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

Iodide manipulation using zinc additives for efficient perovskite solar minimodules

Abstract Interstitial iodides are the most critical type of defects in perovskite solar cells that limits efficiency and stability. They can be generated during solution, film, and device processing, further accelerating degradation. Herein, we find that introducing a small amount of a zinc salt- zinc trifluoromethane sulfonate (Zn(OOSCF3)2) in the perovskite solution can control the iodide defects in resultant perovskites ink and films. CF3SOO̶ vigorously suppresses molecular iodine formation in the perovskites by reducing it to iodide. At the same time, zinc cations can precipitate excess iodide by forming a Zn-Amine complex so that the iodide interstitials in the resultant perovskite films can be suppressed. The perovskite films using these additives show improved photoluminescence quantum efficiency and reduce deep trap density, despite zinc cations reducing the perovskite grain size and iodide interstitials. The zinc additives facilitate the formation of more uniform perovskite films on large-area substrates (78-108 cm2) in the blade-coating process. Fabricated minimodules show power conversion efficiencies of 19.60% and 19.21% with aperture areas of 84 and 108 cm2, respectively, as certified by National Renewable Energy Laboratory (NREL), the highest efficiency certified for minimodules of these sizes