1, Rice Univ, Houston, Texas, United States
3, Northwestern Univesity, Evanston, Illinois, United States
Hybrid (inorganic-organic) halide perovskites have demonstrated an extraordinary potential for clean, sustainable energy technologies and low-cost optoelectronic devices. In spite of the unprecedented progress over the past five years, one of the key challenges that exists in the field today is the stability and reliability of devices when exposed to the environment, external electric-field and light. This vulnerability remains an open question, which might determine the fate of this remarkable material despite excellent properties.
In this talk, I will introduce a new class of hybrid perovskite material known as Ruddlesden-Popper phase layered perovskites (RPLP) as promising alternatives to the bulk hybrid perovskites, which exhibits intrinsic structural stability and unique optoelectronic properties. These are solution-processed quantum wells with a general formula of A’2An-1BnX3n+1 where the inorganic slab (B) is separated by bulky organic molecules (A’). I will first describe the crystal structure and versatility of this novel system through our efforts on synthesis of phase-pure crystals (with well-defined inorganic slab thickness). A major breakthrough was realized through our discovery of growing single-crystalline thin-films using hot-casting method  with the ability to control the orientation at the molecular scale, which facilitates efficient charge separation  and has enabled high-efficiency solar cells. More importantly, we demonstrated that solar cells fabricated using vertically oriented single-crystalline films exhibit technologically relevant stability exceeding 2500 hours of operation under continuous light and 65% relative humidity .
Following this result, our study has shown that the crystalline 2D thin-film with preferred orientation can tremendously enhance carrier injection and transport and lead to strong radiative recombination with high photoluminescence quantum yield. We utilize this property to demonstrate stable light emitting diodes (LEDs) with an external quantum efficiency of electroluminescence approaching 1%, radiance of 35 W/Sr.m2 and without any droop in efficiency with injection currents as large as few Amperes/cm2[3-4]. Furthermore, the LEDs fabricated with phase-pure RPLPs exhibit wide range of color tuning and good stability thus paving the path of realizing the dream of electrically injected lasers using solution-processed semiconductors.
In summary, we believe that these results have opened the door for a wide range of applications using low-temperature processed semiconductor grade single-crystalline thin-films, which will lead to new scientific and technological discoveries.
 Tsai-Nie et al., Science 347, 522 (2015);  H. Tsai et al., Nature 536, 312(2016);  Blancon-Tsai et al., Science 355, 1288 (2017);  H. Tsai et al., Adv. Mater. DIO:10.1002/adma.201704217 (2017)