During the last two decades, organic semiconductor lasers (OSLs) have been attracted much attention due to their advantageous properties such as wavelength tunability in visible range , low cost, flexibility and large area fabrication . These properties make them good candidates for a range of applications including sensing, spectroscopy and optical communication. However, only optically pumped organic lasers have been realized so far and the demonstration of an electrically-driven organic laser diode still remains a very challenging task. To achieve electrically pumped OSLs, much effort has been focused on the reduction of the energy threshold of optically pumped organic lasers by enhancing the gain media properties and optimizing the resonant cavities. The injection of high current density involves Joule heating which causes degradation and breakdown of the device. Moreover, the presence of high charge density induces multiple annihilation processes such as exciton-polaron quenching, polaron absorption and electric field dissociation which are ones of the causes of device external quantum efficiency rolloff. A fundamental understanding of the physical mechanisms governing the device operation is crucial to optimize the device performance and overcome the limitation processes to the achievement of an electrically driven OSLs. Electrical simulation of an organic light emitting diode under high current density is performed in order to predict the current at high voltage. The influence of the various annihilation processes is investigated by solving the exciton continuity equation.
1. O. Mhibik et al., Appl. Phys. Lett. 102, 41112 (2013).
2. C. Ge et al., Opt. Exp. 18, 12980–12991 (2010).
Since the discovery of organic solid-state lasers, great efforts have been devoted to the development of continuous-wave (cw) lasing in organic materials. However, the operation of organic solid-state lasers under optical cw excitation or pulse excitation at a very high repetition rate (quasi-cw excitation) is extremely challenging. In this work, we have demonstrated quasi-continuous-wave (quasi-cw) surface-emitting lasing in a distributed feedback device which combines a second-order grating with an organic thin film of a host material 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) blended with an organic laser dye 4,4’-bis[(N-carbazole)styryl]biphenyl (BSBCz). When pumping the device with optical picosecond pulse excitation, the quasi-cw laser operation maintained up to a repetition rate of 8 MHz. The lasing threshold was around 0.25 J cm−2 which was almost independent of the repetition rates. For our laser devices, the maximum repetition rate (8 MHz) is the highest ever reported, and the lasing threshold (0.25 J cm−2) is the lowest ever reported. These superior quasi-cw lasing characteristics in BSBCz are accomplished by the less generation of triplet excitons via intersystem crossing because a photoluminescence quantum yield of the blend film is nearly 100% and there is no significant spectral overlap between laser and triplet absorption.[1,2] Triplet quenchers, generally used for the fabrication of organic thin-film lasers, were not necessary in our devices because of negligible accumulation of triplet excitons and a small spectral overlap between emission and triplet absorption. Therefore, we believe that BSBCz is the most promising candidate for the first realization of electrically pumped organic laser diodes in terms of optical characteristics. However, electrical characteristics such as charge carrier mobility, charge carrier capture cross section, etc., are also extremely important and will need further investigation and enhancement for realization of electrically pumped organic lasers.
1. Aimono, T.; Kawamura, Y.; Goushi, K.; Yamamoto, H.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071110–071112.
2. Nakanotani, H.; Adachi, C.; Watanabe, S.; Katoh, R. Appl. Phys. Lett. 2007, 90, 231109.
We have recently focused our attention on the application of perovskite materials to a semiconducting layer in field-effect transistors. Because perovskite materials are expected to promise the processability and flexibility inherent to organic semiconductors as well as the superior carrier transport inherent to inorganic semiconductors, we believe that organic semiconductor-like cost-effective, flexible transistors with inorganic semiconductor-like high carrier mobility can be realized using perovskite semiconductors in future. In this study, we have prepared the tin iodide-based perovskite as a semiconducting layer on silicon dioxide layers treated with a self-assembled monolayer containing ammonium iodide terminal groups by spin coating and, then, source-drain electrodes on the perovskite layer by vacuum deposition for the fabrication of a top-contact perovskite transistor. Because of a well-developed perovskite layer formed on the treated substrate and reduced contact resistance resulting from the top-contact structure, we have obtained a new record hole mobility of up to 12 cm2 V–1 s–1 in our perovskite transistors, which is about five times higher than a previous record hole mobility and is considered to be a very good value when compared with widely investigated organic transistors. Along with the high hole mobility, we have demonstrated that this surface treatment leads to smaller hysteresis in output and transfer characteristics and better stress stability under constant gate voltage application. These findings open the way for huge advances in solution-processable high-mobility transistors.