V. S. Kochurov

New conjugated alternating benzodithiophene-containing copolymers with different acceptor units: synthesis and photovoltaic application

Two new alternating low band gap D–A copolymers with different acceptor structures of 4,8-bis-(5-bromothiophene-2-yl)-benzo[1,2,5]thiadiazole (P1) and 4,8-dithiophene-2-yl-benzo[1,2-c;4,5-c′]-bis-[1,2,5]thiadiazole (P2) and a common BDT donor segment have been synthesized under Stille reaction conditions and characterized. The polymers showed good solubility, broad absorption bands and optical band gaps of 1.62 eV and 1.16 eV for P1 and P2, respectively. Bulk heterojunction (BHJ) polymer solar cells based on P1 and P2 as electron donors and fullerene derivatives (PC60BM and PC70BM) as acceptor were fabricated and their photovoltaic response was investigated. The overall power conversion efficieny (PCE) achieved for BHJ solar cells based on P1:PC60BM, P2:PC60BM, P1:PC70BM and P2:PC70BM blends cast from THF solvent is about 2.17%, 0.80%, 3.45% and 1.19%, respectively. The higher PCE for the device based on P1 has been attributed to the high value of hole mobility for P1 as compared to P2 and a larger driving force i.e. LUMO–LUMO offset, for photo-induced charge transfer for P1:PCBM BHJ active layer. The PCE has been further increased up to 5.30% and 1.58% for P1:PC70BM and P2:PC70BM blends cast from DIO/THF solvent, which is attributed to the improved crystallinity and a more balanced charge transport in the device.

Synthesis and characterization of a low band gap quinoxaline based D–A copolymer and its application as a donor for bulk heterojunction polymer solar cells

A new alternating copolymer P comprising of benzo[1,2-b;4,5,b′]dithiophene (BDT) derivative and 4,9-bis-(5-bromothiophene-2-yl)-6,7-di-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DTQx) derivative electron, donating and electron withdrawing units, respectively, has been synthesized by Stille reaction. The copolymer was characterized by TGA, UV-visible absorption and cyclic voltammetry. The optical band gap of P was calculated from the onset wavelength of absorption to be about 1.38 eV. The copolymer P was used as electron donor along with PC60BM or PC70BM as electron acceptors for the fabrication of bulk heterojunction solar cells with configurations of ITO/PEDOT:PSS/P:PC60BM or PC70BM/Al. The power conversion efficiencies (PCE) of the copolymer solar cells blended with PC60BM and PC70BM as electron acceptors, spin cast from THF solvent, were 2.10% and 3.26%, respectively. The PCE device based on the P:PC70BM blend processed from DIO/THF was enhanced up to 4.47%. After optimizing the device parameters, such as blend ratio of P to PCBM and the choice of processing solvent, power conversion efficiency reaches as high as 5.12% for P:PC70BM blend, when processed from DIO/THF solvent, a blend ratio of 1 : 2 w/w and DMSO doped PEDOT:PSS buffer layer is used.

Thienopyrazine or dithiadiazatrindene containing low band gap conjugated polymers for polymer solar cells

Four new low-band-gap alternating copolymers (P-1, P-2, P-3 and P-4) based on electron-rich benzodithiophene and newly developed electron-deficient units, thienopyrazine or dithiadiazatrindene derivatives, were synthesized by Stille polycondensation. All polymers exhibit good solubility in common organic solvents and a broad absorption band in the visible to near-infrared regions. The film optical band gaps of the polymers are in the range of 1.28–2.07 eV and the highest occupied molecular orbital (HOMO) energy levels are in the range of −4.99 eV to −5.28 eV. Bulk heterojunction polymer solar cells (PSCs) of the polymers were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor material, and a power conversion efficiency of 0.80% was realized with P-1 as donor material.

Synthesis, photophysical, and electrochromic properties of new triarylamino-containing polyphenylquinoxalines

New triarylamino-containing bis(α-diketones) are synthesized. On the basis of these compounds, a series of electrochromic organosoluble polyphenylquinoxalines with glass-transition temperatures of 224–315°C are prepared. All polymers intensively fluoresce in solutions and thin films with maxima at 535–600 and 530–560 nm, respectively. Cyclic voltammograms of polyphenylquinoxalines exhibit reversible redox properties in the range E 1/2 = 0.92–1.25 eV. It is shown that, after 15 cycles, all polymers preserve high stability and reversibility of electrochromic characteristics, but their color changes from yellow (neutral form) to wine red (oxidized form).

