Controlled Geometry Layer-by-Layer Quantum Dot Solar Cells

Abstract

Solar cells based on inorganic quantum dots are promising candidates for next generation energy production, because they can be made using low cost materials and processes, and a cell with an absorption corresponding to the solar spectrum can be made using an array of variously sized dots. To understand the relationship between the composition, nanocrystal layer geometry, and performance of the cell, we varied the layer thickness, arrangement, size, and coupling of the quantum dots. We studied the effects of depositing layers of cadmium selenide (CdSe) quantum dots using a controllable annealing technique. Since the nanocrystals are capped with a solubilizing octadecylamine ligand, removal of these ligands was essential to allow multi-layer deposits of CdSe to form. Annealing the CdSe dots at 150oC removed these ligands allowing further deposits to be placed without dissolution occurring. Spin coating and high annealing temperatures between consecutive layer depositions increased the attachment of the CdSe deposits, allowing formation of multilayer films. Surface morphology of the films was characterized using scanning electron microscopy. Ultraviolet–visible spectra of CdSe thin films were taken to monitor its thickness and any change in its optical properties during processing. We also studied the effect of the contact material on cell performance. Working CdSe quantum-dot-based solar cells were fabricated using different configurations of the contact material. We measured our device’s performance under simulated air mass 1.5 G solar illumination to determine its energy conversion efficiency. The presence of top and bottom oxide contacts gave the best electrical performance, with improvement also observed when increased annealing temperatures and mixtures of different sized quantum dot were used. Efficiencies of ~0.1% have been attained. Future goals involve optimizing multilayer quantum dot deposition to achieve higher-powered devices.