Category Advanced Silicon Materials for. Photovoltaic Applications

Technological Constraints

PV manufacturing has its roots in sophisticated semiconductor fabrication technology and most of the thin-fllm PV manufacturing has its origin in liquid-crystal display manufac­turing. Manufacturing compliance is a necessity for the success of any new PV devices. There are basically five steps from the discovery of a new physical phenomenon to a mass-scale manufactured product: the discovery of the new phenomenon, the testing of a working device based on new phenomenon, the development of a manufacturing process, the pilot-plant manufacturing, the mass-scale manufacturing [152]. So far, most of the 3rd-generation solar cells in which Si-NCs play a key role are still at the first two stages...

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Absorption

There are basically two challenges concerning the light absorption of Si-NCs. One is to improve the absorption coefficient of the SRD layer by looking for direct-bandgap transitions. Si-NCs with diameters smaller than 3 nm show much higher absorption coef­ficient than those with diameters higher than 3nm [133]. This has been ascribed to the amorphous structure of the smaller Si-NC [134]. However, the main problem is asso­ciated with the overall optical thickness that, due to the limited Si-NC density in the film in which they are embedded, may be smaller than that of silicon. This is shown in Figure 10.29, where it is evident that only in one data set (that of Ding et al.) the absorp­tion coefficient is higher than that of bulk Si...

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Carrier Transport

The knowledge of the exact mechanism of the electrical transport in Si-NC based cell is fundamental to proper design of the cell and to optimize its performances. A reduced effective carrier mobility (compared with the bulk silicon) is observed since the Si – NCs are embedded into an insulating matrix. The carrier transport in such a composite material is controlled by carrier injection and tunneling among the nanocrystals. In particular, since the geometrical distribution of the Si-NCs is random, one can speculate that carrier motion occurs through percolation. One way to improve the effective carrier mobility and conductivity of the Si-NC:DL composite is to decrease the interdot distance between adjacent Si-NCs.

Consider the SRD/DL superlattice structure shown schematically in Figure 10...

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Size Control

Phase separation is the mechanism associated with the formation of Si-NCs in most of the deposition methods. Usually, a thick SRD layer is deposited onto a proper substrate and then annealed. This leads both to a broad size distribution of Si-NCs and to nonuni­form interdot distances. The former will result in a broad absorption spectrum, which is inappropriate for the subcells of all-silicon tandem cells. The latter impedes good current flow and yields current-focusing effects that deteriorate the lifetime of the layer. A more accurate growth process is to deposit a SRD/DL (DL: dielectric layer) superlattice. The size of the Si-NCs is defined here by the thickness of the SRD sandwiched by DLs...

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Downshifter Cell

One promising application of Si-NCs is for the downshifter solar cells [17], when down­shifting is associated with specific luminescence effects of a downshifting layer, which is deposited on top of the solar cell. Van Sark et al. [112] showed theoretically that a 10% increase in the short-circuit current might be expected by using a quantum-dot-embedded plastic layer on top of an amorphous (a)-Si solar cell. It was also shown that the overall efficiency of a-Si solar cells cannot be improved because of its high spectral response to high-energy photons. This emphasizes the fact that the downshifter concept applies only where the pristine solar cell has a low spectral efficiency. A slight (0.4% against a theoretical 1...

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Multiple-Carrier Generation

Multiple-carrier generation was observed in a MOS cell with the same structure we discussed in Section 10.3.3 but with different active layers, here consisting of a SRO layer with more silicon excess. A superlinear dependence of Isc on the incident light power was observed. This was explained by the presence of the subbandgap interface states and by the multiple-carrier generation process. Such a cell has the potential to be used as a high-efficiency solar cell.

Two SRO layers are used as the active layer of the MOS structure shown in Figure 10.11. The thickness and chemical composition of these two layers, named Г3 and T3N, are reported in Table 10.3. Both layers show PL emission with a peak wavelength larger than 900 nm, which means that the diameter of the Si-NCs is larger than 5nm...

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Intermediate-Band Solar Cells

Subbandgap photoresponse of Si-NCs was found in a metal-oxide-semiconductor (MOS) device. Potentially this would be the premise to realize intermediate-band solar cells. The cross section of the device is shown schematically in Figure 10.11. The active layer is a single SRO layer, which has been deposited on a p-type Si substrate by plasma-enhanced CVD (PECVD), using N2O and SiH4 as precursors, and annealed at 1050 °C for 1h to grow Si-NCs. During deposition, the ratio between the precursor gases N2O and SiH4 was 10. This SRO layer is labeled Г10, hereafter. An n-type poly-Si gate layer (30 nm) was deposited on the SRO layer. The poly-Si was covered by an antireflective coating formed by a 50-nm thick Si3N4 layer and a 120-nm thick SiO2 layer...

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Applications of Si-NCs in the 3rd-Generation Solar Cells

10.3.1 All-Silicon Tandem Solar Cells

Si-NCs are used in all-silicon tandem cells [102, 103]. The idea is to leverage on the bandgap tunability of the Si-NCs. In Figure 10.5, examples of two-cell and three-cell all-silicon tandem solar cell are reported [90]. The theoretical efficiency limits are 42.5% and 47.5% for two-cell and three-cell stacks, respectively [5]. The optimal bandgaps of Si-NCs in order to enhance the efficiency are indicated in Figure 10.5 [90].

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A Si-NC-embedded-SiO2 thin film is a complex composite material, which requires a good control over the Si-NC formation. That is why so far no commercial devices have been realized. In 2008, Cho et al. [104] reported on the PV properties of a phosphorus – doped Si-NC/p-Si device...

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