Baoxiu Mi

Solution-processed copper nanowire flexible transparent electrodes with PEDOT:PSS as binder, protector and oxide-layer scavenger for polymer solar cells

The easy oxidation and surface roughness of Cu nanowire (NW) films are the main bottlenecks for their usage in transparent conductive electrodes (TCEs). Herein, we have developed a facile and scaled-up solution route to prepare Cu NW-based TCEs by embedding Cu NWs into pre-coated smooth poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films on poly(ethylene terephthalate) (PET) substrates. The so obtained Cu NW-PEDOT:PSS/PET films have low surface roughness (∼70 nm in height), high stability toward oxidation and good flexibility. The optimal TCEs show a typical sheet resistance of 15 Ω·sq−1 at high transparency (76% at λ = 550 nm) and have been used successfully to make polymer (poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester) solar cells, giving an efficiency of 1.4%. The overall properties of Cu NW-PEDOT:PSS/PET films demonstrate their potential application as a replacement for indium tin oxide in flexible solar cells.

Polyfluorenes with On-Chain Metal Centers

Polyfluorenes containing metal complexes, especially phosphorescent heavy-metal complexes, are a type of important optoelectronic materials. The present review summarizes the synthesis and optoelectronic properties of polyfluorenes with phosphorescent heavy-metal complexes on the chain. Efficient energy transfer from polymer main-chain to metal centers occurs in these host–guest systems. The promise of strong emission and the easiness for processing in organic electronic devices provide incentives to develop these materials intensively. The range of applications of these materials spans the whole field of interaction between light and electricity. Especially, attention is given to the interesting optoelectronic properties and promising applications in organic light-emitting diodes (OLEDs), memory devices, and sensors.

Copper Mesh Templated by Breath-Figure Polymer Films as Flexible Transparent Electrodes for Organic Photovoltaic Devices

UNLABELLED Metal mesh is a significant candidate of flexible transparent electrodes to substitute the current state-of-the-art material indium tin oxide (ITO) for future flexible electronics. However, there remains a challenge to fabricate metal mesh with order patterns by a bottom-up approach. In this work, high-quality Cu mesh transparent electrodes with ordered pore arrays are prepared by using breath-figure polymer films as template. The optimal Cu mesh films present a sheet resistance of 28.7 Ω·sq(-1) at a transparency of 83.5%. The work function of Cu mesh electrode is tuned from 4.6 to 5.1 eV by Ag deposition and the following short-time UV-ozone treatment, matching well with the PEDOT PSS (5.2 eV) hole extraction layer. The modified Cu mesh electrodes show remarkable potential as a substitute of ITO/PET in the flexible OPV and OLED devices. The OPV cells constructed on our Cu mesh electrodes present a similar power conversion efficiency of 2.04% as those on ITO/PET electrodes. The flexible OLED prototype devices can achieve a brightness of 10 000 cd at an operation voltage of 8 V.

Pure aromatic hydrocarbons with rigid and bulky substituents as bipolar hosts for blue phosphorescent OLEDs

Four pure hydrocarbon molecules of 1,3,5-tris(9-phenyl-9H-fluoren-9-yl)benzene (mTPFB), 4,4′-bis(9-phenyl-9H-fluoren-9-yl)biphenyl (pDPFBP), 1,4-bis(9-phenyl-9H-fluoren-9-yl)benzene (pDPFB) and 1,3-bis(9-phenyl-9H-fluoren-9-yl)benzene (mDPFB), with different connecting modes and cores were prepared in a one-pot reaction. Among them, mTPFB and pDPFDB were newly designed for the study of the structure–property relationship and developing good blue hosts for phosphorescent organic light-emitting devices (PhOLEDs). The four materials show very similar photophysical properties in solution, with almost the same bandgap and triplet energy, as well as similar energy levels. However, their device behavior is quite different with the best performance coming from mTPFB. The conjugation blocked connection mode in the starburst mTPFB molecule with rigid and bulky substituents results in a high triplet energy coupled with good and balanced electron and hole transport, as proved by the internal reorganization energy calculation and mono charge device study, which makes it a good host for a blue phosphor in a PhOLED. The device shows an external quantum efficiency of 15.7% and a current efficiency of 30.6 cd A−1, which is among the best for pure hydrocarbon host based devices.

Electrospray Dense Suspensions of TiO2 Nanoparticles for Dye Sensitized Solar Cells

We report the feasibility of using electrospray to atomize dense suspension of nanoparticles with high solid concentrations. We demonstrate this principle through electrospraying dense suspensions of TiO2 nanoparticles with 40 wt.% in ethylene glycol. A dye sensitized solar cell (DSSC) is fabricated by electrospray deposition and the power conversion efficiency up to 6.81% is demonstrated. This simple, one-step process can fabricate the active layer with uniform thickness and multiple length scales, including 25 nm TiO2 nanoparticles, ∼2 μm micro spherical particles, and ∼20-μm-thick film. A judicious choice of drying temperature is important to ensure complete drying of suspension droplets while avoiding creation of hollow particles, because the hollow particles exhibit significantly lower carrier mobility and short circuit current. The very high solid concentration demonstrated in this work can potentially reduce the manufacturing cost of DSSC because less energy will be wasted on evaporating and/or recycling the organic solvent. In addition, because electrospray is compatible with roll-to-roll process and the yield is scalable through multiplexed electrosprays, the electrospray route is a promising and economically competitive approach for manufacturing DSSCs through spray deposition. Copyright 2013 American Association for Aerosol Research

Structure optimization of organic planar heterojunction solar cells

The management of light absorption in organic photovoltaic cells is of great importance for exciton generation and thus photocurrent. Sufficient light harvesting can be established by localizing the maxima of light absorption density in the region of active layer. Using organic photovoltaic (OPV) devices based on copper phthalocyanine (CuPc) and bulkfullerene (C60) as an example, we demonstrate the methods for localizing the optical interference peaks of the main absorbed light wavelengths inside the respective active layers. The fundamental regulations of the electromagnetic field distribution with thicknesses of the active layers are clarified. The influence of the thickness of the cathode buffer layer on the optimized active layer thicknesses is discussed. Exciton diffusion modelling is combined with optical modelling to give theoretically optimized device structures. The consistency between the results of simulation and experiments is shown, which indicates the validation of the guidance of the modelling work presented in this paper for the design of effective light-using OPV devices.

