We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Two experiments designed to assist in understanding the physics of certain back contacts on p-type CdTe solar-cell devices are described. In the first experiment, x-ray photoelectron and Auger electron spectroscopies are used to show that etching CdTe in HNO3:H3PO4 results in a Te layer on the CdTe surface. In the second experiment, photoemission spectroscopy is used to explore the electronic properties of evaporated Te deposited on thin-film, polycrystalline p-CdTe in an effort to develop a band diagram for the Te/p-CdTe interface. The motivation for developing the band diagram derives from previous observations that chemically etching polycrystalline p-CdTe solar-cell device material before application of the back contact reduces the series resistance of the device. The key results are that the evaporated Te overlayer is p type and that the valence-band offset between Te and p-CdTe is favorable for low-series-resistance contact, ΔEv=0.26±0.1 eV.

No abstract

A fundamental biogeochemical paradox is that nitrogen-rich tropical forests contain abundant nitrogen-fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen-fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen-for-phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen-for-phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen-fixing trees cannot provide a positive nitrogen-phosphorus-carbon feedback to alleviate nutrient limitation of the tropical carbon sink.

No abstract