**3. Basic thin film Si solar cell structure**

Typical a-Si single junction solar cells are composed of five principal layers: Si p-i-n diode sandwiched between two conductive layers. The front TCO forms the front contact, and the

Sharing similar cost percentage of the first three parts with crystalline Si PV systems, the much lower module cost gives thin film PV system lower overall cost and a higher development potential. An increase or decrease of the efficiency of the module implies an increment or a reduction of the BOS and installation costs, respectively. Nevertheless, the financing and inverter cost remain always the same. Therefore, the use of lower efficiency thin film modules are financially more favorable in those cases in which the value of the installed area is not relevant. Thin film panels are thus more applicable to the PV electricity power plants built in remote areas like deserts. Large volume production and deployment is

The cost of thin film modules, in turn is composed of five major components (Jäger-Waldau

1. Material cost (40%). The material consumption is determined by the film growth technology (e.g., PVD vs. CVD), and is also dominated by the module packaging and assembly technology. Special, TCO-coated glass substrates take a significant portion of the direct material cost (25-40%). Assuming similar technology used, the materials cost

2. Equipment related (capital) spending (20%). Initial investment on equipment on a-Si/µc-Si thin film panel manufactures is generally expensive. Upon fixed initial equipment investment, the annual depreciation of equipments is dominated by the deposition materials. The equipment depreciation rate is inversely proportional to the

3. Labor cost (15-17%). The layered, monolithically integrated panel structure minimizes human operation, and the highly automated production methods used in the state-ofthe art thin film PV panel manufactures reduce the labor cost. For a given total production volume, the labor cost is inversely proportional to the process throughput,

4. Energy consumption (15%). Modern PV manufactures use a significant amount of energy to run the factory, including machinery power consumption used for manipulating the substrate, controlling of substrate temperatures, RF power generators, film deposition system, vacuum system, exhaust handling, laser tool, lighting, air conditioning, etc. Once a factory is set up, a large amount of the overhead energy consumption is fixed, and the energy consumption per module is inversely

5. Freight (7-9%). The logistics of shipping and handling of the raw material as well as the assembled module panels take a larger portion of cost in thin film solar panels compared to their crystalline counter parts due to their greater size and weight. Unlike

As seen from the relationship of the thin film PV module cost structure summarized in Fig. 3, the process technology determines the direct material and energy consumption, equipment depreciation and ultimately the panel efficiency, which in turn affect the panel cost. In another word, more advanced module process technology leads to both higher panel efficiency and lower panel cost. Thus in this chapter, we put our focus on the process

Typical a-Si single junction solar cells are composed of five principal layers: Si p-i-n diode sandwiched between two conductive layers. The front TCO forms the front contact, and the

the key factor to fully demonstrate the financial benefit of thin film solar modules.

is inversely proportional to the production volume and panel efficiency.

process throughput and module efficiency.

proportional to the process throughput.

**3. Basic thin film Si solar cell structure** 

extent of automation, and production module efficiency.

the other factors, freight cost is relatively constant for each panel.

details of the manufacturing of modern, large-area a-Si/µc-Si solar panels.

2007):

Fig. 3. Relationship between fabrication process, cost structure, and production of thin film PV modules.

back TCO and the reflector form the back contact (Fig. 4). The Si p-i-n junction absorbs sun light and generates photocarriers, which are collected by the conductive, front and back contacts. The substrate (e.g., glass) provides mechanical support for all the layers. Stacking two a-Si/µc-Si cells on top of each other forms the tandem junction structure, which is also sandwiched between the front and back TCOs.

Depends on the type of substrates on which the films are grown, there are basically two kinds of cell structures. 1) "Substrate" structure, where none-transparent substrates, i.e., metal foils, are used for growing the film stack. Sun light enters the cell from the top of the film stack by going through the top TCO. 2) "Superstrate" structure, where transparent substrates like glass or plastic films are used. Sun light enters the cell through the transparent glass/plastics and the TCO layer. The growth order of the Si p-i-n diodes are reversed in the two structures. The monolithically integrated superstrate type solar cells have superb encapsulation and compatibility with conventional electrical and safety regulations, thus holding a dominant market share.

