optical-properties-of-solar-cell-materials
The optical properties of solar cells often determine the ultimate efficiency of solar cells and are the basis for process design.
(1) Absorption law
When a beam of spectral irradiance I0 is orthogonally incident on the surface of the semiconductor, after subtracting the reflection, the spectral irradiance entering the semiconductor is I0(1-R), and the distance from the front surface is x at the semiconductor. The spectral irradiance Ix is determined by the law of absorption: when the thickness of the lamella is d, we can get a more complete approximation of the transmittance.
Relationship between absorption coefficient and wavelength of single crystal silicon, gallium arsenide and some important solar cell materials
(2) Intrinsic absorption
In atomic images, the intrinsic absorption of silicon can be understood as the fact that a silicon atom is excited by a photon, causing a valence electron to become a free electron while leaving a hole in the covalent bond break. Experiments have found that only those photons whose hu is larger than the forbidden band width Eg can produce intrinsic absorption.
Obviously, the incident photon must satisfy or the frequency of the light in which Vo-- can produce intrinsic absorption (frequency absorption limit); λo-- just the wavelength of the intrinsic absorption of light (wavelength absorption limit).
It is believed that silicon is transparent to infrared light having a wavelength greater than 1.15 μm.
Solar cell equivalent circuit, output power and fill factor
(1) equivalent circuit
In order to describe the operating state of the battery, the battery and load system are often simulated with an equivalent circuit.
1. Constant current source: Under constant illumination, a solar cell in working state does not change its photocurrent with the working state. It can be regarded as a constant current source in the equivalent circuit.
2. Dark current Ibk: Part of the photocurrent flows through the load RL, and the terminal voltage U is established across the load. In turn, it is forward biased to the PN junction, causing a dark current Ibk opposite to the direction of the photocurrent.
3. In this way, the equivalent circuit of an ideal PN homojunction solar cell is drawn.
4. Series Resistance RS: Due to the contact between the front and back electrodes, and the material itself has a certain resistivity, it is inevitable to introduce additional resistors in the base and top layers. When the current flowing through the load passes through them, it will inevitably cause loss. In an equivalent circuit, their total effect can be represented by a series resistor RS.
5. Parallel resistance RSH: Due to the leakage of the battery edge and the metal bridge leakage formed at the micro crack, scratch, etc. when making the metalized electrode, a part of the current that should be passed through the load is short-circuited. The resistor RSH is equivalent.
When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it is obtained:
P in the formula is the output power obtained on the load RL when the solar cell is irradiated.
(2) Output power
When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it can be obtained that P in the formula is the output power obtained on the load RL when the solar cell is irradiated.
When the load RL changes from 0 to infinity, the output voltage U changes from 0 to U0C, and the output current changes from ISC to 0, thereby drawing a load characteristic curve of the solar cell. Any point on the curve is called the working point. The working point and the origin line are called load lines. The reciprocal of the slope of the load line is equal to RL. The horizontal and vertical coordinates corresponding to the working point are the working voltage and working current.
When adjusting the load resistance RL to a certain value Rm, a point M is obtained on the curve, and the product of the corresponding working current Im and the working voltage Um is the largest, that is: Pm=ImUm
Generally, the M point is the optimal operating point (or maximum power point) of the solar cell, Im is the optimal working current, Um is the optimal working voltage, Rm is the optimal load resistance, and Pm is the maximum output power.
(3) Fill factor
1. The ratio of the maximum output power to (Uoc × Isc) is called the fill factor (FF), which is one of the important indicators to measure the output characteristics of solar cells.
2. Filling factor characterizes the advantages and disadvantages of solar cells. Under certain spectral irradiance, the larger the FF, the more "square" the curve and the higher the output power.