In the context of increased space photovoltaic power needs and cost reduction pressures, silicon solar cells spark a new interest. This is even truer if mass-produced silicon technologies can be adapted to the specific constraints of the space environment.
Which solar cell for space missions ?
In space, silicon solar cells suffer from electron and proton radiation induced damages. Use of p-type substrates, front emitters and thin wafers can help to limit this effect. Beyond improving the radiation hardness, thin wafers facilitate design of lightweight and flexible photovoltaic arrays.
In a recent communication, we showed how SHJ cells can be adapted for space applications. Indeed, by using a gallium-doped (p-type) substrate, the cell features a front emitter. Furthermore, both the cell architecture (symmetric structure) and its fabrication process (low temperature, wafers relying on trays) are compliant with very thin (~60 µm) wafers.
Why do we choose TOPCon
The adaptation of TOPCon cells for space raises more challenges. In this case, with p-type wafers, the cell features a rear emitter. Furthermore, the cell fabrication process is less adapted to very thin wafers (high temperature steps, asymmetric structure). In the frame of the CARLAH project, supported by the European Space Agency (ESA), we are developing an alternative TOPCon device which is designed to be better adapted to space missions.
This alternative TOPCon relies on double-side polysilicon on oxide structures. Ultra-thin polysilicon layers are used to limit optical losses. On the front surface, n-type polysilicon layers are formed. Therefore, with Ga-doped substrates, the cell features a front emitter. Interestingly, this solar cell can be obtained via a lean fabrication process, well adapted to thin wafers.
Recently, we successfully prepared 60 µm-thick alternative TOPCon devices (M2 size), by using equipment representative of the manufacturing industry. These devices were electron irradiated and their performances compared to those of conventional p-type PERC. Promising results were obtained since similar post-irradiation PV performances were reached for both technologies, with a device mass about three times lower for the alternative flexible TOPCon cells.
These results were presented at the 13th European Space Power Conference (Elche, Spain). Several paths were identified for further improving the performances.