Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 10(3) A W(-1). The best devices exhibited a normalized detectivity D* of 1.8 x 10(13) jones (1 jones = 1 cm Hz(1/2) W(-1)) at 1.3 microm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 10(12) jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 10(11) jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

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.

PbS colloidal quantum dot photovoltaic devices in a Schottky architecture have demonstrated an infrared power conversion efficiency of 4.2%. Here, we elucidate the internal mechanisms leading to this efficiency. At relevant intensities, the drift length is 10 m for holes and 1 m for electrons. Transport within the 150 nm wide depletion region is therefore highly efficient. The electron diffusion length of 0.1 m is comparable to neutral region width. We quantitatively account for the observed 37% external quantum efficiency, showing that it results from the large depletion width and long carrier lifetime combined.

Crosslinking molecules have recently been combined with colloidal quantum dots to build robust, closely packed, conductive solid-state devices. Ethanedithiol ͑EDT͒ has been used in PbS quantum dot photovoltaic devices to assist in film formation during fabrication. However, there is evidence that EDT influences the electronic properties of the colloidal quantum dot ͑CQD͒ films. We fabricate thin film field-effect transistors and find that EDT treatment increases the majority carrier mobility by a factor of 10. We attribute this increase to a reduction in interparticle spacing which we observe using transmission electron microscopy. However, this increase is accompanied by a decrease in the majority carrier concentration. Using x-ray photoelectron microscopy, we find that EDT reduces the extent of the surface oxidation which is acting as a p-type dopant in these materials. We find that by lightly reoxidizing, we can redope the CQD films and can do so without sacrificing mobility gains.

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.