High‐Molecular‐Weight Regular Alternating Diketopyrrolopyrrole‐based Terpolymers for Efficient Organic Solar Cells

Abstract

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...

Fullerene derivatives as nano-additives in polymer composites

“…It was found that low molecular mass of PBDTTPD resulted in poor efficiency mostly because of the phase separation of the polymer and fullerene derivative and the formation of PC 70 BM-enriched domains, which was avoided in the case of the high-M n polymer. That study 56 and several similar publications 57,58 have demonstrated that the correlation between the BHJ solar cell performance and the molecular mass of the donor polymer may appear to be a promising method for optimization of the solar cell morphology and design of highly efficient structures.…”

Comparing random and regular diketopyrrolopyrrole–bithiophene–thienopyrrolodione terpolymers for organic photovoltaics

“…Owing to its electron deciency, DPP is known for providing access to small band gap polymers with high efficiencies in photovoltaic devices and ambipolar charge transport in eld-effect transistors. [1][2][3][4][5][6] TPD is slightly less electron decient, which results in wide to medium band gap materials. [7][8][9][10] Traditionally, semiconducting copolymers with a push-pull conguration only use the combination of one donor and one acceptor.…”

“…In this approach, two electron rich (or electron poor) building blocks are combined with one electron poor (or electron rich) building block. 2,[11][12][13][14][15] Incorporation of the different co-monomers in the polymer can be achieved in a regular or a (semi)random fashion. One of the principal reasons for using multiple components in the synthesis of semi-conducting polymers is that it can lead to broadening of their absorption spectra, which can be benecial for the harvesting of photons in a photovoltaic cell.…”

Synthesis of N,N′-dialkyl-6,6′-dibromoisoindigo Derivatives by Continuous Flow

“…Among those, the class of organic photovoltaics (OPV) has particular advantages in terms of aesthetics, flexibility, and cost and mainly aims at portable or wearable consumer goods and building/automotive integration [1][2][3][4][5]. OPV device efficiencies have recently made a huge leap forward, reaching~10% for polymerand small-molecule-based solution-processed bulk (single) heterojunction devices [6][7][8][9][10][11][12][13][14][15][16][17]. Nevertheless, some issues concerning cost, efficiency, and reliability still need to be resolved for OPV to become an economically viable technology.…”