What is the photovoltaic effect of solar cells?

A solar cell is a device that uses the principle of photoelectric conversion to convert the light radiated from the sun into electrical energy through a semiconductor material. This photoelectric conversion process is usually called the “photovoltaic effect“. Photovoltaic effect, referred to as photovoltaic effect, refers to the phenomenon that the light produces a potential difference between different parts of an uneven semiconductor or a combination of semiconductor and metal. At present, the high production cost and low efficiency of solar cells have become the bottleneck restricting their popularization and application. Therefore, how to maximize the power generation of solar cells per unit area has become a major research topic in the development of the solar industry. (Welcome to Tycorun Battery to discuss more about batteries with us.)

All matter is composed of atoms, and atoms are composed of nuclei and electrons revolving around the nuclei. Under normal conditions of semiconductor materials, the nuclei and electrons are tightly bound (in a non-conductor state), but under the stimulation of some external factors, the nuclei and electrons The binding force of the electrons is reduced, and the electrons are freed from the shackles of the nucleus and become free electrons, as shown in Figure 1. When sunlight strikes a semiconductor, photons provide energy to electrons, which transition to higher energy states. Among these electrons, the electrons that are available in practical optoelectronic devices are:

Figure 1 - Schematic diagram of electrons breaking free from atomic nucleus and becoming free electrons
Figure 1 – Schematic diagram of electrons breaking free from atomic nucleus and becoming free electrons

(1) Valence band electrons.
(2) Free electrons or holes.
(3) Electrons existing on the impurity level.
The solar cell is composed of a combination of P-type semiconductor and N-type semiconductor. P-type semiconductor (P refers to positive, positively charged) is composed of monocrystalline silicon doped with a small amount of trivalent elements through a special process, which will form a positive band inside the semiconductor. Electric holes. N-type semiconductor (N refers to negative, negatively charged) is composed of single crystal silicon doped with a small amount of pentavalent elements through a special process, which will form negatively charged free electrons inside the semiconductor. When two different types of semiconductor materials, N-type and P-type, come into contact, a built-in electric field is formed at the interface from P-type to N-type due to diffusion and drift. When light shines on the surface of the solar cell, the photons with energy greater than the forbidden band width excite electron and hole pairs. These non-equilibrium minority carriers are separated under the action of the internal electric field, and a potential difference is accumulated at the two poles of the solar cell. , so that the battery can supply current to the external load.

1) Formation of PN junction
In a single crystal semiconductor, one part is P-type semiconductor doped with acceptor impurities, and the other part is N-type semiconductor doped with donor impurities. The transition region near the interface between P-type semiconductor and N-type semiconductor is called PN junction. There are two types of PN junctions: homojunction and heterojunction. A PN junction made of the same semiconductor material is called a homojunction, and a PN junction made of two semiconductor materials with different band gaps is called a heterojunction. Methods of manufacturing PN junctions include alloying, diffusion, ion implantation, and epitaxial growth. Heterojunctions are usually fabricated by epitaxial growth.

There are many positively charged holes and negatively charged ionized impurities in P-type semiconductors. Under the action of an electric field, the holes can move, while the ionized impurities (ions) are fixed. There are many mobile negative electrons and fixed positive ions in N-type semiconductors. When the P-type semiconductor and the N-type semiconductor are in contact, holes diffuse from the P-type semiconductor to the N-type semiconductor and electrons diffuse from the N-type semiconductor to the P-type semiconductor in the vicinity of the interface. Holes and electrons meet and recombine, and the carriers disappear. Therefore, the junction region near the interface lacks carriers, but has charged fixed ions distributed in the space, and this junction region is called the space charge region. The space charge on the P-type semiconductor side is negative ions, and the space charge on the N-type semiconductor side is positive ions. Positive and negative ions generate an electric field near the interface, which prevents further diffusion of carriers and reaches equilibrium.

A homojunction can be doped with a single piece of semiconductor to form P and N regions. Due to the small activation energy AE of impurities, most impurities are ionized into acceptor ions NA- and donor ions ND+ at room temperature. At the interface of the PN region, there is a difference in the concentration of carriers, so they diffuse toward each other. At the moment when the junction is formed, the electrons in the N region are multi-subs, and the electrons in the P-region are minority, so that the electrons flow from the N-region to the P-region, and the electrons and the holes meet and recombine, so that the junction in the original N-region will occur. Nearby electrons become few, leaving the unneutralized donor ion ND+ to form a positive space charge. Similarly, after the holes are diffused from the P region to the N region, a negative space charge is formed by the immobile acceptor ion NA-. Immobile ion regions (also called depletion regions, space charge regions, and barrier layers) are generated on both sides of the interface between the P region and the N region, so a space electric couple layer appears, forming an internal electric field (called a built-in electric field), which has a negative impact on the The diffusion of the multi-carriers in the two regions has a resisting effect, but helps the drift of the minority carriers, until the diffusion flow is equal to the drift flow and reaches equilibrium, and a stable built-in electric field is established on both sides of the interface.

