**4.1.2 Growth of TCO layers**

The TCO layers are typically grown by either CVD or PVD on industrial size substrates. Used as the front TCO layer, SnO2:F layer are normally grown by CVD using SnCl4 precursor gas on glass substrates (Rath et al. 2010). The finished SnO2:F-coated glass is provided as substrates for later PECVD deposition of a-Si/µc-Si layers. Another front TCO candidate is AZO, which is usually grown by CVD processes using precursors like Zn(C2H5)2 or sputter deposition (Agashe et al. 2004). Substrate temperatures are near 150 C for the CVD deposition, and can go as high as 300 C for sputtering.

For many solar panel manufactures, control of the front TCO properties, especially the surface roughness, can be financially unfeasible, thus purchasing glass substrate pre-coated with TCO becomes a good choice. For a-Si absorber layers a high transparency for visible light (wavelength = 400–750 nm) is sufficient, while for solar cells incorporating µc-Si the TCO has to be highly transparent up to the near infrared (NIR) region (400–1100 nm) to accommodate for the wider absorption spectrum of µc-Si. This imposes certain restrictions on the carrier density, *n*, of the TCO material, as increased free carrier absorption leads to a reduction of IR transmission (Agashe et al. 2004). SnO2:F films fulfilling these requirements to a large extent, have been developed by Asahi Glass (Asahi Type U) (Sato et al. 1992). The ZnO crystallite facets imposes diffusive light scattering to the incident sun light, thus enhances the optical absorption with minimal absorbing layer. Further light trapping for long-wavelength light is also achieved in the Asahi high haze, HU-TCO glass, where the TCO surface has two types of textures of different characteristic length thus scattering different portions of light (Kambe et al. 2009).

The back TCO is typically AZO (c.f., Table 1), which is grown in-line with other in-house process steps. In addition, AZO provides excellent long-term stability as the back contact material. Since the back contact is grown on top of finished Si p-i-n stack, the substrate temperature should be kept low (<300 C) to prevent the dopant diffusion from the a-Si/µc-Si n layers. The back AZO contact is typically grown by RF sputtering (Beyer et al. 2007) or low pressure (LP)-CVD (Meier et al. 2010) at temperatures < 150 C.

### **4.1.3 Laser scribing**

A big advantage of the superstrate type a-Si/µc-Si thin film solar panels lies in the monolithically integrated structure, which greatly reduces operational cost and increases production throughput and panel yield by eliminating connection of wafers in the fabrication of crystalline Si PV panels. The thin film panel is scribed into numerous small cells, which are interconnected in series for a high voltage output, which at the same time improves panel yield and lowers the resistive energy loss. The monolithically integrated series connection is realized by a three step patterning process that selectively removes the individual layers, i.e., TCO front contact, thin-film silicon layer stack, and back contact of the solar cell. Highly automated laser scribing patterning is widely used for all patterning steps (c.f., Fig. 5) as it provides precise positioning, high throughput and minimum area losses.

Scribing thin film layers into sections with laser has developed a mature technology applied to large-area superstrate-type module fabrication, where laser beam incidents through the glass substrate. As depicted in Fig. 9, laser scribing mechanism can be described as a fourstep process (Shinohara et al. 2006).

1. Absorption of laser beam. By choosing appropriate laser energy/wavelength, the layer to be scribed absorbs the laser energy following Lambert's law, with more heat created

The TCO layers are typically grown by either CVD or PVD on industrial size substrates. Used as the front TCO layer, SnO2:F layer are normally grown by CVD using SnCl4 precursor gas on glass substrates (Rath et al. 2010). The finished SnO2:F-coated glass is provided as substrates for later PECVD deposition of a-Si/µc-Si layers. Another front TCO candidate is AZO, which is usually grown by CVD processes using precursors like Zn(C2H5)2 or sputter deposition (Agashe et al. 2004). Substrate temperatures are near 150 C

