Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication.

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

Enhanced Photocatalytic and Photovoltaic Performance Arising from Unconventionally Low Donor-Y6 Ratios.

Development of both organic photovoltaics (OPVs) and organic photocatalysts has focused on utilizing the bulk heterojunction (BHJ). The BHJ promotes charge separation and enhances the carrier lifetime, but may give rise to increased charge traps, hindering performance. Here, high photocatalytic and photovoltaic performance is displayed by electron donor-acceptor (D-A) nanoparticles (NPs) and films, using the nonfullerene acceptor Y6 and polymer donor PIDT-T8BT. In contrast to conventional D-A systems, the charge generation in PIDT-T8BT:Y6 NPs is mainly driven by Y6, allowing a high performance even at a low D:A mass ratio of 1:50. The high performance at the low mass ratio is attributed to the amorphous behavior of PIDT-T8BT. Low ratios are generally thought to yield lower efficiency than the more conventional ≈1:1 ratio. However, the OPVs exhibit peak performance at a D:A ratio of 1:5. Similarly the NPs used for photocatalytic hydrogen evolution show peak performance at the 1:6.7 D:A ratio. Interestingly, for the PIDT-T8BT:Y6 system, as the polymer proportion increases, a reduced photocatalytic and photovoltaic performance is observed. The unconventional D:A ratios provide lower recombination losses and increased charge-carrier lifetime with undisrupted ambipolar charge transport in bulk Y6, enabling better performance than conventional ratios. This work reports novel light-harvesting materials in which performance is reduced due to unfavorable morphology as D:A ratios move toward conventional ratios of 1:1.2-1:1.

A Homopolymer of Xanthenoxanthene-Based Polycyclic Heteroaromatic for Efficient and Stable Perovskite Solar Cells.

Highly efficient perovskite solar cells typically rely on spiro-OMeTAD as a hole transporter, achieving a 25.7 % efficiency record. However, these cells are susceptible to harsh 85 °C conditions. Here, we present a peri-xanthenoxanthene-based semiconducting homopolymer (p-TNI2) with matched energy levels and a high molecular weight, synthesized nearly quantitatively through facile oxidative polymerization. Compared to established materials, p-TNI2 excels in hole mobility, morphology, modulus, and waterproofing. Implementing p-TNI2 as the hole transport layer results in n-i-p perovskite solar cells with an initial average efficiency of 24.6 %, rivaling 24.4 % for the spiro-OMeTAD control cells under identical conditions. Furthermore, the p-TNI2-based cells exhibit enhanced thermostability at 85 °C and operational robustness.

Sub-4 nm mapping of donor-acceptor organic semiconductor nanoparticle composition.

We report, for the first time, sub-4 nm mapping of donor : acceptor nanoparticle composition in eco-friendly colloidal dispersions for organic electronics. Low energy scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX) mapping has revealed the internal morphology of organic semiconductor donor : acceptor blend nanoparticles at the sub-4 nm level. A unique element was available for utilisation as a fingerprint element to differentiate donor from acceptor material in each blend system. Si was used to map the location of donor polymer PTzBI-Si in PTzBI-Si:N2200 nanoparticles, and S (in addition to N) was used to map donor polymer TQ1 in TQ1:PC71BM nanoparticles. For select material blends, synchrotron-based scanning transmission X-ray microscopy (STXM), was demonstrated to remain as the superior chemical contrast technique for mapping organic donor : acceptor morphology, including for material combinations lacking a unique fingerprint element (e.g. PTQ10:Y6), or systems where the unique element is in a terminal functional group (unsaturated, dangling bonds) and can hence be easily damaged under the electron beam, e.g. F on PTQ10 donor polymer in the PTQ10:IDIC donor : acceptor blend. We provide both qualitative and quantitative compositional mapping of organic semiconductor nanoparticles with STEM EDX, with sub-domains resolved in nanoparticles as small as 30 nm in diameter. The sub-4 nm mapping technology reported here shows great promise for the optimisation of organic semiconductor blends for applications in organic electronics (solar cells and bioelectronics) and photocatalysis, and has further applications in organic core-shell nanomedicines.

