Although perovskite solar cells (PSCs) are a promising emerging photovoltaic technology, their stability in the presence of oxygen and moisture is of major concern [1]. On the other hand, since they contain lead-based substances, it is of critical importance to make the cells leak-free. As previously described, hermetic encapsulation can be achieved using glass frit as bonding material [2]. In the traditional glass frit bonding process, known as thermo-compressive [3], the entire device should be placed in an oven and heated up to the bonding temperature of the glass frit. This process cannot be used to encapsulate devices containing temperature sensitive materials, like in the case of PSCs.

PSC are fabricated with an electron transport material (ETM), a perovskite absorber and a hole transport material (HTM). Common ETMs such as TiO₂ are thermally stable, however perovskite and HTM layer can thermally decompose at lower temperature. While common perovskites absorber such as MAPbI₃ can withstand temperature up to 150 °C [4], HTM layers such as spiro-OMeTAD and PTAA can decompose at 70 °C [5] and 85 °C [6], respectively. Therefore, to avoid thermal decomposition of PSC components, an encapsulation method with process temperature lower than 85 °C, is required.   

Laser assisted glass sealing of glass substrates has emerged as a very effective process after the pioneering work by Mendes et al. [7]. Laser sealing allows to melt the sealing material (i.e. glass frit) at low processing temperatures, allowing non-contact/remote processing and fast manufacturing process.

Sealing of PSCs with glass frit requires two glass substrates: the cell substrate (i.e. TCO coated glass) and a cover substrate; to reduce the final cost of the sealed device, a soda-lime glass can be used as cover substrate. To avoid possible interaction of the glass frit with the cell components during device fabrication, the glass frit sealing line should be applied on the cover substrate, as the PSC will be assembled on top of the TCO substrate. For laser-sealing, the substrates are place over each other, compressed to the required force and heated up to the process temperature. Afterwards, a focused laser beam is used to locally heat, melt and bond the sealing material.

A new machine for laser-assisted sealing of PSCs was developed. This new machine allows glass sealing devices up to 100 × 100 mm², under an inert atmosphere (i.e. N₂) or air and within a broad range of temperatures – room temperature < T < 150 °C; the process temperature is obtained using a 150 × 150 mm² hot plate heated by cartridge heaters controlled by a PID controller; the laser source is a continuous wave Nd-YAG infrared (λ = 1064 nm) laser source; the laser beam is redirected to the sealing line with a galvanometric 2D scan head; an f-theta lens is used to focus the laser beam on the workpiece; a system based on stainless steel spring applies homogeneous compression of the two glass substrates during sealing process ( < 10 kgf); finite element methods (FEM) were applied for determining the ideal position for the application of the force that promotes more homogeneous bonding; the inert atmosphere is produced inside an airtight stainless steel chamber.

The new patented laser-sealing machine was successfully tested for sealing of empty cells (TCO substrate coated with TiO₂ compact layer) at 85 °C, under inert atmosphere. Moreover, the sealing process is developed to have the scalability potential for future industrialization of large-area hermetically sealed PSCs.