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IEC TS 62789: Photovoltaic Concentrator Cell Specification
Technical specification for concentration photovoltaic cells defining electrical parameters, measurement methods, and performance rating under concentrated light
IEC TS 62789, published in 2014 as a Technical Specification, defines the specifications and measurement methods for concentration photovoltaic (CPV) cells. These specialized solar cells are designed to operate under concentrated sunlight, typically 100 to 1000 suns, and are predominantly based on III-V multijunction semiconductor technology. As the global CPV industry advances toward higher conversion efficiencies exceeding 45%, standardized specification and measurement protocols have become essential for ensuring fair performance comparison, reliable system design, and bankable energy yield predictions.
The standard was developed by IEC Technical Committee 82 (Solar Photovoltaic Energy Systems) Working Group 7 to address the unique requirements of concentrator cells, which differ fundamentally from conventional flat-plate PV cells. CPV cells operate at current densities up to 1000 times higher than conventional cells, requiring specialized measurement techniques and specification formats. The technical specification covers single-junction and multijunction cells across the full range of concentration ratios used in CPV systems, from low-concentration (2-10 suns) to high-concentration (500-1000 suns) applications.
Multijunction CPV cells achieve the highest conversion efficiencies of any photovoltaic technology, with laboratory records exceeding 47% and commercial modules reaching 35-40%. These cells use three or more junctions optimized for different spectral bands of the solar spectrum, capturing more energy than single-junction silicon cells which are fundamentally limited to about 29% theoretical efficiency.
Cell Specifications and Electrical Parameter Definitions
The standard defines a comprehensive set of electrical parameters for concentrator cells, measured under standardized concentration conditions. The reference measurement condition is 1000 W/m2 direct normal irradiance at 25 degrees C cell temperature, using the AM1.5D spectrum (direct normal + circumsolar). Key parameters include the short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF), and maximum power point parameters. Critically, the standard specifies that these parameters must be measured under the actual concentration ratio relevant to the intended application, with the concentration ratio explicitly stated in the specification.
| Parameter | Symbol | Definition | Test Condition |
|---|---|---|---|
| Short-circuit current | Isc | Current at zero voltage under concentration | Specified concentration, 1000 W/m2 |
| Open-circuit voltage | Voc | Voltage at zero current under concentration | Specified concentration, 25 deg C |
| Fill factor | FF | Ratio of Pmax to (Isc x Voc) | Specified concentration |
| Conversion efficiency | eta | Electrical output / optical input power | Specified concentration, AM1.5D |
| Series resistance | Rs | Internal resistance including grid and contact | Dark I-V or illuminated methods |
| Temperature coefficient | alpha, beta | Change in Isc, Voc per degree C | 25-75 deg C range |
| Shunt resistance | Rsh | Parallel leakage resistance | Low-bias dark I-V |
A unique aspect of CPV cell specifications is the explicit requirement for the concentration ratio and spectrum. The short-circuit current of a multijunction CPV cell is determined by the junction with the lowest current under the incident spectrum, making the current in each subcell directly proportional to the spectral content of the concentrated light. IEC TS 62789 requires that measurements be performed using a solar simulator with Class A spectral match to AM1.5D (per IEC 60904-9), with the irradiance level adjusted to match the intended concentration level. For concentrator cells operating above 500 suns, the flash test method is preferred to avoid excessive cell heating during the I-V measurement sweep.
Accurate measurement of CPV cell efficiency requires careful thermal management during testing. At 500 suns concentration, the incident power density on a 1 cm2 cell is 50 W, and without active cooling the cell temperature would rise by over 100 degrees C within seconds, completely invalidating the measurement. Pulsed solar simulators with millisecond pulse widths are essential for high-concentration measurements.
Measurement Methods and Performance Characterization
The standard prescribes detailed measurement procedures for determining concentrator cell performance. The spectral response measurement is particularly important for multijunction cells, as each subcell junction responds to a different part of the solar spectrum. The spectral response is measured using monochromatic light biased with appropriate filtered light to ensure that only the junction under test contributes to the measured photocurrent. For a triple-junction cell (e.g., InGaP/GaAs/Ge), this requires three separate spectral response measurements with carefully selected bias light conditions.
Temperature coefficient characterization under concentration is essential for predicting CPV system performance under real-world operating conditions. Unlike conventional silicon cells where temperature effects are relatively predictable, multijunction CPV cells exhibit temperature coefficients that depend on the specific material system and the spectral balance between junctions. The temperature coefficient of Voc for typical III-V multijunction cells ranges from -3 to -6 mV/deg C per junction, while the temperature coefficient of Isc is positive but relatively small at 0.01-0.05%/deg C. The net effect on efficiency ranges from -0.02 to -0.07%/deg C, significantly lower than the typical -0.4%/deg C for crystalline silicon cells, giving CPV a distinct advantage in hot climates.
