Thermal conductance of single-molecule junctions.

Single-molecule junctions have been extensively used to probe properties as diverse as electrical conduction1-3, light emission4, thermoelectric energy conversion5,6, quantum interference7,8, heat dissipation9,10 and electronic noise11 at atomic and molecular scales. However, a key quantity of current interest-the thermal conductance of single-molecule junctions-has not yet been directly experimentally determined, owing to the challenge of detecting minute heat currents at the picowatt level. Here we show that picowatt-resolution scanning probes previously developed to study the thermal conductance of single-metal-atom junctions12, when used in conjunction with a time-averaging measurement scheme to increase the signal-to-noise ratio, also allow quantification of the much lower thermal conductance of single-molecule junctions. Our experiments on prototypical Au-alkanedithiol-Au junctions containing two to ten carbon atoms confirm that thermal conductance is to a first approximation independent of molecular length, consistent with detailed ab initio simulations. We anticipate that our approach will enable systematic exploration of thermal transport in many other one-dimensional systems, such as short molecules and polymer chains, for which computational predictions of thermal conductance13-16 have remained experimentally inaccessible.

Ionic Ir(III) complex-interfacial layer for efficient carrier collection via induced electric dipole.

The power conversion efficiency (PCE) of polymer solar cells (PSCs) has recently reached >19% through the development of photoactive materials, particularly non-fullerene acceptors. Interfacial layers (ILs) have been another essential factor in optimizing device charge extraction. In this study, we propose a series of ILs, in which ionic iridium(III) (Ir(III)) complexes of different alkali metal cations (Li+, Na+, and K+) enhance the charge collection efficiency between zinc oxide and active layers through an induced internal electric field. The anionic coordinate sphere and counter-cations of the Ir(III) complexes are distributed according to the operating voltage of the PSCs, causing electric dipoles that enhance the internal electric field and charge collection efficiency. Ion species migration in the ILs is confirmed using electrochemical impedance spectroscopy. The PCE of the PM6:Y6-based PSCs was improved from 14.0% to 15.6% by introducing an IL (Ir-K+). Furthermore, the stability of PSCs containing ionic Ir(III) complexes is enhanced significantly under ultraviolet (UV) light and AM 1.5 G one-sun irradiation owing to the intense UV absorption capacity and photo durability of the ILs. A device containing the Ir(III) complex-based ILs retained ∼60% of its initial PCE after UV irradiation, whereas the control device retained only ∼20%.

Self-Healable and Stretchable Ionic-Liquid-Based Thermoelectric Composites with High Ionic Seebeck Coefficient.

The advancement of wearable electronics, particularly self-powered wearable electronic devices, necessitates the development of efficient energy conversion technologies with flexible mechanical properties. Recently, ionic thermoelectric (TE) materials have attracted great attention because of their enormous thermopower, which can operate capacitors or supercapacitors by harvesting low-grade heat. This study presents self-healable, stretchable, and flexible ionic TE composites comprising an ionic liquid (IL), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM:OTf); a polymer matrix, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP); and a fluoro-surfactant (FS). The self-healability of the IL-based composites originates from dynamic ion-dipole interactions between the IL, the PVDF-HFP, and the FS. The composites demonstrate excellent ionic TE properties with an ionic Seebeck coefficient (Si ) of ≈38.3 mV K-1 and an ionic figure of merit of ZTi  = 2.34 at 90% relative humidity, which are higher than the values reported for other IL-based TE materials. The IL-based ionic TE composites developed in this study can maintain excellent ionic TE properties under harsh conditions, including severe strain (75%) and multiple cutting-healing cycles.

FTO-free counter electrodes for dye-sensitized solar cells using carbon nanosheets synthesised from a polymeric carbon source.

