Hongyu Zhen

Full-solution processed, flexible, top-emitting polymer light-emitting diodes based on printed Ag electrodes

Full-solution processed, flexible, top-emitting polymer light-emitting diodes (PLEDs) are fabricated by using low-cost and high-quality printed Ag electrodes. Employing a composite cathode (Ag/ZnO/PEI) and bi-layer anode of WO3/PH1000, the resulting devices exhibit quite comparable device performances to the conventional PLEDs, remarkable flexibility and great potential for large-area production.

Design of asymmetric benzodithiophene based wide band-gap conjugated polymers toward efficient polymer solar cells promoted by a low boiling point additive

In this work, we reported two wide band-gap donor–acceptor (D–A) photovoltaic materials based on asymmetric benzodithiophene units with a bare phenyl or alkoxyl chain modified phenyl group as one of the substitutions. The optical band-gaps of the two polymers are both identified above 1.80 eV. P2 (with a side chain in the phenyl group) reveals slightly red-shifted film absorption spectra compared to polymer P1 attached with the bare phenyl group, which is partly attributed to the better molecular conformations and intermolecular interactions. Polymer solar cells with different acceptor materials are fabricated to systematically evaluate the photovoltaic properties of the two polymers. The results show that P2 based fullerene and fullerene-free solar cells both show superior photovoltaic performance over the devices of P1, mainly attributed to the more favorable heterojunction morphologies and more balanced charge transport. P2 based PSCs with PC71BM as the acceptor demonstrate an enhanced efficiency (8.58%) compared to that of P1 (7.04%). Notably, low boiling point toluene was reported as an effective additive for the first time to optimize the photovoltaic performance of fullerene-free polymer solar cells. The best efficiency of the P2/ITIC based devices reached over 9%, with a VOC of 0.90 V, JSC of 16.70 mA cm−2, and FF of 0.603, suggesting the great potential of low boiling point solvents as the additive toward efficient fullerene-free polymer solar cells.

Synthesis of new conjugated polymers with coordinated praseodymium complexes for polymer memory devices

A series of novel praseodymium(Pr)-coordinated polymers with fluorene, phthalimide and bipyridine moieties in the main chain were synthesized via a coordination reaction and palladium-catalyzed Suzuki coupling polycondensation. Their structures, optical features and memory performance have been well studied. A resistive switching device with the configuration of ITO/polymer/Al was constructed using a spin-coating process. The device exhibits nonvolatile write-once-read-many-times (WORM) memory behavior. Based on the electrochemical properties and theoretical calculations of the polymers, the effects of the phthalimide moiety and neutral Pr complex on the polymer memory device performance were investigated. Our work offers valuable clues on the development of polymer memory devices.

Simple synthesis of novel terthiophene-based D–A1–D–A2 polymers for polymer solar cells

Direct arylation was used to synthesize a series of novel terthiophene (T3)-based D–A1–D–A2 polymers and D–A2–D monomers in fewer synthetic steps. In these T3-based D–A1–D–A2 polymers, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) was selected as the first acceptor A1, octyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,1,3-benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) was selected as the secondary acceptor A2. T2-based polymer with the bithiophene segments (T2) as the donor was synthesized for comparison, too. UV-vis absorption, electrochemical properties, blend film morphology, and photovoltaic properties of the polymers were studied to explore the effects of the oligothiophene unit and secondary acceptor moiety (A2), meanwhile, the fluorine substitution effect was also discussed. It is shown that the change of donor segment from T2 to T3 introduces a difference in the energy levels, crystallinity, polymer:PC71BM morphology and PSC performances between the T2-based and T3-based D–A1–D–A2 polymers. Varying the secondary acceptor (A2) from BT to TPD also promotes the crystallinity and backbone planarity leading to enhanced PSC performances of the T3-based D–A1–D–A2 polymer. Although the effectiveness of fluorine substitution for tuning the UV-vis absorption, energy levels and degree of crystallinity has been demonstrated, the insufficient EDONORLUMO − EPCBMLUMO energy offset and poor miscibility of polymer:PC71BM limit the short circuit current (Jsc). In addition, the highest Jsc of 12.98 mA cm−2 is achieved for P1, while the higher HOMO level limits the open circuit voltage (Voc) and leads to a power conversion efficiency (PCE) of 4.36%.

A synergetic effect of an alkyl-thiophene π-bridge and side chain modification on device performances for stable all-polymer solar cells with high PCE

Tuning the structure of both polymer donors and acceptors, and development of novel matched donor/acceptor pairs are essential steps to enhance the power conversion efficiency (PCE) and stability of all-polymer solar cells (PSCs). Here, two weakly crystalline wide band-gap polymer donors PBDT(T)-HT-TPD and PBDT(ST)-HT-TPD with different side chains were synthesized by incorporating an alkyl-thiophene π-bridge, PBDT(T)TPD-based 2D-benzo[1,2-b:4,5-b]dithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) as reference donors. Three highly crystalline narrow-bandgap polymer acceptors P(NDI2OD-T), P(NDI2OD-Se) and P(NDI2HD-Se) sharing a similar backbone structure were also synthesized and studied. The all-PSCs were fabricated to investigate the synergetic effect of the alkyl-thiophene π-bridge and side chain modification. As a result, the PBDT(T)-HT-TPD/P(NDI2HD-Se) system is an optimal donor/acceptor combination due to its complementary light absorption and well-balanced miscibility/crystallinity, in which the highest power conversion efficiency (PCE) of 6.80% is achieved without extra-treatment. Meanwhile, the stability test of the devices shows that the incorporation of the alkyl-thiophene π-bridge can enhance thermal and storage stability. This work demonstrates that increasing the amorphous region of the donor to an appropriate degree provides an enhancement in the miscibility and morphology stability with highly crystalline acceptors, which will result in stable all-PSCs with high PCE.

Synthesis and photoelectric properties of new Pr-bonded polymers by coordination of isopropyloxide and bipyridine unit

A series of novel praseodymium (Pr)-bonded polymers were successfully synthesized via the coordination reaction and palladium-catalyzed Suzuki coupling reaction of 2,7-dibromo-9,9′-dioctylfluorene and different amounts of 5,5′-dibromo-2,2′-bipyridine. The resulting polymers were characterized by 1H-NMR and GPC. The photoluminescence (PL) and electroluminescence (EL) properties of the resulting polymers were studied to explore the effects of the Pr triisopropyloxide. The results showed that the incorporation of Pr into the polymers caused better coplanarity and effective intermolecular or intramolecular interaction, leading to the higher emission intensity at long-wavelength. Further, it was also found that the emission light color could be tuned from blue to green by introduction of a small amount of Pr into the polymer main chain. A single-layer green emitting EL device based on PF(BipyPr)6 with 6 mol% Pr content was fabricated. The device had a low turn-on voltage of 6 V, a brightness of 705.3 cd·m−2, the maximum luminous efficiency of 1.53 cd·A−1 and the maximum power efficiency of 0.69 lm·W−1.