Photovoltaics

Edmond Becquerel [5], also known as the father of photovoltaics, discovered photovoltaic effect in 1839. He observed that the light on an electrode dipped in acidic medium results in current through the electrode (Figure 6.1). He exposed the electrodes to blue light, ultraviolet, and sunlight.

In 1877, Adams and Day [6] reported the action of light on a bar of crystalline selenium, wherein its resistance was less when bar was exposed to light than it was when kept in dark. The selenium cell is represented through Figure 6.2.

In the year 1883, Fritts [7] proposed first thin film selenium solar photovoltaic cell, depicted in Figure 6.3. Current flows through the contacts to the external circuit. The PV cells were re-creatable and reproducible, but the underling theory was not clear.

Becquerel’s experimental setup

FIGURE 6.1 Becquerel’s experimental setup.

Selenium photovoltaic cell

FIGURE 6.2 Selenium photovoltaic cell.

First thin-film solar photovoltaic cell

FIGURE 6.3 First thin-film solar photovoltaic cell.

The year 1900 saw the birth of quantum mechanism proposed by Max Plank [8], wherein he introduced concept of energy through the equation E=hv. Here, E represents Quanta, a discrete quantized energy packet proportional to frequency represented by v and h = Plank’s constant. He reached this conclusion when he was not able to solve the problem related to thermal radiation by means of traditional classical physics. In 1905, Albert Einstein published wok on photon packets, i.e. light quanta called photon. Through this, he completely laid down the principles of photovoltaic mechanism and the foundations of the semiconductor industry. In 1933, Grondahl [9] described the details behind the manufacturing process of the photoelectric solar cell. It became popular owing to the low manufacturing cost involved. In 1941, Ohl filed the first patent [10] for silicon solar cell. It had an efficiency of less than 1%. Despite it not carrying commercial value, it was a landmark point in history.

In 1954, Chapin et al. [11] proposed a silicon semiconductor solar cell having an efficiency of 6%. This was an offshoot of the observation in which silicon diode produced a significant amount of current and voltage in the presence of light. It was commercialized and can be thought as the beginning of silicon solar cells. After this, the technology saw rapid improvements with a host of semiconductors developed, as summarized in Table 6.3.

Early solar cells were used in telephone repeaters. In 1958, six solar cell panels were first used in space application over the satellite Vanguard 1. It produced a power of 5 mW and was functional for 6 years after the battery was discharged. Nowadays, solar panels have been well understood and accepted, resulting in applications like roof-top solar panels. On July 10,2020, the Prime Minister of India Mr. Narendra Modi inaugurated the Rewa solar power plant, the largest in Asia having a production capacity of 750 MW. Solar power is poised to become a major source of electric supply for this planet.

PV Cell Behavior and V-l Characteristics

PV cell characterization is important from the viewpoint of the analysis and design of PV-based systems. The PV cell is similar in behavior to a p-n junction diode. The incident light raises the valence band gap to the conduction band. Current direction * 1

TABLE 6.3

Summary of Photovoltaic Solar Cell Technology

Group

Photovoltaic Solar Cell Technology

Efficiency (%)

1

(Mono Si) Monocrystalline Silicon

20

(Multi-Si) Polycrystalline

18

(TFSC) Thin Film

18

Note: Group 1 are the mainstay in solar cell as of now.

2

Gallium Arsenide Germanium

30

Copper Indium Gallium Selenide

21

Cadmium Telluride (CdTe)

21

(a-Si) Amorphous Silicon

10

(DSSC) Dye Synthesized

11

Note-. Promising next-generation solar cell technology. Amorphous silicon promises very low production cost.

3

(OPV) Organic solar cell

8

Multi-junction solar cell (InGAP/GaAs/InGaAs)

37

Note-. Organic solar cell has low production cost. Multi junction solar cell claims very high efficiency.

4

Perovskite

Upcoming

Quantum dot

Solar Cell Technology

Note: Perovskites materials were initially used as a coating material for silicon solar cell and were instrumental in a hike of efficiency by 5%.

Quantum dots claims high efficiency as solar cell.

PV cell behavior as a sink

FIGURE 6.4 PV cell behavior as a sink.

PV cell behavior as a source

FIGURE 6.5 PV cell behavior as a source.

PV cell equivalent model

FIGURE 6.6 PV cell equivalent model.

indicates dissipation of power in the diode making it behave like a sink, as shown in Figure 6.4.

However, the desired behavior of the PV cell must be like that of a source, and this is possible in the scenario depicted in Figure 6.5 wherein the current id is flowing from anode to cathode and excess current T is flowing to the external circuit. This is because of the photon current ‘/p’, produced by the PV cell upon exposure to sunlight. The terminal potential is lVak and its direction is also retained. Under dark conditions, the characteristics are equivalent to those of a simple diode but on exposure to solar intensity the V-l characteristics are as described in Figure 6.5. V-I characteristics of solar cell indicate a unique feature of being both the current and voltage source at different instances of time. Rslt and Rs represent the equivalent shunt and series resistances of these sources, respectively. The PV cell model is represented in Figure 6.6 where ip is the photocurrent, and shunt and series non-idealities have been shown by Rsl, and Rs, respectively.

The equation for terminal current and voltage is

The diode current is represented by Here,

-VGo/

/о = KTm e = Reverse saturation current dependent on material, doping, and temperature

К = Constant dependent on dimension and material property VG„ = Numerical equivalent bandgap energy of electron in ev in = 1.5 for silicon

VG„ = 1.16 to 1.21 for silicon. Value is dependent on grade and purification. Therefore,

Equation (6.5) represents the terminal current model of PV cell represented in Figure 6.6.

The variation in V-I characteristics is explained through Point 1, which represents the short-circuit point in PV cell V-I characteristics wherein V„k = 0, Rs Rsh so Equation (6.4) becomes /„. = ip. This condition represents incident solar power and is known as insolation. Point 2 represents the open-circuit point wherein v = Voc and

i = 0. For these conditions, Equation (6.4) becomes Voc = nVT ln^ 'p + '" j. Here, Voc

is related to insolation logarithmically, indicating the change in incident solar power changes the Isc linearly, as represented in Figure 6.7b. p = vi represents the power curve and point 3 represents peak power.

Solar cell efficiency:

p VI

T) = — is cell efficiency at peak power Pm, wherein P„, = "*.

Pin Pin

Influence of Temperature on Solar Cell Characteristics

Effect of temperature on short-circuit current Iu: With increase in temperature, there is reduction in the bandgap energy, allowing more electrons from the valence band to cross to the conduction band, resulting in extra photon current, thereby increasing Isc. Typical change is 0.1%/°K for silicon.

(a) Solar PV cell V-I characteristics representing open-circuit, short-circuit, and maximum power points, (b) Linear versus logarithmic relationship between /„ and V„

FIGURE 6.7 (a) Solar PV cell V-I characteristics representing open-circuit, short-circuit, and maximum power points, (b) Linear versus logarithmic relationship between /„ and V„c.

Effect of temperature on open-circuit voltage Voc:

Here: t) = 2 for silicon, V=volt equivalent of temperature.

For the condition /„ «: I,„

Also,

Differentiating with respect to T

Here Vco « y and VCo is numerical equivalent of bandgap energy.

From Eq. (6.7),

Differentiating with respect to T and considering variation of ip s

Hence, we get

For silicon at 300°K, m = 1.5, Г/ = 2, VCo = 1.16, Vc0 = 0.6,

indicating V

T

Effect of temperature on power:

The coefficient of current and voltage is positive and negative, respectively, effectively giving a negative coefficient of power.

 
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