Highly conductive carbon nanosheets (CNSs) are fabricated using a polymeric carbon source and subsequently applied as the counter electrodes (CNS-CEs) for dye-sensitized solar cells (DSSCs). The CNSs have a similar structure to multilayered graphene, and their high electrical conductivity and electrocatalytic activity enable them to have a dual-function as both CEs and charge supporting electrodes. CNSs form a unique CE material that functions successfully while being metal- and fluorine doped tin oxide (FTO)-free and allowing DSSCs to achieve ∼5% power conversion efficiency. The chemical structure, electrical properties, electrocatalytic activity, and work function of the CNS-CEs prepared under various conditions of carbonization are investigated, and their effects on the performance of the corresponding DSSCs are discussed. Carbonization temperature is shown to have influenced the size of graphitic domains and the presence of heteroatoms and functional groups in CNS-CEs. The change in the graphitic domain size has a marginal influence on the work function of the CNS-CEs and the overpotential for the reduction of the redox couples (I(-)/I3(-)). However, the electrical conductivity of CNS-CEs and the charge transfer resistance at CE/electrolyte interfaces in the DSSCs are considerably influenced by the carbonization condition. Our study shows that CNSs serve as efficient, FTO-free CE materials for DSSCs, and they are appropriate materials with which the effects of the chemical/physical properties of graphene-based materials on the electrode performance of various electrochemical devices may be studied.

Effects of surface characteristics of dielectric layers on polymer thin-film transistors obtained by spray methods.

The effect of surface characteristics of dielectric layers on the molecular orientation and device performance of sprayed organic field-effect transistors (OFETs) obtained by a novel solvent-assisted post-treatment, called the solvent-sprayed overlayer (SSO) method, were investigated. The OFETs were fabricated by the spray method using regioregular poly(3-hexylthiophene) (RR-P3HT) as an active material. The SSO treatment was applied on the as-sprayed active layers to arrange the molecular ordering. Bare thin SiO(2) layers and octadecyltrichlorosilane (OTS)-treated SiO(2) (OTS-SiO(2)) were employed as the dielectric materials. The resulting chain orientation, crystallinity, and device performance were correlated as a function of SSO treatment and dielectric layers. The intrinsic limitation of spray methods for polymer film formation was overcome regardless of the type of dielectric layer using the SSO treatment. The orientation direction of RR-P3HT was controlled by SSO treatment to an edge-on dominant orientation that is preferential for charge transport, regardless of the type of dielectric layer. The crystal growth was further enhanced on the OTS-SiO(2) layers because of the reduced nucleation sites. These effects were successfully reflected in the device performance, including an orders-of-magnitude increase in charge mobility. The SSO method is a powerful external treatment method for reorienting the molecular ordering of solidified active films of OFETs to the preferential edge-on packing. The growth of crystals was further optimized by controlling the surface characteristics of the dielectric layers. The purpose of this study was to find the full capabilities of the SSO treatment method that will facilitate the development of high-throughput, large-area organic electronic device manufacturing.

Self-healable polymer complex with a giant ionic thermoelectric effect.

In this study, we develop a stretchable/self-healable polymer, PEDOT:PAAMPSA:PA, with remarkably high ionic thermoelectric (iTE) properties: an ionic figure-of-merit of 12.3 at 70% relative humidity (RH). The iTE properties of PEDOT:PAAMPSA:PA are optimized by controlling the ion carrier concentration, ion diffusion coefficient, and Eastman entropy, and high stretchability and self-healing ability are achieved based on the dynamic interactions between the components. Moreover, the iTE properties are retained under repeated mechanical stress (30 cycles of self-healing and 50 cycles of stretching). An ionic thermoelectric capacitor (ITEC) device using PEDOT:PAAMPSA:PA achieves a maximum power output and energy density of 4.59 µW‧m-2 and 1.95 mJ‧m-2, respectively, at a load resistance of 10 KΩ, and a 9-pair ITEC module produces a voltage output of 0.37 V‧K-1 with a maximum power output of 0.21 µW‧m-2 and energy density of 0.35 mJ‧m-2 at 80% RH, demonstrating the potential for a self-powering source.