For many solar panel manufactures, control of the front TCO properties, especially the surface roughness, can be financially unfeasible, thus purchasing glass substrate pre-coated with TCO becomes a good choice. For a-Si absorber layers a high transparency for visible light (wavelength = 400–750 nm) is sufficient, while for solar cells incorporating µc-Si the TCO has to be highly transparent up to the near infrared (NIR) region (400–1100 nm) to accommodate for the wider absorption spectrum of µc-Si. This imposes certain restrictions on the carrier density, *n*, of the TCO material, as increased free carrier absorption leads to a reduction of IR transmission (Agashe et al. 2004). SnO2:F films fulfilling these requirements to a large extent, have been developed by Asahi Glass (Asahi Type U) (Sato et al. 1992). The ZnO crystallite facets imposes diffusive light scattering to the incident sun light, thus enhances the optical absorption with minimal absorbing layer. Further light trapping for long-wavelength light is also achieved in the Asahi high haze, HU-TCO glass, where the TCO surface has two types of textures of different characteristic length thus scattering

The back TCO is typically AZO (c.f., Table 1), which is grown in-line with other in-house process steps. In addition, AZO provides excellent long-term stability as the back contact material. Since the back contact is grown on top of finished Si p-i-n stack, the substrate temperature should be kept low (<300 C) to prevent the dopant diffusion from the a-Si/µc-Si n layers. The back AZO contact is typically grown by RF sputtering (Beyer et al. 2007) or

A big advantage of the superstrate type a-Si/µc-Si thin film solar panels lies in the monolithically integrated structure, which greatly reduces operational cost and increases production throughput and panel yield by eliminating connection of wafers in the fabrication of crystalline Si PV panels. The thin film panel is scribed into numerous small cells, which are interconnected in series for a high voltage output, which at the same time improves panel yield and lowers the resistive energy loss. The monolithically integrated series connection is realized by a three step patterning process that selectively removes the individual layers, i.e., TCO front contact, thin-film silicon layer stack, and back contact of the solar cell. Highly automated laser scribing patterning is widely used for all patterning steps (c.f., Fig. 5) as it provides precise positioning, high throughput and minimum area losses. Scribing thin film layers into sections with laser has developed a mature technology applied to large-area superstrate-type module fabrication, where laser beam incidents through the glass substrate. As depicted in Fig. 9, laser scribing mechanism can be described as a four-

1. Absorption of laser beam. By choosing appropriate laser energy/wavelength, the layer to be scribed absorbs the laser energy following Lambert's law, with more heat created

for the CVD deposition, and can go as high as 300 C for sputtering.

low pressure (LP)-CVD (Meier et al. 2010) at temperatures < 150 C.

different portions of light (Kambe et al. 2009).

**4.1.3 Laser scribing** 

step process (Shinohara et al. 2006).

**4.1.2 Growth of TCO layers** 

at the glass-side of the film (Fig. 9a). Typical laser energy is >1×106 W/cm2, and calculation shows more than 80% of that energy is absorbed and converted to heat building up in the film.


It is important to note that the laser scribing removal is not a true thermal process but the mechanical blasting off of the film. By applying different wavelengths of lasers, the laser energy is absorbed by different layers, thus selectively removes those layers without affecting other, underlying layers.

Fig. 9. Laser-scribing mechanism. (a) absorption of laser beam incidents through the glass, (b) decomposition of H from a-Si:H, (c) destruction of the photovoltaic layer and back electrode, (d) film blasted off and formation of heat affected zone (Shinohara et al. 2006).

Making up the interconnection of cell strips, the laser scribing pattern is decisive to the assembled panel performance. Since the total panel area is fixed, the width of cell strips determined by the laser scribing pattern is inversely proportional to the number of cell strips. Laser scribing pattern also affects the number of junctions and total dead area, which both contribute to losses in panel power output. Thus design of the laser scribing pattern is optimized with the width of strip cells and the sheet resistance of the front and back contacts. Precisely scribing fine lines that defines the monolithically integrated thin film solar module, laser scribing technology greatly enhanced the overall panel performance and improves the automation of the process flow. It is an important step in improving the module efficiency

After all film deposition and laser scribing steps conclude, the central PV active region is isolated from the panel edge to avoid electrical shock. In one way, the outside edge of the entire film stack is removed by 10-20 mm of width, called edge deletion. This is typically

