25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

Abstract

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.

How Many Parameters Actually Affect the Mobility of Conjugated Polymers?

“…The results of the model are strikingly similar to the experimental results reported by Blom et al [4,26] for a broad range of chemically different organic semiconductors and also reported in Figure 4. All experimental data can be fitted by an Arrhenius temperature dependence…”

“…It has been argued that in organic diodes the average charge carrier density in a device at zero bias exceeds 1×10 21 m -3 due to the diffusion of charges from the contacts [4]. The presence of such a finite carrier density would then cause the mobility to follow a ln[μ] ∝ 1/T temperature dependence over the temperature range where J-V measurements are usually carried out [26]. However, the model calculations presented here demonstrate that an Arrhenius-like temperature dependence is a fundamental property of transport along polymer chains.…”

“…In the infinite temperature limit the mobility seems to converge to a very limited range of values (100-4000 cm 2 V -1 s -1 with our choice of d 2 M 2 ). and augmented with additional data points [26]. Bottom: mobility vs. 1/T from various models of chains of 1000 monomers with a variety of disorder parameters (see Table 1), including an alternating copolymer (model 14).…”

Simulating charge transport in organic semiconductors and devices: a review

“…Achieving this is a delicate process since one needs to control charge injection at both contacts, charge transport through (and between) energetically disordered OSCs, as well as where excitons are formed, diffuse and decay. Notwithstanding these challenges, there has been significant success in the predictive modelling of OLED [136,428] and WOLED [139] device performance. In this section we will focus on the rate of NGR and device simulation in OLEDs, starting with single materials before moving onto multilayer OLEDs.…”

Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer

“…We speculate that the degradation in PCE is largely due to the formation of deep traps, which is partially arrested by the addition of PMMA. We propose that the deep traps lead to enhanced Shockley-Read-Hall (SRH) recombination [32] which in turn reduces PCE. This is consistent with the observed loss in performance being primarily via J SC and FF.…”