Characterization of phosphorus-doped amorphous carbon and construction of n-carbon/p-silicon heterojunction solar cells

Abstract

Phosphorus (P)-doped carbon (n-C) films are deposited by pulsed laser deposition technique using a camphoric C target. The activation energy increased to approximately 0.23 eV for the film deposited using a 1% P target compared to undoped C film (0.17 eV), after which it decreases with further increase in P content to approximately 0.12 eV for the film deposited from a 7% P target. Study of activation energy reveals that the Fermi level of the n-C film moves from the valence band to near the conduction band edge through the midgap. The quantum efficiency (QE) of the n-C/p-Si cells is observed to improve with P content. The contribution of QE in the lower wavelength region (below 750 nm) may be due to photon absorption by C film and in the higher wavelength region is due to Si substrates. The current-voltage photovoltaic characteristics of n-C/p-Si cells under 1 sun air-mass 1.5 (AM 1.5) illumination condition (100 mW/cm2, 25°C) are improved up to 5% P and deteriorate thereupon. The open circuit voltage (Voc) and short circuit current density (Jsc) vary from 220 to 270 mV and 9 to 12 mA/cm2, respectively. The cell with 5% P yields the highest electrical conversion efficiency, η = 1.25% and fill factor, FF = 53%. The structural, Tauc gap, conductivity and activation energy (together with electron spin resonance spectroscopy) studies reveal successful doping of P in the films deposited from target containing up to 5% P upon modifications in the gap states.