Low band-gap materials

A more effective carrier confinement is offered by a double heterostmcture in which a thin layer of a low band gap material (the active layer) is sandwiched between larger band gap layers. The physical junction between two materials of different band gaps, and chemical compositions, is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in Figure 4. Electrons injected under forward bias across the p—N junction into the lower band gap material encounter a potential barrier, AE at thep—P junction which inhibits their motion away from the junction. The holes see a potential barrier of AE at the N—p heterointerface which prevents their injection into the N region. The result is that the injected minority... 

A Stable Class of Low-Band-Gap Materials 24 Organic Field Effect Transistors (FETs) 25 Synthesis 26 Aldol Route 27... 

As an example, GulnSe2 is a known low band-gap material (1.0 eV) that shows promise for use in solar energy conversion [106]. We can imagine preparing rare-earth based materials using the formulation shown in Table 14.5. 

Why are low band-gap materials not well suited for photovoltaic solar power conversion ... 

Sheina, E.E. et al.. Highly conductive, regioregular aUcoxy-functionalized polythiophenes A new class of stable, low band gap materials, Chem. Mater. 17, 3317-3319, 2005. 

Unfortunately, most of the low band gap materials studied so far have been found to undergo such corrosions. Therefore, there is a need to develop newer materials for developing a photoelectrochemically stable semiconductor. 

Organic r-donors based on tetrathiafulvalene (TTF) and its derivatives have been the subject of numerous studies in organic media, due to their ability to form charge-transfer compounds and their wide range of potential applicability as, for instance, molecular conductors, semiconducting films, electroactive polymers (including low-band gap materials), molecular switches, and superconductors [8]. 

One example of a very low band gap material is polyisothianaphthalene (Figure 17.13). This material has been synthesized. Because of the low band gap, the material is blue in the undoped state. Doping, as usual, moves the absorption to longer wavelengths, but in this case that means out of the visible range. Doped polyisothianaphthalene is a transparent material, and there are many uses for thin film, transparent, conducting materials. You are asked in the Exercises to rationalize the low Eg of polyisothianaphthalene. 

Zhang X, Steckler TT, Dasari RR, Ohira S, Potscavage WJ Jr, Tiwari SP, Coppee S, Ellingta- S, Barlow S, Bredas J-L, Kippelen B, Reynolds JR, Marder SR (2010) Dithienopyrrole-based donor-acceptor copolymers low band-gap materials for charge transport, photovoltaics and electrochromism. J Mater Chem 20 123... 

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