Electroactive Ionenes: Efficient Interlayer Materials in Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs) have the advantages of being lightweight, mechanically flexible, and solution-processable over large areas, and for decades, they have been the focus of the academic and industrial communities. Recent progress in the design of high-performance organic semiconductors and device optimization has promoted solar cell efficiencies of up to 19%, showing great promise for commercialization. Optimally designed OPVs are achieved using a bicontinuous interpenetrating network of donor and acceptor materials in between two charge-collecting electrodes. Charge extraction and transport between metal electrodes and organic semiconductors are crucial to device operation. The energy-level mismatch when metal electrodes and organic semiconductors are in contact usually induces additional energy barriers and resultant inefficient charge transport and collection, leading to charge carrier recombination at the interface and inferior device performance. To align energy levels at the interface, interlayer materials and their integration into devices have emerged as a widely used strategy to promote the performance of solar cell devices. Interlayer materials have the ability to modify the work functions (WFs) of metal electrodes, holding the potential to enhance the built-in electrostatic field (V_bi) of the devices and suppress the charge recombination loss, which is beneficial to improving the open circuit voltage (V_OC), short circuit current density (J_SC), and fill factor (FF) of the solar cells.Organic interlayer materials have recently come into focus for fundamental study and practical development because of their diverse molecular design and superior solution processability. Tremendous effort has been devoted to exploring novel organic interlayer materials to achieve all-solution-processed multilayer solar cells. Such interlayer materials usually have orthogonal solubilities relative to the photoactive layer materials, working as multifunctional interfacial layers to manipulate the mechanical and electrical contacts in solar cell devices. Ionenes are a unique class of polyelectrolytes wherein the ionic species reside within the polymer backbone rather than as pendant groups. In ionenes, the charge density is high in comparison to that of other polyelectrolytes, and the periodicity of the charges is easily controlled, providing a tunable density of dipole moments. Ionenes can be readily synthesized from 3° diamines and α,ω-dihaloalkanes to generate polymer chains of ammonium cations connected by flexible hydrocarbon linkages with mobile anions. However, the requisite building blocks of ionenes are not limited to such molecules. Recent advances in combining ionenes with conjugated molecules to generate electroactive ionenes have catalyzed a great amount of interest in such polymers for organic electronic devices.In this Account, we first introduce the molecular design and synthesis of electroactive ionenes. Following this, we will discuss the mechanism and effect of ionenes on the modification of metal electrodes. We then review the strategies for controlling the morphology of ionene interlayers. Finally, we compare the doping effect, conductivity, and charge transport of some representative ionenes and their performance as interlayers in solar cell devices. We present our current understanding based on recent progress and outstanding issues of interlayer materials in OPVs and to propose future directions and opportunities.