2) PN junction energy band and contact potential difference
Under thermal equilibrium conditions, the junction region has a uniform EF; at the site far from the junction region, the relationship between EC, EF, and EV is the same as the state before the junction is formed. It can be seen from the energy band diagram of the PN junction under thermal equilibrium (Figure 2) that when N-type semiconductor and P-type semiconductor exist alone, there is a certain difference between EFN and EFP. When the N-type semiconductor and the P-type semiconductor are in close contact, electrons flow from the side with the higher Fermi level to the side with the lower Fermi level, and the holes flow in the opposite direction. At the same time, a built-in electric field is generated, and the direction of the built-in electric field is from the N region to the P region. Under the action of the built-in electric field, EFN will move down together with the entire N-region energy band, and EFP will move up together with the entire P-region energy band until the Fermi level is flattened to EFN=EFP and the carriers stop flowing. At this time, the conduction band and the valence band of the junction region are correspondingly bent to form a potential barrier. The height of the potential barrier is equal to the difference between the Fermi levels of the N-type semiconductor and the P-type semiconductor when they exist alone, namely:

Then there are:

In the formula, q is the amount of electrons; VD is the contact potential difference or built-in electromotive force.

Figure 2 - PN junction model and energy band diagram under thermal equilibrium
Figure 2 – PN junction model and energy band diagram under thermal equilibrium

It can be seen that VD is related to the doping concentration. At a certain temperature, the higher the doping concentration on both sides of the PN junction, the greater the VD. For materials with a band gap, ni is small, so VD is also large.

3) PN junction photoelectric effect
When the PN junction is illuminated, both intrinsic and extrinsic absorption of photons will generate photogenerated carriers, but only the minority carriers excited by intrinsic absorption can cause the photovoltaic effect. The photo-generated holes generated in the P region and the photo-generated electrons generated in the N region belong to many electrons, which are blocked by the potential barrier and cannot pass through the junction. Only the photo-generated electrons in the P region, the photo-generated holes in the N region, and the electron-hole pairs in the junction region. When the (minority carrier) diffuses to the vicinity of the junction electric field, it can drift across the junction under the action of the built-in electric field. The photogenerated electrons are pulled to the N region, and the photogenerated holes are pulled to the P region, that is, the electron-hole pair is separated by the built-in electric field. This results in the accumulation of photogenerated electrons near the boundary of the N region and the accumulation of photogenerated holes near the boundary of the P region. They generate a photo-generated electric field opposite to the built-in electric field of the thermally balanced PN junction, which is directed from the P region to the N region. This electric field lowers the potential barrier and reduces the photo-induced potential difference. The P terminal is positive and the N terminal is negative. Therefore, the junction current flows from the P region to the N region, and its direction is opposite to the photocurrent.

In fact, not all photogenerated carriers generated can generate photogenerated current. Let the diffusion distance of holes in the N region be LP during the lifetime, and the diffusion distance of electrons in the P region to be Ln during the lifetime. Because Ln+LP=L is much larger than the width of the PN junction itself, it can be considered that within the average diffusion distance L near the junction, the photogenerated carriers can generate photocurrent. The generated electron-hole pairs that are more than L from the junction area will be recombined in the diffusion process, and have no effect on the photoelectric effect of the PN junction.

An additional current (photocurrent) IP will be generated in the PN under illumination, and its direction is the same as the reverse saturation current I0 of the PN junction, generally IP≥I0. at this time:

In the formula, q is the charge of the electron; V is the terminal voltage of the diode (forward voltage is positive, reverse voltage is negative); T is the thermodynamic temperature during the test, k=8.63×10-6eV/K.
Let IP=SE, S is the effective surface area of ​​the photovoltaic cell, E is the illuminance, then:

When the circuit outside the PN junction under illumination is open, the voltage from the P terminal to the N terminal is the V value when I=0 in the above current equation, that is:

So the open circuit voltage:

In the PN junction under illumination, if the external circuit is short-circuited, the current flowing from the P terminal and returning to the N terminal through the external circuit is called the short-circuit current Isc. The short-circuit current Isc is the I value when V=0 in formula (2-4), that is, Isc=SE.

Voc and Isc are two important parameters of PN junction under illumination. At a certain temperature, oc and illumination E have a logarithmic relationship, but the maximum value does not exceed the contact potential difference VD. Under weak light irradiation, Isc has a linear relationship with E.

The four states of the PN junction are as follows:
(1) Thermal equilibrium state in the absence of light. N-type semiconductors and P-type semiconductors have a unified Fermi level, and the potential barrier height is qVD = EFN-EFP.
(2) The circuit outside the PN junction is open under stable illumination. The photogenerated voltage Voc appears due to the accumulation of photogenerated carriers, there is no longer a unified Fermi level, and the barrier height is q(VD-Voc)
(3) The circuit outside the PN junction is short-circuited under stable illumination. There is no photo-generated voltage at both ends of the PN junction, and the potential barrier height is qVD. 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.
(4) There is light and there is load. A part of the photocurrent establishes the voltage V on the load, and the other part of the photocurrent is offset by the forward current caused by the forward bias of the PN junction, and the potential barrier height is g(VD-Vf).

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