A Cross-Linkable Electron-Transport Layer Based on a Fullerene-Benzoxazine Derivative for Inverted Polymer Solar Cells

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C₆₁ -butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]-phenyl-C₆₁ -butyric methyl ester (PCBM). Thermal cross-linking of the benzoxazine moiety in PCBB resulted in the formation of cross-linked, solvent resistive adhesive films (C-PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT-PCBM polymer blend film upon incorporation of C-PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-PCBM/V₂ O₅ /Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT-PCBM/V₂ O₅ /Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB heterojunctions.

Molecular Engineering on Bis(benzothiophene-S,S-dioxide)-Based Large-Band Gap Polymers for Interfacial Modifications in Polymer Solar Cells

The development of effectively universal interfacial materials for both conventional and inverted polymer solar cells (PSCs) plays a very crucial role in achieving highly photovoltaic performance and feasible device engineering. In this study, two novel alcohol-soluble conjugated polymers (PBSON-P and PBSON-FEO) with bis(benzothiophene-S,S-dioxide)-fused aromatics (FBTO) as the core unit and amino as functional groups are synthesized. They are utilized as universal cathode interfacial layers for both conventional and inverted PSCs simultaneously. Ascribing to the enlarged conjugated planarity and higher electron affinity for an FBTO unit, both PBSON-P and PBSON-FEO exhibit versatile electron-transporting abilities. They show wide band gaps that are important for light absorption in inverted PSCs, at which point PBSON-P and PBSON-FEO are more progressive than some of the reported small band gap cathode interfacial materials. Importantly, PBSON-P and PBSON-FEO display deep highest occupied molecular orbital energy levels, which can block holes at the cathode and thus increase the fill factor. As a result, both conventional and inverted PSCs using PBSON-P and PBSON-FEO as cathode interlayers realize high photovoltaic performance. Therefore, this series of novel polymers are amphibious cathode interfacial materials for high-performance conventional and inverted PSCs.

Fullerene Active Layers for n-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are currently being developed for applications ranging from bioelectronics to neuromorphic computing. We show that fullerene derivatives with glycolated side chains can serve as n-type active layers for OECTs with figures of merit exceeding the best reported conjugated-polymer-based n-type OECTs. By comparing two different fullerene derivatives, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and 2-(2,3,4-tris(methoxtriglycol) phenyl) [60]fulleropyrrolidine (C60-TEG), we find that the hydrophilic glycolated side chains in C60-TEG enable volumetric doping of C60-TEG films. In contrast, the hydrophobic nature of PCBM prevents ions from penetrating into the material. Our results demonstrate that small-molecule semiconductors follow many of the same design principles established for conjugated polymers and can function as high-performing mixed electronic/ionic conductors for efficient, fast OECTs.

Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics

N-doping of conjugated polymers either requires a high dopant fraction or yields a low electrical conductivity because of their poor compatibility with molecular dopants. We explore n-doping of the polar naphthalenediimide-bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole-dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high electrical conductivity of more than 10-1 S cm-1 for a dopant concentration of only 10 mol % when measured in an inert atmosphere. We find that the doped polymer is able to maintain its electrical conductivity for about 20 min when exposed to air and recovers rapidly when returned to a nitrogen atmosphere. Overall, solution coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelectric power factor of up to 0.4 μW K-2 m-1 compared to other NDI-based polymers.

