Yan Yao

University of California, Los Angeles, Los Angeles, California, United States

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Publications (16)98.17 Total impact

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    ABSTRACT: Bandgap and molecular energy level control are of great importance in improving photovoltaic properties of conjugated polymers. A common approach to tuning these parameters is to modify the structure of conjugated polymers by copolymerizing with different units. In this paper, research work focuses on the synthesis of benzo[1,2-b:4,5-b′]dithiophene (BDT) with different conjugated units and their photovoltaic performance. Eight new BDT-based polymers with commonly used conjugated units, including thiophene, benzo[c][1,2,5]thiadiazole (BT), thieno[3,4-b]pyrazine (TPZ), etc., were synthesized. The bandgaps of the polymers were tuned in the range of 1.0-2.0 eV, and their HOMO and LUMO energy levels could also be tuned effectively. The absorption spectra as well as electrochemical and photovoltaic properties of these polymers were investigated systematically. Some units exhibiting the same effect of bandgap lowering exhibited different effects on molecular energy levels of the polymers. For example, the TPZ unit can reduce the bandgap by lowering the LUMO energy level and elevating the HOMO level of the polymer, but the BT unit can lower the bandgap only by depressing the LUMO level. Since open-circuit voltage (V oc) of the heterojunction polymer solar cell is believed to be inversely proportional to the HOMO level of electron donor material, V oc of the devices based on H9, the copolymer of BDT and TPZ, was ca. 0.5 V lower than that of the device based on H7, the copolymer of BDT and BT. The effects of seven commonly used units on bandgap, molecular energy level, and photovoltaic properties of the BDT based polymers are studied and discussed in this paper, which can provide a guideline not only for design of photovoltaic materials but also for materials of various other electronic devices. In addition, the PCE of the device based on PCBM and H6, one of the BDT-based polymers, reached 1.6%, and V oc , I sc , and FF of the device were 0.75 V, 3.8 mA/cm 2 , and 56%, respectively, which indicates that BDT is a promising common unit for photovoltaic conjugated polymers. Since we have developed the synthetic method of the 4,8-bisalkoxy-BDT monomer, the BDT unit will play an important role in future research on conjugated polymer design.
    Macromolecules. 07/2008; 41(16).
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    ABSTRACT: The mixed solvent approach has been demonstrated as a promising method to modify nanomorphology in polymer solar cells. This work aims to understand the unique role of the additive in the mixture solvent and how the optimized nanoscale phase separation develops laterally and vertically during the non-equilibrium spin-coating process. We found the donor/acceptor components in the active layer can phase separate into an optimum morphology with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as solubility and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.
    Advanced Functional Materials 06/2008; 18(12):1783 - 1789. · 9.77 Impact Factor
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    ABSTRACT: Polymer solar cells generally contain a low bandgap p-type conjugated polymer as photosensitizer and an inorganic or organic n-type semiconductor as electron acceptor. We investigated two nanostructured composites of the complementary semiconductors: nanometer-scale blends and interpenetrating nanorod arrays. In the blend approach, new low bandgap polymers were synthesized to match with electron-acceptors such as C<sub>60</sub>-PCBM and C<sub>70</sub>-PCBM for increased absorption efficiency. The nanorod arrays were fabricated by solution-based electrochemical growth. Photovoltaic performance of the composites is also discussed.
    Reliability Physics Symposium, 2008. IRPS 2008. IEEE International; 01/2008
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    ABSTRACT: In this paper, we report a method to increase the open-circuit voltage (VOC) of an organic solar cell by inserting an interfacial layer between the donor and acceptor layers to form a cascade-type energy band structure. We demonstrate its feasibility using a recently reported asymmetric pentacene derivative, tetraceno[2,3-b]thiophene (TT) as a donor material, C60 as an acceptor material, and copper phthalocyanine (CuPc) as the sandwich layer. The VOC was increased from 0.3 V for the device with no CuPc sandwich layer to 0.56 V for 13 nm thick CuPc layer. The power conversion efficiency (PCE) of the device with 13 nm CuPc layer was 1.53% and the fill factor (FF) was 0.64, in comparison to TT/C60 device which had a PCE of 0.78% and a FF of 0.52.
