Application of Spray Pyrolysis Metal Oxide Thin Films for Photovoltaic Applications

Abstract

Thin films of metal oxides have found wide applications in solar cells, microelectronics and optoelectronic devices. Metal oxides are used in photovoltaics devices as a transparent conducting oxide (TCO) electrode on the window layer and, for electrically resistive films, act as a buffer layer when placed between the TCO and semiconductor. The buffer layer imparts several advantages to the final device structure: 1) blocking shunts due to the presence of pinpoints that occur on thinning of the semiconductor window layer (CdS in CdS/CdTe devices); and 2) creating a resistive plug between the electrode and semiconductor layers to eliminate detrimental effects of non-uniformities at the interface. Initial work in the area employed chemical vapor deposition to prepare tin oxide based films, which were found to exhibit excellent buffer layer characteristics. We have chosen to explore the use of spray pyrolysis to deposit thin films of metal oxides because the technology is versatile, simple to implement and of low-cost. The main advantage of spray pyrolysis is the ability to apply any precursor system that can be dissolved or consists of a fluid slurry. Zinc oxide is a good candidate as a transparent conducting oxide (TCO) due to zinc’s high natural abundance, high transparency in the visible region, low resistivity, and low toxicity. The precursor for the spray system was prepared by dissolving zinc acetate dihydrate in solvents such as water and alcohols. Undoped zinc oxide thin films were found to be highly resistive. Adding fluoride anion and/or aluminum cation sources to the precursor solution provided clear films of 4000-8000 A with resistivities of circa 1 ohm cm. Although not of TCO quality these doped zinc oxide films can be employed as a buffer layer between the TCO and CdS layers of a CdS/CdTe based solar cell. Depositing the doped ZnO in a nitrogen gas atmosphere lowers resistivity. In addition fluorine doped tin oxide thin film, a well known TCO, was deposited from dibutyltin diacetate, a chloride-free source for tin; over doping the film provides films with a resistivity appropriate for use as a buffer layer. Zinc oxide and tin oxide were deposited on glass, a transparent and insulating material, so the electro-optical properties of the TCO can be investigated. Surface morphology and crystalline phase of the deposited films were examined by XRD and SEM. While electrical properties were determined by four-probe resistance and thickness measurements. The final proof of usefulness will be the photovoltaic performance of CdS/CdTe devices fabricated with this technique.