GaAs-based Tandem Single-Crystal Cells, a Near Text-Book Example

The concept of the tandem cell is clearly shown in the spectral response curves published by the manufacturer Spectrolab, of the three-junction cell, shown in Figure 7.2.

The complex multijunction GaAs-based cells are at a high state of development, building on experience in the Soviet space program and in the wide application of III—V semiconductor (GaAs) materials in commercial electronics. See Figure 6.5 ff. in Chapter 6. An example close to the record 41.6% efficient cell is shown in Figure 7.3. The three main layers are evident from top to bottom, whose absorption spectra were shown in Figure 7.2. Complexities in this mature technology include the use oflow-resistance PN junctions to series-connect adjacent cells, and provision for back surface fields to inhibit recombination. The problem of maintaining the three cells at optimum operating points consistent with identical current through each junction has been apparently addressed successfully.

Подпись: p-GalnP base

GaAs-based Tandem Single-Crystal Cells, a Near Text-Book Example

GaAs-based Tandem Single-Crystal Cells, a Near Text-Book ExampleGalnP top cell

GaAs-based Tandem Single-Crystal Cells, a Near Text-Book Example

Wide-bandgap tunnel junction

0 12 3 4

Voltage (V)

Подпись: sum of the bandgaps of the three junctions. (Spectrolab press release. http://www. spectrolab.com/DataSheets/TerCel/tercell. pdf)
Подпись: Figure 7.4 Current-voltage curve of three- junction GaAs cell under 350 terrestrial AM 1.5 sun illumination at 25 C. The open-circuit voltage, in principle, approaches 1/e times the

Подпись: Figure 7.5 Measured efficiency as a function of shown here. Note that the peak efficiency is illumination strength, for cell similar to that achieved at illumination on the order of300 shown in Figure 7.3. A slightly different cell has suns. (Spectrolab press release. http://www. achieved 41.6% efficiency as opposed to the 30% spectrolab.com/DataSheets/TerCel/tercell.pdf)

Cells of the type shown in Figure 7.3 are termed “monolithic” and are two-terminal devices. It is evident that these complex devices are at a mature stage of development. What is not clear is what cost levels would be attainable with such devices in mass production. The complexity of the junction device is compounded, in an actual system, with the need for high-accuracy optics, suggested by Figure 5.1, and with a large and accurately tracking mirror or lens system suggested by Figure 5.8. Nevertheless, it is argued that with high concentration the actual cell area needed decreases to the point where the large installation could be cost-competitive with larger area simple flat plate cells such as those made by First Solar. The manufacturer Spectrolab says the tandem cells are useful both for point-focus concentrators and for dense arrays and linear concentrators (Figures 7.4—7.6).

these data support the enhanced power and efficiency in tandem cells predicted for concentrated illumination. (Spectrolab press release. http://www. spectrolab. com/ DataSheets/TerCel/tercell. pdf)

Подпись: Figure 7.6 Measured open-circuit voltage Voc as a function of illumination strength, for cell similar to that shown in Figure 7.3. A slightly different cell has achieved 41.6% efficiency as opposed to the 30% shown here. Note that

It appears largely that the basic theory flowing from the work of Shockley and Quiesser has been validated by the results obtained from the GaAs-based tandem junction cells, as described.

What is not clear is what cost levels would be attainable with such devices in mass production. An attempt at cost analysis has been offered by the Spectrolab Boeing group [90].

Подпись: Figure 7.7 Projected cell costs [90] under different assumptions for efficiency (related to concentration) and volume of production.

The curves in Figure 7.7 are a claim by the manufacturer [90] that costs in a large – scale technology based on concentrating multijunction cells, with attendant tracking mirrors and water-cooling, can be competitive with large-scale installations such as

the one being built by First Solar in China. The market has not supported this claim, and at present there appears to be no large-scale project of this type under construction.

7.1.2

Updated: October 27, 2015 — 12:10 pm