I. O. Konstantinov

Synthesis of new D-A1–D-A2 type low bandgap terpolymers based on different thiadiazoloquinoxaline acceptor units for efficient polymer solar cells

Two low bandgap D-A1–D-A2 conjugated copolymers, namely denoted as P1 (non-fluorine substituted thiadiazoloquinoxaline A2) and P2 (fluorine substituted thiadiazoloquinoxaline A2) with the same D (thiophene) and A1 (benzothiadiazole) groups were synthesized in order to investigate the effect of fluorine atoms on the photovoltaic performance of polymer solar cells. The electrochemical properties demonstrate that the highest occupied molecular orbital (HOMO) energy level lowered from −5.08 eV (for P1) to −5.16 eV (for P2), whereas the lowest unoccupied molecular orbital (LUMO) energy levels remain nearly the same. These copolymers showed strong absorption in the wavelength range 300–1100 nm and have a bandgap of around 1.08 and 1.11 eV for P1 and P2, respectively. After the optimization of the weight ratio and concentration of solvent additives 1-chloronaphathalene (CN), the highest power conversion efficiencies of bulk heterojunction polymer solar cells achieved were up to 5.30% and 7.21% for P1 and P2 as donor and PC71BM as acceptor. The enhanced Voc and Jsc for the P2 based device can be mainly ascribed to the lower HOMO energy levels and higher hole mobility and better morphology of the fluorinated copolymer P2 : PC71BM blend.

Synthesis and photophysical properties of regioregular low bandgap copolymers with controlled 5-fluorobenzotriazole orientation for photovoltaic application

Two copolymers P1 and P2, with regioregular and random structures, respectively, were synthesized and their optical and electrochemical properties (both theoretical and experimental) were investigated. The regioregular copolymer P1 showed a lower optical bandgap and a high degree of crystallinity compared to the random P2 copolymer. These copolymers were used as electron donors along with PC71BM as the electron acceptor for the fabrication of solution processed bulk heterojunction solar cells. After the optimization of the weight ratio between the donor and the acceptor and the concentration of the solvent additive i.e. DIO in chloroform, the solar cells based on regioregular P1 exhibit higher power conversion efficiency (7.66%) than the random P2 (5.33%). The enhancement in the power conversion efficiency has been attributed to the increased hole mobility due to the high regioregularity of the conjugated copolymer backbone and effective ordering between the polymer chains. This work brings forth and establishes the importance of copolymers having a regioregular A–D–A–D structure so as to offer significant performance benefits over the random D–A copolymer. This approach is a promising new route to materials for highly efficient polymer solar cells.

New electron-accepting quinoxalinothiadiazole-containing heterocycles as promising building blocks for organic optoelectronic devices

Two novel quinoxalinothiadiazole-containing dibromides—4,9-dibromo-6,7-bis(9,9-didodecyl7-fluoro-9H-fluoren-2-yl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (11) and 4,9-dibromo-6,7-bis(9,9-didodecyl-5,7-difluoro-9H-fluoren-2-yl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (19)—have been synthesized, which are promising strong electron-accepting building blocks for preparing highly efficient narrow-bandgap D–A conjugated polymers. The composition and structure of monomers 11 and 19 have been proved by elemental analysis data, IR spectroscopy, and 1H and 13C NMR.

Synthesis of new symmetrical carbazole- and fluorene-containing α-diketones

A simple and convenient one-step method for the preparation of symmetrical α-diketones has been proposed. The latter are intended to be used in synthesis of new carbazole- and fluorene-containing quinoxalines showing electroluminescent and photovoltaic properties.

Regular conjugated D–A copolymer containing two benzotriazole and benzothiadiazole acceptors and dithienosilole donor units for photovoltaic application

Herein, we report the synthesis and characterization of a regular D1–A1–D1–A2–D2–A2 conjugated copolymer with an optical bandgap of 1.53 eV, denoted as PTBTfBTzSi consisting of two acceptors i.e. benzothiadizole (A1) flanked with two thiophene donors (D1) and fluorinated benzotriazole (A2) with one more donor dithienosilole (D2) and investigated its optical and electrochemical properties. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of PTBTfBTzSi are estimated from cyclic voltammetry measurements and are −5.38 eV and −3.67 eV, respectively. We have used this copolymer as a donor along with a PC71BM as acceptor for the fabrication of solution processed BHJ polymer solar cells (PSCs). The PSC based on the optimized PTBTfBTzSi:PC71BM active layer (weight ratio 1 : 1.5 and 3 v% 1,8-diiodooctance (DIO)) as a solvent additive in chloroform, showed overall PCE of 8.91% (short circuit current of 14.36 mA cm−2, open circuit voltage of 0.94 V and fill factor of 0.66) which is higher than that of PSC based on a chloroform cast active layer (4.95% with a short circuit current of 9.53 mA cm−2, open circuit voltage of 0.98 V and fill factor of 0.53). The higher PCE is attributed to the balanced charge transport, elevated light harvesting efficiency and more favorable nanoscale morphology and enhanced crystallinity of the active layer processed with the solvent additive. Moreover, the energy loss in our polymer solar cell is 0.59 eV which is one of the lowest values among the most of the efficient polymer solar cells reported so far based on a fullerene based acceptor, to the best of our knowledge.

