptors have smaller HOMO-LUMO gaps, obviously red-shifted absorption bands and stronger light harvesting. The present study revealed that the photoelectric conversion efficiency (PCE) of ZL003 is around 13.42 % with a JSC of 20.21 mA·cm−2, VOC of 966 mV and FF of 0.688 under the AM 1.5G sun exposure, in good agreement with its experimental value (PCE = 13.6 ± 0.2 %, JSC = 20.73 ± 0.20 mA·cm−2, VOC = 956 ± 5 mV, and FF = 0.685 ± 0.005.). With the same procedure, the PCE values for M4, M6, and M7 were estimated to be as high as 19.93 %, 15.38 %, and 15.80 % respectively. Hence, these three dyes are expected to be highly efficient organic sensitizers applied in practical DSSCs.

Panchromatic NIR-Absorbing Sensitizers with a Thienopyrazine Auxiliary Acceptor for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are a cost-effective and highly tunable technology for harvesting solar energy. Current research is focused on the design of near-infrared (NIR, >700 nm) absorbing organic sensitizers to efficiently harvest low-energy photons to increase the efficiency of DSCs. Herein, two NIR sensitizers containing a proaromatic thienopyrazine (TPZ) auxiliary acceptor are synthesized. The optical, electrochemical, and device properties of the sensitizers were studied. The two dyes probe the placement of the TPZ auxiliary acceptor (A′) either between the π-bridge and the acceptor (WM1, D−π–A′−A) or between the donor and the π-bridge (WM2, D–A′−π–A). WM1 and WM2 exhibit absorption maxima at 631 and 604 nm in dichloromethane (DCM), respectively. Both dyes also have higher energy absorption features, which contributes to panchromatic absorption into the NIR region. Optimized devices were sequentially sensitized with Y123 to control the surface conformation of the TPZ dyes, yielding high photocurrent values of 20.3 and 14.4 mA/cm2 for WM1 and WM2, respectively. WM1 exhibits a low-energy NIR incident photon-to-current efficiency (IPCE) onset at ∼900 nm.

Structural and electronic properties of organophosphorus-based systems as sensitizers in solar cells

Considerable attention has been paid to modulating these organic π-conjugates to realize effective and efficient organic photovoltaic by the means of theoretical methods. In respect to this, six commonly used heterocyclic compounds: thiophine (Th), thienopyrazine (TP), benzothiadiazole (BD), quinoxahine (BP), benzobisthiadiazole (BBD), and thenothiadiazole (TD) were co-oligomerized with bisazaphosphole (BAP) and theoretically examined for use in solar cells using density functional theory and time-dependent density functional theory to evaluate their optical, electronic, and light harvesting efficiency, as well as voltaic properties. The results showed that TP, TD, BD, BP, and BDD were preferable for optimization of the bandgaps and molecular energy levels of these organophosphorus-based compounds over Th. heterocyclic compounds. The calculated electron transfer process to the conduction band of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and the subsequent regeneration in BAP–BBD and PBAP (polybisazaphosphole)–TD were possible in organic voltaic cells, making these modeled compounds more proficient solar cell sensitizers. The method used can be explored in understanding the relationship between electronic properties and molecular structure of other materials for electronic devices.

Theoretical Study of the Electronic Structure and Properties of Alternating Donor-Acceptor Couples of Carbazole-Based Compounds for Advanced Organic Light-Emitting Diodes (OLED)

Generally, it is difficult to generate a high-performance pure blue emission organic light-emitting diode (OLED). That is because the intrinsically wide band-gap makes it hard to inject charges into the emitting layer in such devices. To solve the problem, carbazole derivatives have been widely used because they have more thermal stability, a good hole transporting property, more electron rich (p-type) material, and higher photoconductivity. In the present work, novel copolymers containing donor-acceptor-acceptor-donor (D-A-A-D) blue compounds used for OLEDs were investigated. The theory of the geometrical and electronic properties of N-ethylcarbazole (ECz) as donor molecule (D) coupled to a series of 6 acceptor molecules (A) for advanced OLEDs were investigated. The acceptors were thiazole (TZ), thiadiazole (TD), thienopyrazine (TPZ), thienothiadiazole (TTD), benzothiadiazole (BTD), and thiadiazolothienopyrazine (TDTP). The ground state structure of the copolymers were studied using Density Functional Theory (DFT) at B3LYP/6-31G(d) level. Molecular orbital analysis study indicated 3 investigated copolymers (ECz-diTZ-ECz, ECz-diTD-ECz, ECz-diBTD-ECz) have efficient bipolar charge transport properties for both electron and hole injection to the TiO2 conduction band (4.8 eV). In addition, the excited states electronic properties were calculated using Time-Dependent Density Functional Theory (TD-DFT) at the same level. Among these investigated copolymer ECz-diTZ-ECz and ECz-diTD-ECz showed the maximum absorption wavelengths (λabs) with blue emitting at 429 and 431 nm, respectively. The results suggested that selected D-A-A-D copolymers can improve the electron- and hole- transporting abilities of the devices. Therefore, the designed copolymers would be a promising material for future development of light-emitting diodes, electrochromic windows, photovoltaic cells, and photorefractive materials.

Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2. This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.

Simulation study on the effects of chemical structure and molecular size on the acceptor strength in poly(3-hexylthiophene)-based copolymer with alternating donor and acceptor for photovoltaic applications

This research assessed the effects of various chemical structures and molecular sizes on the simulated geometric parameters, electron structures, and spectroscopic properties of single-chain complex alternating donor–acceptor (D–A) monomers and copolymers that are intended for use as photoactive layer in a polymer solar cell by using Kohn–Sham density functional theory with B3LYP exchange-correlation functional. The 3-hexylthiophene (3HT) was selected for electron donor, while eight chemicals, namely thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), benzothiadiazole (BT), thiadiazolothieno-pyrazine (TPD), oxadiazole (OXD) and 5-diphenyl-1,2,4-triazole (TAZ), were employed as electron acceptor functional groups. The torsional angle, bridge bond length, intramolecular charge transfer, energy levels, and molecular orbitals were analyzed. The simulation results reveal that the geometry and electron structure of donor–acceptor monomer and copolymer are significantly impacted by heterocyclic rings, heteroatoms, fused rings, degree of steric hindrance and coplanarity of the acceptor molecular structure. Planar conformation was obtained from the D copolymer, and a pseudo-planar structure with the TD copolymer. The TAZ acceptor exhibited strong steric hindrance due to its bulky structure and non-planarity of its structure. An analysis of the electron structures indicated that the degree of intramolecular electron-withdrawing capability had the rank order TAZ < Z < D < TPD < OXD < TP < BT < TD. The TD is indicated as the most effective acceptor among those that were simulated. However, the small energy gaps of TD as well as TPD copolymer indicate that these two copolymers can be used in transparent conducting materials. The copolymer based on BT acceptor exhibited good intramolecular charge transfer and absorbed at 656 nm wavelength which is close to the maximum flux of solar spectrum. Hence, the BT acceptor functional group provides a compromise in the characteristics of a donor–acceptor copolymer, useful in a polymeric candidate material for the photoactive layer in a polymer solar cell.

Influence of Exciton Diffusion and Charge-Transfer State Dissociation Efficiency on the Short-Circuit Current Densities in Semi-Random Donor/Acceptor Polymer:Fullerene Solar Cells

Four bulk heterojunction (BHJ) polymer:fullerene solar cell systems containing semi-random poly(3-hexylthiophene) (P3HT) based donor/acceptor (D/A) copolymers as donors and phenyl-C61-butyric acid methyl ester (PC61BM) as an acceptor were studied. Even though all three semi-random D/A polymers have broad absorption profiles and high absorption coefficients, benzothiadiazole (BTD) or thienopyrazine (TP) containing polymers P3HTT-BTD and P3HTT-TP have significantly lower short-circuit current densities (Jsc) than P3HT and P3HTT-DPP-10% (containing 10% diketopyrrolopyrrole (DPP)) in BHJ solar cells. As a result, the power conversion efficiencies (PCE) for P3HTT-BTD and P3HTT-TP are below 1% compared to 2.7% for P3HT:PC61BM and 3.1% for P3HTT-DPP-10%:PC61BM (efficiencies are not mismatch corrected and measured after encapsulation and shipment). Complementary optical and numerical simulations, experimental techniques such as external quantum efficiency (EQE) measurements, and transient photoconductivity studies were used to study the polymer:fullerene blends. The efficiency of each step involved in photocurrent generation was calculated, and it was determined that low exciton diffusion efficiency together with geminate and trap-assisted recombinations in deep trapping sites are responsible for the decrease of Jsc in P3HTT-BTD:PC61BM and P3HTT-TP:PC61BM solar cells, whereas they are negligible in the case of P3HT:PC61BM and P3HTT-DPP-10%:PC61BM. The observed drastic influence of relatively small changes in the chemical structure of the studied polymers (only 10% of the total monomer content changes between polymers) on the exciton diffusion efficiency, charge-transfer (CT) state dissociation efficiency, and free polaron collection efficiency emphasizes the importance of a judicious choice of the acceptor monomer in semi-random D/A polymers.

