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

Abstract

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 A_Q = πa_fs = 0.02293 (where a_fs is the fine structure constant). However, our findings contradict the assumption that the absorbance of n layers is simply nA_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 A(n) < nA(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.