Today, Silicon based-solar cells that dominate the photovoltaic market tend to approach their maximum theoretical efficiency of 29.4%. One way to go beyond this limit is to combine the Si absorber with a larger bandgap absorber to obtain a tandem cell that absorbs the sunlight spectrum more efficiently.
Recently, perovskite materials have appeared as good candidates to reach this goal thanks to promising results: up to 28% efficiencies have been reported [1]. Nevertheless, most of results were obtained on small area surface solar cells (i.e., < 1 cm²). One of the challenge for upscaling is the metallization that is usually performed by thermal evaporation. Indeed evaporated contacts suffer from high line resistances due to the low thicknesses deposited (several hundred nm). In addition, metallized surfaces have to be minimized to reduce the shading and hence obtained high currents. If this is not an issue for laboratory devices where low currents are considered thanks to small areas, for industrialization and larger areas, an alternative process has to be considered.
Screen-printing metallization is an attractive solution. Indeed, besides being non-expensive, easy to implement, it is the main technology used nowadays on the solar cell production lines. Therefore, it would facilitate the industrialization of tandem solar cell as an integration of new equipment in the actual Silicon lines. However, the use of screen-printing is not straightforward since high-temperatures are generally required for the paste curing which are not compatible with the stability of the perovskite top-cell. Low-temperature pastes have been already developed for silicon heterojunction (SHJ) cells but their curing temperatures are still too high: about 200°C. Efforts are thus needed to develop a low-temperature screen-printing for tandem integration. Very recently, the potential of screen-printing metallization has been demonstrated with an efficiency of 22.6% over an area of 57.4 cm² for a perovskite/silicon-heterojunction tandem [2].
In this work, we show that low-temperature screen-printing process can be used as metallization for monolithic perovskite/silicon-heterojunction tandem solar cells. Several silver pastes have been investigated and Rlines < 1-10 Ohm/cm have been obtained for curing temperatures < 140°C. The stability of perovskite devices and tandem devices with evaporated contact has been studied and no strong degradation has been observed. The metallization design was optimized considering the electrical and shading aspects. Finally, 9 cm² tandem solar cells with efficiencies over 18% were obtained.