Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics

Abstract

Conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, viewed as side chain dipoles, impart solubility in polar solvents for solution processing, and open opportunities as interfacial components of optoelectronic devices, for example, between metal electrodes and organic semiconductor active layers. Such interlayers are crucial for defining the performance of organic electronic devices, e.g., field-effect transistors (OFETs), light-emitting diodes (OLEDs), and photovoltaics (OPVs), all of which consist of multilayer structures. The interlayers reduce the Schottky barrier height and thus improve charge injection in OFETs and OLEDs. In OPVs, the interlayers serve to increase the built-in electric potential difference (V) across the active layer, ensuring efficient extraction of photogenerated charge carriers. In general, polar and even charged electronically active polymers have gained recognition for their ability to modify metal/semiconductor interfaces to the benefit of organic electronics. While conjugated polyelectrolytes (CPEs) as interlayer materials are well-documented, open questions remain about the role of mobile counterions in CPE-containing devices. CPZs possess the processing advantages of CPEs, but as neutral molecules lack any potential complications associated with counterions. The electronic implications of CPZs on metal electrodes stem from the orientation of the zwitterion dipole moment in close proximity to the metal surface, and the resultant surface-induced polarization. This generates an interfacial dipole (Δ) at the CPZ/metal interface, altering the work function of the electrode, as confirmed by ultraviolet photoelectron spectroscopy (UPS), and improving device performance. An ideal cathode interlayer would reduce electrode work function, have orthogonal processability to the active layer, exhibit good film forming properties (i.e., wettability/uniformity), prevent exciton quenching, possess optimal electron affinity that neither limits the work function reduction nor impedes the charge extraction, transport electrons selectively, and exhibit long-term stability. Our recent discoveries show that CPZs achieve many of these attributes, and are poised for further expansion and development in the interfacial science of organic electronics. This Account reviews a recent collaboration that began with the synthesis of CPZs and a study of their structural and electronic properties on metals, then extended to their application as interlayer materials for OPVs. We discuss CPZ structure-property relationships based on several material platforms, ranging from homopolymers to copolymers, and from materials with intrinsic p-type conjugated backbones to those with intrinsic n-type conjugated backbones. We discuss key components of such interlayers, including (i) the origin of work function reduction of CPZ interlayers on metals; (ii) the role of the frontier molecular orbital energy levels and their trade-offs in optimizing electronic and device p...

Molecular design of interfacial layers based on conjugated polythiophenes for polymer and hybrid solar cells

“…Although the CPEs described above improve the performance of OPV cells when incorporated as ILs, the presence of a mobile counterion which migrates during device operation and redistributes the internal electric field can affect the long‐term photovoltaic performance. In this respect, conjugated polymer zwitterions (CPZs) with cations and anions covalently bonded together to eliminate mobile ions have been developed as ILs in OPV devices . In 2013, Emrick and colleagues reported the synthesis of sulfobetaine‐substituted polythiophene CPZs with methylene ( P14 ) and butylene tethers ( P15 ) separating the zwitterions from the backbone (Scheme ) .…”

Significant Effect of Fluorination on Simultaneously Improving Work Function and Transparency of Anode Interlayer for Organic Solar Cells

“…[27] However, in recent years, with the situation that the highest occupied molecular orbital (HOMO) levels of donors in fullerenefree devices are constantly downshifted for achieving high V oc , [28,29] most previously developed AILs cannot satisfy the new demands for modifying efficient OSCs because the low WFs of these materials give rise to large energy offsets with donors, which causes interfacial barrier for hole collection and leads to serious V oc loss. [31,32] Unlike PEDOT:PSS and metal oxides, the chemical structure of CPEs can be readily modified through synthetic chemistry, which provides great opportunities for improving the WF of AILs. [31,32] Unlike PEDOT:PSS and metal oxides, the chemical structure of CPEs can be readily modified through synthetic chemistry, which provides great opportunities for improving the WF of AILs.…”

“…[31,32] Unlike PEDOT:PSS and metal oxides, the chemical structure of CPEs can be readily modified through synthetic chemistry, which provides great opportunities for improving the WF of AILs. [31,32] Unlike PEDOT:PSS and metal oxides, the chemical structure of CPEs can be readily modified through synthetic chemistry, which provides great opportunities for improving the WF of AILs.…”

Zwitterions for Organic/Perovskite Solar Cells, Light‐Emitting Devices, and Lithium Ion Batteries: Recent Progress and Perspectives

“…Generally, in zwitterion molecules, the mobility of both the charges is almost remained the same and, in this case, a dipole is generated by the partial rearrangement of the ions with side chains either through built-in electric field or through the interaction with the metal electrode at the interface (Figure 1). [90][91][92] Additionally, properties such as solubility and viscosity can be increased by adding the zwitterions into electrolytes because of shielding of dipole-dipole attractions. If the interfacial dipole is less than the difference between WF of metal and electron affinity of zwitterion molecule, the zwitterion molecule will have higher lowest unoccupied molecular orbital (LUMO) energy level from the Fermi level of metal.…”

“…[118] Efficient photoactive materials are responsible for the absorption of more sunlight, generation of efficient charge carriers, and their transport. [35,92] Two kinds of organic electrolyte interlayers have been investigated including polymeric interlayers and small-molecular interlayers. [119] Generally, an interfacial barrier is created due to the mismatch of energy levels if the active materials are directly in contact with the electrodes.…”