The dynamic hysteresis of perovskite solar cells consists of the occurrence of significant deviations of the current density-voltage curve shapes depending on the specific conditions of measurement such as starting voltage, waiting time, scan rate, and other factors. Dynamic hysteresis is a serious impediment to stabilized and reliable measurement and operation of the perovskite solar cells. In this Letter, we formulate a model for the dynamic hysteresis based on the idea that the cell accumulates a huge quantity of surface electronic charge at forward bias that is released on voltage sweeping, causing extra current over the normal response. The charge shows a retarded dynamics due to the slow relaxation of the accompanying ionic charge, that produces variable shapes depending on scan rate or poling value and time. We show that the quantitative model provides a consistent description of experimental results and allows us to determine significant parameters of the perovskite solar cell for both the transient and steady-state performance.

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.

While 2D/3D layered perovskites have been the object of comprehensive researches principally focused on increasing the long-term stability observed in 3D perovskites, significant opportunities are still open concerning the application of different kinds of cations which are outside the sphere of primary amines, which are the cations most usually applied. Our results demonstrate that the materials and the solar cells prepared with dipropylammonium iodide (DipI), a bulky secondary ammonium cation of small size, lead to obtaining not only efficient and thermodynamically stable materials but also robust towards heat stress. Time-resolved studies point out longer carrier´s lifetime for 2D/3D layered perovskites fabricated with this bulky cation than systems based on bulky primary ammonium cations, which allowed us to obtain PCE=12.51% (n=10), 15.78% (n=50) and 17.90% (n=90). We determine that the concentration of perovskite material after 240 min at 100° C is until 575% greater in the 2D/3D perovskite (n=10) than the observed in 3D perovskite films. The material stability also improves the thermal stability of the photovoltaic devices presenting an efficiency drop of just 4% for n=50 and n=10 after thermal annealing while the performance drop for reference 3D samples in the same conditions was of higher than 80%.

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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.