Principles of semiconductor solar photovoltaic cell

## What is the principle of solar cells?

What is the principle of solar cells?

The working principle of solar cells is based on the photovoltaic effect. When light irradiates the solar cell, it will generate a photo-generated current Ipn from the N area to the P area. At the same time, due to the characteristics of the P-N junction diode, there is a forward diode current I_{D}. This current direction is from P

Zone to N zone, opposite to photogenerated current.

Under light conditions, the P-N junction will generate an additional current (photo-generated current) I_{p} whose direction is the same as the reverse saturation current I_{0} of the P-N junction. Generally, I_{p}≥I_{0}.

I=I_{0}e^{qU/(KT)}﹣(I_{0}+I_{p})

Let I_{p}=SE, then

I=I_{0}e^{qU/(KT)}﹣(I_{0}+SE)

The voltage from the P terminal to the N terminal when the circuit outside the P-N junction is open under illumination, that is, the U value when I=0 in the above current equation is the open circuit voltage, which is represented by the symbol Uoc.

0=I_{0}e^{qU/(KT)}﹣(I_{0}+SE)

Uoc=(KT/q )In(SE +I_{0})/I_{0}≈(KT/q)ln(SE/I_{0})

The PN junction under light, when the external circuit is short-circuited, it flows out from the P terminal and passes through the external circuit. The current flowing in from the N terminal is called the short-circuit current. It is represented by the symbol ISC, which is the value of I when U=0 in the above current equation. ISC=SE.

Uoc and ISC are two important parameters of P-N junction under light. At a certain temperature, Uoc has a logarithmic relationship with the illuminance, but the maximum value does not exceed the contact potential difference UD. In low light, ISC has a linear relationship with light intensity. When there is no light in the thermal equilibrium state, the semiconductor has a uniform Fermi level, and the barrier height is qUD=E_{FN}﹣E_{FP}. The circuit outside the PN junction is open under stable light, and the photogenerated voltage appears due to the accumulation of photogenerated carriers. Uoc no longer has a uniform charge level, and the barrier height is q(U_{D}-U_{oc}). The circuit outside the PN junction is short-circuited under stable light. There is no photo-generated voltage at both ends of the PN junction, and the barrier height is qU_{D}. The “photo-generated electron-hole pair” is separated by the built-in electric field and flows into the external circuit to form a short-circuit current. There is light and load, part of the photo-generated current builds up a voltage U_{f} on the load, and the other part of the photo-generated current is offset by the forward current caused by the forward bias of the P-N junction, and the barrier height is q(U_{D}-U_{f}).

When contact between different types of semiconductors (constitute a P-N junction) or between a semiconductor and a metal, diffusion occurs due to the difference in electron (or hole) concentration, forming a barrier at the contact, so this type of contact has unidirectional conductivity. Utilizing the unidirectional conductivity of the P-N junction, semiconductor devices with different functions, such as diodes, triodes, and thyristors, can be made. PN junction also has many other important basic properties, including current-voltage characteristics, capacitance effect, tunneling effect, avalanche effect, switching characteristics, and photovoltaic effect, etc. The current-voltage characteristics are also called rectification characteristics or volt-ampere characteristics, which are PN junctions. The most basic characteristics.

The actual current I obtained by the photovoltaic effect generated by the self-built electric field of the P-N junction is

I=I_{ph}﹣I_{D}=I_{ph}﹣I_{0}{exp[(qU_{D})/(nk_{B}T)]﹣1} ——(1)

In the formula, UD is the junction voltage; I_{0} is the reverse saturation current of the diode; Iph is the photogenerated current proportional to the intensity of the incident light, and its proportional coefficient is determined by the structure of the solar cell and the characteristics of the material; n is the ideal coefficient (value of n), is a parameter indicating the characteristics of the PN junction, usually between 1 and 2; q is the electronic charge; kB is the Boltzmann constant; T is the temperature.

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

I=I_{ph}﹣I_{0}{exp[(qU_{D})/(nk_{B}T)]﹣1} ——(2)

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 (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 Isc. 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, 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 (2)

U∝=[(nk_{B}T)/q]ln(I_{SC}/I_{0} ＋1) —— (3)

Simply put, the open circuit voltage means that the solar cell exposed to light is in an open state, and the photogenerated carriers can only accumulate at the two ends of the P-N junction to generate a photogenerated 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. The load R can be from zero to infinity. When the load R. maximizes the power output of the solar cell, its corresponding maximum power Pm is

P_{m}=_{Im}U_{m } —— (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, and the volt-ampere characteristic curve of the solar cell can be obtained by plotting the working voltage and working current values corresponding to different resistance values.

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 Pm. The working voltage and working current at this time are called the best working voltage and the best working current, which are represented by the symbols Um and Im, respectively.

Define the ratio of the product of Uoc and Isc to the maximum power Pm as the fill factor FF, then

FF=P_{m}/UocI_{sc}=U_{m}I_{m}/U_{oc}I_{sc} ——(5)

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, etc.

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 it is loaded with the best load. The larger the value, the higher the power

The greater the output power of the positive battery. The value of FF is always less than 1, which can be given by the following empirical formula:

FF=[ U_{oc}﹣ln(U_{oc}＋0.72)]/(U_{oc}＋1)

In the above formula, Uoc 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.

η=P_{m}/P_{in}=(I_{m}U_{m})/P_{in}=(FFV_{oc}I_{sc})/P_{in} —— (6)

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

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