3 - Quantum dots synthesis for photovoltaic cells

Abstract

Quantum dots (QDs) are semiconductor nanoparticles that confine the motion of electrons and holes in three spatial directions. The particle size is less than 10−8m. Owing to the direct bandgap characteristics, QDs (low-cost materials) also have strong optical absorption property, thus making them strong candidates for future photovoltaic devices. The first solar cell made from QD material was based on Schottky junction type cell. This cell consists of transparent conducting oxide (TCO) glass, QD, and metal. Potential difference is created at the junction or known as Schottky barrier due to the difference in energy level between the conduction band (CB) of the QD and Fermi level of the metal. Thin insulating layer placed between QD and the metal can improve the performance of the Schottky junction solar cell. Heterojunction QD solar cell has been developed to overcome the problem or limitation in Schottky QD solar cell. The structure can be either planar or bulk heterojunction. These types of cells consist of TCO glass, n-type metal oxide (MO) semiconductor, p-type QD, and metal electrode. Electric field is generated at the MO–QD junction. When the cell is illuminated through the TCO glass, MO and QD, photoelectrons are generated in QD near the MO–QD junction. The generated electrons are then injected into the CB of MO and travel to the external circuit. Another type of QD cells based on single junction is photoelectrochemical QD solar cell which is inspired from dye-sensitized solar cell. Photoelectrochemical QD solar cell uses TCO, MO, QD, electrolyte containing redox mediator and counter electrode (CE) coated with catalyst material. Photoelectrons are generated in QD followed by injection into CB of MO and then they travel to external circuit. Meanwhile, the redox mediator in electrolyte transfers the electrons back to QD from CE. In this chapter, detailed descriptions on QD synthesis and their application in photovoltaic cells are discussed.