Poly(4-vinylpyridine): A New Interface Layer for Organic Solar Cells

Continuous flow fabrication of green graphene oxide in aqueous hydrogen peroxide

Highly processible graphene oxide (GO) has a diversity of applications as a material readily dispersed in aqueous media. However, methods for preparing such free-standing GO use hazardous and toxic reagents and generate significant waste streams. This is an impediment for uptake of GO in any application, for developing sustainable technologies and industries, and overcoming this remains a major challenge. We have developed a robust scalable continuous flow method for fabricating GO directly from graphite in 30% aqueous hydrogen peroxide which dramatically minimises the generation of waste. The process features the continuous flow thin film microfluidic vortex fluidic device (VFD), operating at specific conditions while irradiated sequentially by UV LED than a NIR pulsed laser. The resulting 'green' graphene oxide (gGO) has unique properties, possessing highly oxidized edges with large intact sp² domains which gives rise to exceptional electrical and optical properties, including purple to deep blue emission of narrow full width at half maximum (<35 nm). Colloidally stable gGO exhibits cytotoxicity owing to the oxidised surface groups while solid-state films of gGO are biocompatible. The continuous flow method of generating gGO also provides unprecedented control of the level of oxidation and its location in the exfoliated graphene sheets by harnessing the high shear topological fluid flows in the liquid, and varying the wavelength, power and pulse frequency of the light source.

Entangled ZnO on Ultrathin Hollow Fibers for UV-Aided Pollutant Decomposition

Zinc oxide (ZnO), a widely used ultraviolet (UV) degrading substance, offers high selectivity for wastewater treatment, but the leaching of ZnO into water could cause secondary contamination. Using porous substrates to fix and load ZnO is a promising technical method to improve the water purification efficiency and recycling durability of ZnO. However, limited by the slow kinetics and shielding effects, it is challenging to use traditional techniques to introduce ZnO into the interior of a hollow structure. Here, inspired by an ancient dyeing procedure, we formed a unique single-molecule bio-interfacial entanglement as an absorption layer to capture the catalyst for ZnO electroless deposition (ELD) on the surface of natural ultrathin hollow-structured Kapok fibers. With curcumin serving as a linking bridge, ELD allowed the spontaneous formation of intensive ZnO nanocrystals on both the outer and inner walls. ZnO-kapok as the catalyst for ultraviolet photodecomposition of organic pollutants (methylene blue (MB) and phenol as model pollutants) delivered a decomposition efficiency of 80% and outstanding durability. Further modification of the ZnO-kapok catalyst by doping with reduced graphene oxide (rGO) showed an improvement in photodegradation performance of 90% degradation under 2-h irradiation with 21.85 W/dm² light power. Moreover, to the best of our knowledge, this is the first report featuring ZnO loading on both the outer and inner walls of a fiber-structured hollow kapok material, which provides inspiration for immobilization of metallic oxides on hollow-structured materials for further applications in renewable catalysis, chemical engineering, and energy storage fields.

A Nonionic Alcohol Soluble Polymer Cathode Interlayer Enables Efficient Organic and Perovskite Solar Cells

The choice of interfacial materials and their properties play a critical role in determining solar cell performance and stability. For compatibility with roll-to-roll printing, it is desirable to develop stable cathode interface layers (CILs) that can be processed over the photoactive layer using orthogonal solvents. In this study, an n-type naphthalene diimide core and oligo (ethylene glycol) side-chain-based conjugated polymer is reported as a universal, efficient CIL for organic and perovskite photovoltaics. Besides good thermal stability and easy processing in alcohol/water, the new CIL is found to possess electron transport properties with an electrical conductivity of 2.3 × 10^-6 S cm^-1, enabling its use as a CIL with a film thickness of up to ∼35(±2) nm. Utilizing the new CIL, 16% power conversion efficiency (PCE) is achieved for organic solar cells (OSCs) based on the PM6-Y6 photoactive layer (8.9% PCE for no CIL and 15.1% with state-of-the-art CIL, PDINO), and perovskite solar cells from methylammonium lead iodide yielded a PCE of 17.6%. Compared to the reference devices, the new CIL reduced trap-assisted carrier recombination and increased the built-in potential by 80 mV, simultaneously enhancing all photovoltaic parameters. Moreover, new CIL based devices had better photostability with no burn-in losses.

