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In designing thermoelectric and heat transfer devices on the micrometer scale, the accurate thermal conductivity measurement is very important, and a variety of measurement methods have been developed so far. Among them, the 3ω method is one of the best for conductive wires because it can directly measure thermal conductivity without measuring density or specific heat, and also in the same direction as electrical or thermoelectric property. However, previous studies have not sufficiently considered the effects of ambient pressure and the conductive adhesive used to attach the sample to the electrode, which may hinder accurate measurement. In this study, using a thin gold wire as a test sample, the influence of ambient pressure and the length of conductive adhesive along the sample has been investigated quantitatively as major factors of systematic errors in the 3ω method. When the pressure was increased in the transition flow region, the measured apparent thermal conductivity increased. An analytical model for the low-pressure gas heat conduction is proposed to quantitatively explain the pressure dependence. The measured value also increased when the length of the conductive adhesive exceeded 20% of the sample length. This work has revealed that the ambient should be evacuated to the molecular flow region and the length of conductive adhesive be less than 20% of the sample length. The guidelines proposed here will help researchers in various fields to more accurately determine the thermal conductivity of micrometer-scale wires.

Small bimolecular charge recombination coefficients resulted in all-polymer blend solar cells with high fill factors. The preferred blend morphology was characterized by a well-ordered local structure, exhibiting chain aggregation by both the polymer donor and acceptor.

PCPDTBT, a copolymer of cyclopentadithiophene and benzothiadiazole, is a locally ordered low-bandgap donor–acceptor conjugated polymer that has been used as a hole-transport material for polymer blend solar cells. In this study, the short-range order structure of PCPDTBT in spin-coated PCPDTBT/conjugated polymer blend films has been characterized to elucidate aggregation behaviors that arise when a locally ordered conjugated polymer is blended with other polymers. UV–vis absorption spectroscopy and grazing-incidence wide-angle X-ray scattering measurements revealed that short-range ordering of PCPDTBT is promoted when it is diluted in an amorphous conjugated polymer matrix, poly­(3-methyl-4-hexylthiophene-2,5-diyl) (P3M4HT). In a PCPDTBT/P3M4HT blend film with 10 wt % PCPDTBT, the fraction of PCPDTBT chains that form aggregates increases from 51% (in neat film) to 78%, and the correlation length of the ordered PCPDTBT chains extends from 4.6 nm (in neat film) to 7.7 nm in the chain backbone direction. The polymer matrix-assisted ordering can be explained in terms of suppression of both undesired overlap and interpenetration of PCPDTBT chains; both processes can derange the self-aggregated single-chain order of PCPDTBT. This short-range ordering enhancement would be significant for the design of efficient charge carrier transport in thin-film devices such as polymer/polymer blend solar cells that use low-bandgap donor–acceptor conjugated polymers with various blend ratios as hole- and electron-transport materials.

Herein, the hole transport in a quinoxaline–thiophene based conjugated polymer (PTQ1) mixed with an insulating polystyrene (PS) was studied by macroscopic and local current density−voltage characteristics measurements. As a result, we found that the hole conductivity in PTQ1 : PS blends increases as the weight ratio of PTQ1 is reduced down to 20 wt%. This is mainly ascribed to increases in mobility because the charge carrier density would be constant in the insulating PS matrix. With decreasing PTQ1 weight ratio in the blends, the absorption bandwidth of PTQ1 and additional emission due to excimer decreased, suggesting that interchain interactions are suppressed. By measuring the temperature‐dependent conductivity, we also found that the activation energy for the hole conductivity is smaller in PTQ1 : PS blends than in PTQ1 neat films. These findings suggest that trap sites decrease because of the suppressed interaction between PTQ1 chains in blend films. We also measured conductive atomic force microscope images of the blend films to clarify the local conductive property. For PTQ1 neat films, a low conductive image was observed over the entire film. For PTQ1 : PS blends, on the other hand, many highly conductive spots were locally found. We thus conclude that the dilution of PTQ1 chains in the PS matrix leads to a lower formation of trap sites, resulting in more conductive transport in PTQ1 : PS blends than in PTQ1 neat films.

