Structure-property Correlation of Front Side Metallization Contacts for High-efficiency, Single-crystalline Si Solar Cells

Abstract

Mono-crystalline Si solar cell based on screen-printed front side metallization is the most widely used technology for industrial solar cells. The efficiency of Si solar cells is limited by electrical losses associated with the screen-printed front side contacts. Electrical properties of cells processed with different pastes were considered in detail for selecting cells for microstructural investigations. For such cells temperature dependent I−V curves were acquired and will be discussed. Detailed microstructural and chemical analysis of the front side contacts processed with different pastes yield paste specific microstructural features, which can be correlated to losses related to the series resistance. By this procedure, pastes can be qualified and the cell efficiency can be increased. Results are presented of advanced solar cell characterization applied to high-efficiency p- (18 %) and n- type (20 %) Si solar cells. A new methodology has been set up to investigate the front side metallization of solar cells by employing Scanning Electron Microscopy (SEM) and analytical Transmission Electron Microscopy (TEM). The front side Si/metallization interface consists of a Si emitter containing Ag nanocrystals, a glass layer typically <1 µm thick that contains nano-Ag colloids and on top of it a bulk metallization finger. The important results are: (i) The contact resistance depends on the Si surface orientation. (ii) High-efficiency p- and n-type cells yield similar microstructure of the glass layer and electrical properties of the front side metallization. (iii) The quantitative chemical composition of the glass layer of p- and n-type cells consists of (SiOx)Pb, as main constituent and Zn, Ti, Al, Ag, P and B as minor constituents with mole fractions above the detection limit of EDX. The glass layer is, therefore, considered a dirty semiconductor rather than a perfect insulator. (iv) The presence of a semiconducting rather than an insulating glass layer containing metallic Ag colloids and impurities indicates that the glass layer is part of the relevant current path between the emitter and Ag bulk metallization. This is in contrast to existing models in which the glass layer is considered to be an insulator.