Physics

Dr. Aditya D. Mohite
Los Alamos National Laboratory, New Mexico, United States

Photovoltaic industry to our surprise is one of the most rapidly growing sectors in the United States and is anticipated to capture 10% of the total renewable energy produced by 2020. However, cost/PV efficiency remains one of the key challenges for its ubiquitous deployment. Solution-processing of semiconductor and nanomaterial thinfilms provides a direct route to reduce the costs. However second and third generation solar cell technologies, in spite of extensive development for over 25 years has been limited due to the inadequate power conversion efficiency (PCE~10%) A fundamental critical bottleneck of wet-lab processed thin-film technologies is the material’s polydispersity that leads to the presence of multiple interfaces and defects/trap sites resulting in high carrier losses and low stability. Recent discovery of organic-inorganic (also known as hybrid) perovskites such as CH3NH3PbX3 (X = Cl, Br, I) for photovoltaic devices has reached 22% in merely 3-5 years and offer extraordinary potential for clean sustainable energy technologies and in general low-cost optoelectronic devices. In spite of the recent progress there exists tremendous variability in the structural and optoelectronic properties of the hybrid perovskite thin-films across the globe. Crystallinity, defect density and impurities are in general determining factors for (opto)electronic properties, which are also highly dependent on the materials formation processes. Apart from these the stability and reliability of perovskite based devices remains open questions and perhaps will determine the fate of this remarkable technology in the longer run. In this talk, I will describe our recent work on a novel solution-processing technique termed as “hot-casting” to grow continuous, pin-hole free thin-films of organometallic perovskites with millimeter-scale crystalline grains. Photovoltaic devices show hysteresis-free response, with high degree of reproducibility, thus overcoming a fundamental bottleneck for hybrid perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved chargecarrier mobility in large-grain devices. In addition, I will describe our most recent efforts on understanding the controlling photo-degradation in these systems. Results demonstrate that the large grain-size perovskite thin-films are not limited by detrimental effects such as ion migration or defect assisted trapping generally reported in perovskite thin-film devices allowing us to probe intrinsic photo-physical processes that lead to the degradation of PCE in perovskite solar cells. Finally, I will also describe some new results on layered 2D perovskites with PCE approaching 13% with technologically relevant environmental and photostability.