Coupled 2D quantum dot films for next generation solar cells: electronic structure and anomalous light absorption behaviour

There is an increasing interest into the fabrication of high-dimensionality colloidal quantum dot (CQD) arrays, with long-range periodicity and reduced inter-dot distances. The synthesis of such super-solids, where the dots play the role of conventional atoms in a crystal, is, however, still challenging. This work focuses on understanding the physics of those systems and finding applications for them in solar cells of two different architectures: the hot carrier solar cell and the intermediate band solar cell. We combine the accuracy of the atomistic semiempirical pseudopotential method, at the single-dot level, with the versatility of the tight-binding formalism, for the array calculations, to investigate the electronic structure and optical absorption of individual and stacked 2D InX (X = P, As, Sb) CQD arrays (films), and their dependence on the dot material, the number of layers and the interlayer distance. Our results support the hypothesis of a universal behaviour of absorption in 2D materials, already found in graphene and InAs nanomembranes, where the optical absorption in the region 0.5-1.2 eV is nearly independent of the photon energy and equal to a universal quantum of absorption <i>A</i>_Q = π<i>a</i>_fs = 0.02293 (where <i>a</i>_fs is the fine structure constant). However, our findings contradict the assumption that the absorbance of <i>n</i> layers is simply <i>nA</i>_Q. Indeed, according to our results this conclusion only holds for uncoupled stacked layers, whereas the presence of inter-layer coupling degrades the absorption properties, leading to <i>A</i>(<i>n</i>) < <i>nA</i>(1), questioning the wisdom of the efforts of achieving 3D super-solids if the aim is to improve optical absorption. Additionally, we propose a simplified model that accurately describes the intermediate band structure, useful for device simulations.