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Title: Process Development for High Voc CdTe Solar Cells: Phase I, Annual Technical Report, October 2005 - September 2006

Abstract

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-art 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.

Authors:
;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
903078
Report Number(s):
NREL/SR-520-41525
NDJ-2-30630-18; TRN: US200720%%185
DOE Contract Number:
AC36-99-GO10337
Resource Type:
Technical Report
Resource Relation:
Related Information: Work performed by University of South Florida, Tampa, Florida
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; BUFFERS; CURRENT DENSITY; EFFICIENCY; ELECTRIC POTENTIAL; FILL FACTORS; MANUFACTURING; ORGANIC COMPOUNDS; PERFORMANCE; PRODUCTION; SOLAR CELLS; VOLATILE MATTER; PV; OPEN-CIRCUIT VOLTAGE; THIN FILM; MANUFACTURER; SHORT-CIRCUIT CURRENT DENSITY; FILL FACTOR; HIGH RESISTIVITY TRANSPARENT FILMS; CADMIUM TELLURIDE (CDTE); CONVERSION EFFICIENCY; Solar Energy - Photovoltaics

Citation Formats

Ferekides, C. S., and Morel, D. L.. Process Development for High Voc CdTe Solar Cells: Phase I, Annual Technical Report, October 2005 - September 2006. United States: N. p., 2007. Web. doi:10.2172/903078.
Ferekides, C. S., & Morel, D. L.. Process Development for High Voc CdTe Solar Cells: Phase I, Annual Technical Report, October 2005 - September 2006. United States. doi:10.2172/903078.
Ferekides, C. S., and Morel, D. L.. Sun . "Process Development for High Voc CdTe Solar Cells: Phase I, Annual Technical Report, October 2005 - September 2006". United States. doi:10.2172/903078. https://www.osti.gov/servlets/purl/903078.
@article{osti_903078,
title = {Process Development for High Voc CdTe Solar Cells: Phase I, Annual Technical Report, October 2005 - September 2006},
author = {Ferekides, C. S. and Morel, D. L.},
abstractNote = {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-art 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.},
doi = {10.2172/903078},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}

Technical Report:

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  • 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.
  • First Solar demonstrated a cadmium telluride module with an NREL-confirmed aperture efficiency of 12.2% and stability with accelerated life testing.
  • 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
  • This is a cumulative and final report for Phases I, II and III of this NREL funded project (subcontract # XXL-5-44205-10). The main research activities of this project focused on the open-circuit voltage of the CdTe thin film solar cells. Although, thin film CdTe continues to be one of the leading materials for large-scale cost-effective production of photovoltaics, the efficiency of the CdTe solar cells have been stagnant for the last few years. This report describes and summarizes the results for this 3-year research project.
  • This project, carried out at the Colorado School of Mines, addresses long-term research and development issues related to polycrystalline thin-film solar cells. Our general research approach is based on combining activities aimed at improving cell performance and stability with activities aimed at increasing our fundamental understanding of the properties of materials making up the cells: CdTe, CdS, multilayer back-contact, and transparent conducting oxide (TCO) from contact. We emphasize the relation between structural and electronic materials properties and various processing procedures, as well as the microscopic mechanisms responsible for the cell performance and its degradation. Section 1 presents studies of degradationmore » under stressing of the cells with differently processed CdTe and different back contents. Section 2 presents studies of deep traps in the electrodeposited (ED) CdTe cells performed in collaboration at NREL. Section 3 reports studies of the spectral dependencies of photocurrent, and their spatial distribution over the cross-section of the ED CdTe cells were performed using near-field scanning optical microscopy (NSOM). Section 4 discusses the impedance spectroscopy of CdTe and CdS thin films. Section 5 presents studies of band spectrum and electron scattering in transparent conducting oxides. Appendices present data on personnel involved in the studies, laboratory improvements, and publications.« less