Modification of the Surface Composition of PTB7-Th: ITIC Blend Using an Additive.

We investigated the effect of adding p-anisaldehyde (AA) solvent to the ink containing poly[[2,60-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene(ITIC) on the morphology of the active layer. The present study focuses on determining the effect of the additive on the compositions at the surface of the PTB7-Th: ITIC composite and its morphology, forming one side of the interface of the blend with the MoOX electrode, and the influence of the structural change on the performance of devices. Studies of device performance show that the addition of the additive AA leads to an improvement in device performance. Upon the addition of AA, the concentration of PTB7-Th at the surface of the bulk heterojunction (BHJ) increases, causing an increase in surface roughness of the surface of the BHJ. This finding contributes to an understanding of the interaction between the donor material and high work function electrode/interface material. The implications for the interface are discussed.

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.

A Comparative Study on the Role of Polyvinylpyrrolidone Molecular Weight on the Functionalization of Various Carbon Nanotubes and Their Composites.

Polyvinylidene fluoride (PVDF) nanocomposites filled with polyvinylpyrrolidone (PVP) wrapped carbon nanotubes were prepared via a solution casting technique. The effect of the molecular weight (polymer chain length) of the PVP on the ability to wrap different nanotube structures and its impact towards nanotube dispersibility in the polymer matrix was explored. The study was conducted with PVP of four different molecular weights and nanotubes of three different structures. The composites that exhibit an effective nanotube dispersion lead to a nanotube network that facilitates improved thermal, electrical, and mechanical properties. It was observed that nanotubes of different structures exhibit stable dispersions in the polymer matrix though PVP functionalization of different molecular weights, but the key is achieving an effective nanotube dispersion at low PVP concentrations. This is observed in MWNT and AP-SWNT based composites with PVP of low molecular weight, leading to a thermal conductivity enhancement of 147% and 53%, respectively, while for P3-SWNT based composites, PVP of high molecular weight yields an enhancement of 25% in thermal conductivity compared to the non-functionalized CNT-PVDF composite.

High shear in situ exfoliation of 2D gallium oxide sheets from centrifugally derived thin films of liquid gallium.

A diversity of two-dimensional nanomaterials has recently emerged with recent attention turning to the post-transition metal elements, in particular material derived from liquid metals and eutectic melts below 330 °C where processing is more flexible and in the temperature regime suitable for industry. This has been explored for liquid gallium using an angled vortex fluidic device (VFD) to fabricate ultrathin gallium oxide (Ga2O3) sheets under continuous flow conditions. We have established the nanosheets to form highly insulating material and have electrocatalytic activity for hydrogen evolution, with a Tafel slope of 39 mV dec-1 revealing promoting effects of the surface oxidation (passivation layer).

Cyclic Copper Uptake and Release from Natural Seawater-A Fully Sustainable Antifouling Technique to Prevent Marine Growth.

Unwanted growth of fouling organisms on underwater surfaces is an omnipresent challenge for the marine industry, costing billions of dollars every year in the transportation sector alone. Copper, the most widely used biocide in antifouling paints, is at the brink of a total ban in being used in antifouling coatings, as it has become an existential threat to nontargeted species due to anthropogenic copper inputs into protected waters. In the current study, using a porous and cross-linked poly(ethylene imine) structure under marine and fouling environments, available copper from natural seawater was absorbed and electrochemically released back as a potent biocide at 1.3 V vs Ag|AgCl, reducing marine growth by 94% compared to the control electrode (coupon) at 0 V. The coating can also function as an electrochemical copper sensor enabling real-time monitoring of the electrochemical uptake and release of copper ions from natural seawater. This allows tailoring of the electrochemical program to the changing marine environments, i.e., when the vessels move from high-copper-contaminated waters to coastal regions with low concentrations of copper.

Highly Stable Indacenodithieno[3,2-b]thiophene-Based Donor-Acceptor Copolymers for Hybrid Electrochromic and Energy Storage Applications.

