A new molecular material as a dopant-free hole-transporting layer for stable perovskite solar cells

In this work, a new small molecular material, 9,9'-(pyrene-1,6-diyldimethylylidene)bis[N,N,N ',N '-tetrakis(4-methoxyphenyl)-9H-fluorene-2,7-diamine] (PFD), as a hole-transporting layer was designed for perovskite solar cells (PSCs). PFD has a rigid and flat pyrene unit as the central building block and a symmetric structure to increase the conjugation degree for better carrier conductivity. A power conversion efficiency (PCE) of 18.6% was achieved for PSC based on undoped PFD. When spiro-OMeTAD is further introduced on the top of PFD (PFD/spiro-OMeTAD), a PCE of 21.02% was achieved with a steady-state efficiency of 19.92%, revealing that PFD as an interfacial layer could restrain carrier recombination. Unencapsulated devices based on PFD or PFD/spiro-OMeTAD exhibit excellent stability including long-term stability for over 1400 h under ambient condition, and moisture stability under the "Double 60 program" test simultaneously at 60% RH and 60 degrees C in air. Besides, no obvious degradation was found for the PFD/spiro-OMeTAD device over 200 h under continuous illumination at the maximum power point tracking (MPPT) in N-2 atmosphere. It is thus suggested that PFD could be a promising dopant-free HTM for PSCs, and also acts as an interface layer to suppress the carrier recombination to further improve the device performance.