Introduction
The polymer-based photovoltaic devices present some important advantages, such as the low cost and the easy manufacturing from thin films by chemical/physical vapour deposition, screen-printing, or casting. The band gap of the films can be adjusted by chemical synthesis to convenient values and the carrier mobility can reach 10 cm2/V·s (Dimitrakopoulos & Mascaro, 2001; Kwok, 2003). Therefore, the polymer-based photovoltaic cells are competitive with those based on amorphous silicon, so that the interest for both the properties of the polymer materials and the characteristics of the polymer-based cells increases.
The first structure was a Schottky diode type structure, which has a conversion efficiency under 1%. In 1986, Tang (1986) introduced the planar donor-acceptor heterojunction, which has a higher conversion efficiency. Hiramoto, Fujiwara, and Yokoyama (1991) developed this idea. Tang introduced the concept of Bulk Heterojunction—BH. Xue et al. (2005) introduced the concept of Hybrid Heterojunction (HH) with higher efficiency 5%.
Solar cells based on organic polymers used photoinduced transfer of electrons from semiconductor polymers, donor / acceptor polymers or acceptor molecules (such as C60). Pairs of layers polymer / fullerene have a poor conversion efficiency of solar cell. Nanomorfological control at nanoscale of the separate regions in an interlaced network (BH) significantly increased the conversion efficiency of solar energy cells made of MDMO-PPV/C60. Typical dimensions of the separate regions must be smaller than the exciton diffusion length. On the other hand, bicontinue percolation paths for the transport of charge carriers at the electrodes must be provided in order to increase the transport of charge carriers in organic and polymeric materials, and for mesoscopic order to improve crystallinity. So, an interlaced network at nanoscale with crystalline order of the both constituents is formed (these ones belong to individual sub-networks). Such a network is a convenient structure for the active layer of polymer PV devices. At the same time, forbidden bands of the photoactive layer materials should be chosen in order to absorb more light in solar radiation spectrum.
Using a mixture of MDMO-PPV soluble fullerene derivative (for example PCBM), it is possible to obtain solar cells with a 2.5% conversion efficiency.
It was shown (Shaheen, et al., 2001) that a conversion efficiency of 2.5% (for AM 1.5) can be obtained using chlorobenzenes as a solvent for the deposition by centrifuging in the mass ratio of 1:4 for the MDMO- PPV: PCBM. Using chlorobenzenes instead of toluene nanomorfology changes were remarked and the efficiency conversion increases 3 times . Such BH solar cells contain 80% PCBM . However, the MDMO-PPV polymer could be the main light absorber from these solar cells, because PCBM does not achieve almost no absorption in the visible and near infrared region. So an increase in volume concentration of MDMO-PPV was necessary for a better absorption of sunlight.
The electron mobility from pure PCBM is proved to be higher 
compared to the holes mobility from the MDMO-PPV 
and the holes mobility in the mixture increases with the increasing of fullerene mass, despite of the fact that the addition of fullerene introduces more defects (which would reduce the mobility).
By replacing in PCBM the C60 fullerene with C70 one, the HOMO-LUMO transition would be easier and would increase the absorption of light. An improved efficiency of BH solar cells in isomeric mixture of MDMO-PPV and C70 derivatives was obtained (Wienk, et al., 2003). Mixtures of this kind are suitable when they are prepared from a solution of o-dichlorobenzene (ODCB) and solar cells have a conversion efficiency of 
for PM 1.5.