Doping Strategies for Tetrasubstituted Paracyclophane Hole Transport Layers in Perovskite Solar Cells

Because of its excellent hole conductivity, p-doped 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spiro-bifluorene (spiro-MeOTAD) is commonly deployed for hole transport in organic metal halide perovskite solar cells, but its rather expensive synthesis prompts the research for alternatives. In this work, tetrasubstituted [2.2]paracyclophanes (PCPs) are synthesized and investigated for replacing spiro-MeOTAD. To enhance their conductivity, different doping strategies are followed. Best conductivities are achieved by doping PCP thin films with tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209), matching the conductivity of state-of-the-art p-doped spiro-MeOTAD. Best performance in solar cells is leveraged by doping PCPs with the co-dopants lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP) which are also used to p-dope spiro-MeOTAD thin films in solar cells. Yet, the thermal device stability is maximized upon doping PCPs with FK209 and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F$_4$ TCNQ).