Free-carrier absorption and Burstein-Moss shift effect on quantum efficiency in heterojunction silicon solar cells

Abstract

Effects of the carrier concentration on the dielectric function of indium tin oxide (ITO) films were investigated by spectroscopic ellipsometry using Tauc-Lorentz and Drude oscillator terms. The real and imaginary parts of dielectric function values increased with increasing carrier concentration. Samples with lower refractive indices had (440) preferable crystallographic orientations. A more significant Burstein-Moss (B-M) effect leads to a lower extinction coefficient (k) value in the short-wavelength regions, while more Free-carrier absorption (FCA) leads to a higher k value in the near-infrared region. The electrical properties obtained from optical studies, such as the carrier concentration and the carrier mobility, were compared with those obtained by van der Pauw measurements. The observed discrepancies between optically and electrically obtained values were due to carrier transport and grain boundaries. ITO films with a widened band gap energy due to the B M shift are key to improve quantum efficiency (QE) at short-wavelengths in heterojunction silicon (Hi) solar cells. The reduced QE at long-wavelengths is attributed to optical scattering in the ITO films due to FCA. Despite widening the band gap, the short-circuit current density (J(sc)) decreased from 36.27 mA/cm(2) to 34.17 mA/cm(2) with increasing carrier concentration from 6.15 x 10(20) cm(-3) to 9.84 x 10(20) cm(-3). This result demonstrates that the FCA effect may affect J(sc) more in HJ solar cells. The ITO film, prepared at the lowest carrier concentration, was used as an antireflection layer to fabricate HJ solar cells via optimization of the passivated layer. It resulted in a cell efficiency of 19.12% and an open-circuit voltage of 702 mV. (C) 2014 Elsevier Ltd. All rights reserved.