Category Handbook of the Physics ofThin-Film Solar Cells

Hydrogen Dilution

Hydrogen dilution of the silane gas mixture during a-Si deposition has been found to reduce the density of defect states and improve the stability of the material against light-induced degradation. Solar cells with i – layers deposited using high H2 dilution ratios showed improved performance and stability (Guha et al. 1981; Okamoto et al. 1996).

For higher dilutions, the growing thin film initially adopts an amorphous struc­ture, called the “protocrystalline” regime. As the film thickens, crystallites form in the amorphous matrix (creating a “mixed phase”). The crystallites improve the sta­bility (Kamei et al. 1999). Ultimately, the film becomes entirely nano-crystalline.

Due to their lower absorption with an indirect band-gap ~1...

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Other Deposition Methods

Other deposition processes have been explored for a-Si films. Here only those are listed for which solar cell results were reported. These include

(1) reactive sputter deposition from silicon targets using a mixture of hydrogen and argon (Moustakas et al. 1985);

(2) photo-CVD using ultraviolet excitation and mercury sensitization (Konagai et al. 1987; Rocheleau et al. 1987a, 1987b);

(3) remote plasma chemical vapor deposition (Park et al. 1988);

(4) electron cyclotron resonance (ECR) microwave deposition (Sakamoto 1977; Dalal et al. 1997); and

(5) gas jet deposition (Jones et al. 2000).

These depositions yielded worse a-Si films or solar cells, or they could not be easily scaled to provide large-area uniform films, and therefore, are not used in large-scale a-Si PV production.

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Hot-Wire Chemical Vapor Deposition

With tn improved hot-wire chemical vapor deposition (HWCVD), “device qual­ity” a-Si films were produced (Matsumura 1986; Wiesmann et al. 1979; Mahan et al. 1991). The set-up for a HWCVD system is similar to RF-PECVD except that the RF electrode is replaced with a Pt, W, or Ta filament that is heated to around 1800-2000 °C. In the HW process, SiH4 gas or a mixture of SiH4 and other gases such as H2 or He are used. The silicon radicals then diffuse and deposit onto a substrate placed a few centimeters away and heated to 150-450 °C. Mahan et al. (1991) demonstrated that HWCVD a-Si materials show relatively lower H content in the film and improved stability against light-induced degradation compared with RF PECVD films (Mahan et al. 1991)...

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PECVD at Different Frequencies

Higher excitation frequencies (f = 40-100 MHz) permit the deposition of a-Si and nc-Si films that are more stable at high deposition rates (> 10 A/s) without creating polyhydride powder. In contrast, film and device quality, and stability suffer when deposition rates are larger than >3 A/s by increasing RF power at 13.56 MHz (Sha – farman et al. 2003). High-quality devices have been obtained using VHF deposition by a number of groups (Smets et al. 2008; Takatsuka et al. 2004; Fujioka et al. 2006).

With increasing plasma frequency, the electron energy decreases and its den­sity increases (Wertheimer and Moisan 1985)...

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RF Plasma-Enhanced Chemical Vapor Deposition (RF-PECVD) at 13.56 MHz

The growth of a-Si and nc-Si by PECVD is determined by electron density and energy distribution in the plasma, gas phase reaction chemistry, precursor transport to the growth surface, and surface reactions.

A mixture of SiH4 and H2 is adjusted into a chamber with power from an RF supply. The gas pressure is adjusted for the given RF voltage to initiate the plasma, that ionizes and decomposes the gas. The a-Si:H film grows on a substrate that may be mounted on one or both of the electrodes that are heated to 150-300 °C.

Thermally activated surface diffusion of ad-atoms is used for optimum film qual­ity (Chapman 1980; Luft and Tsuo 1993; Kushner 1988). At lower substrate tem­perature, more H is incorporated that increases the band gap of a-Si:H slightly...

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Depositing Amorphous Silicon

41.5.1 Deposition Techniques

The first preparations of a-Si:H by Chittick and Sterling (1985) and by Spear and LeComber (Spear and LeComber 1972), using a silane-based glow discharge at ra­dio frequencies (RF) is often called plasma enhanced chemical vapor deposition (PECVD). Other deposition methods are using higher frequencies: RF-PECVD with 13.56-MHz that is widely used today. However, emerging film deposition methods for higher deposition rates or to making improved nc-Si:H, have been extensively explored in recent years. These are PECVD with very high frequency (VHF) and hot-wire (HW) catalytic deposition as described below.

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Nanocrystalline Silicon

The same deposition processes that are used to make amorphous silicon can also be used to make hydrogenated “nano-crystalline” silicon (nc-Si:H). Here fine silicon crystallites with a diameter of several nanometers are bound with a hydrogenated amorphous silicon batter. This nano-crystalline silicon is also referred to as “micro­crystalline” silicon for many years.

Below 1.6 eV, nano-crystalline silicon (nc-Si:H) has a stronger optical absorption than a-Si:H, and is similar to c-Si. This makes nc-Si:H interesting as the infrared absorber in multi-junction solar cells, or tandem cells that are sometimes called “micromorph” (Curtins et al. 1987). The Staebler-Wronski degradation is much weaker in nc-Si:H than in a-Si:H, despite the presence of a fraction of a-Si:H.

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Alloying and Optical Properties

The structural and optical properties of a -Si:H can be varied substantially by chang­ing its deposition, such as changing the substrate temperature or the dilution of silane by hydrogen in a plasma deposition. These changes in the microstructure of a-Si:H can cause a change of the optical band-gap between 1.6-1.8 eV (Hama et al. 1983).

Larger band-gap changes can be produced by alloying with Ge, C, O, and N. This is readily accomplished by mixing the silane (SiH4) gas with GeH4, CH4, O2, NO2, or NH3. The resulting alloys have very wide ranges of band-gaps. For example an a-Si1-xGex :H can have on optical band-gaps down to about 1...

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