White emission in polymer light-emitting diodes: color composition by single-layer electroluminescence and external photoluminescence component

Abstract

White emitting devices were obtained through a single component system by combining two materials with different emission properties: a greenish electroluminescent polymer, known as poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9-di-{5׳-pentanyl}-fluorenyl-2,7-diyl)] (PFOFPen), embedded in a diode-like structure (ITO/PEDOT-PPS/PFOFPen/Ca/Al) and a photoluminescent red-emitting material, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). A thin film of the red emitter component (MDMO-PPV) was deposited on a quartz plate and mounted on the opposite face of the glass/ITO substrate. The white color composition (0.35, 0.31) was obtained based on trivial photophysics processes: MDMO-PPV partially absorbs the higher energy electroluminescence (EL) emission coming from the PFOFPen-diode, decaying radiatively and emitting red light via photoluminescence (PL), matching with the transmitted greenish light. White emission is thus accomplished by choosing the proper materials with overlapped absorption spectra, adjusting the absorbance ratio by changing the MDMO-PPV layer film thickness, and the lower energy PL intensity until reaching the CIE white coordinates.