4-vinylpyridine derivatives: Protonation, methylation and silver(I) coordination chemistry

( E)-4-[2-(Pyridin-4-yl)vinyl]benzaldehyde, containing both a 4-vinylpyridine and an aldehyde functionality, is utilized to develop new, highly conjugated chalcone compounds and a bis-Schiff base azine compound. The chalcone-containing compounds are further explored for their protonation, methylation and silver(I) coordination chemistry using the pyridine moiety. In parallel, a cyano-containing analogue, ( E)-4-[2-(pyridin-4-yl)vinyl]benzonitrile is also synthesized and studied for its silver(I) coordination chemistry. These new compounds are fully characterized by mass spectrometry, elemental analysis and spectroscopic techniques. The methylated product of ( E)-1-(9-anthryl)-3-{4-[2-(pyridin-4-yl)vinyl]phenyl}prop-2-en-1-one and a silver complex of ( E)-4-[2-(pyridin-4-yl)vinyl]benzonitrile are structurally determined by X-ray crystallography.

Materials for Interfaces in Organic Solar Cells and Photodetectors

Interface engineering is very important to the high performance of organic optoelectronic devices that are commonly composed of multilayer thin solid films. Interfacial materials are particularly crucial for interface engineering, and a variety of materials have been employed at the interface to accomplish various different functions. This Review summarizes various materials for the interfaces and some of the latest progress in organic solar cells (OSCs) and organic photodetectors (OPDs).

From Straw to Device Interface: Carboxymethyl-Cellulose-Based Modified Interlayer for Enhanced Power Conversion Efficiency of Organic Solar Cells

Advanced interface materials made from petrochemical resources have been extensively investigated for organic solar cells (OSCs) over the past decades. These interface materials have demonstrated excellent performances in OSC devices. However, the limited resources, high-cost, and non-ecofriendly nature of petrochemical-based interface materials restrict their commercial applications. Here, a facile and effective approach to prepare cellulose and its derivatives as a cathode interface layer for OSCs with enhanced performance from rice straw of agroforestry residues is demonstrated. By employing this carboxymethyl cellulose sodium (CMC) into OSCs, a highly efficient inverted OSC is constructed, and a power conversion efficiency (PCE) of 12.01% is realized using poly[(2,6-(4,8-bis(5-(2-ethyl-hexyl)-thiophen-2-yl)-benzo[1,2-b:4,5-b'] dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7-bis(2-ethylhexyl)benzo[1',2'-c: 4',5'-c']dithiophene-4,8-dione): 3,9-bis(2-methylene-((3-(1, 1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d: 2',3'-d']-s-indaceno[1,2-b: 5, 6-b']dithiophene as the active layer, which shows over 9.4% improvement in PCE compared to that of a device without the CMC layer (PCE = 10.98%), especially the enhancement in short-circuit current. The improved current densities and PCEs are attributed to the reduced work function, enhanced absorption, and improved interfacial contact by using CMC and ZnO as co-interface. This approach of fabricating interface materials from biorenewable sources for OSCs is simple, scalable, and cost-effective, representing a promising direction for the development of smart interface and green electronics.

Optimizing Polymer Solar Cells Using Non-Halogenated Solvent Blends

More environmentally friendly polymer solar cells were constructed using a conjugated polymer, poly (2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'h][1,5] naphthyridine-5,10-dione, PTNT, as a donor material in combination with PC71BM as an acceptor in a bulk heterojunction device structure. A non-halogenated processing solvent (o-xylene) and solvent additives that are less harmful to the environment such as 1-methoxynaphthalene (MN) and 1-phenylnaphthalene (PN) were used throughout the study as processing solvents. The most widely used halogenated solvent additives (1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN)) were also used for comparison and to understand the effect of the type of solvent additives on the photovoltaic performances. Atomic force microscopy (AFM) was employed to investigate the surface morphology of the films prepared in the presence of the various additives. The best-performing polymer solar cells provided a high open-circuit voltage of 0.9 V, an efficient fill factor of around 70%, and a highest power conversion efficiency (PCE) of over 6% with the use of the eco-friendlier o-xylene/MN solvent systems. Interestingly, the solvent blend which is less harmful and with low environmental impact gave a 20% rise in PCE as compared to an earlier reported device efficiency that was processed from the chlorinated solvent o-dichlorobenzene (o-DCB).

Ultralow surface energy self-assembled monolayers of iodo-perfluorinated alkanes on silica driven by halogen bonding

Compact self-assembled monolayers (SAMs) of perfluorododecyl iodide (I-PFC12) of reproducible thickness (1.2 nm) are shown to form on silicon wafers. The SAMs have a high fluorine content (95%) and convey an extremely low surface energy to the silicon wafers (4.3 mN m-1), lower than previously reported in the literature for perfluorinated monolayers, and stable for over eight weeks. Shorter chain iodo-perfluorinated (I-PFC8) or bromo-perfluorinated molecules (Br-PFC10) led to less dense layers. The monolayers are stable to heating up to 60 °C, with some loss up to 150 °C. The I-PFC12 monolayer increases the work function of silicon wafers from 3.6 V to 4.4 eV, a factor that could be gainfully used in photovoltaic applications. The I-PFC12 monolayers can be transferred into patterns onto silica substrates by micro-contact printing. The NMR data and the reproducible thickness point to an upright halogen bonding interaction between the iodine in I-PFC12 and the surface oxygen on the native silica layer.

