Heterojunction material

Another example of epitaxy is tin growdi on the (100) surfaces of InSb or CdTe a = 6.49 A) [14]. At room temperature, elemental tin is metallic and adopts a bet crystal structure ( white tin ) with a lattice constant of 5.83 A. However, upon deposition on either of the two above-mentioned surfaces, tin is transfonned into the diamond structure ( grey tin ) with a = 6.49 A and essentially no misfit at the interface. Furtliennore, since grey tin is a semiconductor, then a novel heterojunction material can be fabricated. It is evident that epitaxial growth can be exploited to synthesize materials with novel physical and chemical properties. 

Consequently, interpenetrating phase-separated D/A network composites, i.e. bulk heterojunction , would appear to be ideal photovoltaic materials [5]. By controlling the morphology of the phase separation into an interpenetrating network, one can achieve a high interfacial area within a bulk material. Since any point in the composite is within a few nanometers of a D/A interface, such a composite is a bulk D/A heterojunction material. If the network in a device is bicontinuous, as shown in Figure 15-26, the collection efficiency can be equally efficient. 

The bulk heterojunction material exhibits an ensemble of interface configurations, such that spectroscopic observables and measurable photocurrents reflect statistically averaged properties (with the exception of observations... 

The excellent photosensitivity and relatively high energy conversion efficiencies obtained from the bulk heterojunction materials are promising. The monochromatic power efficiencies for conjugated polymer photovoltaic devices are around... 

I.W. Hwang, D. Moses, and A.J. Heeger, Photoinduced carrier generation in P3HT/PCBM bulk heterojunction materials, J. Phys. Chem. C, 112, 4350-4354 (2008). 

Important progress has been made toward creating bulk D/A heterojunction materials [61,93,94,136].As shown in Figure 8.49, the short circuit current, /sc = 0.5 mA/cm under 20 mW/cm illumination, corresponding to a collection efficiency of t)c = 7.4% of electrons per incident photon [61] is approximately two orders of magnitude higher than that of pure MEH-PPV tunnel diodes as well as of the MEH-PPV/C60 heterojunction device described in the previous section. The electroluminescence quantum efficiency of this blend device was 10 -10 times less than in pure MEH-PPV devices, consistent with the ultrafast photoinduced charge separation which quenches the emission of the donor [61]. 

Figure 8.3 Transient absorption of bulk heterojunction materials e.g. P3HT PCeoBM, PCDTBTPCgoBM, etc.). (Left) Integrated spectral intensity associated with mobile carriers, normalized to the intensity at 100 fs and plotted on a linear scale near zero time delay. (Right) Semi-log plot of the integrated spectral intensity associated with the slower component of the mobile carrier generation process, normalized to the intensity at 100 fs. Dynamics are representative of the limit of low pump intensity. Reproduced with permission from ref. 5. Copyright 2013 American Chemical Society.

Moses, D., Heeger, A.J. Bulk heterojunction materials composed of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b ]thiophene). Ultrafast electron transfer and carrier recombination. J. Phys. Chem. C 112(21), 7853-7857 (2008)... 

Some of tliese problems are avoided in heterojunction bipolar transistors (HBTs) [jU, 38], tlie majority of which are based on III-V compounds such as GaAs/AlGaAs. In an HBT, tlie gap of tlie emitter is larger tlian tliat of tlie base. The conduction and valence band offsets tliat result from tlie matching up of tlie two different materials at tlie heterojunction prevent or reduce tlie injection of tlie base majority carriers into tlie emitter. This peniiits tlie use of... 

The first semiconductor lasers, fabricated from gallium arsenide material, were formed from a simple junction (called a homojunction because the composition of the material was the same on each side of the junction) between the type and n-ty e materials. Those devices required high electrical current density, which produced damage ia the region of the junction so that the lasers were short-Hved. To reduce this problem, a heterojunction stmcture was developed. This junction is formed by growing a number of layers of different composition epitaxially. This is shown ia Figure 12. There are a number of layers of material having different composition is this ternary alloy system, which may be denoted Al Ga his notation, x is a composition... 

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) 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, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

Eig. 10. Schematic of various LED and laser diode stmctures where S signifies material of a lower energy band gap (a) homojunction, (b) double-heterojunction (DH), and (c) multiquantum well (MQW) stmctures. 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

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