ELEC 5564 Electric Power Generation by Renewable Sources - Solar Cell Characteristics

Electrical and Electronic Characteristics of the Solar Cell

Figure a. Dark I-V characteristic of a conventional solar PV.
At Dark (Without illumination).
When the solar cell is not exposed to light, its electrical characteristics (i.e., dark characteristics) resemble that of a pn junction (i.e., large diode).
Dark I-V curve.
Figure b. Equivalent solar PV circuit at dark.

Figure a. Illuminated I-V characteristic of a conventional solar PV.
Upon illumination (solar insolation).
When the solar cell is exposed to light, it starts to generate electrical current (i.e, photocurrent, I_{ph}) by direct conversion of solar radiation into electricity (i.e., photovoltaic effect).
Figure b. Equivalent solar PV circuit at illumination.

The greater the solar irradiance, the higher the amount of current a solar PV cell can generate through the (i.e., photovoltaic effect).
Figure a. Illuminated I-V characteristic of a conventional solar PV.
Figure b. Equivalent solar PV circuit at illumination.

Figure a. I − V characteristics of Si pn junction solar cell under dark conditions and under illumination with light of increasing intensity [1].
Figure b. I-V curve for a solar cell with maximum power indicated by the shaded area[2].

A bulk silicon PV module consists of multiple individual solar cells connected, in series, to increase the power and voltage.
The voltage of a PV module is usually chosen to be compatible with a 12V battery.
An individual silicon solar cell has a voltage at the MPP around 0.5V under 25 °C and AM1.5 illumination.
Taking into account an expected reduction in PV module voltage due to temperature and the fact that a battery may require voltages of 15V or more to charge, most modules contain 36 solar cells in series.
This gives an open-circuit voltage of about 21V under standard test conditions, and an operating voltage at MPP and operating temperature of about 17 or 18V.
Module Circuit Design
The remaining excess voltage is included to account for voltage drops caused by other elements of the PV system, including operation away from maximum power point and reductions in light intensity.
Figure. In a typical module, 36 cells are connected in series to produce a voltage sufficient to charge a 12V battery.

The voltage from the PV module is determined by the number of solar cells and the current from the module depends primarily on the size of the solar cells.
At AM1.5 and under optimum tilt conditions, the current density from a commercial solar cell is approximately between 30 mA/cm2 to 36 mA/cm2.
Single crystal solar cells are often 15.6 × 15.6 cm2, (total current of almost 9 – 10A from a module).
Table. output of typical modules at standard testing conditions.
Figure. A conventional residential module, made of either 60 or 72 cells. There are other sizes such as 96 cell modules but they are much less common.

If all the solar cells in a module have identical electrical characteristics, and they all experience the same insolation and temperature, then all the cells will be operating at exactly the same current and voltage.
In this case, the IV curve of the PV module has the same shape as that of the individual cells, except that the voltage and current are increased.
The overall IV curve of a set of identical connected solar cells is shown below
Module Circuit Design- I-V Characteristics
- N = Number of cells in series;
- M = Number of cells in parallel;
- I_{SC} = Short circuit current;
- V_{OC} = Open circuit voltage;
- MPP = Maximum Power Point;
- V_{MPP} = Voltage at the maximum Power Point;
- I_{MPP} = Current at the maximum Power Point.
- I{SC (total)} = I{SC(cell)}×M
- I{MPP (total)} = I{MPP(cell)}×M
- V{OC (total)} = V{OC(cell)}×N
- V{MPP (total)} = V{MPP (cell)}× N
Figure. I-V Characteristics for N cells in series x M cells in parallel