Effects of Residual Intermediate Phase Buried in MAPbI₃ Films on Photocurrent Generation

Defects and impurities in halide perovskite can have negative effect on device performance and long-term stability. Here, we report an overlooked impurity in MAPbI₃, MA₂Pb₃I₈(DMSO)₂, which reduces photocurrent generation in perovskite solar cells (PSCs). We observe a substantial loss of the external quantum efficiency (EQE) spectrum at 400 nm in PSCs made by photonic curing despite good device performance. This EQE reduction feature has also been reported in the literature. We then find that these samples contain MA₂Pb₃I₈(DMSO)₂ phase, which is a commonly reported intermediate phase after the addition of DMSO in MAPbI₃ precursors. Previously, MA₂Pb₃I₈(DMSO)₂ phase is known to be beneficial by retarding the perovskite growth rate to get high-quality perovskite films. By varying incidence angle in the X-ray diffraction, we show that MA₂Pb₃I₈(DMSO)₂is buried in the substrate/MAPbI₃ interface. Because the high boiling temperature of DMSO, the result could be explained by the solvent DMSO is hard to escape the precursor films. We further study how MA₂Pb₃I₈(DMSO)₂ affects the long-term stability of PSCs. Additionally, the resulting parasitic absorption reduces the photogeneration in the short wavelength region. This is an obstacle while applying PSC as indoor photovoltaics because indoor light-emitting diode spectra are narrower compared to the AM 1.5G. This work is a reminder to the field to rethink the role of MA₂Pb₃I₈(DMSO)₂ in MAPbI₃ films and to investigate a mitigation strategy. <br/>This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Solar Energy Technologies Office Award Number DE-EE0009518.