Florian Maier-Flaig

Multicolor Silicon Light-Emitting Diodes (SiLEDs)

We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.

Preparation of Monodisperse Silicon Nanocrystals Using Density Gradient Ultracentrifugation

We report the preparation of monodisperse silicon nanocrystals (ncSi) by size-separation of polydisperse alkyl-capped ncSi using organic density gradient ultracentrifugation. The ncSi were synthesized by thermal processing of trichlorosilane-derived sol-gel glasses followed by HF etching and surface passivation with alkyl chains and were subsequently fractionated by size using a self-generating density gradient of 40 wt % 2,4,6-tribromotoluene in chlorobenzene. The isolated monodisperse fractions were characterized by photoluminescence spectroscopy and high-angle annular dark-field scanning transmission electron microscopy and determined to have polydispersity index values between 1.04 and 1.06. The ability to isolate monodisperse ncSi will allow for the quantification of the size-dependent structural, optical, electrical, and biological properties of silicon, which will undoubtedly prove useful for tailoring property-specific optoelectronic and biomedical devices.

Colloidally stable silicon nanocrystals with near-infrared photoluminescence for biological fluorescence imaging.

Luminescent silicon nanocrystals (ncSi) are showing great promise as photoluminescent tags for biological fluorescence imaging, with size-dependent emission that can be tuned into the near-infrared biological window and reported lack of toxicity. Here, colloidally stable ncSi with NIR photoluminescence are synthesized from (HSiO1.5)n sol-gel glasses and are used in biological fluorescence imaging. Modifications to the thermal processing conditions of (HSiO1.5)n sol-gel glasses, the development of new ncSi oxide liberation chemistry, and an appropriate alkyl surface passivation scheme lead to the formation of colloidally stable ncSi with photoluminescence centered at 955 nm. Water solubility and biocompatibility are achieved through encapsulation of the hydrophobic alkyl-capped ncSi within PEG-terminated solid lipid nanoparticles. Their applicability to biological imaging is demonstrated with the in-vitro fluorescence labelling of human breast tumor cells.

Single-mode biological distributed feedback laser.

Single-mode second order distributed feedback (DFB) lasers of riboflavin (vitamin B2) doped gelatine films on nanostructured low refractive index material are demonstrated. Manufacturing is based on a simple UV nanoimprint and spin-coating. Emission wavelengths of 543 nm and 562 nm for two different grating periods are reported.

White organic light emitting diodes with enhanced internal and external outcoupling for ultra-efficient light extraction and Lambertian emission

White organic light emitting diodes (WOLEDs) suffer from poor outcoupling efficiencies. The use of Bragg-gratings to enhance the outcoupling efficiency is very promising for light extraction in OLEDs, but such periodic structures can lead to angular or spectral dependencies in the devices. Here we present a method which combines highly efficient outcoupling by a TiO(2)-Bragg-grating leading to a 104% efficiency enhancement and an additional high quality microlens diffusor at the substrate/air interface. With the addition of this diffusor, we achieved not only a uniform white emission, but also further increased the already improved device efficiency by another 94% leading to an overall enhancement factor of about 4.

Looking Inside a Working SiLED

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs.

Synthesis and application of photolithographically patternable deep blue emitting poly(3,6-dimethoxy-9,9-dialkylsilafluorene)s.