Layer-by-layer fabrication of uniformly oriented thin films over large areas by cost-effective solution-based approaches can open new horizons for the realization of high-performance organic circuits in various applications. In this work, fabrication of a large-area ≈40 cm2 film with uniform orientation is reported for poly(3,3‴-dialkylquaterthiophene) (PQT) using a unidirectional floating film transfer method (UFTM). Orientation characteristics and charge transport anisotropy were analyzed using polarized UV-vis spectral mapping and fabrication of bottom-gated organic field-effect transistors (OFETs) from different regions. Films were found to be highly oriented with an optical dichroic ratio of ca. 15. Orientation characteristics reveal that films were highly oriented along the width of the film, covering >70% of the area, and angle-dependent field-effect mobilities are in good agreement with the orientation of the polymer backbones. These highly oriented films resulted in charge transport anisotropy of 8.9. An array of bottom-gated OFETs fabricated along the length of single large-area (≈15 × 2.5 cm2) thin film demonstrated the average field-effect mobility of 0.0262 cm2/(V s) with a very narrow standard deviation of 12.6%. We also demonstrated that film thickness can be easily tuned from 5.6 to 45 nm by increasing the PQT concentration, and field-effect mobility is highly reproducible even when the film thickness is 10 nm. Microstructural characterization of the thus-prepared large-area thin films revealed the edge-on stacked polymer backbones and surface roughness of

To understand the origin of the irreproducibility of the hole injection barrier at the interface between Au top-contact electrodes and organic semiconductors, we performed a systematic study by focusing on the deposition rate of Au. As a benchmarking material system, we fabricated field-effect transistors with pentacene active layers and Au top-contact electrodes evaporated with the deposition rate from 0.5 to 10 A/s. From the contact potential differences (ΔCPDs) between Au and pentacene measured with Kelvin-probe force microscopy, cross-sectional structures with scanning transmission electron microscopy, crystallographic structures with X-ray diffraction, and Au depth profiles with secondary ion mass spectrometry, we concluded that the variation of ΔCPD is due to embedded Au nanoclusters in pentacene with the average diameter of 4–6 nm. These Au nanoclusters, of which depth significantly varies by the Au deposition rate, modulate the ΔCPD and interfere with the reproducibility of hole injection barrier. As a result, gate threshold voltage and field-effect mobility also become irreproducible.

Polymer semiconductor/gate dielectric interface is well known to influence the charge transport behaviors in organic field-effect transistors (OFETs). In the unidirectional floating-film transfer method (UFTM), thin floating films of uniaxially aligned π-conjugated polymers are solidified at the air/liquid interface before transferring them onto device substrates. Therefore, packing structures in the films prepared by UFTM are not affected by the strong interactions with the device substrate as usually seen in other film growth methods. Here, one question arises: will the difference in interactions between the liquid substrate and air impact the orientational structure of the polymer chain near the two surfaces or not? To answer this question, we have studied the charge-transport anisotropy in top- and bottom-gate OFETs by changing the film thickness. As a result, the field-effect carrier mobility and its anisotropy did not significantly differ for both surfaces and were constant against the film thickness. These results indicate that the UFTM can provide unidirectional alignment of polymer chains throughout the film thickness and gives a similar superior performance either for a bottom- or a top-gate OFET.

