In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond approximately 800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.
Living polymerizations involve the creation of polymer chains without significant irreversible chain transfer or chain termination. Such processes are widely used to access well-defined macromolecular materials with controlled architectures, such as block and star polymers. Although this concept was first realized for anionic polymerizations in the 1950s, many key recent advances have been made, most notably in the area of radical polymerization. Here, we report a living photopolymerization that involves photoexcited monomers. Exposure of metal-containing ferrocenophane monomers to Pyrex-filtered light from a mercury lamp (lambda>310 nm) or to bright sunlight in the presence of an anionic initiator leads to living polymerizations, in which the conversion and molecular weight of the resulting polymer can be controlled by the irradiation time. Photoirradiation selectively weakens the iron-cyclopentadienyl bond in the monomer, allowing the use of moderately basic and highly functional-group-tolerant initiators. The polymerization proceeds through attack of the initiator and propagating anion on the iron atom of the photoexcited monomer and, remarkably, the polymerization rate decreases with increasing temperature. Block copolymer formation is possible when the light source is alternately switched on and off in between sequential addition of different monomers, providing unprecedented, photocontrolled access to new types of functional polymers.
Solution-processed thin-film organic, inorganic, and hybrid photovoltaic devices have achieved power conversion efficiencies as high as 5%. However, these devices remain limited by their capture of visible energy; more than a half of the sun's power lies in the infrared. Herein the authors demonstrate photovoltaic devices effective across the visible and all the way out to 1700 nm. Only through the use of ethanedithiol as a bridging molecule to affect interparticle linking were they able to achieve fabrication of smooth, continuous quantum dot films on rough, high-surface area transparent metal oxides. This allowed them to increase light absorption while maintaining efficient charge separation and extraction and at the same time avoiding electrical short circuits. They obtained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaics. The authors demonstrate quantum size-effect tuning of device band gaps relevant to multijunction solar cells.
We report a comparison of photoconductive performance of PbS nanocrystal/polymer composite devices containing either oleic acid-capped or octylamine capped nanocrystals (NCs). The octylamine-capped NCs allow over two orders of magnitude more photocurrent under −1V bias; they also show an infrared photovoltaic response, while devices using oleic acid-capped NCs do not. Further improvement in the photovoltaic performance of films made with octylamine-capped NCs occurs upon thermally annealing the composite layer at 220 °C for 1 h. The procedure leads to a 200-fold increase in short circuit current, a 600-fold increase in maximum power output, and an order of magnitude faster response time.
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