Hierarchically Structured Hole Transport Layers of Spiro-OMeTAD and Multiwalled Carbon Nanotubes for Perovskite Solar Cells

The low electrical conductivity of spiro-OMeTAD hole transport layers impedes further enhancements of the power conversion efficiency (PCE) of perovskite solar cells. We embedded multiwalled carbon nanotubes (MWNTs) in spiro-OMeTAD (spiro-OMeTAD/MWNTs) to increase carrier mobility and conductivity. However, direct electrical contact between CH₃NH₃PbI₃ and the MWNTs created pathways for undesirable back-electron transfer, owing to the large work function of MWNTs, limiting enhancements of the PCE. A hierarchical structure of pure spiro-OMeTAD and spiro-OMeTAD/MWNTs was designed to block back-electron transfer and fully exploit the enhanced charge transport of spiro-OMeTAD/MWNTs. The enhanced fill factor, short-circuit current density, open-circuit voltage, and PCE (15.1) were achieved by using this hierarchical hole transport layer structure (MWNT concentration=2wt). The perovskite solar cells were fabricated by a lowerature solution process, further decreasing their per-Watt cost. Respect the hierarchy! Multiwalled carbon nanotubes (MWNTs) are embedded in spiro-OMeTAD to improve electrical conductivity. Perovskite solar cells employing a hierarchical hole transport layer (HTL) structure, comprising pure spiro-OMeTAD and spiro-OMeTAD/MWNT layers, show better blocking of back-electron transfer while fully exploiting the enhanced conductivity of the spiro-OMeTAD/MWNT material. The power conversion efficiency of these perovskite solar cells is improved, to 15.1.