Energy Focus: Quantum efficiency improved in bulk heterojunction PVs
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Energy Focus Quantum efficiency improved in bulk heterojunction PVs
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lthough bulk heterojunction photo voltaics (BHJs) are attractive as low-cost solar cells due to their ease of processability, devices incorporating lowbandgap quantum dots (QDs) fall short of the performance achieved by analogous blends of polymers and CdSe QDs. Previously, D.S. Ginger, S.A. Jenekhe, and co-researchers at the University of Washington, Seattle, hypothesized that the poor performance by BHJs composed of PbS QDs blended with common conjugated polymers is due to insufficient photoinduced charge transfer at the organic–inorganic interface. Recently, Ginger and co-researchers showed that BHJs made from PbS QDs and new donor–acceptor polymers exhibit efficiencies two orders of magnitude greater than those observed for blends of PbS with conventional host polymers. As reported in the July 14th issue of Nano Letters (DOI: 10.1021/nl1013663; p. 2635), Ginger and co-researchers selected three polymers (PDTPQx, PDTPPPz, and PDTPBT; see figure) to blend with PbS QDs because their ion ization potentials show that their highest occupied molecular orbitals lie within the PbS bandgap. Photoinduced absorption (PIA) spectroscopy shows that in the
Normal modes and density of states achieved in disordered colloidal solids
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olloidal suspensions have been used as model systems in experimental research on the fundamentals of statistical mechanics. In colloidal systems,
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VOLUME 35 • OCTOBER 2010
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with increasing Tb3+ content. Excited at 330 nm, the Commission International de l’Eclairage (CIE) chromaticity coordinates shifted from (0.25, 0.33) to
(0.40, 0.52) as the value of y increased from 0.1 to 0.6 in the Mg(Y3.8-yCe0.2Tby) Si3O13 materials. Melissa A. Harrison
range of 0.8 eV to 2.2 eV BHJ blends of PDTPQx and PbS QDs exhibit new sub-bandgap absorptions by the PDTPQx and a bleach of the polymer’s bandgap transition following photoexcitation, while the PDTPBT blend shows very weak PIA signal, and the PDTPPPz blend shows no detectable PIA signals. Because neat PDTPQx shows no PIA signal, the researchers attribute the blend’s PIA spectrum to long-lived polarons on the PDTPQx polymer chains created by photoinduced electron transfer from the polymer to the PbS QDs, leading them to predict PDTPQx/PbS to perform significantly better in BHJ photodiodes than PbS QDs blended with either of the other two polymers considered. The researchers verified this prediction by showing that the quantum efficiencies exhibited by the PDTPQx/PbS blends are two orders of magnitude higher than those exhibited by blends with the other two polymers. Under simulated AM 1.5 illumination, the researchers estimated a power conversion efficiency (PCE) of about 0.55%, which is modest in comparison to polymerfullerene BHJ cells but is significantly higher than BHJ devices made from previous polymer blends with low-bandgap QDs. The researchers plan to improve performance by increasing the PDTPQx molecular weight to facilitate thicker fil
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