SATURNIA - Highly bifacial IBC cells on glass

Publieke samenvatting / Public summary

The price of c-Si PV modules has fallen so dramatically that the balance-of-system (BOS) rather than the PV modules became dominant in system price. Besides improving the cell efficiency, increasing the system energy yield turned out to be a driver. Higher yield reduces the area-related BOS costs and the levelized cost of electricity (LCoE). A cost effective easy to integrate option to increase energy yield is to use bifacial cells-modules, as they increase the energy yield by 10-30%. On device level, carrier selective polysilicon (polySi) or heterojunction contacts applied to crystalline silicon solar cells show enormous potential for outstanding Voc due to excellent passivation, as demonstrated by lab cells with efficiencies > 26%. However, the performance of cells with heterojunction or polySi contacts in bifacial configurations is limited by parasitic absorption losses in the a-Si:H or polySi, especially in interdigitated back contact (IBC) cells as electron and and hole transport layers are on the rear side. For bifacial IBC cells, an additional challenge is finding a suitable interconnection scheme that combines good conductivity with high rear-side transparency.

To address the challenge of combining high efficiencies on cell level with high energy yields on system level, the Saturnia consortium aims to integrate the best available technological building blocks for bifacial solar cells and modules into a novel device. On cell level, the highest current generation concept (IBC) will be combined with the highest voltage (carrier selective contacts). High bifaciality will be achieved using transparent and stable polysilicon oxide (polySiOx) passivating contacts. In the module, the energy yield will be further increased by integration of advanced anti-reflection coatings and bifacial interconnection directly on the rear glass. Limitations of the current rear IBC or metal wrap through (MWT) module interconnection schemes will be overcome by a metal on glass approach that reduces the rear shading to <10% and offers more flexibility in design and feature resolution. The main goal of the project is to prove the concept of a bifacial IBC device with novel, transparent carrier selective contacts at the rear for high efficiency (>24%) and high energy yield (bifaciality factor >85%) with a new metal on glass interconnection.

Korte omschrijving
The work in the project is divided in two work packages. In WP1, thin polySiOx layers will be deposited by Tempress LPCVD tools on a thin 'tunnel' silicon-oxide layer to form a passivating and carrier selective contact with improved transparency compared to polySi and a-Si:H. Al2O3 to provide additional hydrogenation will be supplied by Levitech. TU/e will work on the hydrogenation of interfaces, as well as deposition of TCO layers. Short and narrow metal lines (fingers) will be printed by TNO. The cell processing and designs will be researched by TUD on lab scale and by TNO on 6-inch wafers with low-cost process technology. In WP2, a busbar pattern will be imprinted by Morphotonics in a suitable, transparent material on the rear glass and subsequently filled with metal paste or powder by TNO. Modelling of the optical and electrical design of the module as well as outlook and cost perspective towards 60-cell modules will be made by TUD and TNO.

This project will deliver two Proof of Concept devices: a bifacial IBC solar cell based on transparent polySiOx passivating contacts and a bifacial module with the Saturnia solar cells based on a new interconnection concept using electrical conductors embedded in a transparent layer on the rear glass.