    Applied Physics Letters 11/2007; 91(22):223508-223508-3. · 3.79 Impact Factor
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    ABSTRACT: Polymer or ''plastics'' solar cells have been an intensively studied area since the discovery of efficient electron transfer between polymers and fullerenes and the introduction of the bulk-heterojunction concept. The last few years have seen significant improvement in plastic solar cell performance through aggressive research on the regioregular poly(3-hexylthiophene) (RR-P3HT) : [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) system. The morphology of the system is controlled through two major strategies which have proven effective in improving the device efficiency—thermal annealing and solvent annealing (slow growth). In this Feature Article, we review the recent progress on this material system. A detailed discussion on thermal annealing and solvent annealing approaches to improve device performance is presented, including a comparison between the two strategies. The effects of these two approaches on improving polymer crystallinity, light absorption in the polymer, carrier transport, blend film nano-morphology, etc. are summarized. We also include a brief discussion on accurate measurement and characterization techniques for polymer solar cells to correctly determine the efficiency by applying spectral mismatch factors. Future directions and challenges on polymer solar cell development are also discussed.
    Journal of Materials Chemistry 03/2007; 17(30). · 5.97 Impact Factor
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    ABSTRACT: An all-polymeric optocoupler has been demonstrated with a polymer light-emitting diode (PLED) as the light source (input unit) and a polymer/fullerene photodiode (PD) as the detector (output unit). The electroluminescence (EL) peak of the PLED is 560 nm, and the entire EL spectrum is within the response range of the PD. The optocoupler can work at low driving voltages, 5 V on the PLED and 0 V on the PD. The output photocurrent increases linearly with input current, and the current density transfer ratio reaches 1.5%. The frequency response of the optocoupler is at 500 kHz. With comparable performance to their inorganic counterparts, the all-polymeric optocouplers demonstrated here will bring the technology of organic photonic devices one step closer to commercialization.
    Applied Physics Letters 01/2007; 90(5):053509-053509-3. · 3.79 Impact Factor
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    Advanced Materials 01/2007; 19(22):3979-3983. · 14.83 Impact Factor
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    ABSTRACT: Efficient polymer solar cells based on a low band gap copolymer poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]- 5<sup>′</sup>,5<sup>″</sup> -diyl} and (6,6)-phenyl- C <sub>71</sub> -butyric acid methyl ester ( C <sub>70</sub> -PCBM) were demonstrated with 2.4% power conversion efficiency under air mass 1.5 G , 100 mW / cm <sup>2</sup> illumination. The broad absorption peak of C <sub>70</sub>- PCBM in 440–530 nm complements the absorption valley (regions between two absorption peaks at 416 and 584 nm ) of the polymer. The external quantum efficiency measurement further demonstrates that this increased absorption contributes significantly to the generation of photocurrent. Morphology studies on the blend films indicated that excellent miscibility between polymer and C <sub>70</sub>- PCBM favors exciton separation. The linear relationship between light intensity and short circuit current density shows efficient and balanced charge transport resulting in increased photocurrent and fill factor.
    Applied Physics Letters 11/2006; · 3.79 Impact Factor
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    ABSTRACT: The effect of side-chains on the molecular weight and the optical and electrical property of a low band gap copolymer poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5"-diyl} (PF-co-DTB) was studied. The decyloxy side-chains help to increase molecular weight (Mw = 115,000) and decrease the band gap (1.78 eV) as well as the oxidation potential (-5.4 eV). Zero-field mobility of 2×10-5 cm2/Vs is measured in hole-only devices. Photovoltaic devices based on PF-co-DTB/fullerene bulk-heterojunction show power conversion efficiency of up to 1.6% under air mass 1.5G, 100 mW/cm2 illumination. Side-chains effect on the photovoltaic devices studies show the trade-off between short circuit current increase and open-circuit voltage drop. Thermal annealing on device performance is also discussed.
    Proc SPIE 11/2006;
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    ABSTRACT: To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum (Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent cathode. In the stacked structure, the open circuit voltage and the short circuit current are twice those of a single cell. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.
