To further increase the efficiency of multijunction thin-film silicon (TF-Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high-quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in-coupling and light trapping for the top amorphous silicon (a-Si:H) solar cell while minimizing the parasitic absorption losses. The crater-shaped micro-textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high-quality nanocrystalline silicon (nc-Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high-quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single-textured state-of-the-art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high-efficiency multijunction solar cells.
different energy bandgaps, multijunction solar cells can reduce the thermalization and nonabsorption losses, meaning less spectral mismatch and a better spectral utilization. The theoretical efficiency limit of the solar cell comprising infinite number of component subcells is 68.2% without concentration and 86.8% with concentration. [3] In practice, the III-V photovoltaic technology represents a very successful demonstration of both strategies. [4,5] Within this category, the benefit of multijunction concept is apparent as the record efficiency of concentrator photovoltaic cells is 29.3% for the single-junction, and grows to 34.2%, 44.4%, and 46.0% for the monolithic two-terminal double-, triple-, and quadruple-junction cells, respectively. [6][7][8][9] Multijunction solar cells can be made with two or more external electrical contacts (terminals). The components in a monolithic two-terminal device are considered to be in series connection. Therefore, the output current of a two-terminal device is constrained by the component which supplies the least photocurrent. Despite the limitation, two-terminal multijunction cells are much more feasible to design and manufacture than the ones with more terminals, thus more practical for applications. This type of two-terminal devices is the subject of this paper. For simplicity, we refer two-terminal multijunction solar cells to as multijunction solar cells, without further specification.While the multijunction III-V solar cells mark the highest achieved power conversion efficiency of photovoltaic cells to date, the multijunction concept has been explored and developed in many other photovoltaic technologies as well. Besides the reduction of losses originating from spectral mismatch, the multijunction concept offers some additional benefits to the thin-film photovoltaics. The effective absorption is split into a few separate layers in different subcells, meaning that each layer can be made thinner for the same total absorption. Such thickness reduction improves the electrical performance when the carrier transportation in the material is a limiting factor. Moreover, the division of photocurrent implies less resistive losses over the electrical interconnections. The thin-film silicon solar cell has a long history of developing multijunction solutions to make use of these advantages. The efficiency improvement by additional subcells has been shown up to the triple-junction configuration. [10][11][12][13][14][15][16][17] In organic photovoltaics, the absorber materials have rather narrow absorption spectra.
We fabricated and studied quadruple‐junction wide‐gap a‐Si:H/narrow‐gap a‐Si:H/a‐SiGex:H/nc‐Si:H thin‐film silicon solar cells. It is among the first attempts in thin‐film photovoltaics to make a two‐terminal solar cell with four different absorber materials. Several tunnel recombination junctions were tested, and the n‐SiOx:H/p‐SiOx:H structure was proven to be a generic solution for the three pairs of neighboring subcells. The proposed combination of absorbers led to a more reasonable spectral utilization than the counterpart containing two nc‐Si:H subcells. Besides, the use of high‐mobility transparent conductive oxide and modulated surface texture significantly enhances the total light absorption in the absorber layers. This work paved the way toward high‐efficiency quadruple‐junction cells, and a practical estimation of the achievable efficiency was given.
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