Hetero-Junctions

There are many published examples in which the coupling of two different materials leads to an increase in the photocatalytic activity. Many of them concern coupling and junctions between different nanopartides, considering also different topologies, like coupled and capped systems [72]. Tentative explanations based on possible heterojunction band profiles are given. However, in-depth analysis of the hetero junction band alignment, the physical structure of the junction, the role of (possible) interfadal traps and of spedfic catalytic properties of the material is still lacking. Some recently published models and concepts based on (nano)junction between different materials are briefly reviewed here. 

These experimental facts indicate that this system can be an excellent model of band-filling control, and will be a good candidate for making a superlattice composed of Mott insulator/superconductor hetero-junctions. It should be emphasized that their lattice parameters are nearly kept constant through such an anion modification, which is the most essential feature for achieving the successful tuning of Tc in an organic superconductor. 

GaAs JEET devices have been tested as gas sensors. An n-type SnO layer was used to form a hetero-junction to the p-type channel region. Palladium on top of the SnO caused the device to detect NO at a device temperature around 200°C [101]. 

Hetero junctions, forming a Schottky barrier like a metal-semiconductor junction, normally change the energy levels of conduction and valence bands. When the Fermi level of the semiconductor equilibrates with the energy level of the redox couple in the solution, the electric energy level at the surface is pinned and a depletion layer is formed. This is postulated since the rectified current can be observed at semiconductor plate electrodes. The bending of the band in the semiconductor at the surface can be described as a solution of the one-dimensional Poisson-Boltzmann equation... 

Overview of interfacial electronic states of polymer hetero junctions... 

Here, a brief summary is provided of the current understanding of the electronic structure properties of the hetero junction interface. We start with the basic picture of the energetics (Sec. 2.1), based on the offset between the frontier orbitals of the two polymer species this offset triggers the dissociation of the photogenerated exciton if the exciton binding energy cb 0.5 eV can be overcome. To refine this picture, explicit electronic structure calculations are necessary, which pose a formidable challenge for the interfacial systems under consideration. Secs. 2.2 and 2.3 summarize recent efforts in this direction [41,43,44,56]. 

Fig. 5.5. Luminescence quenching (bullets, right hand, axis) and short-circuit current Ac (black squares, left hand axis) vs. molar fullerene concentration in a bulk hetero junction composite. The different onsets for percolation for the two phenomena (exciton diffusion versus ambipolar carrier transport) can be clearly seen...

From the previous results it is reasonable to propose the following device model for a bulk hetero junction solar cell (Fig. 5.17) ... 

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 interpenetrating network in bulk hetero junction solar cells [9] helps to overcome the limitations of bilayer systems [25,95] with low mobility materials. However, less is known about the nanometer morphology of an interpenetrating network or the optimum density of donor/acceptor interfacial... 

This view of Voc generation is additionally supported by the fact that the values of the temperature coefficient dUoc/dT = -(1.40-1.65) mVK-1 for the cells under the present study (with bilayer LiF/Al and ITO/PEDOT contacts) coincide with those for polymer/fullerene bulk heterojunction solar cells of the previous generation (with the same components of the active layer but without LiF and PEDOT contact layers) [156]. In this picture, the temperature dependence of Voc is directly correlated with the temperature dependence of the quasi-Fermi levels of the components of the active layer under illumination, i.e., of the polymer and the fullerene. Therefore, the temperature dependence of Voc over a wide range, and in particular V),c(0 K), are essential parameters for understanding bulk hetero junction solar cells. 

MEH-CN-PPV, (b) hetero-junction solar cell, and (c) the light intensity dependence of the cell current and voltage. Reprinted with permission from Nature, Granstrom et al. (1998). Copyright 1998 Macmillan Magazines Limited. 

Polythiophenes and MEH-PPV have been used as hole-transporting layers to replace the electrolyte in cells with TiC>2 electrodes (Gratzel cells). Bulk hetero-junction devices have been made by the hydrolysis in air of titanium(IV) wopropoxide, co-deposited in MDMO-PPV spin-coated films, to form TiC>2 nanoparticles (van Hal et al., 2003). While reasonable peak performance under monochromatic illumination has been reported, the performance as a solar cell does not match that of the photo-electrochemical cells. 

