Semiconductor heterojunction cells

Although conventional solar cells based on silicon are produced from abundant raw materials, the high-temperature fabrication routes to single-crystal and polycrystalline silicon are energy intensive and expensive. The search for alternative solar cells has therefore focused on thin films composed of amorphous silicon and on other semiconductor heterojunction cells (e.g., cadmium telluride and copper indium... 

Rand BP, Burk DP, Forrest SR (2007) Offset energies at organic semiconductor heterojunctions and their infiuence on the open-circuit voltage of thin-film solar cells. Phys Rev B 75 115327... 

The heterojunctions of the polyacetylene were realized not only with inorganic photoconductors but also with organic polymers [139]. The results obtained show good similarity with barrier and heterojunction characteristics for inorganic semiconductors. Photoelectrochemical cell for solar energy conversion with polyacetylene electrodes and Na2S, electrolyte had an efficiency of 1 % at 2.4 eV [140], The complicated phenomena take place at the electrodeelectrolyte interface. 

Typical representatives of this class of homojunction semiconductors are Si several III-V compounds, most prominently GaAs and from the class of ITVI compounds CdTe, since it can be doped p- and ra-type, while others cannot. Several ternary compounds are also used, most prominendy CuInSe2 and similar ternaries. An example for a heterojunction cell is the CdS/CdTe combination (Meyers and Birkmire, 1995). For more details see Green (2001). 

Hsu, L. and Wang, E.Y. (1972) Photovoltaic properties of In203/ semiconductor heterojunction solar cells, in Proc. 13th Photovoltaic Specialists Conference, IEEE, New York, pp. 536-540. 

Dye-sensitized solar cells (DSCs) are a paricularty successful example of a bulk heterojunction cell architecture. A wide bandgap inorganic semiconductor (typically a metal oxide) is sensitized to the solar spectrum by attaching a surface-adsorbed... 

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

Four different types of junctions can be used to separate the charge earners in solar cells (1) a homojunction joins semiconductor materials of the same substance, e.g., the homojunction of a p — n silicon solar cell separates two oppositely doped layers of silicon (2) a heterojunction is formed between two dissimilar semiconductor substances, e.g., copper sulfide, Cu S, and cadmium sulfide, CdS, in CuxS—CdS solar cells (3) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal-insulator-semiconductor junction (MIS), a thin insulator layer, generally less than 0.003-pim thick, is sandwiched between a metal and semiconductor material. 

Scheme 5.9 Scheme of a hybrid photovoltaic cell with an active layer consisting of a composite of a conjugated polymer and semiconductor nanocrystals (so-called bulk heterojunction). 

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