Investigation of Hole-Transfer Dynamics through Simple EL De-Convolution in Non-Fullerene Organic Solar Cells.

In conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, the electron charge transfer state (eECT) that electrons pass through has a great influence on the device's performance. In a bulk-heterojunction (BHJ) system based on a low bandgap non-fullerene acceptor (NFA), however, a hole charge transfer state (hECT) from the acceptor to the donor has a greater influence on the device's performance. The accurate determination of hECT is essential for achieving further enhancement in the performance of non-fullerene organic solar cells. However, the discovery of a method to determine the exact hECT remains an open challenge. Here, we suggest a simple method to determine the exact hECT level via deconvolution of the EL spectrum of the BHJ blend (ELB). To generalize, we have applied our ELB deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). Under the conditions that (i) absorption of the donor and acceptor are separated sufficiently, and (ii) the onset part of the external quantum efficiency (EQE) is formed solely by the contribution of the acceptor only, ELB can be deconvoluted into the contribution of the singlet recombination of the acceptor and the radiative recombination via hECT. Through the deconvolution of ELB, we have clearly decided which part of the broad ELB spectrum should be used to apply the Marcus theory. Accurate determination of hECT is expected to be of great help in fine-tuning the energy level of donor polymers and NFAs by understanding the charge transfer mechanism clearly.

Bias-free solar hydrogen production at 19.8 mA cm-2 using perovskite photocathode and lignocellulosic biomass.

Solar hydrogen production is one of the ultimate technologies needed to realize a carbon-neutral, sustainable society. However, an energy-intensive water oxidation half-reaction together with the poor performance of conventional inorganic photocatalysts have been big hurdles for practical solar hydrogen production. Here we present a photoelectrochemical cell with a record high photocurrent density of 19.8 mA cm-2 for hydrogen production by utilizing a high-performance organic-inorganic halide perovskite as a panchromatic absorber and lignocellulosic biomass as an alternative source of electrons working at lower potentials. In addition, value-added chemicals such as vanillin and acetovanillone are produced via the selective depolymerization of lignin in lignocellulosic biomass while cellulose remains close to intact for further utilization. This study paves the way to improve solar hydrogen productivity and simultaneously realize the effective use of lignocellulosic biomass.

Effect of length and contact chemistry on the electronic structure and thermoelectric properties of molecular junctions.

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (-SH, -NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.

Newly Developed Broadband Antireflective Nanostructures by Coating a Low-Index MgF2 Film onto a SiO2 Moth-Eye Nanopattern.

A newly developed nanopatterned broadband antireflective (AR) coating was fabricated on the front side of a glass/indium tin oxide/perovskite solar cell (PSC) by depositing a single interference layer onto a two-dimensional (2D)-patterned moth-eye-like nanostructure. The optimized developed AR nanostructure was simulated in a finite-difference time domain analysis. To realize the simulated developed AR nanostructure, we controlled the SiO2 moth-eye structure with various diameters and heights and a MgF2 single layer with varying thicknesses by sequentially performing nanosphere lithography, reactive ion etching, and electron-beam evaporation. Optimization of the developed AR nanostructure, which has a 100 nm-thick MgF2 film coated onto the SiO2 moth-eye-like nanostructure (diameter 165 nm and height 400 nm), minimizes the reflection loss throughout the visible range. As a result, the short-circuit current density (JSC) of the newly AR-coated PSC increases by 11.80%, while the open-circuit voltage (VOC) remains nearly constant. Therefore, the power conversion efficiency of the newly developed AR-decorated PSC increases by 12.50%, from 18.21% for a control sample to 20.48% for the optimum AR-coated sample. These results indicate that the newly developed MgF2/SiO2 AR nanostructure can provide an advanced platform technology that reduces the Fresnel loss and therefore increases the possibility of the commercialization of glass-based PSCs.

17% Non-Fullerene Organic Solar Cells with Annealing-Free Aqueous MoO x.

