Towards an electrically pumped polymer laser: Optimization of laser structures using photonic crystals
Final Report Abstract
In this project it was demonstrated that it is possible to combine an ambipolar light-emitting organic field-effect transistor (LEFET) based on the semiconducting polymer poly(9,9- dioctylfluorene-alt-benzothiadiazole) (F8BT) with an efficient light confinement and feedback structure, provided by a rib waveguide architecture with integrated distributed feedback grating. The feedback structure consists of an insulating material with high refractive index such as Ta2O5. An ambipolar F8BT LEFET device with top gate/bottom contact architecture was used as light emis-sion source. Electrons and holes can be accumulated simultaneously within the channel when suitable voltages are applied to the electrodes. Moreover, the recombination zone, where the charge carriers recombine radiatively, can be moved throughout the channel. To realize the idea of using an LEFET with F8BT as current-transporting and light-emitting material in a future electrically pumped laser device, several changes of the processing have to be performed. It was found that the F8BT annealing temperature has a significant impact on the transistor performance as well as the waveguiding performance of the emitted light. Ideally, both these properties have to be optimized at the same time. It was observed that amorphous F8BT is required to induce efficient wa-veguiding of emitted light. Devices annealed to more than 120°C show increased crystallinity, leading to severe light scattering losses. However, transistors with amorphous F8BT were found to exhibit performance cutbacks, which were attributed to injection problems. Charge carrier injection could be effectively tuned and improved, when the injecting gold electrodes are modified by selfassembled monolayers. The performance of the transistors treated with 1-decanethiol reached the level of devic-es based on polycrystalline F8BT. Typical mobilities for electrons and holes were found to be of the order of 10-3 cm2 V-1 s-1. Despite principal suitability for lasing, the required singlet exciton densities could not be achieved via the transistor current, mainly due to low carrier mobilities in F8BT. As the lasing threshold could not be obtained by electrical pumping, the devices were pumped optically to yield laser emission. This way, the influence of the transistor electrodes was investigated. As predicted by simulations, absorption of the propagating light was found to be a limiting factor for the laser efficiency, particularly by the gate electrode. Due to the strong confinement of the Ta2O5 rib waveguide structure, however, it was possi-ble to minimize the threshold enlargement by using silver instead of gold as the gate metal and enlarg-ing the dielectric thickness. The two-dimensional mode confinement also ensures that the lasing thre-shold is unaffected by the integration of the source/drain electrode pattern. Low optical lasing threshold values of about 4.5 μJ cm-2 were achieved in the complete LEFET structure. One major goal of this project was to judge the progress towards the realization of the first electrically driven organic semiconductor laser. If one takes into account the achieved lasing threshold values, in combination with the corresponding ambipolar transistor currents, one is able to estimate that one is still about four orders of magnitude below the necessary singlet exciton density, which is estimated for electrically pumped lasing. As a result of this, it becomes clear that the transistor performance has to be further improved in the future. Nevertheless, it was demonstrated that it is possible to merge an ambipolar LEFET architecture with a distributed feedback structure, which was upgraded by an under-lying waveguide ridge. Therefore, the proposed device architecture constitutes a valuable geometry for the realization of an electrically pumped organic semiconductor laser.
Publications
- Integration of a Rib Waveguide Distributed Feedback Structure into a Light-Emitting Polymer Field-Effect Transistor. Adv. Funct Mater. 2009, published online
M. C. Gwinner, S. Khodabakhsh, M. H. Song, H. Schweizer, H. Giessen, and H. Sirringhaus
(See online at https://doi.org/10.1002/adfm.200801897)