Series resistance characterization is critical for high-concentration operation. The efficiency loss due to series resistance scales with the square of the current density, making it the dominant loss mechanism at 500-1000 suns operation. The standard specifies both dark I-V and illuminated methods for determining series resistance, with the requirement to extract both the lumped series resistance and the distributed resistance components. For cells operating at 1000 suns, the target lumped series resistance should be below 1 mΩ.cm2 to limit resistive losses to less than 10% of the output power. This demanding requirement drives the design of front grid metallization, with finger widths of 5-15 microns and pitch distances of 50-200 microns being typical for high-concentration cells.
Modern triple-junction CPV cells achieve over 46% efficiency under 500-1000 suns concentration. The combination of high efficiency and low temperature coefficient makes CPV an attractive technology for utility-scale solar power generation in high-direct-normal-irradiance regions such as the Middle East, southwestern United States, Australia, and parts of Africa and South America.
Engineering Design Insights for CPV Systems
From a system design perspective, the cell specifications defined by IEC TS 62789 are critical inputs for CPV module and system design. The cell’s angular acceptance characteristics, not explicitly covered by the standard but typically specified by manufacturers, determine the optical tolerance budget for the concentrator optics. High-concentration systems (500-1000 suns) require tracking accuracy of 0.1-0.3 degrees, driving the design of dual-axis trackers and control systems. The cell’s series resistance directly impacts the required grid design and the trade-off between shadowing losses and resistive losses. The cell efficiency under concentration determines the module efficiency and ultimately the levelized cost of electricity.
The standard also provides guidelines for reliability characterization, including thermal cycling and humidity testing adapted for the CPV environment. The cell interconnect design must accommodate thermal expansion mismatches between the semiconductor cell and the substrate, typically using expansion-matched materials or flexible interconnects. Solder fatigue in CPV cell interconnects is a known failure mechanism, as the cells experience daily thermal cycles of 40-60 degrees C as the sun progresses across the sky. The standard recommends accelerated thermal cycling tests with at least 1000 cycles between -40 and +85 degrees C to validate interconnect reliability.
| Cell Type | Junctions | Concentration | Efficiency (Production) | Typical Application |
|---|---|---|---|---|
| Low-concentration Si | 1 (Si) | 2-10 suns | 18-22% | Simple booster reflectors |
| Medium-concentration III-V | 2 (GaAs/Ge) | 100-300 suns | 28-32% | Commercial CPV modules |
| High-concentration triple-junction | 3 (InGaP/GaAs/Ge) | 500-1000 suns | 38-44% | Utility-scale CPV plants |
| Advanced multijunction | 4-6 (InGaP/GaAs/InGaAs/…) | 500-1000 suns | 44-47% | Next-generation systems |
Q1: Why does IEC TS 62789 specify measurement at a specific concentration ratio rather than at 1 sun?
A: CPV cell performance under concentration differs significantly from 1-sun performance due to current-density-dependent effects such as series resistance losses, voltage increase with concentration (Voc increases logarithmically), and heating effects. Specifications are only meaningful at the intended operating concentration.
A: CPV cell performance under concentration differs significantly from 1-sun performance due to current-density-dependent effects such as series resistance losses, voltage increase with concentration (Voc increases logarithmically), and heating effects. Specifications are only meaningful at the intended operating concentration.
Q2: How does spectral mismatch affect CPV cell measurement?
A: Multijunction cells are sensitive to spectral variations because each junction responds to a different wavelength band. Spectral mismatch between the test simulator and actual sunlight can cause significant errors in efficiency measurement. The standard requires spectral mismatch correction using the measured spectral response of each subcell.
A: Multijunction cells are sensitive to spectral variations because each junction responds to a different wavelength band. Spectral mismatch between the test simulator and actual sunlight can cause significant errors in efficiency measurement. The standard requires spectral mismatch correction using the measured spectral response of each subcell.
Q3: Can IEC TS 62789 specifications be compared with conventional PV cell specifications?
A: Direct comparison is not meaningful due to different measurement conditions (concentration ratio, spectrum). CPV cell efficiencies are inherently higher under concentration, but this advantage must be evaluated at the system level considering optical losses, tracking requirements, and system costs.
A: Direct comparison is not meaningful due to different measurement conditions (concentration ratio, spectrum). CPV cell efficiencies are inherently higher under concentration, but this advantage must be evaluated at the system level considering optical losses, tracking requirements, and system costs.
Q4: What is the significance of the temperature coefficient for CPV system design?
A: The lower temperature coefficient of CPV cells compared to silicon is a key advantage, particularly for installations in hot climates. However, thermal management within the CPV module remains critical as the concentrated flux creates high local heat fluxes that must be efficiently dissipated to avoid performance degradation.
A: The lower temperature coefficient of CPV cells compared to silicon is a key advantage, particularly for installations in hot climates. However, thermal management within the CPV module remains critical as the concentrated flux creates high local heat fluxes that must be efficiently dissipated to avoid performance degradation.