Polymer solar cells based on D–A low bandgap copolymers containing fluorinated side chains of thiadiazoloquinoxaline acceptor and benzodithiophene donor units

Three ultralow-bandgap D–A copolymers (P0F, P2F and P4F) based on fluorene-substituted thiadiazoloquinoxaline acceptor and BDT donor units were synthesized and characterized. The effect of the number of fluorine atoms substituted in the fluorene side chain on the optical, electrochemical, crystalline and photovoltaic properties of copolymers was compared. The results showed that the optical properties and lowest unoccupied molecular orbital energy levels of the copolymers are almost the same with an increase in fluorine atoms in the side chain, while the highest occupied molecular orbital (HOMO) energy level is shifted downward. P4F and P2F show better light harvesting abilities, higher crystallinities and low-lying HOMO energy levels than those of P0F. These copolymers were used as donors along with PC71BM as an acceptor for the bulk heterojunction polymer solar cells and optimized by adjusting the donor-to-acceptor weight ratios and solvent vapor treatment. Polymer solar cells based on optimized active layers based on P4F and P2F deliver a power conversion efficiency of 8.15% and 6.38%, respectively, which are higher than that of P0F counterpart (5.04%) due to an increase in all photovoltaic parameters, i.e., short circuit current, open circuit voltage and fill factor, and a very low voltage loss of 0.44 eV.

New iridium-containing conjugated polymers for polymer solar cell applications

A series of novel donor–acceptor (D–A) copolymers P1–P5 with iridium-complexed moieties in their side chains have been synthesized on the basis of a new iridium-containing monomer. The results obtained show that P1–P5 have good thermal stability (317–347 °C) for photovoltaic applications. These copolymers absorb visible light in a broad spectral range up to 680 nm. The optical bandgaps of P1–P5 are in the range of 1.96–2.08 eV, respectively. The HOMO and LUMO energy levels of the polymers P1–P5 estimated from cyclic voltammetry measurements indicate that these copolymers are suitable as electron donors along with PC71BM as an electron acceptor for bulk heterojunction polymer solar cells. BHJ polymer solar cells were developed based on blend compositions (P1–P5):PC71BM. The values of Jsc, Voc, and FF are in the range of 0.95–4.44 mA cm−2, 0.67–0.69 V and 34.6–56.8%, and the power conversion efficiencies (PCE) are in the range of 0.22–1.74%, respectively, the highest value of 1.74% being for P3. Increase of the photovoltaic parameters was achieved with increasing iridium complex percentage in the polymers due to involvement of triplet effects. The improvement in the efficiency of the triplet-forming polymers P2 and P3 in comparison with P1 appears to be due to the formation of triplet excitons in comparison with singlet excitons in the polymer P1 which does not contain heavy metals. With further increase in the content of iridium complex fragments in the polymers, for example, up to 3 mol% for polymer P4, the efficiency falls to 1.23% and further decreased to 0.22% for P5.

New narrow-band-gap thiazoloquinoxaline-containing polymers and their use in solar cells with bulk heterojunction

Two new regioregular polymers P1 and P2 with structure of type D–A1–D–A2 have been prepared. The polymers exhibit strong light absorption in the range 300–1100 nm and have band gaps of 1.09 and 1.11 eV, respectively. The HOMO and LUMO energies for P1 and P2 are–5.08/–3.81 and–5.16/–3.85 eV, respectively. Polymer solar cells (PSC) based on P1: PC71BM (1: 2, v/v) and P2: PC71BM (1: 1, v/v) have open-circuit voltage Voc, short circuit current Jsc, and efficiency of 0.79 and 0.84 V, 8.32 and 9.54 mA/cm2, 3.5 and 4.7%, respectively. The PSC based on P2 exhibits higher characteristics due to the presence of fluorine atoms in the structure: their strong electron-withdrawing properties decrease the HOMO level of polymer P2 as compared with that of P1, which increases the Voc value. Moreover, the formation of additional S∙∙∙F contacts leads to the growth of ordering and crystallinity of polymer P2 as compared with P1, which favors an increase in the values of Jsc and filling factor.

Synthesis and optical and electrochemical properties of 5,6-bis[9-(2-decyltetradecyl)-9H-carbazol-3-yl]naphtho[2,1-b:3,4-b’]dithiophene as a promising building block for photovoltaic applications

A new carbazole-containing naphthodithiophene heteroaromatic compound, 5,6-bis[9-(2-decyltetradecyl)-9H-carbazol-3-yl]naphtho[2,1-b:3,4-b’]dithiophene (M1), was synthesized to be subsequently used as the weak donor structure for the construction of narrow bandgap D–A conjugated polymers in terms of the “weak donor–strong acceptor” concept. The composition and structure of M1 were confirmed by the data of elemental analysis and IR and 1H and 13C NMR spectroscopy. The optical and electrochemical characteristics of the product were measured. The compound M1 was shown to have low-lying HOMO (–5.57 eV), which is favorable for high open-circuit voltage and to be suitable as a promising weak donor moiety in the D–A conjugated polymer for photovoltaic applications.