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In this work we present the manufacturing processes and results obtained from the characterization of heterojunction with intrinsic thin layer solar cells that include a heavily Ti ion implanted Si absorbing layer. The cells exhibit external circuit photocurrent at photon energies well below the Si bandgap. We discuss the origin of this below-bandgap photocurrent and the modifications in the hydrogenated amorphous intrinsic Si layer thickness to increase the open-circuit voltage.

Abstract-Some sputtering processes of GaAs and Ti onto glass, c-Si and c-GaAs substrates have been carried out in order to obtain thin films as candidates to be intermediate band photovoltaic materials. This work presents first results concerning the optical and structural properties of the different deposited thin films.

Composition and optical absorption of thin films\ud of GaAs(Ti) and GaAs, deposited by sputtering on glass\ud substrates under different process conditions, have been\ud investigated. The thin films obtained are typically 200 nm\ud thick. ToF–SIMS measurements show a quite constant\ud concentration and good uniformity of Ti profiles along the\ud GaAs(Ti) layers in all cases and EPMA results indicate that\ud Ti content increases with the substrate temperature in the\ud sputtering process. Measurements of the transmittance and\ud reflectance spectra of the GaAs and GaAs(Ti) thin films have\ud been carried out. In the optical characterization of the films it\ud is found that optical absorption is enhanced in all samples\ud containing Ti. The determination of the optical gap from the\ud optical absorption, shows optical gap variations from 1.15 to\ud 1.29 eV in the GaAs thin films, and from 0.83 to 1.13 eV in\ud the GaAs(Ti) thin films. The differences in absorption and\ud EgTAUC observed between samples of GaAs and GaAs(Ti)\ud are consistent with the presence of an intermediate band.Peer ReviewedPostprint (published version

This work reports fabrication details of heterojunction diodes and solar cells obtained by sputter deposition of amorphous GaAs on p-doped single crystalline Si. The effects of two additional process steps were investigated: A hydrofluoric acid (HF) etching treatment of the Si substrate prior to the GaAs sputter deposition and a subsequent annealing treatment of the complete layered system. A transmission electron microscopy (TEM) exploration of the interface reveals the formation of a few nanometer thick SiO2 interface layer and some crystallinity degree of the GaAs layer close to the interface. It was shown that an additional HF etching treatment of the Si substrate improves the short circuit current and degrades the open circuit voltage of the solar cells. Furthermore, an additional thermal annealing step was performed on some selected samples before and after the deposition of an indium tin oxide (ITO) film on top of the a-GaAs layer. It was found that the occurrence of surface related defects is reduced in case of a heat treatment performed after the deposition of the ITO layer, which also results in a reduction of the dark saturation current density and resistive losses.