Trap-Assisted Dopant Compensation Prevents Shunting in Poly-Si Passivating Interdigitated Back Contact Silicon Solar Cells

Abstract

Using a trap-assisted compensation model, we explain why polycrystalline Si (poly-Si) passivating contacts are able to achieve low leakage current between the doped fingers of interdigitated back contact (IBC) monocrystalline Si solar cells despite mixing of boron and phosphorus dopants in the isolation region. The fill factor of IBC solar cells is strongly affected by the electrical isolation region between n- and p-type fingers, as this region is critical in minimizing shunting losses. During fabrication of monocrystalline Si solar cells with poly-Si passivating contacts, the intrinsic poly-Si isolation region inevitably gets contaminated with both p- and n-type dopants. Using dopant profiles measured with time-of-flight secondary ion mass spectrometry and scanning probe measurements of the isolation region, we demonstrate that despite the dopant spreading during cell processing, a well-compensated region between the doped fingers exists that prevents shunting. The trap-assisted dopant compensation mechanism significantly widens the compensated region to tens of microns, where the residual dopant densities are below the trap density. This enables a high-resistivity region, resulting in low shunt current. Using one-dimensional (1-D) finite element diode simulations, we identify the design parameters and experimental conditions under which a sufficiently resistive region can form. Our measurements of 2-D local resistivity and work function maps across the isolation region using scanning spreading resistance microscopy and Kelvin probe force microscopy demonstrate the existence of a highly resistive, wide compensated region and confirm our proposed compensation mechanism. For our structures, this region is ∼25 μm in width within a ∼150 μm wide finger isolation region with nearly 3 orders of magnitude higher resistivity than the regions dominated by a single type of dopant.

Electron Radiation Effects of Grain-Boundary Evolution on Polycrystalline Silicon in MEMS

“…The increase in defect density and redistribution of defects due to the grain-boundary evolution will also affect the trap-assisted tunneling (TAT) in polycrystalline silicon [ 17 , 18 ]. The influence of grain-boundary evolution on TAT needs further research.…”

Low Breakdown Voltage Solar Cells for Shading Tolerant Photovoltaic Modules

“…Despite diffusion and mixing of dopant atoms during the cell processing steps in the emitter and BSF regions, recombination of charge carriers in the p-i-n junction is strongly limited when the solar cell is forward biased. 39 When the cell is reverse biased, the p-i-n junctions facilitate recombination of the electrons injected at the negative terminal with holes in the emitter. In addition to the avalanche breakdown mechanism, the high doping level in the polysilicon gap region also enables tunneling of carriers at low bias voltages.…”

“…40 However, for the IBC structure shown in Figure 1A, a 2D model is required to simulate horizontal movement of carriers between the BSF and the emitter. 39 The electrical simulations of solar cells in this work were performed using a 2D finite element model in TCAD Sentaurus, which simultaneously solves the Poisson's equation and the charge carrier transport equations. This model has been validated with respect to homojunction, heterojunction, and TOPCon IBC c-Si solar cells.…”