Cohesion and adhesion of nanoporous TiO2 coatings on titanium wires for photovoltaic applications
The working electrode of a dye-sensitized photovoltaic fiber is constituted of a porous TiO2 coated titanium wire. The cohesion and adhesion of such a brittle oxide coating on the ductile metal substrate are identified as crucial factors in maintaining photovoltaic efficiency during textile manufacture and weaving operations. The influence of coating thickness on these factors has been investigated in the present work. The tensile mechanical characterization with in situ microscopic observations shows that two damaging processes are involved. For the smaller thickness, loss of adherence appears to be at the interface and inside the coating bulk. Cracks become visible in a random distribution in size and density and do not cross the entire coating circumference. Large patches of coating are still anchored on the wire. For the larger thickness no cohesive rupture in the coating bulk has been observed. The loss of adherence appears at the interface closed to the cracks and grows rapidly as the strain increases. Numerical investigations based on the finite element method permit to analyze the distribution and the combination of radial interfacial stress and circumferential coating stress and their influence on the observed damage.