Perovskite solar cells involving poly(tetraphenylbenzidine)s: investigation of hole carrier mobility, doping effects and photovoltaic properties

Perovskite solar cells in combination with organic hole transport materials have attracted attention due to their high power conversion efficiencies. Now that these high efficiencies have been reached, it is important to address fundamental questions regarding the requirements of the material properties. Here, we present a detailed study on important properties of the hole transport material such as the influence of the molecular weight, the doping effects on charge carrier mobility and the polarity of the material. A series of poly(tetraphenylbenzidines) (PTPDs) differing in their properties was synthesized via Yamamoto polycondensation. Using space charge limited current (SCLC) measurements, we find that the hole transport mobility is independent of the investigated molecular weight and polarity of the side chains. Doping of the PTPDs with a Co(III)-complex reveals that the charge carrier density increases through an oxidation process. Further, the solar cell performance improves upon doping. After storing the devices, the power conversion efficiencies of the solar cells drastically increase due to improved absorption leading to improved EQE. For example, the best performing cell exhibited a power conversion efficiency of 7.69%. Additionally, the PTPD carrying polar substituents leads to a less pronounced hysteresis effect and a higher stability under illumination compared to the polymer carrying hydrophobic side chains.