Improvement in polymer light-emitting diode performance using polymer heterojunction structure prepared by evaporative spray deposition from ultradilute solution

We successfully demonstrated a significant improvement in polymer light-emitting diode performance by employing a polymer bilayer structure prepared by evaporative spray deposition from ultradilute solution (ESDUS). The bilayer structure consisted of two π-conjugated polymers, where the top layer was deposited from tetrahydrofuran (THF) solution by ESDUS onto the preceding bottom layer composed of the other THF-soluble polymer. The external quantum efficiency (EQE) of the bilayer device is improved markedly in comparison with that of a corresponding single-layer device prepared by conventional spin-coating. Moreover, the EQE of the bilayer device tends to be constant throughout a wide current region, whereas that of single-layer devices increases gradually with current density. These observations indicate that carriers are effectively blocked at the polymer/polymer interface of the bilayer device.