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High photovoltaic device performance is demonstrated in ambient-air-processed bulk heterojunction solar cells having an active blend layer of organic poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state-of-the-art bulk heterojunction devices fabricated in air-free environments. High-resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post-fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an {approx}4 nm aluminum oxide hole-blocking layer present at the electron-collecting contact interface.

High-resolution transmission electron microscopy (HRTEM) and cross-sectional transmission electron microscopy (XTEM) are used to characterize common defects in wurtzite GaN nanowires grown via a vapor-liquid-solid (VLS) mechanism. HRTEM shows that these nanowires contain numerous (001) stacking defects interspersed with small cubic GaN regions. Using XTEM, bicrystalline nanowires are discovered with twofold rotational twin axes along their growth directions, and are found to grow along high-index directions or vicinal to low-index planes. We propose a defect-mediated VLS growth model to qualitatively account for the prevalence of these extended defects, and discuss the implications of these defects for nanowire growth kinetics and device behavior.