Evolution of c-Si PV Cell Technologies: Page 2 of 4
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Anatomy of a Modern c-Si Cell
The structure of a modern c-Si solar cell is a stack of layers built up on either side of a silicon substrate known as the base layer. Silicon crystals are formed by growing a single continuous crystal ingot (monocrystalline silicon) or by producing a solid block of many different crystals (multi-crystalline silicon). The base layer of the solar cell begins as a thinly sliced wafer of c-Si that is charged either positively (p-type) or negatively (n-type). Manufacturers typically dope the p-type wafer with boron, resulting in a net positive charge. Conversely, introducing a negatively charged impurity such as phosphorous results in an n-type wafer with a net negative charge. (P-type solar cells are the most common commercial variety and have held the largest market share among all PV technologies for the last four decades.)
The front surface of the wafer, known as the emitter layer, is formed by injecting an extremely thin layer of dopants, which have a charge opposite that of the base layer. Since the near front surface absorbs a high percentage of light, the primary function of the emitter layer is to absorb incident photons, which in turn generates a pair of oppositely charged carriers. The intersection of the oppositely charged emitter layer and the base layer is known as the p-n junction. This is essentially two physically adjoined layers of silicon crystal with opposite charges, which help separate carriers based on their natural magnetic field.
Bare silicon is highly reflective. Therefore, to reduce the probability that a photon is reflected off the solar cell, manufacturers typically apply two additional layers to the front surface. First, they apply a prismatic texture, resembling miniature pyramids, to redirect photons toward the cell, thereby reducing the likelihood of reflecting light. Second, they typically apply a silicon-nitride AR coating, which helps prevent the unwanted reflection of light in the useful spectrum and acts as an effective front-surface passivation technique.
Finally, they apply metallic contacts to collect the positive and negative carriers. In a traditional c-Si solar cell, to form the rear contact, manufacturers screen-print a continuous solid layer of aluminum paste to the rear side of the solar cell. Also using a screen-printing process, they apply a silver paste to form a grid pattern on the front surface of the solar cell. The size, shape and placement of these grid lines is extremely important to cell performance. On the one hand, the grid lines block sunlight from reaching the surface of the cell, so they require sufficient spacing between them. On the other, if you space the grid lines too far apart, losses may increase due to the inherent resistance of the cell’s semiconductor material.
Physics of a Solar Cell
At its essence, the job of a solar cell is to generate a pair of oppositely charged light-generated carriers and transport these carriers so they can dissipate their power in an external load. As detailed on the PV Education Network website (see Resources), solar cells operate according to four basic steps: Generate light-generated carriers, collect light-generated carriers to generate a current, generate a large voltage across the solar cell, and dissipate power in the load and in parasitic resistances.
The generation of current in a solar cell, known as the light-generated current, involves two key processes. The first is the absorption of incident photons to create a pair of carriers known as an electron-hole pair. If an incident photon has a high enough energy when it impacts an atom in the silicon crystal, it can knock an electron loose from orbit around the silicon atom within the crystal. Once the electron is loose, it is free to move about the semiconductor and participate in conduction. However, when the photon knocks the electron loose from the orbit of the silicon atom, that leaves behind an empty space for another electron to fill. An electron from a neighboring atom can move into this empty space. When this electron moves, it leaves behind another space and so forth.
This continual movement of the space, called a hole, marks the path of a positively charged particle through the crystalline structure. A moving hole is analogous to a bubble in a liquid, except that the crystalline structure does not move as a liquid does. Rather, it is the hole that moves. Because both the electron and the hole can participate in conduction, they are called carriers. Since carriers are moving through a solid crystal material by joining and rejoining neighboring silicon atoms, their path is not as direct as one might think. They tend to bob and weave as they make their way toward the oppositely charged surface of the solar cell.
Before a carrier is collected or swept across the p-n junction, it is not extremely stable. Therefore, the carrier will exist only for a limited period of time, referred to as the carrier lifetime, before recombining with a silicon atom. If the carrier recombines, then the light-generated electron-hole pair disappears without generating current or power.
The second key process for generating current in a solar cell is to prevent carrier recombination. This is accomplished by forming a p-n junction to collect the carriers, which helps to separate the electron from the hole spatially. An electric field that exists at the p-n junction separates the carriers. If the light-generated minority carrier reaches the p-n junction, the electric field sweeps it across the junction, and it becomes a majority carrier. Connect the emitter and base layers, and the light-generated carriers flow through the external circuit, putting the solar cell into operation.