Non-radiative recombination losses in polymer light-emitting diodes

Abstract

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2 0-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the biggest efficiency loss, up to 45%, arises from extrinsic non-radiative recombination via electron traps. The loss caused by exciton quenching at the cathode proves only to be significant for devices thinner than 100 nm. Removal of electron traps by purification is expected to enhance the efficiency of polymer LEDs by more than a factor of two.

Fundamental Tradeoff between Emission Intensity and Efficiency in Light‐Emitting Electrochemical Cells

“…The increase in efficiency at 10 2 s is possibly related to a reduced importance of non-radiative trap-assisted recombination. 18 At relatively high current densities, these traps are saturated so their relative effect on recombination is reduced. Consequently the efficiency is raised at higher current densities.…”

Analyzing degradation effects of organic light-emitting diodes via transient optical and electrical measurements

“…[45][46][47][48][49][50] Although the number of injected electrons and holes does not necessarily change, this will effectively lead to a reduction of the charge balance factor c, because part of the carriers does no longer form excitons. Using the definition of c ¼ j rec /j tot and considering that the (radiative) recombination current (j rec ), i.e., the fraction leading to excitons, is given by the total current (j tot ) reduced by the non-radiative recombination current (j nr ), it follows that c ¼ j tot Àj nr j tot ¼ 1 À j nr j tot .…”

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

“…It has been predicted that a reduction of electron traps by an order of magnitude will lead to a doubling of the efficiency of a PLED. [20] To examine how trap dilution influences the electrical characteristics of PSF-TAD PLEDs, we modeled the PSF-TAD:PFO blend PLEDs and compared the transport parameters with the pristine PSF-TAD PLED that has been described before. [14] The PLED device model is based on a numerical drift-diffusion model, including radiative Langevin recombination and nonradiative trap-assisted recombination, as well as exciton quenching at the electrodes.…”

“…[14] The PLED device model is based on a numerical drift-diffusion model, including radiative Langevin recombination and nonradiative trap-assisted recombination, as well as exciton quenching at the electrodes. [20] For the charge-carrier mobility, the extended Gaussian disorder model is used, that contains as relevant parameters, μ 0 , a mobility prefactor that contains electronic overlap between transport sites, σ, representing the energetic disorder, and a, the distance between two transport sites. [21] Upon dilution, all three parameters can in principle vary: μ 0 and σ can change when dilution with a wide band gap polymer alters the packing of the transporting polymer chains.…”