# Articles about the parameters and introduction of solar cells

1. The parameters of light
(1) Luminous intensity Luminous intensity is abbreviated as light intensity, the international unit is candela (candela), abbreviated as cd, 1cd (ie 1000md) refers to a monochromatic light source (frequency is 540 x 1012Hz, wavelength is 0.550um) in a given direction Luminous intensity per unit solid angle. The unit of luminous intensity was originally defined by candles, and the unit was candlelight. At the 9th International Conference on Metrology in 1948, it was decided to use a black body at the freezing point of platinum as the luminous body, and the unit of luminous intensity was named Candela, which was once called the new candle. In 1967, the 13th International Conference on Weights and Measures made a more rigorous definition of Candela. In 1979, the 16th International Conference on Weights and Measures decided to adopt the current new definition.

(2) Luminous flux The unit of luminous flux is lumens (lm), usually expressed by Φ. It is used to measure the total amount of light emitted. The luminous intensity of the light source is led, which uniformly emits 4π lumens of light energy to the surrounding space.

(3) Illumination Illumination is the intensity of light, and its physical meaning is the luminous flux irradiated on a certain area. The unit of illuminance is the number of lumens (lm) per square meter, also called lux (lux), abbreviated as 1x. When the light intensity of 1lm luminous flux shines on an area of ​​1m2, the illuminance received by the area is 11x.

1. Electrical parameters of solar cells and components
The electrical performance parameters of solar cells and components include: open circuit voltage Uoc, short circuit current Isc, best working voltage Um, best working current Im, maximum power Pm, fill factor FF, conversion efficiency ȵ, series resistance R, and parallel resistance Ra.

These 9 parameters directly or indirectly reflect the electrical performance of the cells and battery components. The data obtained from the test can directly check whether the cells and battery components are qualified.

(1) Open circuit voltage Under standard test conditions, the terminal voltage of the photovoltaic power generation device under no-load (open circuit) conditions is called the open circuit voltage of the solar cell, usually expressed by Uoc.

(2) Short-circuit current Under standard test conditions, the output current of the photovoltaic generator when the terminal voltage is zero, usually expressed by Isc.

(3) Optimal working voltage The voltage corresponding to the maximum power point on the volt-ampere characteristic curve of the solar cell is usually expressed by Um.

(4) Optimal working current The current corresponding to the maximum power point on the volt-ampere characteristic curve of the solar cell is usually expressed by Im.

(5) Maximum power The maximum value of the product of current and voltage on the volt-ampere characteristic curve of a solar cell.

(6) Fill factor, also known as curve factor, refers to the ratio of the maximum power of the solar cell to the product of the open circuit voltage and short circuit current, usually expressed as FF.

(7) Conversion efficiency A solar cell is a semiconductor optoelectronic device that can directly convert solar energy into electrical energy, and can convert solar energy into electrical energy proportionally. The conversion efficiency of solar cells is the ratio of input solar energy to output power.

In order to correctly define the efficiency of solar cells, some necessary conditions need to be attached. The International Electrotechnical Standardization Committee (IEC) stipulates the following: The rated efficiency of ground-use solar cells must be at a temperature of 25°C, a light intensity of 1000W/m² and an air quality that meets IEC regulations. The measurement is carried out under the standard reference light, and these conditions are collectively referred to as the basic state of the test. Therefore, the manufacturer performs factory inspections of the solar photovoltaic modules produced in accordance with the above regulations, and must be clearly marked on the nameplate.

(8) Series resistance Series resistance refers to the resistance in series with the PN junction or MIS junction inside the solar cell, which mainly includes the semiconductor material body resistance, sheet resistance, electrode contact resistance and other parts. In addition, the series resistance of battery components should also include the interconnection bar, bus bar resistance and wire resistance. In the actual measurement, it is impossible to accurately measure the series resistance. This is because the accuracy of the sampling resistance and the test instrument will cause measurement errors, so the test values ​​of various test equipment show large differences.

(9) Parallel resistance Parallel resistance refers to the equivalent resistance between the internal solar cell and the resistance across the two ends of the cell. In actual measurement, it is difficult to accurately measure this value. The reason is the same as measuring series resistance. Generally, the accuracy of parallel resistance is lower than that of series resistance.

(10) Dark current Under light conditions, the forward junction current that is generated inside the solar cell in the opposite direction to the light-generated current is called dark current.

(11) Dark characteristic curve The volt-ampere characteristic curve obtained by applying an external bias to the solar cell under no light conditions is called the dark characteristic curve of the solar cell.

(12) Spectral response Spectral response is also called spectral sensitivity, which refers to the function relationship between the short-circuit current density per unit irradiance and the wavelength at each wavelength.

(13) Module efficiency refers to the conversion efficiency calculated according to the shape (dimension) area of ​​the module (note: aluminum frame).

For example, the component efficiency is calculated as follows (using the example in this book):
The module area is: 1580mm×808mm = 1276640mm²= 1.27664m² The power that should be generated under standard light intensity: 1.27664m²×1000 W/m²= 1276.64W
If the actual power is assumed to be 198w, the battery module efficiency n=198W/1276.64W=15.51%

(14) Relative spectral response Relative spectral response is also called relative spectral sensitivity, which is a normalized spectral response at a specific wavelength (usually the maximum value of the spectral response).