Organometal Halide Perovskites Template Growth for Highly Efficient Light-Emitting and Photovoltaic Devices

Andrea Listorti

The control of 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[1] and light emitting diodes (PeLEDs)[2] are formed from solution, throughout a self-assembly process of their chemical precursors. The resulting polycrystalline film shows often a far from ideal behaviour, due to unsuitable morphology or to an elevated density of electronic defects, direct consequences of a scarcely controllable assembly. Based on our previous findings on the interaction between perovskite precursors and additives,[3-4] and particularly on the hydrogen bond effect upon such interactions, here we explore the use of a tailored biopolymer, the starch, as templating agent for the growth of formamidinium (FA)- and methylammonium (MA)-based tri-iodide perovskite films. We prove how the presence of the macromolecule leads to a fundamental technological advantage allowing a one-step deposition method of an optimum perovskite film, avoiding the complex solvent dripping or sequential two-steps method. Furthermore, it allows a fine tuning of the i) solution viscosity (making it compatible to different large area deposition techniques), of the ii) perovskite grain size and of the iii) film thickness, by simply adjusting the polymer:perovskite relative concentration. 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 and a highly efficient LED (EQE of ~5%) exhibiting outstanding radiance values above 200 W/srŸm2obtained at very high currents (about 1000 mA/cm2) which are among the highest reported radiances for NIR PeLEDs.[5]

The nano and micro-structural changes, as a function of starch concentration, and their influence on optoelectronic properties were studied with SEM, XRD and advanced spectroscopic analyses, confirming that the device performances are strongly related to the composite (starch:perovskite) structure. The overall picture that emerged form the combined investigations is that the presence of starch influences: i) the interface properties within the device stack, ii) the crystallographic behaviour of the perovksite material, iii) the dielectric landscape surrounding the perovskite crystals, leading in case of the PeLED, to an overall enhancement of the perovskite light emission, a minimization of Auger losses at high current regimes, thus an improvement of the device capability in sustaining high carriers densities.[5]