© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Polymer Solar Cells with Diketopyrrolopyrrole Conjugated Polymers as the Electron Donor and Electron Acceptor Weiwei Li, W. S. Christian Roelofs, Mathieu Turbiez, Martijn M. Wienk, and René A. J. Janssen* electron-acceptor polymers are known. Among these are cyano- polyphenylenevinylenes, [24,32] copolymers of benzothiadiazole and fluorene, [21,22,33–36] and copolymers incorporating perylen- ediimide or naphthalenediimide acceptor units. [14–20] Electron- acceptor polymers based on isoindigo [37] or diketopyrrolopyrrole (DPP) [38] have only given low PCEs (<1%) in polymer–polymer solar cells until now. The primary design strategy for an electron-accepting conju- gated polymer is to create a complementary energy level align- ment with an electron-donating polymer, such that it provides enough driving force for exciton dissociation into free charges. The free energy for charge generation is determined by the dif- ference between the exciton energy, or optical gap ( E g ), and the energy of the charge transfer (CT) state at the interface ( E CT ). These energies are related to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the donor and the acceptor. It has been found experimentally that the offsets between the energies of the two HOMO and the two LUMO levels should both be ca. 0.35 eV or more to ensure charge transfer. [39] Energy-level control can be achieved by incorporating suitable electron-donating or elec- tron-deficient moieties in the conjugated backbone. [30] Next to proper energy levels, a high electron mobility is needed for a successful acceptor polymer to ensure efficient electron trans- port to the electrode. It has been shown that trap-free electron transport can only be obtained in conjugated polymers with an electron affinity larger than ca. 3.6 eV. [40] In addition, the elec- tron mobility will be enhanced via interchain interactions and three-dimensional order. Enhanced crystallinity is also ben- eficial in creating efficient percolating pathways for hole and electron transport via microphase separation. [41] Several pub- lications have shown that it is possible to improve the micro- phase separation and the PCE by augmenting the processing solvent with a processing additive, [17,41,42] which is a well-known method used for morphology control in polymer:fullerene solar cells. Our design for an n-type DPP acceptor polymer starts from the copolymer of DPP and terthiophene (PDDP3T). As elec- tron donor, PDPP3T affords a PCE of 7.4% in polymer solar cells when blended with [70]PCBM as the electron acceptor. [6] Although PDPP3T has a relatively low LUMO level (-3.74 eV), its HOMO level (-5.30 eV) is not sufficiently deep to make it a successful electron acceptor in blends with most electron- donor conjugated polymers. To decrease the HOMO level, we replace thiophene by thiazole to form PDPP2TzT ( Figure 1) in which the electronegative imine nitrogens (C=N-C) effectively lower the HOMO and LUMO energy levels. [35,43,44] We find that PDPP2TzT has an electron mobility of 0.13 cm 2 V -1 s -1 in a FET and can be used as electron acceptor in all-polymer Dr. W. W. Li, W. S. C. Roelofs, 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 Dr. M. Turbiez Organic Electronic Materials Basel BASF Schweiz AG Schwarzwaldallee 215, 4002, Basel, Switzerland DOI: 10.1002/adma.201305910 Conjugated polymers based on the diketopyrrolopyrrole (DPP) unit are successfully applied in field-effect transistors (FETs) [1,2] and in polymer solar cells as electron donor. [3,4] Record high mobilities for holes up to 10 cm 2 V -1 s -1 have been realized, par- tially as a result of the tendency of DPP polymers to crystallize. [5] In bulk-heterojunction solar cells, power conversion efficiencies (PCEs) up to 8% with fullerene as electron acceptor have been achieved, exploiting the broad absorption of DPP polymers to the near infrared region. [6] Interestingly, several DPP polymers also show excellent electron mobilities in FETs. [7–11] Combined, these properties raise the question as to whether it is possible to create a bulk-heterojunction solar cell that uses DPP-based semiconducting polymers both as electron donor and as elec- tron acceptor. Intrigued by this question, we designed and syn- thesized a new semiconducting DPP polymer and demonstrate its use as electron acceptor in polymer-polymer solar cells with a second DPP polymer as electron donor. Polymer–polymer solar cells, in which conjugated polymers are used both as electron donor and electron acceptor, are attracting renewed attention. [12–25] Polyera announced obtaining a power conversion efficiency (PCE) of 6.4% for proprietary materials. [26] For known materials, the highest value published to date is PCE = 4.1%. [14] These recent results give credence to the belief that polymer–polymer solar cells can close the gap to the best polymer–fullerene bulk-heterojunction solar cells that have reached PCEs = 9–10%. [27–29] The structural and electronic variation in electron-acceptor conjugated polymers is virtually infinite and offers much wider possibilities for varying the electronic structure and optical bandgap compared with fullerene derivatives. The energy levels, optical bandgap, crystallinity, and charge mobility, can all be adjusted toward improving the power conversion effi- ciencies. Numerous alternating electron push-pull copolymers have been explored as electron donor, [30,31] but - despite the advantages mentioned above - conjugated polymers that can act as electron acceptor in efficient solar cells are rather underdeveloped. [12] So far, only a few successful conjugated Adv. Mater. 2014, DOI: 10.1002/adma.201305910