Zhang Fu-Jun

Organic photovoltaic cells: Novel organic semiconducting materials and molecular arrangement engineering

Organic photovoltaic cells (OPVs) have attracted more and more attention due to its highly potential application to solve the energy crisis considering its advantages, such as low cost and ease of large area production. The power conversion efficiency (PCE) of OPVs has undergone a more than nine-fold increase from ∼1.0% by Tang in 1986 to 9.2% in 2010 announced by Mitsubishi Chemical. The major challenges of obtaining high efficiency OPVs are the synthesis of new narrow band gap materials, controlling molecular arrangement, designing novel configuration cells for better photon harvesting in the active layer. In the article, we summarized the recent progress of novel narrow band gap photovoltaic materials and the effective methods to control the morphology of donor and acceptor in the blend films for high performance of OPVs.

Performance improvement of MEH-PPV:PCBM solar cells using bathocuproine and bathophenanthroline as the buffer layers

In this work, bathocuproine (BCP) and bathophenanthroline (Bphen), commonly used in small-molecule organic solar cells (OSCs), are adopted as the bufier layers to improve the performance of the polymer solar cells (PSCs) based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. By inserting BCP or Bphen between the active layer and the top cathode, all the performance parameters are dramatically improved. The power conversion e‐ciency is increased by about 70% and 120% with 5-nm BCP and 12-nm Bphen layers, respectively, when compared with that of the devices without any bufier layer. The performance enhancement is attributed to BCP or Bphen (i) increasing the optical fleld, and hence the absorption in the active layer, (ii) efiectively blocking the excitons generated in MEH-PPV from quenching at organic/aluminum (Al) interface due to the large band-gap of BCP or Bphen, which results in a signiflcant reduction in series resistance (Rs), and (iii) preventing damage to the active layer during the metal deposition. Compared with the traditional device using LiF as the bufier layer, the BCP-based devices show a comparable e‐ciency, while the Bphen-based devices show a much larger e‐ciency. This is due to the higher electron mobility in Bphen than that in BCP, which facilitates the electron transport and extraction through the bufier layer to the cathode.

Photovoltaic Performance Improvement of P3HT:PCBM Polymer Solar Cells by Annealing Treatment

Several polymer solar cells consisting of ITO/PEDOT:PSS/P3HT:PCBM/Al (indium tin oxide/ poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester/aluminum cathode) were fabricated by spin coating. The influence of annealing temperature on the performance of the polymer solar cells was studied using absorption spectra, photoluminescence spectra, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). These devices were treated at 120 °C for 10 min in an ambient atmosphere and the best power conversion efficiency (PCE) of 2.00% was obtained at an open circuit voltage (Voc) of 0.64 V, a short circuit current density (Jsc) of 10.25 mA·cm-2, and a fill factor (FF) of 38.1%. The intensities of the absorption peaks at 560 and 610 nm increased because of the increased absorption π→π* transition of P3HT after annealing treatment. XRD spectra showed that the intensity of the diffraction peaks at (100) for P3HT increased 1.8 times by comparison with that of the cells that did not undergo annealing treatment. The P3HT:PCBM phase separation increased markedly after annealing treatment, which is valuable for exciton dissociation. FTIR results also showed that the polymer materials did not deteriorate during the annealing treatment process.

Inverted polymer solar cells with TiO 2 electron extraction layers prepared by magnetron sputtering

TiO2 thin films deposited by magnetron sputtering possess excellent optical transmittance, high refractive index, good adhesion and chemical stability. In this manuscript, TiO2 thin films deposited by magnetron sputtering was used for the first time as an electron extraction layer in inverted polymer solar cells (IPSCs), and the effect of the TiO2 thickness on the photovoltaic performance of P3HT:PC61BM IPSCs was investigated. The highest PCE value of 3.75% was obtained when the thickness of TiO2 thin films was in the range between 42 nm and 73 nm. The absorption properties, morphology and structure of the TiO2 films were characterized by UV-Vis spectroscopy, SEM and Raman spectroscopy, and were related to the device performance of P3HT:PC61BM IPSCs. The results indicate that TiO2 films deposited by magnetron sputtering are an excellent electron extraction layer for IPSCs.

