Fan Yang

*Qualcomm MEMS Technologies, Inc. United States* 

#### **1. Introduction**

334 Solar Cells – Thin-Film Technologies

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Providing a sustainable and environment friendly energy source, photovoltaic (PV) power is becoming ever-increasingly important, as it decreases the nation's reliance on fossil-fuel generated electricity. Though widely regarded as a clean and renewable energy source, large scale deployment of PV is still impeded by the fact that the cost of PV energy is generally higher compared to grid electricity. Current development of PV technology is focused on two aspects: 1) improving the efficiency of PV modules and systems and 2) lowering the cost of delivered electricity through decreasing the manufacturing and installation cost. The merit of commercial solar cells aiming at terrestrial application is justified by the cost of unit PV power generation, dollar per watt (\$/Wp), where Wp stands for the peak power generated by the cells.

Since the first practical PV cell grown on Si wafer at the Bell Laboratory in 1954, PV technology has been developed for more than five decades and evolved three "generations" based on different PV materials. The first generation of solar cells use crystalline materials, where the cost of the bulk materials has hit the point that further cost reduction is very difficult (Green 2007). In contrast, the second generation cells use thin film materials, where the required amount of materials is merely a few percent of that of bulk materials, significantly reducing the fabrication cost of this type of cells. The emerging, third generation of PV technology applies new materials and novel device concepts aiming at even higher efficiency and lower cost. At this moment, the commercial PV market is dominated by the first and second generation PV modules, and the third generation cells are still under lab research. As shown in Fig. 1, the efficiency of thin film PV system has improved from ~4 % in 1995 to >11 % in 2010, and will keep increasing to ~12% by 2020, a three-fold improvement compared to the system efficiency back in 1995. During the same timeframe, the cost of thin film PV system drops from ~4 \$/Wp to ~0.5 \$/Wp. Crystalline PV systems, though with higher efficiencies, have higher cost, i.e. 2.5 times the cost of thin film PV system. From the cost and material supply point of view, thin film solar cells will have a long-term development and gradually take more market share from the crystalline cells.

Many thin film materials can be used for PV cells, e.g., Si, CdTe, CIGS or the emerging organic/polymeric materials. Comparing to other materials, thin film Si, including amorphous Si (a-Si) and microcrystalline Si (µc-Si), have the following characters:

1. The PV active Si is the most abundant solid state element on the earth's shell, allowing for practically unlimited production of Si cells.

Fig. 2. Global thin film solar panel manufacturing capacity and compound annual growth

around 10% after 2012. Costs for these technologies are expected to range from 0.80 to 1.20 \$/Wp (Mehta 2010). Consequently, the energy cost pay-back time of these panels will be

For the above mentioned reasons, a-Si/µc-Si solar panels are the mostly produced among all thin film technologies and will stay in large volume production the foreseeable future. This chapter introduces the fundamental thin film PV solar cell structure, the energy conversion physics, and state-of-the-art large scale solar panel manufacturing. Various methods of performance enhancement and cost reduction of large area thin film Si solar cells are focuses

This chapter is organized as follows. Section 1 briefly introduces the history and current production status of a-Si and µc-Si solar panels. Section 2 analyzes the cost structure of typical thin film solar panels and systems. The basic solar cell structures, including the PV active Si p-i-n junction layers and the front and back contact layers, are discussed in Section 3. Next, we describe in details the panel production process in Section 4 and 5. The front end of line (FEOL) processis first introduced, with discussions on CVD deposition of Si layers, physical vapor deposition (PVD) process of transparent conductive oxide (TCO) layers and back contacts, and laser scribing steps. The back end of line (BEOL) process is then described with the introduction of module fabrication, bus line wiring and panel encapsulation. Different process flow configurations are also compared in this part. We

To begin the discussion of the cost of solar panels, we split the cost of thin film PV system

rate (CAGR) by technology, 2006-2013 (estimate) (Young 2010).

shortened to 0.5-2 years.

summary the chapter in Section 5.

**2. Cost structure of PV system** 

1. Planning and financing: 15%

3. Balance of system (BOS) and installation: 10-30%

into four major parts:

2. Inverter: 9-10%

4. Module: 40-66%

of this chapter.

Fig. 1. Photovoltaic (PV) system efficiency and cost. Data from the U.S. Department of Energy.


In addition, production of a-Si/µc-Si solar panels has a low entry barrier, thus making it more acceptable for the emerging PV manufactures. The first thin film Si solar cells were put into production in the 1980's when they were used as power sources for small electronic gadgets. Volume production of a-Si based solar panels started after the year 2000 with the introduction of large-area chemical vapor deposition (CVD) process at these companies: Sharp Corporation, United Solar Ovonic, Kaneka, Mitsubishi Heavy Industries, Ltd, etc. The true burst of Si thin film solar cells, on the other hand, came after 2007 with the "turnkey" (ready to use) thin-film solar manufacturing equipments introduced by Unaxis SPTec (later Oerlikon Solar) (Meier et al. 2007) and Applied Films Gmbh & Co. (later part of Applied Materials Inc.) (Repmann et al. 2007). The idea is that instead of developing the film deposition and module manufacturing technologies by self, the would-be solar maker can buy the full set of equipments together with the process recipes, and start manufacturing panels with relative ease. Each having a designed capacity of 40 – 60 MW, over twenty "turnkey" systems were sold to solar module makers world wide by Oerlikon and Applied Materials by 2010. The fast expansion of production capacity directly induced the drop of a-Si/µc-Si panel cost from around 5 \$/Wp to less than 2 \$/Wp.

