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.

Multiple exciton generation in quantum dots versus singlet fission in molecular chromophores for solar photon conversion

“…(1) in semiconductor nanocrystals (quantum dots (QDs), quantum wires (QWires and quantum rods (QRs)), the exciton multiplication process occurs when the absorbed photons are at least twice the nanocrystal (NC) band gap (E g ) [1][2][3][4][5]; this requirement satisfies Conservation of Energy and the process is called multiple exciton generation (MEG) [2][3][4][5]. In the literature, MEG and CM are used interchangeably.…”

“…There are two general approaches to exceed the Shockley-Queisser (S-Q) thermodynamic limit [1][2][3][4][5][6][7][8][9][10][11][12][13][14] for the efficiency of converting solar radiation into electricalor chemical-free energy via the general mechanism of producing multiple photogenerated excitons (bound electron-hole pairs) from single absorbed photons. If the units for the input and output terms are watts/cm 2 then the conversion efficiency is called the power conversion efficiency (PCE) and the S-Q limit for the standard solar spectrum (AM1.5) at 1 sun intensity is 32% [14].…”

Mechanism and Role of Nanotechnology in Photovoltaic Cells and Applications in Different Industrial Sectors

“…These nano based semiconductor quantum dots are working on the principles of band gaps that are the main design and vital part specifically to convert also longer-wave light and thus increase the efficiency of the solar cells. These semiconductor quantum dots are made through metallic combinations of various elemental compositions [19,22]. The most important metals are Si/Ge or Si/Be Te/Se are considered.…”

Multiple Exciton Generation in Nanostructures for Advanced Photovoltaic Cells

“…Typical values for such Bohr radii are a B (PbSe) = 46nm , a B (PbS) = 20nm and a B (Si) = 4.7nm . 38 QDs of these sizes can now by synthesised easily.…”

“…In this regard, theory suggests that, for this to be the case, the radius of the QD should be less than the bulk exciton Bohr radius, calculated as a B � ε� h 2 /μe 2 , with ε and μ being, respectively, the dielectric constant and the reduced mass of the electron and hole. Typical values for such Bohr radii are a B (PbSe) � 46nm, a B (PbS) � 20nm, and a B (Si) � 4.7nm [40]. QDs of these sizes can now by synthesised easily.…”