Aravind Kumar Chandiran

Halide perovskite materials recently attracted wide attention for light-emitting applications. The intense white light emission and excited state lifetimes greater than 1 μs are the hallmarks of a good light-emitting material. Here, we provide a clear design strategy to achieve both of these aforementioned properties in a single material via the introduction of octahedral asymmetry in halide double perovskites Cs2AgMCl6 through iso-trivalent substitution at the M site. In the substituted Cs2AgMCl6, the presence of mixed M3+ sites distorts the [AgCl6]5- octahedra, affecting the parity of the valence and conduction band edges and thereby altering the optical transitions. The distortion also creates a local polarization that leads to an effective photogenerated carrier separation. Considering perovskite series with three M3+ cations, namely Bi3+, In3+ and Sb3+, the mixed trivalent cationic compounds with specific ratios of In3+ and Bi3+ show white light emission with intensity nearly 150 times larger than that of the parent compounds, and are characterised by excited state lifetimes nearing 1 μs. Using single crystal X-ray diffraction, far-infrared absorption, steady-state and time-resolved photoluminescence, bias-dependent photoluminescence, P-E loop traces and density-functional theory calculations, we hence demonstrate the role of octahedral distortion in enhancing white light emission and excited state lifetimes of halide double perovskites.

Tuning the dimensionality in halide perovskites provides an opportunity to obtain the properties desired for optoelectronic devices. In this work, we demonstrate the dimensional reduction of 3D halide double-perovskite Cs2AgBiBr6 by systematically introducing alkylammonium organic spacer CH3(CH2)nNH3+ (n = 1, 2, 3, and 6) of varying chain lengths. The single crystals of these materials were grown, and their structures were studied at 23 and −93 °C. The ethylammonium cation led to a formation of a 0D material, whereas all the other three higher alkyl ammonium spacers resulted in two-dimensional materials. The parent material possessed symmetric octahedra, whereas the modified samples led to both inter- and intra-octahedral distortion, thereby reducing the symmetry of constituent octahedra. The reduction in dimensionality led to a blue shift in the optical absorption spectrum. All these low-dimensional materials show excellent stability, and they are employed as absorbers for solar photovoltaics.

In this work, stable Cs2AgMCl6(M = Bi,In,Sb) double perovskite materials are successfully employed as photoanodes for solar water oxidation. These materials show an extraordinary oxidative stability, where Cs2AgInCl6 and Cs2AgBiCl6 are stable between 0 and 1.2 V (vs Ag-AgCl) and Cs2AgSbCl6 between 0 and 0.75 V (vs Ag-AgCl) over 100 cycles of electrochemical cycling. This enables us to employ these materials for photoelectrochemical (PEC) water oxidation in CH3CN and H2O, with and without IrOx cocatalyst. All three materials show PEC activity, with Cs2AgInCl6 showing the highest performance. At 1.23 V (vs a reversible hydrogen electrode), a photocurrent density of 0.5 mA cm-2 is observed, and with an applied overpotential of 600 mV, the photocurrent increases to about 0.75 mA cm-2. From the band gap of Cs2AgInCl6, the estimated theoretical maximum current is around 0.82 mA cm-2. So, Cs2AgInCl6-coated IrOx gives nearly 60% and about 90% of the theoretical maximum photocurrent at zero bias and at an applied overpotential, respectively. The device is shown to be stable and reproducibility is confirmed.

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.

Herein, we present a strategy to introduce above-room temperature non-centrosymmetry into two-dimensional halide double perovskites (A′4M′M′′X8) using a halogenated A′-site organic linker, 3-chloro/bromo propyl amine. These crystals exhibit anisotropic polarization with three orders of magnitude variation between different crystallographic axes. The non-centrosymmetry is further confirmed by piezo-force microscopy studies and its role in the thermal and optical properties was investigated.