Progress beyond the state of the art and results

This project will improve the accuracy of the measurements needed for the calibration of reference solar cells, by using the limitations of existing standards as a starting point in order to further reduce the lowest uncertainty in the world. Also, procedures for the evaluation of uncertainties will be established, i.e. brought to standardisation bodies. Furthermore, a metric will be developed to rate the shading sensitivity of PV modules. To improve the use of indoor-PV for the energy supply of IoT devices, the results of this project will contribute to international standardisation. As standardisation has thus far focused on outdoor applications, the development of new calibration facilities for indoor-PV with irradiance levels 100 to 1000 times lower than under standard outdoor conditions goes beyond the state of the art.

The most accurate primary calibrations of WPVS type reference solar cells are performed by PTB. The “world record” lowest uncertainty is 0.4 %, using PTB’s Laser-DSR facility. As these calibrated solar cells act as references and thus as the starting point of the traceability chain for the performance rating of the solar cells and solar modules manufactured by industry, a reduction of this important quantity is crucial.

This project aims to reduce the expanded uncertainty to a value down to 0.35 %, or even below, to successively affect all following calibrations and performance measurements in the traceability chain. In addition, there is a rising demand for calibrations of emerging PV devices (e.g. for perovskite solar cells). Due to their properties (e.g. response time) it is challenging to measure and calibrate these devices with low uncertainties. The uncertainties will be reduced from 3 % to 2 %. The activities within this project will also allow a reduction of the lowest available uncertainty of emerging PV devices.

The standards concerning measurement issues (the IEC 60904 standard series) describe procedures for the determination of I-V curves and of the spectral responsivity of PV devices, but they regularly lack documentation for determination of the uncertainty of the measurements. This determination of the uncertainty is often requested by the users of the standards. The project will remedy the deficiency of the IEC 60904 standard series by writing technical reports on how to calculate the measurement uncertainties.

State of the art energy rating procedures are currently not applicable to the emerging technologies (e.g. perovskite-on-silicon tandem). The existing methods and calculations do not consider the non-uniformity of irradiance in shady urban areas. In this project, a metric will be developed to rate the shading sensitivity of PV modules. This metric will improve the accuracy of energy yield determination.

In the area of indoor-PV, standardisation has just started recently. Indoor PV is used for autarkic energy supply of energy harvesting Internet of Things applications, e.g. for communication and sensor issues. No standards are currently available, although they are important for Industry 4.0 applications. The research activities within this project will contribute significantly to the ongoing standardisation processes by providing the necessary technical input.


Validation of suitable reference devices and measurement procedures

PTB has developed a traceable calibration method for WPVS cells with an uncertainty of 0.35 %. The improvements were achieved by using a better fibre coupling and by improvement on the monitor principle. A final report about this improvement including a comprehensive measurement uncertainty analysis was written.

To develop a traceable calibration method for standard Si modules with an uncertainty of 1.0 % and an extension on perovskite solar cells, TÜBİTAK ordered new simulator (cell simulator) for the measurement of the emerging PV devices. In addition, TÜBİTAK has already reduced its uncertainty to 1.2 % by using spectral

band pass filters for an improved spectral mismatch correction. The uncertainty budget including optical, electrical, thermal, reference standards contributions was prepared by TÜBİTAK. It was also shown that after getting traceability from PTB with the reduced uncertainty of 0.35%, the uncertainty for their calibrations of standard silicon devices will be reduced to below 1.0%. JRC is now ready to measure the spectral responsivity. Work is continuing towards reducing uncertainty below 1.0 %. ISFH updated the measurement routines of their fully integrated measurement system, for combined SR and I-V testing. For multi-junction solar cells LED and filtered halogen bias lights for the top and bottom cells are successfully integrated and tested. Light I-V measurements are performed with an LED-based sun simulator. The optimized measurement software allows adapting the light source to spectra that results in the AM1.5G currents of the individual sub cells. First tests on perovskite on silicon tandem two terminal test cells were successful. FhG performs a manual maximum power point tracking, which takes into account the slow transient response of emerging PV devices, in order to determine the power of these devices. To improve the homogeneity of the bias light, FhG has installed LED bars with central wavelengths of 467, 530 and 850 nm. Non-uniformity at or below 10% according to IEC 60904-8-1 is now fulfilled for large-area emerging PV devices.

