Studies On Photovoltaic Of Light-emitting Electrochemical Cells At Room Temperature

Polymer light-emitting electrochemical cell(LEC) is a new concept based on the traditional PLED and lithium cell.In LEC,the active layer is a conjugated polymer which doped with ion-transport polymer and Li⁺ salt,with an interpenetrating network. When a sufficiently large external bias is applied between the electrodes in a functional LEC(V≥E_g/e,E_g=difference of the HOMO and LUMO of conjugate polymer(CP), e=elementary charge),an electrochemical redox reaction is initiated,achieving the injection of electrons(at the cathode) and holes(at the anode),then a dynamic and reversible p-n junction is created in situ.LEC possess many advantages compared with PLED,such as,low turn-on and operating voltages,insensitivity to electrode materials,insensitivity to film thickness and high efficiency.In this work,we fabricated a sandwich LEC comprises a conjugate polymer(CP) which doped with ion-transport polymer(poly-ethylene oxide,PEO) and Li⁺ salt (lithium trifluoromethanesulfonate,LiCF₃SO₃) as the active layer,the connected electrode is indium-tin oxide and Aluminum.The V_OC and I_SC of the device increasing with the enhancement of the scanning times,and after a period of time,they all arrive at a stabile value,with the voltage scanning from -1 V to 5 V at the moment of measurement in room temperature.From the Current-Time curves and the A.C. impedance spectroscopy,we can see that a certain amount of voltage and time are required to form the p-n junction in room temperature,and the smaller the voltage,the longer the time,vice versa.After the external bias removed,the doping profile changes, and the p-n junction relaxes as the cell discharges,the photovoltaic characteristic of LEC changed when the p-n junction relaxing,this relaxing process needs several minutes.The photovoltaic characteristic of forward,reverse LEC and different film thickness LEC were investigated through applying the same pre-bias and time on room-temperature LEC.From these curves we knew that the forward and reverse LEC have different photovoltaic response;V_OC and I_SC will change when the film thickness of LEC is different.