Structure of the Heterojunction

In the following pictures obtained by scanning electron microscopy the shape of the CdS-Cu S heterojunction formed by topotaxial techniques will be illustrated. The structure of the heterojunction and the surface are of major influence on the optical and electrical data of the cell. 

The real shape of the p-n-junction can be revealed by angle lapping and etching techniques /34/. These investigations are essential for the control of the grain size of the CdS films and the thickness and grain boundary penetration of the CUj S. 

A scheme of the cross-section of a sample lapped at a low angle in combination with different etching techniques illustrates the procedure for the analysis of the structure (see fig. 15), 

Mainly three regions of a sample which has been prepared in this procedure can be distinguished  

III grain boundary Cu S in grain bound. grain boundaries 

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The expected efficiency of photovoltaic conversion is strongly dependent on the dark conductivity, on the processes of carrier photogeneration, on the kinetics of excitons and charge carriers, and can be estimated only in the framework of a complete theory which properly takes into account also the structure of the heterojunction. 

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. 

Structural information on the atomic arrangements at the early stage of formation of metal-metal, metal-semiconductor interfaces and semiconductor-semiconductor heterojunctions is needed along with the determination of the structure of the electron states in order to put on a complete experimental ground the discussion of the formation of solid-solid junctions. Amongst the structural tools that have been applied to the interface formation problem Surface-EXAFS is probably the best... 

Just as described for LEDs in the preceding section, the area of the heterojunction in solar cells with the planar structure shown in Fig. 10.3(a) is defined by the dimensions of the cell. Similarly, this area is dramatically increased by the use of blends of immiscible electron- and hole-transporting polymers (Moons, 2002). Carriers generated at the dispersed interface between the two polymers will be able to diffuse to the electrodes if the network contains... 

Figure 17. Energetic scheme of electron transfer processes taking place in a dye-sensitized heterojunction photovoltaic cell. Also shown is the structure of the spiro-MeOTAD molecule that constitutes an efficient organic hole transport material (HTM).

In fact, there are still some uncertainties about the applicability of Schottky-Mott model to 0/0 heterojunctions. For example, it is not clear whether the energy-level alignment is controlled only by the constituents of the heterojunction and is independent of the substrates and the formation sequence of the junction. In this chapter, we also discuss the electronic structures of some representative 0/0 heterojunctions and investigate the substrate effects on the energy-level alignment. In Section 6.7, we address the implications of the aforementioned for the design of ambipolar OFETs and stacked OLEDs. 

Fig. 6 Scheme for a sensitized nanocomposite of ZnO and CuSCN, as described by O Regan and coworkers [37]. (a) Layered structure of the dye-sensitized heterojunction. 

FigurelS.lO (a) Molecular structures of the n- (b) Schematics of bipolar field-effect transistors channel (BBL) and p-channel polymer based on n/p polymer heterojunctions, (c)...

Figure 3 (a) Molecular structures of the p- and n-type semiconductor materials and (b) layout of the planar heterojunction device... 

Molecular structures of the best performing Pt-containing polymers are shown in Figure 69. A composite of the polymer P-Pt-1 with [60]PCBM was evaluated in bulk heterojunction solar cells which yielded Voc = 820 mV, /sc = 15.4mAcm , FF = 39% and r] = 4.9%. Outstandingly... 

For the heterojunction system CdS-Cu S conversion efficiencies as high as 9 % have been demonstrated, a practical method of fabrication technology has been established and operational tests have been performed /3,4/. The results and experience obtained prove the CdS-Cu S thin film solar cell as an alternative to crystalline cells such as Si. The basic structure of the CdS-Cu S thin film solar cell is illustrated in figure 2. 

So far the physical realization of the heterojunction and its optimization in photovoltaic efficiency have been discussed. The production process of a photovoltaic generator and its economical use have to satisfy a momber of additional criteria which involve a) technological procedures for economic cell and module fabrication allowing large scale production and b) material evaluation for substrates, contacts and encapsulation. These criteria result in a variety of fabrication methods, structures of cells and modules, and materials. 

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