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Performance losses and aging mechanisms are investigated in state-of-the-art PTB7:PC70BM solar cells. Inverted devices incorporating a vanadium pentoxide (V2O5) top contact have efficiencies of 8%. After aging the unencapsulated devices, no changes are observed in the open circuit voltage ( Voc) or short circuit current ( Jsc); however, the fill factor (FF) drops from 0.7 to 0.61. An s-shape initially appears in the J- V curve after aging, which can be reduced by cycling through the J- V curve under illumination. This is discussed in context of the redox properties of V2O5. With impedance spectroscopy, it is demonstrated that changes to the contact interfaces are completely reversible and not responsible for the performance loss. Intensity modulated photocurrent spectroscopy combined with device modeling reveals that the loss in FF is due to trap formation in the active layer. Additionally it is observed that the performance of pristine devices is limited by optical absorption in the thin active layer and the build-up of space charge which hinders carrier extraction. [ABSTRACT FROM AUTHOR]

Two-dimensional (2-D) ray tracing has been used to investigate the influence of inclined cutting front and the cutting edge on the exiting, surplus, laser beam while cutting 6 mm thick stainless steel sheet with a 1070 nm wavelength fibre laser. The ray tracing model, which assumed a Gaussian beam, has investigated the distribution of this surplus beam as a function of kerf wall inclination angle. Multiple reflections are shown to broaden the spatial distribution of the surplus beam. Dross and kerf wall roughness also broaden the spatial distribution of the surplus beam. Comparison of an experimentally obtained beam print from the surplus beam for 1070 nm fibre laser cutting of 6 mm thick stainless steel with modelled results allows inclination of the cut front to be estimated. The experimentally obtained beam print has features consistent with roughness/dross induced beam broadening. [ABSTRACT FROM AUTHOR]