ИСТИНА

Although the efficiency of solar cells based on complex lead iodides has passed the psychological level of 20%, toxicity of lead has raised concerns and now other compounds are scrutinized as potential solar light absorbers to replace their efficient but toxic lead counterparts. So far the photovoltaic efficiency of Bi-based solar cells has reached only 2.1 %,1 but the search for new materials is gaining interest and new compounds have been recently synthesized and examined for potential photovoltaic properties. The intrinsic drawback of complex bismuth halides is their relatively wide band gap of 2.0-2.2 eV that precludes efficient light harvesting;2 therefore, band engineering aiming at finding tools for diminishing the gap thus rendering better charge transfer is coming to the fore. In this report, we present several approaches that have been recently proven to have a reasonable impact on the electronic structure of complex bismuth iodides and indeed lead to narrower band gaps. Firstly, this is condensation of BiI63– octahedra into 1D or 2D polymeric anions; secondly, I2 molecules or I3– anions can be used for stitching the Bi/I substructure; finally, multifold hydrogen bonds and secondary I…I interactions also contribute to narrowing the band gap. We will show examples of such crystal structures, discuss synthesis of these compounds, and present their optoelectronic properties.