#### **3.1 PV active Si p-i-n layers**

The Si p-i-n junction is where the sun light is absorbed and converted to charge carriers, i.e., electrons and holes. Differs from crystalline Si (c-Si), a-Si for PV and other applications (e.g., thin film transistor, TFT) are actually hydrogenated amorphous silicon alloy (a-Si:H, here noted as a-Si for simplicity), in which the H atoms passivate the otherwise high-density Si dangling bonds in pure amorphous Si film that introduce trap states and severely affect the film electrical properties. Normally the H content can be as high as a few percent. The a-Si completely loses the periodical atomic lattice structure; instead, the Si atoms randomly

104 V/cm, comparable to or larger than the thickness of the solar cell film stack. As a result,

Photons absorbed in the heavily doped n- and p- layers, however, don't contribute to the photocurrent as there is no net electric field in the doped layers. As a result, the n- and players are usually less than 20 nm thick to limit photon absorption in these "window" layers. Further reduction of photon absorption in realized by increasing the band gaps of the n- and p-layers, e.g., doping the a-Si or µc-Si window layers with carbon so that they are transparent to the portion of the solar spectrum to be harvested in the i-layer. Total thickness of a typical single or tandem junction cell is less than 2 µm, which is only a few

Though not PV active, the front and back contact layers play important roles on the cell performance. Optical wise, the transparent TCO layers scatter the incident sun light and enhance the optical absorption inside the i-layer. Electrical wise, since the lateral conductance of thin, doped p/n silicon layers is insufficient to prevent resistive losses, the TCO contact layers conduct the photocurrent in the lateral direction to the panel bus lines. The TCO layers

For efficient material usage and fast film deposition, the a-Si/µc-Si absorbers are so thin that the incoming light will not be completely absorbed during one single pass for normal incident rays. Hence, for all absorber materials, optical absorption inside the silicon layers has to be enhanced by increasing the optical absorption path. The difference of index of refraction between the TCO layers and the Si layers, plus the rough interface induce diffusive refraction of incoming light at oblique angles, thus increasing the optical path of solar radiation (Fig. 4). This is typically done by nano-texturing the front TCO electrode to a typical root-mean-square (rms) surface roughness of 40–150 nm and/or nano-textured back reflectors. In the ideal case, these rough layers can introduce nearly completely diffusive

When applied at the front contact, TCO has to possess a high transparency in the spectral region where the solar cell is operating (transmittance > 90% in 350 – 1000 nm), strong scattering of the incoming light, and a high electrical conductivity (sheet resistance < 20 /sq.) (Fortunato et al. 2007). For the superstrate configuration where the Si layers are deposited onto a transparent substrate (e.g., glass) covered by TCO, it has to have at the same time favorable physicochemical properties for the growth of the silicon. For example, the TCO has to be inert to hydrogen-rich plasmas, and act as a good nucleation layer for the growth of the a-Si/µc-Si films. For all thin-film silicon solar cells, scattering at interfaces between neighboring layers with different refractive indices and subsequent trapping of the

TCO is also used between silicon and the metallic contact as a part of the back reflector to improve its optical properties and act as a dopant diffusion barrier. The back TCO layer also prevents reaction between the metal and the a-Si/µc-Si underlayers. Furthermore, applied in a-Si:H/µc-Si:H tandem solar cells, TCO can be used as an intermediate reflector between top and bottom cells to increase the current in the thin amorphous silicon top cell (Yamamoto et al. 2006). Finally, nano-rough TCO front contacts act as an efficient

used for thin film solar cells are doped wide band gap semiconducting oxides.

incident light within the silicon absorber layers is crucial to high efficiency.

antireflection coating due to the refractive index grading at the TCO/Si interface.

transmission or reflection of light (Müller et al. 2004).

the p-i-n type cells have efficient carrier collection efficiency.

percent thick of a c-Si cell.

