Construction and working principle of silicon solar cells
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When two crystalline silicons with the same conductivity type and different doping concentrations are in contact, a concentration junction (also called a gradient junction) with a galvanic double layer and a self-built electric field can also be formed, as shown in Figure 1.
For P-type silicon, the contact barrier height eUg at the junction interface of PP+ concentration at thermal equilibrium is
If a PP+ junction is formed on the N+P junction, the total built-in potential UR of the N+PP+ junction will increase to
Silicon solar cell structure and working principle
Figure 2 shows a schematic diagram of the structure of the most commonly used N+/P crystalline silicon solar cell. Phosphorus is spread on the P-type crystalline silicon wafer to form an N+-type top region, forming a PN+ junction. The surface of the top area is a grid-shaped metal top electrode (also called a positive electrode), the surface is covered with an anti-reflection film, and the back is a metal bottom electrode (also called a back electrode).
When the cell is illuminated, light passes through the anti-reflection film and enters the silicon, and photons with energy greater than the silicon band gap excite photo-generated electron-hole pairs in the N region, the depletion region and the P region. The photo-generated electron-hole pairs entering the depletion region and generated in the depletion region will be immediately separated by the built-in electric field, the photo-generated electrons enter the N region, and the photo-generated holes enter the P region. In the N region, under the action of the built-in electric field, the photogenerated holes (minority carriers) diffused to the boundary of the PN junction cross the depletion region and enter the P region, while the photogenerated electrons (multiple carriers) are left in the N region. Similarly, the photogenerated electrons (minor carriers) in the P region diffuse first and then drift into the N region, while the photogenerated holes (multiple carriers) stay in the P region. Therefore, positive and negative charges are accumulated on both sides of the PN junction, resulting in a photogenerated voltage. When a load is connected, a photocurrent flows from the P region to the N region through the load.
Figure 3 shows the PN junction band diagram of a silicon solar cell. Among them, Fig. 3(a) shows the case of no illumination. At this time, in the thermal equilibrium state, the PN junction has a unified Fermi level, and the potential barrier height is eUn=EFn-EFp.
Figure 3(b) shows the situation under steady light with the battery in an open circuit. At this time, the PN junction is in an unbalanced state, and the photo-generated carriers accumulate to form an open-circuit photovoltage (called open-circuit voltage), so that the PN junction is positively biased, the Fermi level is split, the width of the split is equal to eUoc, and the potential barrier height is e(UD-Uoc). Figure 3(c) shows the condition of the battery in a short-circuit state under stable illumination. The original photogenerated carriers accumulated at both ends of the PN junction recombine through the external circuit, the photovoltage disappears, and the potential barrier height is eUD, the photo-generated carriers in each region are continuously separated by the built-in electric field, and the photocurrent in the short-circuit state (called short-circuit current lsc) is formed through the external wire. Figure 3(d) shows that when there is light and an external load, a part of the photocurrent flows through the load, and a voltage U is established on the load, which is equivalent to applying a forward bias voltage UF to the PN junction; the other part of the photocurrent and the forward current formed by the PN junction under the forward bias voltage UF cancel; the width of the Fermi level split is equal to eU, and the potential barrier height is e(UD-U).