To improve the performance of organic photovoltaics (OPV) for energy harvesting from blue-light, the OPVs (Ag/BCP/C60/BPh-BTBT/10 mol% p-doped BPh-BTBT/ITO) are fabricated with several dopants such as F4TCNQ, C60F36, HATCN, and Mo(tfd)3. The power conversion efficiencies of the OPVs with F4TCNQ, C60F36, HATCN, and Mo(tfd)3-doped BPh-BTBT obtained by white light-emitting diode source are improved with 60, 78, 46, and 49% than those by normal solar light, respectively. Secondary ion mass spectroscopy is performed with the 1-day and 7-days samples after the sample fabrication to observe the dopant distributions in each p-doped layer. Diffusion of dopant is observed only in the F4TCNQ-doped BPh-BTBT.

Orientating conjugated polymers (CPs) in thin films is an attractive technique for obtaining anisotropic properties, resulting in improved device performances. Template-assisted alignment using pre-patterned substrates has been widely used to orient various polymers irrespective of their orientation capability. In this work, we report that using the floating film transfer method, the orientation of guest polymers can be in-situ assisted via blending with a host polymer during film fabrication. Dichroic Ratio (DR) of two guest CPs such as poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] and poly(9,9-di-n-octylfluorenyl-2,7-diyl) were found to be enhanced when blended with host polymer poly(3,3‴-didodecyl-quaterthiophene) (PQT). DR of the host PQT decreases monotonically with an increase in the guest polymer proportion, suggesting that guest polymers are well mixed in the host matrix. The electronic absorption spectra and atomic force microscopy results revealed that blended polymers were well mixed without any phase separation. DR of the guest polymers in the blend was the same even when the overall blend concentration was increased twice to increase the overall film thickness.

Herein, the open-circuit voltage (V[OC]) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of V[OC] in order to discuss the difference in the primary loss mechanism of V[OC] between them. As a result, the photon energy loss for polymer solar cells is in the range of 0.7–1.4 eV, which is ascribed to temperature-independent and -dependent loss mechanisms while that for perovskite solar cells is as small as about 0.5 eV, which is ascribed to a temperature-dependent loss mechanism. This difference is attributed to the different charge generation and recombination mechanisms between the two devices. The potential strategies for the improvement of V[OC] in both solar cells are further discussed on the basis of the experimental data.

Charge transport in intermixed regions of all-polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT; electron donor) with poly[2, 7-(9, 9-didodecylfluorene)-alt-5, 5-(4′, 7′-bis(2-thienyl)-2′, 1′, 3′-benzothiadiazole)] (PF12TBT; electron acceptor) was studied by conductive atomic force microscopy (C-AFM). For a blend film fabricated from a chlorobenzene solution, intermixed regions were detected between the P3HT-rich and PF12TBT-rich domains. The overall hole current in the intermixed regions remained almost constant, both before and after thermal annealing at 80 °C, but it increased in the P3HT-rich domains. For a blend film fabricated from a chloroform solution, the entire observed area constituted an intermixed region, both before and after thermal annealing. The overall hole current in this film was significantly improved following thermal annealing at 120 °C. These finely mixed structures with efficient charge transport yielded a maximum power conversion efficiency of 3.5%. The local charge-transport properties in the intermixed region, as observed via C-AFM, was found to be closely related to the photovoltaic properties, rather than the bulk-averaged properties or topological features.

The charge-transport characteristics of phase-separated blend films of poly(3-hexylthiophene) (P3HT; electron donor) and poly[2, 7-(9, 9-didodecylfluorene)-alt-5, 5-(4′, 7′-bis(2-thienyl)-2′, 1′, 3′-benzothiadiazole)] (PF12TBT; electron acceptor) were visualized by conductive atomic force microscopy (C-AFM). The C-AFM hole-current images clearly showed two phases: an electrically conductive region assigned to the P3HT-rich donor domain and a nonconductive region assigned to the PF12TBT-rich acceptor domain. The hole current in the conductive region was small compared with that of a neat P3HT film with similar thickness, indicating that the P3HT-rich domain contained a large fraction of PF12TBT as a minor component. Thermal annealing initially increased the hole current throughout the P3HT-rich domain because of reorganization of the P3HT chains from their as-cast configurations. Further annealing increased the hole current mainly in the middle of the P3HT-rich domain, but it decreased the hole current in the boundary areas close to the PF12TBT-rich domain owing to the presence of an intermixed region with a gradient of the P3HT/PF12TBT composition ratio. After annealing at temperatures above the glass-transition point of PF12TBT, the widths of the intermixed regions decreased to ∼30 nm as phase separation proceeded with decomposition of the intermixed region. Such variations in the intermixed region, which were electrically resolved by the C-AFM, accounted for the temperature dependence of the photovoltaic properties of P3HT/PF12TBT blend solar cells.

