Subcategory: Materials Science
Joshua J Fox - Pennsylvania State University
Co-Author(s): Xiaotian Zhang, Zakaria Y. Al Balushi, and Joan M. Redwing, Pennsylvania State University, State College, PA
Crystalline silicon solar cells comprise over 90% of the commercial solar cell market, but their conversion efficiencies are limited to a theoretical maximum of ~29%. One pathway to increase efficiency is to form tandem devices that are comprised of a top wider bandgap absorber material and a bottom c-Si photovoltaic device. A theoretical efficiency approaching 44% can be achieved by using a top cell with a bandgap energy of ~1.7 eV. Candidate materials for the 1.7 eV top absorber include dilute nitride III-Vs, CZTS and ZnSiP2, but none of these have yet emerged as the material of choice. Our research is focused on investigating Sn(SxSe1-x)2 ternary alloys, a layered material system with bandgap energies that span the range from 1.1 eV for SnSe2 to 2.2 eV for SnS2, as a potential top absorber material. In addition to having a bandgap energy in the correct range for tandem devices, Sn(SxSe1-x)2 crystallizes as 2-D sheets, bound to one another via van der Waals forces. This is favorable for tandem solar cell applications due to the lack of required lattice matching between the stacked top and bottom absorbing layers which typically limits the choice of material for multi-junction devices. In addition, Sn and S are earth-abundant elements which is important for the development of a sustainable photovoltaic technology. Despite the intriguing potential of these materials, there have been limited studies on the synthesis and properties of Sn(SxSe1-x)2 thin films. Our initial studies have focused on the synthesis of SnSe2 and SnS2 thin films as binary components of the Sn-Se-S system via powder vapor transport (PVT). Source powders are evaporated in a heated horizontal quartz tube and transported downstream resulting in thin film deposition. The film characteristics were evaluated as a function of furnace temperature, growth duration, carrier gas flow rate and substrate type. Most significantly, the orientation of SnSe2 and SnS2 platelets are shown to vary depending on substrate type and position within the reactor. Experiments performed with oxide substrates (sapphire and SiO2) show out of plane platelet orientation likely due to the presence of dangling bonds on the surface which promote attachment of the platelet edges. In contrast, growth on epitaxial graphene which provides a well passivated surface resulted in lateral hexagonal platelets with domain sizes up to ~100 µm which can be coalesced to form continuous films with an RMS roughness of ~0.5 nm over a 5×5 µm scan area. Raman spectroscopy was used to confirm the formation of SnSe2 and SnS2 single phase films. Photoluminescence spectroscopy revealed room temperature emission from the SnS2 at ~1.9 eV which is close to the anticipated bulk bandgap energy. The results demonstrate the importance of surface passivation in obtaining high quality films which will be important for integration on Si. The synthesis and properties of Sn(SxSe1-x)2 films is currently underway and will also be discussed.
Funder Acknowledgement(s): Financial support for this project was provided by the PSIEE seed grant program and supplemental REU/REM support from NSF through PFI:AIR-TT 1414236 and EFRI 2-DARE 1433378.
Faculty Advisor: Joan M. Redwing, email@example.com
Role: Undergraduate research includes growth experiments of the thin film samples and characterization with various microscopy and spectroscopy techniques including SEM, Raman, XRD and more.