Characteristics of a Solar Cell


A solar (photovoltaic) cell is a semiconductor device that absorbs light (solar energy) and converts it directly into electrical energy. The most common solar cell consists of a thin sheet of semiconductor material, composed mainly of monocrystalline silicon doped with III and V valent materials, which when exposed to sunlight absorbs photons with sufficient energy to cause the creation of an electron-hole pair. The difference between a solar cell and a regular photodiode is mainly their application. Solar cells are operated without biasing or with a small forward bias, conditions that create current with a negative sign, thus generating power. Photodiodes are mostly used as sensors or receivers, operated in reverse bias to detect optical signals.

Experiments were carried out on a Solarmodul SM-7546 to determine the I/V characteristics under different temperature and light exposure conditions.

Solar cell used in this experiment.


For these experiments, sourcemeter Keithley 2636A Series Sourcemeter was used as a voltage supply, and also as a current measurement tool.
To measure the performance of the solar cell under different temperatures Vötsch VT4002 climate chamber was used.
The sourcemeter was connected to the solar cell using jumper wires on a classical breadboard. Voltage across the electrodes was swept from negative to positive values for different current ranges to get the best precision of measurements.

I-V charcteristic of a solar cell, with an attempted linear fit to determine \(I_S\).

Python script used for the fit; Raw input data.

This plot represents an I-V curve where the current is shown in logarithmic scale. An ideal diode current-voltage characteristic can be described by the Shockley equation: \begin{equation} I(V) = I_S \cdot (\exp{(\frac{e \cdot V}{n \cdot k_B \cdot T})}-1) \end{equation} where Is represents the reverse saturation current, n the nonideality factor, \(e\) the elementary charge, \(kB\) the Boltzmann constant and \(T\) the temperature. By comparing the ideal curve with the I-V characteristic of the solar cell, we can extract the nonideality factor \(n\). We do that by approximating the measured data as a line in the logarithmic scale and extracting Is as the point where that line crosses the 0 voltage point. Factor \(n\) is then calculated from the Shockley equation.

Extracted data: \begin{align} I_s &= 6.565178400175819\mathrm{e}{-6}~\mathrm{(compared~to~}~2.63453\mathrm{e}{-6}~\mathrm{measured)}\\ n &= 3.295115010931143 \end{align}

In reverse bias condition, there is a very strong current increase, which is dissimilar to the characteristic of a regular diode and deviates from the Shockley Equation by a factor of 100. The literature suggests that this might occur due to combined effect of two currents, where either one can be dominant: \(I_R\) – reverse leakage current and \(I_{Sh}\) - shunt resistance current. Leakage current under reverse bias includes the contributions of diffusion current, space charge generation current; band-to-band tunneling current and thermionic emission current. The shunt resistances are processed-induced, caused by grown-in defects of the material.

I-V charcteristic of a solar cell exposed to light and in darkness, with important points marked.

When the solar cell is exposed to light in a reverse bias condition, the photocurrent becomes the dominant current source. The offset created by the photocurrent pushes the I-V characteristic of the cell into the fourth quadrant, meaning that for positive voltages, the current sign is negative, thus the solar cell is producing power. Three characteristic points for an exposed solar cell are: short circuit current \(I_{SC}\), open circuit voltage \(V_{OC}\) and the point where the cell produces maximal power. The position of these three points depends on environmental (light intensity, temperature) and technological parameters (material of the cell, structure, dimensions, etc.) \begin{align} I_{SC} &= -0.00890048~\mathrm{A}\\ V_{OC} &= 0.991938~\mathrm{V}\\ P_{max} &= -0.0058848289158200005~\mathrm{W~(at~}V = 0.779302~\mathrm{V)} \end{align}

Temperature also plays a role in determining the performance of the solar cell. With temperature increase its output current increases exponentially because of the bandgap narrowing effect, while the voltage output is reduced linearly. The figure below demonstrates a linear voltage shift of the I-V curve for different temperatures.

I-V charcteristic of a solar cell at different temperatures.

Python script used to investigate the temperature dependence of the I-V characteristics of the solar cell.

Raw data of tempereature dependence measurements. 278 K 288 K 298 K 308 K 318 K