Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells

Nonfullerene acceptor based organic solar cells (NF-OSCs) have witnessed rapid progress over the past few years owing to the intensive research efforts on novel electron donor and nonfullerene acceptor (NFA) materials, interfacial engineering, and device processing techniques. Interfacial layers including electron transporting layers (ETL) and hole transporting layers (HTLs) are crucially important in the OSCs for facilitating electron and hole extraction from the photoactive blend to the respective electrodes. In this review, the lates progress in both ETLs and HTLs for the currently prevailing NF-OSCs are discussed, in which the ETLs are summarized from the categories of metal oxides, metal chelates, non-conjugated electrolytes and conjugated electrolytes, and the HTLs are summarized from the categories of inorganic and organic materials. In addition, some bifunctional interlayer materials served as both ETLs and HTLs are also introduced. Finally, the prospects of ETL/HTL materials for NF-OSCs are provided.

Naphthalene-Diimide-Based Ionenes as Universal Interlayers for Efficient Organic Solar Cells

Self-doping ionene polymers were efficiently synthesized by reacting functional naphthalene diimide (NDI) with 1,3-dibromopropane (NDI-NI) or trans-1,4-dibromo-2-butene (NDI-CI) via quaternization polymerization. These NDI-based ionene polymers are universal interlayers with random molecular orientation, boosting the efficiencies of fullerene-based, non-fullerene-based, and ternary organic solar cells (OSCs) over a wide range of interlayer thicknesses, with a maximum efficiency of 16.9 %. NDI-NI showed a higher interfacial dipole (Δ), conductivity, and electron mobility than NDI-CI, affording solar cells with higher efficiencies. These polymers proved to efficiently lower the work function (WF) of air-stable metals and optimize the contact between metal electrode and organic semiconductor, highlighting their power to overcome energy barriers of electron injection and extraction processes for efficient organic electronics.

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes

A new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C₆₀ fullerene monomers into ionene polymers. The power of these novel "C₆₀ -ionenes" for interface modification enables the use of numerous high work-function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C₆₀ -ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non-fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C₆₀ -ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard-soft materials interface to the benefit of organic electronics.