Thin-film silicon solar panels are produced by forming an amorphous silicon (a-Si) film on a glass substrate. Similar to crystalline solar cells, or modules, the a-Si, photovoltaic panels are made from silicon, however, the amount of silicon required is very small: that is, one hundredth or less the amount of the crystalline photovoltaic cells (made from bulk silicon), since only a thin layer of silicon is deposited on a 1.1m x 1.4m glass substrate. For this reason, a-Si thin-film photovoltaic modules have received wide-spread attention as an alternative to crystalline silicon, for various mainstream PV applications.
Thin-film silicon solar panels are produced by forming an amorphous silicon (a-Si) film on a glass substrate. Similar to crystalline solar cells, or modules, the a-Si, photovoltaic panels are made from silicon, however, the amount of silicon required is very small: that is, one hundredth or less the amount of the crystalline photovoltaic cells (made from bulk silicon), since only a thin layer of silicon is deposited on a 1.1m x 1.4m glass substrate. For this reason, a-Si thin-film photovoltaic modules have received wide-spread attention as an alternative to crystalline silicon, for various mainstream PV applications.
Because a-Si photovoltaic modules/panels have lower photoelectric conversion efficiencies than crystalline photovoltaic modules, efforts have been made to develop technologies and manufacturing equipment for producing tandem-type, thin-film photovoltaic panels. Tandem-type panels have a micro-crystalline layer (µc-Si) added to the a-Si layer on the panel. Deposition of the µc-Si layer has been attempted in cluster-type deposition equipment, but these types of systems have negatively impacted the through-put of turn-key manufacturing lines and have greatly increased production costs. This article will cover how a new plasma-enhanced chemical vapor deposition (PECVD) system has been developed that allows for high throughput and highly efficient µc-Si film formation that can allow light from the red-to-infrared wavelengths to be converted to electricity. By incorporating this new PECVD system into a production turnkey line for thin-film silicon photovoltaic panels, power output will be improved by 30%, and manufacturing cost per Watt (Wp) is reduced by 10%; compared with single-junction, a-Si photovoltaic panels.
Tandem-junction thin-film solar panels
Tandem-junction thin-film solar panels are fabricated in much the same fashion as single-junction a-Si panels. Typical high-volume production of both types of panels is done with “turn-key” manufacturing equipment, usually consisting of the following processes: TCO film ablation (laser), deposition of photoelectric conversion films, cell isolation (pin-layer patterning), deposition of rear electrode (reflective film), and cell isolation (laser patterning). The rest of the thin-film panel production process involves: panel cleaning, sealing, wiring, frame and terminal box mounting. The key manufacturing step in a tandem-junction PV panel is the deposition of the µc-Si film.
Figure 1 shows the production process. The glass substrate is a material called “white” glass—it has high transmittance and low iron content. A transparent conductive oxidized film (a TCO), with a slight roughness, called textured TCO, is formed on the glass surface to effectively confine light in the photoelectric conversion films (a-Si and µc-Si) that will be deposited subsequently and sequentially. In some regions, e.g., the Japanese market, glass manufacturers usually provide white glass with a TCO coating, but in other regions, e.g., the European market, there are some cases where the individual cell manufacturer deposits the TCO films on the glass. The TCO film on the glass is ablated with a laser to form several bands of surface electrodes. Sunlight passes through the TCO-coated glass substrate and enters the photoelectric conversion films.
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| Figure 1. Tandem-type thin-film silicon photovoltaic module production. |
The a-Si and µc-Si films are then deposited using two different kinds of plasma CVD systems. First, an amorphous silicon (a-Si) layer is deposited (top cell), followed by microcrystalline silicon (µc-Si) layer (bottom cell). A tandem-junction, thin-film solar cell has the structure of an i-layer sandwiched between p- and n- layers in both the top and bottom cells to effectively separate photo-excited carriers in the photoelectric conversion films. A metal film is deposited with a sputtering system to serve as a rear electrode. The material that is very effective as the rear electrode is silver (Ag). The sputtered silver film is highly reflective and serves to further improve the power generation efficiency of the tandem- junction cell, by effectively reflecting light on this rear electrode surface after it has gone through the photoelectric conversion layer. Although silver is relatively expensive, it has low volume resistivity, and retains its properties over a long period of time. The silver film is also patterned by laser ablation.
