Advances in screen printing metallization for a-Si:H/c-Si heterojunction solar cells

Amorphous / crystalline silicon heterojunction is the most attractive technique to obtain high efficiency solar cells. Usually such a kind of cells is produced starting from n-type silicon wafers, because of several advantages, like the high bulk lifetime and the possibility to easily contact the n-type base with i-n amorphous layers. The emitter is usually covered by Transparent Conductive Oxides (TCO) which works as high conductive layer and Anti Reflection Coating (ARC). The device is completed by a metal grid, made by screen printed silver, sintered at low temperature. Both the TCO and the grid strongly influence the final cell series resistance, and consequently the cell efficiency. When p-type wafer is adopted as substrate for heterojunction cell, the base contact is more difficult to obtain because of the energy bands alignment between the c-Si and the p-type a-Si:H layer. Recently n-type doped SiOx layer has attracted interest as emitter layer in heterojunction device, therefore in this work we show the results obtained on the metallization of n-type SiOx/ptype c-Si heterojunction solar cells by means of low temperature screen printing technique. In particular a new kind of low temperature sintering (<; 200°C) screen printable silver paste has been developed able to ensure high linear conductivity, low specific contact resistivity and strong adhesion to TCO´s. We present electrical characterization using Transfer Length Method (TLM) technique. Since the base contact of SiO_x/c-Si heterojunction is ensured by laser doping technique starting from p-type a-Si:H layer, we also show how the screen printed Ag paste can enhance the base contact of this solar cell.