Haiqiao Wang

Synthesis and electroluminescence of novel copolymers containing crown ether spacers

Three novel copolymers containing alternating 1,4-bis(phenylethenyl)benzene, 1,4-bis(phenylethenyl)-2,5-dimethoxybenzene or 1,5-bis(phenylethenyl)naphthalene chromophores and crown ether (CE) spacers within the polymer backbone were synthesized by the Wittig polycondensation reaction. These copolymers exhibit good thermal stability (decomposition temperature around 380–411 °C). Photoluminescence (PL) and electroluminescence (EL) color can be easily tuned in these copolymers by simply changing the structure of the chromophores. Excimer emission is responsible for the changes in the PL spectra on going from solution to the thin films. Typical light-emitting electrochemical cell (LEC) behavior is observed in the EL devices with these copolymers as both electronic and ionic conductors. The turn-on voltage for emission is at +3.9 V, in forward bias, and at −4.4 V, under reverse bias. Efficient LECs with low turn-on voltages can be demonstrated upon addition of poly(ethylene oxide) (PEO) into the active layers. CE spacers in these copolymers contribute to the increasing ionic conductivity and the improvement in the morphology of the active layers. The d.c. response and a.c. impedance behaviors of the LECs with PEO were investigated, and the results indicate the operation of these LECs corresponds to the electrochemical doping mechanism.

One-step process to fabricate Ag-polypyrrole coaxial nanocables.

Ag-polypyrrole nanocables were fabricated in aqueous solution at room temperature through a redox reaction between silver nitrite and pyrrole, using poly(vinyl pyrrolidone)(PVP) as assistant agent.

Advancements in all-solid-state hybrid solar cells based on organometal halide perovskites

Over the past several years, organic–inorganic hybrid perovskites have gained considerable research attention due to their direct band gap, large absorption coefficient, ambipolar diffusion and long carrier diffusion length, and have revolutionized the prospects of emerging photovoltaic technologies, with the highest power conversion efficiency of over 19% achieved under laboratory conditions. In this perspective, we summarize the recent developments in perovskite solar cells (from April 2009 to December 2014), describe the unique properties of organometal halide perovskites leading to their rapid emergence, and discuss challenges such as stability and environmental issues to be faced in the future.

Rational design on D–A conjugated P(BDT–DTBT) polymers for polymer solar cells

Solution-processable D–A conjugated polymers are proving particularly promising in bulk heterojunction solar cells. Among these, P(BDT–DTBT) and their derivatives with benzodithiophene (BDT) as the donor unit and benzothiadiazole (DTBT) as the acceptor unit are the most commonly studied conjugated polymers due to their excellent photovoltaic properties. There have been a lot of reports recently on the design and structural organization of P(BDT–DTBT) for solar cells, and it has been demonstrated that one of the critical issues for achieving high performance is the rational molecular design of P(BDT–DTBT) polymers. In this review, we focus on the various structural modifications and photovoltaic properties of the resulting P(BDT–DTBT) polymers. We hope that this review will give some inspirations for high-performance P(BDT–DTBT) polymers and be an important guideline for the design of photovoltaic conjugated polymers.

A furan-bridged D-π-A copolymer with deep HOMO level: synthesis and application in polymer solar cells

A novel D-π-A copolymer, PFBT-BDT, based on furan-containing benzothiadiazole (FBT) and benzodithiophene (BDT) was designed and synthesized by Pd-catalyzed Stille-coupling method. The copolymer showed good solubility and film-forming ability combining with good thermal stability. PFBT-BDT exhibited a broad absorption from 300 to 630 nm with an absorption peak centered at 522 nm. The optical band gap (Egopt) determined from the onset of absorption of the polymer film was 1.96 eV. The LUMO and HOMO energy levels of the polymer were estimated to be −3.48 eV and −5.44 eV, respectively. The polymer solar cell fabricated from the blend of the polymer as donor and PC71BM as acceptor exhibited a moderate power conversion efficiency of 2.81% with a high Voc of 0.94 V without annealing and any additives. To the best of our knowledge, this is among the highest Voc values for PSCs based on benzodithiophene derivatives. This work demonstrates that the replacement of thiophene moieties in conjugated polymers with more electron-withdrawing furan moieties is able to significantly lower the HOMO energy levels, and therefore, increase the open circuit voltage of solar cells.

