57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Weilie Zhou, PhD, Advanced Materials Research Institute, University of New Orleans, New Orleans, LA

Summary

A large area of aligned ZnO nanowires array was fabricated by hydrothermal route on an ITO substrate. Cu2ZnSnS4 nanocrystals were synthesized via wet chemistry method, exhibiting a size in the range of 7~15 nm. The synsthesized nanocrystals will be then coated on the surface of the previously synthesized nanowires, in order to form a p-n junction for solar cell applications. The fabrication of the device is in process.

Narrative

The key advantage of using nanowires in solar cells application is the radial charge collection, which allows for relaxing the stringent criterion for material quality, while maintaining the possibility to achieve comparable efficiency to the planar solar cells. Previously, our group focused on the investigation of the nanowires solar cells which core-shell geometry, such as ZnO/ZnSe, ZnO/ZnS and CdSe/ZnTe coaxial nanowires, which has been documented in the former reports. The radial junctions embedded in the core-shell nanowire have demonstrated the advantages in term of materials consumption and light absorption. Nonetheless, the relative thin shell thickness and the defects which inevitably exist in the interfaces greatly limited the energy conversion efficiency of the final devices. On the other hand, the absorption materials used in conventional planar solar cells suffer from the high cost and toxicity. Cu2ZnSnS4, commonly abbreviated as CZTS, has attracted vast attention as an ideal absorption material in solar cell owning to its high absorption coefficient, direct band gap of 1.4~1.5 eV and abundance in earth's crust. In this report, we focus on constructing an environmentally friendly and low cost solar cell by coupling CZTS nanocrystals with ZnO nanowires array. In this device prototype, the carrier separation and collection are facilitated by the radial p-n junction embedded in the nanowire and nanocrystals. All-solution processed technique involved in this project hold the promise to scale up the practical device.

ZnO nanowire array was synthesized by a facile hydrothermal route. First, a ZnO layer of 50 nm was deposited, as a seed layer, on the surface of a clean indium-tin-oxide (ITO) substrate by RF magnetron sputtering. The nanowires were then growth by using an equimolar ratio of hexamethylenetetramine and zinc nitrate hexahydrate as solution precursor. Most ZnO nanowires exhibited tapered morphologies and diameters in the range of 80~180 nm.

Cu2ZnSnS4 nanocrystals were synthesized following the modified hot-injection method. The results are in agreement with the value reported in the literature, as well as the Powder Diffraction File 26-0575 of the International Center for Diffraction Data. Moreover, the single phase nature of the sample was further verified by Raman spectrum and the characteristic peak of a pure CZTS phase, which is centered at 338 cm-1, was observed. The solar cells based on ZnO nanowires and CZTS nanoparticles are being fabricated.