EAGER:TDM Solar Cells: Research on CdSe-Si tandem junction cells

Iowa State University, Ames IA

Investigators

Abstract

Abstract: Non-technical description of the project The conversion of solar energy into electricity using photovoltaic (PV) technology is an important technology with significant potential for reducing global warming. Currently, the costs of the PV solar energy system are dominated by systems related costs, such as structural costs, field wiring and the costs of chemical encapsulant materials and glass. In order to significantly reduce the total costs of solar PV technology, the conversion efficiency needs to be increased from the current ~15-20% to >30%, because most of the systems costs reduce proportionately with an increase inefficiency. The only way to achieve such higher efficiencies is to use a tandem junction solar cell, comprising two materials, one with a high bandgap and one with a lower bandgap. In this project, we will investigate the fundamental properties of an inorganic material, CdSe, then make solar cells in this material, and then make proof-of-concept tandem junction solar cells by combining cells made in this material with cells made in Si. The project will involve the research training of graduate and undergraduate students, including women scientists. The advantage to society is a significant lowering of the cost of solar energy conversion technology, thereby accelerating the deployment of solar energy into the nation's grid. The project is a joint project between Iowa State University and Syracuse University. Technical description of the project: CdSe is an interesting new material with a bandgap (~1.72 eV) which is in the range needed to make tandem cells with Si. But not much is known about the electronic properties of this material, nor have any high efficiency cells been made in this material. A fundamental objective of this project is to study electronic properties of thin films of this material and then make proof-of-concept solar cells in the material. Among the properties to be studied will be doping concentration, mobility of electrons and holes, deep defect densities and recombination phenomena, minority carrier diffusion lengths, electron affinities and energies of band edges. Structural, electronic and optical measurement techniques will be used to measure the relevant properties. CdSe films will be deposited using multi-source evaporation techniques. In addition to studying these fundamental electronic properties, we will make proof-of-concept devices using appropriate heterojunctions. In addition, we will make proof-of-concept tandem cells using tunnel junctions between CdSe and Si. The objective is to demonstrate that one can indeed make tandem junction cells using this new material system. Appropriate heavily doped contact layers will be used on both CdSe and c-Si cells to make efficient tunnel junctions to connect the two cells. The goal is to obtain an open-circuit voltage of ~1.5V in a tandem cell, and also a high fill factor, thereby demonstrating that the tandem junction concept works. The intellectual merit of the project lies in systematically investigating the potential of a stable, inorganic material system for making high efficiency solar cells.