Thickness-Tunable Self-Assembled Colloidal Nanoplatelet Films Enable Ultrathin Optical Gain Media.

We propose and demonstrate construction of highly uniform, multilayered superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using liquid interface self-assembly. These NPLs are sequentially deposited onto a solid substrate into slabs having monolayer-precise thickness across tens of cm2 areas. Because of near-unity surface coverage and excellent uniformity, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin film having 6 NPL layers, corresponding to a mere 42 nm thickness. Furthermore, systematic studies on optical gain of these NPL superstructures having thicknesses ranging from 6 to 15 layers revealed the gradual reduction in gain threshold with increasing number of layers, along with a continuous spectral shift of the ASE peak (∼18 nm). These observations can be explained by the change in the optical mode confinement factor with the NPL waveguide thickness and propagation wavelength. This bottom-up construction technique for thickness-tunable, three-dimensional NPL superstructures can be used for large-area device fabrication.

Theory of plasmonic quantum-dot-based intermediate band solar cells.

High scattering cross section of plasmonic nanoparticles in intermediate band solar cells (IBSCs) based on quantum dots (QDs) can obviate the low photon absorption in QD layers. In this report, we present a modeling procedure to extract the optical and electrical characteristics of a GaAs-based plasmonic intermediate band solar cell (PIBSC). It is shown that metal nanoparticles (MNPs) that are responsible for scattering of incident photons in the absorber layer can lead to photocurrent enhancement, provided that an optimum size and density is calculated. Proper design of QD layers that control the intermediate energy band location, as well as the loss-scattering trade-off of MNPs, can result in an efficiency increase of ∼4.2% in the PIBSC compared to a similar IBSC, and an increase of ∼5.9% compared to a reference GaAs PIN cell. A comprehensive discussion on the effect of intermediate band region width and current-voltage characteristics of the designed cell is presented.