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Standards Development
The proposed document will be part of the IEC 60904 series of standards. This series prescribes and guides the measurement of photovoltaic (PV) device output, specifically the current-voltage (I-V) curve, which is critical to establishing the performance of a PV cell or module for commercial purposes. The project will decide whether the new document should be numbered 60904-X or instead 60904-1-X.
The document will provide guidance for measuring metastable PV devices, beyond what is provided in the IEC 60904 series. The content will be developed from a consensus of stakeholders including testing laboratories, companies and research institutes. The i ntent will be to reduce the presently observed variability in results between different laboratories.
The document will primarily cater for the new perovskite-based PV devices, however it is intended to be applicable to any new PV technologies that exhibit metastability. The methods in the document will apply to single-junction PV devices, however the project will consider that any defined procedure will eventually need to be consistent with the tandem PV device standards IEC 60904-1-1 and 60904-8-1, or otherwise those standards may need to be adjusted for consistency.
The document will focus on metastability-related issues that affect reproducibility of the measured I-V curve, hence it will record consensus and provide guidance on at least the following aspects:
(1) Any mandatory or optional pre-conditioning, including how to demonstrate that the pre-conditioning has been sufficient;
(2) How a dynamic I-V measurement should be performed (except where such documentation already exists in IEC 60904-1) e.g. how to demonstrate that sufficient settling has taken place at each voltage point, and how to deal with situations where such settling is not achieved;
(3) How individual measurements of the key I-V parameters should be performed, specifically Isc, Voc and Pmp (except where such documentation already exists in IEC 60904-1), e.g. how to demonstrate that sufficient settling has taken place, and how to deal with situations where settling is not achieved.
The document will not consider methods designed to produce long-term stabilisation in the device, e.g. technology-specific parameters for the Stabilisation procedure (MQT-19) in IEC 61215.
After 10 years' development the efficiency of laboratory-scale perovskite photovoltaic (PV) devices has reached over 25% for single-junction devices and over 32% for multi-junction devices, based on industrial silicon designs for the bottom cell. Several companies are now developing commercial perovskite PV products and one has claimed their product passes the IEC 61215 type approval tests.
The long term stability of laboratory perovskite PV devices is improving to the point that international inter- comparisons between testing laboratories are now possible. However, these tests have shown that reversible short- and medium-term instability (metastability) remains an obstacle to providing accurate product labelling, and also to reliable measurements before and after stress testing.
The IEC 60904 series includes brief mention of requirements for stabilising devices before testing, and for reporting of measurements under "steady-state conditions". However, no guidance is provided for how to achieve these for particular PV technologies. This situation creates the following problems:
• Labelling that is potentially not indicative of the performance of products in real applications;
• Difficulties for testing laboratories to track their own performance and hence maintain global consistency;
• Complications for before-and-after measurements associated with stress testing for quality and safety.
The academic literature describes several "steady-state" methods such as maximum-power-point-tracking for Pmp, and "dynamic" type measurements for full I-V curves, such that the measurements are representative of performance in the field.
This project seeks to build consensus around those methods and to describe them, or an industrially practical version of them, in a way that may eventually become normative. This will allow fair comparison of labels between new and existing PV products, and between new PV products from different manufacturers, and will begin to address the problems identified above
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