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...

Solution processed polymer tandem solar cells that combine wide and small bandgap absorber layers reach a power conversion efficiency of 7% in a series configuration. This represents a 20% increase compared to the best single junction cells made with the individual active layers and shows that the tandem configuration reduces transmission and thermalization losses in converting sunlight.

Three isomeric π-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C 70 fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells.

To determine the role of photon energy on charge generation in bulk heterojunction solar cells, the bias voltage dependence of photocurrent for excitation with photon energies below and above the optical band gap is investigated in two structurally related polymer solar cells. Charges generated in (poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothia­diazole)] (C‐PCPDTBT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells via excitation of the low‐energy charge transfer (CT) state, situated below the optical band gap, need more voltage to be extracted than charges generated with excitation above the optical band gap. This indicates a lower effective binding energy of the photogenerated electrons and holes when the excitation is above the optical band gap than when excitation is to the bottom of the CT state. In blends of PCBM with the silicon‐analogue, poly[(4,4‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (Si‐PCPDTBT), there is no effect of the photon energy on the electric field dependence of the dissociation efficiency of the CT state. C‐PCPDTBT and Si‐PCPDTBT have very similar electronic properties, but their blends with PCBM differ in the nanoscale phase separation. The morphology is coarser and more crystalline in Si‐PCPDTBT:PCBM blends. The results demonstrate that the nanomorphological properties of the bulk heterojunction are important for determining the effective binding energy in the generation of free charges at the heterojunction.