**3.2 Front and back contacts** 

Fig. 4. Schematic single junction p-i-n a-Si solar cell. *n* stands for the index of refraction.

arrange in space. The lack of lattice structure makes a-Si a direct band gap semiconductor with a band gap of 1.8-1.9 eV at room temperature.

Hydrogenated microcrystalline Si (µc-Si:H, noted as µc-Si for simplicity) has a more complex, phase-mixed structure that consists of the crystalline phase made of silicon nanocrystallites and the amorphous Si matrix. The nanocrystallites grow into conglomerate clusters perpendicular to the film surface, whose diameters are typically between 10 and 50 nm. Embedded in amorphous silicon, the conglomerates are separated by a-Si, grain boundaries and micro-cracks. The band gap of µc-Si is 1.11 eV at room temperature, roughly the same as crystalline Si.

Photon absorption is proportional to the wavelength-dependent absorption coefficient, , of the film. For typical a-Si and µc-Si, is between 102 and 105 cm-1 in the visible range (Shah et al. 2004), which is 10-50 times larger than that of c-Si. Large naturally allows for thinner absorber in solar cells. In the a-Si/µc-Si i-layer, an absorbed photon excites an electron from the valance band to the conduction band, creating a free electron and leaving a hole in the valance band. Due to the amorphous nature of a-Si and µc-Si films, the electrons and holes haves limited diffusion length and short life time. Electronic carrier transport properties are normally characterized by the mobility × lifetime product (µ-product), which is the physical characteristic of both carrier drift and diffusion processes. The measured products of the electron mobility and lifetime, µ00, is 2×10-8 cm2/Vs for a-Si and 1×10-7 cm2/Vs for µc-Si, respectively, much lower than those measured in c-Si wafers (Beck et al. 1996; Droz et al. 2000). The low µ-product in a-Si or µc-Si makes the p-n diode configuration that is widely used in c-Si solar cells unsuitable with these materials, as the photocarrier collection in a p-n diode is diffusion limited. To avoid electron and hole recombination, p-i-n junction is used, where the built-in field drifts electrons towards the n-layer and holes towards the p-layer. The measured electron diffusion length is 2 µm in a-Si and 10 µm in µc-Si under the filed of

n 2 ZnO

Fig. 4. Schematic single junction p-i-n a-Si solar cell. *n* stands for the index of refraction.

Photon absorption is proportional to the wavelength-dependent absorption coefficient,

absorber in solar cells. In the a-Si/µc-Si i-layer, an absorbed photon excites an electron from the valance band to the conduction band, creating a free electron and leaving a hole in the valance band. Due to the amorphous nature of a-Si and µc-Si films, the electrons and holes haves limited diffusion length and short life time. Electronic carrier transport properties are normally characterized by the mobility × lifetime product (µ-product), which is the physical characteristic of both carrier drift and diffusion processes. The measured products of the electron mobility and lifetime, µ00, is 2×10-8 cm2/Vs for a-Si and 1×10-7 cm2/Vs for µc-Si, respectively, much lower than those measured in c-Si wafers (Beck et al. 1996; Droz et al. 2000). The low µ-product in a-Si or µc-Si makes the p-n diode configuration that is widely used in c-Si solar cells unsuitable with these materials, as the photocarrier collection in a p-n diode is diffusion limited. To avoid electron and hole recombination, p-i-n junction is used, where the built-in field drifts electrons towards the n-layer and holes towards the p-layer. The measured electron diffusion length is 2 µm in a-Si and 10 µm in µc-Si under the filed of

al. 2004), which is 10-50 times larger than that of c-Si. Large

arrange in space. The lack of lattice structure makes a-Si a direct band gap semiconductor