Time-resolved spectroscopy is a powerful tool for studying fundamental photophysics in optoelectronic materials on a molecular temporal scale. In this review, we describe transient spectroscopic studies on fundamental photovoltaic conversion processes in polymer solar cells, which consist of a series of conversion processes such as photon absorption, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode. These conversion processes are ultrafast phenomena and are ranging over the wide temporal scale from femtoseconds to microseconds, which can be directly observed by transient spectroscopy.

We developed high-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths. A power conversion efficiency of 6.7% was obtained with an external quantum efficiency over 60% both in the visible and near-infrared regions. Our results demonstrate that the ternary blend all-polymer systems open a new avenue for accelerating improvement in the efficiency of non-fullerene thin-film polymer solar cells.

Polymer solar cells with a thick active layer can generate large photocurrent because they absorb many more photons. In most cases, however, the active layer is as thin as ~100 nm under the optimized condition. In this work, we have studied bimolecular charge recombination in highly efficient polymer solar cells with a thick active layer by transient photovoltage and photocurrent measurements. As a result, we found that the bimolecular recombination rate is smaller by three orders of magnitude than that predicted from the diffusion-limited Langevin recombination. We therefore conclude that such reduced bimolecular recombination enables efficient charge collection even in the solar cell with a thick active layer.

Polymer/polymer blend solar cells based on a blend of two types of conjugated polymers acting as an electron donor (hole transport) and acceptor (electron transport) have recently attracted considerable attention, because they have numerous potential advantages over conventional polymer/fullerene blend solar cells. The highest power conversion efficiency (PCE) was slightly above 2% five years ago, whereas PCEs of beyond 8% are the state-of-the-art today, and the efficiency gap between polymer/polymer and polymer/fullerene systems has closed very rapidly. In this review, we provide an overview of recent progress towards the performance enhancement of polymer/polymer blend solar cells. In addition, we discuss the future outlook and challenges regarding PCEs beyond 10%.

Ternary blend all-polymer solar cells are developed to broaden the absorption bandwidth of the photoactive layer. A wide-bandgap polymer with absorption in the visible region is introduced as a third polymer into a low-bandgap donor/acceptor binary polymer blend showing absorption in the near-infrared (NIR) region. In the ternary blend solar cell, the external quantum efficiency (EQE) is improved in the visible wavelength region, while retaining the excellent EQE of the host binary blend in the NIR wavelength region. These results demonstrate that the use of ternary blends of conjugated polymers containing a wide-bandgap polymer as a visible sensitizer is an effective approach for improving the efficiency of all-polymer solar cells.

Excited-state dynamics in poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1) was studied by transient absorption spectroscopy. Upon photoexcitation at 400 nm, an additional transient species is promptly generated along with singlet excitons and survives up to nanoseconds, while singlet excitons disappear completely. In order to assign the long-lived species, we measured transient absorption spectra over the wide spectral range from 900 to 2500 nm. As a result, we found that the long-lived species is ascribed not to polarons but to triplet excitons, which is formed through the ultrafast singlet fission (SF). We discuss the ultrafast SF mechanism in push-pull low-bandgap polymer PTB1 films on the basis of the excited-state dynamics under various excitation wavelengths and intensities.

