Computational Discovery and Optimization of High-Performance Materials for Intermediate-Band Solar Cells

Intermediate band (IB) solar cells (SC) have been proposed to exceed the theoretical efficiency limit of ~30% affecting commercial devices based on the Shockley-Queisser (SQ) model. The idea is to improve the trade-off between photo-generated current and voltage, by introducing “intermediate” energy levels between the valence (VB) and conduction (CB) band of the photo-converting material. These levels act as stepping stones for the sequential absorption of two sub-gap photons, de facto realizing the multi-gap photo-conversion characteristic of complex multi-junction devices, but with the simple SQ architecture. Although this approach pushes the theoretical efficiency limit to ~45%, IBSCs have failed to outperform SQSCs thus far, mostly because of the unavailability of suitable IB materials. Indeed, these have been realized either as highly mismatched alloys, or by introducing quantum dots into typical SQ semiconductors. Both solutions exhibit issues when it comes to optimizing the optical properties, and, more importantly, ensuring high mobility and lifetime of the photo-generated charge carriers. This hinders real IBSC operation. At the E-MRS 2024 Spring Meeting, I have introduced PhANTOM, a project 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). PhANTOM’s goal is to take advantage of the outstanding property tunability of chalcogenides, to discover Earth-abundant crystals suitable for IBSC operation by state-of-the-art computational materials science approaches. These materials should be intrinsically provided with an IB, to prevent degradation of charge carriers mobility and lifetime, and have highly optimized optoelectronic properties. In my previous talk, I have: 1) introduced material property requirements for IBSC operation; 2) defined a figure of merit to rank materials based on their capacity to meet these requirements; 3) applied this ranking system to ~18500 compounds from online databases; 4) inferred relations between atomic structure and relevant material properties; 5) presented some promising material systems such as I2-VI3 chalcogenides. In this talk, starting from the previously inferred property correlations and identified materials, I will present: 1) new IB material classes obtained by combining crystal-structure-prediction methods, LCAO semi-empirical models, and chemical intuition gained within the project; 2) an accurate analysis of their optoelectronic properties, by approaches based on density-functional-theory and many-body-perturbation-theory; 3) their chemical tuning for IBSC operation by a physics-based optimization framework rooted in the tight-binding-method. This work is expected to provide new materials for IBSCs, whose efficiency can be expected to exceed the SQ limit.