To burn out the defects and improve panel yield, the final FEOL step involves removing cell shunts by reverse biasing the cells, or shunt busting. Shunting in Si thin film solar cells refers to high leakage current in reverse bias, which leads to a loss of power and efficiency. In large scale deposition, pinholes or locally thinner Si layer could form, which allow a connection between the top and bottom contacts, forming partially shorted PV diodes. When applying a reverse bias, larger current is focused at these shunt regions, resulting in local heat generation and consequent burning out of the low resistance pathway. Microscopic observation confirms the change of film morphology and its connection to the

As all cells are readily formed at this stage, electrical and optical inspection of individual cell strips are taken after the shunt busting for quality assurance purposes. This completes the

Panels fabricated at FEOL have to be further shaped and encapsulated to complete the solar panel module at the BEOL steps. Though no more film is deposited in the BEOL steps, these

If the module size is smaller than the substrate, glass with deposited film is first scored and broken into the final panel size, and goes through edge deletion. Then the panel is

According to the laser scribing layout, the two terminal segments of the series connected cell strips are each soldered to a bus line. These two terminal segments serve as the beginning and ending of the series connection of all cell strips on the panel. The cross bus bars are then attached to the terminal bus line and leaves out the final electrical connection to the external

To stand for extreme weather conditions in field usage, the functional films, i.e., TCO layers, a-Si/µc-Si films, metal coatings, and bus lines need good encapsulation to achieve

are important processes to ensure high quality solar panel production.

thoroughly washed for another time and ready for final bus line soldering.

and driving down the module cost independent of the film deposition processes.

done by mechanical grinding or laser scribing (same as P2 laser).

curing of the solar cells (Johnson et al. 2003).

FEOL processing of the solar panel.

circuit.

**4.2.2 Module encapsulation** 

**4.2 Back end of line (BEOL) process** 

**4.2.1 Module fabrication and bus line wiring** 

**4.1.4 Rest of FEOL steps** 

Combination of several laser-scribed layers is used to create interconnection in Si thin-film modules (Fig. 10a). The cell strips are defined by selective ablation of individual layer stacks, and the interconnection between neighboring strip cells are provided by the overlap of conductive layers. In the microscope view of a typical interconnection area (Fig. 10b), P1 is the first laser scribing step that cuts through the front TCO layer, P2 is the second scribing step that cuts through the p-i-n junction layers, P3 is the last step that cuts through the junction layers and the back reflector. The dead-area, i.e., the narrow area between P1 and P3 lines including the HAZ, makes up the interconnection junction but doesn't contribute to photocurrent generation. State-of-the-art laser process can limit the interconnection width to < 350 µm to minimize the dead-area. For a-Si/µc-Si module production, the scribing laser is typically powerful Nd:YVO4 solid-state laser with primary emission at 1064 nm and second harmonic generation at 532 nm. P1 is scribed by the 1064 nm irradiation, in which the strong absorption in TCO results in intensive local heating and explosive TCO evaporation (ablation); the glass that doesn't absorb in this wavelength keeps cool and is free from damage. P2 and P3 are similarly scribed by the 532 nm irradiation. As shown in Fig. 10c, the P3 laser cuts abrupt edges on the a-Si film without leaving any observable damage to the underlying TCO layer. The three laser scribing steps combining the subsequent film deposition steps form differences in the depths of different layers and conductive channels, forming the interconnection region of the cell strips' series connection. Power optimized, high-speed laser scribing technique is already applied in making 5.7 m2 solar panels with exceptional performance (Borrajo et al. 2009).

Fig. 10. (a) Schematic cross-sectional view of Si thin film solar panel showing the sectioned film and laser scribing lines (P1, P2 and P3). (b) Optical microscope image of the laser scribed lines. (c) Scanning electron microscope image of P3 scribed PV layer (Shinohara et al. 2006).

