Solar cells tandem structure

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

FIG. 72. Schematic cross-section of (a) a single junction p-i-n o-Si H superstrata solar cell and (b) a tandem solar cell structure. (From R. E. I, Schropp and M. Zeman. "Amorphous and Microcrystalline Silicon Solar Cells—Modeling, Materials and Device Technology," Kluwer Academic Publishers, Boston, 1998, with permission.)... 

Here we describe the layer structure for single junction as well as for tandem solar cells consisting of a-Si H and pc-Si l I. Further, this section will deal with the stability of silicon thin film solar cells and the possibility to reduce degradation by special design. 

Fig. 8.4. Layer structure of single junction n-i-p (substrate) and p-i-n (superstrate) solar cells. Also included is an amorphous/microcrystalline tandem solar cell structure...

Figure 19.12 Left. Structure of the tandem solar cell based on pentacene and Ceo with a thin, discontinuous evaporated silver layer Incorporated for efficiency improvement. Right Power efficiency versus excitation wavelength for tandem solar cells with 2 nm silver middle layer (solid circles) and without the silver layer (open circles). The right hand scale is for the ratio of efficiency of the two cells to illustrate the significant difference in dependence on excitation wavelength. Reprinted from reference 63 with permission of the SPIE.

High efficiency solar cells are produced from crystalline binary or multinary compounds of elements from Groups III and V, almost entirely for space applications. While a wide range of materials and methods have been employed, the monolithic tandem stack of a Ga cIni (P cell, where x 0.516, above GaAs and Ge cells is a commercially produced, indicative example.Monolithic tandem structures present several challenges. The materials used in the various junctions should have similar thermal expansion coefficients and... 

He J., Lindstrom H., Hagfeldt A. and Lindquist S.-E. (2000), Dye-sensitised nano-structured tandem cell—first demonstrated cell with a dye-sensitised photocathode , Solar Energy Mat. Solar Cells 62, 265-273. 

Figure 7.10 Tandem solar cell structure for polymer blend solar cells, based on the design demonstrated by Hadipour et al. (2006). In this all-solution-processed device, the top cell consists of a polymer PCBM bulk heterojunction with an absorption maximum of 550 nm and preferentially absorbs short-wavelength light, while the bottom cell is made from a bulk heterojunction of PCBM with a red-absortring polymer and absorbs longer-wavelength light. The composite gold-PEDOT PSS internal layer connects the two cells in...

KuboW., Sakamoto A., Kitamura T., Wada Y. and Yanagida S. (2004), Dye-sensitized solar cells improvement of spectral response by tandem structure , J. Photochem. Photobiol. A—Chemistry 164, 33-39. 

Transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes) can be easily fabricated based on inverted architecture and have important application in tandem architectures as well. We can form transparent solar cells by replacing the Al top electrode with 12 nm Au in the inverted structure. The J-V curves for this transparent polymer solar cell, with light incident from ITO and Au side, are shown in Figure 11.17. The difference between the two J-V curves is due to the partial loss by the reflection and absorption at the semitransparent Au electrode. To provide sufficient electrical conductance, Au layer thickness has to be sufficient and the optical loss at Au electrode becomes significant. However, the inverted solar cell structure has the V2O5 layer which is not only transparent but also provides effective protection to the polymer layer. A transparent conductive oxides electrode, such as ITO, can therefore be deposited without compromising device performance. 

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

A two-terminal Alo.isGao.gsAs/Si tandem solar cell was fabricated in order to attain the photocurrent matching between the top cell and the bottom cell. The structure and the current-voltage... 

Figure 12.8 (a) Chemical structures of the materials used in tandem solar cells, (b) Device structure of the high-efficiency tandem solar cell, (c) UV-visible absorption spectra of PBDTT-DPP and P3HT films and the solar... 

Semiconductor-Liquid Junction From Fundamentals to Solar Fuel Generating Structures, Fig. 17 Energy schematic of a Z-scheme analogon light absorber/catalyst structure using a tandem-type cell design n- and p-type semiconductors act as photoanode and photocathode, respectively (see text) VB, CB valence and conduction band edges, respectively n", p" ... 

DBP) as a donor blended with C70 as acceptor and a layer of 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c] [1,2,5] thiadiazol-4-yl)methylene)malononitrile (DTDCTB) as a donor blended with Cgo as acceptor [226]. Such cells are directly competing with solution-processed plastic solar cells consisting of at least one polymeric organic donor and, typically, a molecular organic acceptor, which have also reached similar efficiency levels of 6.1 % [227], 7.4 % [228], 8.7 % [229], and 9.4 % [230] for different material combinations in conventional single-cell device structures, 10.6 % in a tandem cell [231], and even 11.5 % [232] or 11.8 % [233] in triple-junction devices. 

III. DESIGN OF AN OPTIMIZED SOLAR CELL STRUCTURE FOR TANDEM CELL SYSTEMS... 

The company Spectrolab in Cahfornia and several research laboratories have reported efficiencies of over 40%, and theoretically over 45% could be achieved at maximum utilization of the fuU wavelength spectrum of solar radiation by means of multilayer/tandem structures of compound semiconductors and concentration of solar light by mirrors or lenses by factors of 100 to 1000. As a consequence of this concentration approach only very small PV cells are required so that their price is not so critical. 

Figure 5.32 Structure of an oligothiophene-based small molecule with 3-ethyl-rhodanine end-capping units and a 2-D BDT core, used as a donor in both single and tandem junction SM-BHJ solar cells.

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