Recently, lead free all-inorganic double perovskites have revolutionized photovoltaic research, showing promising light emitting efficiency and tunability via modification of inherent structural and chemical properties. Here, we report a combined experimental and theoretical study on the variation of carrier–lattice interaction and optoelectronic properties of Cs2AgIn1–x Bi x Cl6 double perovskite nanocrystals with varying alloying concentrations. Our UV–vis study confirms the parity allowed first direct transition for x ≤ 0.25. Using a careful analysis of Raman spectra assisted with first-principles simulations, we assign the possible three types of active modes to intrinsic atomic vibrations; 2 T2g modes (one for translational motion of “Cs” and other for octahedral breathing), 1 Eg and 1 A1g mode for various stretching of Ag–Cl octahedra. Ab-initio simulation reveals dominant carrier-phonon scattering via Fröhlich mechanism near room temperature, with longitudinal optical phonons being effectively activated around 230 K. We observe a noticeable increase in hole mobility (∼4 times) with small Bi alloying, attributed to valence band (VB) maxima acquiring Bi-s orbital characteristics, thus resulting in a dispersive VB. We believe that our results should help to gain a better understanding of the intrinsic electronic and lattice dynamical properties of these compounds and provide a base toward improving the overall performance of double perovskite nanocrystals.

Solar energy plays an important role in substituting the ever declining source of fossil fuel energy. Finding novel materials for solar cell applications is an integral part of photovoltaic research. Hybrid Lead halide perovskites are one of, if not the most, well sought material in the photovoltaic research community. Its unique intrinsic properties, flexible synthesis techniques, and device fabrication architecture made the community highly buoyant over the past few years. Yet, there are two fundamental issues which still remain a concern i.e. stability in external environment and toxicity due to Pb. This led to a search for alternative materials. More recently, double perovskite (A2BB X6 (X=Cl,Br,I)) materials have emerged as a promising choice. Few experimental synthesis and high throughput computational studies have been carried out to check for promising candidates of this class. The main outcome from these studies, however, can essentially be summarized into two categories, (i) either they have indirect band gap or (ii) direct but large optical band gap which are not suitable for solar device. Here we propose a large set of stable double perovskite materials, Cs2BB X6 (X=Cl,Br,I), which show indirect to direct band gap transition via small Pb +2 doping at both B and B sites. This is done by careful band (orbital) engineering using first principles calculations. This kind of doping has helped to change the topology of band structure triggering the indirect to direct transition which are optically allowed. It also reduces the band gap significantly, bringing it well in the visible region. We also simulated the optical absorption spectra of these systems and found comparable/higher absorption coefficient and solar efficiency with respect to the state of the art photovoltaic absorber material CH3NH3PbI3. A number of materials Cs2(B0.75Pb0.25)(B 0.75Pb0.25)X6 (for various combinations of B, B & X) are found to be promising, some with better stability and solar efficiency than CH3NH3PbI3, but with much less toxicity. Experimental characterization of one of the material, Cs2(Ag0.75Pb0.25)(Bi0.75Pb0.25)Br6 is carried out. Measured properties such as band gap and chemical stability agrees fairly well with the theoretical predictions. This material is shown to be even more stable than CH3NH3PbI3, both under the sufficient humidity (∼55%) and temperature (T=338 K), and hence has potential to become better candidate than the state of the art material. arXiv:1801.07078v4 [cond-mat.mtrl-sci]

Stability of the anode catalysts for PEM water electrolysers can be substantially improved by combining the catalytic component with antimony oxides. However, the mechanisms of the catalyst stabilisation differ depending on the active element used.

We have successfully substituted trivalent Bi3+ with divalent Pb2+ in Cs3Bi2Br9-layered perovskites. Controlled heterovalent Pb substitution in these Cs3Bi2Br9-layered perovskites reduces the band gap because of the emergence of defect states in between the bands. These heterovalent Pb-substituted Cs3Bi2Br9 bulk perovskite compounds are successfully synthesized for the first time by chemical reprecipitation method. X-ray photoelectron spectroscopy analysis indicate that lead substitution in the structure is in Pb2+ form, which creates a charge imbalance in the compound as it replaces Bi3+ from the layered perovskite structure. Such charge imbalance is compensated either by bromine vacancies (VBr) or interstitial cesium (Csi) additions. VBr or Csi in Cs3Bi2Br9 along with PbBi creates defect states in between the bands, which results in redshift in the layered perovskite band. Band structure calculations indeed confirm the onset of such defect states, responsible for the redshift. A more detailed defect physics simulation indicates that the defect complex PbBi + VBr is more probable to form if Pb is rich in the environment, which consequently introduces a few deep level defects responsible for the reduction of the band gap. Understanding of the electronic structure and defect physics of such heterovalent Pb-substituted Cs3Bi2Br9 will strengthen the future photovoltaic and optoelectronic applications.