Thickness dependence of surface morphology and charge carrier mobility in organic field-effect transistors

With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

Ca2BO3Cl:Ce3+, Tb3+: A novel tunable emitting phosphor for white light-emitting diode

Ca2BO3Cl:Ce3+, Ca2BO3Cl:Tb3+, and Ca2BO3Cl:Ce3+, Tb3+ phosphors are synthesized by a high temperature solid-state reaction. The emission intensity of Ce3+ or Tb3+ in Ca2BO3Cl is influenced by the Ce3+ or Tb3+ doping content, and the optimum concentrations of Ce3+ and Tb3+ are 0.03 mol and 0.05 mol, respectively. The concentration quenching effect of Ce3+ or Tb3+ in Ca2BO3Cl occurs, and the concentration quenching mechanism is d—d interaction for either Ce3+ or Tb3+. The Ca2BO3Cl:Ce3+, Tb3+ can produce colour emission from blue to green by properly tuning the relative ratio between Ce3+ and Tb3+, and the emission intensity of Tb3+ in Ca2BO3Cl can be enhanced by the energy transfer from Ce3+ to Tb3+. The results indicate that Ca2BO3Cl:Ce3+, Tb3+ may be a promising double emission phosphor for UV-based white light emitting diodes.

Enhanced performance in organic photovoltaic devices with a KMnO4 solution treated indium tin oxide anode modification

The properties of poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic photovoltaic devices (OPVs) with an indium tin oxide (ITO) anode treated by a KMnO4 solution are investigated. The optimized KMnO4 solution has a concentration of 50 mg/L, and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency (PCE) of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO4, and then the charge collection efficiency is improved.

Characteristics of pentacene organic thin film transistor with top gate and bottom contact

High performance pentacene organic thin film transistors (OTFT) were designed and fabricated using SiO2 deposited by electron beam evaporation as gate dielectric material. Pentacene thin films were prepared on glass substrate with S–D electrode pattern made from ITO by means of thermal evaporation through self-organized process. The threshold voltage VTH was −2.75±0.1V in 0 – −50V range, and that subthreshold slopes were 0.42±0.05V/dec. The field-effect mobility (μEF) of OTFT device increased with the increase of VDS, but the μEF of OTFT device increased and then decreased with increased VGS when VDS was kept constant. When VDS was −50V, on/off current ratio was 0.48 × 105 and subthreshold slope was 0.44V/dec. The μEF was 1.10cm2/(V-s), threshold voltage was −2.71V for the OTFT device.

Performance improvement in pentacene organic thin film transistors by inserting a C60 ultrathin layer

The contact effect on the performances of organic thin film transistors is studied here.A C 60 ultrathin layer is inserted between Al source-drain electrode and pentacene to reduce the contact resistance.By a 3 nm C 60 modification,the injection barrier is lowered and the contact resistance is reduced.Thus,the field-effect mobility increases from 0.12 to 0.52 cm 2 /(V·s).It means that inserting a C 60 ultra thin layer is a good method to improve the organic thin film transistor (OTFT) performance.The output curve is simulated by using a charge drift model.Considering the contact effect,the field effect mobility is improved to 1.15 cm 2 /(V·s).It indicates that further reducing the contact resistance of OTFTs should be carried out.

Electroluminescence quenching mechanism in Rubrene doped host guest system

In this paper, the electroluminescence quenching mechanism in a 5,6,11,12-tetraphenylnaphthacene (Rubrene) doped host-guest system is studied by utilizing a specially designed organic light-emitting diode with an emission layer consisting of a few periodic host/guest structures. Tris-(8-hydroxyquinoline) aluminium (Alq3) and Rubrene are used as the host and the guest, respectively. The thickness variation of the guest layer in each period enables the study of the effect of molecule aggregation, and the thickness variation of the host layer suggests a long range quenching mechanism of dipole–dipole interaction. The long range quenching mechanism is a Forster process, and the Forster radius of Rubrene is between 3 and 10 nm.