Of perhaps even more interest arc the discontinuities in hands at a hetero-junction, the junction between two semiconductors, which is u.sed in electronic devices. The predicted discontinuity in the valence-band maximum (Harrison, 1977 b) is obtained by simply subtracting the corresponding values from Table 10-1. The discontinuity in the conduction bands is obtained by making a correction on each side of the hcterojunclion for the band gap. In alloys, the values from Table... 

The (semi)conductive properties of fullerene derivatives and carbon nanotubes have been a topics of intense recent interest. Unfortunately, at the time of writing it is still not possible to obtain well-defined samples of nano-tubes in sufficient amounts to carry out PR-TRMC measurements. Relatively pure, powder samples of C60 were however available in the mid-1990s and large, if short-lived, PR-TRMC transients could be observed. More recently attention has focused on the methyl ester of phenyl-C61-butyric acid (PCBM) which is a soluble C60 derivative that has found applications as the electron accepting component in hetero-junction materials for solar energy conversion. 

Structural features of thin-film y-Fe203/In203 hetero-junctions and their gas-sensing features have been studied. Two mechanisms of adsorbed CH4 molecule effect on electric conductivity of the l ers depending on operating temperature were proposed. 

Carbon electronics started from the investigation of diamond single crystals (sp -type hybridization) because the diamond crystal structure is similar to that of Si and Ge. It was expected that both p- and n-type doping could be achieved in diamond to obtain the basic element of solid-state electronics that is, the p-n junction. However, the conductivity of only the p-type was realized in diamond and it was the main obstacle to the creation of carbon electronics. Nevertheless, there is an alternative route to the creation of hetero-junctions by use of the highly oriented sp -hybridized carbon films doped by different elements. 

From these analyses, it was found that the 2 nm thick Ir layer had a hetero-junction between the rutile TiOz surface and the large surface area of the exposed Ir metal surface, due to the thin layered structure of Ir. Our work therefore suggests that the activity of CO oxidation was directly associated with the structure of the Ir metal phase having nano-level thickness on the TiOz support, and the selection of the support. 

From the obtained results it is known that the deposited composites film showed a higher photoactivity when compared to the single components due to the availability of numerous interfaces for enhanced charge transfer at the hetero-junction. Effective transport of excitons in conjugated polymers is extremely important for performances of organic light emitting... 

It has been proved that the elimination of lower energetic carriers brings the enhancement of Z and the optimal height of potential barrier within the two band model is given analytically. Such elimination (confinement) of minority carriers has already studied in GaAs/AlAs systems and it has been known that the use of graded alloy composition at interfaces of hetero-junction are important to confine the minority carriers effectively[8] due to decrese in the defect density near interfacial region. 

Martens T, Beelen Z, D Haen J, Munters T, Goris L, Manca J, D Olieslaeger M, Vanderzande D, Schepper LD, Andriessen R (2003) Morphology of MDMO-PPV PCBM bulk hetero-junction organic solar cells studied by AFM, KFM and TEM. In Kafafi Z H, Fichou D (eds) Organic photovoltaics III. SPIE Proc 4801 40... 

Munters T, Martens T, Goris L, Vrindts V, Manca J, Lutsen L, Ceunick WD, Vanderzande D, Schepper LD, Gelan J, Sariciftci NS, Brabec CJ (2002) A comparison between state-of-the-art gilch and sulphinyl synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells. Thin Solid Films 403-404 247... 

Sensing Infrared Light With an Organic/Inorganic Hetero-Junction... 

In this paper it is shown that a p-Si/PCBM hetero-junction features a photovoltaic effect in the infrared regime between photon energies from 0.4 to l.leV (3-1.3 pm) [2],... 

Fig. 1. Left graph shows the chemical structure of the fullerene derivative - PCBM. Right graph shows a schematic cross section of the Al/p-Si/PCBM/Al hetero-junction with an sample photograph [2],...

Fig. 3. Spectrally resolved photo-current of an Al/p-Si/PCBM/Al hetero-junction in the temperature range of 220 to 80 K. The dotted line in the top graph shows the transmission of the at room temperature kept silicon filter [2].

In summary, it has been shown that by infrared radiation charge-carriers can be directly excited across the interface of a silicon/fullerene hybrid hetero-junction. Besides its scientific relevance, the simple fabrication process as well as its compatibility with the well established silicon technology makes the presented hybrid approach a promising candidate for widespread applications. 

Sensing Infrared Light with an Organic/Inorganic Hetero-junction. 153... 

---