In organic-inorganic metal halide perovskite solar cells (PSCs), reported high performance hole transporting materials (HTMs) are mostly limited to spiro compounds and/or arylamine derivatives, which are derived from 2,2′,7,7′-tetrakis-(N,N-di-p-methoxy-phenyl-amine)-9,9′-spiro-bifluorene (Spiro-OMeTAD). These amorphous HTMs typically require additional dopants to increase their hole mobility. However, such Li-salt dopants often accelerate degradation of the device due to their hygroscopic nature. Recently, our group reported a crystalline HTM molecule (IDID-F) containing indolo[3,2-b]indole (IDID) core unit, which has centro-symmetric π-conjugated backbone.  The PSC devices using Li-salt doped IDID-F as HTM showed higher PCE (~19%) and better device stability than those using Spiro-OMeTAD. In addition, even without the use of any dopants, IDID-F shows moderately high PCE over 13% due to its high crystallinity and charge carrier mobility. However, it is still considerably low compared with that of the PSCs using dopants. It was thus speculated that the strong secondary interactions between the IDID-F molecules are positive in developing high crystallinity but are negative in significantly limiting its solubility, which causes poor film morphology and high series resistance to induce low fill factor and thus low PCE. Therefore, the development of IDID-based HTMs possessing high solubility with controlled crystallinity is demanded and actually targeted in this work for highly efficient dopant-free PSCs.
Here, we report new IDID-based crystalline HTMs bearing various alkyl chains for non-doped PSCs. The new IDID derivatives show improved solubility and processability that can ensure uniform film morphologies while maintaining high crystallinity to induce high hole mobility. The PSC devices using these new IDID derivatives as an HTM exhibit excellent PCE over 18% without any additional dopants as well as good device stability. In addition, it should also be noted that the IDID derivatives show one of the highest open circuit voltage (~1.1 V) among the reported methylammonium lead iodide-based PSCs, which is attributed to their deep lying HOMO level (~5.20 eV). The steady-state photoluminescence (PL) and time-resolved PL studies clearly revealed that the IDID derivatives have much improved hole extracting and transporting capabilities compared with those of Spiro-OMeTAD and previously reported IDID-F. To elucidate the origin of the superior hole transport ability, we analyzed the films of IDID derivatives by grazing incidence wide angle X-ray scattering method. The clear diffraction patterns observed from the IDID derivatives indicates high crystallinity of the films.
 I.Cho, Chem. Sci., 2017, 8, 734-741