Xingzhu Wang

Metallophosphors of platinum with distinct main-group elements: a versatile approach towards color tuning and white-light emission with superior efficiency/color quality/brightness trade-offs

A new series of phosphorescent platinum(II) cyclometalated complexes with distinct electronic structures has been developed by simple tailoring of the phenyl ring of ppy (Hppy = 2-phenylpyridine) with various main-group moieties in [Pt(ppy-X)(acac)] (X = B(Mes)2, SiPh3, GePh3, NPh2, POPh2, OPh, SPh, SO2Ph substituted at the para position). Their distinctive electronic characters, resulting in improved hole-injection/hole-transporting or electron-injection/electron-transporting features, have confined/consumed the electrons in the emission layer of organic light-emitting diodes (OLEDs) to achieve good color purity and high efficiency of the devices. The maximum external quantum efficiency of 9.52%, luminance efficiency of 30.00 cd A−1 and power efficiency of 8.36 lm W−1 for the OLEDs with Pt-B (X = B(Mes)2) as the emitter, 8.50%, 29.74 cd A−1 and 19.73 lm W−1 for the device with Pt-N (X = NPh2), 7.92%, 22.06 cd A−1 and 13.64 lm W−1 for the device with Pt-PO (X = POPh2) as well as 8.35%, 19.59 cd A−1 and 7.83 lm W−1 for the device with Pt-SO2 (X = SO2Ph) can be obtained. By taking advantage of the unique electronic structures of the Pt-Ge (X = GePh3) and Pt-O (X = OPh) green emitters and the intrinsic property of blue-emitting hole-transport layer of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), single-dopant white OLEDs (WOLEDs) can be developed. These simple WOLEDs emit white light of very high quality (CIE at (0.354, 0.360), CRI of ca. 97 and CCT at 4719 K) even at high brightness (>15000 cd m−2) and the present work represents significant progress to address the bottle-neck problem of WOLEDs for the efficiency/color quality/brightness trade-off optimization that is necessary for pure white light of great commercial value.

Polymer solar cells based on very narrow-bandgap polyplatinynes with photocurrents extended into the near-infrared region

The synthesis, characterization and photophysics of some solution-processable intensely coloured polyplatinynes functionalized with the thienopyrazine-thiophene hybrid spacer and their model molecular complexes are described. These metallated polymers possess extremely low bandgaps of 1.47-1.50 eV, which extend towards the near-infrared (NIR) range of the solar spectrum, and represent the lowest optical bandgaps ever reported for metallopolyynes in the literature. Both polymers can be used to fabricate efficient solar cells with power conversion efficiencies (PCEs) of up to 0.63% under air mass (AM1.5) simulated solar illumination, and the possibility of covering the 600-900 nm solar-radiation range to harvest photocurrent has been demonstrated. The influence of the thienyl core as well as its substituent group, on the optical and photovoltaic behavior of these metallopolymers was investigated in detail. The power dependencies of the solar cell parameters (including the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also studied. The present work offers an attractive avenue towards conjugated materials with broad solar absorptions and demonstrates the potential of metallopolyynes for both visible and NIR light power generation.

Dithienosilole-bridged small molecules with different alkyl group substituents for organic solar cells exhibiting high open-circuit voltage

Three new small organic molecules, I, II and III, consisting of dithienosilole as the central core, bithiophene bridge with different alkyl group substituents, and octyl cyanoacetate or dicyano unit as different end units, have been designed and synthesized. The thermal, optical, electrochemical and photovoltaic properties of these three compounds have been investigated. The solubility, absorption, energy levels and band gaps of these materials were effectively tuned by different alkyl groups substituted on the thiophene unit and/or different electron-withdrawing end groups. Bulk heterojunction solar cells with molecules I–III as electron donors and PC60BM ([6,6]-phenyl-C60-butyric acid methyl ester) as an election acceptor exhibited power conversion efficiencies of 3.27, 2.88 and 3.81% for I, II and III, respectively. All of these solar cells showed very high Voc values of 0.89–0.92 V, and the high Voc is consistent with the low-lying HOMO level of the donor. These compounds also have low LUMO levels which ensure effective charge transfer from the donor to the fullerene acceptor. The structure–photovoltaic property relationships of these donor materials were investigated and discussed.