Diblock Copolymer PF-b-PDMAEMA as Effective Cathode Interfacial Material in Polymer Solar Cells

An alcohol-soluble diblock copolymer poly[2,7-(9,9-dihexylfluorene)]₁₅-block-poly[2-(dimethylamino)ethyl methacrylate]₇₅ (denoted as PF₁₅-b-PDMAEMA₇₅) was employed as the cathode interfacial layer (CIL) in p-i-n polymer solar cells (PSCs). PF₁₅-b-PDMAEMA₇₅ contains a conjugated rigid block and a nonconjugated flexible block grafted with polar amino groups, and it can effectively lower the work function of the Al cathode and decrease the series resistance of the devices. When applied as the CIL in PSCs based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl C71 butyric acid methyl ester, the champion power conversion efficiency of 8.80% was achieved, which is slightly higher than that of the PSCs using the well-known poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] as CIL under our experimental conditions, and much better than that of PSCs using Ca as CIL. The improvement of the performance is mainly attributed to the enhanced open-circuit voltage and fill factor. To the best of our knowledge, this is the first time a diblock copolymer has been used as a CIL in PSCs, and this study may provide a novel avenue for the design and synthesis of interfacial materials for high-performance PSCs.

Photo-cross-linked perylene diimide derivative materials as efficient electron transporting layers in inverted polymer solar cells

We present an efficient and stable interfacial material based on a water-soluble perylene diimide derivative functionalized with ionic and methacrylate groups (abbreviated as PDIM), which can be stabilized by the photo-polymerization of diacrylate groups at both ends of the side chain in the PDIM. The characteristics of the photo-cross-linked PDIM films were examined using absorption spectra, cyclic voltammetry, work function, and surface morphology. The feasibility of the photo-cross-linked PDIM films as a novel electron transporting layer (ETL) in polymer solar cells (PSCs) was also investigated. The PTB7-Th:PC₇₁BM-based PSC using the PDIM as the ETL achieved the excellent power conversion efficiency of 9.44% similar to the conventional polyethylenimine ethoxylated (PEIE) and better than ZnO. Furthermore, the PSC with the PDIM films exhibited a similar lifetime to that of the PEIE-based device. This approach suggests that the photo-cross-linked PDIM film could be regarded as a promising interfacial material for fabricating highly efficient PSCs.

Efficient and Reversible Electron Doping of Semiconductor-Enriched Single-Walled Carbon Nanotubes by Using Decamethylcobaltocene

Single-walled carbon nanotubes (SWCNTs) offer great potential for field-effect transistors and integrated circuit applications due to their extraordinary electrical properties. To date, as-made SWCNT transistors are usually p-type in air, and it still remains challenging for realizing n-type devices. Herein, we present efficient and reversible electron doping of semiconductor-enriched single-walled carbon nanotubes (s-SWCNTs) by firstly utilizing decamethylcobaltocene (DMC) deposited by a simple spin-coating process at room temperature as an electron donor. A n-type transistor behavior with high on current, large I _on /I _off ratio and excellent uniformity is obtained by surface charge transfer from the electron donor DMC to acceptor s-SWCNTs, which is further corroborated by the Raman spectra and the ab initio simulation results. The DMC dopant molecules could be reversibly removed by immersion in N, N-Dimethylformamide solvent, indicating its reversibility and providing another way to control the carrier concentration effectively as well as selective removal of surface dopants on demand. Furthermore, the n-type behaviors including threshold voltage, on current, field-effect mobility, contact resistances, etc. are well controllable by adjusting the surface doping concentration. This work paves the way to explore and obtain high-performance n-type nanotubes for future complementary CMOS circuit and system applications.

Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells

A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds with polar phosphine oxide (P═O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity of the TAZ unit and P═O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P═O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl]:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PTB7:PC₇₁BM) system with PBTAZPOBr as CIL displayed simultaneously enhanced open-circuit voltage, short-circuit current density, and fill factor, whereas the power conversion efficiency increased from 5.42% to 8.04% compared to that of the pristine Al device. The outstanding performance of PBTAZPOBr is attributed not only to the polar pendant groups of BDT unit but also to the TAZ unit linked with the P═O group of PBTAZPOBr, demonstrating that functionalized TAZ building blocks are very promising cathode interfacial materials (CIMs). The design strategy proposed in this work will be helpful to develop more efficient CIMs for high performance PSCs in the future.

