Abstract/Details

Electrical characterization of thin film CdTe solar cells


2007 2007

Other formats: Order a copy

Abstract (summary)

Thin film CdTe based solar cells have the potential for high efficiency and have been investigated for 30 years due to their variations in flexibility in manufacturing technology, rapid deposition, and an excellent match to the solar spectrum. Despite these promising attributes, these devices have not reached their full potential. Their large scale implementation is limited by several factors including back contact and stability issues. The device performance is not well co-related with different design parameters and to date there is no generally accepted model for carrier transport.

Fabricating ohmic contacts to a p-type CdTe cells, due to its high work function, has been a long-standing problem in the photovoltaic industry. Many different contact materials and processes have been developed, but still, the contacts at their best are quasi-ohmic. In this work, a semi-transparent back contact using Cu doped ZnTe film was developed by galvanic deposition. The bifacial device configuration with transparent ZnTe:Cu back contacts permitted us to perform bifacial device characterization by illuminating the device through either the front or back contact or simultaneously through both. This approach to device characterization using a bifacial device is novel contribution in the field of thin film CdTe photovoltaic characterization techniques because it allows us to determine the fundamental minority carrier transport parameters such as the diffusion length (L) and depletion width (W) and separate effect of front junction from the back contact. The cells used for this project were produced by different methods but had similar efficiencies ∼10-12%, even though the processing conditions and back contact fabrication techniques and materials varied.

Bifacial current voltage (JV) analysis of illuminated devices indicated that the back contact was photosensitive. Experimental results prove that CdTe device operation is controlled solely by the primary CdS/CdTe heterojunction. Detailed bifacial spectral analysis on devices made with different CdTe thicknesses and different applied biases was used to quantify the transport parameters L and W. CdTe devices with absorber thickness in the range of 3-8μm were investigated. The diffusion lengths derived were in the range of 0.6-0.7μm. In CdTe devices the absorption co-efficient is in the range of 105 1/cm, W in the range of 2-4μm. As a result the absorption depth αW > 1, and all the carrier generation and absorption occur in the depletion region even for forward bias of 0.5V. Bifacial spectral response characterization results prove that for CdTe solar cells, the operation is determined by voltage dependent current collection and not diffusion length.

Laboratory simulated accelerated test life conditions were used to investigate the cell degradation phenomenon, with direction towards identifying device degradation mechanisms. Stressing results show that primary degradation in xvii efficiency occurs due to degradation in the open circuit voltage (Voc) and the fill factor (FF). Cyclic stress, where in the devices were subjected to alternating light and dark light bias cycles and switching applied bias during stress experiments performed on CdTe devices allowed us to identify and partially explain transient degradation and recovery mechanism in CdTe devices. Results from stressing experiments performed on CdTe devices made with different back contact materials showed that cells with ZnTe:Cu based contacts had minimum degradation in Voc. Bifacial JV analysis performed on these stressed devices revealed a photo-sensitive back contact.

Temperature dependence of the open circuit voltage, with different back contacts was studied under different illumination conditions. Measurements showed there were two distinct regions, one above 220K where open circuit voltage linearly increases with temperature, and one below 200K where Voc becomes nearly independent of both temperature and light intensity. A model explaining this "Voc saturation" behavior has been proposed in this document.

Photovoltaic device modeling results obtained using AMPS (Analysis of microelectronic and photonic structures) suggest that the dominant recombination mechanism is the SRH recombination through midgap states.

Indexing (details)


Subject
Electrical engineering
Classification
0544: Electrical engineering
Identifier / keyword
Applied sciences; Cadmium telluride; Electrical; Solar cells; Thin film
Title
Electrical characterization of thin film CdTe solar cells
Author
Desai, Darshini
Number of pages
320
Publication year
2007
Degree date
2007
School code
0060
Source
DAI-B 67/12, Dissertation Abstracts International
Place of publication
Ann Arbor
Country of publication
United States
University/institution
University of Delaware
University location
United States -- Delaware
Degree
Ph.D.
Source type
Dissertations & Theses
Language
English
Document type
Dissertation/Thesis
Dissertation/thesis number
3247712
ProQuest document ID
304861336
Copyright
Database copyright ProQuest LLC; ProQuest does not claim copyright in the individual underlying works.
Document URL
http://search.proquest.com/docview/304861336
Access the complete full text

You can get the full text of this document if it is part of your institution's ProQuest subscription.

Try one of the following:

  • Connect to ProQuest through your library network and search for the document from there.
  • Request the document from your library.
  • Go to the ProQuest login page and enter a ProQuest or My Research username / password.