Intermediate Band PV Materials: Proposal of New Systems and Experimental Realizations

The intermediate band (IB) solar cell, based on the absorption of two low energy photons to promote one electron across the full bandgap, has been proposed to increase PV efficiency by a factor above 1.5. Using DFT calculations we show here that silicon materials with certain elements inserted and which have been experimentally prepared and reported in the literature, develop an IB electronic structure able to implement an IB solar cell. In particular, this happens for Ti atoms introduced via ion implantation and occupying interstitial sites, and for S or Se atoms inserted in lattice sites by similar methods. In addition, we present new experimental data for another IB material previously proposed and synthesized by us, viz. vanadium-substituted In2S3: using a photocatalytic method, in which photogenerated electrons and holes are transferred to molecules at the surface of this material (made in powder form and suspended in an aqueous solution) producing thus chemical reactions, we show that insertion of V in In2S3 allows to generate electrons and holes, which can be extracted at the material surface, also with photons having energy lower than the In2S3 bandgap width. This material could thus be used to build an IB thin film PV cell.