Slowed Recombination via Tunable Surface Energetics in Perovskite Solar Cells

Metal halide perovskite semiconductors have the potential to reach the optoelectronic quality of meticulously grown inorganic materials, but with a distinct advantage of being solution processable. Currently, perovskite performance is limited by charge carrier recombination loss at surfaces and interfaces. Indeed, the highest quality perovskite films are achieved with molecular surface passivation, for example with n-trioctylphosphine oxide, but these treatments are often labile and electrically insulating. As an alternative, the formation of a thin 2D perovskite layer on the bulk 3D perovskite reduces non-radiative energy loss while also improving device performance. But, thus far, it has been unclear how best to design and optimize 2D/3D heterostructures and whether critical material properties, such as charge carrier lifetime, can reach values as high as ligand-based approaches. Here, we study perovskite devices that have exhibited power conversion efficiencies exceeding 25% and show that 2D layers are capable of pushing beyond molecular passivation strategies with even greater tunability. We set new benchmarks for photoluminescence lifetime, reaching values > 30 {\mu}s, and perovskite/charge transport layer surface recombination velocity with values < 7 cm s^{-1}. We use X-ray spectroscopy to directly visualize how treatment with hexylammonium bromide not only selectively targets defects at surfaces and grain boundaries, but also forms a bandgap grading extending > 100 nm into the bulk layer. We expect these results to be a starting point for more sophisticated engineering of 2D/3D heterostructures with surface fields that exclusively repel charge carriers from defective regions while also enabling efficient charge transfer. It is likely that the precise manipulation of energy bands will enable perovskite-based optoelectronics to operate at their theoretical performance limits.Comment: Main text: 15 pages, 4 figures. Supporting Information: 31 pages, 19 figure

How machine learning can help select capping layers to suppress perovskite degradation

Environmental stability of perovskite solar cells (PSCs) has been improved by trial-and-error exploration of thin low-dimensional (LD) perovskite deposited on top of the perovskite absorber, called the capping layer. In this study, a machine-learning framework is presented to optimize this layer. We featurize 21 organic halide salts, apply them as capping layers onto methylammonium lead iodide (MAPbI₃) films, age them under accelerated conditions, and determine features governing stability using supervised machine learning and Shapley values. We find that organic molecules’ low number of hydrogen-bonding donors and small topological polar surface area correlate with increased MAPbI₃ film stability. The top performing organic halide, phenyltriethylammonium iodide (PTEAI), successfully extends the MAPbI₃ stability lifetime by 4 ± 2 times over bare MAPbI₃ and 1.3 ± 0.3 times over state-of-the-art octylammonium bromide (OABr). Through characterization, we find that this capping layer stabilizes the photoactive layer by changing the surface chemistry and suppressing methylammonium loss.NSF (Award DMR-1419807)NSF (Grant CBET-1605547)Skoltech (Grant 1913/R)DOE (Award DE-EE0007535)ISN (Grant W911NF-13-D-0001)NASA (Grant NNX16AM70H

An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss

Stabilization of the crystal phase of inorganic/organic lead halide perovskites is critical for their high performance optoelectronic devices. However, due to the highly ionic nature of perovskite crystals, even phase stabilized polycrystalline perovskites can undergo undesirable phase transitions when exposed to a destabilizing environment. While various surface passivating agents have been developed to improve the device performance of perovskite solar cells, conventional deposition methods using a protic polar solvent, mainly isopropyl alcohol (IPA), results in a destabilization of the underlying perovskite layer and an undesirable degradation of device properties. We demonstrate the hidden role of IPA in surface treatments and develop a strategy in which the passivating agent is deposited without destabilizing the high quality perovskite underlayer. This strategy maximizes and stabilizes device performance by suppressing the formation of the perovskite δ-phase and amorphous phase during surface treatment, which is observed using conventional methods. Our strategy also effectively passivates surface and grain boundary defects, minimizing non-radiative recombination sites, and preventing carrier quenching at the perovskite interface. This results in an open-circuit-voltage loss of only ∼340 mV, a champion device with a power conversion efficiency of 23.4% from a reverse current–voltage scan, a device with a record certified stabilized PCE of 22.6%, and enhanced operational stability. In addition, our perovskite solar cell exhibits an electroluminescence external quantum efficiency up to 8.9%. ©2019Institute for Soldier Nanotechnology (Grant W911NF-13-D-0001)NASA (Grant NNX16AM70H)DOE Division of Materials Sciences and Engineering (Award DE-FG02-07ER46454)NSF (Grant CBET-1605495

Solvent-Engineering Method to Deposit Compact Bismuth-Based Thin Films: Mechanism and Application to Photovoltaics

Bismuth-based materials have been studied as alternatives to lead-based perovskite materials for photovoltaic applications. However, poor film quality has limited device performance. In this work, we developed a solvent-engineering method and show that it is applicable to several bismuth-based compounds. Through this method, we obtained compact films of methylammonium bismuth iodide (MBI), cesium bismuth iodide (CBI), and formamidinium bismuth iodide (FBI). On the basis of film growth theory and experimental analyses, we propose a possible mechanism of film formation. Additionally, we demonstrate that the resultant compact MBI film is more suitable to fabricate efficient and stable photovoltaic devices compared to baseline MBI films with pinholes. We further employed a new hole-transporting material to reduce the valence-band offset with the MBI. The best-performing photovoltaic device exhibits an open-circuit voltage of 0.85 V, fill factor of 73%, and a power conversion efficiency of 0.71%, the highest reported values for MBI-based photovoltaic devices