How are Solar Cells Made? Silicon vs. Perovskite Production

Solaires Team | Victoria, B.C. | JULY 2022

Solar energy is captured by a solar panel which captures sunlight and converts it to electricity. While generating electricity from solar panels produces no carbon emissions, solar panels still have an important environmental impact when extracting metals, purifying metals, manufacturing the panels, and waste disposal at the end of their life.

Conventional solar panels use crystalline silicon (Si), which must be produced from highly pure silicon, known as metallurgical grade. Metallurgical grade Si (MG-Si) is primarily sourced from quartz (silicon dioxide) rocks, though it needs a lot of processing to go from rock to silicon. To extract silicon, the oxygen in silicon dioxide is removed by mixing it with carbon and heating it in an electric furnace to temperatures higher than 2,000 oC. At those temperatures, carbon is oxidised by silicon dioxide, forming carbon dioxide, which leaves high purity silicon at the furnace's bottom.

While this silicon is acceptable for most silicon products, like automotive parts, semiconductor silicon still requires further processing. In a solar cell, only one of every million atoms can be something other than silicon which means purity of 99.9999%. To achieve that, MG-Si is further processed in hydrochloric acid, to remove impurities such as iron and aluminium. The product silicon-acid is then reduced in H2 at over 1000 oC for hundreds of hours to finally produce semiconductor grade silicon. In all, producing the feedstock for Si photovoltaics takes hundreds of hours at extremely high temperatures.

The purified silicon is melted and crystallised into ingots. The ingots are sliced into wafers, between 100 and 500 micrometres thick. This surface of the sliced Si wafer is lightly roughed up to reduce surface reflectivity, and coated with an electrically active material to improve conductivity. Then, metal contacts are deposited by screen printing to collect the charges generated by the Si wafer. The solar cells are connected in series, to form the solar panel. The entire assembly is then encapsulated with ethylene vinyl acetate to protect the final structure of the solar panel.

Researchers have continued to search for new and improved materials for solar panels. One promising and emerging material is called perovskite. Perovskite is a general class of crystalline materials, where different chemicals can be harnessed to alter the characteristics of a perovskite crystal. Much of the photovoltaic research over the last decade has focused on altering chemical formulas to improve perovskite characteristics. Perovskites are much better at absorbing sunlight than crystalline silicon. As a result, perovskites are crystallised as a very thin film, typically 300-900 nanometers thick, roughly one-thousand times thinner than silicon layers.

Building a thin-film perovskite solar cell begins with a substrate that everything will be sequentially coated on. The substrate can be as rigid as a silicon cell or glass, but it can also be flexible, which increases the possible use-cases of thin-film solar cells. A pair of conductive materials are printed on top of the substrate, followed by the photo-active perovskite layer. Perovskite precursors are dissolved in a solvent, which is then deposited onto a substrate in very small amounts using printing techniques common in the electronic and metallic industries. The solvent evaporates as the solution dries, leaving only the very thin perovskite crystal layer which acts as the active solar absorption layer in solar cells to convert sunlight into electricity. Once the perovskite thin-film has crystallised, another conductive thin film is printed on the perovskite, followed by an electrode, completing the solar architecture. To make the perovskite module, the entire film is laser-scribed in different steps to make perovskite solar cells and connect them in series. Finally, the solar modules are encapsulated as a panel, similar to the final steps of silicon solar panel fabrication. This entire process can be carried out near room temperature.

The simplicity and energy efficiency of the perovskite fabrication process is very important because the energy used during fabrication is a huge contributor to greenhouse gas emissions. It is much simpler, less energy-intensive, and requires fewer raw materials to fabricate perovskite solar cells than silicon counterparts. Since the manufacturing energy used is the primary source of emissions from solar cell production, the reduction in energy enabled by using perovskites is crucial in advancing the solar industry to become even more sustainable.