Recent interest in quantum dots (QDs) stems from the plethora of potential applications that arises from their tunable absorption and emission profiles, high absorption cross sections, resistance to photobleaching, functionalizable surfaces, and physical robustness. The emergent use of QDs in biological imaging exploits these and other intrinsic properties. For example, quantum confined Stark effect (QCSE), which describes changes in the photoluminescence (PL) of QDs driven by the application of an electric field, provides an inherent means of detecting changes in electric fields by monitoring QD emission and thus points to a ready mean of imaging membrane potential (and action potentials) in electrically active cells. Here we examine the changing PL of various QDs subjected to electric fields comparable to those found across a cellular membrane. By pairing static and timeresolved PL measurements, we attempt to understand the mechanism driving electric-field-induced PL quenching and ultimately conclude that ionization plays a substantial role in initiating PL changes in systems where QCSE has traditionally been credited. Expanding on these findings, we explore the rapidity of response of the QD PL to applied electric fields and demonstrate changes amply able to capture the millisecond timescale of cellular action potentials.
The electronic structure of the interface formed by Mg deposition onto 2,5-bis(6’-(2’,2"-bipyridyl))-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) was investigated using ultraviolet, inverse, and X-ray photoemission spectroscopies. PyPySPyPy is of interest for use as an electron injection/transport layer in high efficiency organic light-emitting diodes. Upon deposition of Mg onto PyPySPyPy there is a shift of the occupied energy level structure to higher binding energy, away from the Fermi level, and appearance of two energy levels within the energy gap of PyPySPyPy. The lowest unoccupied molecular orbital is also shifted to higher binding energy.
We report the performance of molecular organic light-emitting diodes (MOLEDs) using silole derivatives as emissive and electron transport materials. Two siloles, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP) and 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane (2PSP), with high PL quantum yields of 94% and 85%, respectively, were used as emissive materials. Another silole, namely 2,5-bis-(2',2"-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was used as the electron transport material. MOLEDs using these two siloles and NPB as the hole transport material show a low operating voltage of approximately 4.5 V at a luminance of 100 cd/m2 and high external electroluminescence (EL) quantum efficiencies of 3.4% and 3.8%, respectively, at 100 A/m2. MOLEDs based on PPSPP exhibit a red-shifted EL spectrum which is assigned to an exciplex formed at the PPSPP:NPB interface.
Using a combination of ultraviolet and x-ray photoelectron spectroscopies (UPS, XPS), we have studied the relative energy level alignment of two phosphorescent guest molecules, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3), doped in an electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3), and in a hole transport host, 4,4'-bis(carbazol-9-yl)biphenyl (CBP). In each of the guest-host systems, we find that the vacuum levels of the guest and the host molecules align, and that the position of the highest occupied molecular orbital (HOMO) of the phosphorescent guest is independent of the guest molecule concentration (0.8 - 56% by mass) in the composite films. These results are used to shed light on possible electroluminescence mechanism(s) in the emissive layer of an organic light-emitting device utilizing the studied guest-host structures.
Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) aqueous dispersion (PEDOT:PSS) is a highly conductive polymer. Attempts to utilize them as an anode in an organic light emitting diode have been made but a satisfactory result was not obtained due to low device efficiency caused by the relatively high surface resistance and poor optical transparency. We recently reported that the conductivity of the PEDOT:PSS (Baytron P by Bayer) was dramatically increased without losing the optical transparency by addition of a small amount of a polyalcohol. Here, we present the improved I-V-L characteristics of the molecular organic light emitting diodes fabricated using the highly conductive and transparent PEDOT:PSS as an anode.
We report a photoemission study of the interfaces between spin-cast films of a new variation of a polymer blend consisting of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4'-bis(carbazol-9-yl)biphenyl (CBP),N,N'-diphenyl-N,N'-bis(1-naphthyl)-1-1'biphenyl-4,4'di amine (NPD) and N,N'-diphenyl-N,N'-bis(3methylphenyl)-1,1'-biphenyl-4,4'-dia mine (TPD). The hole injection barrier, as deduced from photoemission spectroscopy, is 0.5 - 0.9 eV at the PEDOT-PSS / HTL interface, which compares very well with the previously reported barrier heights for oxygen plasma -treated indium-tin oxide (ITO)/NPD and ITO/TPD interfaces, and which is, most notably, a factor of two smaller than barriers measured for a PEDOT-PSS/hole-transporting luminescent polymer, e.g. poly(bis-(2-dimethyloctylsilyl)-1,4-phenylvinylene, interface. The measured energy barriers imply a sufficiently efficient charge injection at the studied PEDOT-PSS/HTL interface, which is very encouraging for further development of anode structures based on similar conducting polymer blends and chemically modified structures to be utilized in molecular organic light-emitting device applications.
The charge transfer (CT) process in organic semiconductor thin film structures is an important problem for applications such as photoreceptors and light-emitting devices. The operation of a photoreceptor structure is based on a CT process between a donor molecule and an acceptor transport molecule. We have investigated such a structure formed by vacuum-grown thin films of two organic molecules, N,N'-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP) and N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'- biphenyl-4,4'-diamine (TPD), with femtosecond time-resolved photoemission spectroscopy. By measuring the lifetimes of the excited electron states in the mixtures of these molecules we are able to determine the time-scale for the electron transfer (ET) between the excited states of the TPD and DPEP molecules. We show that the biexponential ET dynamics consist of a short component of less than 100 fs and a long component of several hundreds fs in length.
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