Band Gap Engineering via Controlling Donor–Acceptor Compositions in Conjugated Copolymers

Varying composition of π-donor/acceptor moieties has been considered as an effective strategy for fine-tuning of the electronic properties of D-A conjugated copolymers. In this study, the change of optoelectronic properties with the change of donor/acceptor ratios is investigated on the basis of first-principles density functional calculations. Copolymers containing moieties of similar π-electron donating and/or accepting capabilities, e.g., thiophene (T)-methoxythiophene (OT), exhibit a linear dependence of electronic properties (especially, HOMO/LUMO, band gap, and bandwidth) on the D/A content. In contrast, for strong D/A contrast systems, e.g., thiophene (T)-thienopyrazine (TP), the electronic properties vary nonlinearly with D/A compositions. However, when the block size of one parent monomer in a strong D/A contrast system is fixed, the variation of electronic properties shows a remarkable linear correlation against D/A compositions. We found that the deviation of electronic properties from a linear composition dependence is dominated by the strength of orbital interactions between D and A. Weak orbital interactions between D and A moieties tend to lead to a nonlinear composition dependence. Our results provide useful insights for band gap tuning through the adjustment of D/A compositions in D-A conjugated copolymers.

Theoretical Study Of The Geometric, Electronic Structure And Properties Of Alternating Donor-acceptor Conjugated Oligomers: Carbazole (Cbz)-based 3,4-ethylenedioxythiophene (Edot)

In this work the geometrical and electronic properties of (Carbazole-3.4-Ethylenedioxythiophene) (Cbz-Edot) based alternating donor-acceptor conjugated oligomers were studied by the density functional theory (DFT) at the B3LYP level with 6-31G(d) basis set. The acceptors investigated include thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), benzothiadiazole (BT) and thiadiazolothienopyrazine (TPD). A low band gap will be expected in polymers containing donor-acceptor (D-A) repeating units. In order to predict the band gaps for guiding the synthesis of novel materials with low band gaps, we apply quantum-chemical techniques to calculate the band gaps in several oligomers. The results have been compared with those of thiophene and 3,4-ethylenedioxythiophene polymers with donor-acceptor fragment. The lowest excitation energies (Eex) and the maximal absorption wavelength (λabs) are studied using the time dependent density functional theory (TD-DFT), method. The electronic transitions of the absorption spectrum derived by TD-DFT method give useful structural and electronic information for designing novel conducting organic polymer materials. The theoretical results suggest that both the acceptor strength and the stable geometry contribute significantly to the electronic properties of alternating donor-acceptor conjugated copolymers.

Solar cells based on semi-random P3HT analogues containing dithienopyrrole: influence of incorporating a strong donor

Novel semi-random poly(3-hexylthiophene) (P3HT) based polymers P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP containing the strong donor dithienopyrrole (DTP) as well as different acceptors (benzothiadiazole (BTD), thienopyrazine (TP) or diketopyrrolopyrrole (DPP)) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. All polymers (except for the all donor polymer P3HTT-DTP) show considerably broadened absorption compared to P3HT due to the donor-acceptor effect and their multichromophoric nature. The introduction of the strong donor DTP leads to increased HOMO energies and thus decreased open-circuit voltage (Voc) (compared to previously published semi-random polymers) as well as an amorphous character of P3HTT-DTP, P3HTT-BTD-DTP and P3HTT-TP-BTD resulting in low hole mobilities and moderate solar cell efficiencies (0.18% to 0.36%). The exception is P3HTT-DPP-DTP, which is semi-crystalline and has a high hole mobility of 1.4×10−4 cm2 ?startVend?−1 s−1 comparable to P3HT, as well as increased photocurrent (10.7 mA/cm2) due to broad and uniform photoresponse up to 850 nm leading to a promising non-optimized device efficiency of 2.83%.

Synthesis and Characterization of Bridged Bithiophene-Based Conjugated Polymers for Photovoltaic Applications: Acceptor Strength and Ternary Blends

Six of three-component donor−acceptor random copolymers P1−P6, symbolized as (thiophene donor)m−(thiophene acceptor)n, were rationally designed and successfully synthesized by the palladium-catalyzed Stille coupling. The 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) unit serves as the donor for P1−P4, while the benzothiadiazole (BT), quinoxaline (QU), dithienoquinoxaline, and thienopyrazine (TP) units are used as the acceptor for P1, P2, P3, and P4, respectively. P5 and P6 are structurally analogous to P1 and P2 except for using the dithieno[3,2-b:2′,3′-d]silole (DTS) unit as the donor. Because the band gap lowering ability of the acceptor units in the polymer is in the order TP > BT > QU presumably due to the quinoid form population in the polymers, the optical band gaps can be well adjusted to be 1.2, 1.6, and 1.8 eV for P4, P1, and P2, respectively. It is found that the two bridged bithiophene units, CPDT and DTS, have similar steric and electronic effects on the P1 and P5 as well as P2 and P6, respect...

