A detailed analysis of the energy levels configuration existing in the band gap of supersaturated silicon with titanium for photovoltaic applications

Abstract

The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (10 13 cm -2 and 10 14 cm -2 ) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around E c -425 and E c -275 meV for implantation doses of 10 13 cm À2 and 10 14 cm À2, respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, E c and E c -450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known, the Meyer-Neldel rule typically appears in processes involving multiple excitations, like carrier capture and emission in deep levels, and it is generally observed in disordered systems. The obtained Meyer-Neldel energy value, 15.19 meV, is very close to the value obtained in multicrystalline silicon samples contaminated with iron (13.65 meV), meaning that this energy value could be associated to the phonons energy in this kind of substrates. V C 2015 AIP Publishing LLC. [http://dx

Application of Neon Ion Implantation to Generate Intermediate Energy Levels in the Band Gap of Boron-Doped Silicon as a Material for Photovoltaic Cells

“…Despite the fact that from a theoretical point of view, the principle of operation of IBSC-type cells has been comprehensively described in [18], the problem of practical implementation of this concept in the form of a semiconductor material with an intermediate energy bandwidth, ensuring effective conversion of solar energy into electricity, still remains valid. Therefore, the mechanism of the formation of intermediate bands in semiconductors, as well as its utilitarian aspects, is the subject of many research papers, including a few selected current research directions based on the use of ion implantation [19][20][21][22][23][24][25]. One of them is an approach that includes the use of ion implantation to introduce dopants of very high concentration into the semiconductor substrate [25] or subjecting the silicon layer to implantation with metal ions of very high doses [20,23].…”

“…Therefore, the mechanism of the formation of intermediate bands in semiconductors, as well as its utilitarian aspects, is the subject of many research papers, including a few selected current research directions based on the use of ion implantation [19][20][21][22][23][24][25]. One of them is an approach that includes the use of ion implantation to introduce dopants of very high concentration into the semiconductor substrate [25] or subjecting the silicon layer to implantation with metal ions of very high doses [20,23].…”

Influence of Substrate Type and Dose of Implanted Ions on the Electrical Parameters of Silicon in Terms of Improving the Efficiency of Photovoltaic Cells

“…They are characterized by an intermediate energy band in the silicon band structure (although the idea of IBSC cells is not limited to silicon as the base material). Many studies have shown [28,29], that the implantation of a high resistivity silicon substrate with titanium ions in selected cases contributes to a significant increase in the absorption of photons whose energy is below the band gap width. The titanium ion implantation of the silicon substrate is aimed at the implementation of theoretical assumptions regarding the construction and structure of IBSC intermediate band photovoltaic cells, according to which the efficiency limit of such a cell is 63.2% [27], which is much higher than the limit of 47.1% [16] for conventional research solar cells with a single energy gap (Multijunction (4-junction or more, Concentrator)) [30].…”

Application of Ion Implantation for Intermediate Energy Levels Formation in the Silicon-Based Structures Dedicated for Photovoltaic Purposes

“…Moreover, this may be be possible for other elements such as chalcogens or transition metals. As a result, on the basis of experimental research conducted for titanium-implanted silicon, a detailed model of intermediate energy levels configuration was proposed [15]. The developed model theoretically explains the phenomena of energy level splitting, band gap narrowing and suppressing carrier recombination as a physical consequence of high impurities concentration.…”

“…In this context, it is reasonable to investigate if it is possible to induce a shift in the value of E g using appropriately configured ion implantation and whether such operation will influence the performance of PV cells. Since the ion implantation and post-implantation treatment are the processes that require predetermining of a number of correlated variables, the aspect of consideration was a matter of many experimental and theoretical studies [11][12][13][14][15].…”