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State-of-the-art hybrid perovskites have demonstrated excellent functionality in photovoltaics and light-emitting applications, and have emerged as a promising candidate for exciton-polariton (polariton) optoelectronics. In the strong coupling regime, polariton formation and Bose-Einstein condensation (BEC) have been demonstrated at room-temperature in several perovskite formulations. Thermodynamically, low-threshold BEC requires efficient scattering to $k_{||}$ = 0, and many applications demand precise control of polariton interactions. Thus far, the primary mechanisms by which polaritons relax in perovskites remains unclear. In this work, we perform temperature-dependent measurements of polaritons in low-dimensional hybrid perovskites with high light-matter coupling strengths ($\hbar \Omega_{Rabi}$ = 260$\pm$5 meV). By embedding the perovskite active layer in a wedged cavity, we are able to tune the Hopfield coefficients and decouple the primary polariton relaxation mechanisms in this material for the first time. We observe the thermal activation of a bottleneck regime, and reveal that this effect can be overcome by harnessing intrinsic scattering mechanisms arising from the interplay between the different excitonic species, such as biexciton-assisted polariton relaxation pathways, and isoenergetic intracavity pumping. We demonstrate the dependence of the bottleneck suppression on cavity detuning, and are able to achieve efficient relaxation to $k_{||}$ = 0 even at cryogenic temperatures. This new understanding contributes to the design of ultra-low-threshold BEC and condensate control by engineering polariton dispersions resonant with efficient relaxation pathways, leveraging intrinsic material scattering mechanisms for next-generation polariton optoelectronics., Comment: *Corresponding authors: danedeq@mit.edu, bulovic@mit.edu

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting-recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to on-demand tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing-coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components-additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices., Comment: Chemical Society Reviews, 2021, Advance Article

Layered lead halide A'2An-1PbnI3n+1 perovskites (2D LHPs) are attracting considerable attention as a more stable alternative with respect to APbI3 parent phases - a promising material for a new generation of solar cells. However, a critical analysis on the photostability of 2D perovskites homologous row is still lacking. In this work, we perform a comparative study of BA2MAn-1PbnI3n+1 (BA - butylammonium, MA - methylammonium) 2D LHPs with different layer number n = 1-3, considered as study-case systems, and MAPbI3, as a reference. We discuss a stability testing protocol with general validity, comparing photometrical determination of iodine-containing products in nonpolar solvents, X-ray diffraction, and photoluminescence spectroscopy. The resulting photostability trends in inert atmosphere demonstrate a nonmonotonic dependence of degradation rate on the perovskite layer thickness n with a "stability island" at n > 3 which is caused by a combination of antibate factors of electronic structure and chemical composition in the family of 2D perovskites. We also identify a critical oxygen concentration in the surrounding environment that affects the mechanism and strongly enhances the rate of layered perovskites photodegradation.

Hybrid perovskite solar cells, with their power conversion efficiency (PCE) exceeding 22%, have been representing a revolutionary concept for future energy power generation. Although listed among the Top 10 Emerging Technologies of 2016, device longevity is the actual bottleneck for their real uptake in the market. Here we design an ultra-stable molecular junction of two/three dimensional (2D/3D) perovskites. It consists of a 2D (HOOC(CH2)2NH3)2PbI4, anchored at the oxide substrate, that templates the growth of a highly ordered 3D CH3NH3PbI3 perovskite stabilizing in the orthorhombic phase, even at room temperature. The unique and exceptional 2D/3D structure yields 14.6% PCE in solar cells with Spiro-OMeTAD and Au, and 12.9% PCE in hole-conductor free architecture. Aiming at the up-scaling of this technology, we realize 10x10 cm2 large-area photovoltaic modules by a low-cost, fully printable, industrial-scale process delivering 11.2% PCE. We demonstrate a record stability in the PCE of 5,000 hours, setting the direction for the new generation of carbon free energy., Comment: 13 pages, 4 figures