Manufacturing of photovoltaic products can be broadly grouped into two sectors: the production of “crystalline silicon” (c-Si) based products and the production of “thin film” products. Thin film production creates solar modules directly from raw materials, while c-Si production involves discrete processes for ingot, wafer, cell and module manufacturing. Historically, c-Si has been used in most solar modules.
Manufacturing of photovoltaic products can be broadly grouped into two sectors: the production of “crystalline silicon” (c-Si) based products and the production of “thin film” products. Thin film production creates solar modules directly from raw materials, while c-Si production involves discrete processes for ingot, wafer, cell and module manufacturing. Historically, c-Si has been used in most solar modules. It yields stable products with good efficiencies (15–22 percent, half to three-quarters of the theoretical maximum) and uses process technology developed from the huge knowledge base of the microelectronics industry. Each c-Si cell generates about 0.5V, so a number of cells are usually soldered together in series to produce a module with an output to charge a 12V battery or for connection to a power inverter. Thin Film uses strongly light-absorbing compounds (most commonly amorphous silicon (a-Si), cadmium telluride (CdTe), or copper indium (gallium) diselenide (CIS or CIGS)). Each of these compounds is amenable to large area deposition (on to substrates of one square meter or greater) and hence high-volume manufacturing is possible. Although they are less efficient (production modules range from 5 to 12 percent), thin films are potentially cheaper than c-Si because they have lower materials costs and a larger substrate size. However, conventional c-Si manufacturing technology has continued its steady improvement year by year and its production costs are still falling. The following description focuses on c-Si cell production and is illustrated in Figure A below.
A c-Si cell manufacturing plant starts with a purified silicon wafer, doped with boron, as raw material. It takes each wafer through a processing sequence to create working solar cells.
Figure A – Surface damage removal and texturization
The wafers are first passed through steps to remove surface and edge irregularities, clean the wafers and produce a uniform non-reflective texture on the surface. The surface of each wafer is then exposed to a phosphorous mist/gas and the wafers are heated in a furnace to cause the phosphorus atoms to be diffused into the crystalline silicon. This forms the solid-state chemistry in the wafer that allows it to generate electricity when exposed to light. A by-product of the diffusion process is the creation of a phospho-silicate glass (PSG) layer on the surface that must be removed in an etching bath. The next step is to create an antireflective coating on the wafers consisting of a transparent layer of Silicon Nitride or one of a number of compounds known as “Transparent Conductive Oxides”, which aid light capture by reducing reflections and quelling certain surface electrical activity. The final steps of the process is the printing process where electrode grids made from silver paste are screen printed onto the top of the cell and two contact strips are applied to the back of the wafer (a strip of silver and a layer of aluminum) to produce the back surface field. The printed wafer goes into a ~900ºC furnace to cause the printed layer to sink into the silicon wafer. A procedure to prevent short-circuiting of the cell (called “edge isolation”) will also be performed by one of a variety of methods by completion of this step. The finished cells are tested by exposing them to a bright light source and sorted into grades.