Back contact improvement is one of the most crucial issues for the realization of highly efficient n-CdS/p-CdTe solar cells. Conventional methods for making a sufficiently ohmic contact to p-CdTe usually involve a solution etching process and a Cu doping process, which are known to negatively affect the device reliability.
To resolve this problem, a low-resistance back contact for n-CdS/p-CdTe solar cells has been developed, which utilizes a vapor-deposited transition metal oxide (TMO) thin film as the back contact buffer layer between p-CdTe and the back electrode. The usefulness of TMO is attributed to its unusually high work function which is needed to match that of p-type CdTe in producing a contact of low resistance. As one major representative of TMO materials, molybdenum oxide (MoOx) has been investigated as a novel buffer in this thesis.
First, the processes for making low resistance contact with MoO x buffer are explored. To achieve a good ohmic contact to p-CdTe, a water rinse step is necessary in order to remove surface residues from the CdTe surface prior to MoOx deposition. In addition, different methods for depositing MoOx films have been examined, including thermal evaporation and DC sputtering methods. With MoOx as the high work function buffer, various metals can be used as the electrode to realize an ohmic back contact to p-CdTe. Other advantages of the MoOx buffer include dry application by vacuum deposition, and thus it is particularly suitable for the fabrication of ultra-thin CdTe solar cells without introducing additional shorting defects.
Second, the mechanism by which MoOx improves the performance of the back contact is studied through the investigation of Ni/CdTe and Ni/MoO x/CdTe interfaces with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Contrasting differences in the interface formation and core level structure were observed, indicating that the presence of a thin MoOx inter-layer was effective in reducing the unfavorable reaction between Ni and the native tellurium oxide at the CdTe surface.
Finally, the stability of CdTe cells with MoOx buffer is studied. MoOx is found to effectively stabilize the device performance, especially the open-circuit voltage (Voc) of the CdTe solar cell during thermal stress tests. The cell with a Ni-only electrode rapidly degrades due to Ni diffusing into the CdTe film and Ni abstracting Te from CdTe. With the addition of a layer of MoOx as the buffer layer, the diffusion of Ni and the reaction between Ni and CdTe are alleviated, resulting in significantly improved device stability.
To summarize, an efficient and stable back contact has been developed through the application of a high work function MoOx as the buffer for p-CdTe. As a result, n-CdS/p-CdTe solar cells with improved efficiency and stability have been realized.
|Advisor:||Tang, Ching W.|
|Commitee:||Funkenbusch, Paul D., Hirschman, Karl D., Shestopalov, Alexander A.|
|School:||University of Rochester|
|Department:||Hajim School of Engineering and Applied Sciences|
|School Location:||United States -- New York|
|Source:||DAI-B 74/07(E), Dissertation Abstracts International|
|Subjects:||Alternative Energy, Chemical engineering, Electrical engineering|
|Keywords:||Back contact, Cadmium sulfide, Cadmium telluride, Device stability, Molybdenum oxide, Solar cells, Thin films|
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