Columnar liquid crystals as stability enhancing hole transport layers for Ruddlesden–Popper perovskite optoelectronics
Metal halide perovskites are highly promising materials for next generation optoelectronic devices, but they suffer from moisture-driven and thermal degradation. We focus on the prototypical light-absorbing methylammonium lead(II) iodide (MAPbI3) crystal that degrades predominantly into lead iodide (PbI2). In this work, we perform a systematic comparison of the structural and optical properties between perovskites coated with spiro-OMeTAD and a discotic liquid crystal (DLC) 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HAT5). Butylammonium iodide (BAI) is used as an additive to make hybrid mixed dimensional (MD) and two-dimensional perovskites. We perform aging studies in a lab environment over a period of 45 days and two sets of conditions are tested; the first set is exposed to humidity that varies between 58 and 36%, and the second set was exposed to the environment and additionally was heated at 65 °C in the atmosphere for four hours every day. Our results demonstrate that the DLC HAT5 molecule results in improved long-term stability of hybrid MD BAI substituted Ruddlesden–Popper perovskites. Some concerns about low phase transition temperatures in working devices are also highlighted. The enhanced stability of perovskite-based optoelectronic devices with the use of HATn HTLs provides a promising molecular engineering approach that can be applied in stable next-generation perovskite optoelectronic devices.