Combination of several laser-scribed layers is used to create interconnection in Si thin-film modules (Fig. 10a). The cell strips are defined by selective ablation of individual layer stacks, and the interconnection between neighboring strip cells are provided by the overlap of conductive layers. In the microscope view of a typical interconnection area (Fig. 10b), P1 is the first laser scribing step that cuts through the front TCO layer, P2 is the second scribing step that cuts through the p-i-n junction layers, P3 is the last step that cuts through the junction layers and the back reflector. The dead-area, i.e., the narrow area between P1 and P3 lines including the HAZ, makes up the interconnection junction but doesn't contribute to photocurrent generation. State-of-the-art laser process can limit the interconnection width to < 350 µm to minimize the dead-area. For a-Si/µc-Si module production, the scribing laser is typically powerful Nd:YVO4 solid-state laser with primary emission at 1064 nm and second harmonic generation at 532 nm. P1 is scribed by the 1064 nm irradiation, in which the strong absorption in TCO results in intensive local heating and explosive TCO evaporation (ablation); the glass that doesn't absorb in this wavelength keeps cool and is free from damage. P2 and P3 are similarly scribed by the 532 nm irradiation. As shown in Fig. 10c, the P3 laser cuts abrupt edges on the a-Si film without leaving any observable damage to the underlying TCO layer. The three laser scribing steps combining the subsequent film deposition steps form differences in the depths of different layers and conductive channels, forming the interconnection region of the cell strips' series connection. Power optimized, high-speed laser scribing technique is already applied in making 5.7 m2 solar panels with

P1

50 µm

Fig. 10. (a) Schematic cross-sectional view of Si thin film solar panel showing the sectioned film and laser scribing lines (P1, P2 and P3). (b) Optical microscope image of the laser scribed lines.

(c) Scanning electron microscope image of P3 scribed PV layer (Shinohara et al. 2006).

(c)

Dead

Active area area

P2

Dead area

P3 P2 P1

exceptional performance (Borrajo et al. 2009).

(a)

Back contact P3

Glass

p-i-n junction layers Front contact

(b)

Making up the interconnection of cell strips, the laser scribing pattern is decisive to the assembled panel performance. Since the total panel area is fixed, the width of cell strips determined by the laser scribing pattern is inversely proportional to the number of cell strips. Laser scribing pattern also affects the number of junctions and total dead area, which both contribute to losses in panel power output. Thus design of the laser scribing pattern is optimized with the width of strip cells and the sheet resistance of the front and back contacts. Precisely scribing fine lines that defines the monolithically integrated thin film solar module, laser scribing technology greatly enhanced the overall panel performance and improves the automation of the process flow. It is an important step in improving the module efficiency and driving down the module cost independent of the film deposition processes.

### **4.1.4 Rest of FEOL steps**

After all film deposition and laser scribing steps conclude, the central PV active region is isolated from the panel edge to avoid electrical shock. In one way, the outside edge of the entire film stack is removed by 10-20 mm of width, called edge deletion. This is typically done by mechanical grinding or laser scribing (same as P2 laser).

To burn out the defects and improve panel yield, the final FEOL step involves removing cell shunts by reverse biasing the cells, or shunt busting. Shunting in Si thin film solar cells refers to high leakage current in reverse bias, which leads to a loss of power and efficiency. In large scale deposition, pinholes or locally thinner Si layer could form, which allow a connection between the top and bottom contacts, forming partially shorted PV diodes. When applying a reverse bias, larger current is focused at these shunt regions, resulting in local heat generation and consequent burning out of the low resistance pathway. Microscopic observation confirms the change of film morphology and its connection to the curing of the solar cells (Johnson et al. 2003).

As all cells are readily formed at this stage, electrical and optical inspection of individual cell strips are taken after the shunt busting for quality assurance purposes. This completes the FEOL processing of the solar panel.

#### **4.2 Back end of line (BEOL) process**

Panels fabricated at FEOL have to be further shaped and encapsulated to complete the solar panel module at the BEOL steps. Though no more film is deposited in the BEOL steps, these are important processes to ensure high quality solar panel production.

#### **4.2.1 Module fabrication and bus line wiring**

If the module size is smaller than the substrate, glass with deposited film is first scored and broken into the final panel size, and goes through edge deletion. Then the panel is thoroughly washed for another time and ready for final bus line soldering.

