Inside Front Cover: Controlling Morphology in Polymer–Fullerene Mixtures (Adv. Mater. 2/2008)

On p. 240, Klaus Meerholz and co‐workers show control over the aggregation of P3HT in solution by mixing a dipolar, but miscible solvent to the coating solution. The resulting nanoparticle dispersions are stable and allow a quantitative comparison of the absorption spectra of amorphous and aggregated P3HT. These results are interesting not only because they allow control of morphology on the nanometer scale, but they also show a path to low‐cost morphological control of large‐area films, which is an essential step for the commercialization of plastic PV devices.     

Impact of Solvent Additive on Carrier Transport in Polymer:Fullerene Bulk Heterojunction Photovoltaic Cells

The effects of a solvent additive, 1,8‐diiodooctane (DIO), on both hole and electron transport are investigated in a state‐of‐the‐art bulk‐heterojunction (BHJ) system, namely PTB7:PC₇₁BM. For a polymer:fullerene weight ratio of 1:1.5, the electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC₇₁BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 and 5 vol%, the electron and hole mobilities are balanced. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of the electrons. In addition, there exist alternative donor–acceptor ratios to achieve optimized PTB7:PC₇₁BM based solar cells. It is demonstrated that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.