Nelson E. Coates

Solution-Processed Inorganic Solar Cell Based on in Situ Synthesis and Film Deposition of CuInS2 Nanocrystals

A novel, reproducible, and simple solution-based process for the fabrication of CuInS(2) absorber layers and CdS buffer layers for photovoltaics is presented. In this process, a precursor solution is deposited on a substrate, after which sintered NCs are formed in situ at temperatures as low as 250 degrees C. Solar cell efficiencies of up to 4% have been demonstrated using this novel fabrication method.

Effect of Interfacial Properties on Polymer–Nanocrystal Thermoelectric Transport

The electrical behavior of a conducting-polymer/inorganic-nanowire composite is explained with a model in which carrier transport occurs predominantly through a highly conductive volume of polymer that exists at the polymer-nanowire interface. This result highlights the importance of controlling nanoscale interfaces for thermoelectric materials, and provides a general route for improving carrier transport in organic/inorganic composites.

Carrier generation and transport in bulk heterojunction films processed with 1,8-octanedithiol as a processing additive

Improved performance of polymer-based solar cells based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzo-thiadiazole)] PCPDTBT has been obtained by using 1,8-octanedithiol (ODT) as a processing additive in the polymer solution used to spin cast the bulk heterojunction films. Although ultrafast spectroscopy studies indicate that the carrier losses are reduced in the films processed with ODT [similar to the reduced carrier losses after thermal annealing of bulk heterojunction (BHJ) materials made from P3HT:PCBM], the magnitude of the reduction is not sufficient to explain the observed factor of 2 increase in the power conversion efficiency. From the analysis of carrier transport in field effect transistors, we find increased electron mobility in the PCPDTBT:PC70BM composites when fabricated with ODT, which is indicative of enhanced connectivity of PC70BM networks. The improved electron mobility appears to be the primary origin of the improved power conversion ...

Diffusion and drift in terahertz emission at GaAs surfaces

We study terahertz (THz) emission from GaAs as a function of photon energy and electric field. THz radiation arises from transport of photogenerated charge in an electric field and by hot carrier diffusion (the photo-Dember effect). These mechanisms can be separated by experiments in which either the electric field or the kinetic energy of the carriers is varied. For electric fields E∼4 kV/cm, we find that the electric field controls THz emission for carrier temperatures kBTC⩽0.1 eV, while hot-carrier diffusion dominates for kBTC≈1 eV. Both mechanisms contribute at intermediate fields and carrier temperatures. Our results are consistent with estimates of the relative magnitudes of these two effects.

Efficacy of TiOx optical spacer in bulk-heterojunction solar cells processed with 1,8-octanedithiol

The efficacy of TiOx as an optical spacer in bulk-heterojunction (BHJ) solar cells made from regioregular poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester films processed with 1,8-octanedithiol is investigated. The results indicate that the enhanced efficiency resulting from the TiOx optical spacer is anticorrelated to the concentration of the processing additive. The use of the processing additive induces surface roughness on the BHJ film and thereby diminishes the efficacy of the optical spacer. Solar cells fabricated with both the optical spacer and 1,8-octanedithiol yield efficiencies that are superior to those with either process alone.

Visible-Near Infrared Absorbing Dithienylcyclopentadienone-Thiophene Copolymers for Organic Thin-Film Transistors

Structural design, synthesis, and characterization of a series of organic semiconductors consisting exclusively of dithienylcyclopentadienone subunits within a polythiophene backbone are described as the first example for organic electronic devices. The donor (thiophene)-alt-acceptor (cyclopentadienone) copolymers exhibit a substantial p-carrier mobility in OFET but an unexpected noncorrelation between absorption and photoconductivity.

Engineering Synergy: Energy and Mass Transport in Hybrid Nanomaterials

An emerging class of materials that are hybrid in nature is propelling a technological revolution in energy, touching many fundamental aspects of energy-generation, storage, and conservation. Hybrid materials combine classical inorganic and organic components to yield materials that manifest new functionalities unattainable in traditional composites or other related multicomponent materials, which have additive function only. This Research News article highlights the exciting materials design innovations that hybrid materials enable, with an eye toward energy-relevant applications involving charge, heat, and mass transport.

Solution‐Based In Situ Synthesis and Fabrication of Ultrasensitive CdSe Photoconductors

Solution-based semiconductor fabrication offers a low cost route for mass-producing optoelectronic devices. [ 1–3 ] Here, we present a simplifi ed solution-based in situ synthesis and fabrication method demonstrated for CdSe fi lms, where both the quantum dot synthesis and sintering occur simultaneously on a substrate in ambient atmosphere. The CdSe fi lms fabricated using this method exhibit high photoconductive gain and a relatively large signal bandwidth, stemming from a large photogenerated carrier mobility–lifetime ( μ τ ) product. We present a structural characterization of the CdSe fi lms, results of photoconductivity studies, and an analysis connecting the structural properties of the fi lm with the high photoconductive response. Our structural characterization indicates a high degree of crystalline order that results in a relatively large carrier mobility, and the temperature-dependent transient photoconductivity data suggest that sensitizing traps are responsible for the extended carrier lifetime. Photoconductive devices comprising inorganic quantum dots (QDs) have been developed extensively in recent years. [ 1 , 4–11 ]

Terahertz and infrared transmission of an organic/inorganic hybrid thermoelectric material

We report terahertz and infrared transmission measurements of a high-performance thermoelectric material containing tellurium nanowires in a conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) matrix. The DC electrical conductivity of the hybrid material (41 S/cm) is approximately one hundred times that of pure PEDOT:PSS and more than 400 times that of a film of pure tellurium nanowires, while the terahertz-frequency (THz) conductivity of PEDOT:PSS and the hybrid material are comparable at f ∼ 2THz. A frequency-dependent conductivity model indicates that the increased DC conductivity of the hybrid material results from an increase in the DC charge mobility rather than in the free charge density. We suggest that the increased DC conductivity of the hybrid material results from an increase in linkage between PEDOT domains by the tellurium nanowires.

Amorphous dithenylcyclopentadienone-carbazole copolymer for organic thin-film transistors

We developed a new high performance amorphous donor–acceptor conjugated copolymer consisting of a dithienylcylclopentadienone subunit as an electron acceptor and carbazole derivative as an electron donor. X-Ray diffraction analysis shows no scattering patterns, indicating that a disordered amorphous solid is formed. Atomic force microscopy (AFM) images show an amorphous surface mophology regardless of the annealing temperature. A high on/off current ratio of approximately 106 and high field effect mobility of 2.2 × 10−2 cm2 V−1 s−1 were obtained with stable output characteristics. The high performance of the amorphous copolymer is ascribed to the relatively low activation energy and low characteristic temperature obtained from a low temperature transport analysis, reflecting that localization of the charge carrier is not substantial in the film.