Date of Award

10-4-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Tar-pin Chen

Second Advisor

Jingbiao Cui

Abstract

The extremely large energy consumption rate on Earth today is likely to exhaust the conventional energy resources such as fossil fuels and coal in the near future. The need for new renewable energy sources becomes inevitable and urgent. Among various new energy resources, solar energy is the most promising one because it is abundant and environmentally clean. The Sun light can be converted into electricity by using solar cell devices. Even though the solar cell panels made of thin films are in competition with conventional single crystal solar cells, only four thin film solar cells, i.e. amorphous silicon (a- Si), polycrystalline Si heterojunctions, CdS/CdTe, and CdS/CuInSe2, have achieved industrial production. Among these the chalcopyrite Cu(In/Ga)(Se/S)2 (CIGS) is a most promising one, because it has the highest light absorption coefficient (1 × 105/cm) and an energy efficiency as high as 20% today, the highest next only to the single crystalline Si solar cells. The main purpose of this research is to study the CIGS thin films using a sputtering process using a quaternary target made of Cu:In:Ga:Se at a ratio of 25:17.5:7.5:50 at %. Even though the sputtered CIGS films have a crystalline structure, they still need to be annealed in order to improve their grain size for solar cell devices fabrication. In order to grow large CIGS crystals, we have studied different annealing processes such as sulfurization under vacuum and selenization in ambient argon gas atmosphere. In the sulfurization study, which was done under vacuum and at 650 °C, morphology and composition of the CIGS absorber layer, and annealing temperature had a great impact on crystal growth. However, the devices didn’t achieve the desired energy conversion efficiency., For samples made by the selenization process which were carried out at 500 °C in an argon-filled tube under an ambient atmosphere pressure. The effect of annealing time and temperature have significantly improve the film morphology and grain size.The samples have achieved good crystalline structures and a conversion efficiency of 5.02%. It was found that many parameters such as the morphology of the thin films, the thickness of the absorber layer, the annealing time, and the annealing temperature played important roles in improving device performances. To sum up, CIGS have been successfully fabricated and characterized. This was done by revealing the sputtering process using a CIGS quaternary target. It is found that after the annealing processes the device performances was improved.

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