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We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.

We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.

Photoconductive atomic force microscopy is used to investigate nanoscale incident photon-to-current efficiency spectra of polymer bulk heterojunction solar cells based on poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Nanoscale external quantum efficiency reveals the complex morphology of MDMO-PPV:PC71BM films cast from toluene solution. Not only electron transfer from the photoexcited donor to the fullerene but also hole transfer process from photoexcited fullerene to the donor phase due to highest occupied molecular orbital offset is observed. The difference in performance between toluene and chlorobenzene-cast devices is explained by the variation in relative contributions from two charge transfer mechanisms.

Electroabsorption spectroscopy was used to directly probe the electric fields in a polymer light emitting diode that utilizes a conjugated polyelectrolyte electron transporting/injection layer. The electric field in the emitting layer was found to be negligible at applied biases greater than the built-in field of the device. Holes injected at these biases accumulate at the emitting layer/conjugated polyelectrolyte interface and screen the field from the emitting layer to the conjugated polyelectrolyte layer. In conjunction with mobile ions that redistribute the field in the conjugated polyelectrolyte layer, this leads to greatly improved electron injection from high work function cathodes.

The authors poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester present a technique for modifying the internal structure of bulk heterojunction films which yields significantly improved photoconductivity. This method comprises the addition of alkyl thiol molecules to the polymer/fullerene solution prior to spin coating. Based on the steady-state and transient photoconductivity measurements, the photoresponsivity is an order of magnitude greater than that in films spun from pristine solvent; additionally, the carrier mobility measurements indicate significant increase in the hole mobility, consistent with the enhanced structural order inferred from x-ray diffraction and optical absorption measurements.