Intermediate-Band Solar Cells - From Atoms to Devices Through Computational Materials Design

Intermediate band (IB) solar cells (SC) have been proposed to outperform commercial devices based on the Shockley-Queisser (SQ) model. By introducing “intermediate” energy levels between the valence (VB) and conduction (CB) band of the photo converting material, the trade-off between photo-generated current and voltage is improved, while preserving the same simple device architecture. This is due to the activation of the sequential absorption of two sub-gap photons, and the collection of the corresponding photo-generated charge carriers. The theoretical power conversion efficiency limit thus increases from ~30% to ~45%. Despite their enormous potential, IBSCs have failed to outperform SQSCs thus far, because of the lack of materials with optimal properties. In fact, IB photo-converters are typically realized as highly mismatched alloys (HMA), or III-V semiconductors “doped” with quantum dots. In addition to complex manufacturing, the disorder-/defect- based nature of these solutions degrades charge transport properties to such an extent that true IBSC operation is hindered. To overcome this problem, though facing a very tough materials design challenge, one could envision the realization of materials naturally born with an intermediate band, that is, having the IB electronic structure in their intrinsic state. In this talk, I will present the research activities carried out in this direction, with focus on the highly tunable and industrially appealing class of chalcogenides. In particular, I will discuss: 1) material property requirements and approaches to quantify the quality of a material for IBSC operation; 2) screening of large material property databases and infererence of correlations between atomic structure and relevant optoelectronic properties; 3) identification of promising IB chalcogenides by combining the gained chemical intuition with crystal structure prediction methods and semi-empirical band structure models; 4) characterization of these promising materials by state-of-the-art DFT and many-body-perturbation-theory methods, and their optimization through tight-binding approaches; 5) some basic solar cell design incorporating these optimized IB materials, to exceed the SQ limit. The work has been carried out as part of the project PhANTOM, funded by the Italian Ministry of University and Research (MUR) through the National Recovery and Resilience Plan (PNRR) with the call Young Researchers – Seal of Excellence (CUP number: E13C22002920006).