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光伏中的填充因子

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The Fill Factor (FF) is a key parameter that determines the quality of a solar cell. It is the ratio of the maximum power a cell can produce ([latex]P_{max}[/latex]) to the theoretical power if it were an ideal voltage and current source ([latex]V_{oc} \times I_{sc}[/latex]). A higher fill factor indicates lower parasitic resistance losses and a more ‘square’ I-V curve.

The Fill Factor (FF) is a crucial figure of merit for a solar cell, providing a quick measure of its overall performance and quality, independent of its size or the light intensity. It is defined by the equation: [latex]FF = \frac{P_{max}}{V_{oc} \times I_{sc}} = \frac{V_{mp} \times I_{mp}}{V_{oc} \times I_{sc}}[/latex]. Here, [latex]V_{oc}[/latex] is the open-circuit voltage (the voltage across the cell with no load), and [latex]I_{sc}[/latex] is the short-circuit current (the current through the cell when the terminals are shorted). The product [latex]V_{oc} \times I_{sc}[/latex] represents the theoretical maximum power the cell could deliver if its I-V curve were a perfect rectangle.

In reality, the I-V curve is curved due to the diode characteristics and parasitic resistances. The point on this curve where the product of voltage and current ([latex]V \times I[/latex]) is at its maximum is the Maximum Power Point (MPP). The voltage and current at this point are [latex]V_{mp}[/latex] and [latex]I_{mp}[/latex], respectively. The Fill Factor essentially measures how ‘square’ the I-V curve is. An ideal cell would have an FF of 1 (or 100%), but practical values for commercial silicon cells are typically in the range of 0.75 to 0.85 (75% to 85%).

The Fill Factor is highly sensitive to the parasitic series ([latex]R_s[/latex]) and shunt ([latex]R_{sh}[/latex]) resistances. A high series resistance reduces the current at the maximum power point and ‘softens’ the knee of the I-V curve, lowering the FF. A low shunt resistance provides a leakage path for current, reducing [latex]I_{mp}[/latex] and the slope of the I-V curve near [latex]I_{sc}[/latex], which also lowers the FF. Therefore, monitoring the Fill Factor is an effective way to diagnose degradation or manufacturing defects in solar cells.