SEMI Publishes Eight New Photovoltaic Standards
By Kevin Nguyen, SEMI International Standards
SEMI is pleased to announce the publication of eight new PV standards, which were approved during the recent Photovoltaic Standards committee meeting at Intersolar North America in July 2010. The new SEMI Standards include guides and specifications for bulk gases and liquid chemicals used in PV cell manufacturing as well as two new test methods for PV silicon materials.
Guides for Bulk Gases and Specifications for Liquid Chemicals Used in PV Manufacturing
- PV5-1110 Guide for Oxygen (O2), Bulk, Used In Photovoltaic Applications
- PV6-1110 Guide for Argon (Ar), Bulk, Used In Photovoltaic Applications
- PV7-1110 Guide for Hydrogen (H2), Bulk, Used In Photovoltaic Applications
- PV8-1110 Guide for Nitrogen (N2), Bulk, Used In Photovoltaic Applications
- PV11-1110 Specifications for Hydrofluoric Acid, Used In Photovoltaic Applications
- PV12-1110 Specifications for Phosphoric Acid, Used In Photovoltaic Applications
Whether used as cleaning agents, for surface texturing, diffusion, or etching, chemicals play a huge role in producing efficient and reliable cells. Overspecification adds unnecessary cost to these chemicals while underspecification may negatively impact the process of manufacturing an efficient PV cell. Thus, it is imperative to establish industry standards to eliminate variations while maintaining the consistency of the exact chemical needs. PV manufacturers and chemical suppliers alike will benefit from using these standards; manufacturers will know what to expect and suppliers will know what quality to produce.
Two years ago a survey was distributed to the PV industry to prioritize chemicals in need of standardization. This survey provided clear guidance to the SEMI PV Gases and Liquid Chemicals Purity Task Force (TF), and led to the six approved standards above. To produce standards based on the list of prioritized chemicals, the TF had discussions monthly via teleconference and met face-to-face during several SEMI Standards meetings. Key contributors to the TF effort included Air Liquide, Air Products, George Fischer, Solvay, KGM Chemicals, Voltaix, Pall, and Solarworld. Using existing semiconductor standards as a basic template, the TF adjusted parameters to reflect industry input. After development, ballots were distributed to the global PV Standards Committee membership and gained consensus within one year.
The TF is now working on Hydrochloric Acid, Nitric Acid, Silane, Anhydrous Ammonia and Helium. Its next meeting will be in conjunction with the Europe Fall Standards Meetings, on October 19 at the Solarwatt facility in Dresden, Germany. Following this meeting, the TF will next assemble at the NA Fall Standards meetings taking place at SEMI HQ in San Jose, November 9-10. Participation from the industry is highly encouraged, particularly from end-users, who are asked to join this important group and provide input to address the essential elements necessary for chemical materials standardization.
Test Methods for PV Silicon Materials
- PV9-1110 Test Method for Excess Charge Carrier Lifetime in PV Silicon Materials by Non-Contact Measurements of Photoconductivity Decay By Microwave Reflectance
- PV10-1110 Test Method for Instrumental Neutron Activation Analysis (INAA) Of Silicon
Electrical, chemical and structural characteristics of silicon materials for PV applications are critical to cell performance. While standardized test methods for semiconductor materials have been published for several decades, until recently test methods for PV materials have not available due to different commercial practices and a wider range of silicon feedstock for single and multicrystalline in wafer, brick and ingot configurations. However, the SEMI International PV Analytical Test Methods Task Force (TF) has now successfully developed two new test methods.
These test methods were circulated among PV and instrument manufacturers such as Semilab, Evans Analytical, Sinton Instruments, Bosch Solar, Applied Materials, Kobelco, Solarworld, SUMCO, Covalent Materials, Wacker, REC Silicon, Brewer Science, Hemlock Semiconductor, Fraunhofer and many others for consensus building. Constructive feedback was received and incorporated, and within one year, these test methods were successfully approved by the global SEMI PV Standards Committee.
PV9-1110 describes the method of measuring decay time of excess carrier lifetime, which is a critical indicator of PV solar cell performance. For high efficiency cells the decay characteristics must be carefully controlled to obtain the desired high performance device. The procedure is based on the microwave photoconductance decay method in which the decay of the conductance after photoexcitation is determined by the decay time of the photogenerated excess carriers.
PV manufacturers can use this Standard to set minimum acceptable criteria on starting silicon materials to eliminate poor material at the beginning of the manufacturing cycle, leading to significant cost and time savings.
PV10-1110 is a highly sensitive method for multi-element quantitative and qualitative analysis of various silicon materials in the form of powder, granules, chunks or wafers. The growing volume production of single crystalline and multicrystalline silicon requires higher quality control. However, there was previously no industry standard that explicitly defined sample preparation, measurement procedure and data analysis using Instrumental Neutron Activation Analysis (INAA) techniques. With PV10-1110, PV silicon manufacturers can avoid different and multiple analyses for material preparation, translating to time and cost efficiencies.
These two new test methods join the previously published PV1-0709 Test Method For Measuring Trace Elements In Silicon Feedstock For Silicon Solar Cells By High-Mass Resolution Glow Discharge Mass Spectrometry, and the TF is now working toward completion of a Test Method for Contactless Excess-Charge-Carrier Recombination Lifetime Measurement In Silicon Wafers, Ingots, and Bricks Using An Eddy-Current Sensor. This ballot will be adjudicated at the next PV Committee meeting in conjunction with the Europe Fall Standards Event, and the TF will also report on the progress of Document 4675A, Test Method for the Measurement of Elemental Impurity Concentrations in Silicon Feedstock by Bulk Digestion, Inductively-Coupled-Plasma Mass Spectrometry.
Standards Watch - September 2010