The approaches to stabilize the perovskite structure of formamidinium lead iodide (FAPI) commonly result in a blue-shift of the band gap, which limits the maximum photo-conversion efficiency.Here, we report the use of PbS colloidal quantum dots (QDs) as stabilizing agent, preserving the original low band gap of 1.5 eV,. The surface chemistry of PbS plays a pivotal role, by developing strong bonds with the black phase but weak ones with the yellow phase. As a result, stable perovskite FAPI black phase can be formed at temperatures as low as 85°C in just 10 minutes, setting a record of concomitantly fast and low temperature formation for FAPI, with important consequences for industrialization. FAPI thin films obtained through this procedure reach an open circuit potential (Voc) of 1.105 V -91% of the maximum theoretical Voc-and preserve the efficiency for more than 700 hours. These findings reveal the potential of strategies exploiting the chemi-structural properties of external additives to relax the tolerance factor and optimize the optoelectronic performance of perovskite materials.

The Pb substitution in quantum dots (PQDs) with lesser toxic metals has been widely searched to be environmentally friendly, and be of comparable or improved performance compared to the lead-perovskite.

The optoelectronic properties of metal chalcogenide colloidal nanoplatelets are often interpreted in terms of excitonic states. However, recent spectroscopic experiments evidence the presence of trion states, enabled by the slow Auger recombination in these structures. We analyze how the presence of an additional charge in trions modifies the emission energy and oscillator strength as compared to neutral excitons. These properties are very sensitive to dielectric confinement and electronic correlations, which we describe accurately using the image-charge and variational Quantum Monte Carlo methods in effective mass Hamiltonians. We observe that the giant oscillator strength of neutral excitons is largely suppressed in trions. Both negative and positive trions are red-shifted with respect to the exciton, and their emission energy increases with increasing dielectric mismatch between the platelet and its surroundings, which is a consequence of the self-energy potential. Our results are consistent with experiments in the literature and assess the validity of previous theoretical approximations.

PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew−Burke−Ernzerhof functional for solids and with the GW approximation, including spin−orbit interactions. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We derive effective masses for conduction and valence bands and deduce that quantum confinement along the a crystallographic axis (short axis of the NPL) reinforces the indirect band gap but that along b and c axes favors a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k•p theory, show that excitonic effects are strong, with binding energies of about 150 meV.