A Nonconjugated Zwitterionic Polymer: Cathode Interfacial Layer Comparable with PFN for Narrow-Bandgap Polymer Solar Cells

Electronic Tuning of Monolayer Graphene with Polymeric "Zwitterists"

Work function engineering of two-dimensional (2D) materials by application of polymer coatings represents a research thrust that promises to enhance the performance of electronic devices. While polymer zwitterions have been demonstrated to significantly modify the work function of both metal electrodes and 2D materials due to their dipole-rich structure, the impact of zwitterion chemical structure on work function modulation is not well understood. To address this knowledge gap, we synthesized a series of sulfobetaine-based zwitterionic random copolymers with variable substituents and used them in lithographic patterning for the preparation of negative-tone resists (i.e., "zwitterists") on monolayer graphene. Ultraviolet photoelectron spectroscopy indicated a significant work function reduction, as high as 1.5 eV, induced by all polymer zwitterions when applied as ultrathin films (<10 nm) on monolayer graphene. Of the polymers studied, the piperidinyl-substituted version, produced the largest dipole normal to the graphene sheet, thereby inducing the maximum work function reduction. Density functional theory calculations probed the influence of zwitterion composition on dipole orientation, while lithographic patterning allowed for evaluation of surface potential contrast via Kelvin probe force microscopy. Overall, this polymer "zwitterist" design holds promise for fine-tuning 2D materials electronics with spatial control based on the chemistry of the polymer coating and the dimensions of the lithographic patterning.

Zwitterions: promising interfacial/doping materials for organic/perovskite solar cells

This review summarizes the recent progress in zwitterionic materials through the concepts of interfacial dipoles and passivating defects.

Alcohol-Soluble Cross-Linked Poly( nBA) n- b-Poly(NVTri) m Block Copolymer and Its Applications in Organic Photovoltaic Cells for Improved Stability

In this study, a series of alcohol-soluble cross-linked block copolymers (BCPs) consisting of poly( n-butyl acrylate) (poly( nBA)) and poly( N-vinyl-1,2,4-triazole) (poly(NVTri)) blocks with different individual functions and lengths are designed and developed. These presynthesized cross-linked BCPs (PBA _n-Tri _m) were, for the first time, revealed to exhibit many advantages in serving as the electron-extraction layer (EEL) for organic photovoltaics (OPVs). The cross-linked BCPs possessed intense ionic functionality, showing well capability to form effective interfacial dipoles at the indium tin oxide interface to facilitate the charge extraction at the corresponding interface. Furthermore, it also consisted a core-shell structure, wherein the polar poly(NVTri) core was well protected by the poly( nBA) shell to endow improved robustness against solvent erosion and thermal/photo inputs. Consequently, the PBA₇₀-Tri₃₀ device yielded a decent power conversion efficiency of 8.03% with a V_oc of 0.83 V, much exceeding the performance of the control device without using any EEL. Moreover, this device showed superior thermal stability/photostability. More than 80% of its initial performance was retained after being heated at 60 °C for 1000 h or exposed under continuous illumination (1 sun) for 1000 h, greatly surpassing the lifetime of the control device and the reference device using a common poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) EEL. The results revealed the merit of using cross-linked BCPs in improving the long-term stability of OPVs.

Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient Organic Solar Cells

A novel nonconjugated polymer named poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) was designed and synthesized. The PAMPS-Na has good solubility in polar solvents, such as water, methanol, and ethanol, which can be used as the cathode buffer layer in organic solar cells (OSCs) through solution processing without damaging the underlying active layer. Moreover, it was found that PAMPS-Na can significantly decrease the Al work function when it was modified with Al. To reveal its universal application in organic photovoltaic devices, a variety of photovoltaic donor materials, including two medium-band gap polymers, a wide-band gap polymer, and a small molecule donor were employed to fabricate OSCs. Compared with OSCs with Ca/Al electrode, the devices based on PAMPS-Na/Al exhibited higher photovoltaic performance, mainly because of the increased short-circuit current. Additionally, OSCs with PAMPS-Na/Al displayed better ambient stability than devices with Ca/Al. It is also interesting to find that the performance of the devices can tolerate a wide change of PAMPS-Na's thickness, enabling the potential for large-scale fabrication of OSCs. The results suggest that PAMPS-Na is a promising candidate as the cathode buffer layer to improve the efficiency and stability of OSCs.