Imidazole-Functionalized Fullerene as a Vertically Phase-Separated Cathode Interfacial Layer of Inverted Ternary Polymer Solar Cells

By using a facile one-pot nucleophilic addition reaction, we synthesized a novel imidazole (IMZ)-functionalized fullerene (C₆₀-IMZ), and applied it as a third component of inverted ternary polymer solar cells (PSCs), leading to dramatic efficiency enhancement. According to FT-IR, XPS spectroscopic characterizations, and elemental analysis, the chemical structure of C₆₀-IMZ was determined with the average IMZ addition number estimated to be six. The lowest unoccupied molecular orbital (LUMO) level of C₆₀-IMZ measured by cyclic voltammetry was -3.63 eV, which is up-shifted relative to that of 6,6-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM). Upon doping C₆₀-IMZ as a third component into an active layer blend of poly(3-hexylthiophene) (P3HT) and PC₆₁BM, the power conversion efficiency (PCE) of the inverted ternary PSCs was 3.4% under the optimized doping ratio of 10 wt %, dramatically higher than that of the control device ITO/P3HT:PC₆₁BM/MoO₃/Ag based on the binary P3HT:PC₆₁BM blend (1.3%). The incorporation of C₆₀-IMZ results in enhancement of the absorption of P3HT:PC₆₁BM blend film, increase of the electron mobility of the device, and rougher film surface of the P3HT:PC₆₁BM active layer beneficial for interfacial contact with the Ag anode. Furthermore, C₆₀-IMZ doped in P3HT:PC₆₁BM blend may migrate to the surface of ITO cathode via vertical phase separation as revealed by XPS depth analysis, consequently forming a cathode interfacial layer (CIL), which can lower the work function (WF) of ITO cathode. Thus, the interfacial contact between the active layer and ITO cathode is improved, facilitating electron transport from the active layer to ITO cathode. The effectiveness of C₆₀-IMZ as a vertically phase-separated CIL on efficiency enhancement of inverted ternary PSCs is further verified by doping it into another active layer system comprised of a low-bandgap conjugated polymer, poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7), blended with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM). Under the optimized C₆₀-IMZ doping ratio of 10 wt %, the PCE of the PTB7:PC₇₁BM-based inverted ternary PSC device reaches 5.3%, which is about 2 times higher than that of the control binary device (2.6%).

Pyridine-incorporated alcohol-soluble neutral polyfluorene derivatives as efficient cathode-modifying layers for polymer solar cells

A class of novel alcohol-soluble polyfluorene derivatives with a pyridine group incorporating at the side chains of fluorene is developed to modify the cathode interfaces of both conventional and inverted polymer solar cells.

Measurement of exciton diffusion in a well-defined donor/acceptor heterojunction based on a conjugated polymer and cross-linked fullerene derivative

We designed a well-defined donor/acceptor heterojunction for measuring exciton diffusion lengths in conjugated polymers. To obtain an insoluble electron acceptor layer, a new cross-linkable fullerene derivative (bis-PCBVB) was synthesized by functionalizing [6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) with two styryl groups. The spin-coated bis-PCBVB film was cross-linked in situ by heating at 170 °C for 60 min. Surface characterizations by UV-visible absorption, atomic force microscopy, and photoelectron yield spectroscopy revealed that a smooth and solvent-resistant film (p-PCBVB) was obtained. In bilayer films with a donor conjugated polymer, poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thienyl)-2',1',3'-benzothiadiazole)] (PF12TBT), spin-coated on top of the p-PCBVB acceptor layer, the photoluminescence (PL) of the PF12TBT was effectively quenched. This is because the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the p-PCBVB film are nearly the same as those of the parent bis-PCBM spin-coated film. On the basis of the PL quenching results, the exciton diffusion length and exciton diffusion coefficient in the PF12TBT were evaluated to be 11 nm and 9.8 × 10(-4) cm(2) s(-1), respectively.

