The advantages and limitations of photovoltaic solar modules for energy generation are reviewed with their operation principles and physical efficiency limits. Although the main materials currently used or investigated and the associated fabrication technologies are individually described, emphasis is on silicon-based solar cells. Wafer-based crystalline silicon solar modules dominate in terms of production, but amorphous silicon solar cells have the potential to undercut costs owing, for example, to the roll-to-roll production possibilities for modules. Recent developments suggest that thin-film crystalline silicon (especially microcrystalline silicon) is becoming a prime candidate for future photovoltaics.

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Photovoltaic Technology: The Case for Thin-Film Solar Cells

The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon c-Si surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon a-Si:H on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by a the minimization of the density of recombination centers and b the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV.

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Model for a-Si: H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

This chapter covers the current use and challenges of thin-film silicon solar cells, including conductivities and doping, the properties of microcrystalline silicon (the role of the internal electric field, shunts, series resistance problems, light trapping), tandem and multijunction solar cells (a-Si:H/a-Si:H tandems, triple-junction amorphous cells, microcrystalline–amorphous or “micromorph” tandems), module production (deposition of thin-film layers, substrate materials, laser scribing, cell interconnection, and module encapsulation), and module performance.

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Thin-Film Silicon Solar Cells

A plasma CVD process comprises conducting film formation in a reaction chamber capable of being substantially vacuumed in which a plurality of cylindrical substrates are spacedly arranged on a concentric circle in said reaction chamber such that a desired discharge space is formed at the central position of the inside of said reaction chamber and a cathode electrode is disposed at the central position of said discharge space, by introducing a film-forming gas into said discharge space and applying a high frequency power from a high frequency power source to said cathode electrode to produce plasma between said plurality of cylindrical substrates and said cathode electrode, whereby forming a deposited film on the surface of each of said plurality of cylindrical substrates, characterized in that an earth shield comprising a non-magnetic material and a soft magnetic material or an insulating material being stacked is disposed at each of the opposite end portions of said cathode electrode, and a very-high-frequency energy of a frequency range of 60 MHz or more from said high frequency power source is applied to said cathode electrode to produce plasma in said reaction chamber whereby forming a deposited film on the surface of each of said plurality of cylindrical substrates. And a VHF plasma CVD apparatus suitable for practicing the plasma CVD process.

Plasma CVD process using a very-high-frequency and plasma CVD apparatus

A plasma processing apparatus wherein a layer structure consisting of plural layers formed in stack one upon another on a semiconductor wafer placed on the sample holder located in the process chamber, is etched with plasma generated in the process chamber by supplying high frequency power to the electrode disposed in the sample holder, the apparatus comprising a ring-shaped electrode disposed above the electrode and around the periphery of the top portion of the sample holder, an outer circumferential ring of dielectric material disposed above the ring-shaped electrode and opposite to the plasma, and a power source for supplying power at different values to the ring-shaped electrode depending on the sorts of layers of the layer structure.

Plasma Processing Apparatus

The effect of plasma excitation frequency on the deposition rate and on the optical and electrical properties of amorphous silicon film is studied over the range 25–150 MHz. Deposition rates as high as 21 A/sec are obtained at ∼70 MHz, which is a factor of 5–8 larger than typical rates obtained for the conventional 13.56-MHz silane glow-discharge system. Only minor changes occur in the defect density (as measured by the photothermal deflection spectroscopy method), the optical bandgap, and the electrical conductivity over this frequency range. In a preliminaryinterpretation given here, the large variation of the deposition rate as a function of excitation frequency is explained in terms of changes in the electron energy distribution function.

Influence of plasma excitation frequency for a -Si:H thin film deposition

From comparison of PDS (Photothermal Deflection Spectroscopy) spectra with photoconductivity (mobility-lifetime product) data over four decades of sample thickness we find experimental evidence for the presence of material inhomogeneities along the film growth axis.

Study of surface/interface and bulk defect density in a-Si:H by means of photothermal deflection spectroscopy and photoconductivity☆

The preparation of hydrogenated amorphous silicon by the radio-frequency (RF) glow discharge technique is shown to be strongly dependent on the plasma excitation frequency. We have investigated the influence of this parameter on the deposition rate, on the hydrogen content and on the opto-electronic properties of the amorphous silicon films, over the range 25 to 150 MHz. A large variation of the deposition rate is observed, while most of the material properties remain practically unchanged. The results should be of considerable interest for mass production of low-cost amorphous silicon thin film devices.

M. Favre

Papers

Influence of plasma excitation frequency for a -Si:H thin film deposition

Study of surface/interface and bulk defect density in a-Si:H by means of photothermal deflection spectroscopy and photoconductivity☆

Influence of Plasma Excitation Frequency on Deposition Rate and on Film Properties for Hydrogenated Amorphous Silicon

High-rate deposition of hydrogenated amorphous silicon by the VHF-GD method

High-rate deposition of hydrogenated amorphous silicon by the VHF-GD method