Liangliang Han

Hyperconjugated side chained benzodithiophene and 4,7-di-2-thienyl-2,1,3- benzothiadiazole based polymer for solar cells

A novel donor–acceptor (D–A) copolymer (P3TBDTDTBT), including hyperconjugated side chained benzodithiophene as a donor and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) as an acceptor, was designed and synthesized. Due to the introduction of the hyperconjugated side chain, the resultant polymer exhibited good thermal stability with a high decomposition temperature of 437 °C, a low band-gap of 1.67 eV with an absorption onset of 742 nm in the solid film, and a deep highest occupied molecular orbital (HOMO) energy level of −5.26 eV. Finally, the polymer solar cell (PSC) device based on this polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) showed the best power conversion efficiency (PCE) of 3.57% with an open-circuit voltage (Voc) of 0.78 V, a short-circuit current density (Jsc) of 8.83 mA cm−2 and a fill factor (FF) of 53%.

High efficiency solution-processed two-dimensional small molecule organic solar cells obtained via low-temperature thermal annealing

A new two-dimensional (2D) organic small molecule, DCA3T(T-BDT), was designed and synthesized for solution-processed organic solar cells (OSCs). DCA3T(T-BDT) exhibited a deep HOMO energy level (−5.37 eV) and good thermal stability. The morphologies of the DCA3T(T-BDT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blends were investigated by atomic force microscopy and the crystallinity was explored by X-ray diffraction (XRD) and 2D grazing incidence wide-angle X-ray scattering (GIWAXS), respectively. The morphologies of the blends were strongly influenced by the blend ratio of DCA3T(T-BDT):PC61BM and annealing temperature. The effect of thermal annealing on the photovoltaic performance of DCA3T(T-BDT)-based small molecule organic solar cells (SMOSCs) was studied in detail. When DCA3T(T-BDT) was used as a donor with PC61BM as an acceptor, high efficiency SMOSCs with a power conversion efficiency of 7.93%, a high Voc of 0.95 V, Jsc of 11.86 mA cm−2 and FF of 0.70 were obtained by a thermal annealing process at only 60 °C, which offers obvious advantages for large scale production compared with solvent additive or interfacial modification treatment.

Very high-efficiency organic light-emitting diodes based on cyclometallated rhenium (I) complex

A phosphorescent (Ph) cyclometallated rhenium (I) (ReI) complex was synthesized by reacting 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline with pentacarbonylbromorhenium in refluxing toluene solutions. The precipitates were easily sublimed to obtain a pure electrically neutral carbonyl diamine ReI complex, (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)Re(CO)3Br. The Re complex was used as an orange emitting dopant in 4,4′-N,N′-dicarbazole-biphenyl host to fabricate Ph organic light-emitting diodes (PhOLEDs). The maximum electroluminescence (EL) efficiency and luminance of 21.8cd∕A and 8315cd∕cm2 were harvested, respectively, which were the highest EL results among the PhOLEDs based on ReI complexes. The improvements of the EL performances could be ascribed to the synergistic effects of together incorporation of two reciprocally repulsive phenyl and methyl groups on the backbone of 1,10-phenanthroline molecule of the ligand.

Solution-processed, indacenodithiophene-based, small-molecule organic field-effect transistors and solar cells

Two indacenodithiophene-based molecules with different side chains, BTIDT-C6 and BTIDT-OC12, have been designed and synthesized for solution-processed, small-molecule organic solar cells (OSCs) donor materials. By optimizing the side chains, the hole mobility of the materials is modulated, which has been proven by the organic field-effect transistor (OFET) performances. Solar cells based on BTIDT-C6 show a power conversion efficiency (PCE) of 4.83%. To the best of our knowledge, this is the first report about indacenodithiophene-based, solution-processed, small-molecule OFETs, and it is also one of the highest PCE reports for the indacenodithiophene-based, solution-processed, small-molecule OSCs. This report makes indacenodithiophene-based small molecules the third type of high-efficiency (5% PCE), solution-processed, small-molecule OSCs donor materials, in addition to benzodithiophene (BDT) and dithienosilole (DTS).

Efficient polymer solar cells based on a new benzo[1,2-b:4,5-b ']dithiophene derivative with fluorinated alkoxyphenyl side chain

A novel fluorine-containing benzo[1,2-b:4,5-b′]dithiophene (BDT) derivative (BDTPF) was designed to construct a donor–acceptor (D–A)-structured polymer (PBDTPF-DTBT) with the electron-withdrawing unit 4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (DTBT). The resulting polymer exhibits a broad absorption spectrum, relatively low lying HOMO energy level (−5.39 eV) and a good film-forming ability. The field-effect mobility of PBDTPF-DTBT is 0.034 cm2 V−1 s−1. Bulk heterojunction organic solar cells (OSCs) based on PBDTPF-DTBT and PC71BM were prepared and showed a good photovoltaic performance with power conversion efficiency (PCE) of 7.02%. This work demonstrates that a BDT unit with fluorinated alkoxyphenyl side chains is a promising candidate as an electron-rich building block for high performance solution-processed OSCs.

