PbS nanocrystal ͑nc-PbS͒-C 60 photovoltaic devices are demonstrated, in which nc-PbS function as electron donors, showing infrared photosensitivity up to 1600 nm. Annealing nc-PbS is proved to remove capping oleic acid ligands, studied using x-ray photoelectron spectroscopy, significantly improving the short circuit current, open circuit voltage, and fill factor. The device performance is rationalized by quantum confinement in nc-PbS and energy level alignment at the heterojunction based on direct measurements of nc-PbS ionization potential using ultraviolet photoelectron spectroscopy.

We report the use of PbS nanocrystals within a hybrid device that emits 1.2 m electroluminescence with an external quantum efficiency of 1.15% corresponding to an internal quantum efficiency of ϳ5 % -12% thus demonstrating a viable, low-cost, highly efficient near infrared organic electroluminescent device. Direct generation of the excited state on the nanocrystal result in eliminating competing processes that have previously led to the low reported efficiencies in near-infrared light emitting devices. Furthermore, the emission wavelength can be tuned to cover a wide range of wavelengths including the 1.3-1.5 m region without significant change of the efficiency. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2909589͔Organic molecules are generally optically active in the range of wavelengths below 1 m due to the fundamental properties of delocalized systems. Even when modified through substitution, or ligation of heavy metal atoms, significant extension of optical activity beyond 1 m is difficult. Near-infrared ͑NIR͒ organic/polymer devices emitting electroluminescence ͑EL͒ above 1 m would have a number applications including ͑bio͒sensing, waveguide and free space chip-to-chip optical circuits, and telecommunications. These factors have driven significant research into extending the optical activity of organic materials into this wavelength region despite the inherent difficulties.The most studied approach has been the use of organolanthanide complexes in which intra-atomic 4f-4f transitions are used to obtain characteristic narrow emission in the NIR from ions such as Er 3+ and Nd 3+ . 1-3 However, devices based on this concept have proved to be highly inefficient ͑Ͻ0.01% ͒ due to poor excitation of the lanthanide ion and strong competing nonradiative relaxation mechanisms. 4,5 The development of colloidal quantum dots consisting of semiconductor nanocrystals 6-8 has provided an alternative approach toward obtaining a low cost NIR electroluminescent device. The ability to directly modify the bandgap through selection of the nanocrystal material and diameter allows for emission to be obtained from the visible and through the NIR region. The incorporation of nanocrystals within organic devices as the emitting center allows low-cost solution processing to be used and direct access to the NIR region. To date, the highest reported external quantum efficiency ͑EQE͒ from such hybrid devices are 0.5% ͑Ref. 9͒ ͑emitting ϳ1.3 m͒ and 0.27% ͑Ref. 10͒ ͑emitting ϳ1.15 m͒ utilizing a MEH-PPV-PbS and MEH-PPV-InAs composite architecture. In these devices, the excited nanocrystal state is achieved through exciton transfer from nearby molecules. As a result, the quantum efficiency relating to emission originating from the nanocrystals is critically dependent upon this energy transfer step which must compete against radiative and nonradiative recombination of excitons prior to energy transfer. Given the short range dependence of Förster or Dexter energy transfer, systems that rely upon these methods to excite the nanocryst...

Abstract-Solution processed photovoltaic devices are an attractive alternative to costly inorganic semiconductor based conventional photovoltaics. Solution processable organic photovoltaic systems are affected by low carrier mobility, lifetime issues under ambient conditions and limited optical absorption due to the high bandgaps of organic materials. Nanostructured inorganic materials promise to alleviate some of these drawbacks, by enabling the hybrid systems to perform better in a commercial perspective. This review examines four key areas of hybrid organic-inorganic photovoltaic systems. These are metal oxide-organic, carbon nanotube-organic, semiconductor nanowire-organic and semiconductor nanocrystal-organic systems, which are showing growing importance and potential in the literature. Recent advances in terms of device performance for these respective topics are reviewed, along with an outlook for each hybrid system.

Cyclic voltammetry was used to measure PbS nanocrystal ͑PbS-NC͒ energy levels. Accuracy of these measurements was justified by electron transfer experiments, with well known fullerene derivatives, which included photoluminescence quenching experiments and current density-voltage measurements of hybrid photovoltaic devices. It is believed that these energy level measurements, carried out on PbS-NCs under ambient conditions, would provide valuable information for the design and fabrication of optimized hybrid nanoelectronic devices. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2964203͔ Narrow bandgap colloidal semiconductor nanocrystals ͑NCs͒ are actively researched for their size depended optical properties and solution processibility. 1,2 NCs can be incorporated with organic semiconductors to fabricate low cost, flexible, and large area optoelectronics and photonic hybrid devices. 3,4 A fundamental drawback in the design of hybrids is the accurate understanding of energy level positions, which govern charge transfer at NC-organic interfaces. Mostly, NC band diagrams reported in literature for hybrid devices are obtained by calculations, based on bulk ionization potential ͑IP͒ and electron affinity ͑EA͒. 1 Direct measurement of the IP, using photoemission spectroscopic techniques, has been reported for PbS-NCs. 3 However, a major problem encountered in such studies was the detrimental effect caused by the passivating organic ligands, capping the NCs, on photoemission measurements and subsequent analysis. Insulating organic ligands caused charge buildup on the substrates as well as screened NCs from exposure to ultraviolet photons, severely affecting the accuracy of the measurements. Consequently, these NCs were annealed in ultrahigh vacuum to overcome such problems and the calculated energy levels showed reasonable agreement to the device performance. However, these annealing techniques were reported to alter the surface ͑bulk͒ condition as well as the stoichiometry of the NCs from its condition present at device fabrication. 3 Therefore, less invasive techniques such as electrochemistry have to be employed for measurement of these NC energy levels. Kucur et al. reported IP and EA values for different size CdSe NCs using cyclicvoltammetry. 5 Furthermore, differential pulse voltammetry, which demonstrate greater sensitivity than cyclicvoltammetry, has been used previously for determining the subtle changes of the NC energy levels, with different ligands, proving the versatility of electrochemical techniques. 6 In this paper, we utilize a technique of electrochemistry to investigate the energy levels of PbS-NCs and probe the accuracy of the obtained values by charge transfer experiments. Current density-voltage ͑JV͒ characteristics of hybrid photovoltaic ͑PV͒ devices fabricated using C 60 and soluble C 60 derivative of ͓6,6͔-phenyl-C 61 -butyric acid methyl ester ͑PCBM͒, and photoluminescence ͑PL͒ quenching experiments were used for determining the electron transfer ͑ET͒ from PbS-NCs justifying the energy ...

A near-infrared sensitive hybrid photovoltaic system between PbS nanocrystals (PbS-NCs) and C(60) is demonstrated. Up to 0.44% power conversion efficiency is obtained under AM1.5G with a short circuit current density (J(sc)) of 5 mA cm(-2) when the PbS-NC layer is treated in anhydrous methanol. The observed J(sc) is found be approximately one-third of the maximum expected from this hybrid configuration, indicating the potential for further optimization. Crucial for device operation, a smooth film of nanocrystals is seen to form on the hole transporting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer deposited on the transparent electrode, facilitated through an ionic interaction between nanocrystal capping ligands and the PEDOT:PSS. The formation of the open circuit voltage in this system is seen to be influenced by an interfacial dipole formed at the hole-extracting electrode, providing insights for further optimization.