Low-Cost Tandem Technology: Amorphous Silicon:H-Based Solar Cells

Подпись: Figure 7.11 J—V characteristics [92] of single cells and tandem cell with PCPDTBT:PCBM and P3HT:PC70BM composites under AM 1.5 G illumination from a calibrated solar simulator

We have spoken earlier about the optical absorption of crystalline Si as being relatively small because of the indirect bandgap. This was shown in Figure 6.3 and led to the

with irradiation intensity of100 mW/cm2(about one sun). The tandem cell shows short-circuit current density 7.8 mA/cm2, open-circuit voltage 1.2 V, and power efficiency 6.5%.

texturing of the surfaces to increase the distance a photon could travel inside the solar cell structure. Amorphous silicon can easily be deposited in thin layers and has a much higher optical absorption. This means that an amorphous silicon cell need not be as thick as a crystalline solar cell, a double advantage in reducing the cost of such a cell. The amorphous form of silicon, however, has inferior electrical properties, compared to crystalline silicon. Primarily, the carriers are less mobile because the material is not periodic in the location of the atoms. Related to the p-i-n nature of the junctions in these cells, one can refer to Figure 6.2a. The diffusion length described in Chapter 3 is short in the amorphous material, and a gradient of doping acting as a local electrical field is introduced to separate the photogenerated holes and electrons.

Several forms of amorphous silicon can be rather easily deposited that have characteristically different bandgaps. This facilitates making a tandem cell, based on the same principles as were described in connection with the GaAs triple-junction solar cells. Microcrystalline silicon, amorphous silicon, amorphous silicon passiv­ated with hydrogen “a-Si:H”, and Si:Ge all can be incorporated and have differing energy gap values. Amorphous silicon is slightly unstable, tending to the more stable crystalline state, and requires steps including hydrogen passivation, to fill dangling Si bonds in the incompletely filled bond set around the occasional atom, and also postfabrication annealing to allow a part of the reconstruction to occur before the device is put to use.

It should be said that these various forms of thin-film silicon solar cells are widely produced and employed in devices of all sorts. Where the efficiency is not a key parameter, useful products find many applications, from hand calculators to wrist – watches to semitransparent glass panels that may be used in sky-lights or in a sun roof for a car. Also, especially connected with the United Solar Ovonics, LLC, company of Troy, Michigan, are inexpensive roofing tiles with built-in solar cells. The cells shown in Figure 7.12 are among the most efficient, approaching 11% efficiency.

The structures described here are wide-area devices produced on flexible stainless steel backing of 5 mil (0.127 mm) thickness and 35.6 cm width. This is a production process in which rolls of stainless steel foil are sequentially processed. Noting from Figure 7.12, the reflective coating Ag/ZnO is first applied, to reflect back into the cell light that has not been absorbed. The second full roll process was deposition of the three-layer silicon structure, as the foil advances down a line of processing stations including a radio frequency (rf) glow discharge to deposit Si at a thickness rate of 3A/s and a linear foil speed of 2.3’/min. This was followed by a full roll deposition of the transparent conductive oxide (TCO) to cover the structure. The roll was then cut into individual cells of dimension 35.6 cm x 23.6 cm, to which were applied wires and bus bars to collect current. The efficiencies measured on these large-area junctions were near 10.4% under one sun illumination, corresponding to open-circuit voltage 2.2 V and short-circuit current 5.7 A. (The best efficiency for cells of the type shown in Figure 7.13 is reported as 13% [94].) The large-area cells [93] are joined to form laminated modules for which the stable output power under one sun is expected to be 151W. The larger open-circuit voltage, 2.2 V, coming from the series connection of junctions, enables a single cell to electrolyze water, which requires about 1.93 V, as

Подпись: Grid Подпись: Wire

Low-Cost Tandem Technology: Amorphous Silicon:H-Based Solar CellsTransparent Conductive Oxide
p-type microcrystalline Si Alloy
l-type amorphous Si:H Alloy
n-type amorphous Si:H Alloy
p-type microcrystalline Si Alloy
l-type amorphous Si:Ge:H Alloy
n-lype amorphous Si:H Alloy
p-type microcrystalline Si Alloy
l-type amorphous Si:Ge:H Alloy
п-type amorphous Si:H Alloy
Textured Back Reflector Ag/ZnO
Stainless Steel Substrate
(a)

surface-textured TCO
ZnO:AI

a-Si:H top. absorber

a-SiGe:H middle absorber

Подпись: Figure7.12 (a) Structureofatandem solarcell constructed with amorphous layers of Si and Si-Ge alloys [93]. Note that “insulator-type layers” are present in the structure, in which an effective electric field is maintained to counter Подпись: the inherently low electron and hole mobilities in amorphous silicon and its alloys. (b) A similar tandem silicon cell including surface texturing to trap light [94].

t/c-Si:H bottom absorber

Low-Cost Tandem Technology: Amorphous Silicon:H-Based Solar Cells

described in Chapter 9. Single-junction silicon cells have an open-circuit voltage typically less than 0.6 V. Tandem silicon cells are used in the “artificial leaf” that we will describe in Chapter 9.

This kind of product could be used on the rooftops of New York City as was discussed in Chapter 5. This manufacturer, Ovonics, LLC, uses silicon, which is benign environmentally and of unlimited supply (sand is silicon dioxide), and this manufacturer also has a long history of providing solar tiling for roofs of apartment buildings. It appears that these flexible solar panels are available in 10 MW quantities. On the other hand, a lower price and higher efficiency ~12.5% might well be obtained with thin-film single-junction CdTe cells, also available in large quantities.

7.2

Updated: October 27, 2015 — 12:10 pm