Understanding polymer degradation under voltage stress in PV applications
AC Partial Discharge (PD) specification has been a key design parameter for PV backsheets, as called for in IEC 61730-1. In previous work, we have studied how the structure of polymeric materials can impact their behavior in this test. In this paper, we shift the focus to consideration of potential root causes which can lead to electrical degradation in actual usage (e.g. a PV module producing DC power), and the efficacy of the AC partial discharge test in air and oil to identify these. The suitability of utilizing AC voltage stresses in PV is also challenged in this paper. New experimental evidence is presented as to why testing partial discharge with AC with specimens immersed in the medium is not suitable for testing of solar backsheet materials. PD measurements can be very different for tests performed in air versus oil, and can lead to different pass/fail conclusions for the same material. When tested in oil, materials with voids had a much lower Ve than materials with no voids; in air, there was no difference in Ve for a material prepared with and without voids. With regard to AC versus DC testing the time to breakdown versus voltage was measured for both; these are not linearly related: the AC and DC breakdown voltages for one material study were 25 kV(AC) and 85 kV (DC), more than 3× difference; the voltages which led to breakdown after 10000 seconds were 5 kV(AC) and 50 kV (DC), a 10× difference. The issues raised in this study are active areas of discussion for the TC 82 WG2 Standards Committee.
Comparison of breakdown-voltage of contemporary and veteran photovoltaic backsheets
Abstract The ability of designated electrically insulating materials in a module, including the backsheet, to act as insulators is a key safety requirement for photovoltaic technology. The recently developed test standards IEC TS 62788-2 (including a method for direct-current breakdown-voltage) and IEC TS 62788-7-2 (for ultraviolet [UV] weathering) are examined for the first time together in this study. Thirty-six contemporary backsheets were compared using the breakdown-voltage test, before and after UV weathering, according to the recent A3 test condition. Twenty-eight backsheets, extracted from a variety of veteran photovoltaic modules, were also examined using the breakdown test. Additional characterizations were performed to interpret the breakdown-voltage results, including: surface and cross-sectional optical microscopy failure analysis; cross-sectional scanning electron (SEM) microscopy with EDS energy-dispersive X-ray spectroscopy (EDX) failure analysis; Fourier-transform infrared spectroscopy (FTIR) to identify the base resin and its chemical integrity; and differential scanning calorimetry (DSC) to identify phase transitions that might adversely affect degradation during weathering. The electrical insulation for the backsheets is discussed relative to: the industry safety requirement; features observed to facilitate the degradation of insulation; the validity of the artificial UV weathering test; the validity of the sheet geometry used for specimens in this study; and the validity of subsequent mechanical testing.
Resonant Auger decay study of C1s -> pi* core-excited OCS
The present work aims at characterizing short-lived C1s((-1)) pi*((1)) core-excited states of the OCS molecule based on the analysis of the vibrational fine structure and lineshape profiles of the high-resolution resonant Auger decay spectra recorded at the excitation energies along the C1s -> pi* resonance in the binding energy region 15-19eV. Very different behavior in terms of lineshape and resonant enhancement is observed for the (A) over tilde, (B) over tilde and (C) over tilde final states. This is explained by (1) the variation in the C-O bond lengths for the states involved in the electronic relaxation and (2) different contributions in terms of Mulliken population to the molecular orbitals determining the electronic character of the corresponding states. Since the final-state geometries are known from a number of previous experiments and ab initio calculations, the geometry of the C1s((-1)) pi*((1)) intermediate states can be predicted in analogy with e.g. the N-2, CO2 and N2O molecules. (C) 2009 Elsevier B.V. All rights reserved. (Less)