At the moment thin film Si cells, including a-Si and µc-Si, take the largest market share (more than half of total production volume) among all types of thin film cells. Close to 5 GW of a-Si/µc-Si panels were manufactured in 2010, and will keep similarly large market share to at least 2013 (Fig. 2) (Young 2010). It is also noted from the same figure that the production volume of a-Si panels has an impressive compound annual growth rate (CAGR) of 42%, highest among all thin film PV technologies. Currently a significant amount of Si thin film panels are single-junction a-Si panels, whose efficiency will gradually increase to 8% - 8.5%. By adopting the a-Si/µc-Si multi-junction cells, panel efficiency will move up to

Fig. 1. Photovoltaic (PV) system efficiency and cost. Data from the U.S. Department of Energy.

3. Process of a-Si/µc-Si thin films takes the advantage of the highly mature semiconductor

4. a-Si is a metastable material, and the initial cell performance of a-Si based cells degrades under illumination and then stabilizes, known as the Staebler–Wronski effect (Kolodziej

In addition, production of a-Si/µc-Si solar panels has a low entry barrier, thus making it more acceptable for the emerging PV manufactures. The first thin film Si solar cells were put into production in the 1980's when they were used as power sources for small electronic gadgets. Volume production of a-Si based solar panels started after the year 2000 with the introduction of large-area chemical vapor deposition (CVD) process at these companies: Sharp Corporation, United Solar Ovonic, Kaneka, Mitsubishi Heavy Industries, Ltd, etc. The true burst of Si thin film solar cells, on the other hand, came after 2007 with the "turnkey" (ready to use) thin-film solar manufacturing equipments introduced by Unaxis SPTec (later Oerlikon Solar) (Meier et al. 2007) and Applied Films Gmbh & Co. (later part of Applied Materials Inc.) (Repmann et al. 2007). The idea is that instead of developing the film deposition and module manufacturing technologies by self, the would-be solar maker can buy the full set of equipments together with the process recipes, and start manufacturing panels with relative ease. Each having a designed capacity of 40 – 60 MW, over twenty "turnkey" systems were sold to solar module makers world wide by Oerlikon and Applied Materials by 2010. The fast expansion of production capacity directly induced the drop of a-

At the moment thin film Si cells, including a-Si and µc-Si, take the largest market share (more than half of total production volume) among all types of thin film cells. Close to 5 GW of a-Si/µc-Si panels were manufactured in 2010, and will keep similarly large market share to at least 2013 (Fig. 2) (Young 2010). It is also noted from the same figure that the production volume of a-Si panels has an impressive compound annual growth rate (CAGR) of 42%, highest among all thin film PV technologies. Currently a significant amount of Si thin film panels are single-junction a-Si panels, whose efficiency will gradually increase to 8% - 8.5%. By adopting the a-Si/µc-Si multi-junction cells, panel efficiency will move up to

2. Si has no toxicity and is environmental friendly.

Si/µc-Si panel cost from around 5 \$/Wp to less than 2 \$/Wp.

and display industries.

2004).

Fig. 2. Global thin film solar panel manufacturing capacity and compound annual growth rate (CAGR) by technology, 2006-2013 (estimate) (Young 2010).

around 10% after 2012. Costs for these technologies are expected to range from 0.80 to 1.20 \$/Wp (Mehta 2010). Consequently, the energy cost pay-back time of these panels will be shortened to 0.5-2 years.

For the above mentioned reasons, a-Si/µc-Si solar panels are the mostly produced among all thin film technologies and will stay in large volume production the foreseeable future. This chapter introduces the fundamental thin film PV solar cell structure, the energy conversion physics, and state-of-the-art large scale solar panel manufacturing. Various methods of performance enhancement and cost reduction of large area thin film Si solar cells are focuses of this chapter.

This chapter is organized as follows. Section 1 briefly introduces the history and current production status of a-Si and µc-Si solar panels. Section 2 analyzes the cost structure of typical thin film solar panels and systems. The basic solar cell structures, including the PV active Si p-i-n junction layers and the front and back contact layers, are discussed in Section 3. Next, we describe in details the panel production process in Section 4 and 5. The front end of line (FEOL) processis first introduced, with discussions on CVD deposition of Si layers, physical vapor deposition (PVD) process of transparent conductive oxide (TCO) layers and back contacts, and laser scribing steps. The back end of line (BEOL) process is then described with the introduction of module fabrication, bus line wiring and panel encapsulation. Different process flow configurations are also compared in this part. We summary the chapter in Section 5.
