Photovoltaic Module Reliability. John H. Wohlgemuth
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Название: Photovoltaic Module Reliability
Автор: John H. Wohlgemuth
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119459026
isbn:
A variety of materials have been used commercially to make solar cells. The list below discusses those that have been successfully utilized in terrestrial PV modules and are available commercially today.
Crystalline silicon materials: Crystalline silicon materials were the first utilized for fabrication of solar cells by Bell Laboratories [7]. They were also the first used for space applications and then the first commercialized for remote power terrestrial applications. Crystalline silicon technology has been, and still is, the dominant commercial PV material, with more than 90% of the terrestrial PV market in 2018 [4].
Crystalline Si is a wafer‐based technology. Today, all of the commercial wafers are cut from ingots. There are two different types of crystalline silicon materials, single or mono‐crystalline and multi‐crystalline. They differ in the way the silicon ingots are grown. The single‐crystal ingots are grown by a crystal growth process called Czochralski (CZ), where a single‐crystal seed is dipped into a bath of molten silicon. As the seed is withdrawn, the silicon grows the same orientation as the seed. This is the same process that is used to make the wafers for many semiconductor devices. Single‐crystal ingots are round, but round wafers do not pack very well into rectangular modules so, in most cases, the round wafers are cut into square or pseudo‐square wafers for PV. Multi‐crystalline Si is grown using direction solidification in a crucible or mold. This process was developed specifically for PV in the late 1970s [15]. According to Paula Mints [4], in 2018 multi‐crystalline Si technology accounted for slightly more than 50% of the worldwide shipments of PV modules, while mono‐crystalline Si accounted for approximately 45%.
Figure 1.1 Cross‐sectional drawing of screen‐printed cry‐Si solar cell.
For both mono‐ and multi‐crystalline Si, the ingots are cut into wafers using wire saws. The resultant wafers are then processed into cells and the cells incorporated into modules in very similar ways. There are a number of cell processes in use today. Let's take a look at screen‐print technology (one of the simpler methods) since most of the cell components will be similar no matter which cell design is used. Figure 1.1 shows a diagram of the important features of a screen‐printed cry‐Si solar cell.
The solar cell is fabricated on the Si wafer. A p‐n junction is grown in the front (sun side) usually by diffusion. Metal grid lines are printed on the front to collect the current generated by the cell. The back side is also metallized. The drawing shows continuous back metallization though a grid can also be used on the back. Finally, because bare silicon is quite reflective, an antireflective coating (AR) is usually applied to the front. For mono‐crystalline Si the front surface is usually textured to increase optical absorption. This is one of the reasons that mono‐crystalline cells are usually a few percent higher in efficiency than multi‐crystalline cells. It is important to remember the different components in the cell when addressing reliability, because it is usually not the Si wafer or the p‐n junction that degrade, but rather the contacts, the interconnections between cells or even the AR coating.
Screen‐printed cry‐Si cells are typically fabricated on 15.6 × 15.6 cm wafers. Over the years, the wafer thickness has been slowly reduced to lower the cost of the Si used in the module. Today, Si wafers are typically less than 200 μm thick, which makes them more susceptible to breakage than the thicker wafers that were used in the earlier days of PV. Today most commercial screen‐printed cells have a conversion efficiency in the range of 15–19% with the multi‐crystalline Si cells at the low end and monocrystalline Si cells at the high end.
A number of commercial cell manufacturers have utilized specialty structures in order to increase cell performance. Several examples are:
SunPower has commercialized cells with all of the contacts on the back of the cell. This cell structure eliminates front surface shadowing and can provide improved collection of the high currents associated with large area cells. This cell structure requires silicon substrates with very high lifetimes and excellent front surface passivation. SunPower offers their commercial cells with conversion efficiency up to 22.7% [16].
The HIT (Heterojunction with intrinsic thin layer) cell uses heterojunctions between a‐Si and crystalline silicon to produce a much higher voltage than the standard p‐n junction. So HIT cells have high efficiency with research cells reported at 25.6% [17] and commercial modules available from Panasonic with reported efficiencies of 19.7% [18].
One of the latest methods for increasing cell efficiency is the use of PERC Technology (Passivated emitter rear cell). PERC cells use the same screen‐printed front surface as standard screen‐print technology, but the rear is modified by replacing the full metal coverage with a passivated dielectric layer with small area back contacts. PERC improves the cells by reducing the back‐surface recombination of carriers and improving the reflection of long wavelength light back into the cells [19]. Solar World has reported PERC cell efficiencies of 22% [20]. There have been a number of forecasts that PERC will gain significant market share in the next few years.
Each solar cell produces a voltage determined by the semiconductor junction. For crystalline Si the typical p‐n junction cell has an open circuit voltage of 0.6–0.7 V and a peak power voltage of around 0.5 V. Therefore, to reach useful voltages a number of cells are connected together in series into a module. Each cell has metal contacts typically copper ribbons attached to the front metal grid and then to the back metallization of the next cell in the string. Ribbons from the front of the cell before it in the string are attached to its back metallization. In this way cell voltages are combined to reach useful levels. Today, most cry‐Si power modules have 60 or 72 cells in series. The next section will talk about the module packaging in more detail.
Thin Films Materials: In the thin‐film approach, the various layers of a PV device are deposited directly onto a substrate or superstrate. The advantages of thin films are the potential ability to:
Dramatically reduce the amount of material utilized. In some cases, the active semiconductor only needs to be a fraction of a micron thick in order to absorb most of the incident sunlight.
Directly integrate into a higher voltage module, thereby eliminating much of the handling and labor necessary to produce cry‐Si modules.
For monolithically integrated thin film modules, the deposition processes and the integration processes are described in Table 1.1 and the resultant structure shown in cross section in Figure 1.2. Figure 1.2 is not drawn to scale as the thin film layers and the three different scribe lines are exaggerated in size so you can see them. In thin film devices, the semiconductors have considerably higher resistance than cry‐Si so metal grids do not work well. Instead, a Transparent Conductive Oxide (TCO) is often used on the semiconductor surface to provide a conductive path as well as serving as the AR. Because the TCO layers are not as conductive as a metal grid, thin film solar СКАЧАТЬ