A charge transport layer based on transition metal-oxides prepared by an anhydrous sol-gel method normally requires high-temperature annealing to achieve the desired quality. Although annealing is not a difficult process in the laboratory, it is definitely not a simple process in mass production, such as roll-to-roll, because of the inevitable long cooling step that follows. Therefore, the development of an annealing-free solution-processable metal-oxide is essential for the large-scale commercialization. In this work, a room-temperature processable annealing-free "aqueous" MoO x solution is developed and applied in non-fullerene PBDB-T-2F:Y6 solar cells. By adjusting the concentration of water in the sol-gel route, an annealing-free MoO x with excellent electrical properties is successfully developed. The PBDB-T-2F:Y6 solar cell with the general MoO x prepared by the anhydrous sol-gel method shows a low efficiency of 7.7% without annealing. If this anhydrous MoO x is annealed at 200 °C, the efficiency is recovered to 17.1%, which is a normal value typically observed in conventional structure PBDB-T-2F:Y6 solar cells. However, without any annealing process, the solar cell with aqueous MoO x exhibits comparable performance of 17.0%. In addition, the solar cell with annealing-free aqueous MoO x exhibits better performance and stability without high-temperature annealing compared to the solar cells with PEDOT:PSS.

Stable and Colorful Perovskite Solar Cells Using a Nonperiodic SiO2/TiO2 Multi-Nanolayer Filter.

While research on building-integrated photovoltaics (BIPVs) has mainly focused on power-generating window applications, the utilization of other underutilized surface areas in buildings, including exteriors, facades, and rooftops, has still not been fully explored. The most important requirements for BIPVs are color, power conversion efficiency (PCE), and long-term stability. In this work, we achieved colorful (red, green, blue, RGB) perovskite solar cells (PSCs) with minimized PCE loss (<10%) and enhanced photostability by exploiting the optical properties of nonperiodic multi-nanolayer, narrow-bandwidth reflective filters (NBRFs). The NBRFs were fabricated by multilayering high-index TiO2/low-index SiO2 in a nonperiodic manner, which allowed devices to demonstrate various colors with effectively suppressed unwanted baseline ripple-shape reflectance. The PCEs of PSCs with nonperiodic RGB-NBRFs were 18.0%, 18.6%, and 18.9%, which represent reductions of only 10%, 7%, and 6% of PCE values, respectively, compared to a black control PSC (20.1%). Moreover, the photostability of the PSCs was substantially improved by using the NBRFs because of ultraviolet blocking in the TiO2 layers. The G-PSC retained 65% of the initial PCE after 60 h of continuous illumination (AM 1.5G one sun) at the maximum power point, whereas the black PSC retained only 30%. Aesthetic color value, low PCE loss, and enhanced photostability of PSCs were simultaneously achieved by employing our NBRFs, making this a promising strategy with potential applicability in power-generating building exteriors.

High-Efficiency Air-Stable Colloidal Quantum Dot Solar Cells Based on a Potassium-Doped ZnO Electron-Accepting Layer.

High-efficiency colloidal quantum dot (CQD) solar cells (CQDSCs) with improved air stability were developed by employing potassium-modified ZnO as an electron-accepting layer (EAL). The effective potassium modification was achievable by a simple treatment with a KOH solution of pristine ZnO films prepared by a low-temperature solution process. The resulting K-doped ZnO (ZnO-K) exhibited EAL properties superior to those of a pristine ZnO-EAL. The Fermi energy level of ZnO was upshifted, which increased the internal electric field and amplified the depletion region (i.e., charge drift) of the devices. The surface defects of ZnO were effectively passivated by K modification, which considerably suppressed interfacial charge recombination. The CQDSC based on ZnO-K achieved improved power conversion efficiency (PCE) of ≈10.75% ( VOC of 0.67 V, JSC of 23.89 mA cm-2, and fill factor of 0.68), whereas the CQDSC based on pristine ZnO showed PCE of 9.97%. Moreover, the suppressed surface defects of ZnO-K substantially improved long-term stability under air. The device using ZnO-K exhibited superior long-term air storage stability (96% retention after 90 days) compared to that using pristine ZnO (88% retention after 90 days). The ZnO-K-based device also exhibited improved photostability under air. Under continuous light illumination for 600 min, the ZnO-K-based device retained 96% of its initial PCE, whereas the pristine ZnO-based device retained only 67%.