We have systematically investigated a family of newly proposed twodimensional MA 2 N 4 materials (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; A = Si, Ge) using first-principles calculation. We categorize the potential of these materials into three different applications based on accurate simulation of band gap (using Hybrid HSE06 functional) and the associated descriptors. Three candidate materials (MoGe 2 N 4 , HfSi 2 N 4 , and NbSi 2 N 4 ) turn out to be extremely promising for three different applications. MoGe 2 N 4 and HfSi 2 N 4 monolayers show strong optical absorption in the visible range, including high transition probability from the valence to conduction band. The GW+BSE calculations confirm a strong excitonic effect in both the systems. With a band gap of 1.42 eV, multilayer MoGe 2 N 4 shows reasonably large simulated efficiency (∼15.40%) and hence can be explored for possible photovoltaic applications. High optical absorption, suitable band gap/edge positions, and the CO 2 activation make HfSi 2 N 4 monolayer a promising candidate for photocatalytic CO 2 reduction. NbSi 2 N 4 , on the other hand, belongs to a new class of spintronic material called a bipolar magnetic semiconductor, recommended for spin-transport-based applications.

Structural stability, electronic structure and optical properties of CH3NH3BaI3 hybrid perovskite is examined from theory as well as experiment. Solution-processed thin films of CH3NH3BaI3 exhibited a high transparency in the wavelength range of 400 nm to 825 nm (1.5 eV to 3.1 eV for which the photon current density is highest in the solar spectrum) which essentially justifies a high bandgap of 4 eV obtained by theoretical estimation. Also, the XRD patterns of the thin films match well with the {00l } peaks of the simulated pattern obtained from the relaxed unit cell of CH3NH3BaI3, crystallizing in the I4/mcm space group, with lattice parameters, a = 9.30Å, c = 13.94Å. Atom projected density of state and band structure calculations reveal the conduction and valence band edges to be comprised primarily of Barium d -orbitals and Iodine p-orbitals, respectively. The larger band gap of CH3NH3BaI3 compared to CH3NH3PbI3 can be attributed to the lower electro-negativity coupled with the lack of d -orbitals in the valence band of Ba 2+ . A more detailed analysis reveals the excellent chemical and mechanical stability of CH3NH3BaI3 against humidity, unlike its lead halide counterpart, which degrades under such conditions. We propose La to be a suitable dopant to make this compound a promising candidate for transparent conductor applications, especially for all perovskite solar cells. This claim is supported by our calculated results on charge concentration, effective mass and vacancy formation energies.PACS numbers: 81.10.Dn, 61.50.Ah, 61.10.Nz, 42.70.Qs, Recently, compounds in the organic-inorganic halide perovskite family (AB X 3 : A is an organic cation, B is an inorganic cation, and X is a halide element) have garnered a lot of attention in the solar photovoltaic community. This is due to their superior optoelectronic properties, easy synthesis techniques and variety of compounds that can be obtained via simple substitutions of the A, B and X ions. Specifically, solar cells, with (CH 3 NH 3 ) + as the A cation and Pb 2+ as the B cation, have shown a rapid growth in the solar-to-electricity power conversion efficiency.1-5 The lead halide perovskite solar cell was first introduced by Kojima et al in 2009, wherein it was used in a dye-sensitized solar cell architecture.6 Much of the research in recent times has focused on solid-state cells with different architectures, hole transport layers, compositional engineering, and synthesis techniques.1,3,7-10 Even then, there are some caveats associated with the various components of the CH 3 NH 3 PbX 3 -based solar cells: the stability of the absorber material in ambient conditions and the presence of Pb to name a couple. Active research to address these problems is being conducted worldwide through suitable replacements to both the CH 3 NH 3 and Pb cations.Tunability of the properties by changing the constituent elements gives this class of material more scope of research and applicability.11-13 Such tunability in the bandgap has been observed in the oxide perovskites, wher...

Discovery of high-performance materials remains one of the most active areas in photovoltaics (PV) research. Indirect band gap materials form the largest part of the semiconductor chemical space, but predicting their suitability for PV applications from first-principles calculations remains challenging. Here, we propose a computationally efficient method to account for phonon-assisted absorption across the indirect band gap and use it to screen 127 experimentally known binary semiconductors for their potential as thin-film PV absorbers. Using screening descriptors for absorption, carrier transport, and nonradiative recombination, we identify 28 potential candidate materials. The list, which contains 20 indirect band gap semiconductors, comprises well-established (3), emerging (16), and previously unexplored (9) absorber materials. Most of the new compounds are anion-rich chalcogenides (TiS3 and Ga2Te5) and phosphides (PdP2, CdP4, MgP4, and BaP3) containing homoelemental bonds and represent a new frontier in PV materials research. Our work highlights the previously underexplored potential of indirect band gap materials for optoelectronic thin-film technologies.