Understanding the improved stability of hybrid polymer solar cells fabricated with copper electrodes

It is known that atmospheric oxygen is essential for realizing the photovoltaic properties of P3HT-TiO₂-based hybrid polymer solar cells because oxygen vacancies created in TiO₂ can become recombination sites for charge carriers, causing photovoltaic properties like open-circuit voltage (V(oc)) to decline quickly in an inert atmosphere. We demonstrate here that using an annealed Cu layer as hole collecting electrode results in a remarkably stable hybrid solar cell that maintains its photovoltaic parameters during 1 h of continuous testing in an inert atmosphere. An analysis of the data from photovoltaic device performance tests and X-ray photoelectron spectroscopy (XPS) attributes this improvement to the tendency of Cu to form sulfide-like complexes with the S atoms on P3HT, thereby inducing a chemically driven vertical segregation of P3HT toward the hole-collecting metal electrode. Additionally, XPS depth profiling analysis shows that Cu atoms can diffuse up to the TiO₂ layer and assist in filling up oxygen vacancies on the TiO₂ surface, thus eliminating defects that can act as donors of free electrons and degrade photovoltaic performance in an inert atmosphere. We analyze these improvements by examining in situ the effect of Cu on the P3HT and TiO₂ layers and on the organic-inorganic interface formed between them inside a hybrid solar cell.