58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Narrative: Dye sensitized solar cells (DSSCs) have been developed into one of the most attractive third generation photovoltaic devices, with easy fabrication and relatively high conversion efficiency. A typical DSSC is composed of three parts: a dye-sensitized semiconductor photoanode, an electrolyte, and a counter-electrode. So far, titanium dioxide (TiO2) nanostructured photoanodes are the most promising semiconductor materials in DSSCs to promote electrons from the ground state to an excited state, in the dyes, and then transfer them to the conduction band of the photoanodes. Different dyes have been investigated to absorb broader bands to improve the light absorption. High electrocatalytic activity for efficient reduction of the charge carriers in the electrolyte is also indispensable, which requires a high quality counter electrode with a good electronic conduction. Generally, platinum (Pt) is used as a counter electrode, where electrons from the external circuit are collected and catalyse the reduction of the redox electrolyte. However, catalytic materials with low cost, low toxicity and environmental abundance are always highly favored to replace Pt (scarce and expensive) despite its high conductivity and good electrocatalytic activity. Quaternary chalcogenide Cu2ZnSnS4 (CZTS) was first found as a p-type semiconductor with suitable properties for solar cell applications. The advantages of this quaternary semiconductor are its earth-abundance and ideal direct band gap of about 1.5 eV and its high absorption coefficient ( larger than 1 x 10-4 cm-1), enabling a wide range of applications, such as light absorber material for thin film photovoltaics, thermoelectric material, etc. In this period, ligand free vertically aligned Cu2ZnSnS4 nanoplates were directly synthesized on fluorine doped tin oxide substrate using pulsed laser deposition (PLD) method, forming a nanoplate array. The array follows a two-step growth by first forming a Cu2ZnSnS4 thin film ( about 100 nm), followed by a vertical nanoplate formation. The nanoplates are about 20 nm thick and 300 nm high with a petal-like shape. Furthermore, the nanoplate array was integrated in a dye sensitized solar cell as a counter electrode with a power conversion efficiency of 3.65%, which is comparable to that of a conventional sputtered Pt counter electrode (3.33%) and higher than that fabricated with a “classical” Cu2ZnSnS4 thin film (2.83%). The Cu2ZnSnS4 nanoplate array is proved to be suitable for counter electrode fabrication to achieve Pt-free dye sensitized solar cell, which could significantly cut down the cell cost and provide environmentally friendly photovoltaic devices. In this no-cost extension period, Besides the PI, the ACS PRF fund to support one graduate student and one undergraduate student to work on solar cell study, which helped the PI established environmental friendly three-dimensional nanowire array fabrications for photovoltaics application.