The Bloom of Perovskite Optoelectronics: Fundamental Science Matters

Fundamental Science Matters In the past decade, halide perovskites (HPs) have gained enormous attention due to their rapidly advanced performance in solar cells, starting with a first reported power conversion efficiency (PCE) of 3.8% in 2009 to a certified 23.7% now. The exceptional performance of perovskite solar cells (PSCs) is attributed to the fascinating physical properties of HP materials such as long photocarrier diffusion lengths and large absorption coefficients, which also make HPs promising for a lot of other optoelectronic applications beyond solar cells (e.g., light-emitting diodes, photodetectors, X-ray scintillators). Recently, the HP community convened at the 2018 Materials Research Society (MRS) Fall Meeting (Nov. 26−30, 2018, Boston, MA, United States). The perovskite-themed “twin” symposia, ET04: Perovskite Solar Cells Challenges and Opportunities and ET05: Fundamental Aspects of Halide Perovskite (Opto)electronics and Beyond, were among the busiest events of the MRS meeting with ∼500 abstracts in total. The “twin” symposia also formed an unprecedented joint session “The Past, Present, and Future of Halide Perovskites” featuring five keynote speakers (David Mitzi, Tsutomu Miyasaka, Nam-Gyu Park, Anders Hagfeldt, and Kai Zhu) followed by a panel discussion on the most debated topics (stability, toxicity, upscaling, defects) in the perovskite field. The latest results and research trends of both fundamental and device aspects of HPs (see Figure 1) were intensely discussed and analyzed across the two symposia. Here, we present a brief summary of the highlights in both perovskite symposia, aiming to inform the perovskite and broader materials community of important ongoing developments and limitations regarding HPs for greater exploitation of these fascinating optoelectronic materials. Defect and Ions. Defect chemistry in a crystal lattice is what accounts for the difference between ideal and real crystals. The deviation from perfect crystallinity affects theoretical calculations and renders the underlying assumptions not completely applicable. Meanwhile, the impact of structural defects is enormous on basic material properties of perovskites and can appear in perovskite structures with different dimensionalities with the need for controlled passivation treatment. Filippo De Angelis highlighted some new results regarding defect/ion migration and polarons. In his work, he considered Frenkel defects as the major defect type expected in lead iodide perovskite structures. Electronic structure calculations suggested that large amounts of iodide anions exist at grain boundaries of perovskite materials, which thus indicated an increased ion migration and the principal surface locations of Frenkel defects. In addition, De Angelis showed that iodine interstitials can produce deep traps, which was verified by spectroscopic characterizations in collaboration with Annamaria Petrozza from Istituto Italiano di Tecnologia. Besides Frenkel defects, there are plenty of other possible defects in material lattices, such as linear, planar, or 3D bulk defects, with each defect requiring specific treatment for effective passivation. Nitin P. Padture demonstrated the importance of functionalizing the grain boundaries of perovskite methylammonium lead iodide (CH3NH3PbI3, or MAPbI3) thin films via structurally incorporating triblock copolymer Pluronic P123. In the meantime, Xun Xiao adopted argon plasma treatment as an effective method to systematically control defects at the surface of perovskite films. In Xiao’s study, he concluded that only Pb defects remained after argon plasma treatment, which made it highly targetable with specific passivation treatment. Profoundly, the argon treatment and surface passivation synergistically led to greater charge carrier mobility and thus consequently resulted in higher PCEs of solar cell devices. Moreover, Jacob Tse-Wei Wang introduced another passivation approach by coating a formamidinium chloride (HC(NH2)2Cl, FACl) layer on perovskite film. X-ray diffraction (XRD) measurements confirm that in the case of an additional layer of FACl the formation of PbI2 can be prevented, while grazing incidence X-ray diffraction further confirmed that the FACl was not leaching into the perovskite bulk structure. The FACl crystallization on top of the perovskite layer resulted in less nonradiative recombination due to lower defect density. Therefore, a slower degradation process can be observed. Yabing Qi also touched upon the subject of Cl− addition and showed an improvement of the PV