Measurement of contact resistivity at metal-tin sulfide (SnS) interfaces

Abstract

We measured the contact resistivity between tin(II) sulfide (SnS) thin films and three different metals (Au, Mo, and Ti) using a transmission line method (TLM). The contact resistance increases in the order Au < Mo < Ti. The contact resistances for Au and Mo are low enough so that they do not significantly decrease the efficiency of solar cells based on SnS as an absorber. On the other hand, the contact resistance of Ti to SnS is sufficiently high that it would decrease the efficiency of a SnS solar cell using Ti as a back contact metal. We further estimate the barrier heights of the junctions between these metals and tin sulfide using temperature-dependent TLM measurements. The barrier heights of these three metals lie in a narrow range of 0.23-0.26 eV, despite their large differences in work function. This Fermi level pinning effect is consistent with the large dielectric constant of SnS, and comparable to Fermi-level pinning on Si. The contact resistivity between annealed SnS films and Mo substrates under light illumination is as low as 0.1 X cm 2 .

Ultrathin Solar Cells Based on Atomic Layer Deposition of Cubic versus Orthorhombic Tin Monosulfide

“…Insensitivity to the metal work function may be due to Fermi-level pinning, which results from a reservoir of interfacial midgap states that trap charge at the surface and effectively constrain the Fermi level to within a narrow energy range in the semiconductor bandgap . Fermi-level pinning has previously been observed for metal/SnS contacts produced in a similar manner . It is also interesting that the short-circuit current and FF with Ti are somewhat higher than those for Mo back contacts, the more common choice.…”

“…For α-SnS, various metal back contacts have previously been evaluated in theoretical calculations and experiments, ,, with ohmic behavior observed for Al, In, Sn, Cu, Zn, Ti, and graphite. Meanwhile, Ni and Mo were found to be less suitable because of unfavorable band alignment and/or inhomogeneities and resistive oxide, resulting in tunneling-assisted recombination. , In the case of Mo, it has been argued that the metal sulfide layer which is likely to be formed at the interface of an ALD-grown (500 nm) SnS film and Mo is more favorable for conduction, and one of the highest efficiencies accomplished in an α-SnS solar cell configuration, of 4.4%, did indeed use a Mo back contact . For π-SnS, the influence of the metal back contact has been much less researched.…”

“…The difference compared to α-SnS could also be inherent to the π-SnS phase or stem from deviations in stoichiometry . Annealing would likely increase both mobility and carrier concentration …”

“…Another important aspect is that SnS-based solar cells often rely on device structures established for other, more common thin-film solar cell materials such as CIGS. − The interface properties and band alignment of these configurations are not necessarily optimal. Studies have indeed pointed to an unusually low ionization potential resulting in wide conduction band offsets (CBOs) for SnS devices in typical thin-film solar cell stacks. , Yet other research ascribes the main issue to a strong Fermi-level pinning, which prevents appropriate CBO and band bending to be established at the front junction. , …”

Framework to predict optimal buffer layer pairing for thin film solar cell absorbers: A case study for tin sulfide/zinc oxysulfide

“…The molybdenum (Mo) back contact is ohmic with SnS, as demonstrated by transmission line contact resistance measurements [24]. The recombination velocity at the Mo back contact is assumed to be high, 10 7 cm/s, in the present-day scenario and low 10 2 cm/s for the high-efficiency scenario.…”