Heralded as a major scientific breakthrough of 2013, organic/inorganic lead halide perovskite solar cells have ushered in a new era of renewed efforts at increasing the efficiency and lowering the cost of solar energy. As a potential game changer in the mix of technologies for alternate energy, it has emerged from a modest beginning in 2012 to efficiencies being claimed at 20.1% in a span of just two years. This remarkable progress, encouraging at one end, also points to the possibility that the potential may still be far from being fully realized. With greater insight into the photophysics involved and optimization of materials and methods, this technology stands to match or even exceed the efficiencies for single crystal silicon solar cells. With thin film solution processability, applicability to flexible substrates, and being free of liquid electrolyte, this technology combines the benefits of Dye Sensitized Solar Cells (DSSCs), Organic Photovoltaics (OPVs), and thin film solar cells. In this review we present a brief historic perspective to this development, take a cognizance of the current state of the art, and highlight challenges and the opportunities.

Perovskite based solar cells have demonstrated impressive performances. Controlled environment synthesis and expensive hole transport material impede their potential commercialization. We report ambient air synthesis of hole transport layer free devices using ZnO-GO as electron selective contacts. Solar cells fabricated with hole transport layer free architecture under ambient air conditions with ZnO as electron selective contact achieved an efficiency of 3.02%. We have demonstrated that by incorporating GO in ZnO matrix, low resistivity electron selective contacts, critical to improve the performance, can be achieved. We could achieve max efficiency of 4.52% with our completed devices for ZnO: GO composite. Impedance spectroscopy confirmed the decrease in series resistance and an increase in recombination resistance with inclusion of GO in ZnO matrix. Effect of temperature on completed devices was investigated by recording impedance spectra at 40 and 60 oC, providing indirect evidence of the performance of solar cells at elevated temperatures.

A B S T R A C T The well-known trade-off between strength and fracture toughness in bulk specimens is often used to explain the low fracture toughness of very thin ductile face-centred cubic metal specimens, but this interpretation contradicts the relative length scales of thickness-dependent strength and thickness-dependent fracture toughness. This study uses the concept of similitude to demonstrate that linear elastic fracture mechanics analysis of 25.4 μm thick annealed aluminium is invalid, although the resulting fracture toughness measurements fit well with the existing literature and idea of a strength/fracture toughness trade-off. Similarly, an elastic plastic fracture mechanics analysis is sensitive to out-of-plane deformation that cannot be practically eliminated or corrected for with a model. However, a plastic collapse analysis using a critical net section stress criterion is demonstrably valid by the concept of similitude, is insensitive to out of plane deformation, and agrees with the evidence of extensive plasticity in the fracture surfaces.Keywords ductile tearing; fracture mechanics; fracture toughness; plastic collapse; stable crack growth; thin sheet. N O M E N C L A T U R Ea = crack length in SE(T) specimen or half crack length in M(T) specimen Atot = total work applied to specimen A tot ¼ ∫ Δ 0 Pd Δ b = uncracked ligament length B, B 0 , B * = thickness, minimum thickness for plane strain, critical thickness J, J max = J-integral, maximum valid J measurement for a given specimen goemetry K = stress intensity factor K Ic , K R = fracture toughness in mode I loading, propagation toughness in K-R analysis L À = mean lineal intercept grain size LEFM, EPFM = linear elastic fracture mechanics, elastic-plastic fracture mechanics M(T) = middle crack tension specimen geometry P = force applied to specimen R = load ratio σ min /σ max used in fatigue precracking SE(T) = single edge crack tension specimen geometry t = traction acting normal to J-integral path Γ u = displacement vector at a point along path Γ w = width of SE(T) or half width of M(T) specimens W = strain energy density x, y = in-sheet coordinate axes perpendicular and parallel to tensile stress, respectively Y = geometric correction factor used in computation of K ¼ σ Y ffiffiffiffiffiffi π a p Correspondence: C. L. Muhlstein.

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