Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends

Journal article, 2006

The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values. [on SciFinder (R)]