Heterojunctions molecular 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). 

Dale L. Martin, Molecular Beam Epitaxy of IV-VI Compound Heterojunctions Robert L. Gunshor, Leslie A. Kolodziejski, Arto V. Nurmikko, and Nobuo Otsuka, Molecular Beam Epitaxy of II-VI Semiconductor Microstructures... 

Peumans, P. Uchida, S. Forrest, S. R. 2003. Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films. Nature 425 158-162. 

Schubert M, Yin CH, Castellani M, Bange S, Tam TL, Sellinger A, Horhold HH, Kietzke T, Neher D (2009) Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells. J Chem Phys 130 094703... 

Xue JG, Rand BP, Uchida S, Eorrest SR (2005) A hybrid planar-mixed molecular heterojunction photovoltaic cell. Adv Mater 17 66... 

Xue JG, Uchida S, Rand BP, Forrest SR (2004) Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions. Appl Phys Lett 85 5757... 

Vandewal K, Tvingstedt K, Gadisa A, Inganas O, Manca JV (2010) Relating the open-circuit voltage to interface molecular properties of donor acceptor bulk heterojunction solar cells. Phys Rev B 81 125204... 

Fig. 2 Molecular structure of a model F8BT(top) TFB(bottom) polymer heterojunction in the eclipsed stacking configuration, see also Fig. 3. In the actual polymer, the residues are R = C8Hi2 in the calculations reported here, R = H was used (Adapted from Ref. [43]).

In Refs. [50-53], two levels of analysis were successively addressed (i) a two-state XT-CT model which is able to capture the basic features of the phonon-mediated exciton dissociation process (ii) a three-state XT-IS-CT model which also comprises an intermediate state (IS), i.e., an additional charge transfer state whose presence can have a significant influence on the dynamics, see Fig. 6. In the latter case, comparative calculations for several interface configurations were carried out, leading to a realistic, molecular-level picture of the photophysical events at the heterojunction. In the following, we start with a summary of the findings reported in Refs. [50,51], where the two-state model was explored (Sec. 5.1). Following this, we address in more detail the analysis of Refs. [52,53] for the three-state model (Sec. 5.2). 

Experimental determinations of barrier heights on oxide semiconductor interfaces using photoelectron spectroscopy are rarely found in literature and no systematic data on interface chemistry and barrier formation on any oxide are available. So far, most of the semiconductor interface studies by photoelectron spectroscopy deal with interfaces with well-defined substrate surfaces and film structures. Mostly single crystal substrates and, in the case of semiconductor heterojunctions, lattice matched interfaces are investigated. Furthermore, highly controllable deposition techniques (typically molecular beam epitaxy) are applied, which lead to films and interfaces with well-known structure and composition. The results described in the following therefore, for the first time, provide information about interfaces with oxide semiconductors and about interfaces with sputter-deposited materials. Despite the rather complex situation, photoelectron spectroscopy studies of sputter-deposited... 

Photovoltaic Devices with OPV4—Ceo- The increased lifetime of the charge-separated state, which extends into the millisecond time domain, opens the possibility of using the OPVrt-Coo dyads as the active material in a photovoltaic device. As an important difference with previous bulk heterojunction cells, the covalent linkage between donor and acceptor in these molecular dyads restricts the dimensions of the phase separation between the oligomer and the fullerene that could freely occur in blends of the individual components. This can be considered as a primitive attempt to obtain more ordered and better-defined phase-separated D-A networks. 

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