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Journal of Electrochemistry ›› 2005, Vol. 11 ›› Issue (1): 1-7.

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Studies on Polymer Light-emitting Electrochemical Cells ——Introduction of the Studies on the Electrochemistry of Conducting Polymers in CAS Key Laboratory of Organic Solids (Ⅱ)

LI Yong-fang   

  1. (CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
  • Received:2005-02-28 Revised:2005-02-28 Online:2005-02-28 Published:2005-02-28

Abstract: Polymer light-emitting electrochemical cell (LEC) is composed of a conjugated polymer(CP) / polymer electrolyte blend film sandwiched between an ITO anode and an Al cathode. When a bias voltage greater than Eg/e (where Eg is the energy gap of the CP, e is the electron charge) is applied on the two electrodes, the CP in the polymer blend film at the anode side will be oxidized (to form a p-doped region), and that at the cathode side will be reduced (to form a n-doped region), then a p-i-n junction will be produced. Light-emission occurs in the intrinsic region between the p- and n- doped layers. In comparison with the single layered polymer light-emitting diode (PLED), LEC possesses the advantages of low turn-on voltage and high quantum efficiency of its electroluminescence. But there are some drawbacks in the LEC, including slow response due to the slow ionic motion and the phase separation problem between the CP and the ionic-conducting polymer (such as PEO). Our research on the LEC focused on the electrochemical studies of the LEC and on the improvement of the device performance by overcoming the drawbacks of the LEC. Our main achievements in the studies of LECs are as follows: 1 We measured the cyclic voltammograms of a series of PPV derivatives, and calculated the HOMO and LUMO energy levels and energy gaps of the conjugated polymers. And the solid state electrochemical properties of LEC were studied for the first time. Accordingly, the electrochemical doping mechanism of the electroluminescence of LEC was confirmed. 2 The p-i-n junction structurd of the LECs was studied and confirmed by ac impedance measurements which show the flattened semicircle in its impedance plots and capacitance increase with the increase of the bias voltage. 3 In order to overcome the phase separation problem in the LECs, several kinds of bi-functional conjugated polymers possessing both electroluminescent and ionic-conductive abilities were designed and synthesized, including the PPV and PPP derivatives with PEO side chains, the block copolymers with its main chain comprised of luminescent conjugated segments and ionic-conductive segments (PEO or crown ether). The low turn-on voltage and high electroluminescent (EL) efficiency of the LECs with the bifunctional copolymers were realized: the LECs emit blue-green light with the turn-on voltage of 2.6~2.8 V and the highest EL efficiency of the LEC based on the copolymer of DMSN-TEO reached 4.2 cd/A. (4) The imidazolium salts ionic liquids with the melting points from room temperature to 80℃ were synthesized and applied in the fabrication of LECs as an electrolyte. The room temperature quasi- frozen p-i-n junction LECs were realized with the blend of MEH-PPV and the imidazolium salts ionic liquids of \\ (mp: 73 ℃)and \\ (mp: 83 ℃)as the active layer. The frozen p-i-n junction LECs emit light with fast response and high luminance.

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