In-operando dipole orientation for bipolar injection from air-stable electrodes into organic semiconductors

TL;DR

This paper introduces a dipolar doping strategy using TMPE-OH in organic semiconductors to enable efficient charge injection from air-stable electrodes without additional layers or ionic additives.

Contribution

It demonstrates that in-operando dipole reorientation in a single-layer device improves charge injection and device performance, offering a practical alternative to existing methods.

Findings

Dipolar compounds reorient under applied voltage, enabling immediate luminance.

The dipole-doped OLED achieves efficiencies comparable to devices with dedicated injection layers.

The approach simplifies device architecture while maintaining high performance.

Abstract

Efficient charge-carrier injection from air-stable electrodes into organic semiconductors (OSCs) is essential for fabricating solution-processed organic optoelectronic devices under ambient conditions. Today, this is typically achieved by incorporating doped OSC interlayers, introducing self-assembled dipole monolayers, or adding mobile ions to the active material (AM). Here, we demonstrate an alternative approach that eliminates the need for additional injection layers or ionic additives. We achieve this by blending the dipolar compound TMPE-OH into the electroluminescent polymer Super Yellow (SY) and depositing this sole AM between two air-stable electrodes, forming a single-layer, dipole-doped OLED (D-OLED). By tracking its transient voltage-luminance response, performing impedance spectroscopy, and comparing these characteristics with two other single-layer device concepts, i.e. a neat-SY OLED without a dipolar compound and a light-emitting electrochemical cell (LEC) containing mobile ions, we can establish that the auxiliary dipoles in the D-OLED reorient under the applied driving voltage, enabling immediate luminance turn-on and lowering the injection barriers at both electrodes. Finally, we demonstrate that the D-OLED achieves current efficacies comparable to those of SY OLEDs incorporating dedicated injection layers or LECs. Our study establishes dipolar doping as a practical strategy for efficient bipolar charge injection from air-stable electrodes in solution-processed organic semiconductor devices.