Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent.Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potentialinduced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

We report on a direct measurement of two-dimensional potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on grain boundaries, and the grain boundary is positively charged. The local built-in potential on the grain boundary is expected to increase the minority-carrier collection area from one to three dimensional. In addition, a work function decrease induced by Na on the film surface was observed.

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The photoelectrochemlcal properties of vacuum-deposited thin films of titanyl phthalocyanine (TiOPc) are described. Continuous films between 60 and 400 nm in thickness show behavior like that of a highly doped p-type semiconductor, with open-circuit photopotentials (K^) that vary linearly in magnitude with the formal potential of the contacting redox couple (up to 0.7 V). Comparison of photocurrent action spectra and transmission absorbance spectra showed that two distinct phases of TiOPc were deposited from vacuum, having different absorbance maxima at ca. 720 and 820 nm. The TiOPc phase with the more red-shifted absorbance appeared to possess higher photoactivity. X-ray photoelectron spectroscopy (XPS) of both TiOPc and vanadyl phthalocyanine films (VOPc) showed clearly the presence of extra oxygen forms for films vacuum deposited in moderate ( 6 Torr) vacuums, as opposed to the stoichiometric quantities of oxygen found in films deposited under ultrahigh vacuum.

The amine density of 3-aminopropyldimethylethoxysilane (APDMES) films on silica is controlled to determine its effect on DNA probe density and subsequent DNA hybridization. The amine density is tailored by controlling the surface reaction time of (1) APDMES, or (2) n-propyldimethylchlorosilane (PDMCS, which is not amine terminated) and then reacting it with APDMES to form a mixed monolayer. High-resolution X-ray photoelectron spectroscopy (XPS) is used to quantify silane surface coverage of both pure and mixed monolayers on silica; the XPS data demonstrate control of amine density in both pure APDMES and PDMCS/APDMES mixed monolayers. A linear correlation between the atomic concentration of N atoms from the amine and Si atoms from the APDMES in pure APDMES films allows us to calculate the PDMCS/APDMES ratio in the mixed monolayers. Fluorescence from attached DNA probes and from hybridized DNA decreases as the percentage of APDMES in the mixed monolayer is decreased by dilution with PDMCS.