Toward Intrinsic Intermediate-Band Materials for Solar Cells

Intermediate-band (IB) solar cells (SC) have been proposed to exceed the theoretical efficiency limit of commercial devices based on the Shockley-Queisser (SQ) model, also referred to as single-junction (SJ) SCs. SJSCs employ a single-gap semiconductor to convert sunlight photons into free electrons and holes, whose ordered motion toward opposite terminals establishes a net electrical current. By scaling inversely with the number of photo-generated carriers and directly with the energy available from each one of them, the single-gap imposes a trade-off between current and voltage that limits the achievable efficiency to ~30%. To exceed this limit, multi-junction (MJ) SCs combining multiple single-gap devices have been proposed. Yet, they rely on complex and expensive architectures that render their cost-effectiveness unclear despite their outstanding efficiency of ~45%. IBSCs are based on the same architecture as SJSCs, but introduce an “intermediate band” of energy levels between valence (VB) and conduction (CB) bands of the photo-converting material, to enable sequential sub-gap two-photon absorption. This adds sunlight conversion at lower energies without affecting the maximum energy available from each charge carrier, still given by the VB-CB gap. The improved current-voltage trade-off dramatically increases the efficiency limit to ~45%, with a device that uniquely combines the multi-gap photo-conversion of MJSCs and the systemic simplicity of SJSCs. Despite their outstanding potential, IBSCs have not outperformed SJSCs so far, mostly because of sub-optimal properties of the photo-converting material. Indeed, IB absorbers are usually realized by doping a single-gap semiconductor. On the one hand, the very high doping concentration required to form the IB degrades the electrical properties. On the other one, SJSC semiconductors are often used because of technological convenience, but their VB-CB gap is too small (<1.6eV) for the IB paradigm (optimal value ~2eV). In this contribution, I will discuss the realization of earth abundant intrinsic (doping-free) IB materials such as sulfides and selenides, potentially unaffected by the aforementioned electrical issues, and the optimization of their optical properties. In particular, I will: 1) examine IB property requirements and define suitable figure of merits to rank materials for IB operation; 2) present promising material classes identified by screening online databases; 3) analyze the interplay between atomic structure and relevant material properties, to provide material design guidelines and obtain optimized materials, on the basis of optically accurate DFT simulations and interpolation schemes based on the tight-binding method. 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).