We demonstrate tandem and triple-junction polymer solar cells with power conversion efficiencies of 8.9% and 9.6% that use a newly designed, high molecular weight, small band gap semiconducting polymer and a matching wide band gap polymer.

Diketopyrrolopyrrole-based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open-circuit voltage below 0.6 eV.

The recent significant increase in power conversion efficiency (PCE) of polymer-fullerene solar cells largely originates from the successful development of new electron donor polymers. The donor-acceptor (D-A) or push-pull design, where electron-rich and electron-deficient units alternate along the copolymer chain-is commonly used to tune the HOMO and LUMO energy levels and the optical band gap of these polymers. [1,2] While structure-property relationships for energy levels are well-established, these are less clear for the actual photovoltaic performance. Creating morphologies in which nanometer-sized, interconnected, semi-crystalline domains of both polymer and fullerene exist seems crucial for high photovoltaic performance. [3,4] These semi-crystalline domains optimize the conjugation along the polymer backbone and allow delocalizing the carrier wave functions to assist efficient charge separation. [5] A high molecular weight and a tendency to crystallize are important in achieving such morphologies.Herein we present the advantageous effect of high molecular weight and refined energy level control through the synthesis of a regular alternating D 1 -A-D 2 -A terpolymer and demonstrate its superior performance in polymer-fullerene solar cells compared to the corresponding D 1 -A and D 2 -A copolymers. The regular alternating D 1 -A-D 2 -A design motif presents a versatile way to fine-tune energy levels and the optical band gap. Compared to random alternation of D 1 and D 2 with A, the regular D 1 -A-D 2 -A alternation allows quantifying the exact chemical composition. [6][7][8][9] Furthermore, regular alternation of units along the polymer chain reduces local variations in HOMO and LUMO energy levels that broaden the density of states and reduce charge carrier mobility. [10] The new terpolymer uses diketopyrrolopyrrole (DPP) as the electron-deficient unit (A), alternating with electron-rich terthiophene (D 1 = 3T) and thiophene-phenylene-thiophene (D 2 = TPT) segments in a regular fashion: PDPP3TaltTPT (Scheme 1). The DPP unit has previously been copolymerized with several different electron-rich units, providing polymers with excellent performance in photovoltaic cells and fieldeffect transistors. [4,[11][12][13][14][15][16][17][18] The choice for the 3T and TPT segments is based on our previous work on the individual PDPP3T and PDPPTPT polymers (Scheme 1), for which we obtained favorable PCEs of 4.7 % and 5.5 %. [11,12] Herein we demonstrate that by improving the polymerization reaction of PDPP3T, PDPPTPT, and of the new PDPP3TaltTPT, a dramatic enhancement of the PCEs to 7.1 %, 7.4 %, and 8.0 %, respectively, can be achieved. These PCEs are the highest values reported for DPP-based polymers to date.Compared to previous synthesis, [11,12] the improved crosscoupling polymerization procedure involves a slight decrease in the amount of palladium (4-6 mol % vs. 8-9 mol %) and using a higher triphenylphosphine to palladium ligand ratio (Pd/PPh 3 of 1:2 vs. 1:1.2). The higher ligand ratio serves to prevent decompos...

Conjugated polymers have been extensively studied for application in organic solar cells. In designing new polymers, particular attention has been given to tuning the absorption spectrum, molecular energy levels, crystallinity, and charge carrier mobility to enhance performance. As a result, the power conversion efficiencies (PCEs) of solar cells based on conjugated polymers as electron donor and fullerene derivatives as electron acceptor have exceeded 10% in single-junction and 11% in multijunction devices. Despite these efforts, it is notoriously difficult to establish thorough structure-property relationships that will be required to further optimize existing high-performance polymers to their intrinsic limits. In this Account, we highlight progress on the development and our understanding of diketopyrrolopyrrole (DPP) based conjugated polymers for polymer solar cells. The DPP moiety is strongly electron withdrawing and its polar nature enhances the tendency of DPP-based polymers to crystallize. As a result, DPP-based conjugated polymers often exhibit an advantageously broad and tunable optical absorption, up to 1000 nm, and high mobilities for holes and electrons, which can result in high photocurrents and good fill factors in solar cells. Here we focus on the structural modifications applied to DPP polymers and rationalize and explain the relationships between chemical structure and organic photovoltaic performance. The DPP polymers can be tuned via their aromatic substituents, their alkyl side chains, and the nature of the π-conjugated segment linking the units along the polymer chain. We show that these building blocks work together in determining the molecular conformation, the optical properties, the charge carrier mobility, and the solubility of the polymer. We identify the latter as a decisive parameter for DPP-based organic solar cells because it regulates the diameter of the semicrystalline DPP polymer fibers that form in the photovoltaic blends with fullerenes via solution processing. The width of these fibers and the photon energy loss, defined as the energy difference between optical band gap and open-circuit voltage, together govern to a large extent the quantum efficiency for charge generation in these blends and thereby the power conversion efficiency of the photovoltaic devices. Lowering the photon energy loss and maintaining a high quantum yield for charge generation is identified as a major pathway to enhance the performance of organic solar cells. This can be achieved by controlling the structural purity of the materials and further control over morphology formation. We hope that this Account contributes to improved design strategies of DPP polymers that are required to realize new breakthroughs in organic solar cell performance in the future.