Leizhi Sun

Enhancing the efficiency of SnS solar cells via band-offset engineering with a zinc oxysulfide buffer layer

SnS is a promising earth-abundant material for photovoltaic applications. Heterojuction solar cells were made by vapor deposition of p-type tin(II) sulfide, SnS, and n-type zinc oxysulfide, Zn(O,S), using a device structure of soda-lime glass/Mo/SnS/Zn(O,S)/ZnO/ITO. A record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S). Increasing the sulfur content in Zn(O,S) raises the conduction band offset between Zn(O,S) and SnS to an optimum slightly positive value. A record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37 exhibits short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) of 19.4 mA/cm2, 0.244 V, and 42.97%, respectively, as well as an NREL-certified total-area power-conversion efficiency of 2.04% and an uncertified active-area efficiency of 2.46%.

3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.

Band alignment of SnS/Zn(O,S) heterojunctions in SnS thin film solar cells

Band alignment is critical to the performance of heterojunction thin film solar cells. In this letter, we report band alignment studies of SnS/Zn(O,S) heterojunctions with various compositions of Zn(O,S). Valence band offsets (VBOs) are measured by femtosecond laser pump/probe ultraviolet photoelectron spectroscopy (fs-UPS) from which conduction band offsets (CBOs) are calculated by combining with band gaps obtained by optical transmission/reflection measurements. The SnS/Zn(O,S) heterojunctions with S/Zn ratios of 0.37 and 0.50 have desirable small positive CBOs, while a ratio of 0.64 produces an undesirable large positive CBO. The results are consistent with the device performance of SnS/Zn(O,S) solar cells.

A path to 10% efficiency for tin sulfide devices

We preform device simulations of a tin sulfide (SnS) device stack using SCAPS to define a path to 10% efficient devices. We determine and constrain a baseline device model using recent experimental results on one of our 3.9% efficient cells. Through a multistep fitting process, we find a conduction band cliff of -0.2 eV between SnS and Zn(O,S) to be limiting the open circuit voltage (VOC). To move towards a higher efficiency, we can optimize the buffer layer band alignment. Improvement of the SnS lifetime to >1 ns is necessary to reach 10% efficiency. Additionally, absorber-buffer interface recombination must be suppressed, either by reducing recombination activity of defects or creating a strong inversion layer at the interface.

Impact of H 2 S annealing on SnS device performance

Tin sulfide is regarded as a possible earth-abundant alternative for chalcogenide thin film photovoltaics. The material has strong absorption in the visible wavelength region and the possibility for high carrier mobility. We review recent progress for SnS solar cell efficiencies. Annealing in H2S gas and surface passivation of SnS are thought to be two key components that increase efficiency of SnS devices. An efficiency of η = 3.88% [1] was achieved via thermal evaporation, a manufacturing-friendly deposition method.

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

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 un...