Stable doping of indacenodithieno[3,2-b]thiophene (IDTT) structures enables easy color tuning and significant improvement in the charge storage capacity of electrochromic polymers, making use of their full potential as electrochromic supercapacitors and in other emerging hybrid applications. Here, the IDTT structure is copolymerized with four different donor-acceptor-donor (DAD) units, with subtle changes in their electron-donating and electron-withdrawing characters, so as to obtain four different donor-acceptor copolymers. The polymers attain important form factor requirements for electrochromic supercapacitors: desired switching between achromatic black and transparent states (L*a*b* 45.9, -3.1, -4.2/86.7, -2.2, and -2.7 for PIDTT-TBT), high optical contrast (72% for PIDTT-TBzT), and excellent electrochemical redox stability (Ired/Iox ca. 1.0 for PIDTT-EBE). Poly[indacenodithieno[3,2-b]thiophene-2,8-diyl-alt-4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole-7,7'-diyl] (PIDTT-EBzE) stands out as delivering simultaneously a high contrast (69%) and doping level (>100%) and specific capacitance (260 F g-1). This work introduces IDTT-based polymers as bifunctional electro-optical materials for potential use in color-tailored, color-indicating, and self-regulating smart energy systems.

Diffusion-Limited Crystallization: A Rationale for the Thermal Stability of Non-Fullerene Solar Cells.

Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature Tg of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high Tg of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above Tg, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high- Tg materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.

Role of Molecular Weight in Polymer Wrapping and Dispersion of MWNT in a PVDF Matrix.

The thermal and electrical properties of a polymer nanocomposite are highly dependent on the dispersion of the CNT filler in the polymer matrix. Non-covalent functionalisation with a PVP polymer is an excellent driving force towards an effective dispersion of MWNTs in the polymer matrix. It is shown that the PVP molecular weight plays a key role in the non-covalent functionalisation of MWNT and its effect on the thermal and electrical properties of the polymer nanocomposite is reported herein. The dispersion and crystallisation behaviour of the composite are also evaluated by a combination of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).

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).

Building intermixed donor-acceptor architectures for water-processable organic photovoltaics.

A modified synthesis method for aqueous nanoparticle printing inks, based upon vacuum-assisted solvent removal, is reported. Poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester nanoparticle inks were prepared via this modified miniemulsion method, leading to both an improvement in photoactive layer morphology and a substantial reduction in the ink fabrication time. A combination of UV-visible spectroscopy, photoluminescence spectroscopy and scanning transmission X-ray microscopy measurements revealed a nanoparticle morphology comprising highly intermixed donor-acceptor domains. Consistent with these measurements, dynamic mechanical thermal analysis of the nanoparticles showed a glass transition temperature (Tg) of 104 °C, rather than a pure polymer phase or pure fullerene phase Tg. Together the spectroscopy, microscopy and thermomechanical data indicate that rapid solvent removal generates a more blended nanoparticle morphology. As such, this study highlights a new experimental lever for optimising nanostructure in the photoactive layer of nanoparticulate organic photovoltaic devices by enabling highly intermixed donor-acceptor architectures to be built from customised nanoparticulate inks.

Recent Advances in n-Type Polymers for All-Polymer Solar Cells.

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

Design, Synthesis and Computational Study of Fluorinated Quinoxaline-Oligothiophene-based Conjugated Polymers with Broad Spectral Coverage.

Donor-acceptor (D-A) copolymers typically show two absorption peaks in the visible region, flanking a valley region of limited absorptivity. One strategy for more panchromatic light harvesting is to incorporate side-groups orthogonal to the polymer backbone, which enable 2D π conjugation and can give rise to additional absorption peaks. Here we design and synthesize two D-A polymers which both carry a fluorinated quinoxaline acceptor unit, but while P1 includes a benzodithiophene donor moiety with thiophene side-groups (2D-BDT), the P2 polymer lacks 2D conjugation in its simpler pentathiophene donor segment. The P1 polymer consequently shows an atypical absorption profile with more panchromatic absorption with no apparent valley in the spectrum. In order to understand the structure-electronic relations, the optical and electrochemical properties were predicted using a previously developed computational approach. The predicted optical properties show very good agreement with the experimental results. Solar cells made from P1 show a short-circuit current more than twice as large as P2, attributed to its enhanced spectral coverage. However, poor fill factors limit the preliminary power conversion efficiencies to 3.3 % for P1 and 1.0 % for P2 as blended with PCBM[70] in a 1 : 1.5 (w/w) ratio.

