(Invited) Metallization: The Future of Highly Reliable Fire-through-Dielectric-Contacts to Silicon Solar Cells

Abstract

The reliability of any electron device depends on the functionality of the metal contact to the semiconductor. Thus, the transport mechanism at the semiconductor/metal interface is dictated by the barrier height posed by the difference in the work function of these materials. A low barrier height is desirable to avoid the power loss in the form of heat at the interface, which often lead to delamination of the metal contacts and failure. However, it is highly improbable for the two work functions to ever match, therefore, the doping on the semiconductor is modified to reduce the barrier height. For lithography process which uses evaporated/plated metal contact, the barrier height is often reduced by heavily doping the semiconductor underneath the metal contact and treated in a reducing ambient to remove the Schottky barrier after metal contact process step. However, for the commercial silicon solar cell which relies on fire-through-dielectric-metal contacts with Ag, Ag coated Cu or Cu thick film metal pastes, there is glass at the semiconductor/metal interface, which is part of the thick film metal paste, that adds to the contact resistance. Since solar cell requires the emitter to be transparent in order to avoid the loss of photons, the metal paste must therefore be modified to reduce the barrier height to enable the field emission carrier transport at the interface. This paper will therefore focus on the efforts to understand the characteristics of thick film metal pastes that would lead to a highly reliable fire-through-dielectric-metal contacts.The thick film metal pastes for solar cell metal contacts consists three main components: (i) organics, (ii) glass frits and (iii) the metal powder. The morphology and particle size of the metal powders and glass frits are critical to the contact and gridline resistances. The viscosity and rheology of the paste among other parameters ensure the printability of continuous grid lines. The sintering of the gridline is impacted by the particle size and morphology of the metal powders and glass frits. The percentage of the glass frits determines how fast the underlying dielectric is etched during the sintering step. After the sintering, the proximity of the metal particle to the pn junction of the device dictates the quality of the formulated metal thick film paste. To understand the relationship between the thick film metal paste and the device parameters, the semiconductor/metal interface is often characterized through SEM/EDS to assess the (i) glass thickness, (ii) metal crystallites (iii) effectiveness of glass frit in etching the underlying dielectric, and (iv) the compounds formed and their distribution in the contact. All these characteristics are pertinent to the barrier height that determines the carrier transports at the metal/semiconductor interface.Thus, for a thick film metal paste to be compatible with the fire-through-dielectric contact (i) the glass frits should be able to uniformly etch the underlying dielectric, (ii) must produce a thin glass layer at the semiconductor/metal interface for low barrier height, (iii) must result in an ideality factor of close to unity for the device and (iv) high shunt resistance value. However, because of the high cost of Ag, there is an intensified effort to investigate the alternatives such as Cu and Ag coated Cu to achieve the above characteristics for cost effective metallization of solar cells. This paper will compare and contrast the emerging alternatives with the state-of-the-art Ag paste contacts.