Silicon wafers and silicon solar cells are typical representatives of semiconductor materials and semiconductor devices, respectively. Semiconductor characteristic parameters measure and characterize the performance of materials and their devices. Due to the fact that charge carriers are the functional carriers of semiconductor materials and devices, their movement generates currents and electric fields. At the same time, charge carriers have characteristics such as luminescence and thermal radiation. Therefore, charge carrier parameters are the basis for characterizing the transport characteristics of charge carriers in semiconductor materials and devices, and are an important component of the characteristic parameters of silicon wafers and silicon solar cells. When silicon wafers are processed and manufactured to form silicon solar cells, the difference in pn junction and Fermi level leads to carrier separation and voltage formation, which in turn reflects and affects the volt ampere characteristics of solar cells through electrical performance parameters such as saturation current, fill factor, and photoelectric conversion efficiency. Based on the above analysis, the main characteristic parameters of silicon wafers include carrier parameters.
Carriers are divided into majority carriers and minority carriers, including electrons and holes. The diffusion and drift of charge carriers form the basis for current transmission in semiconductor devices. Carrier transport parameters are basic parameters that describe the motion and concentration of carriers, mainly including carrier lifetime, diffusion coefficient, and front and back surface recombination rates. These parameters directly reflect the physical and electrical properties of semiconductor materials, affecting carrier concentration and mobility; Doping concentration is another important parameter that determines the carrier concentration, affecting parameters such as material resistivity and carrier lifetime, and determining device performance.
Carrier Concentration
Most semiconductor devices are minority carrier devices, such as silicon solar cells. The carrier parameters mentioned later in this article are all minority carrier parameters. In the state of thermal equilibrium, semiconductors have equal concentrations of holes and electrons, and are in a steady state; When subjected to external stimuli (such as light, electricity, heat, etc.), semiconductors are in a non-equilibrium state, with an increase in both electrons and holes, forming excess charge carriers. The lifetime of charge carriers refers to the average duration of existence of excess charge carriers, and the concentration of charge carriers follows an exponential decay law.
Carrier Lifetime
The carrier lifetime can be divided into radiative recombination lifetime, Auger recombination lifetime, and Shockley Read Hall (SRH) recombination lifetime based on the type of carrier recombination. Carrier lifetime is an important parameter reflecting the defect concentration of materials and devices, as well as an important indicator for measuring device switching speed, current gain, voltage, and other characteristics. It also plays an important role in the electro-optical and photoconversion efficiency of optoelectronic devices such as semiconductor lasers, photodetectors, and solar cells.
Surface Recombination Rate
Carriers recombine both inside and on the surface of the material. Surface recombination velocity (s) is a physical quantity that describes the speed at which charge carriers recombine on a surface. The longer the surface composite life, the lower the surface composite rate, and conversely, the higher the surface composite rate. Surface roughness, surface dangling bonds, and other surface physical properties and states are key factors affecting surface recombination rate. The surface recombination rate is an important performance parameter that characterizes the surface quality of materials.
Effective Lifespan
The effective carrier lifetime is a parameter that combines the bulk lifetime and surface recombination lifetime, and is a characterization of the overall carrier lifetime of a specific sample. At present, most detection technologies detect the carrier lifetime as the effective carrier lifetime, which cannot separate the bulk lifetime and surface recombination rate. Therefore, it is difficult to analyze the effects of surface treatment processes, bulk defects, and doping on the performance of silicon wafers and solar cells one by one.
Diffusion Coefficient
The diffusion coefficient (D) is a physical quantity that characterizes the speed at which charge carriers pass through an interface per unit time and area. The diffusion coefficient and carrier lifetime jointly determine the carrier diffusion length, which is a typical parameter for evaluating material properties. The longer the carrier diffusion length, the better the material quality; For solar cells, the longer the carrier diffusion length, the better the carrier separation and collection efficiency, and the higher the photoelectric conversion efficiency.
Doping
Doping: A Key Step in Semiconductor Manufacturing
Doping is a necessary step in the formation of functional semiconductors, and doping concentration has a significant impact on resistivity and carrier transport parameters. Intrinsic semiconductors, i.e. undoped semiconductors, have very high electrical resistivity at room temperature. As the doping concentration increases, the electrical resistivity decreases, and the carrier lifetime and diffusion length gradually decrease.