Unsymmetric platinum(II) bis(aryleneethynylene) complexes as photosensitizers for dye-sensitized solar cells.

Four new unsymmetric platinum(II) bis(aryleneethynylene) derivatives have been designed and synthesized, which showed good light-harvesting capabilities for application as photosensitizers in dye-sensitized solar cells (DSSCs). The absorption, electrochemical, time-dependent density functional theory (TD-DFT), impedance spectroscopic, and photovoltaic properties of these platinum(II)-based sensitizers have been fully characterized. The optical and TD-DFT studies show that the incorporation of a strongly electron-donating group significantly enhances the absorption abilities of the complexes. The maximum absorption wavelength of these four organometallic dyes can be tuned by various structural modifications of the triphenylamine and/or thiophene electron donor, improving the light absorption range up to 650 nm. The photovoltaic performance of these dyes as photosensitizers in mesoporous TiO(2) solar cells was investigated, and a power conversion efficiency as high as 1.57% was achieved, with an open-circuit voltage of 0.59 V, short-circuit current density of 3.63 mA cm(-2), and fill factor of 0.73 under simulated AM 1.5G solar illumination.

Study of Arylamine-Substituted Porphyrins as Hole-Transporting Materials in High-Performance Perovskite Solar Cells

To develop new hole-transporting materials (HTMs) for efficient and stable perovskite solar cells (PSCs), 5,10,15,20-tetrakis{4-[N,N-di(4-methoxylphenyl)amino-phenyl]}-porphyrin was prepared in gram scale through the direct condensation of pyrrole and 4-[bis(4-methoxyphenyl)amino]benzaldehyde. Its Zn(II) and Cu(II) complexes exhibit excellent thermal and electrochemical stability, specifically a high hole mobility and very favorable energetics for hole extraction that render them a new class of HTMs in organometallic halide PSCs. As expected, ZnP as HTM in PSCs affords a competitive power conversion efficiency (PCE) of 17.78%, which is comparable to that of the most powerful HTM of Spiro-MeOTAD (18.59%) under the same working conditions. Meanwhile, the metal centers affect somewhat the photovoltaic performances that CuP as HTM produces a lower PCE of 15.36%. Notably, the PSCs employing ZnP show a much better stability than Spiro-OMeTAD. Moreover, the two porphyrin-based HTMs can be prepared from relatively cheap raw materials with a facile synthetic route. The results demonstrate that ZnP and CuP can be a new class of HTMs for efficient and stable PSCs. To the best of our knowledge, this is the best performance that porphyrin-based solar cells could show with PCE > 17%.

Synthesis of Star‐Shaped Poly(ϵ‐caprolactone)‐b‐Poly(styrene) Block Copolymer by Combining Ring‐Opening Polymerization and Atom Transfer Radical Polymerization

Newly designed star‐shaped block copolymers made of poly(ϵ‐caprolactone) (PCL) and polystyrene (PS) were synthesized by combining ring‐opening polymerization (ROP) of ϵ‐caprolactone (CL) and atom transfer radical polymerization (ATRP) of styrene (St). The switch from the first to the second mechanism was obtained by selective transformation of “living” radical sites. First, tri‐ and tetrafunctional initiators were used as an initiator for the “living” ring opening polymerization (ROP) of ϵ‐caprolactone producing a hydroxyl terminated three or four arm star‐shaped polymer. Next, the OH end groups of PCL star branches were derivatized into 2‐bromoisobutyrate groups which gave rise to the corresponding tri‐ and tetrabromoester ended‐PCL stars; the latter served as macroinitiators for the ATRP of styrene at 110°C in the presence of CuBr/2,2‐bipyridine (Bipy) catalyst system affording star‐shaped block copolymers PCLn‐b‐PSn (n=3 or 4). The samples obtained were characterizated by 1H‐NMR spectroscopy and GPC (gel permeation chromatograph). These copolymers exhibited the expected structure. The crystallization of star‐shaped block copolymers was studied by DSC (differential scanning calorimetry). The results show that when the content of the PS block increased, the Tm of the star‐shaped block copolymer decreased.

