Low-bandgap semiconductor

The heterocyclic PAEs are useful for low-bandgap applications, as n-type semiconductors, and in sensory applications. Again, as long as the alkynylated or iodinated monomers are available, the synthesis of the corresponding PAE is not a problem, and either the Pd-catalyzed couplings or alkyne metathesis can be utilized toward that end. 

While EQN (1) can be used to verify an NEA for wide bandgap semiconductors, another aspect that signals the presence of an NEA is the appearance of a sharp peak at the low kinetic energy end of the spectrum. This feature is attributed to electrons thermalised to the conduction band minimum. For a positive electron affinity, these electrons would be bound in the sample and not observed in the spectrum. 

The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

The structure of the low bandgap polymeric semiconductor and the dopant dye is plotted in Fig. 5.19. The average thickness of the active layers, determined by AFM measurements, is between 80 and 110 nm. In order to obtain a better understanding of the transport behavior of polymer blends, low temperature studies of cells with pristine MDMO-PPV and MDMO-PPV/PTPTB 1 1 (wt. %) with Au electrodes were carried out. Au has a high work function and should therefore be a good hole injection contact and provide a high barrier for electron injection. The device will therefore be a hole-only device, as described earlier in this chapter [14]. 

Polymeric solar cells with a bandgap > 2 eV are spectrally so badly mismatched to the solar spectrum that their efficiency is severely restricted. It is essential to develop polymeric semiconductors with lower bandgap. Low bandgap polymeric semiconductors behave similarly in conjunction with fullerenes as n-type semiconductors (acceptors). 

Semiconductors are low bandgap insulators. Low is defined qualitatively, so that an appreciable density of electrons can be thermally excited into the conduction band at temperatures that are technologically relevant. In silicon, a large gap semiconductor Eg = 1.12eV exp(—Eg/Agf) 1.6 x 10 at 300K), this density is very small at room temperature. Germanium (Eg = 0.67) and indium-antimonide (InSb, Eg = 0.16 eV exp(—Eg/ksT 2 x 10 at 300 K) are examples of lower gap semiconductors. For comparison, in diamond Eg = 5.5 eV. 

Mercury cadmium teiluride (HgCdTe) is a direct bandgap semiconductor widely used as a material for infrared detectors due to his narrow variable band gap. The achievement of high-performance detectors depends critically on a low surface recombination velocity of the minority carriers. The chemical growth of a passivation oxidized superficial layer in an aqueous Fe(CN)g3- basic solution is studied in this work. The depth profiles of the different elements in the oxidized layer superficial layer and its thickness are studied by X-ray photoelectron spectroscopy. The electrical properties of the interface are evaluated from MIS devices. The conditions of oxidation have been optimized. 

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