Magnetoresistance and Carrier Collection in Bifacial Multicrystalline-Si Solar Cells: Role of Base Thickness and Illumination Mode

Abstract

We investigate how weak transverse magnetic fields (B = 10⁻⁴ to 10⁻³ T) influence bifacial multicrystalline-silicon photovoltaic cells operated near open-circuit conditions, focusing on the photovoltage (Vph), photocurrent density (Jph), and the apparent series resistance (Rs) as functions of base thickness (e = 100–400 μm) and illumination mode (front, rear, dual). Jph–Vph characteristics reveal a systematic increase of Rs with B, whose magnitude depends strongly on e and illumination: the effect is maximal under rear illumination, mitigated under dual illumination, and moderate under front illumination. Under rear illumination, increasing e markedly reduces carrier collection; for illustration, Jph and Vph drop from ≈10 mA·cm⁻² and 0.34 V to ≈0 mA·cm⁻² and 0.23 V when e increases from 100 to 400 μm. To quantify this behavior, we analyze Rs(e, B) and the absolute change ΔRs (maximized over the tested field window). Because the carrier-collection velocity (CCV) can vary with operating conditions, we report extractions in which CCV is either freely fitted or held fixed at 60.256 cm·s⁻¹ to isolate the magnetic contribution to Rs. The interpretation follows a magnetoresistive framework: the Lorentz force reduces the effective mobility μ(B) and, via the Einstein relation, the diffusion coefficient D(B). In the Drude limit for B ⟂ J, the longitudinal diffusion follows D∗ (B)=D/[1+(μB)²], implying a shortened diffusion length and reduced conductance. At millitesla fields, however, the intrinsic reduction of D is slight; the dominant contributions to the “apparent” Rs arise from surfaces and contacts (passivation quality, metallization), current-spreading in the base, extraction geometry (sheet vs. bulk paths), and injection level. A joint parametric extraction of {J0, n, Rs , Rsh} by illumination mode and thickness is essential to explain high-voltage slopes and to link transport parameters to performance. These findings identify practical levers, thickness selection, improved surface passivation, optimized contact design, and illumination strategy, to mitigate magnetically induced increases in Rs. They also motivate future work on field orientation, temperature dependence, and spatial mapping of Rs to localize resistive bottlenecks and guide device engineering.