Platinum-Based Poly (Aryleneethynylene) Polymers Containing Thiazolothiazole Group with High Hole Mobilities for Field-Effect Transistor Applications

Two solution-processable platinum-acetylide polymers functionalized with the electron-deficient thiazolothiazole spacer are synthesized and show absorption features spanning from 320 to 600 nm and optical bandgaps of 2.15 and 2.05 eV. The spin-coated polymer thin films of both materials exhibit p-channel field-effect charge transport characteristics with impressive peak field-effect charge-carrier mobilities of (2.1-2.8) × 10(-2) cm(2) V(-1) s(-1) and on/off ratios of (0.8-1.0) × 10(5) for the holes. The high hole mobility value reported for one of the polymers is among the highest reported for metallopolyynes to date. It is also shown that the hole mobility can be notably increased by extending the conjugation length of the chain from the monothienyl to the bithienyl segment on each side of the thiazolothiazole ring.

A visible-near-infrared absorbing A–π2–D–π1–D–π2–A type dimeric-porphyrin donor for high-performance organic solar cells

Most of the currently available small molecule bulk heterojunction organic solar cells (BHJ OSCs) only utilize visible light and, to further increase the efficiency, the development of new organic materials that harvest near-infrared (NIR) light to produce an electric current is essential. Herein, a new A–π2–D–π1–D–π2–A type dimeric porphyrin-cored small molecule (CS-DP) is designed, synthesized and characterized. The use of CS-DP with a narrow bandgap (Eg) (1.22 eV) and the deep energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) affords the highest power conversion efficiency of 8.29% in BHJ OSCs with PC71BM as an acceptor, corresponding to a short circuit current of 15.19 mA cm−2, an open circuit voltage of 0.796 V and a fill factor of 70% under AM 1.5G solar irradiation. The high device performance is attributed to the visible-near-infrared light-harvesting capability of CS-DP, and the super low energy loss feature. The energy loss (Eloss) lies between 0.43 and 0.51 eV in the system, which is related to the very small energy offset of the LUMOs between the CS-DP donor and PC71BM (ΔELUMO = 0.06 eV). The value of ΔELUMO, which is considered as a driving force for the photoinduced charge separation, is much smaller than the empirical threshold of 0.3 eV, but would not be a limiting factor in the charge separation process. The results indicate that there may be room for further improving the PCE for low bandgap dimeric porphyrin systems.

Study on Synthesis of Star‐Shaped Poly(ethylene oxide) by Atom Transfer Radical Polymerization

Star‐shaped poly(ethylene oxide) (PEO) was prepared by atom transfer radical polymerization (ATRP) with a 2‐bromoisobutyryl PEO ester as a macroinitiator. Divinylbenzene (DVB) and ethylene glycol dimethacrylate were employed as the coupling reagents. Several factors pertinent to star polymer formation are: type of coupling reagents and solvents, feed ratio of DVB to the macroinitiator, and reaction time. These were studied and used to optimize the star formation process. The optimum yield of star polymer was ca. 90–98%.

Chemically driven supramolecular self-assembly of porphyrin donors for high-performance organic solar cells

Supramolecular self-assembly of a novel acceptor–π–porphyrin–π–acceptor compound driven by intermolecular sulfur–sulfur interactions leads to the formation of J-aggregates composed of helical nanowire structures. These nanowires have exhibited interesting optical activity, and when utilized as a molecular electron donor with a fullerene acceptor in bulk heterojunction organic solar cells, a high photovoltaic power conversion efficiency of over 8% has been achieved.