Ternary Blend Composed of Two Organic Donors and One Acceptor for Active Layer of High-Performance Organic Solar Cells.

Ternary blends composed of two donor absorbers with complementary absorptions provide an opportunity to enhance the short-circuit current and thus the power conversion efficiency (PCE) of organic solar cells. In addition to complementary absorption of two donors, ternary blends may exhibit favorable morphology for high-performance solar cells when one chooses properly the donor pair. For this purpose, we develop a ternary blend with two donors (diketopyrrolopyrrole-based polymer (PTDPP2T) and small molecule ((TDPP)2Ph)) and one acceptor (PC71BM). The solar cell made of a ternary blend with 10 wt % (TDPP)2Ph exhibits higher PCE of 7.49% as compared with the solar cells with binary blends, PTDPP2T:PC71BM (6.58%) and (TDPP)2Ph:PC71BM (3.21%). The higher PCE of the ternary blend solar cell is attributed mainly to complementary absorption of two donors. However, a further increase in (TDPP)2Ph content in the ternary blend (>10 wt %) decreases the PCE. The ternary blend with 10 wt % (TDPP)2Ph exhibits well-developed morphology with narrow-sized fibrils while the blend with 15 wt % (TDPP)2Ph shows phase separation with large-sized domains, demonstrating that the phase morphology and compatibility of ternary blend are important factors to achieve a high-performance solar cell made of ternary blends.

Medium Bandgap Conjugated Polymer for High Performance Polymer Solar Cells Exceeding 9% Power Conversion Efficiency.

Two medium-bandgap polymers composed of benzo[1,2-b:4,5-b']dithiohpene and 2,1,3-benzothiadiazole with 6-octyl-thieno[3,2-b]thiophene as a π-bridge unit are synthesized and their photovoltaic properties are analyzed. The two polymers have deep highest occupied molecular orbital energy levels, high crystallinity, optimal bulk-heterojunction morphology, and efficient charge transport, resulting in a power conversion efficiency of as high as 9.44% for a single-junction polymer solar-cell device.

Performance enhancement of planar heterojunction perovskite solar cells by n-doping of the electron transporting layer.

Herein we report a simple n-doping method to enhance the performance of perovskite solar cells with a planar heterojunction structure. Devices with an n-doped PCBM electron transporting layer exhibit a power conversion efficiency of 13.8% with a remarkably enhanced short-circuit current of 22.0 mA cm(-2) as compared to the devices with an un-doped PCBM layer.

Highly Crystalline Low Band Gap Polymer Based on Thieno[3,4-c]pyrrole-4,6-dione for High-Performance Polymer Solar Cells with a >400 nm Thick Active Layer.

Two thieno[3,4-c]pyrrole-4,6-dione (TPD)-based copolymers combined with 2,2'-bithiophene (BT) or (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TV) have been designed and synthesized to investigate the effect of the introduction of a vinylene group in the polymer backbone on the optical, electrochemical, and photovoltaic properties of the polymers. Although both polymers have shown similar optical band gaps and frontier energy levels, regardless of the introduction of vinylene bridge, the introduction of a π-extended vinylene group in the polymer backbone substantially enhances the charge transport characteristics of the resulting polymer due to its strong tendency to self-assemble and thus to enhance the crystallinity. An analysis on charge recombination in the active layer of a solar cell device indicates that the outstanding charge transport (µ = 1.90 cm(2)·V(-1)·s(-1)) of PTVTPD with a vinylene group effectively suppresses the bimolecular recombination, leading to a high power conversion efficiency (PCE) up to 7.16%, which is 20% higher than that (5.98%) of the counterpart polymer without a vinylene group (PBTTPD). More importantly, PTVTPD-based devices do not show a large variation of photovoltaic performance with the active layer thickness; that is, the PCE remains at 6% as the active layer thickness increases up to 450 nm, demonstrating that the PTVTPD-based solar cell is very compatible with industrial processing.

Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71.

Fluorinated n-type conjugated polymers are used as efficient electron acceptor to demonstrate high-performance all-polymer solar cells. The exciton generation, dissociation, and charge-transporting properties of blend films are improved by using these fluorinated n-type polymers to result in enhanced photocurrent and suppressed charge recombination.

