Silicon-based solar cells Characteristics and production processes ...

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Silicon-based solar cells – characteristics and production processes Fig. 24. Front (a) and rear side of the solar cell (b) with the surface area of 164 cm 2 , produced on mc-Si silicon, with the construction adequate to the scheme in Fig.23. VI. Current-voltage characteristics measurements One of the basic measurements of photovoltaic cells is the measurement of the light current-voltage characteristics (I-V). In the measuring system, one can distinguish three basic elements which determine the measurement quality, that is the light source, the measuring system, as well as the table and the contact probes. The I-V characteristics must be measured under strictly defined conditions for the given radiation spectrum and temperature, under so called STC (Standard Test Condition). The applied standard also includes the use of solar light simulators Class A, with the AM1.5 spectral matching tolerance within the rage of 0,75 ÷ 1,25 and with the acceptable inhomogeneity of the illumination intensity of ± 2 %, on the surface of the solar cell illuminated by the radiation with the intensity of 1000 W/m 2 , and with the temperature of the cell equaling 25 o C [58]. Fig. 25. Solar simulator and I-V characteristic of a illuminated solar cell. 37
Silicon-based solar cells – characteristics and production processes The numerical matching of the I-V characteristics with a single or double diode model makes it possible to calculate directly the value of the photocurrent, the voltage, the dark current, as well as the parallel and series resistance of the cell [59]. Fig. 26. Scheme of the equivalent electrical circuit in a double diode model for an actual silicon solar cell; designation in accordance with the dependence (20). The mathematical model which most thoroughly describes the non-linear I-V characteristics of a solar cell is the equation (20) with seven parameters, which one can obtain by applying Kirchhoff’s Law to the equivalent circuit presented in Fig. 26 [60]. ( V IR ) ⎞ ⎤ ⎡ ⎛ q( V+ IR ) ⎡ ⎛ q + ⎤ s s ⎞ ⎛ V+ IR s ⎞ I = I ⎢ ⎜ ⎟ − ⎥ − ⎢ ⎜ ⎟ − ⎥ − ⎜ ⎟ ph − IS1 exp 1 IS 2 exp 1 (20) ⎣ ⎝ A1kT ⎠ ⎦ ⎣ ⎝ A 2kT ⎠ ⎦ ⎝ R sh ⎠ or ⎛ V+ IR ⎞ s I = Iph − I1− I 2 − ⎜ ⎟ (21) ⎝ R sh ⎠ and I 1 = I where: I ( V IR ) ⎡ ⎛ q + ⎢exp ⎜ ⎣ ⎝ A1kT ⎞ ⎤ ⎟ −1⎥ ⎠ ⎦ s S 1 , 2 ( ) ⎡ ⎛ q V+ IR ⎤ I = s ⎞ I ⎢exp ⎜ ⎟ − 1⎥ (22a) , (22b) S 2 ⎣ ⎝ A 2kT ⎠ ⎦ - current possible to obtain from the solar cell I - photocurrent generated by electromagnetic radiation ph I 1 - dark diffusion current I 2 - dark generation-recombination current I S1 - saturation current of dark current diffusion component I S2 - saturation current of dark current generation-recombination component V - voltage possible to obtain from the solar cell R s - cell’s series resistance 38
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Silicon-based solar cells Characteristics and production processes ...

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