Silicon-Bridged donor-acceptor compounds: synthesis and nonlinear optical properties

This thesis describes the synthesis and the spectroscopic and nonlinear optical characterization of a large series of donor- and acceptor-substituted diphenylsilanes with the structure DPh-(SiMez).-PhA and fragments thereof. Nonlinear optics (NLO) deals with the interaction of electromagnetic fields (light) with matter so as to generate new electromagnetic fields altered from the incident field with respect to phase, frequency, amplitude or other propagation characteristics. One of the applications involving nonlinear optical phenomena is frequency doubling of near infrared laser light (frequency a) by a nonlinear optical material into blue light. ... Zie: Summery

Radiative efficiency of lead iodide based perovskite solar cells

The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate

Photorefractive polymers with low intrinsic trap density

Two novel photorefractive polymers are presented, based on the charge transport molecule N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD). In one polymer the TPD unit is chemically modified so that it can function both as charge transport and as electro-optic molecule. In the other polymer the TPD is incorporated into the polymeric backbone and provides the charge transport and functions as a host for the dispersed electro-optic molecules. In both types of polymers the trap density is very low, which causing a 90 degrees phase shift between the refractive index grating and the illumination pattern and a rather small photorefractive performance. The trap density can in the case of the bifunctional molecule based polymer be increased by adding small amounts of N,N,N',N'-tetramethyl-paraphenylene diamine (TMPD) which has a lower ionization energy than the bifunctional molecule. This results in a strong increase of photorefractive performance and simultaneously in a lowering of the phase-shift. The ionization energy of some molecules used in photorefractive polymers is determined using gas-phase ultraviolet photoelectron spectroscopy and cyclovoltammetry. Using these ionization energies the process of the space-charge field formation can be understood on the basis of the components of the photorefractive polymer.</p

Effect of Plasticization on the Performance of a Photorefractive Polymer

A systematic study of the effect of plasticization on the photorefractive performance of a poly(N-vinylcarbazole) (PVK) based polymer composite is presented. It is shown that N-ethylcarbazole (ECZ) can be used as an efficient plasticizer, leading to a large increase of the gain coefficient and the diffraction efficiency, which arises solely due to an improvement in the orientational mobility of the dispersed nonlinear optical molecules. Phase separation, which is observed at samples with a high ECZ concentration, sets the upper limit. The steady state and the kinetics of the photorefractive gratings in the plasticized samples are discussed. The measurements suggest a high value of the saturation field and a response time that is limited by photogeneration. A configuration is described, where prisms are employed to couple light into and from the sample, resulting into a higher two-beam coupling gain and diffraction efficiency.

Transient photorefractive gratings in polymers

The transient behavior of photorefractive gratings in the polymer composite poly(N-vinyl carbazole) (PVK), 2,4,7-trinitro-9-fluorenone doped with the electro-optic molecules 4- (hexyloxy)nitrobenzene or N,N-diethyl-para-nitroaniline is presented. It is shown that despite the dispersive character of charge transport in polymers, information that allows the estimation of the hole drift mobility is obtained. The change in the hole drift mobility for samples with various trap densities, induced by 4-(diethylamino)benzaldehyde diphenylhydrazone, was measured