Organic/Organic Cathode Bi-Interlayers Based on a Water-Soluble Nonconjugated Polymer and an Alcohol-Soluble Conjugated Polymer for High Efficiency Inverted Polymer Solar Cells

Charge-Compensated N-Doped π-Conjugated Polymers: Toward both Thermodynamic Stability of N-Doped States in Water and High Electron Conductivity

The understanding and applications of electron-conducting π-conjugated polymers with naphtalene diimide (NDI) blocks show remarkable progress in recent years. Such polymers demonstrate a facilitated n-doping due to the strong electron deficiency of the main polymer chain and the presence of the positively charged side groups stabilizing a negative charge of the n-doped backbone. Here, the n-type conducting NDI polymer with enhanced stability of its n-doped states for prospective "in-water" applications is developed. A combined experimental-theoretical approach is used to identify critical features and parameters that control the doping and electron transport process. The facilitated polymer reduction ability and the thermodynamic stability in water are confirmed by electrochemical measurements and doping studies. This material also demonstrates a high conductivity of 10-2  S cm-1  under ambient conditions and 10-1  S cm-1  in vacuum. The modeling explains the stabilizing effects  for various dopants. The simulations show a significant doping-induced "collapse" of the positively charged side chains on the core bearing a partial negative charge. This explains a decrease in the lamellar spacing observed in experiments. This study fundamentally enables a novel pathway for achieving both thermodynamic stability of the n-doped states in water and the high electron conductivity of polymers.

Alcohol-Soluble Zwitterionic 4-(Dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate Small Molecule as a Cathode Modifier for Nonfullerene Acceptor-Based Organic Solar Cells

A new zwitterionic small molecule 4-(dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate (PAS), synthesized from 2-dimethylaminopyrindine (2-DMAP), was developed for the ITO cathode modifier. PAS and 2-DMAP dissolved in methanol can form a thin layer on ITO cathode by a simple spin-coating process. The heteroatom moieties in 2-DMAP (sp² and sp³ nitrogen) and PAS (sp² nitrogen and sulfonate ion) can coordinate to the ITO surface and decrease the ITO work function by the induced surface dipoles. The fullerene-based (PBDTT-FTTE:PC₇₁BM) inverted OSCs using PAS and 2-DMAP interlayer can achieve PCEs of 8.95 and 8.26%, respectively, which are superior to the devices without a modifier (PCE = 3.25%) and comparable to the corresponding ZnO-based device (PCE = 8.57%). Nevertheless, 2-DMAP, like other nitrogen-containing polymer interlayer materials, turns out to be not applicable to inverted organic solar cells (I-OSCs) with IT-4F as the n-type electron acceptor because the amino group of 2-DMAP can act as a nucleophile to attack the end-group of IT-4F at the interface. The decomposition of IT-4F by 2-DMAP was carefully proved to be via retro-aldol condensation. As a result, the device (PBDBT-F:IT-4F) modified with 2-DMAP displayed a low PCE of 7.34%. The zwitterionic PAS with reduced nucleophilicity and basicity can modify the ITO surface without decomposing IT-4F. The PBDBT-F:IT-4F-based device modified with PAS maintained a high PCE of 11.41%. Most importantly, the PAS-based device using the well-known Y6 acceptor (PBDBT-F:Y6) can achieve a PCE of 13.82%. This new interfacial material can be universally applied to I-OSCs employing various A-D-A-type acceptors installed with the electrophilic 1,1-dicyanamethylene-5,6-difluoro-3-indanone (FIC) end-group.

Circularly polarized luminescence based chirality transfer of the chiral BINOL moiety via rigid π-conjugation chain backbone structures

Three kinds of chiral BINOL-based polymers could be synthesized by polymerization in a Pd-catalyzed cross-coupling reaction.

High Performance Small-Molecule Cathode Interlayer Materials with D-A-D Conjugated Central Skeletons and Side Flexible Alcohol/Water-Soluble Groups for Polymer Solar Cells

A new class of organic cathode interfacial layer (CIL) materials based on isoindigo derivatives (IID) substituted with pyridinium or sulfonate zwitterion groups were designed, synthesized, and applied in polymer solar cells (PSCs) with PTB7:PC₇₁BM (PTB7: polythieno[3,4-b]-thiophene-co-benzodithiophene and PC₇₁BM: [6,6]-phenyl C71-butyric acidmethyl ester) as an active layer. Compared with the control device, PSCs with an IID-based CIL show simultaneous enhancement of open-circuit voltage (V_oc), short-circuit current (J_sc), and fill factor (FF). Systematic optimizations of the central conjugated core and side flexible alcohol-soluble groups demonstrated that isoindigo-based CIL material with thiophene and sulfonate zwitterion substituent groups can efficiently enhance the PSC performance. The highest power conversion efficiency (PCE) of 9.12%, which is 1.75 times that of the control device without CIL, was achieved for the PSC having an isoindigo-based CIL. For the PSCs with an isoindigo-based CIL, the molecule-dependent performance property studies revealed that the central conjugated core with D-A-D characteristics and the side chains with sulfonate zwitterions groups represents an efficient strategy for constructing high performance CILs. Our study results may open a new avenue toward high performance PSCs.

Doping ZnO with Water/Alcohol-Soluble Small Molecules as Electron Transport Layers for Inverted Polymer Solar Cells

By doping ZnO with porphyrin small molecules (FNEZnP-OE and FNEZnP-T) as cathode electron transport layers (ETLs), the inverted polymer solar cells (i-PSC) with PTB7:PC₇₁BM (PTB7: polythieno[3,4-b]-thiophene-co-benzodithiophene, PC₇₁BM:[6, 6]-phenyl-C₇₁-butyric acid methyl ester) as the active materials exhibit enhanced device performance. While the power conversion efficiency (PCE) of the PSCs with pure ZnO ETL is 7.52%, that of the devices with FNEZnP-T-doped ZnO ETL shows a slightly improved PCE of 8.09%, and that of the PSCs with FNEZnP-OE-doped ZnO ETL is further enhanced up to 9.24% with an over 20% improvement compared to that with pure ZnO ETL. The better performance is contributed by the better interfacial contact and reduced work function induced by 9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorenes and 3,4-bis(2-(2-methoxy-ethoxy)-ethoxy)-phenyls in the porphyrin small molecules. More importantly, the PCE is still higher than 8% even when the thickness of FNEZnP-OE-doped ZnO ETL is up to 110 nm, which are important criteria for eventually making organic photovoltaic modules with roll-to-roll coat processing.

Sulfonate anionic small molecule as a cathode interfacial material for highly efficient polymer solar cells

A non-amino small molecule, TBT-a, with sulfonate anionic pendants was developed as a cathode interlayer in efficient PSCs.