Pseudo-Para-Substituted [2.2]Paracyclophanes for Hole Transport in Perovskite Solar Cells

2,2 ',7,7 '-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9 '-spirobifluorene (spiro-OMeTAD) is the prevalent hole transport layer in perovskite solar cells (PSCs) with regular device architecture. Yet, its spirobifluorene core and multistep synthesis make it rather expensive. For the further technological success of PSCs, novel scalable and inexpensive alternative hole transport layers are needed. Herein, a study of the structure-property relations of pseudo-para-substituted [2.2]paracyclophanes is presented. Eight different hole transport materials are synthesized via double CH activation, eliminating metal-containing substituents for cross-coupling reactions. The ionization potentials (IPs) of the disubstituted paracyclophanes (DiPCPs) are examined by photoelectron spectroscopy in air, cyclic voltammetry and theoretical calculations. Through variation of donor groups and pi-linkers, IPs that span a range from 5.14 to 5.86 eV are achieved, demonstrating high customizability. From the eight novel materials, five showed good solubility and are implemented into PSCs. The solar cells with a hole transport layer of undoped 4,16-di(4-(2-thienyl)-N,N-bis(4-methoxyphenyl)aniline)[2.2]paracyclophane (DiPCP-2) exhibit a power conversion efficiency of 12.7% +/- 0.4%. The facile synthesis of DiPCP-2 enables an estimated cost reduction by two thirds compared to spiro-OMeTAD.

Automated summary

In this study, eight different hole transport materials were synthesized via double CH activation. By varying the donor groups and pi-linkers, a wide range of ionization potentials (IPs) were achieved, demonstrating high customizability. One of the materials, DiPCP-2, showed good performance as a hole transport layer in PSCs, and its facile synthesis enables a cost reduction by two thirds.