Enhancement of photovoltaic performance by increasing conjugation of the acceptor unit in benzodithiophene and quinoxaline copolymers

Copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c]phenazine (TQ2), were synthesized via a Stille coupling reaction. By increasing the conjugation in the TQ2 unit, the polymer based on TQ2 showed a narrower band gap (Eg), a lower highest occupied molecular orbital energy level and enhanced interchain π–π interactions. Polymer solar cells based on TQ2 showed a simultaneous enhancement of the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) compared with polymer solar cells based on the TQ1 polymer. A good power conversion efficiency of 4.24% was achieved by solar cells based on the TQ2 polymer and [6,6]-phenyl-C71-butyric acid methyl ester composite. These preliminary results indicate that increasing the acceptor unit (quinoxaline) conjugation is an effective way of improving the performance of polymer solar cells.

Highly sensitive organic ultraviolet optical sensor based on phosphorescent Cu (I) complex

Ultraviolet light-sensitive organic optical sensor based on photovoltaic diode was demonstrated by using a phosphorescent Cu complex and a diamine derivative as electroacceptor and electrodonor, respectively. The Cu complex is Cu(DPEphos)((Bphen))BF4, in which DPEphos and Bphen denote 6,7-dicyanodipyrido [2,2-d:2′,3′-f] quinoxaline and bathophenanthroline. And the diamine derivative, m-MTDATA, is 4, 4′,4″-tris-(2-methylphenyl phenylamino) triphenylamine. The sensor is highly sensitive to UV light band from 300to420nm while it has almost no response to the visible light, and under illumination of 365nm light with power of 1.7mW∕cm2, the sensor exhibits an open circuit voltage of 1.86V, a short circuit current of 105.3μA∕cm2, a fill factor of 0.246, and a power conversion efficiency of 2.83%. The dependences of ultraviolet responsive sensitivity on illumination intensity and working temperature were also discussed.

High-Performance Small Molecule/Polymer Ternary Organic Solar Cells Based on a Layer-By-Layer Process.

UNLABELLED The layer-by-layer process method, which had been used to fabricate a bilayer or bulk heterojunction organic solar cell, was developed to fabricate highly efficient ternary blend solar cells in which small molecules and polymers act as two donors. The active layer could be formed by incorporating the small molecules into the polymer based active layer via a layer-by-layer method: the small molecules were first coated on the surface of poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) ( PEDOT PSS), and then the mixed solution of polymer and fullerene derivative was spin-coated on top of a small molecule layer. In this method, the small molecules in crystalline state were effectively mixed in the active layer. Without further optimization of the morphology of the ternary blend, a high power conversion efficiency (PCE) of 8.76% was obtained with large short-circuit current density (Jsc) (17.24 mA cm(-2)) and fill factor (FF) (0.696). The high PCE resulted from not only enhanced light harvesting but also more balanced charge transport by incorporating small molecules.

4,7-Di-2-thienyl-2,1,3-benzothiadiazole with hexylthiophene side chains and a benzodithiophene based copolymer for efficient organic solar cells

In this work, a hexylthiophene substituted thiophene-bridge is used to connect benzodithiophene (BDT) and benzothiadiazole (BT) segments to build a new photovoltaic polymer (PBDT-DTTBT). The highest occupied molecular orbital (HOMO) level of the polymer is decreased by 0.2 eV to −5.47 eV compared to the reference polymer without a hexylthiophene side chain, which may be ascribed to the conformational torsion caused by the steric hindrance from the bulky hexylthiophene side group. The band gap of the polymer shows no obvious change compared to a polymer with a same backbone without a hexylthiophene side chain (around 1.7 eV). Polymer solar cells (PSCs) based on PBDT-DTTBT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency (PCE) of 6.19% with an open-circuit voltage (Voc) of 0.80 V, a short-circuit current density (Jsc) of 12.72 mA cm−2 and a fill factor (FF) of 60.97%. The work provides a new method to design new conjugated polymers with a deep HOMO level and a low band gap for high Voc PSCs.

The reduced triplet-triplet annihilation of electrophosphorescent device doped by an iridium complex with active hydrogen

We demonstrate high efficiency electrophosphorescent devices doped by bis(2-phenyl-benzoimidazole) iridium (III) acetylacetonate [Ir(ppi)2acac] with active hydrogen. At 100 mA/cm2 2 wt % Ir(ppi)2acac doped device offers an external quantum efficiency (ηext) of 5.1% and an efficiency roll-off of 28.2%, which decreases for 33.5% and 39.1% compared with that of 2 and 6 wt % fac-tris(2-phenylpyridine) iridium (III) [fac-Ir(ppy)3] doped reference devices, respectively. 7 wt % Ir(ppi)2acac doped device behaves a maximum ηext of 13.4% and a ηext of 8.0% at a luminance of 10 000 cd/m2. Both the ηext are higher than that of the reference device. The improvement in electroluminescent performance was assumed to be the very short triplet lifetime (3.74 ns) induced by the active hydrogen atom.