Small molecules based on thieno[3,4-c]pyrrole-4,6-dione for high open-circuit voltage (VOC) organic photovoltaics: effect of different positions of alkyl substitution on molecular packing and photovoltaic performance.

Two different thienopyrroledione (TPD)-based small molecules (SMs) with different alkyl substitution positions were synthesized, and their photovoltaic properties are measured and compared to examine the effect of the alkyl substitution position on their optical, electrochemical, and photovoltaic properties. The use of TPD as an electron-accepting unit in conjugated SMs effectively lowers the highest occupied molecular orbital (HOMO) energy levels of the conjugated SMs and leads to high open-circuit voltage (VOC). The two SMs with n-hexyl group substituted at different positions exhibit almost identical optical and electrochemical properties in the pristine state. However, the crystallographic and morphological characteristics of the two SMs are significantly different, because they are blended with PC71BM. The SM in which n-alkyl groups are substituted at the central accepting unit exhibits a power conversion efficiency (PCE) of 6.0% with VOC=0.94 V, which is among the highest PCE values of TPD-based SM devices, whereas the SM with n-alkyl groups being substituted at the chain ends shows a moderate PCE value of 3.1%.

Synthesis of pyridine-capped diketopyrrolopyrrole and its use as a building block of low band-gap polymers for efficient polymer solar cells.

A new building block for low band-gap polymers, diketopyrrolopyrrole (DPP) flanked by pyridine (PyDPP), has been synthesized via a simple synthetic route. PyDPP was polymerized with bithiophene (BT) to afford a low band-gap copolymer (PBTPyDPP) which was used as an electron donor of the active layer in polymer solar cells. The solar cell device based on PBTPyDPP exhibited a promising PCE of 4.9% with a high VOC over 0.9 V, which is one of the highest values among DPP-based polymer solar cells.

A fluorinated phenylene unit as a building block for high-performance n-type semiconducting polymer.

Copolymers composed of diketopyrrolopyrrole and phenylene units with different numbers of fluorine subsitution are synthesized. When the effect of the number of fluorine substitution on the n-channel transporting property is investigated, the polymer with four fluorine substitutions exhibits the best n-type charge-transporting behavior with an electron mobility of 2.36 cm(2) V(-1) s(1).

Facile method to functionalize graphene oxide and its application to poly(ethylene terephthalate)/graphene composite.

Graphene oxide (GO) prepared in bulk quantities by oxidation of graphite with strong oxidants contains many hydrophilic groups, such as hydroxyl, epoxy, and carboxyl acid. We present a method to efficiently convert these hydrophilic groups into alkyl and alkyl ether groups by a one step reaction of bimolecular nucleophilic substitution with alkyl bromide. The functionalized graphene oxide (fGO) can be homogeneously dispersed as exfoliated monolayers in various organic solvents without degradation of size and shape of graphene oxide sheet. The degree of substitution reaction of each hydrophilic group in GO with alkyl bromide is quantitatively determined by comparing the deconvoluted O 1s X-ray photoelectron spectrum of GO with that of fGO. Addition of a small amount of fGO in poly(ethylene terephthalate) (PET) improves remarkably tensile and gas barrier properties of PET/fGO composite due to homogeneous dispersion of fGO sheets in PET matrix.

A low band-gap polymer based on unsubstituted benzo[1,2-b:4,5-b']dithiophene for high performance organic photovoltaics.

A low band-gap conjugated polymer, PBDTDPP, composed of unsubstituted benzo[1,2-b:4,5-b']dithiophene and diketopyrrolo[3,4-c]pyrrole was synthesized. The deep HOMO level of PBDTDPP enhances the V(OC) of a PSC up to 0.82 V and exhibits a PCE of 5.16%, while alkoxy substituted PBDTDPP-OR yields a PCE of 2.24% with a V(OC) of 0.61 V.

Bilayer order in a polycarbazole-conjugated polymer.

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

Unidirectionally aligned line patterns driven by entropic effects on faceted surfaces.

