RESUMO
The methylammonium (MA) and formamidinium (FA) are the most commonly used organic cations in perovskite solar cells (PSCs), whereas the impact of size and polarity differences between these two on the photovoltaic performances has been rarely revealed. Herein, we systematically investigated the phase distribution, optoelectronic and stability properties of FA-MA mixed perovskites. To identify the phase homogeneity, depth-dependent grazing-incidence wide-angle x-ray scattering measurements were employed, which demonstrates that the mixed cation perovskite possesses a FA-rich phase on the film surface and the bottom is comprised of MA-rich phase. Additionally, upon long-time illumination, a new PL peak is appeared at 778 nm, representing the generation of MA-rich phase induced by ion migration. It is worth noting that the phase splitting and inhomogeneous phase distribution would not bring any obvious detrimental effects to the photovoltaic performances and stability properties. Through judiciously tuning the cation proportion in pure-iodide perovskite, the additive-free PSCs achieve an efficiency as high as 20.7%. Furthermore, the PSCs with a broad range of FA/MA ratios show improved humidity/thermal/light stability despite the phase inhomogeneity. Therefore, the work shows that the MA and FA cations have a high compatibility in perovskite structure and the precise ratio control can further improve the performances.
RESUMO
Metal halide perovskite light emitting diodes (PeLEDs) have recently experienced rapid development due to the tunable emission wavelengths, narrow emission linewidth and low material cost. To achieve state-of-the-art performance, the high photoluminescence quantum yield (PLQY) of the active emission layer, the balanced charge injection, and the optimized optical extraction should be considered simultaneously. Multiple chemical passivation strategies have been provided as controllable and efficient methods to improve the PLQY of the perovskite layer. However, high luminance under large injection current and high external quantum efficiency (EQE) can hardly be achieved due to Auger recombination at high carrier density. Here, we decreased the electron injection barrier by tuning the Fermi-level of the perovskite, leading to a reduced turn on voltage. Through molecular doping of the hole injection material, a more balanced hole injection was achieved. At last, a device with modified charge injection realizes high luminance and quantum efficiency simultaneously. The best device exhibits luminance of 55,000 cd m-2, EQE of 8.02% at the working voltage of 2.65 V, current density of 115 mA cm-2, and shows EQE T50 stability around 160 min at 100 mA cm-2 injection current density.
