View article

Therein, hole transporting materials (HTMs), as an important interface contact between the perovskite crystals and the metal electrode (back contact), have been desirably developed and investigated. The structures of the HTM play crucial roles in determining the hole transfer kinetics at the interface between HTM and perovskite, as well as affecting the rate of hole transport through the HTM to the metal contact. These in turn affect the interfacial charge recombination and the charge collection efficiency.[11, 12] Therefore, a good HTM should possess the following features:(1) high solubility in organic solvents (such as chlorobenzene, toluene) is favorable to the spin-coating process;(2) the highest occupied molecular orbital (HOMO) of the HTM must be above the valence band of the perovskite in order to achieve efficient hole injection;(3) high conductivity is required to reduce the charge transport resistance;(4) excellent thermal and photochemical stability. In this respect, organic small molecular HTMs have attracted great attention due to their large availability of solution-processing and the ease of structural modification, although many conducting polymers [13] and inorganic semiconductors [14, 15] were reported to be efficient HTMs for PSCs. Nevertheless, the state-of-the-art organic small molecule, spiro-OMeTAD, is the most commonly used and efficient HTMs in PSCs by showing high PCEs of more than 15% in many cases. However, its known features, such as the complex synthesis, high cost, and low conductivity, could impede its large-scale application in PSCs.[16] Therefore, many types of small molecule HTMs without spiro motif have …