The front electrode (TCO), the photoelectric conversion layer (a-Si + µc-Si), and rear electrode (Ag) are all patterned by laser ablation, in this fashion, and bands of power generating elements are connected in series.
PECVD systems are the key
Two types of PECVD systems (CCV and CIM) are key tools for producing tandem-junction solar cells/panels. The power generating layers of amorphous silicon and microcrystalline silicon serve as the top and bottom layers of the cell. To make a solar cell with high power conversion efficiency, it is necessary to slowly deposit the i- layer in both the top and bottom cells. In the top cell (a-Si layer), a deposition that is too fast will increase dangling bonds in the silicon film and cause holes and the recombination of electrons, and as a result the power generation efficiency degrades, as shown in Fig. 2. This is also true for the bottom cell. It is also very important to prevent the doping gasses used for the p- and n- layers from entering the i- layer. In addition to these technical elements, high productivity is an indispensable prerequisite for producing tandem-junction PV panels, at the lowest possible manufacturing cost. For these reasons, a multi-chambered, in-line method has been adopted for the primary plasma CVD system. The primary plasma CVD system deposits the a-Si layer with good p-i-n junction formation (top cell). Productivity is also greatly improved by simultaneously processing two glass substrates, as shown in Fig. 3.
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| Figure 2. Amorphous silicon solar cells. |
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| Figure 3. Sectional view of in-line plasma CVD system. |
The µc-Si layer (bottom cell), in contrast, has a low optical absorption coefficient and needs to be 5 to 10 times as thick as the a-Si film. In volume production, this thickness requirement has lead to a “slow-down” of manufacturing productivity, or has required the need for several cluster-type plasma CVD systems (at much added cost) to keep productivity rates up, for a tandem-junction production line.
Several research efforts have been undertaken to increase the deposition rates for the microcrystalline silicon film layer. One technique that accelerates the deposition rate uses high electric power, but this technique tends to decrease crystallinity and electrical properties, due to high levels of ion bombardment. A technique used to reduce ion bombardment involves adding a very high excitation frequency that is well above the conventional excitation frequency of 13.56MHz, but this tends to cause an irregular (unstable) plasma. Until recently, it was difficult to produce tandem-junction panels, in large volumes, with stable solar cell characteristics.
A new plasma CVD system
A new type of secondary PECVD system has been developed to increase the productivity for the tandem-junction production line. The system combines unique parallel plate and capacity-coupling type electrodes, for processing, with the high productivity feature of simultaneously processing six (6) glass substrates at a low deposition rate. It has been confirmed that tandem-junction solar cell panels (1.1mx1.4m), composed of a-Si films and microcrystalline silicon films, generated electricity of 130W or more, after light degradation (Fig. 4). A typical single-junction panel (of similar dimensions) produces about 100W of electricity. Tandem-junction cell technology can improve the annual production capacity of a production line from 25MW to 32.5MW, or about a 30% increase in production efficiency.
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| Figure 4. Characteristics of tandem thin-film silicon solar cells. |
References
1. Y. Sunaga, “Thin-Film Modules Production Turnkey Line,” ULVAC Technical Jour., No.71E, Oct., 2009.
2. Y. Shimizu, “Tandem-Type Thin-film Silicon Photovoltaic Module Production,” Jour. of the Japanese Assn. for Crystal Growth, Jan. 2010 issue (Vol. 36-4).
David J. Mount is manager, marketing & business development, at Ulvac Technologies, Inc., 401 Griffin Brook Drive, Methuen, MA 01844; ph.: 978-686-7550; [email protected].
