Short-Step Synthesized Dopant-Free Spiro-Type Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells

Hole-transporting materials (HTMs) play an important role in charge extraction, interfacial recombination, and device stability in perovskite solar cells (PSCs). For example, 2,2 ',7,7 '-tetrakis(N,N-di-p-methoxyphenylamino)-9,9 '-spirobifluorene (Spiro-OMeTAD) is one of the most frequently used HTMs. However, dopants such as lithium bis(trifluoromethylsulfonyl)imide are necessary to achieve high power-conversion efficiency (PCE), and these dopants have been speculated to cause severe instability issues in PSCs. Recently, dopant-free organic HTMs have emerged as highly desirable materials for stable and efficient n-i-p-type PSCs; however, the literature contains relatively few reports of such dopant-free organic HTMs. In addition, the synthesis of dopant-free organic HTMs often leads to further complications. Herein, we synthesized spiro-type HTMs SF67 and SF71, which are nondoped HTMs with halogen and N,N-dimethylamino substituents as electron-withdrawing and -donating groups, respectively. The advantage of SF67 and SF71 is that they are easy to synthesize because of their two-step synthesis starting from commercially available precursors. By introducing two kinds of substituents with different electronic properties, we could adjust the highest occupied molecular orbital (HOMO) to an energy level appropriate for PSCs. Moreover, both SF67 and SF71 can cover the surface of the perovskite uniformly without forming pinholes. Compared with Spiro-OMeTAD, they also exhibit a superior hole extraction ability. Consequently, the PCEs of devices based on SF67 and SF71 were 19.6% and 19.5%, respectively. In addition, the thermal stability of the PSCs based on nondoped SF67 and SF71 at 85 degrees C in ambient air was superior to that of a PSC based on nondoped Spiro-OMeTAD. Therefore, both SF67 and SF71 are expected to be effective high-performance dopant-free HTMs for PSCs.