A star-shaped conjugated molecule featuring a triazole core and diketopyrrolopyrrole branches is an efficient electron-selective interlayer for inverted polymer solar cells

A novel triazole-cored, star-shaped, conjugated molecule (TDGTPA) has been synthesized for use as an electron-selective interlayer in inverted polymer solar cells (PSCs). This star-shaped molecule comprised a triazole unit as the central core, 2,5-thienyl diketopyrrolopyrrole units as π-conjugated bridges, and tert-butyl-substituted triphenylamine units as both end groups and donor units. The inverted PSC had the device structure indium tin oxide/ZnO/TDGTPA/poly(3-hexylthiophene) (P3HT)/fullerene derivative (PC₇₁BM)/MoO₃/Ag. Inserting TDGTPA as the electron-selective layer enhanced the compatibility of the ZnO-based electron transport layer and the P3HT:PC₇₁BM blend-based photoactive layer. The low energy of the lowest unoccupied molecular orbital (−3.98 eV) of TDGTPA was favorable for electron transfer from the photoactive layer to the ZnO layer, thereby enhancing the photovoltaic performance of the PSC. The photo-conversion efficiency of the device incorporating TDGTPA as the electron-selective layer was 15.8% greater than that of the corresponding device prepared without it.

A novel triazole-cored, star-shaped, conjugated molecule (TDGTPA) has been synthesized for use as an electron-selective interlayer in inverted polymer solar cells (PSCs).

Doping ZnO Electron Transport Layers with MoS2 Nanosheets Enhances the Efficiency of Polymer Solar Cells

In this study, we incorporated molybdenum disulfide (MoS₂) nanosheets into sol-gel processing of zinc oxide (ZnO) to form ZnO:MoS₂ composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS₂ composite film from 4.45 to 4.22 eV by varying the content of MoS₂ up to 0.5 wt %, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene- alt-(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl] (PTB7-TH)/phenyl-C₇₁-butryric acid methyl ester (PC₇₁BM). In particular, the power conversion efficiency (PCE) of the PTB7-TH/PC₇₁BM (1:1.5, w/w) device incorporating the ZnO:MoS₂ (0.5 wt %) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL, a relative increase of 15%. Incorporating a small amount of MoS₂ nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy, synchrotron grazing incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS₂ composite films.

Compositional engineering of acceptors for highly efficient bulk heterojunction hybrid organic solar cells

The wet chemical synthesis of chromium oxide (Cr₂O₃) nanoparticles (NPs) and its application in active layer of inverted bulk heterojunction organic solar cells is documented in this research. Chromium oxide NPs of 10-30 nm size range having a band gap of 2.9 eV were successfully synthesized. These NPs were used in inverted organic solar cells in amalgamation with P3HT:PCBM and PTB7:PCBM polymers. The fabricated hybrid devices improves PCE significantly for P3HT:PCBM and PTB7:PCBM systems. The photophysical energy levels, optoelectrical properties and microscopic images have been systematically studied for the fabricated devices. The introduction of Cr₂O₃ nanoparticles (NPs) enhances light harvesting and tunes energy levels into improved electrical parameters. A clear red shift and improved absorption have been observed for ternary blended devices compared to that observed with controlled organic solar cells. Apparently, when the amount of NPs in the binary polymer blend exceeds the required optimum level, there is a breakdown of the bulk heterojunction leading to lowering of the optical and electrical performance of the devices.

Polymer brush on surface with tunable hydrophilicity using SAM formation of zwitterionic 4-vinylpyridine-based polymer

A zwitterionic vinylpyridine-based polymeric SAM was assembled on different surfaces to obtain tunable hydrophilicity.

ITO Modification for Efficient Inverted Organic Solar Cells

We demonstrate a facile approach to designing transparent electron-collecting electrodes by depositing thin layers of medium and low work function metals on top of transparent conductive metal oxides (TCOs) such as ITO and FTO. The modified electrodes were fairly stable for months under ambient conditions and maintained their electrical characteristics. XPS spectroscopy data strongly suggested integration of the deposited metal in the TCO structure resulting in additional doping of the conducting oxide at the interface. Kelvin probe microscopy measurements revealed a significant decrease in the ITO work function after modification. Organic solar cells based on three different conjugated polymers have demonstrated state of the art performances in inverted device geometry using Mg- or Yb-modified ITO as electron collecting electrode. The simplicity of the proposed approach and the excellent ambient stability of the modified ITO electrodes allows one to expect their wide utilization in research laboratories and electronic industry.