Organic light-emitting diodes (OLEDs) with doublet-spin radical emitters have emerged as a new route to efficient display technologies. In contrast to standard organic semiconductors, radical materials have unpaired electrons. This feature results in the most well-known examples of organic radicals being where they are reactive species in chemical reactions 1 . Stabilised radicals can be used in optoelectronic applications which exploit their optical and spin properties, allowing up to 100% internal quantum efficiency (IQE) for electroluminescence 2 . Highly efficient OLEDs have been demonstrated which operate in the doublet-spin electronic state manifold with doublet emission 2,3 . The radical-based devices present a departure from the singlet-and triplet-level considerations which impose efficiency limits in OLEDs for typical organic semiconductors (25% IQE). This Perspective focuses on radical doublet emitters for optoelectronics, outlining how the photo-and spin-physics of unpaired electron systems present new avenues for research in light-emitting applications. 3 I. DOUBLET EMISSION FOR OLED DEVICESElectron and hole recombination from conduction and valence bands results in light emission for semiconductor systems. These electron-hole excited states are known as excitons. Organic semiconductors intrinsically screen the coulombic interaction of electric charges less than their inorganic counterparts, a result of the lower dielectric constant in organic molecular solids 4 . The stronger interaction of charges within organic semiconductors can give rise to Frenkel excitons, where electron-hole pairs are more tightly bound than those in the Wannier-Mott excitons found in more classical inorganic semiconductors. Frenkel excitons generally have stronger transition dipole moments for more efficient light emission in optoelectronic devices. The 'organic' approach allows more flexible manufacture of light-emitting layers than devices based on III-nitride semiconductors 5,6 , as well as easily tuneable properties from chemical synthesis.However, strong coulomb interactions in organic semiconductors also impose efficiency limits for light emission from charge recombinationa consequence of the quantum-mechanical spin properties of singlet (S1) and triplet (T1) excitons. Singlet and triplet electronic states have total spin quantum numbers, S = 0 and S = 1, respectively. Due to a singlet ground state in typical organic semiconductors, triplet excitons should be dark and non-emissive due to the rule of spin conservation in transitions for light emission. As triplet excitons are formed in 75% of charge recombination events for such organic semiconductors 4 , spin statistics would limit the electroluminescence efficiency of OLEDs to 25%. The generally larger coulomb interaction in organic semiconductors compared to inorganic systems results in a larger exchange interaction and singlet-triplet exciton energy gap; triplets act as excitonic, non-luminescent traps if their emissive properties are not enhanced because T1 exciton...
In this work w e demonstrate the effectiveness of both x-ray diffraction and x-ray reflectivity in t h e structural characterization of semiconductor structures By combining information from both techniques the abruptness of t h e interfaces for Si,_.Ge, structures. with x = 0.1-0.57, may be determined. For superlattice structures with x < 0.3 both types of interlace were found to have a root mean square (RMS) roughness of 0.5 f 0.3 nm. For a Si/Si,.,,Ge,.,, superlattice structure the interfaces are found to have differing roughnesses. For the SiGe-on-Si interface the RMS roughness is found to be 0.5 0.2 nm; however, t h e Si-on-SiGe interface has a larger value of roughness, 1.0 0.3 nm. This roughness at the Si-on-SiGe interface is found to be dependent on t h e G e content of t h e layer and it is shown by transmission electron microscopy analysis to be long ranged (about 70 n m ) and wavy at t h e interface.
The application of grazing incidence X-ray reflectivity measurements to the characterization of metallic multilayers is described. Use of simulation permits interface roughness to be determined to a precision of about 0.1 nm r.m.s. from the specular reflectivity profile. Measurement of the diffuse scatter permits interdiffusion to be distinguished in principle from interface roughness. Localization of strong diffuse scatter around the Bragg peaks of superlattices provides evidence for coherency in the roughness through the superlattice thickness.
Epitaxial films of Hg1-xMnxTe (MMT), between 2 and 10 mu m thick, have been grown by MOVPE on GaAs and CdZnTe substrates and their perfection assessed by high-resolution X-ray scattering and topography. The full width at half height maximum (FWHM) was found to vary inversely with the MMT layer thickness, independently of Mn concentration or substrate type. A general increase in integrated intensity with thickness was observed, though with substantial variations between individual data points. Double-axis topography and triple-axis scattering showed that the greatest contribution to the double-axis FWHM came from tilts between subgrains within which the strain was small. Little difference was seen either in the double-axis FWHM or triple-axis isointensity contours between thick MMT layers grown on GaAs or CdZnTe substrates. The model of Ayers et al. does not appear to be applicable to the system, which incorporates a CdTe buffer layer on the GaAs.
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