The Effect of H‐ and J‐Aggregation on the Photophysical and Photovoltaic Properties of Small Thiophene–Pyridine–DPP Molecules for Bulk‐Heterojunction Solar Cells

Abstract

(1 of 12) 1605779 structure, and the perfectly reproducible synthetic procedures that largely avoid the variability between different batches seen in polymerizations. Recently, successful small-molecule donor systems have been reported that reach high power conversion efficiencies (PCEs) in organic solar cells. [2] The combination of a judicious molecular design with the optimal material processing conditions and device engineering remains a critical point for the success in the photoenergy conversion process. Rational molecular design is of great importance for further improving the photovoltaic performance of small-molecule bulk-heterojunction organic solar cells. However, achieving a high performance employing newly designed small-molecule materials remains a challenge because it is difficult to predict in sufficient detail how mole cules assemble in thin films and how this affects photovoltaic performance.When small-molecule chromophores assemble in the solid state, they often form H-type or J-type aggregates, depending on the relative alignment of the transition dipole moments on adjacent molecules. In an H-aggregate, molecules stack predominantly face-to-face, while J-aggregates form when molecules primarily stack in a head-to-tail arrangement. The formation of such aggregates has important consequences for the energies of the excited states and the oscillator strengths of the transitions to these states from the ground state. Consequently, H-and J-aggregation can strongly modify optical absorption and the photoluminescence spectra. To contribute to our understanding of the effect of molecular packing on photovoltaic performance, it is of interest to assemble one type of molecule in different packing modes.In this contribution, we do that by using molecules based on diketopyrrolopyrrole (DPP) flanked by two 5-(thiophen-2-yl) pyridin-2-yl units in which solubilizing hexyl side chains are introduced on the free positions of the peripheral thiophene units (Scheme 1). The DPP fragment has been extensively explored for organic semiconductors, because of its strong optical absorption and excellent charge transport properties, [3] especially for the design of materials for organic photovoltaic applications. [4] The electron density of the DPP core can be modified by employing different (hetero)aromatic flanking groups. The combination of DPP with two flanking pyridin-2-yl moieties leads to the simultaneous decrease of the frontier energy levels due to the electron-withdrawing nature of

Tuning the Optical and Self‐Assembly Properties of Diketopyrrolopyrrole Semicarbazone Derivatives through Hydrogen Bonding

“…When DPPSC and BrDPPSC solutions were warmed up to temperatures close to the boiling point of the solvents, a hypsochromic shift and a variation in the intensity ratio of the vibronic peaks (0‐0 and 0‐1 transitions), were detected (Figure S2a–h). This effect has been observed previously in other DPP ‐based dyes without hydrogen bonds, in the solid state. It can be related to the presence of two types of aggregates when the strength of the intermolecular excitonic coupling is similar to the electron‐vibrational coupling.…”

A Nonfullerene Acceptor Containing Rhodanine and Barbituric Acid End Groups for Use in Organic Photovoltaic Devices

“…The photovoltaic properties of the as-cast and SVA-treated devices are summarized in Table 2. 23 A similar tendency was also observed in our previous work on the bithiophene−rhodanine-based NFA T2-ORH. 21,22 The PTB7-Th:ORH-T2-OBAR devices showed a broad EQE response from 400 to 750 nm, with two EQE peaks centered at 440 nm and 710 nm; these peaks correspond to the broad absorption regions in the UV-Vis spectra of ORH-T2-OBAR (400-600 nm) and PTB7-Th (600-750 nm), respectively, indicating charge generation via both hole and electron transfer.…”

Harnessing Brightness in Naphthalene Diimides

“…The formation of such aggregates has important consequences for the energies of the excited states and the oscillator strengths of the transitions to these states from the ground state. Consequently, H‐ and J‐aggregation can strongly modify optical absorption and emission spectra . Intermolecular co‐facial π–π interactions have been utilised both to increase and decrease fluorescence emission.…”