THESSO – Simultaneous Co-Diffusion Processes for n-Type Solar Cells

In comparison to industrially produced solar cells made of p-type silicon, solar cells of n-type silicon have the potential for higher efficiency values. However, the process to manufacture highly efficient n-type solar cells is complex, as it typically needs several sequential masking and diffusion steps to form heavily n-doped or p-doped regions. One possibility to reduce the number of process steps required is to use simultaneous co-diffusion processes, in which previously deposited layers containing dopants serve as a diffusion source parallel to the diffusion from the vapour phase. The heavily doped regions are then formed in a single hightemperature step.

At present, several different concepts for solar cells based on n-type silicon wafers are being intensively investigated. Most of them feature heavily doped regions with different dopants on the front and back-surfaces of the wafer. Depending on the solar cell concept, diverse specifications are made for the surface concentration and depth of the doping profile. The heavily doped regions are typically formed by diffusion from the vapour phase. For simultaneous co-diffusion processes, a solid diffusion source in the form of a thin film is deposited onto the sample already before the high-temperature step, e.g. by plasma-enhanced chemical vapour deposition (PECVD) or atmospheric pressure chemical vapour deposition (APCVD).

The choice of processing gases during deposition allows phosphorus silicate (PSG) or boron silicate glasses (BSG) to be deposited as the solid diffusion source with different dopant concentrations, which then causes n-type or p-type doping of the wafer in the subsequent high-temperature step.

In combination with conventional vapour phase diffusion, e.g. diffusion of POCl3 or BBr3, different depth profiles of various dopants can be generated simultaneously and very cost-effectively in a single high-temperature step on a single sample by adapting the processing parameters for this method (Fig. 2). Industrial quartz tube furnaces are available in PV-TEC at Fraunhofer ISE for such processes (Fig. 1). By  combining the diffusion from three different sources in one single high temperature step, back-junction back-contact (BJ-BC) solar cells with 19.8 % conversion efficiency have been processed.

About THESSO

In the THESSO project, materials, technologies and solar cell structures that show promise for increasing the efficiencies of industry monocrystalline solar cells up to 22 % are developed. For multicrystalline solar cells, peak efficiencies of 20 % and average efficiencies of 19.5 % are targeted. These solar cells shall be fabricated using wafers from silicon blocks that were produced at Fraunhofer ISE.

In total, four different areas are to be addressed: materials, technology, solar cell structures and modules. Work in each area will be carried out in parallel but also interactively. The developments will be accompanied by characterizations at all levels as well as by simulations. Additionally an economic analysis of the technologies and solar cell processes is to be undertaken. The developed solar cell processes will be produced at the pilot level in the PV-TEC laboratory (throughput > 1000 wafer/hour) at near-industry production, thus facilitating the continual development of existing processes and process sequences.  During the project, demonstration modules with target efficiencies of 20 % will be manufactured. In this way it can be guaranteed that the investigated solar cell processes are principally suitable for use in PV modules.

"Simultaneous Co-Diffusion Processes for n-Type Solar Cells” is only one of many sub-projects within the THESSO project. Other sub-projects are: