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ABSTRACT: Polymeric photovoltaic (PV) cells based on poly(3-hexylthiophene-2,5-diyl):[6, 6]-phenyl C 61 butyric acid methyl ester (P3HT:PCBM) with the cathode confinement in the thermal treatment show better performance than the PV cells without the cathode confinement in the thermal treatment. The functions of the cathode confinement are investigated in this paper by using X-ray photoelectron spectroscopy, atomic force microscopy, optical absorption analysis, and X-ray diffraction analysis. It is found that the cathode confinement in the thermal treatment strengthens the contact between the active layer and the cathode by forming Al–O–C bonds and P3HT-Al complexes. The improved contact effectively improves the device charge collection ability. More importantly, it is found that the cathode confinement in the thermal treatment greatly improves the active layer morphology. The capped cathode effectively prevents the overgrowth of the PCBM molecules and, at the same time, increases the crystallization of P3HT during the thermal treatment. Thus, a better bicontinuous interpenetrating network is formed, which greatly reduces the exciton loss and improves the charge transport capability. Meanwhile, the enhanced crystallites of P3HT improve the absorption property of the active layer. All these aforementioned effects together lead to the great performance improvement of polymeric PV cells.
IEEE Transactions on Electron Devices 01/2011; 58. · 2.32 Impact Factor
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ABSTRACT: Experimental results show that the V <sub>OC</sub> of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the V <sub>OC</sub> of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of V <sub>OC</sub> on the electrodes is proposed. It is found that the V <sub>OC</sub> of the two types of PV cells follows the same mechanism and is mainly determined by the light-injected carriers at the donor/acceptor (D/A) interface and the electrodes. However, the distinct device structures make the boundary conditions in layered and bulk HJ PV cells different, which leads to the different dependences of V <sub>OC</sub> on the electrodes. The layered HJ PV cells have geometrically ¿flat¿ D/A and metal/organic (M/O) interfaces (the interface near the electrode), which makes the effective thickness from the D/A interface to the M/O interface large. Thus, there is a low electric field at the M/O interface and, then, a very small barrier lowering. Under this condition, the light-injected carriers at the D/A interface tend to ¿pin¿ the Fermi level of the electrodes. As a result, V <sub>OC</sub> shows only a very weak dependence on the work function of the electrodes. However, the formation of the interpenetrating network in bulk HJ PV cells greatly decreases the D and A domain dimensions and induces the ambipolar carrier distribution in the blend layer. This will cause very large barrier lowering at the M/O interface when there is a high barrier. Under this condition, the light-injected carriers at the D/A interface can no longer ¿pin¿ the electrode Fermi level. Thus, a strong dependence of V <sub>OC</sub> on the electrodes for bulk HJ PV cells is observed.
IEEE Transactions on Electron Devices 03/2010; · 2.32 Impact Factor
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ABSTRACT: An organic-based diode-memory device that has a bistable memory function and a high rectification ratio has been studied. The diode-memory device is fabricated by incorporating an organic-based diode component in series with a polymer memory component. The organic-based diode-memory device performs well as a reliable rectifying memory device, achieving an excellent on/off current ratio of 10<sup>6</sup> and a high rectification ratio of 10<sup>3</sup>. The conduction models are also fitted to study the proposed conductivity mechanism of the rectifying memory device. The demonstrated organic-based diode-memory device is very promising for use in a passive matrix crossbar polymer memory array.
IEEE Electron Device Letters 06/2009; · 2.85 Impact Factor
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ABSTRACT: A significant increase in open circuit voltage (VOC) is obtained in the polymer-fullerene bulk heterojunction solar cell by using the e-beam deposited Al cathode. Compared with the device with the thermal evaporated Al cathode, an obvious enhancement of VOC from 596 to 664 mV is obtained, which makes the overall device power conversion efficiency improved by 12.4% (from 3.79% to 4.26%). Electrical characterizations suggest that the energetic particles in the e-beam deposition induce deep interface hole traps in the poly(3-hexylthiophene-2,5-diyl) (P3HT), while leaving the fullerene unaffected. The deep trapped holes near the P3HT/cathode interface can induce the image negative charges in the cathode and thus form “dipoles.” These dipoles lead to the lowering of the Al effective work function and cause the enhancement of VOC.
Applied Physics Letters 03/2009; 94(10):103305-103305-3. · 3.84 Impact Factor
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ABSTRACT: A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl- C <sub>61</sub> -butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54 mA / cm <sup>2</sup> and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.
Applied Physics Letters 08/2008; · 3.84 Impact Factor
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ABSTRACT: We proposed and demonstrated a simple tandem structure of organic photovoltaic (PV) cell for efficient light harvesting. In this device structure, a soluble fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is employed simultaneously to form a bilayer heterojunction PV subcell with the underlying copper phthalocyanine (CuPc) and a bulk heterojunction PV subcell with blended poly(3-hexylthiophene-2,5-diyl) (P3HT). In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. The enhanced short circuit current density (JSC = 8.63 mA/cm2) and power conversion efficiency (PCE) (2.79%) of the tandem structure are nearly the sum of those of the stand-alone cells of CuPc/PCBM (JSC = 2.09 mA/cm2, PCE = 0.43%) and P3HT:PCBM (JSC = 6.87 mA/cm2, PCE = 2.50%).
Applied Physics Letters 02/2008; 92(8):083310-083310-3. · 3.84 Impact Factor
