Ultrafast differential transmission spectroscopy was employed to study the carrier dynamics in InGaN/GaN multiple quantum wells with high inverted hexagonal pits density due to threading dislocation. By monitoring the temporal evolution of the excitonic absorption spectrum, a reduction of the quantum-confinement Stark shift was observed due to the photo-induced in-well field screening at low carrier densities and excitonic absorption quenching at high carrier densities. By comparing the differential absorption spectra at various injected carrier densities, the in-well field screening effect was distinguished from excitonic bleaching.

We investigated the photovoltaic response of nanocomposites made of colloidal, infrared-sensitive, PbSe nanocrystals (NCs) of various sizes and conjugated polymers of either regioregular poly (3-hexylthiophene) (RR-P3HT) or poly- (2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEH-PPV). The conduction and valence energy levels of PbSe NCs were determined by cyclic voltammetry and revealed type II heterojunction alignment with respect to energy levels in RR-P3HT for smaller NC sizes. Devices composed of NCs and RR-P3HT show good diode characteristics and sizable photovoltaic response in a spectral range from the ultraviolet to the infrared. Using these materials, we have observed photovoltaic response at wavelengths as far to the infrared as 2 μm (0.6 eV), which is desirable due to potential benefits of carrier multiplication (or multi-exciton generation) from a single junction photovoltaic. Under reverse bias, the devices also exhibit good photodiode responses over the same spectral region.