What is the photovoltaic effect of solar cells?

What is the photovoltaic effect of solar cells?

Solar cells are devices that use the principle of photoelectric conversion to convert solar radiation light into electrical energy through semiconductor materials. This photoelectric conversion process is usually called the “photovoltaic effect.” Photovoltaic effect is abbreviated as photovoltaic effect, which refers to the phenomenon that light causes a potential difference between different parts of a non-uniform semiconductor or a combination of semiconductor and metal. At present, the high production cost and low efficiency of solar cells have become a bottleneck restricting their promotion and application. Therefore, how to make solar cells emit the maximum amount of electricity per unit area has become a major research topic in the development of the solar energy industry.

All matter is composed of atoms. Atoms are composed of nuclei and electrons rotating around the nucleus. In the normal state of semiconductor materials, the nucleus and electrons are tightly combined (in a non-conductor state), but under the stimulation of some external factors, the nucleus and electrons The binding force of electrons decreases, and the electrons get rid of the bondage of the nucleus and become free electrons, as shown in Figure 1. When sunlight shines on the semiconductor, the photon provides energy to the electrons, and the electrons will transition to a higher energy state. Among these electrons, the available electrons in the optoelectronic devices actually used are:

What is the photovoltaic effect of solar cells?
Figure 1 The electron gets rid of the bondage of the nucleus and becomes a free electron

(1) Valence band electronics.
(2) Free electrons or holes.
(3) Electrons present in the impurity energy level.

The electrons available in solar cells are mainly valence band electrons. The process of valence band electrons obtaining light energy and transitioning to the conduction band determines the absorption of light (called intrinsic or intrinsic absorption).

Solar cells are composed of P-type semiconductors and N-type semiconductors. P-type semiconductors (P means Positive, positively charged) are composed of single crystal silicon mixed with a small amount of trivalent elements through a special process, which will be formed inside the semiconductor Positively charged holes; N-type semiconductor (N refers to Negative, negatively charged) is composed of single crystal silicon mixed with a small amount of pentavalent elements through a special process, which will form negatively charged free electrons inside the semiconductor. When N-type and After P-type semiconductor materials of two different types are in contact, due to diffusion and drift effects, a built-in electric field from P-type to N-type is formed at the interface. When light shines on the surface of the solar cell, photons with energy greater than the forbidden band excite electron and hole pairs. These unbalanced minority carriers separate under the action of the internal electric field and accumulate and form at the two poles of the solar cell. The potential difference, so that the solar cell can provide current to the external load, as shown in Figure 2. A simple installation of conventional solar cells is shown in Figure 3.

What is the photovoltaic effect of solar cells?
Figure 2 Schematic diagram of solar cell composition
What is the photovoltaic effect of solar cells?
Figure 3 Simple device of conventional solar cell

1) The formation of PN junction
In a single crystal semiconductor, part of the doped acceptor impurity is a P-type semiconductor, and the other part doped with a donor impurity is an N-type semiconductor, the transition area near the interface between the P-type semiconductor and the N-type semiconductor is called a PN junction. There are two types of PN junctions: homojunction and heterojunction. PN junctions made of the same semiconductor material are called homojunctions, and PN junctions made of two semiconductor materials with different band gaps are called heterojunctions. There are alloying methods, diffusion methods, ion implantation methods, and epitaxial growth methods for manufacturing PN junctions, and epitaxial growth methods are usually used to manufacture heterojunctions.

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 immobile. There are many movable negative electrons and fixed positive ions in N-type semiconductors. When the P-type and N-type semiconductors 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 near the interface. Holes and electrons meet and recombine, and the carriers disappear. Therefore, in the junction region near the interface, there is a distance lack of carriers, but there are fixed ions distributed in the space, which is called the space charge region. The space charge on the side of the P-type semiconductor is negative ions, and the space charge on the side of the N-type semiconductor is positive ions. The positive and negative ions generate an electric field near the interface, which prevents the carriers from further diffusing and reaches equilibrium.