Hydrogenated microcrystalline Si (µc-Si:H, noted as µc-Si for simplicity) has a more complex, phase-mixed structure that consists of the crystalline phase made of silicon nanocrystallites and the amorphous Si matrix. The nanocrystallites grow into conglomerate clusters perpendicular to the film surface, whose diameters are typically between 10 and 50 nm. Embedded in amorphous silicon, the conglomerates are separated by a-Si, grain boundaries and micro-cracks. The band gap of µc-Si is 1.11 eV at room temperature, roughly

Back reflector

n-Si

i-Si

p-Si TCO

Glass

is between 102 and 105 cm-1 in the visible range (Shah et

, of

naturally allows for thinner

n 1.5

with a band gap of 1.8-1.9 eV at room temperature.

the same as crystalline Si.

the film. For typical a-Si and µc-Si,

n 2

n 4

104 V/cm, comparable to or larger than the thickness of the solar cell film stack. As a result, the p-i-n type cells have efficient carrier collection efficiency.

Photons absorbed in the heavily doped n- and p- layers, however, don't contribute to the photocurrent as there is no net electric field in the doped layers. As a result, the n- and players are usually less than 20 nm thick to limit photon absorption in these "window" layers. Further reduction of photon absorption in realized by increasing the band gaps of the n- and p-layers, e.g., doping the a-Si or µc-Si window layers with carbon so that they are transparent to the portion of the solar spectrum to be harvested in the i-layer. Total thickness of a typical single or tandem junction cell is less than 2 µm, which is only a few percent thick of a c-Si cell.

#### **3.2 Front and back contacts**

Though not PV active, the front and back contact layers play important roles on the cell performance. Optical wise, the transparent TCO layers scatter the incident sun light and enhance the optical absorption inside the i-layer. Electrical wise, since the lateral conductance of thin, doped p/n silicon layers is insufficient to prevent resistive losses, the TCO contact layers conduct the photocurrent in the lateral direction to the panel bus lines. The TCO layers used for thin film solar cells are doped wide band gap semiconducting oxides.

For efficient material usage and fast film deposition, the a-Si/µc-Si absorbers are so thin that the incoming light will not be completely absorbed during one single pass for normal incident rays. Hence, for all absorber materials, optical absorption inside the silicon layers has to be enhanced by increasing the optical absorption path. The difference of index of refraction between the TCO layers and the Si layers, plus the rough interface induce diffusive refraction of incoming light at oblique angles, thus increasing the optical path of solar radiation (Fig. 4). This is typically done by nano-texturing the front TCO electrode to a typical root-mean-square (rms) surface roughness of 40–150 nm and/or nano-textured back reflectors. In the ideal case, these rough layers can introduce nearly completely diffusive transmission or reflection of light (Müller et al. 2004).

When applied at the front contact, TCO has to possess a high transparency in the spectral region where the solar cell is operating (transmittance > 90% in 350 – 1000 nm), strong scattering of the incoming light, and a high electrical conductivity (sheet resistance < 20 /sq.) (Fortunato et al. 2007). For the superstrate configuration where the Si layers are deposited onto a transparent substrate (e.g., glass) covered by TCO, it has to have at the same time favorable physicochemical properties for the growth of the silicon. For example, the TCO has to be inert to hydrogen-rich plasmas, and act as a good nucleation layer for the growth of the a-Si/µc-Si films. For all thin-film silicon solar cells, scattering at interfaces between neighboring layers with different refractive indices and subsequent trapping of the incident light within the silicon absorber layers is crucial to high efficiency.

TCO is also used between silicon and the metallic contact as a part of the back reflector to improve its optical properties and act as a dopant diffusion barrier. The back TCO layer also prevents reaction between the metal and the a-Si/µc-Si underlayers. Furthermore, applied in a-Si:H/µc-Si:H tandem solar cells, TCO can be used as an intermediate reflector between top and bottom cells to increase the current in the thin amorphous silicon top cell (Yamamoto et al. 2006). Finally, nano-rough TCO front contacts act as an efficient antireflection coating due to the refractive index grading at the TCO/Si interface.

