Solar cells, also known as photovoltaic cells, directly convert sunlight into electrical energy. Measuring the efficiency of solar cells typically involves assessing the power of incident sunlight using a radiometer and determining the electrical power output at the maximum power point. However, this process faces challenges due to the dependency of cell performance on the solar spectrum, which varies with seasonal changes, geographic location, and weather conditions. These factors, combined with calibration errors in radiometers, can lead to inconsistent and inaccurate measurements.
To mitigate such issues, most manufacturers use solar simulators to test solar cell efficiency in controlled environments. These simulators are calibrated using standard cells that align with the spectral distribution of sunlight under standard conditions.
Common Pitfalls in Testing Amorphous Silicon Thin-Film Solar Cells
Some laboratories and testing agencies use crystalline silicon cells as reference standards to evaluate amorphous silicon thin-film cells. This practice often results in significant measurement errors, leading to doubts about the performance of amorphous silicon cells.
International Standards for Testing Solar Cells
To ensure consistent and reliable comparisons, international testing standards define specific conditions for solar cell evaluation:
Spectrum: AM1.5
Irradiance: 1000 W/m²
Temperature: 25°C
AM1.5 refers to the solar spectrum when sunlight passes through the atmosphere at an angle corresponding to a zenith angle of 48.2°.
For accurate measurements, two key conditions must be met:
The spectral response of the reference cell and the test cell must align within a specified range, which is typically achieved by using reference and test cells made from the same semiconductor material and similar manufacturing processes.
The light source in the simulator must closely match the spectral composition of the AM1.5 standard.
Special Considerations for Amorphous Silicon Cells
Amorphous silicon cells differ significantly from crystalline silicon cells in terms of material and spectral response. Here are key considerations for accurate testing:
Irradiance Calibration:
Use an amorphous silicon reference cell specifically designed for calibrating irradiance. Using crystalline silicon cells for this purpose can lead to meaningless results due to spectral mismatch. Even if an ideal light source were available, ensuring accurate results in typical lab or production environments remains challenging.
Light Source Selection:
The solar simulator should use a light source with a spectral range between 300 nm and 800 nm that closely matches the AM1.5 spectrum. Common xenon lamp simulators often have an infrared-rich spectrum (800 nm to 1100 nm) that deviates from the standard, causing significant mismatches.
Spectral Response:
The spectral response of a solar cell refers to the number of charge carriers generated per photon at a given wavelength. Amorphous silicon cells have a spectral response range of 400 nm to 800 nm, compared to 400 nm to 1100 nm for crystalline silicon cells. When testing amorphous silicon cells using simulators calibrated with crystalline silicon standards, the infrared-rich spectrum (800 nm to 1100 nm) contributes to the current of crystalline cells but not to amorphous cells. This results in a severe underestimation of the amorphous silicon cell’s current and overall performance.
Additionally, the spectral response of amorphous silicon cells is influenced by factors such as bias light and voltage, making it critical to account for these variables under non-standard conditions.
Accurate testing of amorphous silicon thin-film solar cells requires careful attention to irradiance calibration, light source selection, and spectral response alignment. Adhering to these guidelines ensures reliable results and avoids the errors associated with improper calibration methods.
