Influence of Fluorination and Molecular Weight on the Morphology and Performance of PTB7:PC 71 BM Solar Cells Xiaoxi He, † Subhrangsu Mukherjee, ‡ Scott Watkins, § Ming Chen, § Tianshi Qin, § Lars Thomsen, ∥ Harald Ade, ‡ and Christopher R. McNeill* ,⊥ † Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom ‡ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States § CSIRO Materials Science and Engineering, Private Bag 10, Clayton South MDC, Victoria 3168, Australia ∥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3169, Australia ⊥ Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia * S Supporting Information ABSTRACT: The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance domi- nates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combina- tion of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC 71 BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC 71 BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC 71 BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg mol −1 are required for DIO to be effective in reducing the PC 71 BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination; an efficiency of 4.6% is achieved for a degree of fluorination of only 5.3%. ■ INTRODUCTION Steady progress is continuing in the development of polymer solar cells with efficiencies of over 9% reported for single- junction devices and over 10% for tandem cells. 1−3 Cells based on the thienothiophene-benzodithiophene copolymer PTB7 4 (see Figure 1 for chemical structure) exhibit the highest literature-reported single-junction efficiencies 1 in blends with the fullerene derivative PC 71 BM. Blending of semiconducting polymers with high electron affinity fullerene derivatives is necessary to drive exciton dissociation, with bound excitons being the primary product of photogeneration. 5 The polymer acts as the electron donor (or hole acceptor) while the fullerene derivative acts as the electron acceptor (or hole donor). The active layer of polymer−fullerene solar cells is prepared by dissolving polymer and fullerene in a common solvent and casting a blended thin film either by spin-coating (typical for laboratory experiments) or any number of printing techniques (more appropriate for a manufacturing setting). The micro- structure of the resultant layer is sensitive to the solution deposition process; fine-tuning of the active layer morphology is required to optimize device performance. 6 For the PTB7:PC 71 BM system 4 (along with several other high- performance systems 7−9 ), small amounts of so-called solvent additives are added to the host solvent to achieve an optimal microstructure. Specifically, it has been shown that adding ∼3 vol % of the additive 1,8-diiodooctane (DIO) to solutions leads to a significant boost in device performance attributed to a reduction of the size of PC 71 BM domains from ∼200 nm to ∼20 nm. 4 The effectiveness of DIO has been linked to its ability to selectively dissolve PC 71 BM aggregates in solution. 10 Domains larger than ∼20 nm are generally not beneficial to device performance as the limited exciton diffusion length of organic semiconductors (∼10 nm) requires domains of order 10−20 nm for efficient exciton harvesting. 5 Domains that are too small can lead to reduced performance because of increased recombination. 11 Fundamental to understanding the high performance of PTB7-based solar cells is the influence of the chemical and Received: February 4, 2014 Revised: April 7, 2014 Published: April 11, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 9918 dx.doi.org/10.1021/jp501222w | J. Phys. Chem. C 2014, 118, 9918−9929