As previously discussed (c.f., Section 2), manufacturing of the solar panels directly determines the cost of modules, which takes 40-66% of the overall PV system cost. This section focuses on the factory panel production, and addresses various methods of panel

> (Optional) Panel cut & break

seaming, edge deletion and washing

Bus wire attachment

Back glass loading

Back glass washing

Epoxy film cut and lay-up

Lamination

Autoclave curing

Junction box attachment

Rail bonding

Final quality inspection & binning

Fig. 5. Flow diagram of typical thin film solar panel production, comprising of both the front end of line (FEOL) and back end of line (BEOL) technologies and processes (Bhan et al.

Development in the a-Si and µc-Si thin film process technology combined with the booming PV market resulted in the fast expansion of a-Si/µc-Si based solar panel manufacturing after 2007. This industry largely benefits from the lab demonstration of thin film solar cells on small size substrates, as well as the large-area thin film deposition techniques developed for thin-film transistor liquid crystal display (TFT-LCD) industry. The growth of high-quality Si thin films for PV applications shares many of the skill sets required for growing Si TFT films, and using similar large-area thin film deposition chambers (Yang et al. 2007). In fact, both thin film solar "turnkey" equipment providers, Oerlikon and Applied Materials, have been manufacturing large-scale TFT-LCD deposition systems for years before becoming thin

**4. Factory panel production** 

TCO glass loading

TCO glass seaming and washing

P1 laser scribing of TCO, and post-washing

Deposition of Si P-I-N stack by PECVD

P2 laser scribing of Si film

Back contact deposition

P3 laser scribing of back contact

Quality assurance & shunt removal

film solar equipment providers.

2010).

efficiency improvement and cost reduction.

FEOL BEOL

The front and back TCO layers are at the same time electrodes that collect photogenerated carriers. As a semiconductor, the optical transparency and the electrical conductivity are closely related to the band gap structure of the TCO. The short-wavelength cutoff of the transmission spectrum corresponds to the oxide band gap, whereas the long-wavelength transmission edge corresponds to the free carrier plasma resonance frequency. On the other hand, electron conduction in TCO is achieved by degenerate doping that increases the free carrier density and moves the Fermi level into the conduction band. High carrier density and carrier mobility are thus required for TCO layers. There is, however, a tradeoff between high optical transmittance and low electrical resistance. Increasing electron carrier density decreases resistivity but also increases the plasma oscillation frequency of free carriers, thus shifting the IR absorption edge towards the visible. The transmission window is thus narrowed as a result of improved conductivity.


Table 1. Different TCOs employed in Si thin film solar cells. RF, radio frequency. LPCVD, low-pressure chemical vapor deposition. APCVD, atmosphere-pressure CVD.

The most-widely used TCOs in Si thin film solar cells are doped SnO2 (i.e., SnO2:F) and ZnO (i.e., ZnO:Al) due to their temperature and chemical stabilities. Compared to the more conductive alternative indium-tin-oxide (ITO), they offer a much lower cost by avoiding the use of the costly In. At the same time, surface roughness induced by the crystalline texture of SnO2 and ZnO is widely applied for increasing the optical absorption. These three typical TCOs are compared in Table 1.

The reflector layer on top of the back TCO can be Ag, Al, or white paint in a "superstrate" cell, and is the metal foil itself or another Ag/Al coating on the foil in a 'substrate' type cell. Ag is typically used in laboratory research work, while Al is more often used in mass production modules due to its lower cost and better properties in removing module shunts. Back contacts of Oerlikon's thin film panels, on the other hand, use proprietary white paint as the reflector (Meier et al. 2005). The white paint can be rolled on or screen printed directly onto the TCO. At the same time, it offers the following advantages (Berger et al. 2007): 1) high optical reflectance over a broad wavelength band, 2) optimal light scattering pattern which is generally beneficial for solar cells because this maximizes the fraction of photons that are trapped inside the solar cell due to total internal reflection at both cell surfaces, 3) pigmented materials have the potential of low cost. In certain instances, the white paint is a better surface reflector than Al, or TCO/Al reflector.
