Solution processable polypyrrole nanotubes as an alternative hole transporting material in perovskite solar cells

The major challenge towards the commercialization of perovskite solar cells is to find an alternative stable, low-cost, and solution-processible hole transporting material to unstable and expensive 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine) 9,9'- spirobifluorene. Despite having all prerequisite properties like the ease of synthesis, low cost, tunability of hole conductivity, the conducting polymers such as polypyrrole are hardly being explored extensively as hole-transporting materials because of their insolubility in common organic solvents. Here, a colloidal solution of hydrophobic polypyrrole nanotubes co-doped with sodium dodecylbenzenesulfonate has been developed and explored for the hole transporting layer in methylammonium lead iodide-based perovskite solar cells. The best device has shown a power conversion efficiency of 7.3% at ambient conditions and without sealing. In terms of stability, the unsealed polypyrrole-based devices maintaining 85% of the initial efficiency at relative humidity 75-80 % with over 120 h have outperformed the unencapsulated poly(3-hexylthiophene)- based devices (average power conversion efficiency similar to 11 %) fabricated using the same procedure and measured under same conditions. Frequency response analysis and electroluminescent imaging reveal that the photovoltaic performance of the polypyrrole-based cells can further be improved by reducing series resistance arising due to hole transporting layers. These studies show the polypyrrole colloids as a promising low-cost solution for the hole transporting layer in perovskite solar cells.