Poly(silafluorene)s (PSFs) are promising light-emitting materials with brilliant solid-state blue luminescence, high quantum efficiency, excellent solubility, and improved thermal and chemical stability. PSFs are reported to have high electron affinity and conductivity originating from σ*-π* conjugation between the σ*-antibonding orbital of the exocyclic Si-C bond and the π* antibonding orbital of the butadiene fragment, a promising characteristic for improved charge carrier balance in OLEDs. In this paper, we present a protocol for photopatterning derivatives of poly(3,6-dimethoxy-9,9-dialkylsilafluorenes) with resolutions exceeding 10 μm. The procedure begins by converting polymers (Mn = 50-55 kg/mol, PDI = 1.8) with cyclohexenyl and norbornenyl containing side chains to their respective epoxides using the Prilezhaev reaction and m-chloroperoxybenzoic acid (m-CPBA). Using the I-line (365 nm) of a Karl Suss MA6 mask aligner, a 1 s UV light exposure of the photoacid generator (PAG) bis(4-tert-butylphenyl)iodonium hexafluoro-phosphate (DtBPI-PF6) generates sufficient protons to catalyze epoxide ring-opening and form a bridging network of covalent C-O bonds which renders the material insoluble in developing solvents such as toluene or THF. The resultant cross-linked material possess characteristic blue photoluminescence with solid state quantum yields >80%. Polymer films have excellent transparency (with a measured Eg ≈ 3.0 eV). Energy levels determined using cyclic voltammetry were -5.7 and -2.7 eV for HOMO and LUMO, respectively. Additionally, several device applications are demonstrated which incorporate cross-linked films. These include examples of solid state lasing in the region of 420-450 nm from cross-linked films on second order corrugated silica substrates (Λ = 200 nm). OLEDs were also prepared with a cross-linked emitting layer as part of a trilayer device which we report to have a maximum external quantum efficiency of 3.2% at 33 mA/cm(2) and a stable blue-violet emission with an electroluminescence maximum at 410 nm. Photopatternable PSF epoxides are also shown to be efficient hosts for Förster energy transfer and we provide examples of pattern layers incorporating small molecule emitters which emit in both the red and green while blue emission of the host is effectively suppressed.

Novel nano- and micro-textures for highly efficient outcoupling in white organic light emitting diodes

We demonstrate two approaches to significantly enhance the outcoupling in organic light emitting diodes. Nanostructures in combination with a microlens array or spherical texturing lead to efficiency enhancement factors of ~4 and ~3.7, respectively.

Multicolor silicon-quantum-dot light-emitting diodes

Quantum dots (QDs) based on different elements and compounds feature great promise for novel applications such as biolabeling or optoelectronic devices. A team of researchers recently reported efficient light-emitting diodes based on semiconductor QDs,1 while another group realized tunable QD lasers.2 However, the toxicity of the elements used for these devices—such as cadmium sulfide, cadmium selenide, and their lead-containing counterparts—is a severe drawback for many applications and may even impede the commercialization of these novel QDbased applications.3 Among the more promising candidates, silicon quantum dots (SiQDs) seem to be ideally suited to this task due to silicon’s non-toxicity, earth abundance, and its domination of the microelectronics and photovoltaics industry. As bulk silicon is an indirect semiconductor, it is only possible to achieve significant light emission under strong confinement conditions occurring for SiQDs with a size of about 5nm or less.4 Recent reports of very high photoluminescence quantum yields,5, 6 novel synthesis routes, colloidal stability, and lack of cytotoxicity of SiQDs7, 8 have done much to encourage study of LEDs based on these nanoparticles.9–11 Using separation by size-selective precipitation, as recently reported,5 we were able to synthesize allylbenzene-capped SiQDs of different sizes and achieve surprisingly narrow size distributions. In our study, the resulting quantum dots showed bright and intense photoluminescence (PL) with peaks at 680nm, 650nm and 625nm, corresponding to approximately 1.8nm-, 1.6nm-, and 1.3nm-sized SiQDs, respectively.5 We used these novel, efficiently luminescent, and colloidally stable SiQDs as part of a light-emitting material in hybrid organic-QD-based light-emitting diodes: see Figure 1(a) for a schematic representation of the device architecture. Figure 1. Device architecture and electroluminescence (EL) measurements. (a) Schematic representation of the stack of silicon-quantumdot light-emitting diodes (SiLEDs).12 (b) EL intensity over time at constant current of 1.6mA/cm2 for size-separated and not-sizeseparated silicon quantum dots for light emission, respectively. (c) EL spectra as a function of the applied voltage (3.5, 4.5, 8, and 10V). We achieved a reduced shift of the emission wavelength of about 15nm using size-separated samples. The inset shows a shift of the EL maximum averaged over three different SiLEDs. LiF/Al: Lithium fluoride/aluminium. TPBi: 1,3,5-tris(N-phenylbenzimidazol2,yl)benzene. SiQDs: Silicon quantum dots. Poly-TPD: Poly[N,N’bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine]. PEDOT:PSS: Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate). ITO: Indium tin oxide. a.u.: Arbitrary units.