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 (F4TCNQ). Novel tetrasubstituted paracyclophanes as hole transport materials for organic metal halide perovskite solar cells are synthesized and their properties are investigated in‐depth. By using MIS‐CELIV characterization and density functional theory the charge carrier mobilities of the pristine and doped paracyclophane hole transport materials are investigated and their properties and thermal stability are qualified in methylammonium lead iodide solar cells.