Optimum PCBM Concentration

Optical measurements showed that a small concentration of 2% is effective in quenching the PL of MDMO but a high concentration of more than 67% is required to increase the photoconductivity or the short circuit current of a BHJSC. The solar cells efficiency measurements show that the optimum PCBM concentration is 80%. See next section. 

The short circuit current /sc and the open circuit voltage V0c are shown in Fig. 5.19. It is seen that the short circuit current is strong function of the composition. The open circuit voltage is also dependent on PCBM concentration. It is difficult to conclude whether or not open circuit voltage is entirely dependent on bulk heterojunctions. The open circuit voltage depends on short circuit current and also on dark saturation current. The dependence of dark saturation current on PCBM concentration is not known. It is clear from the figure that the optimum PCBM concentration is 80% where the short circuit current is maximum. 

Recently, a new method for the preparation of active layers of polymeric solar cells without the need for thermal post-treatment to obtain optimal performance was presented by Berson et al. [263]. P3HT nanofibers were obtained in highly concentrated solutions, which enabled the fabrication of nanostructured films on various substrates. By mixing the nanofibers with a molecular acceptor such as PCBM in solution, it was possible to obtain in a simple process a highly efficient active layer for organic solar cells with a demonstrated PCE of up to 3.6%, which was achieved with an optimum composition of 75 wt% nanofibers and 25 wt% P3HT. 

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