| AVS 64th International Symposium & Exhibition | |
| Thin Films Division | Wednesday Sessions |
| Session TF-WeM |
| Session: | Thin Film for Photovoltaics |
| Presenter: | Stephen Exarhos, University of California, Riverside |
| Authors: | S.A. Exarhos, University of California, Riverside E. Palmes, University of California, Riverside R. Xu, University of California, Riverside L. Mangolini, University of California, Riverside |
| Correspondent: | Click to Email |
Cu2ZnSnS4 (CZTS), a potential low-cost, earth-abundant absorber material, has been under intense investigation for prospective use in thin film photovoltaic (PV) devices. Its high optical absorption coefficient and variable band gap also have potential applications in other areas including photocatalysis, thermoelectrics, and energy storage. Theoretically predicted physical properties, however, have not been realized to date owing to the lack of control over chemical and microstructural heterogeneity and grain size and grain boundary chemistry. Using a modified aerosol spray pyrolysis technique, we have grown phase-pure 25 nm CZTS nanoparticles having no organic ligand contaminant. This is desirable in that grain boundary chemistry is known to degrade both optical and electrical response. Many applications demand that these nanoparticles be consolidated into relatively thick, large grain size films by means of a high temperature annealing treatment that drives impurities to interfaces thereby degrading properties. For PV applications, large grains are required to significantly reduce exciton recombination at grain boundaries. While the particles produced here do not sinter uniformly into polycrystalline thin films, we have shown that introducing a thin amorphous oxide to the surface of the particles does indeed promote uniform grain growth to create thin films with the optimal morphology for absorber layers in PV devices. The improvement is attributed to an increased retention of volatile SnS, a byproduct of CZTS decomposition, which occurs at high temperature. We have also controlled grain growth by incorporating Na2S into the film prior to annealing by means of a facile dipping technique – sodium is known to enhance grain growth in this material, though its incorporation is generally uncontrolled. Additionally, we present data from in situ Raman spectroscopy measurements designed to simulate the standard CZTS thin film annealing process. Using this approach, we have characterized the time/temperature effects on the vibrational spectrum of annealed CZTS films at high temperature in static variable atmospheres of sulfur vapor with and without a thin surface oxide interfacial coating. This technique is also used to monitor the cation site exchange process (copper and zinc) in the lattice that appears to be dependent on the rate of cooling following high temperature annealing.