Enhancing the performance of solution-processed n-type organic field-effect transistors by blending with molecular "aligners"

A novel approach to enhancing the performance of solution-processed n-type organic field-effect transistors by using trace amounts of molecular "aligners" to manipulate the assembly of "matrix" molecules in thin films is demonstrated. The device performance is one order of magnitude higher in 1wt% blended thin films than that in neat films, which correlates to an induced change of preferred orientation of the in-plane π-stacking molecules upon blending.

Light manipulation for organic optoelectronics using bio-inspired moth's eye nanostructures

Organic-based optoelectronic devices, including light-emitting diodes (OLEDs) and solar cells (OSCs) hold great promise as low-cost and large-area electro-optical devices and renewable energy sources. However, further improvement in efficiency remains a daunting challenge due to limited light extraction or absorption in conventional device architectures. Here we report a universal method of optical manipulation of light by integrating a dual-side bio-inspired moth's eye nanostructure with broadband anti-reflective and quasi-omnidirectional properties. Light out-coupling efficiency of OLEDs with stacked triple emission units is over 2 times that of a conventional device, resulting in drastic increase in external quantum efficiency and current efficiency to 119.7% and 366 cd A⁻¹ without introducing spectral distortion and directionality. Similarly, the light in-coupling efficiency of OSCs is increased 20%, yielding an enhanced power conversion efficiency of 9.33%. We anticipate this method would offer a convenient and scalable way for inexpensive and high-efficiency organic optoelectronic designs.

Doping of fullerenes via anion-induced electron transfer and its implication for surfactant facilitated high performance polymer solar cells

Simple and solution-processible tetrabutyl-ammonium salts (TBAX) can dope fullerene and its derivatives to achieve conductive thin films (σ as high as 0.56 S/m). The electron transfer between the anions of TBAXs and n-type semiconductors induces doping without encountering any harsh activation. These provide valid support for the surfactant interfacial doping of fullerene in polymer solar cells for enhanced device performance.

Tuning the Dirac point in CVD-grown graphene through solution processed n-type doping with 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole

Controlling the Dirac point of graphene is essential for complementary circuits. Here, we describe the use of 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) as a strong n-type dopant for chemical-vapor-deposition (CVD) grown graphene. The Dirac point of graphene can be tuned significantly by spin-coating o-MeO-DMBI solutions on the graphene sheets at different concentrations. The transport of graphene can be changed from p-type to ambipolar and finally n-type. The electron transfer between o-MeO-DMBI and graphene was additionally confirmed by Raman imaging and photoemission spectroscopy (PES) measurements. Finally, we fabricated a complementary inverter via inkjet printing patterning of o-MeO-DMBI solutions on graphene to demonstrate the potential of o-MeO-DMBI n-type doping on graphene for future applications in electrical devices.

Solution-processible highly conducting fullerenes

n-Doping of solution-processible organic semiconductors: highly conductive fullerenes are demonstrated through solution-processed fulleropyrrolidinium iodide (FPI) and FPI-doped PCBM to reach a high conductivity (3.2 S/m). The n-doping proceeds via anion-induced electron transfer between the iodide on FPI and the fullerene in the solid state.

Compensating poly(3-hexylthiophene) reveals its doping density and its strong exciton quenching by free carriers

Adding increasing quantities of an n-type compensating dopant, cobaltocene, to poly(3-hexylthiophene) reveals an almost perfect mirror symmetry between the conductivity and the luminescence intensity. The sharp minimum/maximum shows that the uncompensated p-type doping density is 1.2 × 10(18) cm(-3) and that excitons are strongly quenched by free charge carriers, not by bound charges.

2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide as a new air-stable n-type dopant for vacuum-processed organic semiconductor thin films

2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) was synthesized and employed as a strong n-type dopant for fullerene C(60), a well-known n-channel semiconductor. The coevaporated thin films showed a maximum conductivity of 5.5 S/cm at a doping concentration of 8.0 wt% (14 mol%), which is the highest value reported to date for molecular n-type conductors. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.