Effect of CsF buffer layer on charge-carrier mobility in organic light-emitting diodes based on a polyfluorene copolymers by admittance spectroscopy

Insert an ultrathin insulating layer at the organic/metal interface is a promising way to increase the device efficiency of organic light-emitting diodes (OLEDs). Here we have fabricated OLEDs, which were sandwich structure with Al/poly [9,9-dioctylfluorene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole] (PFO-DBT15)/PEDOT/indium tin oxide (ITO) OLEDs, with and without ultrathin CsF buffer layer at the organic/metal interface. Special attention was paid to polymer/electrode interface modification. Transport of carriers in copolymer with and without CsF buffer layer was investigated by means of admittance spectroscopy, respectively. We compare the charge-carrier mobilities of double-carrier (with Al/CsF cathode) and hole-only (with Al cathode) devices. CsF buffer layer is shown to significantly influence the electron mobilities while the hole mobilities are left unchanged and thereby carrying out a better balance of carrier in the device. The diffusion of CsF from electrode into copolymer is clearly observed by scanning electron microscopy and energy dispersive spectrometer, which resulting in enhance electric injection and improve the carrier balance in a double carrier device.