    Proc SPIE 11/2006;
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    ABSTRACT: The authors investigate the effect of self-organization by controlling the growth rate on the performance of polymer/fullerene bulk-heterojunction solar cells. The effect of growth rate on the morphology of the active layer is studied by atomic force microscopy technique. The electrical characterization by dark current and photocurrent measurements is performed. The hole mobility in the polymer increases by about two orders in magnitude and the carrier transport becomes highly balanced. Increased exciton generation rate, more efficient electron-hole pair dissociation, higher carrier mobility, and balanced carrier transport in the active layer explain the enhancement in the short-circuit current and fill factor.
    Applied Physics Letters 09/2006; · 3.79 Impact Factor
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    ABSTRACT: Low band gap conjugated polymers with proper energy levels for charge transfer are required to achieve high-efficiency polymer solar cells. We report the synthesis and characterization of two new regioregular copolymers that are based on 3-alkoxythiophene monomers: poly(3-octylthiophene-2,5-diyl-co-3-decyloxythiophene-2,5-diyl) (POT-co-DOT) and poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5' '-diyl} (PF-co-DTB). Compared to the alkyl substituents, the alkoxy side chains on the thiophene units can effectively lower the band gap of copolymers and enhance the charge transfer to electron acceptors such as (6,6)-phenyl C(61)-butyric acid methyl ester (PCBM). The chemical structure and regioregularity of the copolymers were confirmed by NMR. Both copolymers are readily soluble in organic solvents and form high-quality thin films. Electrochemical and photophysical studies reveal band gaps of 1.64 eV for POT-co-DOT and 1.78 eV for PF-co-DTB. Bulk heterojunction photovoltaic devices were fabricated using blends of these copolymers with PCBM as the active layer, ITO-glass as the anode, and aluminum as the cathode. Power conversion efficiency of 1.6% was obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from a solar cell with an active layer containing 20 wt % PF-co-DTB and 80 wt % PCBM. Regioregular poly(3-decyloxythiophene-2,5-diyl) (P3DOT) was also studied for comparison purposes.
    Journal of the American Chemical Society 08/2006; 128(27):8980-6. · 10.68 Impact Factor
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    ABSTRACT: Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100 mW/cm2.
    Applied Physics Letters 02/2006; 88(7). · 3.79 Impact Factor
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    ABSTRACT: We demonstrate a multiple-device stacked structure of polymer solar cells for efficient light harvesting. Two polymer photovoltaic cells are stacked together and connected in series or in parallel to achieve a tandem structure. In this two-cell structure, a multilayer semitransparent electrode, made of lithium fluoride (LiF)/aluminum (Al)/gold (Au), is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent cathode. Upon stacking, the open-circuit voltage and the short-circuit current are almost doubled compared to a single cell.
    Applied Physics Letters. 01/2006; 88(6):064104.
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    ABSTRACT: Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.
    Nature Material 10/2005; 4(11):864-868. · 35.75 Impact Factor
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    ABSTRACT: Regioregular poly3-hexylthiophene RR-P3HT is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. In this manuscript, we report polymer photovoltaic devices based on RR-P3HT:methanofullerene 6,6-phenyl-C 61 -butyric acid methyl ester PCBM 1:1 weight-ratio blend. We studied the effects of annealing temperature and time on the device performance for devices annealed before and after cathode deposition. Thermal annealing shows significant improvement in the performance for both types of annealing conditions, with postproduction annealing being slightly better. For devices with a 43-nm-thick active layer, maximum power conversion efficiency PCE of 3.2% and fill factor up to 67% is achieved under Air Mass 1.5, 100-mW/ cm 2 illumination. We performed atomic force microscopy and ultraviolet-visible absorption spectroscopy on the P3HT:PCBM films to explain the effect of thermal annealing. By keeping the optimized thermal annealing condition and by varying the active layer thickness, we fabricated devices with PCE up to 4.0%, which is the highest efficiency reported so far for this system. © 2005 American Institute of Physics.
    Journal of Applied Physics 01/2005; 98. · 2.21 Impact Factor