Junction formation with HWCVD and TCAD model of an epitaxial back contact solar cell

Abstract

In this paper, we present morphological and electrical characteristics of a junction formed of Si p-type films deposited on an n-type silicon wafer using a hot wire chemical vapor deposition (HWCVD) tool. We describe the fabrication process and study the influence of diborane flow and postprocess annealing in improving junction characteristics. Our morphological studies undertaken using atomic force microscopy show that the initial deposition suffered from voids rather than being a uniform film; however, this improves significantly under our annealing treatment. The improvement in morphology was observed in the electrical characteristics, with estimated $V-{\text{oc}}$ doubling and rectification of the junction improving by several orders of magnitude. Fitting of the current-voltage curves to a two-diode model showed that increasing the diborane flow in the process helps reduce the saturation current and ideality factors, while increasing the shunt resistance. Electrochemical capacitance-voltage (ECV) and quasi-steady-state photoconductance measurements are used to characterize the deposited films further. A solar cell device with a silicon epitaxy emitter is modeled using industry-standard 3-D modeling tools and input parameters from experimental data, and the impact of defects is studied. A potential efficiency approaching 25% is shown to be feasible for an optimized device.