ELECTRONIC DOPING OF AMORPHOUS CARBON THIN FILMS

Abstract

This paper reports on the successful deposition of boron ( B )-doped p-type ( p-C:B ) and phosphorus ( P )-doped n-type ( p-C:P ) carbon ( C ) films, and the fabrication of p-C:B on silicon ( Si ) substrate ( p-C:B/n-Si ) and n-C:P/p-Si cells by a pulsed laser deposition (PLD) technique using a graphite target at room temperature. The boron and phosphorus atoms incorporated in the films were determined by X-ray photoelectron spectroscopy (XPS) to be in the range of 0.2–1.75 and 0.22–1.77 atomic percentages, respectively. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination conditions (100 mW/cm2, 25°C). The open circuit voltage (V oc ) and short circuit current density (J sc ) for p-C:B/n-Si are observed to vary from 230 to 250 mV and from 1.5 to 2.2 mA/cm2, respectively; they vary from 215 to 265 mV and from 7.5 to 10.5 mA/cm2, respectively, for n-C:P/p-Si cells. The p-C:B/n-Si cell fabricated using the target with the amount of boron by 3 weight percentages (Bwt%) showed the highest energy conversion efficiency, η = 0.20% and fill factor, FF = 45%. The n-C:P/p-Si cell fabricated using the target with the amount of 7 Pwt% showed the highest η = 1.14% and FF = 41%. The quantum efficiency (QE) of the p-C:B/n-Si and n-C:P/p-Si cells were observed to improve with Bwt% and Pwt%, respectively. The contribution of QE in the lower wavelength region (below 750 nm) may be due to photon absorption by the carbon layer, in the higher wavelength region it was due to the Si substrates. In this paper, the dependence of the boron and phosphorus content on the electrical and optical properties of the deposited films and the photovoltaic characteristics of the respective p-C:B/n-Si and n-C:P/p-Si heterojunction solar cells are discussed.