Solar energy is one of the biggest enablers of a carbon neutral, more ecological future. As a technology, it is already proven to be viable. However, with the increasingly electric-driven automotive industry, constantly growing electronics industry and proliferation of smart devices that all require electricity, the solar energy industry needs new solutions. To keep up and surpass the evergrowing electrical needs, ALD is just the technology to do this.
ALD made its breakthrough in silicon (Si) solar cell manufacturing a few years ago. Nowadays, the most efficient large-scale manufactured Si solar cells, so-called PERC (passivated emitter and rear) cells utilize a thin layer of ALD Al2O3 passivation layer to prevent charge carrier recombination. With ALD passivation, cell efficiencies can be increased from ca. 20% to 22–23%. There are already several dozen GW manufacturing capacity of ALD-passivated PERC cells in China alone.
ALD Al2O3 is an ideal passivation material for Si solar cells, due to its high intrinsic negative charge density. Gas-phase, moderate temperature ALD process is gentler to the bulk Si, and the excellent film quality, uniformity, conformality and structural integrity enable use of thinner films compared to other coating methods. Al2O3 is also one of the most well-known, fast, simple and cost-efficient ALD processes which advocates its use in this high throughput, mass volume application.
Also other ALD materials can be used, and have been widely investigated, for solar cell passivation, and the method provides a wide variety of options for both p- and n-type silicon based cells.
ALD can potentially improve other solar cell types as well, such as CIGS (copper-indium-gallium-selenide) thin film solar cells, and newer inventions such as perovskite, black Si, organic, quantum dot, dye-sensitized, and hybrid/tandem solar cells. Here, the ‘softness’ of the ALD deposition and the unmatched conformality of ALD films make the method ideal as several new solar cell types employ either high aspect ratio nanostructures, porous layers or sensitive materials such as organic compounds or plastics. ALD can be used to deposit moisture barrier layers, buffer layers, passivation layers, anti-reflection layers or transparent conductive oxides such as ITO (indium-doped tin oxide) as current collectors.