Aalto and PTB have purchased emerging perovskite modules and cells from Solaronix to be used in a round-robin intercomparison. A comparison protocol was written. A round-robin based on reference cells has already started. The other round-robin using Perovskite modules is about to start in January 2023.

To adapt, develop, test and improve reference cells for use in the calibration of emerging PV devices, a screening analysis has been carried out by FhG. An extensive set of perovskite top and Si bottom cell spectral responsivities as well as reference cell spectral responsivities have been considered and the matching index between them has been calculated. A KG3 filter has yielded best spectral match for perovskite top cells, RG695- or RG715-filtered A+ reference cells show best agreement for Si bottom cells. Additional cells with RG715 filter were manufactured. FhG has furthermore investigated the irradiation-induced increase of the filter temperature. By means of an analytical model, the effect was reproduced and its effect on different measurement configurations (DSR method, flash I-V measurement, steady-state I-V measurement) quantified. Additional measurement uncertainties have been determined to be 0.2 % for RG695- and 0.3 % for RG715- and RG780-filtered reference cells. Thus, filter and solar cell combinations optimal for perovskite/Si tandem solar cells have been identified and the corresponding reference cells have been produced. The reference solar cells have been calibrated by PTB.

Evaluation of uncertainty sources related to the output power and energy of PV modules

To determine the measurement uncertainties related to the output power of PV modules within the IEC 61853-3 energy rating standard, as a first step an intercomparison regarding the Climate Specific Energy Rating (CSER) calculation was performed. An initial blind comparison using PV module data experimentally determined by TÜV Rheinland revealed discrepancies of 14.7 % rel. in the calculated CSER of the different participants. Four subsequent intercomparison phases were performed, during which best practices were established and ambiguous steps in the procedure were clarified. Thereby, the deviations could be reduced to below 0.1 %. The good agreement was confirmed by analysing the data of a second PV module in a final blind intercomparison. A joint publication on the results of the intercomparison and best practices in the calculation of the CSER of PV devices was written and presented at EU-PVSEC-37 and accepted for publication in IEEE Journal of Photovoltaics. 5 project collaborators (PV Performance Labs, NPL, DTU, TNO, Univ. Delft) participated at the round robin. The draft spreadsheet for uncertainty calculation is available and has been discussed at the 27 m project meeting with all partners, especially PTB, JRC, SUPSI, ITRI, LNE and Certisolis.

Definition of a quality metric for the sensitivity of PV modules power output in shady locations

To develop a quality metric defining the shading tolerance of PV modules, indoor- and outdoor facilities were adapted for the measurement of the shading tolerance. A metric was developed and tested using the facilities. A presentation for internal discussion was prepared. A quality metric has been presented at WCPEC8 in September 2022 and is currently being tested by several laboratories in an intercomparison campaign.

Characterisation and classification of PV-based energy-harvesting devices for Internet of Things (IoT) applications

As a summary of the state of the art, a chapter on the “Characterization and Power Measurement of IPV Cells” was written and published. In addition, several facilities for the measurement of the special indoor characteristics, needed for characterisation of the energy-harvesting devices for IoT applications, were built and characterisation measurements were performed. A possible set of indoor-adapted measurement conditions to be met by the facilities was published in the book. Aalto has studied various cells used for energy harvesting, and selected one Epishine cell, and one Amorphous silicon cell to be used in a round-robin intercomparison. The cells (including a reference cell to go with the energy harvesting cells) have been preliminarily tested for dynamic spectral responsivity, and electrical parameters at varied illuminance levels of 20 lx - 1200 lx.