State fullerene/polymer solar cells

Lee J, Vandewal K, Yost SR, Bahlke ME, Goris L, Baldo MA, Manca JV, Van Voorhis T (2010) Charge transfer state versus hot exciton dissociation in polymer-fullerene blended solar cells. J Am Chem Soc 132 11878... 

Fullerene/Conjugated Polymer Composite for the State-of-the-Art Polymer Solar Cells... 

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). 

Fig. 15 Charge-transfer state electroluminescence (EL) for several polymer fullerene blends used in donor/acceptor organic solar cells. Adapted with permission from [184]. Copyright 2009 American Chemical Society...

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. 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

An alternative inexpensive organic polymer-based photovoltaic solar cell has been invented. In this device, p-type and n-type semiconductors are sequentially stacked on top of each other. In such devices, absorption of a photon by a ji-conjugated polymer results in the formation of an excited state, where coulom-bicaUy bound electron-hole pair (exciton) is created. This exciton diffuses to a region of interface of n-type semiconductor where exciton dissociation takes place and transport of charge to the respective electrodes occurs. For example, the photo-induced electron transfer from a donor layer (p-type) to acceptor layer (n-type) takes place in a polymer/fullerene-based organic bilayer solar cell, MDMO-PPV PCBM, with power conversion efiiciency of 2.5 % (Fig. 11.8) [13]. 

The capability of drift-diffusion models can be increased by introducing localized states into the band gap. Recently, several studies have introduced single trap levels [83] as well as distributions of localized states in order to describe the results of transient and steady state experiments on polymer fullerene solar cells [54, 84-89]. Most of these models use a Shockley-Read-Hall type occupation statistics for the localized states, which we will discuss in more detail in Sect. 2.3 and the Appendix 2 before discussing some of the implications of this model in the case studies in Sect. 4.1. 

---