A simple, versatile approach to the directed self-assembly of block copolymers into a macroscopic array of unidirectionally aligned cylindrical microdomains on reconstructed faceted single crystal surfaces or on flexible, inexpensive polymeric replicas was discovered. High fidelity transfer of the line pattern generated from the microdomains to a master mold is also shown. A single-grained line patterns over arbitrarily large surface areas without the use of top-down techniques is demonstrated, which has an order parameter typically in excess of 0.97 and a slope error of 1.1 deg. This degree of perfection, produced in a short time period, has yet to be achieved by any other methods. The exceptional alignment arises from entropic penalties of chain packing in the facets coupled with the bending modulus of the cylindrical microdomains. This is shown, theoretically, to be the lowest energy state. The atomic crystalline ordering of the substrate is transferred, over multiple length scales, to the block copolymer microdomains, opening avenues to large-scale roll-to-roll type and nanoimprint processing of perfectly patterned surfaces and as templates and scaffolds for magnetic storage media, polarizing devices, and nanowire arrays.

Effect of nanoscale SubPc interfacial layer on the performance of inverted polymer solar cells based on P3HT/PC71BM.

The effect of a nanoscale boron subphthalocyanine chloride (SubPc) interfacial layer on the performance of inverted polymer solar cells based on poly (3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) was studied. When a 1 nm SubPc layer was introduced between the active layer (P3HT:PC(71)BM) and MoO(x) in the device with ITO/ZnO/P3HT:PC(71)BM/SubPc/MoO(x)/Al configuration, the power conversion efficiency (PCE) was increased from 3.42 (without SubPc) to 3.59%. This improvement is mainly attributed to the enhanced open-circuit voltage from 0.62 to 0.64 V. When the Flory-Huggins interaction parameters were estimated from the solubility parameters through the contact angle measurement, it revealed that the interaction between SubPc and PC(71)BM is more attractive than that between SubPc and P3HT at the interface of P3HT:PC(71)BM/SubPc, through which charges are well transported from the active layer to the anode. This is supported by a decrease of the contact resistance from 5.49 (SubPc 0 nm) to 0.94 MΩ cm (SubPc 1 nm). The photoelectron spectra provide another evidence for the enhanced PCE, exhibiting that the 1 nm thick SubPc layer extracts more photoelectrons from the active layer than other thicknesses.

One-pot synthesis of hybrid TiO2-polyaniline nanoparticles by self-catalyzed hydroamination and oxidative polymerization from TiO2-methacrylic acid nanoparticles.

A simple self-catalyzed hydroamination method for creating hybrid TiO(2)-polyaniline core-shell nanoparticles (NP) has been shown. Hybrid NPs with a range of possible sizes are afforded in high yield under mild reaction conditions and simultaneously show improved charge transport and electrochromic behavior compared to either polyaniline alone or physically blended with TiO(2).

Supramolecular assembly of end-functionalized polymer mixtures confined in nanospheres.

Supramolecular assembly of functionalized polymers, capable of forming block copolymer-like molecular clusters, has emerged as a promising alternative for creating nanoscopically ordered structures. Here, we demonstrate that nanospheres, which have intriguing internal nanodomains and controllable surface functionality, can be fabricated by supramolecular assembly of two complementarily end-interacting species of mono-end-functionalized polymers using the self-organized precipitation (SORP) method. An exotic internal morphology, hierarchically organized structure of perforated spherical layers, was formed inside the nanosphere prepared from the stoichiometric mixture of the end-functionalized polymers, which is due to the formation of diblock-like supramolecules and their packing frustration in the spherically confined nanospace. When the mixing ratio of the two end-functionalized polymers differs from the stoichiometric ratio, the nanoparticle surface is enriched with an excess of unpaired functionalized groups, which therefore provides us with a useful way to precisely control the surface functionality of the nanoparticles.

Fabrication of highly conductive and transparent thin films from single-walled carbon nanotubes using a new non-ionic surfactant via spin coating.

Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for fabricating high-quality single-walled carbon nanotube (SWCNT) films by a simple spin coating method. The absence of charge repulsion between SWCNT/surfactant complexes successfully leads to formation of a dense network of SWCNTs on the substrate through a single deposition of spin coating. When the SWCNT film was treated with nitric acid and thionyl chloride after washed with dichloromethane and water, a high-performance SWCNT film with the sheet resistance of 59 ohm/sq and the transparency of 71% at 550 nm was successfully obtained. Since the SWCNT film exhibits a high value of σ(dc)/σ(ac) (∼17) and excellent dimensional stability after releasing from the substrate, the film can be used as a transparent electrode in flexible optoelectronic devices.

Secondary water pore formation for proton transport in a ClC exchanger revealed by an atomistic molecular-dynamics simulation.

Several prokaryotic ClC proteins have been demonstrated to function as exchangers that transport both chloride ions and protons simultaneously in opposite directions. However, the path of the proton through the ClC exchanger, and how the protein brings about the coupled movement of both ions are still unknown. In this work, we use an atomistic molecular dynamics (MD) simulation to demonstrate that a previously unknown secondary water pore is formed inside an Escherichia coli ClC exchanger. The secondary water pore is bifurcated from the chloride ion pathway at E148. From the systematic simulations, we determined that the glutamate residue exposed to the intracellular solution, E203, plays an important role as a trigger for the formation of the secondary water pore, and that the highly conserved tyrosine residue Y445 functions as a barrier that separates the proton from the chloride ion pathways. Based on our simulation results, we conclude that protons in the ClC exchanger are conducted via a water network through the secondary water pore, and we propose a new mechanism for the coupled transport of chloride ions and protons. It has been reported that several members of ClC proteins are not just channels that simply transport chloride ions across lipid bilayers; rather, they are exchangers that transport both the chloride ion and proton in opposite directions. However, the ion transit pathways and the mechanism of the coupled movement of these two ions have not yet been unveiled. In this article, we report a new finding (to our knowledge) of a water pore inside a prokaryotic ClC protein as revealed by computer simulation. This water pore is bifurcated from the putative chloride ion, and water molecules inside the new pore connect two glutamate residues that are known to be key residues for proton transport. On the basis of our simulation results, we conclude that the water wire that is formed inside the newly found pore acts as a proton pathway, which enables us to resolve many problems that could not be addressed by previous experimental studies.

Morphology control of a polythiophene-fullerene bulk heterojunction for enhancement of the high-temperature stability of solar cell performance by a new donor-acceptor diblock copolymer.

A well defined diblock copolymer (P3HT-b-C(60)) based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was synthesized via two controlled polymerization steps and used as a compatibilizer for the P3HT/PCBM blend, which has widely been used as an active layer in bulk heterojunction polymer solar cells. The addition of a small amount of P3HT-b-C(60) results in not only the reduction of phase size of P3HT/PCBM blend but also the suppression of macrophase separation for long-time thermal annealing owing to the preferential location of the diblock copolymers at the interface between P3HT and PCBM phases. The morphology change with the annealing time is closely related to the change of the power conversion efficiency (PCE) of solar cells: the PCE of P3HT/PCBM greatly decreases with increasing annealing time while the addition of P3HT-b-C(60) significantly reduces the decrease of PCE for long-time thermal annealing.

Chloride ion conduction without water coordination in the pore of ClC protein.

In the present work, we have found by an atomistic molecular dynamics simulation that hydrogen atoms originating from the residues of a prokaryotic ClC protein (EcClC) stabilize the chloride ion without water molecules in the pore of ClC protein. When the chloride ion conduction is simulated by pulling a chloride ion along the pore axis, the free energy barrier for chloride ion conduction is calculated to be low (4 kcal/mol), although the chloride ion is stripped of its hydration shell as it passes through the dehydrated pore region. The calculation of the number of hydrogen atoms surrounding the chloride ion reveals that water molecules hydrating the chloride ion are replaced by polar and non-polar hydrogen atoms protruding from the protein residues. From the analysis of the pair interaction energy between the chloride ion and these hydrogen atoms, it is realized that the hydrogen atoms from the protein residues stabilize the chloride ion at the dehydrated region instead of water molecules, by which the energetic penalty for detaching water molecules from the permeating ion is compensated.