Influences of fluorination on homoleptic iridium complexes with C^N=N type ligand to material properties, ligand orientation and OLED performances

Two new iridium complexes with C∧N=N type ligand (i.e., Ir(BFPPya)3{tris[3,6-bis(4-fluorophenyl)pyridazine]iridium(III)} and Ir(BDFPPya)3{tris[3,6-bis (2,4-di-fluorophenyl)pyridazine]iridium(III)}) attaching with fluorine atoms, were synthesized and the effects of fluorination on the material properties and device performance were investigated. Compared with our previously reported fluorine-free analogue material, that is Ir(BPPya)3{tris[3,6-bis(phenyl)pyridazine]iridium(III)}, blue shifts in the emission spectra as well as in the long wavelength region of the absorptions were observed. The photoluminescence quantum yield (PLQY) (0.44 and 0.84 vs 0.29), phosphoresces lifetime (0.88 and 1.31 vs 0.66 ms), and oxidation potential (1.10 and 1.37 vs 0.95 V) increased obviously after fluorinating the ligand. In contrast, the thermal stability of the iridium complexes decreased slightly (Td: 435 and 402 vs 440 °C). In the density functional theory (DFT) calculations, by comparing the steric shape of the three ligands within one optimized molecule, orientational differences among the complexes were observed. In OLED device studies, bluish green electroluminescence with peak emission of 500 nm, using the electron-transporting host of TPBI [2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl- 1H-benzimidazole)] and the most fluorinated dopant of Ir(BDFPPya)3, was achieved with maximum efficiency of 20.3 cd/A. On one hand this efficiency is not satisfactory considering a high PLQY of 0.84. On the other hand with the similar device structure, that the (HOMO-LUMO)s of all the dopants are wrapped within that of the host TPBI, and all the triplet energies of the dopants are smaller than that of the host TPBI, it is abnormal that the ordering of device efficiencies is contradictory to that of PLQY. Assisting with the phosphorescent spectrum of TPBI and the absorption spectra of the dopant, the contradiction was interpreted reasonably.

Order-enhanced silver nanowire networks fabricated by two-step dip-coating as polymer solar cell electrodes

The efficient utilization of Ag NW by avoiding their aggregation and multi-layer stacking in their networks can improve the optical and electrical properties of the derived transparent conductive electrodes (TCEs). In this work, we develop a two-step dip-coating process to prepare Ag NW networks on polyethylene terephthalate (PET) substrates using three types of Ag NW with length/diameters (L/D) ratio of 100, 400 and 1000 respectively. The NW with small or medium L/D (100 and 400) are likely to form order-enhanced networks since they are rigid and easy to be separated. The TECs prepared by this approach present much higher performance than those prepared by spray coating. The optimal Ag NW-based TCEs show a typical sheet resistance of 35 Ω sq−1 with high transparency of 92%. The polymer solar cells constructed on the resulting TCEs can give an efficiency of 3.28%, demonstrating the potential application of our Ag NW/PET films as the ITO alternative in flexible solar cells.

A thermal stable cathode buffer based on an inexpensive tetranuclear zinc(II) complex for organic photovoltaic devices

An inexpensive material, i.e., tetranuclear zinc(II) complex, (Zn4O(AID)6) [AID = 7-azaindolate], was utilized as a cathode buffer in organic photovoltaic (OPV) devices, leading to the improvement of device performance. Compared to OPV devices based on a conventional cathode buffer of TPBi (1,3,5-tris(2-N-phenylbenzimidazolyl)benzene), although the freshly prepared devices showed similar performance, when heated to a series of high temperatures under air, the short circuit current and the open circuit voltage of the Zn4O(AID)6 devices dropped more slowly, indicating the superiority of using Zn4O(AID)6 as a cathode buffer over TPBi in OPV devices.

Interfacial engineering of graphene for highly efficient blue and white organic light-emitting devices

Graphene as anodes of flexible organic light-emitting devices (OLEDs) has intrinsic drawbacks of a low work function and a high sheet resistance although it can eliminate the brittle feature of ITO. Chemical doping as a conventional approach is universally used to decrease the sheet resistance and adjust the work function of graphene electrodes, but it suffers from instability problems due to the volatility of chemical species. Here, an insulated poly(4-styrenesulphonate) (PSS) modification layer is firstly coated on the graphene surface along with improved air-stability and hole-injection ability via interfacial dipoles. Besides, the utilization of PSS is beneficial to reduce the leakage current of OLEDs. Then a gradient injection layer of poly(3,4-ethylenedioxythiophene):PSS (PEDOT:PSS)/tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer-doped PEDOT:PSS is covered onto the PSS-modified graphene to further promote hole injection and improve carrier balance inside OLEDs. With above interfacial modification technique, very high efficiencies of 201.9 cd A−1 (76.1 lm W−1, 45.2%) and 326.5 cd A−1 (128.2 lm W−1, 99.5%) for blue and white emissions are obtained, which are comparable to the most efficient display and lighting technologies so far.