Principles of semiconductor solar photovoltaic cell

## What is the equivalent circuit of a solar cell?

The solar cell can be represented by a circuit composed of a series resistance R_{s} caused by a PN junction diode VD, a constant current source I_{ph}, and an electrode of the solar cell, and a parallel resistance R_{sh} corresponding to the PN junction leakage current, as shown in Figure 1. It is the equivalent circuit of the solar cell. From the equivalent circuit diagram, the relationship between the current and voltage at both ends of the solar cell can be obtained as

In order to make the solar cell output more power, the series resistance R_{s} must be reduced as much as possible, and the parallel resistance R_{sh} must be increased.

Of course, judging from the previous formula, the open circuit voltage of the solar cell is determined by the photogenerated current and the saturation current. As for the ideal

The saturation current I_{s} of the P-N diode can be used

To express. Where q_{0} represents the unit electricity, n_{i }represents the intrinsic carrier concentration of the semiconductor, N_{D} and N_{A} represent the concentration of donor and acceptor, respectively, D_{n} and D_{p} represent the dispersion coefficients of electrons and holes, respectively, τ_{n} and τ_{p} represent electrons and The recombination time of the electric hole. Of course, the above expression assumes that the N-type area and the P-type area are both wide. Generally, in solar cells using P-type substrates, the N-type area is very shallow, and the above expression needs to be modified.

When light irradiates a solar cell, a photo-generated current is generated, and the photo-generated current is the closed-circuit current in the relationship between the current and voltage of the solar cell. Here we will give a brief description of the origin of the photo-generated current. The generation rate of carriers per unit volume (unit: m^{﹣3}·s^{-1}) is determined by the light absorption coefficient, which is

In the formula, α is the light absorption coefficient; φ_{inc} is the incident photon intensity (or called photon flux density); R is the reflection coefficient. Therefore φ_{inc}(1-R) represents the intensity of incident photons that are not reflected. The three main mechanisms for the generation of photocurrent I_{L} are: the diffusion current I_{p} of minority carrier electrons in the P-type region, the diffusion current I_{n} of minority carrier holes in the N-type region, and the diffusion of electrons and holes in the space charge region. Drift current I_{sc}. Therefore, the photo-generated current can be expressed as

In the formula, L_{n} and L_{p }are the diffusion lengths of electrons in the P-type region and holes in the N-type region, respectively: W is the width of the space charge region.

Summarizing these results, a simple expression of the open circuit voltage can be obtained:

In the formula, r_{rec} is the recombination rate per unit volume of “electron-hole pairs”. Of course, this is a natural result, because the open circuit voltage is equal to the Fermi energy difference between electrons and holes in the space charge region, and the Fermi energy difference between electrons and holes is determined by the carrier generation rate and recombination rate.

As can be seen from the above introduction, if the solar cell is in an open circuit state, then the photogenerated electrons and photogenerated holes separated by the built-in electric field will accumulate in the P and N regions on both sides of the space charge region to form a photogenerated voltage. . If a load is connected, there will be a “photo-generated current” passing through, thus converting light energy into electrical energy. This is how solar cells work. This working principle determines that the solar cell has no chemical pollution and is a safe and reliable green energy source, which is beneficial to environmental protection and human health.

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