Krishnamoorthy Thirumal

Bio: Krishnamoorthy Thirumal is an academic researcher from Nanyang Technological University. The author has contributed to research in topics: Perovskite (structure) & Photovoltaic effect. The author has an hindex of 10, co-authored 14 publications receiving 721 citations.

Rb as an Alternative Cation for Templating Inorganic Lead-Free Perovskites for Solution Processed Photovoltaics

Abstract: Even though perovskite solar cells have reached 22% efficiency within a very short span, the presence of lead is a major bottleneck to its commercial application. Tin and germanium based perovskites failed to be viable replacements due to the instability of their +2 oxidation states. Antimony could be a possible replacement, forming perovskites with structure A3M2X9. However, solution processing of Cs, organic ammonium based Sb perovskites result in the formation of the dimer phase with poor charge transport properties. Here we demonstrate that Rb can template the formation of the desired layered phase irrespective of processing methodologies, enabling the demonstration of efficient lead-free perovskite solar cells.

Computational Study of Halide Perovskite-Derived A2BX6 Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

Abstract: The electronic structure and energetic stability of A2BX6 halide compounds with the cubic and tetragonal variants of the perovskite-derived K2PtCl6 prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A2BX6 compounds with A = K, Rb, and Cs; and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se, and Te; and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For varia...

Morphology-Independent Stable White-Light Emission from Self-Assembled Two-Dimensional Perovskites Driven by Strong Exciton–Phonon Coupling to the Organic Framework

Abstract: Hybrid two-dimensional (2D) lead halide perovskites have been employed in optoelectronic applications, including white-light emission for light-emitting diodes (LEDs). However, until now, there have been limited reports about white-light-emitting lead halide perovskites with experimental insights into the mechanism of the broadband emission. Here, we present white-light emission from a 2D hybrid lead chloride perovskite, using the widely known phenethylammonium cation. The single-crystal X-ray structural data, time-resolved photophysical measurements, and density functional theory calculations are consistent with broadband emission arising from strong exciton–phonon coupling with the organic lattice, which is independent of surface defects. The phenethylammonium lead chloride material exhibits a remarkably high color rendering index of 84, a CIE coordinate of (0.37,0.42), a CCT of 4426, and photostability, making it ideal for natural white LED applications.

Tunable room-temperature spin-selective optical Stark effect in solution-processed layered halide perovskites

Abstract: Ultrafast spin manipulation for opto-spin logic applications requires material systems that have strong spin-selective light-matter interaction. Conventional inorganic semiconductor nanostructures [for example, epitaxial II to VI quantum dots and III to V multiple quantum wells (MQWs)] are considered forerunners but encounter challenges such as lattice matching and cryogenic cooling requirements. Two-dimensional halide perovskite semiconductors, combining intrinsic tunable MQW structures and large oscillator strengths with facile solution processability, can offer breakthroughs in this area. We demonstrate novel room-temperature, strong ultrafast spin-selective optical Stark effect in solution-processed (C6H4FC2H4NH3)2PbI4 perovskite thin films. Exciton spin states are selectively tuned by ~6.3 meV using circularly polarized optical pulses without any external photonic cavity (that is, corresponding to a Rabi energy of ~55 meV and equivalent to applying a 70 T magnetic field), which is much larger than any conventional system. The facile halide and organic replacement in these perovskites affords control of the dielectric confinement and thus presents a straightforward strategy for tuning light-matter coupling strength.

Multidimensional Perovskites: A Mixed Cation Approach Towards Ambient Stable and Tunable Perovskite Photovoltaics.

Abstract: Although halide perovskites are able to deliver high power conversion efficiencies, their ambient stability still remains an obstacle for commercialization. Thus, promoting the ambient stability of perovskites has become a key research focus. In this review, we highlight the sources of instability in conventional 3 D perovskites, including water intercalation, ion migration, and thermal decomposition. Recently, the multidimensional perovskites approach has become one of the most promising strategies to enhance the stability of perovskites. As compared to pure 2 D perovskites, multidimensional perovskites typically possess more ideal band gaps, better charge transport, and lower exciton binding energy, which are essential for photovoltaic applications. The larger organic cations in multidimensional perovskites could also be more chemically stable at higher temperatures than the commonly used methylammonium cation. By combining 3 D and 2 D perovskites to form multidimensional perovskites, halide perovskite photovoltaics can attain both high efficiency and increased stability.

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Understanding the physical properties of hybrid perovskites for photovoltaic applications

Abstract: This Review summarizes advances in understanding the unique physical properties of hybrid perovskites that enable the fabrication of high-efficiency solar cells with high open-circuit voltages, which is crucial for their further development towards commercialization.

Toward Lead-Free Perovskite Solar Cells

Abstract: Since the first reports of solar cells with power conversion efficiencies around 10% in 2012, the science and technology of perovskite photovoltaics has been progressing at an unprecedented rate. The current certified record efficiency of 22.1% makes perovskites the first solution-processable technology to outperform multicrystalline and thin-film silicon. For this technology to be deployed on a large scale, the two main challenges that need to be addressed are the material stability and the toxicity of lead. In particular, while lead is allowed in photovoltaic modules, it would be desirable to find alternatives which retain the unique optoelectronic properties of lead halide perovskites. Here we offer our perspective on the most exciting developments in the materials science of new halide perovskites, with an emphasis on alternatives to lead. After surveying recent developments of new perovskites and perovskite-related materials, we highlight the potential of halide double perovskites. This new family of...

2D Ruddlesden-Popper Perovskites for Optoelectronics

Abstract: Conventional 3D organic-inorganic halide perovskites have recently undergone unprecedented rapid development. Yet, their inherent instabilities over moisture, light, and heat remain a crucial challenge prior to the realization of commercialization. By contrast, the emerging 2D Ruddlesden-Popper-type perovskites have recently attracted increasing attention owing to their great environmental stability. However, the research of 2D perovskites is just in their infancy. In comparison to 3D analogues, they are natural quantum wells with a much larger exciton binding energy. Moreover, their inner structural, dielectric, optical, and excitonic properties remain to be largely explored, limiting further applications. This review begins with an introduction to 2D perovskites, along with a detailed comparison to 3D counterparts. Then, a discussion of the organic spacer cation engineering of 2D perovskites is presented. Next, quasi-2D perovskites that fall between 3D and 2D perovskites are reviewed and compared. The unique excitonic properties, electron-phonon coupling, and polarons of 2D perovskites are then be revealed. A range of their (opto)electronic applications is highlighted in each section. Finally, a summary is given, and the strategies toward structural design, growth control, and photophysics studies of 2D perovskites for high-performance electronic devices are rationalized.

White-Light Emission from Layered Halide Perovskites.

Abstract: ConspectusWith nearly 20% of global electricity consumed by lighting, more efficient illumination sources can enable massive energy savings. However, effectively creating the high-quality white light required for indoor illumination remains a challenge. To accurately represent color, the illumination source must provide photons with all the energies visible to our eye. Such a broad emission is difficult to achieve from a single material. In commercial white-light sources, one or more light-emitting diodes, coated by one or more phosphors, yield a combined emission that appears white. However, combining emitters leads to changes in the emission color over time due to the unequal degradation rates of the emitters and efficiency losses due to overlapping absorption and emission energies of the different components. A single material that emits broadband white light (a continuous emission spanning 400–700 nm) would obviate these problems.In 2014, we described broadband white-light emission upon near-UV excita...