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.