Performance Improvement in Low-Temperature-Processed Perovskite Solar Cells by Molecular Engineering of Porphyrin-Based Hole Transport Materials.

Porphyrin derivatives have recently emerged as hole transport layers (HTLs) because of their electron-rich characteristics. Although several successes with porphyrin-based HTLs have been recently reported, achieving excellent solar cell performance, the chances to improve this further by molecular engineering are still open. In this work, Zn porphyrin (PZn)-based HTLs were developed by conjugating fluorinated triphenylamine (FTPA) wings at the perimeter of the PZn core for low-temperature perovskite solar cells (L-PSCs). The fluorinated PZn-HTLs (PZn-2FTPA and PZn-3FTPA) exhibited superior HTL properties compared to the nonfluorinated one (PZn-TPA). Moreover, their deeper highest occupied molecular orbital energy levels were beneficial for boosting open-circuit voltages, and their enhanced face-on stacking improved the hole transport properties. The L-PSC using PZn-2FTPA achieved the highest performance of 18.85%. Thus far, this result is one of the highest reported power conversion efficiencies among the PSCs using porphyrin-based HTLs.

Development of n-Type Porphyrin Acceptors for Panchromatic Light-Harvesting Fullerene-Free Organic Solar Cells.

The development of n-type porphyrin acceptors is challenging in organic solar cells. In this work, we synthesized a novel n-type porphyrin acceptor, PZn-TNI, via the introduction of the electron withdrawing naphthalene imide (NI) moiety at the meso position of zinc porphyrin (PZn). PZn-TNI has excellent thermal stability and unique bimodal absorption with a strong Soret band (300-600 nm) and weak Q-band (600-800 nm). The weak long-wavelength absorption of PZn-TNI was completely covered by combining the low bandgap polymer donor, PTB7-Th, which realized the well-balanced panchromatic photon-to-current conversion in the range of 300-800 nm. Notably, the one-step reaction of the NI moiety from a commercially available source leads to the cheap and simple n-type porphyrin synthesis. The substitution of four NIs in PZn ring induced sufficient n-type characteristics with proper HOMO and LUMO energy levels for efficient charge transport with PTB7-Th. Fullerene-free organic solar cells based-on PTB7-Th:PZn-TNI were investigated and showed a promising PCE of 5.07% without any additive treatment. To the best of our knowledge, this is the highest PCE in the porphyrin-based acceptors without utilization of the perylene diimide accepting unit.

High-Performance Near-Infrared Absorbing n-Type Porphyrin Acceptor for Organic Solar Cells.

While the outstanding charge transport and sunlight-harvesting properties of porphyrin molecules are highly attractive as active materials for organic photovoltaic (OPV) devices, the development of n-type porphyrin-based electron acceptors has been challenging. In this work, we developed a high-performance porphyrin-based electron acceptor for OPVs by substitution of four naphthalene diimide (NDI) units at the perimeter of a Zn-porphyrin (PZn) core using ethyne linkage. Effective π-conjugation between four NDI wings and the PZn core significantly broadened Q-band absorption to the near infrared region, thereby achieving the narrow band gap of 1.33 eV. Employing a windmill-structured tetra-NDI substituted PZn-based acceptor ( PZn-TNDI) and mid-band gap polymer donor (PTB7-Th), the bulk heterojunction OPV devices achieved a power conversion efficiency (PCE) of 8.15% with an energy loss of 0.61 eV. The PCE of our PZn-TNDI-based device was the highest among the reported OPVs using porphyrin-based acceptors. Notably, the amorphous characteristic of PZn-TNDI enabled optimization of the device performance without using any additive, which should make industrial fabrication simpler and cheaper.

