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.

Designing a photocatalyst material with reduced recombination of photogenerated charges is one of the most important aspects of hydrogen generation through solar water splitting. Here, we report hydrogen generation using the TiO2/ultrathin g-C3N4 (U-g-CN) heterostructure fabricated using a unique in situ thermal exfoliation process. Multilayer g-CN is converted into U-g-CN having a high surface (∼190 m2/g) area by calcination at ∼550 °C through oxygen-induced exfoliation, which also forms a robust heterostructure with TiO2. In addition, the presence of g-CN also inhibits further growth of TiO2 nanoparticles, thereby retaining a high specific surface area. The presence of U-g-CN causes a redshift (∼0.13 eV) in the absorption edge of heterostructure compared to that of bare TiO2, which extends the light absorption capability. Addition of 40 wt. % of multilayer g-CN to TiO2 shows an enhanced H2 evolution rate, which is ∼15 times and ∼4 times higher compared to that of bare TiO2 and U-g-CN, respectively. Photoluminescence (PL) and time-resolved PL (TRPL) studies indicate a reduced recombination rate of photogenerated charge carriers with an increase in the average lifetime from 10.53 (TiO2) to 13.32 ns (TiO2/U-g-CN40). The interfacial charge transport characteristics studied through impedance spectroscopy reveal a reduced charge transfer resistance at the semiconductor-electrolyte interface, which facilitates faster charge separation due to the heterostructure formation. The band edge positions are estimated through flatband potential from the Mott-Schottky measurements and optical absorption data, indicating a type-II heterojunction. More light absorption and enhanced separation of photogenerated charges at the heterojunction interface lead to better photocatalytic H2 generation.

Storing solar energy using a stable visible light absorbing Cs2AgBiBr6 double perovskite is achieved using a photoelectrochemical (PEC) device with cobalt complexes and methyl viologen redox mediators. Under illumination, a potential gain of nearly 500 mV is achieved for charging. The charge-discharge cycling was carried out, and using in situ emission and FTIR studies, the self-discharge and solvent crossover were investigated. This journal is

Halide perovskite materials with ABX3 show excellent photovoltaic properties, but their stability remains a serious concern. On the other hand, the double perovskite with two formula unit A2BB′X6 shows excellent stability, but their visible light absorption properties are inferior, which restricts them from being employed in solar energy conversion devices. The incorporation of dopant ions in the crystal structure is found to be an effective strategy to introduce the visible light absorption to these materials. In this work, copper is systematically substituted in the place of Ag+ (B site) in three different halide double perovskites, Cs2Ag(Bi/In/Sb)Cl6, and we explored their structural, optical, and electrochemical properties in detail. The role of copper in tuning the properties is found to be different in each of these materials. Only a small fraction of the dopant could be introduced in Cs2AgInCl6 (CAIC) and Cs2AgSbCl6 (CASC), and no copper could be incorporated in Cs2AgBiCl6 (CABC). The presence of Cu+ in the CAIC and CASC crystals (i) distorted the octahedra formed by [InCl6]3- and [SbCl6]3- and (ii) introduced states above the valence band affecting the optical properties. The Cu 3d orbitals are introduced very close to the valence band of CAIC, leading to a change in the absorption onset from 400 to 580 nm. However, for CASC the copper orbitals are pushed toward the conduction band, resulting in near IR absorption with an onset at ∼1300 nm. The color of CAIC changed from white to yellow, and CASC changed from yellow to black upon substitution of copper. Combining structural, optical, and electrochemical characterization experiments, we explained the role of copper in improving the visible light absorption in halide double perovskites.

Structural distortion in halide perovskites is important to tune the optical properties of the materials. The octahedra formed by metal cations and halide anions in these classes of materials remain symmetric; however, the introduction of asymmetry provides enormous opportunities to improve the photoluminescence emission and excited-state lifetimes for their application in white light emitters. In this work, we have systematically introduced asymmetry in vacancy-ordered halide triple perovskite materials Cs3M2X9 (M = Bi3+, Sb3+; X = Cl−, Br−, I−) by mixing trivalent sites in three different halide compounds. The Raman and FT-far-IR measurements were used to investigate the distortion introduced in these materials. The distortion is shown to (i) enhance self-trapped excitonic emission, which is broad and intense leading to emission in the complete visible region and (ii) improve excited-state lifetimes. This strategy to create distortion and its proven ability to improve light emission will find application in light-emitting diodes.

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.

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.

Photoelectrochemical water oxidation is a challenging reaction in solar water splitting due to the parasitic recombination process, sluggish catalytic activity, and electrode stability. Oxide semiconductors are stable in an aqueous medium but show huge charge carrier recombination. Creation of a heterojunction is found to be effective for extracting the photogenerated electrons/holes before they recombine to the ground state. In this work, we created a heterojunction of BiVO4 with vacancy-ordered halide perovskite Cs2PtI6 and used it as a photoanode in PEC water oxidation. Cs2PtI6 is the only halide perovskite that is found to be extremely stable even in strong acids and bases. We utilized the stability of this material and its panchromatic visible light absorption property and made the first unprotected heterojunction dual-absorber photoanode for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO4 gave 0.6 mA cm-2 photocurrent density, whereas the BiVO4/Cs2PtI6 heterojunction shows 0.92 mA cm-2. With the addition of IrOx cocatalyst, at 1.23 V (vs RHE), the heterojunction gave ∼2 mA cm-2. To obtain 2 mA cm-2 photocurrent, pure BiVO4 requires 560 mV overpotential, whereas the heterojunction requires 250 mV. The increase in the photocurrent arises from the increase in the efficiency of charge separation from BiVO4 to Cs2PtI6 and the complementary absorption offered by the latter.

Rhenium sulfide (ReS2) is an exciting two-dimensional (2D) material employed in various optoelectronic applications due to panchromatic visible absorption properties. However, unlike other transition-metal dichalcogenides (TMDs) such as Mo(S/Se)2 and W(S/Se)2, ReS2 has not been explored for solar energy conversion applications. In this work, ReS2 nanolayers are employed as photoabsorber in both liquid and solid-state dye-sensitized type solar cells, in combination with TiO2 and SnO2 as electron transporting layers (ETLs). Various devices with compact/mesoporous TiO2 and mesoporous SnO2 ETLs were tested for photovoltaic performance with spiro-OMeTAD hole conductor as well as with different redox mediators. While no photovoltaic response was seen for mesoporous TiO2 (both liquid and solid-state), a clear photovoltaic performance was observed for mesoporous SnO2 solid-state devices. This was attributed to the energy level alignments unsuitable for charge carrier injection from ReS2 to ETL in the former and suitable in the latter cases. In the case of only compact layer TiO2 ETL, devices in both solid and liquid-state configurations exhibited photovoltaic response with latter showing higher photocurrent than the former, due to additional role played by the energy level of redox mediator. While impedance measurements revealed limitations in conductivity of ReS2 photoabsorber, presence of photovoltaic response in them indicate the significant role played by the energy level alignment of individual components. The presence of photovoltaic effect for compact layer TiO2 devices, in spite of unsuitable energy level alignment seen for TiO2, is attributed to the charge carrier injection assisted by the trap states.