Efficient Near-Infrared Electroluminescence at 840 nm with "Metal-Free" Small-Molecule:Polymer Blends.

Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light-emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn-on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole-based emitter and a novel indacenodithiophene-based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution-processed active layer, not leveraging triplet-assisted emission.

High-Performance Organic Photodetectors from a High-Bandgap Indacenodithiophene-Based π-Conjugated Donor-Acceptor Polymer.

A conjugated donor-acceptor polymer, poly[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro- s-indaceno[1,2- b:5,6- b']dithiophene-2,7-diyl- alt-5-(2-ethylhexyl)-4 H-thieno[3,4- c]pyrrole-4,6(5 H)-dione-1,3-diyl] (PIDT-TPD), is blended with the fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) for the fabrication of thin and solution-processed organic photodetectors (OPDs). Systematic screening of the concentration ratio of the blend and the molecular weight of the polymer is performed to optimize the active layer morphology and the OPD performance. The device comprising a medium molecular weight polymer (27.0 kg/mol) in a PIDT-TPD:PC61BM 1:1 ratio exhibits an external quantum efficiency of 52% at 610 nm, a dark current density of 1 nA/cm2, a detectivity of 1.44 × 1013 Jones, and a maximum 3 dB cutoff frequency of 100 kHz at -5 V bias. These results are remarkable among the state-of-the-art red photodetectors based on conjugated polymers. As such, this work presents a functional organic active material for high-speed OPDs with a linear photoresponse at different light intensities.

High Performance All-Polymer Photodetector Comprising a Donor-Acceptor-Acceptor Structured Indacenodithiophene-Bithieno[3,4-c]Pyrroletetrone Copolymer.

The synthesis of an acceptor polymer PIDT-2TPD, comprising indacenodithiophene (IDT) as the electron-rich unit and an interconnected bithieno[3,4-c]pyrrole-4,4',6,6'-tetrone (2TPD) as the electron-deficient unit, and its application for all-polymer photodetectors is reported. The optical, electrochemical, charge transport, and device properties of a blend of poly(3-hexylthiophene) and PIDT-2TPD are studied. The blend shows strong complementary absorption and balanced electron and hole mobility, which are desired properties for a photoactive layer. The device exhibits dark current density in the order of 10-5 mA/cm2, external quantum efficiency broadly above 30%, and nearly planar detectivity over the entire visible spectral range (maximum of 1.1 × 1012 Jones at 610 nm) under -5 V bias. These results indicate that PIDT-2TPD is a highly functional new type of acceptor and further motivate the use of 2TPD as a building block for other n-type materials.

Isothermal Crystallization Kinetics and Time-Temperature-Transformation of the Conjugated Polymer: Poly(3-(2'-ethyl)hexylthiophene).

Thermal annealing strongly impacts the nano- and microstructure of conjugated polymers. Despite the fundamental importance for the resulting optoelectronic behavior of this class of materials, the underlying crystallization processes have not received the same attention that is encountered in other disciplines of materials science. The question arises whether classical treatment of nucleation and growth phenomena is truly applicable to conjugated polymers? Here, the isothermal crystallization behavior of the conjugated polymer poly(3-(2'-ethyl)hexylthiophene) (P3EHT) is monitored with differential scanning calorimetry (DSC). Avrami analysis reveals growth- and nucleation-limited temperature regimes that are separated by the maximum rate of crystallization. The molecular weight of the polymer is found to strongly influence the absolute rate of crystallization at the same degree of undercooling relative to the melting temperature. A combination of optical microscopy and grazing-incidence wide-angle X-ray scattering (GIWAXS) confirms that the resulting nano- and microstructure strongly correlate with the selected isothermal annealing temperature. Hence, this work establishes that classical nucleation and growth theory can be applied to describe the solidification behavior of the semicrystalline conjugated polymer P3EHT.