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Title: Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006

Abstract

We describe the results of our continuing study of deep electronic states controlling open-circuit voltage in CdTe/CdS thin-film solar cells (Task 1). The study includes: (1) analysis of factors affecting trap signatures derived from admittance spectroscopy and capacitance transients measurements, such as activation-energy capture cross-sections and trap-density estimates, and (2) comparative studies of cells received from four different sources and prepared with significant variations in cell structure and processing procedures.

Authors:
; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
898922
Report Number(s):
NREL/SR-520-41097
ADJ-2-30630-05; TRN: US200708%%142
DOE Contract Number:
AC36-99-GO10337
Resource Type:
Technical Report
Resource Relation:
Related Information: Work performed by Colorado School of Mines, Golden, Colorado
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; ACTIVATION ENERGY; CAPACITANCE; CROSS SECTIONS; ELECTRIC POTENTIAL; PROCESSING; SOLAR CELLS; SPECTROSCOPY; TRANSIENTS; SOLAR ENERGY; ADMITTANCE SPECTROSCOPY; CAPACITANCE TRANSIENTS; DEEP ELECTRONIC STATES; CHEMICAL-BATH DEPOSITION; POST-DEPOSITION ANNEALING; GAS-TRANSPORT DEPOSITION; GAS-JET SYSTEM; OPTIMIZATION; Solar Energy - Photovoltaics

Citation Formats

Beach, J., Seymour, F. H., Kaydanov, V. I., and Ohno, T. R.. Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006. United States: N. p., 2007. Web. doi:10.2172/898922.
Beach, J., Seymour, F. H., Kaydanov, V. I., & Ohno, T. R.. Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006. United States. doi:10.2172/898922.
Beach, J., Seymour, F. H., Kaydanov, V. I., and Ohno, T. R.. Mon . "Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006". United States. doi:10.2172/898922. https://www.osti.gov/servlets/purl/898922.
@article{osti_898922,
title = {Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress; Annual Technical Report, 1 November 2005 - 31 October 2006},
author = {Beach, J. and Seymour, F. H. and Kaydanov, V. I. and Ohno, T. R.},
abstractNote = {We describe the results of our continuing study of deep electronic states controlling open-circuit voltage in CdTe/CdS thin-film solar cells (Task 1). The study includes: (1) analysis of factors affecting trap signatures derived from admittance spectroscopy and capacitance transients measurements, such as activation-energy capture cross-sections and trap-density estimates, and (2) comparative studies of cells received from four different sources and prepared with significant variations in cell structure and processing procedures.},
doi = {10.2172/898922},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Technical Report:

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  • This report describes basic issues behind CdTe/CdS cell performance and stability, such as the nature and electronic properties of impurities and defects that control the majority carrier concentration, mechanisms of dopant compensation, recombination processes, their nature and properties, migration and transformation of defects under various processing, stress, and operating conditions. We believe that a better basic understanding of the specific influence of grain boundaries, especially for fine-grain materials such as those making up CdTe-based cells, is now one of the most important issues we must address. We need to clarify the role of grain boundaries in forming the film electronicmore » properties, as well as those of the p-n junction.« less
  • The focus of this project is the open-circuit voltage of the CdTe thin-film solar cell. CdTe continues to be one of the leading materials for large-scale cost-effective production of photovoltaics, but the efficiency of the CdTe solar cell has been stagnant for the last few years. At the manufacturing front, the CdTe technology is fast paced and moving forward with U.S.-based First Solar LLC leading the world in CdTe module production. To support the industry efforts and continue the advancement of this technology, it will be necessary to continue improvements in solar cell efficiency. A closer look at the state-of-the-artmore » performance levels puts the three solar cell efficiency parameters of short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) in the 24-26 mA/cm2, 844?850 mV, and 74%-76% ranges respectively. During the late 1090s, efforts to improve cell efficiency were primarily concerned with increasing JSC, simply by using thinner CdS window layers to enhance the blue response (<510 nm) of the CdTe cell. These efforts led to underscoring the important role 'buffers' (or high-resistivity transparent films) play in CdTe cells. The use of transparent bi-layers (low-p/high-p) as the front contact is becoming a 'standard' feature of the CdTe cell.« less
  • The objectives of this program are to develop alternate heterojunction partner layers (buffer layers) for high efficiency CuInSe{sub 2}-based thin-film solar cells, and improve the understanding of how these layers and the details of processing affect cell performance. Investigations have primarily involved three tasks: (1) MOCVD growth of non-cadmium containing buffer layers; (2) optimized processing of buffer layers for high efficiency solar cells; and (3) electrical and physical characterization of layers and devices. Investigations of alternative buffer layers emphasized studies of ZnO grown by MOCVD. Using CIS substrates obtained form Siemens for process development, it was determined that growth proceduresmore » that resulted in good results with Siemens CIS (non-sulfur containing material) substrates also worked well with NREL CIGS material. A two step process was developed for growing highly resistive ZnO buffer layers (i-ZnO). In particular, after growing 100 to 150 {angstrom} of ZnO at 250 C, an additional 600 {angstrom} to 800 {angstrom} were grown at 100 C. Collaboration with NREL resulted in a n-ZnO/i-ZnO/CIGS cell which was determined to have a total area efficiency of 12.7%, and an active area efficiency greater than 13%. After growing i-ZnO with the two-step process onto NREL CIGS material, the i-ZnO/CIGS film structure was sent to NREL for deposition of a TCO, namely, conducting ZnO (n-ZnO). Collector grids and a MgF{sub 2} AR coating were also deposited at NREL. Low level efforts were devoted to studies of ZnSe and InSe buffer layers. A total area efficiency of 9.5% was achieved for a completed ZnSe/CIS cell making use of a RF sputtered ZnO for a TCO. Investigations of In{sub x}Se{sub y} (InSe) buffer layers were also initiated this past year.« less
  • A major effort during this subcontract period has been to evaluate the microcrystalline Si material under development at United Solar Ovonics Corporation (USOC). This material is actually a hydrogenated nanocrystalline form of Si and it will be denoted in this report as nc-Si:H. Second, we continued our studies of the BP Solar high-growth samples. Third, we evaluated amorphous silicon-germanium alloys produced by the hot-wire chemical vapor deposition growth process. This method holds some potential for higher deposition rate Ge alloy materials with good electronic properties. In addition to these three major focus areas, we examined a couple of amorphous germaniummore » (a-Ge:H) samples produced by the ECR method at Iowa State University. Our studies of the electron cyclotron resonance a-Ge:H indicated that the Iowa State a Ge:H material had quite superior electronic properties, both in terms of the drive-level capacitance profiling deduced defect densities, and the transient photocapacitance deduced Urbach energies. Also, we characterized several United Solar a Si:H samples deposited very close to the microcrystalline phase transition. These samples exhibited good electronic properties, with midgap defect densities slightly less than 1 x 1016 cm-3 in the fully light-degraded state.« less
  • The project describes long-term research and development issues related to polycrystalline thin-film solar cells. The general research approach is based on combining activities aimed at improvement of cell performance and stability with activities aimed at increasing fundamental understanding of the properties of materials making up the cells: CdTe, CdS, multi-layer back contact, and transparent conducting oxide (TCO) front contact. The authors emphasize the relation between structural and electronic material properties and various processing procedures, as well as the microscopic mechanisms responsible for the cell performance and its degradation. Major results and conclusions of this project include: (1) Stress tests ofmore » the cells under various stress conditions revealed conditions providing the most severe degrading of different cell parameters; (2) Consecutive stress testing under different bias revealed some reversible effects; (3) Preliminary analysis of the data obtained demonstrated a significant role of electromigration of the charged defects/impurities; (4) Some new approaches for the cell characterization and the data analysis were developed and checked experimentally; (5) New stress test experiments were planned for continued studies of degradation mechanisms.« less