According to the laser scribing layout, the two terminal segments of the series connected cell strips are each soldered to a bus line. These two terminal segments serve as the beginning and ending of the series connection of all cell strips on the panel. The cross bus bars are then attached to the terminal bus line and leaves out the final electrical connection to the external circuit.

#### **4.2.2 Module encapsulation**

To stand for extreme weather conditions in field usage, the functional films, i.e., TCO layers, a-Si/µc-Si films, metal coatings, and bus lines need good encapsulation to achieve

Schematics Fig. 12a Fig. 12b Fig. 12c

Examples Oerlikon Solar customers United Solar, ECD, Xunlight Applied Materials

System footprint 6 m × 8.6 m (KAI 1200) 6 m × 90 m Variable sizes

Substrate Glass, 1.1 m × 1.25 m Stainless steel roll,

Same equipment can be used for multiple depositions

Favors slow depositions that require long residence time.

Requires strict scheduling and control. Minimal energy integration.

products can be easily accommodated. Possible of making multiple solar panels with different

Tolerable to significant equipment fouling because cleaning / fixing of equipment is a standard operating procedure. Throughput can be affected when individual plasma-box fails in the process tower.

Table 2. Comparison of three thin film solar module process flow types

Continuous deposition of the multilayer structure is realized in a roll-to-roll manner, which ensures stable chamber conditions for consistent film growth for large volume production. United Solar, Energy Conversion Devices (ECD), and Xunlight took this type of growth configuration. For example, the ECD 30 MW a-Si process line consists of nine seriesconnected chambers with gas gates that isolate dopant gases between chambers (Fig. 11b)

Easily modified to produce different solar

cell structures

Product demand Changing demand for

structures.

Batch Process Continuous Process Hybrid Process

20 MW/yr 30 MW/yr 40-55 MW/yr

36 cm x 2.6 km

Moderate operational flexibility but often leads to inefficient capital use.

Recipe of the entire line is fixed. Equipments are optimized for minimal operating conditions

Slow depositions require large equipments and slow

Reduces fugitive energy losses by avoiding multiple heating and cooling cycles

Difficult to make changes as the process recipes are fixed

for the entire line.

Significant fouling in continuous operations is a serious problem and difficult to handle. Sometimes significant fouling requires shutting down of the entire production line.

process flow.

customers

Glass, 2.2 m × 2.6 m

Easily modified to produce different solar cell structures.

Slow process is shared by parallel chambers for high throughput.

Changing demand for products can be easily accommodated. Possible of making multiple solar panels with different structures at the same time.

Fouling chamber can be by-passed or replaced with similar chambers, thus minimizing the adverse effect to the throughput.

Scheduling and synchronization of chambers are optimized by artificial intelligence.

Same equipment can be used for multiple depositions

Process flow types

Production volume

Operational flexibility

Standardized equipment

Rate of deposition

affects throughput

Processing efficiency

Equipment fouling

for long panel lifetime. The most common encapsulation method for panels with the glass substrate is to use another piece of glass to cover the functional films. The gap between the two glass plates is filled with an epoxy (ethylene vinyl acetate, EVA, or polyvinyl butyral, PVB) film, which not only insulates the functional films against reactants like oxygen and moisture, but also mechanically strengthens the rigidness of the finished panel. Quality of the module encapsulation is directly associated with the failures of panels in the field. Judgment of the encapsulation properties includes low-interface conductivity, adequate adhesion of encapsulants to glass as a function of in-service exposure conditions, and low moisture permeation at all operation temperatures (Jorgensen et al. 2006).

The panel then passes through a laminator where a combination of heated nip rollers removes the air and seals the edges. The lamination film at the same time provides electrical insulation against any electric shock hazard. At the exit of the laminator conveyer, the modules are collected and stacked together on a rack for batch processing through the autoclave where they are subjected to an anneal/pressure cycle to remove the residual air and completely cure the epoxy. Finally, a junction box is attached to the cross bus wire and sealed on top of the hole of the back glass and is filled with the pottant to achieve a complete module integrity.

The fully processed module is then tested for output power, *I*SC, *V*OC, and other characteristics under a solar simulator. Then it is labeled, glued to the supporting bars, and packaged. At this point, the full panel assembly is finished.
