Person:

Sinsermsuksakul, Prasert

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 ((J_{sc})), open circuit voltage ((V_{oc})), and fill factor (FF) of (19.4 mA/cm^{2}), 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%.

Thin-film solar cells made from earth-abundant, inexpensive, and nontoxic materials are needed to replace the current technologies whose widespread use is limited by their use of scarce, costly, and toxic elements. Tin monosulfide (SnS) is a promising candidate for making absorber layers in scalable, inexpensive, and nontoxic solar cells. SnS has always been observed to be a p-type semiconductor. Doping SnS to form an n-type semiconductor would permit the construction of solar cells with p-n homojunctions. This paper reports doping SnS films with antimony, a potential n-type dopant. Small amounts of antimony ((\sim 1%)) were found to greatly increase the electrical resistance of the SnS. The resulting intrinsic SnS(Sb) films could be used for the insulating layer in a p-i-n design for solar cells. Higher concentrations ((\sim 5%)) of antimony did not convert the SnS(Sb) to low-resistivity n-type conductivity, but instead the films retain such a high resistance that the conductivity type could not be determined. Extended X-ray absorption fine structure analysis reveals that the highly doped films contain precipitates of a secondary phase that has chemical bonds characteristic of metallic antimony, rather than the antimony–sulfur bonds found in films with lower concentrations of antimony.

Thin film solar cells made from earth-abundant, non-toxic materials are needed to replace the current technology that uses (Cu(In,Ga)(S,Se)2) and (CdTe), which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide ((SnS)). In this report, pure, stoichiometric, single-phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of (bis(N,N′-diisopropylacetamidinato)tin(II)) ([Sn(MeC(N-^{i}Pr){2})_{2}]) and hydrogen sulfide ((H_2 S)) at low temperatures (100 to 200 (°C)). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption ((\alpha > 10^4 cm^{−1})) is observed throughout the visible and near-infrared spectral regions. The films are p-type semiconductors with carrier concentration on the order of (10^{16} cm^{−3}) and hole mobility (0.82–15.3cm^{2}V^{−1}s^{−1}) in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in-plane direction.

To sustain future civilization, the development of alternative clean-energy technologies to replace fossil fuels has become one of the most crucial and challenging problems of the last few decades. The thin film solar cell is one of the major photovoltaic technologies that is promising for renewable energy. The current commercial thin film PV technologies are based on (Cu(In,Ga)Se_2) and CdTe. Despite their success in reducing the module cost below $1/Wp, these absorber materials face limitations due to their use of scarce (In and Te) and toxic (Cd) elements. One promising candidate for an alternative absorber material is tin monosulfide (SnS). Composed of cheap, non-toxic and earth-abundant elemental constituents, SnS can potentially provide inexpensive PV modules to reach the global energy demand in TW levels. Because of the high volatility of sulfur and various oxidation states of tin, non- stoichiometric chemical composition, traces of other phases ((i.e. Sn, Sn_2S_3, and SnS_2)), and elemental impurities (e.g. oxygen) are usually observed in SnS films obtained from various reported deposition techniques. First, we present a process to prepare pure, stoichiometric, single-phase SnS films from atomic layer deposition (ALD). The as-deposited SnS films exhibit several attractive properties, including suitable energy band gaps ((E_{g,}~ 1.1 – 1.3 eV)), a large absorption coefficient ((\alpha > 10^4 cm^{˗1})), and a proper carrier concentration (([p] ~ 10^{15} – 10^{16} cm^{˗3})). Then, heterojunction solar cells were fabricated from p-type SnS and n-type zinc oxysulfide (Zn(O,S)). A record high active-area efficiency of 2.46 % was achieved via conduction band offset engineering by varying the oxygen-to-sulfur ratio in Zn(O,S). Finally, we address two approaches potentially used for improving a device efficiency of the SnS solar cell. First, via doping to create an n-type SnS, a p-n homojunction device could be made. We present the processes and the results of doping SnS films with antimony and chlorine, potential n-type dopants. Second, by post-deposition heat treatment, an improvement in the transport properties of SnS film can be achieved. We discuss the effect of temperature and an annealing ambient ((N_2, H_2S), and sulfur) on grain growth and the electrical properties of annealed SnS films.

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.

Thin-film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. The effects of deposition temperature and annealing conditions of the SnS absorber layer were studied for solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO. Solar cells were further optimized by varying the stoichiometry of Zn(O,S) and the annealing conditions of SnS. Post-deposition annealing in pure hydrogen sulfide improved crystallinity and increased the carrier mobility by one order of magnitude, and a power conversion efficiency up to 2.9% was achieved.

((Sn,Al)O_x) composite films with various aluminum (Al) to tin (Sn) ratios were deposited using an atomic layer deposition technique. The chemisorption behavior of cyclic amide of tin(II) and trimethylaluminum were analyzed by Rutherford backscattering spectroscopy. Both precursors showed retarded and enhanced chemisorption on (Al_2O_3) and (SnO_2) surfaces, respectively. The films show highly anisotropic electrical conductivity, i.e., much higher resistivity in the direction through the film than parallel to the surface of the film. The cause of the anisotropy was investigated by cross-sectional transmission electron microscopy, which showed a nanolaminate structure of crystalline (SnO_2) grains separated by thin, amorphous (Al_2O_3) monolayers. When the Al concentration was higher than ( \sim 35) atom percent, the composite films became amorphous, and the vertical and lateral direction resistivity values converged toward one value. By properly choosing the ratio of (SnO_2) and (Al_2O_3) subcycles, controlled adjustment of film electrical resistivity over more than 15 orders of magnitude was successfully demonstrated.