Efficient Solution‐Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor–Acceptor Dyes

Abstract

Research on small-molecule-based organic semiconductors has undoubtedly been strongly influenced by xerographic photoconductors like triarylamines, the first important organic electronic materials in market products.[1] Their development was strongly influenced by the Bässler model, which provided a rationale for the design of amorphous organic photo-and semiconductors.[2] According to this model, only compounds that lack dipole moments are considered promising for charge-carrier transport because the increased energetic disorder associated with dipole moments is thought to impede charge hopping. Recently, we questioned this paradigm in the field of organic photovoltaics (OPV) and successfully implemented highly dipolar merocyanine dyes as active components for light harvesting as well as exciton and hole transport in solution-cast bulk heterojunction (BHJ) solar cells.[3] The rationale behind our concept [4] was that highly dipolar donor-acceptor (D-A) substituted p systems (also called push-pull dyes) self-assemble into centrosymmetric dimers, [5] thus effectively eliminating molecular dipole moments on the supramolecular and material levels.[6] Two drawbacks of our BHJ materials, however, limited the acceptance of our concept so far. Firstly, the best solar cells were obtained for merocyanine dyes whose molecular scaffolds were equipped with rather bulky substituents that interfere with close face-to-face antiparallel dimerization.[3] Secondly, the power-conversion efficiencies (h) under standard AM1.5, 100 mW cm À2 simulated solar illumination conditions for solution-cast BHJ cells with fullerenes-although significantly advanced by more sophisticated vacuum processing [7] -could not be improved beyond 2.6 %, which is significantly lower than the best solutionprocessed small-molecule-based BHJ devices fabricated with A-D-A and D-A-D chromophores, for example, acceptorsubstituted oligothiophenes (up to 3.7 %) [8] and triarylamines (up to 4.3 %), [9] diketopyrrolopyrroles (up to 4.4 %), [10] and squaraines (up to 5.2 %).[11] Herein, we introduce dipolar D-A dyes with flat structures that undoubtedly form centrosymmetric dimers [5] with perfectly cancelled dipole moments in the solid state. Solution-processed BHJ solar cells derived thereof exhibit power-conversion efficiencies up to 4.5-5.1 % (dependent on light intensity), clearly placing D-A dyes now among the top-performing small molecules in the field of organic photovoltaics.Scheme 1 outlines the synthetic route that follows our earlier work on merocyanine dyes for photorefractive materials [12] and the simple access to 5-dialkylamino-thiophene-2-carbaldehydes by Hartmann.[13] Detailed synthetic procedures and characterization data are described in the Supporting Information.The optical properties of the synthesized dyes were investigated by UV/Vis and electro-optical absorption spectroscopy.[14] Furthermore, cyclic voltammetry was performed for each dye to obtain information about their highest occupied molecular orbital (HOMO) and lowest unoccupied molecu...

Interrelation between Crystal Packing and Small‐Molecule Organic Solar Cell Performance

“…The bulk characteristics in turn depend on non-bonding intermolecular interactions, which lead to specific molecular order, packing, and morphology in the solid state. [4][5][6] A good performance of BHJ SMOSCs is only guaranteed if an interpenetrating network of the oligomeric donor (D) and an electron-acceptor (A), typically a fullerene derivative, is formed at the nanoscale as the photoactive layer. [7][8][9] Moreover, the size of the D and A homophases is crucially important for the diffusion of excitons to the D-A interface and after charge separation for effective charge transport to the electrodes.…”

Indoline as electron donor unit in “Push–Pull” organic small molecules for solution processed organic solar cells: Effect of the molecular π-bridge on device efficiency

“…85 All molecules used in this study (Scheme 1) were syn-86 thesized using the indoline moiety as secondary electron 87 donor unit due to its good donating ability and stability 88 among other chemical groups, more frequently used 89 donors such as triphenylamine. Indoline based molecules 90 have been previously reported in both smOSC and dye-91 sensitized solar cells [34,35]; however this kind of asym-92 metric structures are, in comparison, less explored in the 93 smOPV field [35][36][37][38][39][40][41]. In order to make a more comprehen-94 sive study, all of these molecules were designed using the 95 same donor (indoline) and acceptor moiety (dicyanovinyl); 96 and, on the contrary, the p-bridge structure was changed.…”

“…An initial analysis suggests that the main limitation is the device photocurrent due to 35 the device film thickness. Yet, charge transfer dynamics were studied to correlate charge 36 loss mechanisms to p-bridge structural variations and, moreover, mobility measurements 37 were also carried out to fully explain these device limitations. device loss mechanisms.…”

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

“…4.8d): These include pentacene (PEN), sexithiophene (6T), zinc phthalocyanine (ZnPc), the merocyanine dye EL86 [128], and the acceptor-substituted oligothiophene D5M [9]. Of these materials, only EL86 and D5M have a ground-state dipole moment due to their donor-acceptor (DA) and cis-acceptor-donor-acceptor (ADA) architecture.…”