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.

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.