Half of the sun's power lies in the infrared. As a result, the optimal bandgaps for solar cells in both the single-junction and even the tandem architectures lie beyond 850 nm. However, progress in low-cost, large-area, physically flexible solar cells has instead been made in organic and polymer materials possessing absorption onsets in the visible. Recent advances have been achieved in solution-cast infrared photovoltaics through the use of colloidal quantum dots. Here we report stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared. The use of a strongly bound bidentate linker, benzenedithiol, ensures device stability over weeks. The devices reach external quantum efficiencies of 46% in the infrared and 70% across the visible. We investigate in detail the physical mechanisms underlying the operation of this class of device. In contrast with drift-dominated behavior in recent reports of PbS quantum dot photovoltaics, we find that diffusion of electrons and holes over hundreds of nanometers through our PbSe colloidal quantum dot solid is chiefly responsible for the high external quantum efficiencies obtained in this new class of devices.

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.

We report the realization of large-area, communications-wavelength electro-optic modulators made via simple solution-casting onto an arbitrary substrate. The devices employ colloidal quantum dots synthesized in, and processed from, the solution phase. Devices exhibit greater than 30% modulation depth at the 1.55 microm eye-safe wavelengths of interest in free-space optical communications. The devices retain considerable modulation depth beyond 1 MHz.