Exciton states in lead selenide (PbSe) and lead sulfide (PbS) quantum dots have been studied extensively. However, relatively less attention has been paid to the states within the quantum dot bandgap. Our experimental results have revealed a single in-gap state which bears confinement dependence yet cannot be explained by dark exciton theory, nor is it a trap state related to quantum dot surface defects as previously observed. A detailed analysis of the temperature dependence of photoluminescence, Stokes shift, absorption and photoinduced absorption indicates the unconventional GS is a new state of a trapped exciton in a QD film. With appropriate design engineering, these trapped excitons might be harvested in solar cells and other optoelectronic devices.

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We report the transient positive photo annealing effect in which over 600% boost of power conversion efficiency was observed in inverted organic photovoltaic devices (OPV) made from P3HT/PCBM by spray method, after 2 hrs of constant solar AM 1.5 irradiation at low temperature. This is opposite to usual photodegradation of OPV, and cannot be explained by thermal activation alone since the mere temperature effect could only account for 30% of the enhancement. We have investigated the temperature dependence, cell geometry, oxygen influence, and conclude that, for p-doped active layer at room temperature, the predominant mechanism is photo-desorption of O2, which eliminates electron traps and reduces space charge screening. As temperature decreases, thermal activation and deep trap-state filling start to show noticeable effect on the enhancement of photocurrent at intermediate low temperature (T = 125 K). At very low temperature, the dominant mechanism for photo annealing is trap-filling, which significantly reduces recombination between free and trapped carriers. At all temperature, photo annealing effect depends on illumination direction from cathode or anode. We also explained the large fluctuation of photocurrent by the capture/reemit of trapped electrons from shallow electron traps of O2- generated by photo-doping. Our study has demonstrated the dynamic process of photo-doping and photo-desorption, and shown that photo annealing in vacuum can be an efficient method to improve OPV device efficiency.

We have developed an innovative way to fabricate organic solar arrays for application in dc power supplies for electrostatic microelectromechanical systems devices. A solar array with 20 miniature cells interconnected in series was fabricated and characterized. Photolithography was used to isolate the individual cells and output contacts of the array, whereas the thermal-vacuum deposition is employed to make the series connections of the array. With 1mm2 for single cell and a total device area of 2.2cm2, the organic solar array based on bulk heterojunction structure of π-conjugated polymers and C60 derivative [6,6]-phenyl C61 butyric acid methyl ester produced an open-circuit voltage of 7.8V and a short-circuit current of 55μA under simulated air mass (AM) 1.5 illumination with an intensity of 132mW/cm2. The procedure described here has the full potential for use in future fabrication of microarray with the size as small as 0.01mm2.

A laser-assisted spray process was developed to deposit surfactant-free PbSe quantum dot (QD) films directly on a substrate. These QDs are in close contacts with each other, forming a percolation path for charge transport. Absorption spectroscopy confirmed the quantum confinement of the deposited particles. Room temperature current-voltage measurements across a 2 μm tunnel junction formed by the QDs produced a power-law dependence of the form I∝V2.25, which describes a percolation path of dimensionality slightly above two dimensions. Temperature dependent conductance studies showed thermally activated transport at high temperatures and temperature independent tunneling, followed by previously unobserved metallic conduction at low temperatures.