photovoltaic effect

Thread Content

Photovoltaic effect The photovoltaic effect, referred to as the photovoltaic effect, 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. There are several mechanisms for generating this potential difference, the main one being due to the existence of a barrier layer. The following takes the PN junction as an example. PN Junction in Thermal Equilibrium Formation of PN Junction: A homojunction can be formed by doping a piece of semiconductor into P and N regions. Since the activation energy ΔE of impurities is very small, almost all impurities are ionized into acceptor ions NA- and donor ions ND+ at room temperature. Because there is a concentration difference of carriers at the interface of the PN region, they must diffuse toward each other. Imagine that at the moment the junction is formed, the electrons in the N region are majority carriers, and the electrons in the P region are minority carriers, causing electrons to flow from the N region into the P region. When electrons and holes meet, they recombine, so that there are very few electrons near the junction that was originally the N region, leaving unneutralized donor ions ND+ to form a positive space charge. Similarly, after holes diffuse from the P region to the N region, a negative space charge is formed by the immobile acceptor ion NA-. Immovable ion regions (also called depletion regions, space charge regions, barrier layers) are generated on both sides of the interface between the P region and the N region, and a space electric double layer appears, forming an internal electric field (called a built-in electric field). This electric field resists the diffusion of the majority carriers in the two regions and helps the drift of the minority carriers until equilibrium is reached when the diffusion flow is equal to the drift flow, and a stable built-in electric field is established on both sides of the interface. PN junction model and energy band diagram under thermal equilibrium PN junction energy band and contact potential difference: Under thermal equilibrium conditions, the junction region has a uniform EF ； At locations far away from the knot area, the relationship between EC, EF, and Eν is the same as before knot formation. From the energy band diagram, 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 flow from the higher Fermi level to the lower Fermi level, and 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 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. In the junction area, the conduction band and valence band bend accordingly, forming a potential barrier. The barrier height is equal to the difference in Fermi level when N-type and P-type semiconductors exist alone.: qUD=EFN-EFP gets UD=(EFN-EFP)/qq: Electronic power UD: Contact potential difference or built-in potential for states outside the depletion region: UD=(KT/q)ln(NAND/ni2) NA, ND, ni: Acceptor, donor, intrinsic carrier concentration. It can be seen that UD is related to doping concentration. At a certain temperature, the higher the doping concentration on both sides of the PN junction, the greater the UD. For materials with a forbidden bandwidth, ni is smaller, so UD is also larger. PN junction PN junction photoelectric effect under light: When the PN junction is illuminated, both the intrinsic absorption and extrinsic absorption of photons by the sample will generate photogenerated carriers. However, only minority carriers excited by intrinsic absorption can cause photovoltaic effects. Because the photogenerated holes generated in the P region and the photogenerated electrons generated in the N region are multi-carriers, they are blocked by the potential barrier and cannot pass the junction. Only the photogenerated electrons in the P region, the photogenerated holes in the N region, and the electron-hole pairs (minority carriers) in the junction region can drift across the junction under the action of the built-in electric field when they diffuse near the junction electric field. Photo-generated electrons are pulled toward the N region, and photo-generated holes are pulled toward 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 region and the accumulation of photogenerated holes near the boundary of the P region. They generate a photogenerated electric field opposite to the direction of the built-in electric field of the thermally balanced PN junction, and its direction is from the P region to the N region. This electric field reduces the potential barrier, and the reduction is the photogenerated potential difference. The P terminal is positive and the N terminal is negative. As a result, 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 contribute to the photocurrent. Assume that the diffusion distance of holes in the N region within the lifetime τp is Lp, and the diffusion distance of electrons in the P region within the lifetime τn is Ln. Ln+Lp=L is much larger than the width of the PN junction itself. Therefore, it can be considered as attaching * * All photogenerated carriers generated within the uniform diffusion distance L contribute to the photocurrent. The generated electron-hole pairs whose position is more than L from the junction area will all be recombined during the diffusion process and will not contribute to the photoelectric effect of the PN junction. PN junction current equation under light: Compared with the thermal equilibrium, when there is light, an additional current (photocurrent) Ip will be generated in the PN junction, and its direction is the same as the reverse saturation current I0 of the PN junction. Generally, Ip ≥ I0. At this time, I=I0eqU/KT - (I0+Ip) Let Ip=SE, then I=I0eqU/KT - (I0+SE) Open circuit voltage Uoc: The voltage between the P terminal and the N terminal when the external circuit of the PN junction under light is open, that is, the U value when I=0 in the above current equation: 0=I0eqU/KT - (I0+SE) Uoc=(KT/q)ln(SE+I0)/I0≈(KT/q)ln(SE/I0) Short circuit current Isc: When the external circuit of a PN junction under light is short-circuited, it flows out from the P terminal, passes through the external circuit, and flows into the N terminal, which is called the short-circuit current Isc. That is, the I value when U=0 in the above current equation, we get Isc=SE. Uoc and Isc are two important parameters of the PN junction under light. At a certain temperature, Uoc has a logarithmic relationship with the illumination E, but the maximum value does not exceed the contact potential difference UD. Under weak light, Isc has a linear relationship with E. a) In the thermal equilibrium state without light, the NP-type semiconductor has a unified Fermi level and the barrier height is qUD=EFN-EFP. b) Under stable illumination, the external circuit of the PN junction is open. Due to the accumulation of photo-generated carriers, the photo-generated voltage Uoc no longer has a unified Fermi level, and the barrier height is q (UD-Uoc). c) Under stable illumination, the external circuit of the PN junction is short-circuited. There is no photogenerated voltage at both ends of the PN junction, and the barrier height is qUD. The photogenerated electron-hole pairs are separated by the built-in electric field and then flow into the external circuit to form a short-circuit current. d) There is light and a load. Part of the photocurrent establishes a voltage Uf 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. The barrier height is q (UD-Uf).