EAGER: Tandem solar cells of two dissimilar material systems

North Carolina State University, Raleigh NC

Investigators

Abstract

ECCS-1665211 Bedair, S. M. North Carolina State University EAGER: Tandem Solar Cells of Two Dissimilar Material Systems ABSTRACT Tandem solar cells are an approach capable of achieving a conversion efficiency higher than 45%. This approach involves stacking several cells with different properties for the optimum utilization of the solar spectrum. Currently all satellite and space communication systems are powered by tandem solar cell structures. However these structures suffer from several limitations such as cost and material related issues. They thus cannot compete for terrestrial applications which are currently dominated by silicon and inexpensive cell technology with efficiencies less than 20%. The goal of this EAGER project is to develop the science and technology to allow the high efficiency tandem cell concept to be applied to cells made from inexpensive material systems. We propose a new approach that relies on intermetallic bonding (IMB) to assemble two or more cells that may be composed of dissimilar material systems into a tandem cell structure. This IMB approach can impact several different fields such as LEDs emitting at different wavelengths and dual wavelength photodetectors. Graduate and undergraduate students involved in this project will gain experience in device fabrication, testing and modeling. Students will be part of research activities that can impact the national goals of renewable energy. The IMB approach relies on the adhesion properties of indium (In) metal. Indium films will be deposited on the existing grid contacts of various solar cells. Bonding between the cells will take place by applying pressure at room temperature, resulting in In-metal bonding between the two cells. The research program will focus on the development of the IMB technique, aligning the bottom contact of the top cell and top contact of the bottom cells, finding the optimum conditions for both mechanical strength and a low resistance connecting junction with minimum shadowing losses. We will investigate MJ structures from off-the-shelf bottom cells [Si (1.1 eV) or CIGS (1.0 eV)] and epitaxially grown junctions from the PI's lab for the top cell based on InGaP/multiple quantum well structures or AlGaAs to tune the band gap between 1.55 and 1.7 eV. The above combinations can approach efficiencies close to 30%, in a two terminal device structure. We have successfully applied the IMB technique to bond GaAs/InGaP and Si/GaAs showing both mechanical stability and electrical conductivity. We have shown that the IMB offers a very low resistivity interconnects and was successfully applied to demonstrate the proof of concept in the case of GaAs/Si tandem cell.