Quantum Confined Semiconductors for Enhancing Solar Photoconversion through Multiple Exciton Generation

Abstract

Quantum-confined semiconductor nanostructures that have at least one dimension small enough to confine the wavefunction of an electron to a size comparable or less that its Bohr radius provide new ways to control solar energy conversion not achievable in thin film or bulk semiconductors. The nanostructures are synthesized in solution-phase chemical reactions, producing stable colloidal solutions, where the reaction conditions can be modified to produce a variety of shapes, compositions, and structures with well-controlled size. If the semiconductor nanostructure is confined in one dimension, quantum films, wells, or discs are produced. Quantum wires or rods (QRs) result from two-dimensional confinement, and quantum dots (QDs) are three-dimensionally confined nanostructure. Combining two or more semiconductors either as alloys or as nano-heterostructures allows for further control over energy flow. There are various strategies to incorporate these novel structures into suitable solar conversion systems and some of these have the potential to convert sunlight more efficiently than the Shockley–Queisser (S-Q) limit of ∼33% and thus may become viable third generation photovoltaic (TGPV) cell architectures. Here we review two such approaches. (1) Multiple exciton generation (MEG) is a process where absorption of one high-energy photon produces multiple charge carriers available for power generation and has recently been observed in PbSe QD-based solar cells demonstrating that one of the tenets of the SQ limit can be overcome. (2) Solution processed multi-junction QD-based solar cells.