Raman spectral changes of PEDOT–PSS in polymer light-emitting diodes upon operation

The micro-Raman spectra of a double-layer light-emitting diode fabricated with poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT–PSS) as a hole-injection layer and poly(2,7-(9,9-dioctylfluorene)-alt-benzothiadiazole) (F8BT) blended with poly(9,9-dioctylfluorene) (PF8) as an emissive layer have been measured with excitation at 633 nm. After the operation of the device, the intensities of the bands due to PEDOT have increased. These increases in Raman intensity have been attributed to the reduction (dedoping) of PEDOT chains during operation on the basis of the results of electrochemical oxidation and reduction of a PEDOT–PSS film. The reduction of PEDOT chains is most likely one of the intrinsic factors of the degradation of polymer light-emitting diodes.