Metal halide perovskite single crystals (MHPSCs) are gaining enormous attention in the energy research community due to their impressive responses both in optical sensing and in photovoltaics. The switching from polycrystalline to monocrystalline morphology, not only allows to maintain the outstanding properties that characterize perovskite materials, but also enhances them. However, the poor control over the thickness and size during growing methods leads to considerable differences between surface and bulk responses. Impedance spectroscopy (IS) has been revealed as a powerful technique to understand the kinetics governing polycrystalline perovskite materials. The ionic migration, trap states, and recombination mechanisms occurring in both bulk and surface of the MHPSCs, need to be analyzed in depth to exploit their full potential. Here, we highlight the importance of IS to further advance our knowledge about monocrystalline perovskite materials, bringing to the table the relevance of other small perturbation techniques to complement the IS.

Metal halide perovskites (MHPs) have recently emerged as an ideal semiconductor for photovoltaic application. Much of the advantageous properties of perovskite is dominated by its large charge carrier mobility (μ)...

Quasi-two-dimensional (2D) Ruddlesden–Popper (RP) perovskites are currently considered as the material of choice for the next-generation light-emitting diodes (LEDs) due to their superior optoelectronic properties. Despite their spectacular external quantum efficiency, the excessive surface defect states generated due to the reduced crystal size and phase impurity limit their radiative recombination efficiency. In the present work, we have shown the order of magnitude enhancement of radiative emission in butylamine (BA)-based quasi-2D perovskite (BA)2(MA) n−1Pb n Br3n+1 after passivating with two different Lewis basesa small organic molecule triphenylphosphine oxide (TPPO) and an insulating polymer polymethyl methacrylate (PMMA). The reduction in crystal grain size was observed after passivation, attributed to the complexation of the passivating molecules (PM) on the surface and nanocrystal pinning (A-NCP) phenomena. Both the steady-state and time-resolved photoluminescence study confirmed significant enhancement in fluorescence intensity and improved average lifetime (τavg. = 19.4 ns) after surface passivation. The interaction mechanism between the layered perovskite and PMs was probed with FTIR spectroscopy, XPS, and KPFM study. All these studies confirmed that the CO group in PMMA and PO group in TPPO deactivate the acceptor-type defects (uncoordinated Pb2+ and Br vacancies) in these RP perovskites. Furthermore, the stability of the passivated film enhanced significantly, as confirmed by contact angle measurement. Our study establishes that uncoordinated Pb2+ passivation by a Lewis base provides a viable strategy for photoluminescence (PL) lifetime, intensity, and stability enhancement in quasi-2D perovskite films.

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Two-dimensional (2D) metal halide perovskites have recently emerged as promising photovoltaic materials due to their superior ambient stability and rich structural diversity. However, power conversion efficiencies (PCEs) of the 2D perovskites solar cells (PSCs) still lag behind their three-dimensional (3D) counterpart, particularly due to the anisotropy in the charge carrier mobility and inhomogeneous energy landscape. A promising alternative is Dion−Jacobson (D−J) phase quasi-2D perovskite, where the bulky organic diammonium cations are introduced into inorganic frameworks to remove the weak van der Waals interactions between interlayers and to improve the open-circuit voltage (V oc ). Although the D−J phase 2D perovskite shows a homogeneous energy landscape and better charge transport, their poor crystallinity and existence of higher trap states remain a major challenge for the development of high-efficiency solar cells device. To address this issue, here, we report the eclipsed D−J phase 2D perovskite using 1,5-diaminonaphthalene cation and subsequently treated the film with ammonium thiocyanate (NH 4 SCN) additive to further improve the film crystallinity, out-of-plane orientation, and carrier mobility. We observe that 2 mol NH 4 SCN surface treatment in NDA-based D−J phase perovskite leads to better film morphology and improved crystallinity, as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Time-resolved photoluminescence (TRPL) spectroscopy and steady-state space charge limited current (SCLC) mobility measurement reveal a significant reduction of trap-assisted nonradiative recombination and improvement of carrier mobility in the thiocyanate-passivated perovskite. Consequently, the PCE of the NH 4 SCN-treated (NDA)(MA) 3 (Pb) 4 (I) 13 perovskite device enhanced nearly 46% from 10.3 to 15.08%. We have further studied intensity-dependent J−V characteristics, which demonstrate the reduction of ideality factor, confirming the effective suppression of trap-assisted nonradiative recombination, consistent with the transient PL results. Electrochemical impedance spectroscopy (EIS) confirms the improved charge carrier transport in NH 4 SCN additive-treated devices. Interestingly, our additive-engineered unsealed perovskite devices retained 75% of their initial efficiency after 1000 h of continuous storage under 60% relative humidity. This study opens up the strategy for developing high-efficiency and stable 2D perovskite solar cells.