Solid heterojunction

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Suspensions of semiconductors with heterojunctions formed by CdS or solid solution ZnyCdi-yS and Cu , S have been prepared and tested as photocatalysts for photochemical hydrogen production [278]. With platinized powders of Zno.nCdo.ssS/CujS in solution containing both S and SOj ions, hydrogen was generated concomitantly with thiosulfate ions with quantum yield of about 0.5. 

N. Holonyak, Jr. and M. H. Lee, Photopumped III-V Semiconductor Lasers H. Kressel and J. K Butler, Heterojunction Laser Diodes A Van der Ziel, Space-Charge-Limited Solid-State Diodes P. J. Price, Monte Carlo Calculation of Electron Transport in Solids... 

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... 

Bach et al. have successfully introduced the concept of a solid p-type semiconductor (heterojunction), with the amorphous organic hole-transport material 2,2, 7,7 -tetrakis (, V, V-di-/ -methoxyphcnyl-aminc)9,9 -spirobifluorenc [96]. This hole-conducting material allows the regeneration of the sensitizers after electron injection due to its hole-transport properties. Nevertheless, the incident photon-to-current conversion efficiencies using complex 22 as a charge-transfer sensitizer... 

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

Figure 15 (a) A schematic of the process for fabricating nanotube/carbide heterojunction by means of solid- solid reaction. M represents Si or Ti and MC represents formation of metal carbide, (b) TEM images showing SiC nanorods interfaced with SWNT bundles and a single SWNT. (Reprinted with permission from Y. Zhang, T. Ichihashi, E. Landree, F. Nihey, and S. lijima, Science, 1999, 285, 1719. 1999 AAAS)... 

Photovoltaic devices are based on the concept of charge separation at an interface of two materials having different conduction mechanisms, normally between solid-state materials, either n- and p-type regions with electron and hole majority carriers in a single semiconductor material, heterojunctions between different semiconductors, or semiconductor-metal (Schottky) junctions. In photoelectrochemical cells, the junctions are semiconductor-electrolyte interfaces. In recent years, despite prolonged effort, disillusion has grown about the prospects of electrochemical photo-... 

Charge separation across a solid-state heterojunction... 

For the QD-sensitised cell, QDs are substituted for the dye molecules, as shown in Fig. 3.19b they can be adsorbed from a colloidal QD solution (Zaban et al, 1998) or produced in situ (Weller, 1991 Liu and Kamat, 1993 Vogel et al, 1994 Hoyer and Konenkamp, 1995). Successful PV effects in such cells have been reported for several semiconductor QDs including InP, CdSe, CdS, PbS and InAs (Weller, 1991 Liu and Kamat, 1993 Vogel et al, 1994 Hoyer and Konenkamp, 1995 Zaban et al, 1998 Nozik, 2001a Robel et al, 2006 Yu et al, 2006 Tachibana et al, 2007). Possible advantages of QDs over dye molecules are the tunability of optical properties with size, better heterojunction formation with solid hole conductors, and the unique potential capability of the QD-sensitised solar cell to produce quantum yields greater than one by MEG (inverse Auger effect) (Nozik, 2002). 

Figure 6.20 Spectral quantum collection efficiency in a CdTe /Xi02 heterojunction cell for various bias voltages. The solid hues give a best fit to the experimental data, based on a minority-carrier diffusion length of 130 nm (Ernst, 2001).

Ti02 heterojunction solar cells obtained by atomic layer deposition , Thin Solid Films 431-432, 492-496. 

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