In a laboratory setting, the efficiency of a photovoltaic cell is measured by exposing it to standardized, controlled light and temperature conditions that simulate sunlight, precisely measuring its electrical output, and then calculating the ratio of the maximum electrical power it can produce to the power of the light incident upon its surface. This process, governed by strict international standards like IEC 60904, is far more complex than simply pointing a cell at the sun; it involves sophisticated equipment to ensure accuracy and repeatability, allowing for fair comparisons between different cell technologies from labs around the world. The resulting efficiency percentage is the single most critical metric for evaluating a cell’s performance and potential for commercial application.
The heart of any PV characterization lab is the solar simulator. This isn’t a miniature sun but a high-powered lamp system designed to mimic the sun’s spectrum as closely as possible. The gold standard is the Air Mass 1.5 Global (AM1.5G) spectrum, which represents the sunlight reaching the earth’s surface after passing through an atmosphere 1.5 times thicker than when the sun is directly overhead. This spectrum is defined in the ASTM G-173 standard and includes both direct and diffuse sunlight. Solar simulators are classified (Class A, B, or C) based on how well they match this spectrum, their spatial uniformity across the test area, and their temporal stability. A Class A simulator has the highest performance in all three categories. The light intensity, or irradiance, is calibrated to 1000 Watts per square meter (1000 W/m²), which is considered “one sun” for standard testing conditions.
The cell under test is placed on a temperature-controlled stage, typically set to 25°C (77°F). This is a crucial part of the Standard Test Conditions (STC), as a cell’s efficiency decreases as its temperature increases. The stage ensures the measurement is consistent and not skewed by thermal effects. The entire setup is often contained within a dark enclosure to block any ambient light that could contaminate the measurement.
To measure the cell’s electrical behavior, it is connected to a precision instrument called a source measure unit (SMU) or a dedicated current-voltage (I-V) tracer. This instrument does two key things: it can apply a variable voltage across the cell’s terminals and measure the resulting current with extreme precision, or vice-versa. By sweeping the voltage from zero (a short circuit) up to the point where no current flows (an open circuit), the instrument plots the cell’s I-V curve.
The I-V curve is the fundamental fingerprint of a solar cell’s performance. From this curve, several key parameters are extracted:
- Short-Circuit Current (Isc): The current flowing when the voltage is zero. This is directly proportional to the number of photons the cell absorbs and converts into electrons.
- Open-Circuit Voltage (Voc): The voltage across the cell when no current is flowing. This is a measure of the inherent voltage potential of the semiconductor material.
- Maximum Power Point (Pmax or MPP): The point on the I-V curve where the product of current and voltage is at its highest. This is the maximum power the cell can deliver.
- Fill Factor (FF): A measure of the “squareness” of the I-V curve, calculated as FF = Pmax / (Isc x Voc). A higher fill factor (closer to 1) indicates a cell with lower internal electrical losses.
Here is a typical set of data for a high-quality monocrystalline silicon cell measured under STC:
| Parameter | Symbol | Typical Value | Unit |
|---|---|---|---|
| Short-Circuit Current | Isc | 6.5 | Amps (A) |
| Open-Circuit Voltage | Voc | 0.66 | Volts (V) |
| Current at Max Power | Imp | 6.2 | Amps (A) |
| Voltage at Max Power | Vmp | 0.57 | Volts (V) |
| Maximum Power | Pmax | 3.53 | Watts (W) |
| Fill Factor | FF | 82.5 | % |
With these values, the final efficiency (η, or eta) is calculated using the formula:
η = (Pmax / (Irradiance × Cell Area)) × 100%
If our example cell has an area of 243 cm² (a common size for lab cells), we first convert the area to square meters: 0.0243 m². The incident light power is 1000 W/m² × 0.0243 m² = 24.3 Watts. The efficiency is then (3.53 W / 24.3 W) × 100% = 14.5%.
But the analysis doesn’t stop with a single light intensity. To understand real-world performance, labs perform light intensity dependency tests, measuring efficiency at various irradiance levels from 200 W/m² to 1000 W/m². This reveals how a cell performs on cloudy days or during early morning and late evening. Furthermore, the cell’s performance is highly temperature-dependent. The temperature coefficient of power is measured, which is typically around -0.3% to -0.5% per degree Celsius for silicon cells. This means for every degree above 25°C, the cell’s power output drops by that percentage. On a hot rooftop where the cell might reach 65°C, the power loss can be significant, around 12-15% compared to its STC rating.
For the most advanced research, particularly for multi-junction cells used in concentrator and space applications, spectral response or External Quantum Efficiency (EQE) measurements are essential. An EQE system uses a monochromator to shine very specific, narrow wavelengths of light onto the cell, one at a time, and measures the electrical current generated. This produces a graph showing how efficiently the cell converts each color of light into electricity. This is vital for diagnosing losses and optimizing cell designs, as it can pinpoint if a particular layer of the cell is not absorbing light effectively. For a multi-junction cell, which is essentially a stack of different cells each tuned to a different part of the spectrum, EQE measurement is the only way to accurately determine the performance of each individual sub-cell.
Calibration is the unsung hero of lab measurements. The reference cell used to set the simulator to “one sun” intensity is itself a primary reference cell calibrated against a national standard, such as those maintained by the National Renewable Energy Laboratory (NREL) in the USA or the Physikalisch-Technische Bundesanstalt (PTB) in Germany. These primary standards are calibrated using absolute radiometers in natural sunlight, creating a traceable chain of measurement that ensures a 22% efficient cell measured in a lab in China is directly comparable to a 22% efficient cell measured in a lab in Europe. This global standardization is what drives innovation and fair competition in the solar industry.
Finally, it’s important to distinguish between cell efficiency and module efficiency. The lab-measured cell efficiency is a peak, idealized value. When cells are wired together and encapsulated behind glass to form a commercial solar panel, losses occur due to the gaps between cells, the resistance of the interconnecting wires, and the absorption and reflection of light by the glass and encapsulant materials. Therefore, a module’s efficiency is always several percentage points lower than the efficiency of the individual cells from which it is made. A module using cells with 22% efficiency might have a final module efficiency of only 19-20%.