We describe results of our investigations of the structural, optical, and electronic properties of PbS-QD films fabricated using layer-by-layer dip coating based on 1,2-ethanedithiol as an insolubilizing agent. Our investigations extend to a study of the photovoltaic properties of heterojunction thin film solar cells fabricated by sputter-deposition of a CdS ntype thin film followed by deposition of a PbS-QD thin film. Our CdS/PbS-QD solar cells exhibit open circuit voltage in excess of previously reported PbS-QD solar cells. Under standard simulated AM1.5G illumination, we observe short circuit current density as high as 12 mA cm<sup>-2</sup>, open circuit voltage as high as 0.65 V, and a maximum efficiency of 3.3%.
We examine ultrafast photoconductivity in functionalized pentacene single crystals using optical-pump terahertz-probe techniques. The 0.5 ps rise time observed in the photoconductive transients, which is limited by the response time of the terahertz pulse setup, suggests that mobile charge carriers are a primary photoexcitation. The peak of the photoconductive signal increases as the temperature decreases due to higher carrier mobilities at lower temperatures. A lower limit for the carrier mobility of 1.6 cm<sup>2</sup>/Vs at 10 K and 0.2 cm<sup>2</sup>/Vs at room temperature is obtained. We further show that the absorption edge near the pump excitation wavelength of 800 nm remains temperature independent, and is therefore not a contributing factor in our observation of larger transient signals at lower temperatures. After an initial fast decay, a power-law decay is observed in the tail of the transient response from 2 to 600 ps. The dependence of the photoconductive response on the pump fluence and the electric field amplitude of the terahertz pulse are examined.
Finally, we show some preliminary results of transient photocurrent measurements on contact-biased samples using a fast oscilloscope with a system rise time of about 50 ps.