Category High-Efficiency Solar Cells

Future Perspectives

With today’s record efficiency of 44.0 % under a concentration of 942 suns [38] III-V solar cells have not reached their full potential. The multi-junction concept has the perspective of obtaining efficiencies above 45 % and maybe even 50 % under concentration. New designs for cells with more than three junctions require the use of thin-film cells in some form [27, 59, 60] either as a free standing thin-film cell or in combination with cells grown on different wafer materials through a layer transfer process.

A basic method for obtaining a thin-film cell by etching/polishing of the wafer after growth is widely used [1, 28, 29], but this is an expensive method because the wafer, which has a large contribution in the total cell cost, is lost...

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Substrate Reuse

One of the aims of ELO is to allow multiple reuse of the wafer after lift-off to reduce cell cost. Ideally, a minimum of wafer re-preparation between consecutive lift-offs is required. However, exposure to the HF solution increases the surface roughness of the wafer on a nanometer scale. The roughness is about 0.3 nm for new so-called epi-ready wafers and between 2 and 4 nm for wafers that have been subjected to a 20 % HF solution for 21 h. In addition during ELO reaction residues are left on the surface [56], mainly elemental arsenic. During storage of the etched wafer under ambient conditions, from this arsenic As2O3 particles are formed...

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New Device Designs

Wafer-based cells have a full area back contact. A thin-film cell allows access to the backside of the device. This makes it possible to apply a metal grid on front and back side. In this design the cell can be used as a bifacial cell [45] where illumina­tion takes place from both top and bottom side using a relatively cheap mirror setup.


Fig. 21.10 Examples of thin-film cell designs: semitransparent InGaP cell (left) and bifacial GaAs cell (right)

Such a cell can also be applied as semitransparent cell for mechanical stacking on a lower bandgap cell. An example is a thin-film InGaP cell combined with a Si cell [46] or an InGaAsP/InGaAs tandem cell grown on InP combined with a GaAs cell [47]. In Fig. 21.10 an InGaP and GaAs semitransparent cell are shown...

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Low Power to Weight Ratio/Flexibility

For space solar cells high efficiency, high radiation resistance, a low weight, and flexibility are the desired features. Because of their highest power output wafer – based III-V cells have been dominant over other cell types since the late 1990s. Wafer-based cells are mounted on a rigid honeycomb structure to prevent the wafers from breaking, resulting in relatively high weight. A thin-film cell on a flexible host substrate does not require a rigid panel assembly. This introduces new low weight options for panel design. In the past, other attempts were made for thin – film space cells which used a-Si or CIGS materials...

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Thin-Film III-V Cell Development

The first good quality thin-film III-V cell, a 4 cm2 GaAs cell with a 23.3 % efficiency, was made by Kopin in 1990 [25] at a time when the best wafer-based GaAs cell performance was 25.1 %. The thin-film was not lifted off chemically like in ELO but mechanically using the CLEFT (Cleavage of Lateral Epitaxial Films for Transfer) technique [26]. The cell was used in a mechanical tandem in combination with CIS bottom cell. The total efficiency was 25.8 % which made it the best mechanically stacked tandem cell at that time intended for use in space. No progress in CLEFT was reported after 1990.


Fig. 21.8 Flexible ELO thin-film cells with a metal foil backing


Fig. 21...

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Epitaxial Lift-Off Cells

Conventional triple junction cells grown on Ge have their limitations. They are inflexible, brittle, and relatively heavy. Therefore wafer-based cells require some kind of structural support to prevent damage. Thin-film III-V cells produced with the ELO technique offer new opportunities for device design, based on the fact that the thin-film carrier can be selected on its material properties rather than crystal growth demands. Figure 21.8 shows an example of flexible thin-film cells with a metal backing. Substrate reuse and thus lower cell cost has been the main driver for the development of ELO thin-film cells, but apart from this there are many interest­ing new concepts and applications which are not possible for wafer-based cells.

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ELO Methods for Small Area Devices

Alternative approaches for ELO are directed to the production of microchips using a transfer-printing technique to peel and print a large number of small thin-film structures onto glass or plastic [23]. The layer stack for this method is identical to other ELO methods: a device structure that is grown over an AlAs release layer, only in this approach the AlAs layer is much thicker (1 ^m) than generally applied for the full area lift-off. In this method not the entire wafer area is lifted off, but small area device structures. To do this, before lift-off the material is separated into square blocks by vertical etching through the device structure to enable exposure of the release layer to HF by immersion...

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Key Process Parameters

For the application of ELO the process needs to have a sufficiently high etch rate. It is and always will be a relatively slow process. With an etch rate of less than 1 mm/h it initially took more than a day to lift-off a 2 in. diameter thin-film

Подпись: Ve
Подпись: [HF] Rd + Rr Подпись: (21.2)

structure. Therefore an important goal was to speed up the process to a time scale of hours for lifting of a 4 in. thin-film, which is in the same order as the deposition time of the solar cell layer structures by MOCVD...

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