**6. Conclusions**

22 Will-be-set-by-IN-TECH

and untreated cells, dark IV curves were recorded for all 4 types of treated cells shown in Fig.

Fig. 17. The improvement in charge carrier mobility in treated (annealed films and films fabricated with processing additive) compared to pristine films demonstrated by two methods: dark current-voltage injection and CELIV. (a) log-lin plot showing the rectification ratio in forward and reverse bias and insignificant differences in leakage current in reverse bias. (b) log-log plot in forward bias showing much higher injection current levels in treated blends. (c) faster carrier extraction in treated films compared to pristine directly measured by CELIV current transients. Improvement in the carrier mobility can be seen from the shift in the position of extraction maximum, while experimental conditions (film thicknesses and applied voltages ) were kept similar. Thermal annealing of films fabricated with processing additive results in no change in performance. Reprinted with permission from (Pivrikas et

The dark current in the region of negative applied voltage (the reverse bias, positive voltage on Al, negative on ITO), is similar in all cells, showing that current injection is contact limited. A significant rectification ratio is observed for all types of studied cells. The dark leakage

Due to the different nanomorphologies of the interpenetrating network, the dark conductivity is expected to increase in the cells with higher conversion efficiency, because of improved conductivity of the films (assuming the injection is not limited by the contact). The dark injection current in forward bias is observed to be significantly higher in treated cells. In Fig. 17 (b) the dark injection current in forward bias is plotted in log-log scale for all devices. Faster charge carrier mobilities in all cells were estimated from these dependences using the Mott-Gurney Law. As can be directly seen from the magnitude of injection current, the highest mobility was observed in the films with chemical additives, confirming the beneficial effect of chemical additives for charge transport in bulk-heterojunction solar cells. From CELIV measurements, shown in Fig. 17 (c) it was demonstrated that charge carrier mobility is mainly

However, the charge carrier recombination processes in operating devices has yet to be clarified. It was shown that the typically expected Langevin bimolecular charge carrier recombination can be avoided in highly efficiency P3HT and PCBM blends.(Pivrikas et al., 2005) Non-Langevin carrier recombination was shown to be crucially important in low mobility organic photovoltaic devices, since the requirement for the slower carrier mobility can be reduced without recombination losses. This implies that close to unity Internal quantum efficiency can be reached in low bandgap organic materials with very low carrier mobility if reduced bimolecular recombination (non-Langevin) is present in the device.

al., 2008). Copyright 2008, with permission from Elsevier.

current in reverse bias is rather high, but similar for all cells.

reponsible for improvements in OPV performance.

17.(Pivrikas et al., 2008)

The film nanomorphology of bulk heterojunction solar cells determines the power conversion efficiency through photophysical properties such as light absorption, exciton dissociation, charge transport and recombination. The nano-morphology can be controlled by a variety of different methods. Thermal annealing of fabricated solar cells can be successfully substituted with slow drying of the solvent or chemical additives. These methods induce the phase separation between the donor and acceptor in the bulk-heterojunction, which results in red-shifted light absorption, improved exciton dissociation, faster charge carrier transport, and reduced recombination. Segregated donor-enriched and/or acceptor-enriched phases can be formed resulting in an interpenetrating bicontinuous network with the domain sizes comparable to the exciton diffusion length. Interconnected pathways for electromn and hole transport to the electrodes are required. This structure is essential for the photovoltaic performance of polymer-based solar cells. Therefore, reproducible, low cost nano-structure control is crucially important for fabrication of high efficiency OPV suitable for commercialization. In order to be able to control and predict the film nano-morphology of novel materials, an understanding of the material parameters governing the phase separation is required.
