Characterization parameters of solar cells

The working principle of solar cells is based on the photovoltaic effect. When the light irradiates the solar cell, it will produce a photo-generated current I_ph from the N zone to the P zone. At the same time, due to the characteristics of the PN junction diode, there is a forward diode current I_D. The direction of this current is from the P area to the N area, which is opposite to the photogenerated current. Therefore, the actual current I obtained is

In the formula, U_D is the junction voltage; I₀ is the reverse saturation current of the diode; I_ph is the photogenerated current proportional to the intensity of the incident light, and its proportionality factor is determined by the structure of the solar cell and the characteristics of the material; n is the ideal coefficient (value of n), which is a parameter indicating the characteristics of the PN junction, usually between 1 and 2; q is the electronic charge; k_B is the Boltzmann constant; T is the temperature.

If the series resistance R_S of the solar cell is neglected, U_D is the terminal voltage U of the solar cell, then the formula (1-1) can be written as

When the output terminal of the solar cell is short-circuited, U=0 (U_D≈0), and the short-circuit current can be obtained by formula (1-2)

I_sc =I_ph

Simply put, the short-circuit current is the maximum current measured when the solar cell is short-circuited from the outside, expressed by I_sc. It is the maximum current that the photocell can obtain in the external circuit under a certain light intensity. Regardless of other losses, the short-circuit current of the solar cell is equal to the photo-generated current Iph, which is proportional to the intensity of the incident light. When the output terminal of the solar cell is open circuit, I=0, the open circuit voltage can be obtained by formula (1-1) and formula (1-2)

Simply put, the open-circuit voltage means that the solar cell exposed to light is in an open-circuit state, and the photo-generated carriers can only accumulate at the two ends of the PN junction to generate a photo-generated electromotive force. At this time, the potential difference measured at the two ends of the solar cell is represented by the symbol U_oc.

When the solar cell is connected to the load R, the resulting load volt-ampere characteristic curve is shown in Figure 1-10. The load R can be from zero to infinity. When the load R_m maximizes the power output of the solar cell, its corresponding maximum power P_m is

                                     P_m=I_mU_m                               (1-4)

In the formula, I_m and U_m are the best working current and the best working voltage respectively.

When the solar battery is connected to the load, a current flows through the load. This current is called the working current of the solar battery, also called the load current or output current. The voltage across the load is called the working voltage of the solar cell. The working voltage and working current of the solar cell change with the load resistance. The volt-ampere characteristic curve of the solar cell can be obtained by making the working voltage and current values corresponding to different resistance values into a curve (Figure 1).

If the selected load resistance value can maximize the product of output voltage and current, the maximum output power is obtained, which is represented by the symbol P_max. The working voltage and current at this time are called the best working voltage and the best working current, which are represented by the symbols U_mp and I_mp, respectively.

Define the ratio of the maximum power P_m to the product of U_oc and I_sc as the fill factor FF, then

FF is an important characterization parameter of solar cells. The larger the FF, the higher the output power. FF depends on the incident light intensity, the band gap of the material, ideal coefficient, series resistance and parallel resistance.

The fill factor FF is an important parameter to measure the output characteristics of the solar cell. It is the ratio of the maximum output power to the product of the open circuit voltage and the short circuit current. It is the characteristic that represents the maximum power that the solar cell can output when the load is optimal. The larger the value, the greater the output power of the solar cell. The value of FF is always less than 1, which can be given by the following empirical formula:

In the formula, U_oc is the normalized open circuit voltage.

The photoelectric conversion efficiency of a solar cell refers to the maximum energy conversion efficiency when the optimal load resistance is connected to the external circuit, which is equal to the ratio of the output power of the solar cell to the energy incident on the surface of the solar cell. The conversion efficiency of photovoltaic cells directly converting light energy into useful electrical energy is an important parameter for judging battery quality, which is represented by η.

That is, the ratio of the maximum output power of the battery to the incident light power.