**2.2 Organic bulk heterojunction solar cells**

416 Solar Cells – New Aspects and Solutions

been shown to be compatible with solution-processing at room temperature, for instance, by high-throughput printing techniques. Processing on flexible substrates is possible, thus allowing for roll-to-roll manufacturing as well as influencing the properties of the finished electronic devices. The recent considerable achievements in terms of power conversion efficiency have been made possible now by more than 15 year long research and development on solution-processed organic solar cells. Nevertheless, in order to let the scientific progress be followed by a commercial success, further improvements in term of

In this chapter, we will briefly introduce the basic working principles of organic solar cells and present an overview of the most often studied PPV-type materials as applied within the

The first organic solar cells consisted of a single layer of photoactive material sandwiched between two electrodes of different work functions (Chamberlain, 1983; Wohrle & Meissner, 1991). However, due to the high binding energy of the primary photoexcitations, the separation of the photogenerated charge carriers was so inefficient that far below 1% power

The next breakthrough was achieved in 1986 by introducing the bilayer heterojunction concept, in which two organic layers with specific electron or hole transporting properties were sandwiched between the electrodes (Tang, 1986). In this organic bilayer solar cell were consisting of a light-absorbing copper phthalocyanine layer in conjunction with an electronegative perylene carboxylic derivative. The differing electron affinities between these two materials created an energy offset at their interface, thereby driving exciton

The efficiencies of the first organic solar cells reported in the 1980s were about 1% at best at that time. Primarily, this is due to the fact that absorption of light in organic materials almost always results in the production of a mobile excited state, rather than free electronhole pairs as produced in inorganic solar cells. This occurs because in organic materials the weak intermolecular forces localize the exciton on the molecules. Since the exciton diffusion lengths in organic materials are usually around 5-15 nm (Haugeneder et al., 1999), much shorter than the device thicknesses, exciton diffusion limits charge-carrier generation in these devices because most of them are lost through recombination. Photogeneration is

The discovery of ultrafast photoinduced electron transfer (Sariciftci et al., 1992) from a conjugated polymer to buckminsterfullerene (C60) and the consequent enhancement in charge photogeneration provided a molecular approach to achieving higher performances from solution-processed systems. In 1995 the first organic bulk heterojunction organic solar cell was fabricated based on a mixture of soluble *p*-phenylene-vinylene (PPV) derivative with a fullerene acceptor (Yu et al., 1995). In 2001, Shaheen et al. obtained the first truly promising results for bulk heterojunction organic solar cells when mixing the conjugated polymer poly(2-methoxy-5-(3',7'-dimethyl-octyloxy)-*p*-phenylene vinylene) (MDMO-PPV) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) yielding a power

therefore a function of the available mechanisms for excitons dissociation.

conversion efficiency of 2.5% (Shaheen et al., 2001).

efficiency and device lifetime have to be made.

conversion efficiency could be achieved.

photoactive layer.

**2.1 A brief history** 

dissociation.

**2. Organic solar cells** 

The sequential process involved in the light into electricity conversion can be summarized by the following steps: First, incident light is absorbed within the photoactive layer leading to the created of a bound electron-hole pairs (singlet excitons); the created excitons start to diffuse within the donor phase leading to charge separation; the separated charge carriers are transported to the corresponding electrodes.

Fig. 1. (a) Schematic device structure and (b) energy diagram for an organic bilayer solar cell

Figure 1 (a) shows the simplest structure of an organic bilayer solar cell appears to be the superposition of donor and acceptor materials on top of each other, providing the interface needed to ensure the charge transfer. The schematic energy diagram of such an organic bilayer solar cell is depicted in Figure 1 (b). The excitons photogenerated in the donor or in the acceptor can diffuse to the interface where they are dissociated. According to the Onsager theory (Onsager, 1938) that can be invoked as a first approximation in organic semiconductors, photoexcited electrons and holes, by virtue of the low dielectric constant intrinsic to conjugated polymers, are coulombically bound. Due to the related exciton binding energy, which with around 0.5 eV is much larger than the thermal energy, the photogenerated excitons are not easily separated. Once excitons have been generated by the

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 419

Fig. 3. MIM picture for a polymer diode under different operating modes. (a) open circuit

injection is hindered by the field present in the device (Dennler & Sariciftci, 2005).

When polarized in the forward direction (high work function electrode (ITO) connected to (+) and low work function electrode (Al) connected to (-)) as in Figure 3 (c), electrons can be injected from the Al electrode to ITO electrode and holes from ITO electrode to Al electrode. The effective field in the device will ensure the drift of electrons from Al electrode to ITO electrode and hole from ITO electrode to Al electrode. Finally, when the device is polarized in the reverse direction (ITO connected to (-) and Al connected to (+)) (Figure 3 (d), charge

condition, (b) short circuit condition, (c) forward bias, (d) reverse bias.

absorption photons, they can diffuse over a length of approximately 5-15 nm (Haugeneder et al., 1999). Since the exciton diffusion lengths in conjugated polymers are less than the photon absorption length, the efficiency of a bilayer cell is limited by the number of photons that can be absorbed within the effective exciton diffusion range at the polymer/electron interface. This limits drastically the photocurrent and hence the overall efficiency of the organic bilayer solar cells. To overcome this limitation, the surface area of the donor/acceptor interface needs to be increased. This can be achieved by creating a mixture of donor and acceptor materials with a nanoscale phase separation resulting in a threedimensional interpenetrating network: the "bulk heterojunction solar cells" (Figure 2).

Fig. 2. (a) Schematic device structure and (b) energy diagram for an organic bulk heterojunction solar cell

The discovery of 1-(3-methoxycarbonyl)propyl-1-phenyl[6]C61 (PCBM) (Hummelen et al., 1995), a soluble and processable derivative of fullerene C60, allowed the realization of the first organic bulk heterojunction solar cell by blending it with poly(2-methoxy-5-(2'-ethyl-hexoxy)- 1,4-phenylene-vinylene) (MEH-PPV) (Yu et. Al., 1995). Figure 2(b) demonstrates the schematic energy diagram of an organic bulk heterojunction solar cell. Contrary to Figure (1b), excitons experience dissociation wherever they are generated within the bulk. Indeed, the next interface between donor and acceptor phases is present within the exciton diffusion length everywhere in the device. After having been generated throughout the bulk, the free carriers have to diffuse and/or be driven to the respective electrodes (Dennler & Sariciftci, 2005).
