Alfred P. Marchetti

The effect of size restriction on silver bromide. A dramatic enhancement of free exciton luminescence

Abstract The effect of size restriction on the low temperature emission of silver bromide is examined. The critical size for the onset of detectable free exciton emission is determined. The effect of deliberately introduced iodide traps on free exciton luminescence was examined. Several reasons for the enhancement of the free exciton are discussed.

Optical properties of nanocrystalline silver halides

Abstract A review of quantum confinement effects in nanocrystals of silver bromide (AgBr) and silver iodide (AgI) is presented. AgBr is an indirect gap semiconductor while AgI has a direct band-to-band lowest energy transition. An examination of the low-temperature optical properties of quantum confined AgBr grown using a variety of synthetic techniques will be made. The dynamics of some of the involved excitonic processes will be measured and discussed in reference to a possible breakdown in the momentum selection rules as the nanocrystals are made smaller. Other explanations for this behavior such as impurity exclusion and surface effects will also be considered, as will the dynamics associated with the trapping of excitons at intrinsic iodide impurities in AgBr. Absorption measurements on AgI nanocrystals will be discussed and compared with the exciton photophysics in AgBr. Both AgBr and AgI display an increasing blue shift of their luminescence, arising from the recombination of excitons, as the crystallite size decreases. The luminescence intensity arising from this process increases with decreasing size in AgBr but it disappears in small crystals of AgI. This leads to the conclusion that in the latter material nonradiative decay channels are opening up as the size decreases.

Integer charge transfer states in organic light-emitting diodes: Optical detection of hole carriers at the anode∣organic interface

Hole carriers that reside at the anode∣hole-transport-layer (HTL) interface, creating a dipole layer, have been observed in organic light-emitting diode (OLED) and model devices using charge modulation spectroscopic techniques. These interfacial carriers have been observed with various HTL materials and several anode compositions at voltages such that the HTL is reverse-biased. The cation density at an ITO∣CFx∣NPB interface is estimated to be approximately 1×1013/cm2, or approximately 10% of a molecular layer. [NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl. CFx is a fluorocarbon polymer.] The cations represent the integer charge-transfer states whose presence has been inferred from ultraviolet photoelectron spectroscopy studies of various conductor∣organic interfaces. Anions of an organic hole-injecting material, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) were also observed. Conditions near the injecting contacts often determine the electric field profile and drive voltage of an OL...

Luminescence Properties of Indirect Bandgap Semiconductors: Nanocrystals of Silver Bromide

Abstract Using a modified reverse micelle protocol with acetyl bromide as the bromide source, AgBr nanocrystals can be grown over a size range from 30 A radius to bulk. These clusters exhibit quantum confinement indicated by an increasing blue shift of the free exciton emission peak as a function of size. As the cluster size decreases, a concomitant decrease occurs in the apparent radiative lifetime. This is predicted as the k selection rules break down at small crystal diameters. However, the effects are quite modest. indicating that even at these smallest sizes, the indirect character of the transition is largely retained.

Permanent polarization and charge distribution in organic light-emitting diodes (OLEDs): Insights from near-infrared charge-modulation spectroscopy of an operating OLED

Vapor-deposited Alq3 layers typically possess a strong permanent electrical polarization, whereas NPB layers do not. (Alq3 is tris(8-quinolinolato)aluminum(III); NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl.) The cause is a net orientation of the Alq3 molecules with their large dipole moments. Here we report on consequences for an organic light-emitting diode (OLED) with an NPB hole-transport layer and Alq3 electron-transport layer. The discontinuous polarization at the NPB|Alq3 interface has the same effect as a sheet of immobile negative charge there. It is more than compensated by a large concentration of injected holes (NPB+) when the OLED is running. We discuss the implications and consequences for the quantum efficiency and the drive voltage of this OLED and others. We also speculate on possible consequences of permanent polarization in organic photovoltaic devices. The concentration of NPB+ was measured by charge-modulation spectroscopy (CMS) in the near infrared, where the NPB+ has a stro...

23.1: Invited Paper: Luminescence Quenching by Charge Carriers in Organic Light-Emitting Diodes

We report photoluminescence quenching studies on model DCJTB-doped Alq3 electroluminescent devices. The spectroscopy versus doping density suggests that solvatochromic effects are minor but that energy transfer plays a large role. Quenching of DCJTB fluorescence is due to Forster energy transfer to DCJTB cations, and measurements of the bleaching recovery dynamics show that charge extraction from ionized DCJTB sites is slow and consistent with biexponential kinetics. Temperature-dependent studies find effective activation barriers for detrapping that are too small to be consistent with simple HOMO offsets between host and dopant.

Implications of Delayed Luminescence for Conjugated Polymer Photophysics

We observe unusual decay dynamics of persistent luminescence in a model oligofluorene. Generation of interchain triplet polaron pairs can account for the dynamics but not the reduction of photoluminescence in films relative to dilute solution.

Quantum Confinement Effects on 100–400 Å Diameter Silver Bromide Microcrystals

Previous work on quantum confined AgBr microcrystals has been limited to either large (500-1000 A diameter) aqueous samples or extremely small (60-100 A diameter) inhomogeneously sized micellar samples. This has prevented an investigation of the effect of size restriction on the phonon assisted transitions of the indirect exciton. A non-aqueous preparation was developed, capable of providing more homogeneous size distributions of AgBr microcrystals in the 100-400 A diameter range. Low temperature luminescence studies were performed to determine the changes in energy and intensity associated with these excitonic transitions. The radiative lifetimes of some of the excitonic processes were measured in order to obtain a more global perspective on the photophysics of AgBr.