The effect of molecule size and shape on free charge generation, transport and recombination in all-thiophene dendrimer:fullerene bulk heterojunctions
Charge mobilities of all-thiophene dendrimers were studied using the charge extraction by linearly increasing voltage technique (CELIV). Four dendrimers differing in size, shape and with and without trimethylsilyl (TMS) protecting groups have been compared yielding important structure–property correlations: The hole mobility of the dendrimers with 21 or 42 thiophene units was found to be very similar and around 10-5 cm2 V-1 s-1, indicating that shape and size of the studied dendrimers have little effect on their charge transport properties. Hole mobility is decreased by a factor of 3–10 by incorporation of TMS groups, which is attributed to the intrinsically slower electron transfer rates between the TMS-protected dendrimer molecules. Bulk heterojunction solar cells using 1:2 mixtures of the thiophene dendrimers and the electron acceptor [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) were fabricated and their performances were analyzed. The decreased hole mobility in the blend, the unbalanced electron and hole mobility and a rather short charge carrier lifetime measured by charge extraction and transient absorption techniques are thought to be important factors that limit overall power conversion efficiency in theses devices. Improved free charge generation with increasing dendrimer size was observed, which was related to increased separation distance between electron and hole pairs.