Herein, we theoretically and experimentally investigated the mechanisms of charge separation processes of organic thin-film solar cells. PTB7, PTB1, and PTBF2 have been chosen as donors and PC₇₁BM has been chosen as an acceptor considering that effective charge generation depends on the difference between the material combinations. Experimental results of transient absorption spectroscopy show that the hot process is a key step for determining external quantum efficiency (EQE) in these systems. From the quantum chemistry calculations, it has been found that EQE tends to increase as the transferred charge, charge transfer distance, and variation of dipole moments between the ground and excited states of the donor/acceptor complexes increase; this indicates that these physical quantities are a good descriptor to assess the donor–acceptor charge transfer quality contributing to the solar cell performance. We propose that designing donor/acceptor interfaces with large values of charge transfer distance and variation of dipole moments of the donor/acceptor complexes is a prerequisite for developing high-efficiency polymer/PCBM solar cells.

The growth of hole-transporting nanostructures of regioregular poly(3-hexylthiophene) (P3HT) films with thermal annealing were examined by conductive atomic force microscopy (C-AFM). The C-AFM current images visualized spatially inhomogeneous hole transport in the film on a nanometer scale, with relatively low conductive regions and high conductive domains. The high conductive domains were attributed to the relatively high density regions of P3HT nanocrystallites in the film, which were determined during spin-coating. The current images obtained from the same area of the P3HT film showed that thermal annealing improved the hole-transporting property of the film on average, but both the size and spatial distributions of the relatively high conductive domains in the as-spun film remained almost the same even after annealing. Furthermore, we found that the increase in current flow proceeded mostly in the relatively high conductive domains. In these domains, the electrical connectivity among the crystalline phases was effectively improved by the growth of individual nanocrystallites, leading to the formation of preferred hole-transporting pathways in the direction of film thickness.

We have successfully measured electron transport nanostructures of conjugated polymer thin films by conductive atomic force microscopy, using an air-stable electron-injecting electrode coated with ethoxylated polyethylenimine. Electron- and hole-transport networks in donor/acceptor polymer blends can be selectively observed by using an appropriately coated electrode. This approach enables us to visualize phase-separated nanostructures of donor/acceptor polymer blends for thin-film electronic devices based on their semiconducting properties.

Efficient exciton collection at charge-generation sites is one of the key requirements for the improvement in power conversion efficiency (PCE) of organic solar cells, because only excitons arriving at a donor/acceptor interface can be dissociated into free charge carriers. We evaluated the effective diffusion length in poly(3-hexylthiophene) (P3HT) by using donor/acceptor bilayers with two different exciton-quenching acceptors. One is an insoluble fullerene polymer (p-PCBVB), which is an efficient electron-accepting material with negligible absorption in the visible region. The other is a low-bandgap polymer, poly[(4, 4-bis(2-ethylhexyl)-dithieno[3, 2-b:2', 3'-d]silole)-2, 6-diyl-alt-(2, 1, 3-benzothiadiazole)-4, 7-diyl], (PSBTBT). This polymer has a large absorption band in the near-IR region, which overlaps well with the emission band of P3HT. The effective diffusion length of P3HT excitons is evaluated to be 15nm for P3HT/p-PCBVB bilayers and improved to 30nm for P3HT/PSBTBT bilayers. This improvement is ascribed to long-range energy transfer from P3HT to PSBTBT. This finding suggests that the effective diffusion length of P3HT excitons can be increased through long-range energy transfer by incorporating PSBTBT into P3HT/PCBM blends.

Ternary hybrid solar cells based on zinc oxide with wide bandgap poly(3-hexylthiophene) (P3HT) and narrow bandgap poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (PTQ1) exhibit improved photovoltaic performance compared to that of individual binary hybrid solar cells. The increase in the photocurrent is partly due to the complementary absorption bands, which can extend the light-harvesting range from visible to near-infrared regions, and partly due to efficient energy transfer from P3HT to PTQ1, by which P3HT excitons are more efficiently collected at the PTQ1/ZnO interface and hence convert to charge carriers effectively. Furthermore, the improvement in the fill factor may be due to efficient hole transfer from PTQ1 to P3HT with higher hole mobility, and thereby, hole polarons are more efficiently collected on the electrode.

