Advanced Amorphous Silicon(2)

1.Increase of the conversion ef?ciency of a-Si:H solar cells.Based on fundamental considera-

tions,major performance improvement is expected in the near future from an increase in the current of thin?lm silicon solar cells[23].This increase has to result from improved light management schemes such as light trapping and reduction of light absorption losses.For solar cells deposited on glass plates,also called superstrate type cells,the development of a TCO front electrode material with an optimal surface morphology that results in improved light scattering properties is essential.Essential for solar cells deposited on(?exible)opaque carriers,often denoted as substrate type solar cells,is improvement of the texturing and re?ectivity of the back contact.Ongoing attention has to be paid to further improvement of the optoelectronic quality of a-Si:H and a-SiGe:H absorbers,the doped layers and the interfaces between the doped layers and intrinsic absorbers.

176THIN FILM SOLAR CELLS

2.Elimination of the light induced degradation known as the Staebler–Wronski effect[24].

This effect is responsible for a decrease in the initial performance of an a-Si:H solar module of typically15–30%.Full understanding of the Staebler–Wronski effect in a-Si:H based materials is necessary for fabricating a-Si:H with improved stability against light exposure.

The light induced degradation of the modules can be suppressed by using thin absorber layers.However,the use of the thin absorbers strongly depends on the implementation of ef?cient light trapping techniques in the solar cells,which have to provide for suf?cient absorption in these layers.

3.The deposition rate of absorber layers is required to be10to20?A/s in order to limit

the investment in the a-Si:H deposition machine,which is strongly re?ected in the cost of the modules.The central question regarding the deposition rate is how to avoid the increased light induced degradation of?lms deposited at elevated deposition rates[25].

In addition to the radio frquency(rf)PECVD technique,several deposition techniques are being intensively investigated capable of fabricating absorber layers with suf?cient quality at high deposition rates,such as very high frequency PECVD,hot wire CVD,and expanding thermal plasma CVD.

4.The choice of mass production technology.Although the deposition of a-Si:H based layers

is the most important part of solar cell fabrication,complete production includes several fabrication steps which substantially contribute to the total cost of the solar module.These include the deposition of the TCO front electrode,the deposition of the multilayer back electrode,laser scribing for the subcell series connection,and encapsulation and framing.

The solar cell structure and module design determine the choice of sequence of fabrication steps and the deposition and processing techniques to be applied.At present,there are three major approaches to depositing a-Si:H based layers:the one chamber batch process,the multichamber process,and the roll-to-roll process.The advantages and disadvantages of particular a-Si:H production systems are discussed in reference[18].The general trend is to increase the substrate size,which reduces the cost per unit area,by lowering the relative contribution of the edges.The experience gained in the display industry regarding the deposition of a-Si:H on large area substrates is being transferred to solar cell production.

The general requirements for the production machines include suf?cient reliability of the deposition process,high production uptime,high yield and the right choice of procedure for cleaning the process chambers.

5.Lowering of material costs.The material costs contribute considerably to the overall cost

of a-Si:H modules.A substantial part is formed by the cost of the substrate carrier,the glass plate or high temperature resistant polymer foil.Therefore,cheaper thin metal foils that also allow the use of the continuous roll-to-roll technology are a preferable choice.In case of?exible modules,usually a relatively thick?uoropolymer type encapsulant is applied in order to guarantee a module lifetime of20years.The encapsulant dominates the cost of the module and the development of a cheap encapsulant is therefore one of the most important cost issues.The choice of substrate carrier determines the acceptable process temperatures and the sequence of processing steps.The choice of gasses for the deposition of a-Si:H based layers,their purity and the gas utilization also have?nancial consequences.For example, the use of germane for multijunction solar cells with a-SiGe:H absorbers can substantially increase the material costs.

ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES177 5.3HYDROGENATED AMORPHOUS SILICON

In order to understand the design and operation of a-Si:H based solar cells,which are different from those for crystalline silicon solar cells,we will summarize the structural and material properties of a-Si:H and compare them to those of single crystal silicon.Some widely used techniques to characterize a-Si:H are also described in this section.

5.3.1Atomic structure

Figure5.1illustrates the difference in atomic structure between single crystal silicon and a-Si:H.Figure5.1a shows the structure of single crystal silicon schematically.Each silicon atom is covalently bonded to four neighboring atoms.All bonds have the same length,and the angles between the bonds are equal.The number of bonds of an atom with its immediate neighbors in the atomic structure is called the coordination number or coordination.Thus,in single crystal silicon,the coordination number for all silicon atoms is four;we can also say that silicon atoms are fourfold coordinated.A unit cell can be de?ned,from which the crystal lattice can be reproduced by duplicating the unit cell and stacking the duplicates next to each other.Such a regular atomic arrangement is described as a structure with a long range order.

Figure1b illustrates that a-Si:H does not exhibit a structural order over a long range. Nevertheless,there is a similarity in atomic con?guration on a local atomic scale,where most silicon atoms have covalent bonds with four neighbors.Though a-Si:H lacks the long range order,it has the same short range order as single crystal silicon.This conclusion about the bonding structure in a-Si:H has been obtained from X-ray diffraction measurements[26].The small deviations in bonding angles and bonding lengths between the neighboring atoms in a-Si:H lead to a complete loss of the locally ordered structure on a scale exceeding a few atomic distances.The resulting atomic structure of a-Si:H is called a continuous random network.

(a)(b)

unpassivated

dangling bond

Figure5.1Schematic representation of the atomic structure of(a)single crystal silicon,(b)hydro-genated amorphous silicon.

178THIN FILM SOLAR CELLS

Due to the short range order,the common semiconductor concept of the energy state bands, represented by the conduction and valence bands,can still be used in a-Si:H.

The larger deviations in bonding angles and bonding lengths between the neighboring atoms in a-Si:H result in so-called weak or strained bonds.When enough energy is available, for example in the form of heat,weak bonds can easily be broken.This process leads to the formation of defects in the atomic network.We note that in the continuous random network, the de?nition of a defect is modi?ed with respect to the crystal structure.In a crystal,any atom that is out of place in a lattice forms a defect.In the continuous random network,an atom cannot be out of place.Because the only speci?c structural feature of an atom in the continuous random network is the coordination to its neighbors,a defect in a-Si:H is a coordination defect [26].This happens when an atom has too many bonds or too few.In a-Si:H,defects are mainly silicon atoms that are covalently bonded to only three silicon atoms(threefold coordinated)and have one unpaired electron,a so-called dangling bond.Since this con?guration is the dominant defect in a-Si:H,the defects in a-Si:H are often related to dangling bonds.Some dangling bond defects are depicted in Figure5.1b.Another defect con?guration is a silicon atom bonded to ?ve silicon atoms(?vefold coordinated).This con?guration is referred to as a?oating bond.

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