Fatima Bencheikh

Toward continuous-wave operation of organic semiconductor lasers

Organic semiconductor laser operating in the quasi-CW regime at 80 MHz and under 30 ms long pulse photoexcitation is demonstrated. The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasi–continuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture.

Long-lived efficient delayed fluorescence organic light-emitting diodes using n-type hosts

Organic light-emitting diodes have become a mainstream display technology because of their desirable features. Third-generation electroluminescent devices that emit light through a mechanism called thermally activated delayed fluorescence are currently garnering much attention. However, unsatisfactory device stability is still an unresolved issue in this field. Here we demonstrate that electron-transporting n-type hosts, which typically include an acceptor moiety in their chemical structure, have the intrinsic ability to balance the charge fluxes and broaden the recombination zone in delayed fluorescence organic electroluminescent devices, while at the same time preventing the formation of high-energy excitons. The n-type hosts lengthen the lifetimes of green and blue delayed fluorescence devices by > 30 and 1000 times, respectively. Our results indicate that n-type hosts are suitable to realize stable delayed fluorescence organic electroluminescent devices.OLEDs based on thermally activated delayed fluorescence have shown high fluorescence efficiency but poor lifetime. Herein, Cui et al. demonstrate that the use of n-type host molecules can increase the device lifetime by 30 times and 1000 times for green and blue OLEDs, respectively.

Influence of the organic film thickness on the second order distributed feedback resonator properties of an organic semiconductor laser

We report on the cavity numerical characterization of a second order one-dimensional distributed feedback organic laser. The gain medium containing 6 wt. % of 4,4′-bis[(N-carbazole)styryl]biphenyl) in a 4,4′-bis[9-dicarbazolyl]-2,2′-biphenyl) host is vacuum deposited to form an organic thin film on a SiO2 grating. The influence of the organic film thickness on the properties of the resonant cavity is investigated through numerical calculations of both the confinement factor Γ and the Q-factor. The Q-factor is obtained using two methods, one by calculating the eigenmodes of the resonant cavity and the other by calculating the reflection spectrum. It was found that while the Γ increases with the organic film thickness, the Q-factor shows a non-monotonic function with a maximum value for a thickness of 200 nm.

Enhanced Electroluminescence from Organic Light-Emitting Diodes with an Organic-Inorganic Perovskite Host Layer

The development of host materials with high performance is essential for fabrication of efficient and stable organic light-emitting diodes (OLEDs). Although host materials used in OLEDs are typically organics, in this study, it is shown that the organic-inorganic perovskite CH3 NH3 PbCl3 (MAPbCl3 ) can be used as a host layer for OLEDs. Vacuum-evaporated MAPbCl3 films have a wide band gap of about 3 eV and very high and relatively balanced hole and electron mobilities, which are suitable for the host material. Photoluminescence and electroluminescence take place through energy transfer from MAPbCl3 to an organic emitter in films. Incorporation of an MAPbCl3 host layer into OLEDs leads to a reduction of driving voltage and enhancement of external quantum efficiency as compared to devices with a conventional organic host layer. Additionally, OLEDs with an MAPbCl3 host layer demonstrate very good operational stability under continuous current operation. These results can be extensively applied to organic- and perovskite-based optoelectronics.

Electrical simulation of organic light emitting diode under high current injection (Conference Presentation)

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 [1], low cost, flexibility and large area fabrication [2]. 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).