© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1565 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Effect of the Fibrillar Microstructure on the Efficiency of High Molecular Weight Diketopyrrolopyrrole-Based Polymer Solar Cells Weiwei Li, Koen H. Hendriks, Alice Furlan, W. S. C. Roelofs, Stefan C. J. Meskers, Martijn M. Wienk, and René A. J. Janssen* Dr. W. W. Li, K. H. Hendriks, A. Furlan, W. S. C. Roelofs, Dr. S. C. J. Meskers, Dr. M. M. Wienk, Prof. R. A. J. Janssen Molecular Materials and Nanosystems Eindhoven University of Technology P.O. Box 513, 5600, MB Eindhoven, The Netherlands E-mail: r.a.j.janssen@tue.nl DOI: 10.1002/adma.201304360 Bulk heterojunction polymer solar cells based on conjugated polymers as electron donor and fullerene derivatives as elec- tron acceptor have been extensively investigated in the last decade and power conversion efficiencies (PCEs) reached up to 9.2% for single layer absorbers [1] and 10.6% for tandem cells. [2] In developing more efficient conjugated materials for solar cells, several polymer semiconductors have been synthe- sized in which alternating electron-rich and electron-deficient conjugated units are used to control the energy levels and optical band gap. [3] Moreover, a high charge carrier mobility is advantageous for a successful material. [4–6] High mobilities are achieved for materials with three-dimensional order that exhibit semicrystalline film morphologies. Recently, it is becoming increasingly clear that several different structural and molec- ular parameters are crucial ingredients for obtaining high PCEs in polymer solar cells. [7] Creating a rigid conjugated backbone to enhance crystallinity and charge transport, [8–10] introducing solubilizing side chains, [11] improving molecular weight, [12–14] and forming a micro-phase separation in bulk heterojunction films with the fullerene acceptor, [15,16] all seem required to approach the intrinsic limits of energy conversion efficiencies in polymer solar cells too. [17] This makes designing and synthe- sizing of superior materials a challenging task for which accu- rate guidelines are yet to be determined. The rigid backbone of conjugated polymers and their char- acteristic feature to aggregate via π- π stacking impart a reduced solubility on these materials in common organic solvents. By introducing alkyl chains the solubility is increased, but the electrically insulating side chains influence charge transport, aggregation behavior, and the bulk heterojunction morphology when blending the polymer with a fullerene derivative in the photoactive layer of a solar cell. [18–23] It is well established that the solubilizing side chains play a crucial role in forming the phase separation and bulk heterojunction morphology of con- jugated polymer – fullerene blends, [24,25] affecting exciton dif- fusion, charge dissociation, and charge transport. Even though accurate design rules for the optimal morphology are elusive, micro-phase separation with domain sizes on the order of 10 nm seems to be beneficial. Control of the domain size can possibly be obtained by adjusting the solubilizing side chains in combination with processing conditions. Short side chains reduce solubility and increase the tendency to crystallize. On the other hand, longer side chains enhance solubility and create more disorder, but -in contrast- also allow achieving high molecular weights during synthesis, which also can lead to improved aggregation. [14,26] Hence, the optimal length of side chains is not a simple choice and likely needs to be opti- mized for each specific conjugated backbone to achieve the best performance. In the past five years conjugated push-pull polymers that use diketopyrrolopyrrole (DPP) unit as electron withdrawing unit have emerged as promising semiconductors for application in field-effect transistors [27] and polymer solar cells. [28] DPP- based conjugated polymers with side chains such as ethylhexyl (EH), [29] butyloctyl (BO), [30–32] hexyldecyl (HD), [14,33,34] octyldo- decyl (OD), [16,35–37] and linear alkyl chains [38] have been applied in solar cells with PCEs up to 8%. [39] To obtain more insight into the effect of the side chains on the performance of organic solar cells we compare PDPPTPT polymers with HD, OD, and DT (decyltetradecyl) side chains of different length ( Figure 1). The synthesis of the polymers has been optimized to achieve high molecular weights. [39] The polymers were applied in bulk het- erojunction solar cells with [70]PCBM ([6,6]-phenyl-C 71 -butyric acid methyl ester) as acceptor, and provide PCEs ranging from 3.2% to 7.4%. We will show that the differences in PCE can be directly related to the microstructure of the bulk heterojunction as determined by transmission electron microscopy (TEM). More specifically the external quantum efficiency and photovol- taic performance correlate with the width of the long fibrils that form in blends of the PDPPTPT polymers with [70]PCBM. For fibrils that are significantly wider than ∼10 nm, reduced exciton dissociation into free charges limits the performance, because not all excitons diffuse to the interface with [70]PCBM. Steady state and time-resolved fluorescence spectroscopy support this conclusion. The results demonstrate that in designing conju- gated polymers for polymer solar cells, the nature of the solubi- lizing side chains is a crucial parameter. The three PDPPTPT polymers were prepared by Suzuki poly- merization using a catalyst system based on Pd 2 (dba) 3 as source of palladium with PPh 3 as ligand (See Supporting Information for details). The molecular weight of the polymers was deter- mined by gel permeation chromatography (GPC) using o-DCB as eluent at 80 °C and a low polymer concentration of 0.06 mg Adv. Mater. 2014, 26, 1565–1570