Fabrication and Photoelectrical Characteristics of Polymer-Organic-Blend Photodetectors
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Fabrication and Photoelectrical Characteristics of Polymer-Organic-Blend Photodetectors Difei Qi, Kody Varahramyan, and Sandra Selmic Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA ABSTRACT Polymer optoelectronics and microelectronics have been recognized as next generation technologies. One of the widely investigated materials for photodiode, LED and solar cell applications is the insoluble conjugated polymer poly(p-phenylene vinylene) or PPV. In this paper we present experimental results of a blended polymer-organic compound photodiode. This diode is based on a soluble derivative of PPV, poly(2-methoxy-5- (2,9-ethyl-hexyloxy)-1,4phenylene vinylene) or MEH-PPV, and the organic material ethyl viologen dibromide or EVD. In making the photodiodes, solutions of MEH-PPV and EVD were spin-coated on indium tin oxide coated glass substrates. The thicknesses of these polymer-organic thin films were approximately 190 nm. An aluminum cathode was deposited by thermal evaporation. These devices were illuminated under monochromatic light in UV and visible range wavelengths. These thin polymer-organic blend photodiodes have shown an eight-fold increase in responsivity and quantum efficiency compared to pure MEH-PPV photodiode devices. The increase in photoconductivity of blended MEH-PPV:EVD photodiodes may be due to charge transfer by EVD dication. The results from this work clearly demonstrate the application of the reported approach for the realization of polymer photodiodes with increased photoconductivity characteristics. INTRODUCTION Since the discovery of conducting polymers, intensive research has resulted in emerging polymer-based electronic and optoelectronic devices, e.g. light emitting diodes (LEDs) [1][2], photodiodes [3], photovoltaic cells [4][5], and field effect transistors [6]. The poly(2-methoxy-5(2,9-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV), a soluble derivative of PPV, is widely used in polymer LEDs and photovoltaic cells due to its ease of processing and significantly higher hole mobility than electron mobility [7]. The MEH-PPV acts as an electron donor with a relatively low quantum efficiency of 0.07% [8] due to a limited exciton diffusion length, molecular structure distortion, and relatively low hole mobility compared to inorganic solid state semiconductor materials like Si. However, the relatively low conductivity of intrinsic MEH-PPV limited the quantum efficiency of MEH-PPV devices. A donor-acceptor heterostructure was used to enhance charge dissociation in photodiodes and photovoltaic diodes [2][3][6][9]. Thereby, donors should have a low ionization potential and acceptors should have a high electron affinity. When donor and acceptor molecules are mixed, the electrons excited from donors are energetically favorable for transferring electrons to acceptors. Conversely, holes are transferred to the anode by hopping. Acceptors like C60, PCBM and CN-PPV, have been demonstrated to greatly improve the charge carrier dissociation in blended structures [9]-[1
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