Efficient polymer solar cells based on a broad bandgap D–A copolymer of “zigzag” naphthodithiophene and thieno[3,4-c]pyrrole-4,6-dione

A promising broad bandgap copolymer, PzNDTTPD, based on a rigid planar “zigzag” naphthodithiophene unit, was designed and synthesized. The BHJ solar cells based on PzNDTTPD:PC71BM blends afforded a power conversion efficiency of 5.3% with a high Voc over 0.9 V.

Effects of fluorination on the properties of thieno[3,2-b]thiophene-bridged donor–π–acceptor polymer semiconductors

Thieno[3,2-b]thiophene-bridged polymer semiconductors, P(BDT-TT-HBT) and P(BDT-TT-FBT), combining a benzo[1,2-b:4,5-b′]dithiophene donor unit and a benzothiadiazole or fluorinated benzothiadiazole acceptor unit, respectively, were designed and synthesized. The introduction of fluorine substituents remarkably influenced the molecular architecture, optical, electrochemical, and morphological properties of the polymers, as well as the optoelectronic performance of the devices made from these materials. The introduction of fluorine substituents on the benzothiadiazole unit not only down-shifted the HOMO energy level of the organic semiconductor but also enhanced the intra- and intermolecular interactions of the resulting conjugated polymer. As a result, the open-circuit voltage and mobility of corresponding devices based on the fluorinated polymer were enhanced markedly. Power conversion efficiencies of the polymer solar cells based on P(BDT-TT-HBT) and P(BDT-TT-FBT) reached 4.37% and 3.56%, with open circuit voltages of 0.72 and 0.81 V, respectively. The fluorinated polymer exhibited much higher mobilities (4.1 to 6.3 times) than the non-fluorinated polymer, reaching 0.017 cm2 V−1 s−1.

The role of conjugated side chains in high performance photovoltaic polymers

Four new D–A type copolymers, namely, PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT, were designed and synthesized to investigate the influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All the four copolymers have an identical conjugated backbone comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying conjugated side chains, p-alkoxyphenyl or 2-alkylthienyl, attached to the donor and acceptor units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic voltammetry, density functional theory (DFT), grazing incidence X-ray scattering (GIXS), transmission electron microscope (TEM) and photovoltaic measurements, the difference in conjugated side chain modulation led to totally different physicochemical properties. Among the four copolymers, PBDT-DFQX-TT exhibits the broadest absorption spectrum, the most close-packed structure as well as a finest fibril structure when blended with PC71BM. After systematic device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) photovoltaic devices based on the blends of PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT with PC71BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively. By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl ones, PCEs was increased by 250% from 3.96% to 7.68%. To date, PBDT-DFQX-TT is one of a few Qx-based PSCs that exhibits PCE exceeding 7.5%, and the results suggest that simultaneously modulating the conjugated side chains on both donor and acceptor units of copolymers could be an effective strategy for constructing high performance photovoltaic copolymers.

Self n-doped [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium) ethyl ester diiodides as a cathode interlayer for inverted polymer solar cells

A series of self n-doped fullerene ammonium derivatives have been synthesized and confirmed with electron paramagnetic resonance and conductivity measurements. The existence of stable C60O2˙− anion radical in these materials resulted in intrinsically high conductivities between 1.05 × 10−2 and 1.98 × 10−2 S cm−1. Among fullerenes with different numbers of ammonium and counter anions, [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)-ethyl ester diiodides (PCBDANI) showed the best solvent resistance, which was confirmed by the measurement of film thickness and corresponding UV-vis absorption before and after rinsing with dichlorobenzene. Most importantly, the inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high power conversion efficiency even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer. Thus large-area devices via printing this interlayer or printing on this interlayer could become feasible.

Ordered macroporous titania photonic balls by micrometer-scale spherical assembly templating

Highly ordered macroporous photonic balls (i.e. inverse opaline structure) composed of titania frameworks were fabricated by using a titania precursor templated around polystyrene spheres which had been assembled into polymer photonic balls (i.e. opaline structure). Narrow disperse polymer photonic balls consisting of monodisperse surface-modified polystyrene (PS) latex particles were prepared by utilizing a suspension system. The diameters of the opaline balls can be controlled in a range of a few or a few tens of micrometers. The macroporous titania structure made by this method was well-defined because the PS spheres making up the polymer photonic balls were close-packed and ordered in three-dimensions. Furthermore, crystalline types of titania (anatase or rutile) were readily adjusted through tuning the calcination temperatures, so the macroporous titania inverse opaline balls composed of anatase or rutile can be used for various applications.