Synthesis and photovoltaic properties of new donor-acceptor benzodithiophene-containing copolymers

Four new alternating narrow band-gap copolymers containing benzodithiophene, 4,8-dithiophen-2-yl-benzo[1,2-c;4,5-c′-bis[1,2,5]thiadiazole, 4,9-bis(thiophen-2-yl)-6,7-di(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline, 5,8-dibromo-2,3-bis(5-octylthiophen-2-yl)quinoxaline, and 4,7-bis(5-bromothiophen-2-yl)benzo[1,2,5] thiadiazole units are synthesized under Stille reaction conditions. The structures, molecular masses, and physical properties of the copolymers are studied via 1H NMR spectroscopy, GPC, cyclic voltammetry, and thermomechanical and thermogravimetric analyses. The polymers show solubility and a broad absorption region (with the band gap in the range from 0.81 to 1.53 eV). All of the polymers are photostable in air, and their levels of the highest occupied molecular orbital vary from −4.98 to −5.30 eV. Polymer solar cells based on these copolymers as donors and fullerene PC60BM as an acceptor show open-circuit voltages in the range 0.16–0.61 V, and the efficiencies of the devices are in the range 0.02–0.49%.

New narrow-band-gap conjugated copolymers based on benzodithiophene: Synthesis and photovoltaic properties

Four new alternating narrow-band-gap copolymers based on benzodithiophene are synthesized under conditions of the Stille reaction. In comparison to benzodithiophene homopolymers, all these copolymers show good solubility in common organic solvents and broad absorption in the visible spectrum of sunlight. The band gaps of the polymer films, as estimated from the cyclic voltammetry data, vary within 1.34–2.28 eV, and the position of the highest occupied molecular orbital lies in the range 4.99–5.72 eV. The values of the open-circuit voltage and the efficiency of polymer solar cells based on the copolymers are within 0.22–0.65 V and 0.02–0.48%, respectively.

Novel electron-withdrawing π-conjugated pyrene-containing poly(phenylquinoxaline)s

65 In recent time, conjugated polymers have attracted considerable attention of researchers not only from scientific but also from a practical viewpoint due to their fundamental optoelectronic properties and application as electroactive materials for organic light emitting diodes [1], field effect transistors [2], and photovoltaic devices [3]. Conjugated polymers are prepared by Stille, Suzuki, and Yamamoto reactions using nickel (Yamamoto) and palladium (Suzuki and Stille) catalysts [4, 5]. However, the high cost of the processes and difficulties of purification from metal catalyst residues, which are undesirable impurities for optoelectronic devices, are considerable drawbacks of these reactions. New noncatalytic methods for the synthesis of high purity conjugated polymers should be developed to solve these problems. It has also been found that intense electroluminescence and high charge mobility in a polymer require a high electron– hole conduction of a medium due to charge carriers mobility and a balance of injection of carriers of both signs from opposite electrodes into the emission poly mer layer. However, the majority of conjugated poly mers provide hole transport that causes imbalance of charge injection and a poor quantum yield. The search for new efficient electron transport polymers is necessary to improve the characteristics of electrooptical devices, which is provided, in particu lar, by the introduction of electron withdrawing frag ments—triazole [6], oxodiazole [7], quinoline [8], and quinoxaline [9]—into macromolecules. The lat ter are the least studied and show higher electron withdrawing properties due to the presence of four electron withdrawing heteroatoms. π Conjugated thiophene containing poly(phenylquinoxaline)s (PPQs) have been obtained recently. These PPQs behave as excellent electron transport materials for organic light emitting diodes [10]. We prepared new conjugated pyrene containing PPQs as high purity polymers by noncatalytic polycondensation reaction to provide the balance of the hole and electron con duction. The introduction of flat structures, such as pyrene, into a conjugated polymer is supposed to enhance intermolecular electronic interaction in the solid state and result in formation of π–π stacking structures. In continuation of our studies [11, 12], we prepared and studied new bipolar pyrene containing PPQs as efficient electron hole and transport materi als and thereby synthesized, previously unknown bis(α diketone)s IV and VIII.

