Zaifang Li

Design and synthesis of solution processable small molecules towards high photovoltaic performance

We successfully synthesized a series of novel symmetrical solution processable small molecules (APPM, AAPM and ATPM) consisting of the electron-accepting moiety (2-pyran-4-ylidenemalononitrile) (PM) and the electron-donating moiety (triphenylamine) linked by different electron-donating moieties (phenothiazine, triphenylamine and thiophene) through a Suzuki coupling reaction. Differential scanning calorimetry (DSC) measurement indicates that APPM and AAPM shows relatively high glass-transition temperature of ca. 137 °C and 163 °C, while the melting point of ATPM is at ca. 164 °C. UV-vis absorption spectra show that the combination of the PM moiety with moieties with a gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, which leads to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Both cyclic voltammetry measurement and theoretical calculations displayed that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction (BHJ) photovoltaic devices with a structure of ITO/PEDOT/PSS/small molecules/PCBM/LiF/Al were fabricated by using the small molecules as donors and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as acceptor. Power conversion efficiencies (PCE) of 0.65%, 0.94% and 1.31% were achieved for the photovoltaic devices based on APPM/PCBM, AAPM/PCBM and ATPM/PCBM under simulated AM 1.5 illumination (100 mW cm−2), respectively. The open circuit voltage of 1.0 V obtained from the device based on ATPM/PCBM is one of the highest values for organic solar cells based on solution processable small molecules.

Theoretical Design Study on Photophysical Properties of Light-emitting Pyrido[3,4-b]pyrazine-based Oligomers

The electronic structures, charge injection and transport, and absorption and emission properties of four series of dimethylpyrido[3,4-b]pyrazine-based oligomers (5-(5,5-dimethyl-5H-dibenzo[b,d]silol-3-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (SPP)n, (5-(dibenzo[b,d]thiophen-3-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (TPP)n, (5-(9,9-dimethyl-9H-fluoren-2-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (FPP)n, (2-(2,3-dimethylpyrido[3,4-b]pyrazin-5-yl)-9-methyl-9H-carbazole)n (PPC)n were investigated by the density functional theory approach. The ground-state geometries of (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4) were optimized at the B3LYP/6–31G(d) level. The energies of the HOMO, LUMO and HOMO–LUMO energy gaps of (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4) were obtained by a linear extrapolation method. Further, calculations of ionization potential, electronic affinity and reorganization energy were used to evaluate charge injection and transport abilities. For (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4), the time-dependent density functional theory (TDDFT) calculation results revealed that the absorption peaks can be characterized as π–π* transitions and are coupled with the location of electron density distribution change in different repeat units. All the primary theoretical investigations are intended to establish structure–property relationships, which can provide guidance in designing and preparing novel efficient organic light-emitting materials with a high performance.

Synthesis and properties of novel polymers based on PD electron-withdrawing unit

In this article, we designed and synthesized a series of 5-(2,6-dimethyl-4H-pyran-4-ylidene)-1,3-diethyl-2-thioxodihydropyrimidine- 4,6(1H, 5H)-dione (PD) unit based polymers (PFTDT, CZTDT, PHTDT and THTDT) for the first time. In these polymers, fluorene, 2,7-carbazole, phenothiazine and thiophene are employed as electron-donating groups and PD as electron-withdrawing group. TGA measurements demonstrated that these polymers possess good thermal stability (all above 377 °C). Very broad absorption spectrum was also obtained from the polymer THTDT (300–850 nm). CV characterization found that these polymers owned low highest occupied molecular orbital (HOMO) energy levels (–5.39 eV for THTDT,–5.49 eV for CZTDT and–5.78 eV for PFTDT) except for PHTDT (–5.17 eV). The geometry and electronic properties of PFTDT, CZTDT, PHTDT and THTDT were investigated by means of theoretical calculation. All the above advantages demonstrate that PD based polymers could be candidates for electronic devices.

Substituent effects on the properties of fluorene-thieno[3,4-b]pyrazine derivatives for light-emitting applications

Five thieno[3,4-b]pyrazine-based model compounds were studied to explore the effects of the substituent groups (alkyl or aryl) on the structure, atomic charge, optical properties, ionization potential (IP), electron affinity (EA), and reorganization energy. Theoretical calculations were carried out by density functional theory (DFT) using the B3LYP hybrid function combined and CAM-B3LYP with the 6-31G(d) basis set. The lowest-lying absorption and emission spectra of 9,9′-diethylhexylfluorene-alt-5,7-dithien-2-yl-thieno[3,4-b] pyrazine (FDDTTP) with alkyl groups showed a blue-shift, while those of FDDTTP with aryl groups exhibited a red-shift. The results agree well with analytical data from reorganization energies. IPs are brought down by both alkyl and aryl groups. However, EAs are raised only by aryl units. The results indicate that aryl groups are more helpful in forming excitions for FDDTTP molecules. Consequently, FDDTTP with aryl groups are more efficient acceptor segments for designing donor–acceptor copolymers than those with alkyl groups.