The homojunction can be doped with a piece of semiconductor to form the P region and the N region. Because the impurity activation energy ΔE is very small, the impurities are usually ionized into acceptor ions NA and donor ions N+D at room temperature. There is a difference in the carrier concentration at the interface of the P and N regions, so they must diffuse toward each other. Imagine that at the moment when the junction is formed, the electrons in the N area are many electrons, and the electrons in the P area are large and few, so that the electrons flow from the N area to the P area, and the electrons and holes meet and recombine, so that the original N area is There are few electrons near the junction surface, leaving the unneutralized donor ion N+D to form a positive space charge. Similarly, after holes diffuse from the P area to the N area, the immobile acceptor ion NA forms a negative space charge. Immovable ion regions (also called depletion regions, space charge regions, and barrier layers) are generated on both sides of the interface between the P and N regions, so a space galvanic layer appears, forming an internal electric field (and a built-in electric field), this electric field It has an inhibitory effect on the diffusion of multiple sons in the two regions, and has a helpful effect on the drift of minority carriers, until the diffusion flow is equal to the drift flow, 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 the conditions of thermal equilibrium, the junction area has a unified EF: in the part far away from the junction area, the relationship between EC, EF and EV is the same as the state before the formation of the junction. It can be seen from Figure 4 that when N-type and P-type semiconductors exist alone, there is a certain difference between EFN and EFP. When the N-type and P-type are in close contact, electrons will flow from the higher Fermi level to 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 area to the P area. Under the action of the built-in electric field, EFN will move down together with the energy band of the entire N region, and EFP will move up together with the energy band of the entire P region until the Fermi level is leveled to EFN=EFP and the carriers stop flowing. At this time, the conduction band and valence band in the junction area bend correspondingly, forming a potential barrier. The barrier height is equal to the difference between the Fermi energy levels when N-type and P-type semiconductors exist alone:
have to:
In the formula, q is the electric quantity of electrons; VD is the contact potential difference or the built-in potential.

What is the photovoltaic effect of solar cells?
Figure 4 PN junction model and energy band diagram under thermal equilibrium

For states outside the depletion zone:
In the formula, NA, NP and ni are acceptor, donor, and intrinsic carrier concentration, respectively.

It can be seen that VD is related to doping concentration. At a certain temperature, the higher the doping concentration on both sides of the PN junction, the greater the VD. The ni of the forbidden bandwidth material is small, so the VD is also large.

3) PN junction photoelectric effect
When the PN junction is exposed to light, both the intrinsic absorption and extrinsic absorption of photons will produce photo-generated carriers. But it is only the minority carriers excited by intrinsic absorption that can cause the photovoltaic effect. The photo-generated holes generated in the P-zone and the photo-generated electrons in the N-zone are multiple electrons, and they are all blocked by the barrier and cannot pass through the junction. Only the photogenerated electrons in the P region, the photogenerated holes in the N region and the electron-hole pairs in the junction region can drift through the junction under the action of the built-in electric field when they diffuse to the vicinity of the junction electric field. The photo-generated electrons are drawn to the N region, and the photo-generated holes are drawn to the P region, that is, the electron-hole pairs are separated by the built-in electric field. This results in the accumulation of photogenerated electrons near the boundary of the N zone and the accumulation of photogenerated holes near the boundary of the P zone. They generate a light-generated electric field that is opposite to the built-in electric field of the thermally balanced PN junction, and its direction is from the P area to the N area. This electric field lowers the potential barrier and reduces the photo-generated potential difference. Because the P terminal is positive and the N terminal is negative, the junction current flows from the P area to the N area, and its direction is opposite to the direction of the photocurrent.

In fact, not all photogenerated carriers generated can generate photocurrent. Suppose the diffusion distance of holes in the N zone during the lifetime τP is LP, and the diffusion distance of the electrons in the P zone during the lifetime τn is Ln. Ln+Lp=L is much larger than the width of the PN junction itself. Therefore, it can be considered that within the average diffusion distance L near the junction, the generated photo-generated carriers can generate photocurrent. The generated electron-hole pairs whose position is more than L from the junction area will be completely recombined during 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 junction under illumination, and its direction is the same as the direction of the reverse saturation current I0 of the PN junction, generally IP≥I0. at this time:

Let IP=SE, then:

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

Open circuit voltage Voc:

When the PN junction under the light is short-circuited in the external circuit, the current flowing from the P terminal through the external circuit and flowing from the N terminal is called the short-circuit current ISC. That is, the value of I when V=0 in the above current equation, ISC=SE.

VOC and ISC are two important parameters of the PN junction under light. At a certain temperature, VOC has a logarithmic relationship with the illuminance E, but the maximum value does not exceed the contact potential difference VD. In low light, there is a linear relationship between ISC and E. The four states of PN junction are as follows:
(1) In the thermal equilibrium state without light, N-type and P-type semiconductors have a uniform Fermi energy level, and the barrier height is qVD = EFN-EFP.
(2) When the circuit outside the PN junction is open under stable light, the photo-generated voltage Voc appears due to the accumulation of photo-generated carriers, there is no longer a uniform Fermi level, and the barrier height is q (VD-Voc).
(3) The external circuit of 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 qVD. The photo-generated electron-hole pairs are separated by the built-in electric field and flow into the external circuit to form a short-circuit current.
(4) There is light and a load, a part of the photocurrent builds up a voltage Vf 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 barrier height is q (VD-Vf).