Synthesis and photovoltaic properties of new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b′′] trithiophene donor and different acceptor units (P1 and P2)

Two new conjugated D–A copolymers P1 (PBTT-TQ) and P2 (PBTT-TPz) based on the same benzo[1,2-b:3,4-b′:6,5-b′′] trithiophene (BTT) donor and different acceptor monomers 4,9-bis(5-bromithiophen-2-yl)-6,7-bis(2-ethylhexyl)[1,2,5] thiadiazolo[3,4-g] quinoxaline (TQ) and 5,7-bis(5-bromothiophen-2-yl)-2,3-bis(5-octylthiophen-2-yl) thieno[3,4-b] pyrazine (TPy), respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), 1H NMR, UV-Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. The optical bandgaps of P1 and P2 measured from the onset of optical absorption of these copolymers in thin films were 1.14 eV and 1.38 eV, respectively. Photovoltaic properties of the bulk heterojunction (BHJ) devices were studied using P1 and P2 as the donor and PC61BM or PC71BM as the acceptor with weight ratios of polymer : PC61BM of 1 : 1, 1 : 2 and 1 : 3. The optimized photovoltaic devices fabricated with active blend layers of P1:PC71BM (1 : 2) and P2:PC71BM (1 : 2) cast from o-diclorobenzene (o-DCB) showed PCEs of 2.05% and 3.14%, respectively, whereas P2:PC61BM and P2:PC71BM yielded PCEs of 1.44% and 2.11%, respectively. The PCEs of BHJ solar cells based on P1:PC71BM and P2:PC71BM processed from 1-chloronapthanene (CN)/o-DCB solvent were further improved up to 5.28% and 2.6%, respectively.

Synthesis of n-type conjugated polymers for bulk heterojunction solar cells

A new conjugated alternating donor-acceptor copolymer is synthesized from perylene diimide and the benzodithiophene derivative via the Suzuki reaction. Temperatures corresponding to the 10% weight loss of the copolymer in air and argon are 336 and 362°C. The copolymer is soluble in common organic solvents. The maxima of absorption spectra of the copolymer in chloroform solution and thin films are at 456 and 567 and at 444 and 567 nm, respectively. The optical band gap, as calculated from the onset of the absorption spectrum, is 1.74 eV, and the electrochemical band gap is 1.89 eV. The energies of HOMO and LUMO derived from the onset of the first oxidation and reduction potential of the cyclic voltamperogram are −5.97 and −4.08 eV. It is found that the solar cell with copolymer-to-poly(3-hexylthiophene) = 1: 1 features the best characteristics: The open circuit voltage is 0.54 V, the short circuit current is 0.976 mA/cm2, the fill factor is 0.397, and the efficiency is 0.14%. The conjugated polymer based on perylene diimide belongs to the class of n-type polymers and shows promise as an acceptor material for all-polymer bulk heterojunction solar cells.

Synthesis of new conjugated copolymers containing 4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene/5,7-bis(3,4-diethylthien-2-yl)-2,3-diphenylthieno[3,4-b]pyrazine and 4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b′]dithiophene/4,6-di(3,4-diethylthien-2-yl)-thieno[3,4-c][1,2,5]thiadiazole derivatives for photovoltaic applications

Two new donor-acceptor narrow-band-gap photovoltaic copolymers containing various electron-acceptor functional groups, such as thienopyrazine and thienodithiazole derivatives, are synthesized under the conditions of the Suzuki polycondensation reaction and characterized via 1H NMR spectroscopy and GPC. The optical, electrochemical, and photovoltaic properties of the polymers are examined in detail. Both copolymers have good solubilities, high thermal stabilities, broad absorption regions (350–950 nm), relatively low levels of the highest occupied molecular orbital (−5.01 and −5.02 eV for polymers I and II, respectively), and narrow band gaps (1.31 and 1.41 eV, respectively). Photovoltaic devices based on a polymer-II-PC60BM (1: 3, wt/wt) mixture under AM 1.5 irradiation of 100 mV/cm2 exhibit a power-conversion efficiency of ∼0.29%, a short-circuit current of 1.45 mA/cm2, an open-circuit voltage of V oc = 0.67 V, and a fill factor of 0.30. These results are evidence that thienopyrazine functional groups are attractive as electron-acceptor building blocks in organic electronics.