Hybrid metal halide perovskites such as CH3NH3PbI3 and CH3NH3PbI3-xClx have emerged as a new class of photovoltaic materials showing remarkable progress in power conversion efficiency (PCE) with certified values over 20.1% [1]. This fast advancement has been achieved due to the improved understanding of perovskite solar cell operation enabling the optimization of the device architecture and fabrication. Spiro-OMeTAD is used in the overwhelming majority of cases as hole transporting material (HTM). However, the spirobifluorene core in the spiro-OMeTAD molecule requires a multi-step synthesis, which results in high production costs of the final compound, making it by far the most expensive ingredient of the perovskite solar cell. Another disadvantage is the fact the 4-tert-butylpyridine, an additive commonly used to improve the conductivity of spiro-OMeTAD, can dramatically reduce the chemical stability of hybrid perovskites [2]. Moreover, acetonitrile used as the solvent for the other widely applied additive Li-TFSI can also corrode hybrid perovskites [3]. Therefore, the development of cheaper alternatives to spiro-OMeTAD, which at the same time do not contribute to perovskite degradation, is of prime importance.

Several semiconducting polymers have been investigated as HTMs but usually display inferior performance [4] and suffer from batch-to-batch variations. In contrast, a number of small molecules such as simple arylamines or oligothiophene- have been reported showing PCEs of 13.4% [5].

In this communication we will present a new family of small molecules with a polyaromatic core and synthesized through a facile synthetic route, allowing their preparation in large scale. We investigated their performance as HTM in CH3NH3PbI3 perovskite solar cells. Spiro-OMeTAD was used as reference device for comparison. Their HOMO levels determined by cyclic voltammetry vary from 5.07 to 5.24 eV depending on the molecular structure, i.e. close to the value of spiro-OMeTAD (5.0-5.2 eV) [5,6]; their band gap energies lie in the range of 2.9-3.6 eV. The obtained current-voltage characteristics show a clear relationship between molecular structure / electronic properties of the small molecules and the solar cell parameters measured. Finally, the influence of HTM thickness and doping are investigated. The presented approach can serve as a platform for the design and synthesis of novel HTMs with optimized properties when used in combination with different kinds of hybrid perovskites.