In contrast, in an amorphous structure the order difference between monocrystalline and polycrystalline solar cells pdf atomic positions is limited to short range. Polycrystalline cells can be recognized by a visible grain, a “metal flake effect”. Polycrystalline silicon can be as much as 99.
A rod of semiconductor-grade polysilicon. The deposition of polycrystalline silicon on plastic substrates is motivated by the desire to be able to manufacture digital displays on flexible screens. Si material to above the melting point of silicon, without melting the entire substrate. The molten silicon will then crystallize as it cools. In order to create devices on polysilicon over large-areas however, a crystal grain size smaller than the device feature size is needed for homogeneity of the devices. One of its primary uses is as gate electrode material for MOS devices.
Polysilicon may also be employed as a resistor, a conductor, or as an ohmic contact for shallow junctions, with the desired electrical conductivity attained by doping the polysilicon material. When polysilicon and a-Si devices are used in the same process this is called hybrid processing. For the first time, in 2006, over half of the world’s supply of polysilicon was being used by PV manufacturers. 2013 the number increased to over 100 manufacturers. Critical process variables for polysilicon deposition include temperature, pressure, silane concentration, and dopant concentration. Wafer spacing and load size have been shown to have only minor effects on the deposition process.
Based on this equation, the rate of polysilicon deposition increases as the deposition temperature increases. There will be a minimum temperature, however, wherein the rate of deposition becomes faster than the rate at which unreacted silane arrives at the surface. Beyond this temperature, the deposition rate can no longer increase with temperature, since it is now being hampered by lack of silane from which the polysilicon will be generated. Such a reaction is then said to be ‘mass-transport-limited. When a polysilicon deposition process becomes mass-transport-limited, the reaction rate becomes dependent primarily on reactant concentration, reactor geometry, and gas flow.
When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, then it is said to be surface-reaction-limited. A deposition process that is surface-reaction-limited is primarily dependent on reactant concentration and reaction temperature. Deposition processes must be surface-reaction-limited because they result in excellent thickness uniformity and step coverage. C is too slow to be practical. C, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion. Pressure can be varied inside a low-pressure reactor either by changing the pumping speed or changing the inlet gas flow into the reactor.
If the inlet gas is composed of both silane and nitrogen, the inlet gas flow, and hence the reactor pressure, may be varied either by changing the nitrogen flow at constant silane flow, or changing both the nitrogen and silane flow to change the total gas flow while keeping the gas ratio constant. Polysilicon doping, if needed, is also done during the deposition process, usually by adding phosphine, arsine, or diborane. Adding phosphine or arsine results in slower deposition, while adding diborane increases the deposition rate. The deposition thickness uniformity usually degrades when dopants are added during deposition. UMG-Si greatly reduces impurities in a variety of ways that require less equipment and energy than the Siemens process.
In 2008 several companies were touting the potential of UMG-Si in 2010, but the credit crisis greatly lowered the cost of polysilicon and several UMG-Si producers put plans on hold. The Siemens process will remain the dominant form of production for years to come due to more efficiently implementing the Siemens process. 6,000 tonnes by the end of 2010. 5 years with boron at 0.