We analyze the 3D structure of a 120×38 nm disc-shaped region of a PbSe QD epi-SL using full-tilt high-angle annular dark-field electron tomography. The high spatial resolution enables determination of the center-of-mass coordinates of all 1846 QDs in the sample as well as the size and shape of the thousands of epitaxial connections (necks) between the QDs. A map of the neck network is used to quantify relationships between neck number (the number of necks each QD possesses), average neck diameter, QD location in the film, and the nearest neighbor inter-QD distance and distance distribution. An electronic model demonstrates how neck distribution and SL defects reduce charge mobility.
The largest need in organic electronic devices is a universal method to produce micro- to nano-scale features for devices from semiconducting polymers cheaply, and at scale. We recently developed a solution method to optically pattern conjugated polymers with resolution that exceeds the linear Abbe diffraction limit. We examine the relationship between optical write intensity, write speed, and write wavelength on the resulting pattern fidelity. Finite element modeling reveals that nearly all patterning occurs as a result of local heating and superlinear resolution is a result of a highly non-linear dissolution rate for the polymer as a function of temperature. This result is general to any conjugated polymer. We used this new technique to fabricate P3HT/F4TCNQ nanowires We also demonstrate that a P3HT nanowires can be doped and de-doped from solution without changing the dimension of the wire.
Sequential solution doping is a processing technique that allows a conjugated polymer film to be doped from a solvent that will not dissolve the polymer. We present here a method to predict the film doping level in cm-3 from the solution concentration used to dope the film. We show using four polymers and three different and newly synthesized dopants that the doping level can me modeled using a simple Langmuir isotherm. In addition, analysis of the UV/vis spectra shows filling of the density of states. Polymers with a sharper band edge demonstrate much high conductivity for the same hole density. We analyze a series of DPP polymers and show how the polymer order changes as a function of the doping level. A second recent discovery is that the anion in sequentially doped films can be exchanged with another anion after doping. This means that the reactive molecule used to doped the polymer can be removed and replaced with a different ion that is not reactive. We present a multi-ion Langmuir isotherm model and show that the film doping level in mixed ion solutions can also be predicted.
Organic electronics promise to provide flexible, large-area circuitry such as photovoltaics, displays, and light emitting diodes that can be fabricated inexpensively from solutions. A major obstacle to this vision is that most conjugated organic materials are miscible, making solution-based fabrication of multilayer or micro- to nanoscale patterned films problematic. Here we demonstrate that the solubility of prototypical conductive polymer poly(3-hexylthiophene) (P3HT) can be reversibly “switched off” using high electron affinity molecular dopants, then later recovered with light or a suitable dedoping solution. Using this technique, we are able to stack mutually soluble materials and laterally pattern polymer films using evaporation of dopants through a shadow mask or with light, achieving sub-micrometer, optically limited feature sizes. After forming these structures, the films can be dedoped without disrupting the patterned features; dedoped films have identical optical characteristics, charge carrier mobilities, and NMR spectra as as-cast P3HT films. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.
Although organic light emitting diodes are generally well characterized, their mechanism of decay, e.g. formation of black spots, is still not fully understood. Here we present a new technique allowing for deeper insight into the degradation process of an OLED by measuring its photovoltaic properties. The results show the possibility to record maps of crucial photovoltaic values with a lateral resolution of 50 microns. Based on these results, we propose a mechanism for the decay process. Black spots in the device are formed during the fabrication process, and the lifetime is determined by the active materials' chemical stability.
At present, heterojunction polymer solar cells are typically fabricated with an active layer thickness of approximately 80 nm to 100 nm. This active layer thickness has traditionally been chosen based upon convenience and empirical results. However, a detailed mechanistic study of the effects of active layer thickness on the short circuit current and efficiency has never been performed for polymer solar cells. We demonstrate that using the high mobility materials regio regular poly(3-hexylthiophene and [6,6]-phenyl (P3HT) and C61-butyric acid methyl ester (PCBM), that high efficiency solar cells can be fabricated with active layer thickness greater than 100 nm. Devices with an active layer thickness of 200 nm are fabricated with a power efficiency of 4.1% under AM1.5 illumination at and intensity of 80 mW/cm2. In addition, we explain the variation in short circuit current density as a function of thickness using calculations of the distribution of the optical electric field intensity as a function of device thickness.
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