Fullerene conjugated polymer heterojunctions

A significant increase in the forward current and in the FF is observed for conjugated polymer/fullerene bulk heterojunction solar cells upon insertion of a thin layer of LiF between the organic layer and the Al electrode (negative electrode of the solar cell), as shown in Fig. 5.37a and b. 

The observed experimental result that Voc decreases linearly for bulk heterojunction solar cells allows us to conclude that, at least in the high temperature range (T > 200 K), these solar cells may be described by a diode model with Ip exp(E/kT). Here E is a parameter analogous to Eg for conventional semiconductors. For conjugated polymer/fullerene bulk heterojunction solar cells, E should correspond to the energy difference between the HOMO level of the donor and the LUMO level of the acceptor components of the active layer [as also suggested by the extrapolated value of V oc(0 K)]. 

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

Hoppe H, Arnold N, Meissner D, Sariciftci NS (2003) Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells. Sol Energy Mater Sol Cells 80 105... 

Hoppe H, Niggemann M, Winder C, Kraut J, Hiesgen R, Hinsch A, Meissner D, Sariciftci NS (2004) Nanoscale morphology of conjugated polymer/fullerene based bulk-heterojunction solar cells. Adv Fund Mater 14 1005... 

Hoppe H, Glatzel T, Niggemann M, Hinsch A, Lux-Steiner MC, Sariciftci NS (2005) Kelvin probe force microscopy study on conjugated polymer/fullerene bulk heterojunction organic solar cells. Nano Lett 5 269... 

H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner, N. S. Sariciftci, Nanoscale Morphology of Conjugated Polymer/ Fullerene-Based Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2004, 14, 1005-1011. 

Brabec, C.J., G. Zerza, G. Cerullo, S. De Silvestri, S. Luzzati, J.C. Hummelen, and N.S. Sariciftci. 2001. Tracing photoinduced electron transfer process in conjugated polymer/fullerene bulk heterojunctions in real time. Chem Phys Lett 340 232. 

Hoppe, H., N. Arnold, D. Meissner, and N.S. Sariciftci. 2004. Modeling of optical absorption in conjugated polymer/fullerene bulk heterojunction plastic solar cells. Thin Sol Films 451-452 589. 

Mozer, A.J., G. Dennler, N.S. Sariciftci, M. Westerling, A. Pivrikas, R. Osterbacka, and G. Juska. 2005. Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells. Phys Rev B 72 035217. 

Miniaturization of the conjugated polymer/Ceo heterojunction devices described above has been realized by Rubner and co-workers [154-156,159]. Very uniform and pinhole-free films with charged, functionalized, conjugated polymers and fullerenes were demonstrated using the techniques of self-assembly [154-157]. Fabrication of 15 nm PPV light emitting diodes using the self-assembly of the precursor polymer on the substrate (followed by conversion to PPV) have been reported [154-157]. 

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). 

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. 

A similar temperature dependence of Isc, Voc, and r) is also reported for the lower mobility generation of solar cells, based on interpenetrating networks of conjugated polymers with fullerenes, but processed from solvents so that the initial efficiency is < 1% [156]. This behavior is discussed extensively in the section dealing with Isc. A positive temperature coefficient is also observed for the efficiency of Cgo single-crystal photoelectrochemical cells [160]. Finally, a temperature dependence of Isc qualitatively similar to that shown in Fig. 5.47a and 5.48 is also observed for organic solar cells based on Zn-phthalocyanine (ZnPc)/perylene (MPP) heterojunctions [161]. 

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