Aravind Kumar Chandiran

The stability of the absorber materials in an aqueous medium is the key to developing successful photoelectrochemical (PEC) solar fuel devices. The halide perovskite materials provide an opportunity to tune desired optoelectronic properties and show very high photovoltaic power conversion efficiency. However, their stability is poor as they decompose instantly in an aqueous electrolyte medium. Here the most stable vacancy ordered double perovskites Cs2PtCl6 and Cs2PtBr6, which remain intact in a wide range of pH values between 1 and 13 is reported. These materials also possess excellent absorption properties covering a significant portion of the visible spectrum. Like conventional ABX3 materials, these ultrastable materials offer tunability in optical properties via mixed halide sites. Through anion exchange, the conversion of Cs2PtCl6 to Cs2PtBr6 through core–shell conversion mechanism is shown. The latter led to the formation of type-II heterostructures. The electrochemical properties of these materials are investigated in detail and their ability to carry out solar water oxidation on an unprotected photoanode, with photocurrent density of >0.2 mA cm−2 at 1.23 V (vs. RHE) is demonstrated.

In this work, osmium-based vacancy-ordered double perovskites Cs2OsX6 (X = Cl−, Br−, I−) are reported and the role of halides on stability, optical and photoelectrochemical properties is investigated. All these three materials crystallize in a cubic phase like Cs2SnX6 or Cs2PtX6 and possess extraordinary stability in ambient conditions and remain stable in strong acids and bases (from pH 1 to 11). One of the unique properties of these materials is that they show panchromatic visible and NIR absorption (up to 1200 nm) through ligand-to-metal [X (Cl−/Br−/I−) to Os] charge transfer and another NIR absorption between 1800 and 2500 nm due to Os d-d transition. Their pH stability and panchromatic light absorption properties enabled them to be employed as photoanodes in PEC water-splitting devices.

Light induced changes in the optoelectronic properties affect the performance and the stability of halide perovskites. In this work, we report the real-time visualization of the photobrightening (PLB) effect using confocal laser scanning microscopy wherein the photon induced enhancement in photoluminescence is observed and their role in conductivity and photovoltaic properties are studied. The methodology is inspired from the Fluorescence Recovery After Photobleaching (FRAP) technique that is traditionally used to study biological cells. The role of composition, and surface/grain boundaries of perovskites, wavelengths, and intensity of illuminating photons, and time of illumination on the photobrightening or photobleaching is thoroughly investigated. The CH3NH3PbI3 exhibits a dominant photobrightening effect, with green photons showing more PLB than blue or red photons. The study of PLB between films and single crystals clearly shows the effect is a surface phenomenon. The presence of mixed iodide/bromide or pure bromide in the halide site and formamidinium or cesium in the A site suppressed the PLB. The strain relaxation in the organic site is found to be responsible for the PLB effect, and it enhanced the overall conductivity in material leading to better photovoltaic performance.

Oxide ferroelectric materials based on the ABO3 structure possess net electric polarization at zero applied fields that give rise to new photovoltaic concepts. One of the peculiar properties specific to ferroelectric materials is the ‘anomalous photovoltaic effect’ (APVE), where the photovoltage of the single junction device exceeds the bandgap. Like many next-generation photovoltaic concepts, namely hot-electron harvesting, multi-exciton generation, and up- or down-conversion, the ferroelectric photovoltaics can lead to a phenomenal revolution in the solar cell domain. Although APVE is observed in oxides, these materials possess a large optical bandgap and low charge carrier diffusion length, limiting their ability for further exploration in high performance solar cells. Recently, a new class of organic-inorganic halide perovskite (OIHP) materials with exceptional structural tunability and extraordinary optoelectronic properties has emerged. These materials were successfully employed in solar cells, and have shown excellent power conversion efficiency of over 25%. A sub-class of OIHPs based on a two-dimensional layered structure was shown to possess a non-centrosymmetric structure with net ferroelectric polarization. These materials provide a new opportunity to explore the anomalous photovoltaic effect and potentially improve the conversion efficiency. There are at least 28 different layered halide perovskites with 23 unique organic cations reported to possess above-room-temperature ferroelectrics. In this review, we have analyzed all of these systems and presented three different design strategies to introduce polarization in the perovskite crystal structure: (i) alloying of organic cations that possess a net dipole, (ii) halogen substitution in organic linkers, and (iii) the use of homochiral polar molecules. In the second half of this review, we discuss the application space made possible by these ferroelectric semiconductors, namely photodetectors, solar cells, light-emitting diodes, and piezoelectric detectors. We conclude this review with a roadmap for employing these 2D-layered halide perovskite ferroelectric materials for highly efficient optoelectronic devices, specifically solar cells.