Efficient Blue Polymer Light-Emitting Diodes with Electron-Dominated Transport Due to Trap Dilution

As is common for many conjugated polymers used in light-emitting diodes (PLEDs), the charge transport in blue-emitting polyspirobifluorene (PSF) copolymerized with the hole transport unit - N,N,N'N'-tetraaryldiamino (TAD) biphenyl - is dominated by holes. Although the free electron mobility is an order of magnitude higher than the hole mobility, the electron transport is strongly hindered by traps. By diluting PSF-TAD with the wide band gap polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), the effect of electron trapping can be nearly eliminated. As a result, the transport in the PSF-TAD:PFO blend becomes electron dominated. Due to the higher electron mobility, PLEDs made from these blends exhibit higher current and light-output as compared to hole-dominated PLEDs made from pristine PSF-TAD. The reduced amount of electron traps enhances their efficiency from 2 cd A(-1) for the hole-dominated PLED to 5.3 cd A(-1) for the electron-dominated blend PLED.