Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon 1 . However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance [2][3][4][5][6][7][8] . Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantumdot light-emitting diodes that operate at infrared wavelengths with radiances (6.4 W sr 21 m 22 ) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3 nm by varying the lengths of the linker molecules from three to eight CH 2 groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850 nm.Colloidal quantum dots have been proposed for the development of low-temperature solution-processed quantum-dot devices, including next-generation photovoltaics, photodetectors and lightemitting diodes (LEDs) [1][2][3][4][5][6][7][8][9][10][11] . In particular, the development of high-power, efficient and low-cost infrared LEDs will further progress in applications such as night vision, optical communications and sensing. Early efforts to exploit quantum dots in LEDs were based on hybrid device structures in which the quantum dots were interfaced with conjugated polymers. Quantum dots with long capping ligands were either mixed with an organic host or directly sandwiched between organic carrier-transporting layers to form the LED structure 4,5,8 . The operating mechanism of such devices is based mainly on Förster transfer, in which exciton energy transfers from the organic host to the quantum dots by means of a dipole-dipole interaction. Owing to the long capping ligands and low carrier mobility of the organic materials, these devices suffer from low current density, charge injection imbalance and exciton ionization caused by large applied bias voltages 12 .Recently, an infrared quantum-dot LED based on direct exciton generation through carrier injection achieved 1.15% external quantum efficiency (EQE), but the organic carrier-injection layer limited the current density and, as a result, the radiance 4). In visiblewavelength quantum-dot LEDs, inorganic charge-transport layers (ZnO:SnO 2 alloy for electrons and NiO for holes) have recently been used to increase the current density to a few amperes per square centimetre, with a consequent significant improvement in radiance 6 . These results directly reflect the impro...

We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date.

Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.