Thin-layer solar cells (i. e. cells that are only a few micrometres thick) made of semiconductor materials with differing band gap energy EG can be stacked (optical series connection, see Figure 3.34).
Figure 3.33 Close-up of a concentrator cell installation where sunlight is concentrated using mirrors that focus their light onto the solar cell at the point. The relatively large (black) cooling elements for each of these solar cells are clearly visible (Courtesy of DOE/NREL)
Figure 3.34 Structure and spectral energy absorption of an a-Si and CIS tandem cell, which allows for optimized use of the solar spectrum. The front cell F uses the energy represented by the vertically hatched area (blue curve in Figure 3.19), while the back cell R uses the energy represented by the horizontally hatched area, which is the energy portion with higher l that was not yet used by the front cell (total potential energy production of the back cell alone shown by dark red curve in Figure 3.19).
If Eg for each junction is defined in such a way that the front solar cell (i. e. the cell facing the Sun) has the highest EG and the back cell the lowest, the energy from the photons not absorbed by the front layers can be used by the layers that are further back, since photons with h ■ n < EG simply pass through semiconductor material. This of course increases the spectral efficiency of the entire arrangement, as a comparison of Figures 3.34 and 3.22 clearly shows.
The relationships shown in Figure 3.34 are obtained with an arrangement such as a two-layer tandem solar cell made of a-Si (thin-film silicon, TFS), where EG ~ 1.75 eV, and CIS, EG ~ 1 eV. As can be seen in Figure 3.28, the spectral quantum efficiencies of both materials complement each other very well. As early as 1986, Arco Solar made such experimental tandem solar cells whose efficiency Zpv was 14% with a 65 m2 area [3.3].
The concept of series connecting both tandem cell layers not only optically but also electrically has great appeal, as it greatly simplifies the manufacturing process (see Figure 3.36). However, with this concept it is difficult to achieve that the bottom cell always produces the exact same current as the top cell via the residual light that filters down, since the current must always be the same for series connections. If this is not the case, the weaker cell determines overall current, thus immediately reducing efficiency. This problem can be solved by electrically insulating the two optically series-connecting solar cells using lightcoupling film, which allows for series connection of solar cell groups comprising different types of solar cells with about the same voltage. These groups can then be parallel connected without any difficulty using a four-terminal arrangement. This concept was used in the Arco Solar a-Si/CIS tandem solar cells mentioned above (see Figure 3.35).
Transparent contact materials are used on both the front and back of tandem solar cells to allow sunlight to reach the rear cell. For this purpose in most cases tin and zinc oxide layers are used. Only for the rear contact of the back cell a metal contact can be used.
Optical and electrical series connection has now also been successfully realized. Figure 3.36 shows the cross-section of such a tandem cell with optical and electrical series connection.
Uni-Solar sells a triple-cell module (three cells with varying levels of band gap energy and arranged one behind the other) made of amorphous silicon (with slightly varying additives in the amorphous silicon
Figure 3.35 Structure of an experimental Arco Solar four-terminal tandem solar cell made of a-Si and CuInSe2 (CIS) (after [3.3]). In this arrangement, the two cells are electrically insulated from each other, thus allowing for the use of any connection method desired
cells), which, apart from being somewhat more efficient than individual amorphous cells, is above all far more stable. Figure 3.51 in Section 3.5 shows a cross-section of such a cell.
The procedure described in Section 3.4.2 also allows for the determination of the theoretical efficiency Zt of tandem solar cells with two p-n junctions, i. e. electrically stand-alone (four-terminal) cells. At STC, Zt for such a cell reaches a maximum of around 40% with EG ~ 1.75 eV and EG ~ 1 eV in the front and back cell respectively (see Example 4 in Section 3.7).
The highest efficiency achieved by a lab tandem cell to date (GalnP and GaAs) in the AM1.5 spectrum with 1 kW/m2 at 25 °C is 30.3%, while the highest efficiency achieved to date for a lab triple cell (GalnP, GaAs and Ge) under the same conditions is 32%. The area of each of these cells was around 4 cm2 [3.2].
Particularly high efficiency levels can be reached by combining the two efficiency optimization techniques, i. e. by building tandem or triple concentrator cells. The currently confirmed record efficiency for a tandem concentrator cell (GaAs and GaSb, AZ — 0.05 cm2) with four terminals (as in Figure 3.35) with 100-fold sunlight (G — 100kW/m2) is Zpv — 32.6% [3.2]. In 2005, some vendors were hoping that by the end of 2006 triple concentrator cells would be available that could provide 40% efficiency and that by 2010, 45% efficiency could even be achieved [3.4]. In July 2008, under 140-fold sunlight a triple concentrator cell made of GaInP, GaAs and GaInAs with AZ ~ 0.1 cm2 attained ZpV — 40.8% [3.2], while in January 2009 under 454-fold sunlight a triple cell made of GaInP, GaInAs and Ge with AZ ~ 0.05 cm2 reached Zpv — 41.1% [3.11]. By splitting the light into three different spectral ranges and using cells specially designed for them, Zpv — 42.7% and 43% were reached in 2007 and 2009 respectively without unduly elevated solar concentration [3.12].
Although research on tandem and triple solar cell techinques is still in its infancy, it is safe to assume that intensive R&D will ultimately allow for considerably higher solar cell efficiency. However, it will be a while longer until such advances can be applied to industrially produced solar cells.