Improved carrier balance and polarized in-plane light emission at full-channel area in ambipolar heterostructure polymer light-emitting transistors

We report polarized in-plane emission at full-channel area in ambipolar heterostructure organic light-emitting transistors (OLETs) utilizing bilayer oriented fluorene-type polymers. The present work demonstrates that carrier balance between bilayers is a key factor in achieving in-plane emission at almost full-channel area when hole transport is dominant in the upper layer, which acts as an electron blocking layer, and electrons are injected into the lower layer. Hole accumulation in the upper layer affects electron injection from the electrode to the lower layer and the probability of recombination of holes in the upper layer with electrons in the lower layer near the bilayer interface. For OLETs having an oriented bilayer with the channel direction parallel to orientation of the polymer chains, a uniform in-plane emission pattern, an increased electroluminescence intensity, and a high external quantum efficiency can be achieved by improving the hole mobility in the upper layer and optimizing the carrier balance. These results are expected to be very useful for the development of surface micro-light sources. (C) 2016 Elsevier B.V. All rights reserved.