Polymer Photodetector Developed with High Detectivity Across Broad Spectral Range
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Polymer Photodetector Developed with High Detectivity Across Broad Spectral Range Photodetectors sensitive to the full UVvisible to near-infrared spectrum are very desirable for scientific and technological applications, but conventional semiconductor detectors are limited to narrower subbands of the spectrum. Full coverage thus requires separate sensors for different subbands, which is a complicated and costly solution at best. As published in the September 25 issue of Science (DOI: 10.1126/science.1176706; p. 1665), a group of researchers have addressed this problem with a new polymer photo detector (PPD) with broad spectral response and unusually high detectivity. X. Gong and A.J. Heeger from the University of California in Santa Barbara and CBrite Inc., Y. Cao from South China University of Technology, and their colleagues have built a PPD that delivers a response from 300 nm to 1450 nm in a device that incorporates a small bandgap conjugated polymer with a fullerenederivative in the active region. Gong and co-workers fabricated their
PPD by spin-casting a mixture of poly(5,7bis(4-decanyl-2-thienyl)-thieno (3,4-b) diathiazole-thiophene-2,5) (PDDTT) and (6,6)-phenyl-C61-butyric acid methyl ester (PC60BM) between an InSnO3 (ITO) anode and Al cathode. The PDDTT:PC 60BM forms an interpenetrating donor and acceptor bulk heterojunction network in the active layer, allowing efficient photogeneration and collection of carriers. The researchers said that shot noise from dark current limits detectivity and, under this assumption, attempt to maximize detectivity by incorporating additional materials in a multilayer geometry to block or suppress dark current. They fabricated a number of devices including layers of various polymers and fullerene-derivatives, and measured the photocurrent and dark current of each device at -100 mV bias. From these data they calculated the detectivity and found the best response from devices with layers of polystyrene-N, N-diphenyl-N,N-bis(4-n-butylphenyl)(1,10-biphenyl)-4,4-diamine-perfluorocyclobutane (PS-TPB-PFCB) and C60. The layering order of this device was ITO/PEDOT/PS-TPD-PFCB/PDDTT:
Pinning Down Superconductivity to a Single Layer Using precision techniques for making superconducting thin films layer-by-layer, G. Logvenov, A. Gozar, and I. Božovi´c of Brookhaven National Laboratory have identified a single layer responsible for one such material’s ability to become superconducting. The technique, described in the October 30 issue of Science (DOI: 10.1126/science.1178863; p. 699), could be used to engineer ultrathin films with “tunable” superconductivity for higher efficiency electronic devices. “We wanted to answer a fundamental question about such films,” said physicist and group leader Ivan Božovi´c. “Namely: How thin can the film be and still retain high-temperature superconductivity?” The thinner the material (and the higher its transition temperature to a superconductor), the greater its potential for applications where the superconductivity can be controlled by an external el
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