Electronic line-up in light-emitting diodes with alkali-halide/metal cathodes

Abstract

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…[76,121] Several groups have made efforts regarding the construction of a band diagram, based on indirect evidence, such as transient photocurrent [171] and surface potential characterization. [180] On the other hand, electroabsorption spectroscopy [185,186] has proven to be a powerful approach to study the internal electric field and to characterize the interfacial barrier modulation under working conditions. [187] Li et al [115] found that there is a shift of the built-in potential during the device scanning.…”

Photoelectron Spectroscopy of the Contact between the Cathode and the Active Layers in Plastic Solar Cells: The Role of LiF

“…8,19) The use of both LiF and PEDOT-PSS in this way has proven effective in photovoltaic cells 20) and polymer light emitting diodes. 21,22) In fact, the use of alkali halide sandwich layers to improve injection at the cathode electrode has been used successfully in a wide variety of organic electronic devices, e.g., refs. 21, 23-26. In this work the cathode side is studied, particularly the beneficial effect that the LiF layer has on the interface between the Al and the active organic layer.…”

Improved electron injection in polymer light-emitting diodes using anionic conjugated polyelectrolyte

“…In particular, the insertion of an inorganic or organic electron injection layer ͑EIL͒/transport layer between the cathode and emissive layer can provide an intermediate step for facilitating electron injection. [10][11][12] One example is the use of lithium fluoride ͑LiF͒. When incorporated into polymer light-emitting diodes ͑PLEDs͒, the LiF layer improves the device performance by increasing the electron attenuation length, by suppressing the interfacial reactions of the metal electrode with emissive layer during the cathode evaporation, and/or by reducing the barrier height of the electron injection through formation of a low work function interfacial layer at the cathode.…”

“…When incorporated into polymer light-emitting diodes ͑PLEDs͒, the LiF layer improves the device performance by increasing the electron attenuation length, by suppressing the interfacial reactions of the metal electrode with emissive layer during the cathode evaporation, and/or by reducing the barrier height of the electron injection through formation of a low work function interfacial layer at the cathode. 10,[13][14][15] The recent research using cationic systems [16][17][18] implies that anionic conjugated polyelectrolytes might offer several interesting opportunities for use in polymer-based optoelectronic devices. In light-emitting electrochemical cells ͑LECs͒, the mobile ions enable redox doping and the formation of ohmic contacts.…”