Abstract
Electrical transport parameters for active layers in silicon (Si) wafer solar cells are determined from free carrier optical absorption using non-contacting optical Hall effect measurements. Majority carrier transport parameters [carrier concentration (N), mobility (μ), and conductivity effective mass (m*)] are determined for both the n-type emitter and p-type bulk wafer Si of an industrially produced aluminum back surface field (Al-BSF) photovoltaic device. From measurements under 0 and ±1.48 T external magnetic fields and nominally “dark” conditions, the following respective [n, p]-type Si parameters are obtained: N = [(3.6 ± 0.1) × 1018 cm−3, (7.6 ± 0.1) × 1015 cm−3]; μ = [166 ± 6 cm2/Vs, 532 ± 12 cm2/Vs]; and m* = [(0.28 ± 0.03) × me, (0.36 ± 0.02) × me]. All values are within expectations for this device design. Contributions from photogenerated carriers in both regions of the p-n junction are obtained from measurements of the solar cell under “light” 1 sun illumination (AM1.5 solar irradiance spectrum). From analysis of combined dark and light optical Hall effect measurements, photogenerated minority carrier transport parameters [minority carrier concentration (Δp or Δn) and minority carrier mobility (μh or μe)] under 1 sun illumination for both n- and p-type Si components of the solar cell are determined. Photogenerated minority carrier concentrations are [(7.8 ± 0.2) × 1016 cm−3, (2.2 ± 0.2) × 1014 cm−3], and minority carrier mobilities are [331 ± 191 cm2/Vs, 766 ± 331 cm2/Vs], for the [n, p]-type Si, respectively, values that are within expectations from literature. Using the dark majority carrier concentration and the effective equilibrium minority carrier concentration under 1 sun illumination, minority carrier effective lifetime and diffusion length are calculated in the n-type emitter and p-type wafer Si with the results also being consistent with literature. Solar cell device performance parameters including photovoltaic device efficiency, open circuit voltage, fill factor, and short circuit current density are also calculated from these transport parameters obtained via optical Hall effect using the diode equation and PC1D solar cell simulations. The calculated device performance parameters are found to be consistent with direct current-voltage measurement demonstrating the validity of this technique for electrical transport property measurements of the semiconducting layers in complete Si solar cells. To the best of our knowledge, this is the first method that enables determination of both minority and majority carrier transport parameters in both active layers of the p-n junction in a complete solar cell.
Highlights
Minority carrier transport parameters critically affect operation and performance of many p-n junction semiconductor devices including bipolar transistors and solar cells[1,2,3,4]
The bus bar front electrical contacts are avoided by the probing beam, and the metal finger bars occupy ~7% of the surface area of the probing beam spot. Contributions from these finger bars are not included in the optical model as the majority of the beam spot samples the surface of the Si solar cell
As optical Hall effect measurements are non-contacting and sensitive to the optical response of multiple layers in a sample stack, it has the potential to determine transport parameters of the individual layers in multilayer PV device structures, which may be inaccessible by direct measurements such as the electrical Hall effect
Summary
Minority carrier transport parameters critically affect operation and performance of many p-n junction semiconductor devices including bipolar transistors and solar cells[1,2,3,4]. In this work optical Hall effect measurements of a Si wafer solar cell under nominally dark and 1 sun illumination conditions are used to determine both majority and minority carrier transport properties. Optical Hall effect via magnetic field dependent frequency domain terahertz (THz) range spectroscopic ellipsometry is a potential alternative method that enables extraction of majority carrier transport parameters such as conductivity effective mass (m*), carrier concentration (N), and mobility (μ) of individual layers even in the complex multilayer structures, but without making physical contact[17,18,19,20,21,22,23,24]. Majority carrier N, μ, and m* are determined for both the n-type emitter and p-type wafer Si regions of an industrially produced Al-BSF Si solar cell from THz spectral range optical Hall effect measurements performed in nominally “dark” conditions. Experimental error on solar cell performance parameters from J-V are determined from one standard deviation of the measurement
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