Heterojunction Silicon Based Solar Cells

Abstract

Heterojunction (HJ) silicon solar cells use crystalline silicon wafers for both carrier transport and absorption, and amorphous and/or microcrystalline thin silicon layers for passivation and junction formation. The top electrode is comprised of a transparent conductive oxide (TCO) layer in combination with a metal grid. Heterojunction silicon solar cells have attracted a lot of attention because they can achieve high conversion efficiencies, up to 25%, while using low temperature processing, typically below 200 °C for the complete process. Low processing temperature allows handling of silicon wafers of less than 100 μm thick while maintaining a high yield. In this chapter the best wafer-based homojunction and heterojunction crystalline silicon solar cells are compared, and the advantages of heterojunction silicon solar cells related to the processing of the junction and solar cell operation are explained. The development and recent status of HIT (Heterojunction with Intrinsic Thinlayer) silicon solar cells at the company Sanyo are presented. In order to reduce cost of the HIT solar cells, Sanyo is focusing on reducing the thickness of the silicon wafer. In 2009 the company demonstrated 22.8% conversion efficiency and record high open circuit voltage of 0.743 V on a solar cell based on a 98 μm thick wafer with a total area of 100.3 cm2. Achievements from other research groups such as Tokyo Institute of Technology (Tokyo Tech) and the National Institute of Advanced Industrial Science and Technology (AIST) in Japan, the National Renewable Energy Laboratory (NREL) in the U.S.A., Helmholtz Zentrum Berlin (HZB) and Frauhofer institute for Solar Energy Systems (Frauhofer ISE) in Germany, L’Institut National de l’Energie Solaire (INES) in France, Neuchatel PV-lab of Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland, National Agency for New Technologies, Energy and the Environmentand (ENEA) in Italy and Mingdao University in China are presented. The research activities and results achieved with heterojunction silicon solar cells in the Netherlands are also reported. Challenges to further improve the performance of heterojunction silicon solar cells by minimizing the optical, recombination, and resistance losses in heterojunction silicon solar cells are discussed. These challenges deal with wafer cleaning, suppression of epitaxial growth, controlling thin silicon layer thickness, reduction of absorption losses in thin silicon layers and transparent conductive oxide, surface texturing and the improvement of grid electrodes.