Double perovskites overtaking the single perovskites: A set of new solar harvesting materials with much higher stability and efficiency

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 that still remain a concern, i.e., the stability in external environment and the toxicity due to Pb. This led to a search for alternative materials. More recently, double perovskite [A(2) B B' X-6 (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 an indirect band gap or (ii) a direct but large optical band gap, which is not suitable for solar devices. Here we propose a large set of stable double perovskite materials, Cs-2 B B' X-6 (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 the band structure, triggering an indirect to direct transition that is optically allowed. It also reduces the band gap significantly, bringing it well into the visible region. We also simulated the optical absorption spectra of these systems and found a comparable/higher absorption coefficient and solar efficiency with respect to the state of the art photovoltaic absorber material CH3NH3PbI3 . A number of materials Cs-2(B0.75Pb0.25)(B-0.75'-Pb-0.25)X-6 (for various combinations of B, B', and 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 materials, Cs-2(Ag0.75Pb0.25)(Bi0.75Pb0.25)Br-6, is carried out. The measured properties such as band gap and chemical stability agree fairly well with the theoretical predictions. This material is shown to be even more stable than CH3NH3PbI3, both under sufficient humidity (similar to 55%) and temperature (T = 338 K), and hence has the potential to become a better candidate than the state of the art materials.