Photovoltaic converters are silicon-based semiconductor devices, converting solar energy into direct electric current. They are basic elements of a device, which we call solar battery.
Growing silica monocrystals by drawing, solar battery manufacturing, solar modules lamination – all these industrial processes are impossible without vacuum technology.
The most critical and expensive part of any solar cell is a silicon slab. A 30 Torr vacuum is necessary on the silica monocrystals growing stage.
The key element of construction details of crystallic silicon photovoltaic converters is a pn-junction. Usually in solar cells manufacturing p-type conductivity starting slabs are used. For this purpose silica is doped with the necessary admixtures, such as Borium, as early as the slab growing stage. Therefore to create an n-layer in it it is necessary to inject into one of the cell’s surfaces another admixture, which would compensate Borium function and saturate the semi-conductor with n-type charge carrier.
It can be done by injecting phosphor or another suitable admixture from a corresponding part of Mendeleev's table into silica.
One of the most conventional and economically feasible methods of saturation of silica with phosphor is diffusion, i.e. the process, under which phosphor flows into semi-conductor under high temperatures. Conventionally phosphor diffusion is carried out in tube-type or conveyor furnaces under temperatures around 800 °С in low vacuum conditions. In the first case slabs are placed into quartzous holders and the tube is filled with phosphor-containing substance vapors.
Solar cell design involves presence of pn-junction near one of its surfaces, which is called the exterior or the work face. Another surface is called backside. Usually there is a pickup grid on the exterior face and a flush contact on the backside. As n-layer, formed by diffusion, is highly saturated with admixtures, it has good electric conductivity. During diffusion this layer is formed not only on the exterior face of the slab but also on its edges and along the perimeter of the backside. In addition electrical interlocking between exterior and backside pickup pins takes place. Usually this problem is solved by plasma-chemical etching, which is a plasmic treatment of a stack of tightly compacted silica plates under vacuum.
Due to the structure the slab surface reflection is reduced on average from 35% to 11%. It means that one tenths of the radiation falling into the surface of a solar cell will still be reflected back and will not be able to participate in electric current generation process. In order to further decrease these losses, classified as optical, on the next technological operation a so called antireflection coating (ARC) is applied to the work face of the solar cell. Silicon nitride is usually applied by PECVD, i.e. plasma-enhanced chemical vapor deposition, in special tube-type furnaces in certain vacuum conditions.
The exterior face primarily serves for maximum absorption of radiation, emitted on it, what defines technical requirements for contact plating. For this very reason the pin, located on the work face of the solar unit, is manufactured in the form of a grid, usually consisting of 2-3 wide contact pads and several dozens of thin pickup lines, located orthogonally to the wide ones. This process is run under high vacuum conditions.It is obvious, that any industrial product has to be carefully before being delivered to the customer and solar elements are not exception in this case.