Small band gap conjugated polymers based on thiophene–thienopyrazine copolymers

AbstractIn this study, five small band gap thiophene (TH)–thienopyrazine (TP) conjugated copolymers were synthesized by Stille‐coupling reaction. The polymer structures consisted of one to four thiophene rings with the TP of different side groups provided a systematical investigation on the structure–electronic property relationship. The absorption maxima of the polymer films decreased from 850 to 590 nm as the thiophene moieties increased from thiophene to quaterthiophene. The optical and electrochemical band gaps of the studied poly[2,3‐didodecyl‐5‐(thiophen‐2‐yl)thieno[3,4‐b]pyrazine] (PTHTP‐C12) were 0.97 and 0.78 eV, respectively, indicating a significant intramolecular charge transfer. The theoretical geometry and electronic properties of the TH‐TP copolymers by the density functional theory at the B3LYP level and 6‐31G(d) basis set suggested that the bond length alternation enlarged with enhancing the thiophene content and resulted in the variation on the polymer band gap. The relatively small theoretical effective mass of poly(TH‐alt‐TP) also indicated its potential applications for field transistor applications. Our study demonstrates the tunable electronic properties of small band gap copolymers by the thiophene content and the resulted geometry variation. Such polymers could be potentially used for near‐infrared electronic and optoelectronic devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5872–5883, 2007

Theoretical Electronic Structure and Properties of Alternating Fluorene–Acceptor Conjugated Copolymers and Their Model Compounds

The theoretical geometries and electronic properties of fluorene (F) based alternating donor–acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6–31G or 6–31G** basis set. The acceptors investigated in this study include thiazole (TZ), thiadiazole (TD), thienopyrazine (TP), thienothiadiazole (TT), thiadiazolothienopyrazine (TTP), quinoxaline (Q), benzothiadiazole (BT), pyrazinoquinoxaline (PQ), thiadiazoloquinoxaline (TQ), and benzobisthiathiadiazole (BB). The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e., HOMO, LUMO level, and band gap. The geometries of fluorene-based donor–acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene–acceptor conjugated copolymers could be tuned by the geometries or acceptor strength. Hence, these proposed copolymers could have potential applications as light-emitting diodes (LEDs), transparent conductor, or photovoltaic devices.

Does the donor–acceptor concept work for designing synthetic metals?

Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained. Four of the acceptors, BBT, TP, TT, and TTP, give rise to copolymers with band gaps that are smaller than that of polyacetylene. BBT and TTP copolymers with PY in 1:2 stoichiometry are predicted to be synthetic metals. Band-gap reductions result from upshifts of HOMO energies and much smaller upshifts of LUMO values. The smallest band gaps are predicted with TTP, since changes in LUMO energies upon copolymerization are particularly small. The consequence of the small interactions between LUMO levels of donor and acceptor are vanishingly small conduction bandwidths.

Electronic structure and properties of alternating donor–acceptor conjugated copolymers: 3,4-Ethylenedioxythiophene (EDOT) copolymers and model compounds

The electronic structure and properties of 3,4-ethylenedioxythiophene (EDOT) based alternating donor–acceptor conjugated copolymers and their model compounds were studied by the density functional theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The acceptors investigated include thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), thiadiazolothienopyrazine (TPD), quinoxaline (BP), benzothiadiazole (BD), pyrazinoquinoxaline (BPP), benzobisthiadiazole (BDD), and thiadiazoloquinoxaline (BDP). The torsional angle, intramolecular charge transfer, bridge bond length, and bond length alternation were analyzed and correlated with the electronic properties. It was found that the geometries of the donor–acceptor materials were significantly affected by the ring size and intramolecular charge transfer. The HOMO level, LUMO level, and band gap of the model compounds were well correlated with the acceptor strength. However, the electronic properties of the copolymers did not vary systematically with the acceptor strength due to the change in geometry from model compound to polymer. The aromatic geometry of EDOT–TP model compound is transformed to quinoid in the corresponding copolymer and results in a small band gap ( E g) of 0.97 eV. Large intramolecular charge transfer and the small bond length alternation in the EDOT–BDP copolymer resulted in an E g of 0.7 eV. Hence, these two polymers could have potential applications for transparent conductors or photovoltaic devices. The small effective masses and large HOMO and LUMO bandwidths of PEDOT–TP and PEDOT–BDP make them potential materials for ambipolar thin film transistors. The theoretical results suggest that both the acceptor strength and the stable geometry contribute significantly to the electronic properties of alternating donor–acceptor conjugated copolymers.