Semiconductor Composites for Solid-State Lighting

Inorganic-organic semiconductor heterostructures have been subjects of interest for solid-state lighting (SSL) in the last decades due to the potential for simultaneous utilization of the complementary favorable properties of two distinct material classes for producing light. This work proposes a hybrid composite active layer design for light emission which combines inorganic and organic semiconductors. Particular to this composite design, light- emitting organic semiconductor molecules are embedded in an ambipolar charge- transporting inorganic semiconductor matrix. The embedded molecules serve as the radiative recombination sites for charge carriers that are injected into the matrix. Coming up with a composite layer may seem to be just a plug-and-play of different inorganic and organic semiconductor combinations. However, there are several fundamental requirements to make the concept work. Very important factor in realizing high efficiency devices are the properties of the interfaces between the different components because they often govern the physical properties of the overall structure. With this in mind, the energy level alignments at the interfaces of the candidate material combinations and the different chemical interactions taking place at such interfaces were investigated by photoelectron spectroscopy (PES). For the energy level alignment, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the organic dye should be situated in between the valence and conduction bands of the inorganic semiconductor. This alignment provides the necessary energetic driving forces for electron and hole transfers from the charge-transporting inorganic matrix to the light-emitting organic molecules. The PES investigations were focused on the interface formations of these two types of materials in three deposition configurations, namely: the inorganic/organic bilayer, the composite layer and the organic/inorganic bilayer. Additionally, the interface between the inorganic and organic part has to be of high purity and electronic perfection, as otherwise defect states will destroy the coupling of the two materials. By interpreting PES data, we also determined and described the chemical composition of the interface and the resulting coupling of the two materials on the atomic scale. Furthermore, as one of the advantages of hybrid designs, energy transfer from the matrix to the dopant is another possible route for obtaining light emission from the organic dopant. Therefore, another factor to look into would be the resonance between the exciton energies of the inorganic and organic materials. With this in mind and in conjunction with available experimental methods, the room-temperature (RT) photoluminescence (PL) of the hybrid composite active layer was investigated using a high energy laser for exciting the inorganic matrix. The idea here is to exclusively excite the matrix; if there is a light emission coming from the dopant, then it could hint at two possible mechanisms: one is energy transfer and the other is charge transfer from the excited matrix to the dopant. As one of the candidate material combinations, zinc selenide (ZnSe) and a red emitter, Ir(BPA), were investigated. Bilayer and composite thin films of ZnSe and Ir(BPA) organic light emitter were prepared in situ by UHV thermal evaporation technique. The measured energy band alignments for the ZnSe/Ir(BPA) bilayer and ZnSe+Ir(BPA) composite reveal that the HOMO and LUMO of the organic dye are positioned within the ZnSe bandgap. For the initial steps of ZnSe deposition on a dye film to form Ir(BPA)/ZnSe bilayers, zinc atoms were found to intercalate into the dye film leaving behind an excess of selenium at the interface that partly reacts with dye molecules. PES of the composites shows the same chemical species suggesting a similar mechanism. This mechanism leads to composite films with increased content of amorphous phases in the inorganic matrix. The PL spectra of composite films showed a relative enhancement of the emissions coming from the films with low dye concentrations as compared to the films with higher concentrations. This enhancement may hint at a charge transfer and/or an energy transfer from ZnSe to Ir(BPA). Proof of concept for the novel composite design is provided by a device that was fabricated with an active layer that is composed of alternating layer sequences of ZnSe and Ir(BPA). Weak areal emission and red intermittent sparks were visually observed from the device. Overall, several challenges in realizing the hybrid composite active layer design were uncovered during the course of this work. From the results of the PES measurements on several material combinations, it is found that the position of the Fermi level in the pristine inorganic semiconductor and the pristine organic light emitter strongly directs the resulting energy level lineup in the bilayer and composite upon thermodynamic Fermi level equalization at the interfaces. A large interface dipole formation at the material interface can positively reinforce the desired alignment for the proposed composite design. In addition, reactive interfaces of certain compositions are formed rather than the desired abrupt material interface. This reactive interface can hinder efficient charge and energy transfers, impede charge carrier conduction and supply nonradiative recombination sites. Furthermore, alternatives for a less-chemically reactive deposition method are limited by the availability of techniques that are compatible for inorganic-organic layered and composite depositions