(Invited) Functional Additives for Halide Perovskites Tailored Deposition for Highly Efficient Light-Emitting and Photovoltaic Devices

Abstract

The control over Organometal Halide Perovskites (OHPs) formation is a fundamental requirement foreseeing the exploitation of these outstanding, eclectic materials in optoelectronics. OHPs, extensively used in solar cells and light emitting diodes (PeLEDs) are deposited from solution on a target substrate and formed throughout a self-assembly process of their chemical precursors [1][2]. The resulting polycrystalline film shows often a far from ideal behaviour, due to unsuitable morphology and high defect density, direct consequences of a scarcely controllable assembly. We present here the exploitation of a tailored biopolymer, starch, as beneficial additional component of formamidinium (FA)- and methylammonium (MA)-based tri-iodide perovskite films. We prove how the macromolecule by establishing specific supramolecular interactions with OHPs precursors allows the deposition of an optimal film via single-step deposition method. Noticeably this is a fundamental technological advantage in comparison to standard multistep deposition approaches. Furthermore, it allows a fine tuning of i) solution viscosity (making it compatible with different large area deposition techniques), of ii) perovskite grain size and of iii) film thickness, parameters that all depends to the polymer:perovskite:solvent relative concentrations. Very importantly the presence of the biopolymer is also improving the stability of the polycrystalline film thanks to two fundamental properties. The polymer being an electronic insulator reduce the impact of the internal electric field over OHPs mobile ions; in addition the polymer is also permeable to ions (ionic conductor), thus by buffering those prevent ions migration/segregation across perovskite grains, one of the most important material degradation mechanism under device working conditions.We validated our approach by embedding these composites in photovoltaic (PV) and light-emitting (PeLED) devices. As result we obtained an inverted, planar, low-temperature processed solar cell showing a remarkable 17% efficiency [3] and a highly efficient LED (EQE of ~5%) exhibiting outstanding radiance values above 200 W/sr·m2 obtained at very high currents (about 1000 mA/cm2) which are among the highest reported radiances for NIR PeLEDs [4].