11% Organic Photovoltaic Devices Based on PTB7-Th: PC71BM Photoactive Layers and Irradiation-Assisted ZnO Electron Transport Layers.

The enhancement of interfacial charge collection efficiency using buffer layers is a cost-effective way to improve the performance of organic photovoltaic devices (OPVs) because they are often universally applicable regardless of the active materials. However, the availability of high-performance buffer materials, which are solution-processable at low temperature, are limited and they often require burdensome additional surface modifications. Herein, high-performance ZnO based electron transporting layers (ETLs) for OPVs are developed with a novel g-ray-assisted solution process. Through careful formulation of the ZnO precursor and g-ray irradiation, the pre-formation of ZnO nanoparticles occurs in the precursor solutions, which enables the preparation of high quality ZnO films. The g-ray assisted ZnO (ZnO-G) films possess a remarkably low defect density compared to the conventionally prepared ZnO films. The low-defect ZnO-G films can improve charge extraction efficiency of ETL without any additional treatment. The power conversion efficiency (PCE) of the device using the ZnO-G ETLs is 11.09% with an open-circuit voltage (VOC), short-circuit current density ( JSC), and fill factor (FF) of 0.80 V, 19.54 mA cm-2, and 0.71, respectively, which is one of the best values among widely studied poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]: [6,6]-phenyl-C71-butyric acid methyl ester (PTB7-Th:PC71BM)-based devices.

Fullerene-Free Organic Solar Cells with an Efficiency of 10.2% and an Energy Loss of 0.59 eV Based on a Thieno[3,4-c]Pyrrole-4,6-dione-Containing Wide Band Gap Polymer Donor.

Although the combination of wide band gap polymer donors and narrow band gap small-molecule acceptors achieved state-of-the-art performance as bulk heterojunction (BHJ) active layers for organic solar cells, there have been only several of the wide band gap polymers that actually realized high-efficiency devices over >10%. Herein, we developed high-efficiency, low-energy-loss fullerene-free organic solar cells using a weakly crystalline wide band gap polymer donor, PBDTTPD-HT, and a nonfullerene small-molecule acceptor, ITIC. The excessive intermolecular stacking of ITIC is efficiently suppressed by the miscibility with PBDTTPD-HT, which led to a well-balanced nanomorphology in the PBDTTPD-HT/ITIC BHJ active films. The favorable optical, electronic, and energetic properties of PBDTTPD-HT with respect to ITIC achieved panchromatic photon-to-current conversion with a remarkably low energy loss (0.59 eV).

Diphenyl-2-pyridylamine-Substituted Porphyrins as Hole-Transporting Materials for Perovskite Solar Cells.

The susceptibility of porphyrin derivatives to light-harvesting and charge-transport operations have enabled these materials to be employed in solar cell applications. The potential of porphyrin derivatives as hole-transporting materials (HTMs) for perovskite solar cells (PSCs) has recently been demonstrated, but knowledge of the relationships between the porphyrin structure and device performance remains insufficient. In this work, a series of novel zinc porphyrin (PZn) derivatives has been developed and employed as HTMs for low-temperature processed PSCs. Key to the design strategy is the incorporation of an electron-deficient pyridine moiety to down-shift the HOMO levels of porphyrin HTMs. The porphyrin HTMs incorporating diphenyl-2-pyridylamine (DPPA) have HOMO levels that are in good agreement with the perovskite active layers, thus facilitating hole transfers from the perovskite to the HTMs. The DPPA-containing zinc porphyrin-based PSCs gave the best performance, with efficiency levels comparable to those of PSCs using spiro-OMeTAD, a current state-of-the-art HTM. In particular, PZn-DPPA-based PSCs show superior air stability, in both doped and undoped forms, to spiro-OMeTAD based devices.