Technology


Silicon Heterojunction Technology (HJT) uses very high quality of passivation achieved by the deposition of ultra-thin layers of amorphous silicon in the nanometre range on both sides of very well cleaned mono crystalline silicon wafers. In contrast to standard technology the formation of the p-n junction that is required to separate the generated eletrical carriers is also achieved by the deposition of ultra-thin amorphous silicon layers. The cells process is completed by the depostion of transparent conductive oxides that allow for an excellent metallization. The metallization can be done by a standard screen printing which is widely used in industry for the majority of cells or with innovativ technologies as will be investigated in the Swiss-Inno HJT project.

 

HIGH EFFICIENCY
 

Schematic cross section of Meyer Burger’s HJT solar cell

 

The Swiss-Inno HJT project enabled the production on the developed platforms of HJT solar cells with up to 23.1 % in efficiency, on 15.6 × 15.6 cm2 Cz commercial wafers. This high efficiency, with conversion of almost up to one quarter of the solar energy hitting the solar cell into electricity, is achieved with production processes requiring fewer steps in comparison to other crystalline silicon technologies.

The main limiting factor for the efficiency of standard silicon solar cells originates from the recombination losses of carriers between the silicon of the cell and the metallization of the cell (which is required to drive the generated current out of the cell). Maximum efficiencies of standard cell technologies are typically 17% to 18 % for multi-crystalline silicon and 19% to 20% for mono-crystalline silicon. The Silicon Heterojunction Technology (HJT) demonstrates very high level of silicon surface passivation and low losses at the contact to the cell metallization. This is achieved by the deposition of ultra-thin layers (few nanometers) of amorphous silicon on both sides of carefully cleaned mono-crystalline silicon wafers. In contrast to standard technology the formation of the p-n junction that is required to separate the generated electrical carriers is also achieved by the deposition of ultra-thin doped amorphous silicon layers. The cell process is completed by the deposition of transparent conductive oxides that allow for lateral transport and for efficient contacting to the cell metallization. This metallization is not directly contacting the silicon wafer but contacts through the TCO and a-Si:H layers, limiting the aforementioned recombination losses.


Process sequence of differing PV technologies

 

The Swiss-Inno HJT project focuses on the development of advanced metallization materials and schemes, to limit its cost and further increase performance. Standard screen printing technology, widely used in industry for the majority of solar cells, is further developed for printing continuous finger as fine as possible to reduce the silver laydown (cost reduction) and increase the current density (efficiency enhancement). This shows a high potential when combined with the innovative SmartWire interconnection technology, as cells with less than 40 mg of Ag could be demonstrated.  Innovative technologies are investigated with the development of industrially compatible processes for plating of fine copper lines onto Silicon Heterojunction solar cells TCO top surface. Research is conducted to drive down the production cost, to guarantee reliability and high adhesion of the copper lines, and to enhance efficiency with the application of fine lines (< 30 µm wide).

Silicon Heterojunction Solar cell with fine-line-printed fingers, interconnected with a SmartWire foil which embeds copper wires of  200 µm in diameter and a low temperature alloy coating.

 

 

Silicon Heterojunction Solar Cell with Copper plated fingers.

 

HIGH ENERGY YIELD