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.

We have fabricated ternary blend solar cells based on poly(3-hexylthiophene) (P3HT) as a donor material, poly{[ N , N ′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt -5,5′-(2,2′-bithiophene)} (N2200) as an acceptor material, and a silicon naphthalocyanine derivative (SiNc) as a near-IR sensitizer. In order to discuss how molecular structures impact the dye sensitization in P3HT/N2200 solar cells, we studied two SiNc molecules with different axial groups: SiNc10 with decyldimethylsilyl oxide and SiNc6 with trihexylsilyl oxide. As a result, P3HT/N2200/SiNc10 ternary solar cells exhibited a power conversion efficiency of 1.4%, which is improved by 73% compared to P3HT/N2200 binary solar cells and also by 17% compared to P3HT/N2200/SiNc6 ternary solar cells. We discuss the origin of the improvement in the device performance in terms of dye location in ternary blend films.

The charge generation and recombination dynamics in fullerene-attached poly(3-hexythiophene) (P3HT)-based diblock copolymer were studied in comparison with those in blend films of P3HT and a fullerene derivative (PCBM) in order to understand the potential advantage of diblock copolymer-based polymer solar cells. Upon photoexcitation, P3HT singlet excitons are promptly converted to P3HT polarons with a time constant of ∼30 ps in both P3HT-PCBM diblock copolymer and P3HT/PCBM blend films. This similar charge generation dynamics is indicative of analogous phase-separated morphology both in these films on a scale of nanometers. After the charge generation, a part of polarons in disorder phases geminately recombine to the ground state in diblock copolymer films, while no geminate recombination is observed in blend films. This geminate recombination loss is probably due to defects of phase-separated structures in diblock copolymer films. On the other hand, charge carrier lifetime is as long as 15 μs in diblock ...

Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly(9,9′-di-n-octylfluorene-ran-N,N′-bis(4-n-butylphenyl)-N,N′-diphenyl-1,4-benzenediamine) (F8-PDA) and poly(9,9′-di-n-octylfluorene-ran-N,N′-bis(4-t-butylphenyl)-N,N′-diphenyl-9,10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 × 108 s–1. On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet–triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is ...

Two new trinuclear Mn complexes containing naphthalene moieties, [Mn3(μ3-O)(μ-O2CCH2-1-naph)6(py)3] (1) and [Mn3(μ3-O)(μ-O2CCH2-2-naph)6(py)3] (2) (naph-1-CH2CO2H = 1-naphthylacetic acid, naph-2-CH2CO2H = 2-naphthylacetic acid, py = pyridine), were synthesized. The X-ray crystallography of 1, the EXAFS analyses of 1 and 2, and the magnetic susceptibility data for these complexes showed that they are trinuclear complexes containing a [Mn3(μ3-O)(μ-O2CR)6] core with mixed-valence Mn3(II, III, III) centers. CVs exhibited an oxidation peak ascribed to MnIII3/MnIIMnIII2 and a reduction peak ascribed to MnIIMnIII2/MnII2MnIII in CH2Cl2. In the fluorescence spectra, the fluorescence intensities of 1 and 2 were smaller than those of the naphthylacetic acids and depended on the dielectric constant of solvent. These results suggested that there is electron transfer between the manganese centers and the naphthyl moieties. The fluorescence decays of 1 and 2 in CH3CN were also faster than the decays in the toluene solutions; this result parallels the lower intensity of the static fluorescence spectra in solvents with a higher dielectric constant. The fluorescence decay curves of 1 and 2 were fitted by two lifetimes, τ1 and τ2, suggesting that electron transfer occurs from the monomer (τ1) and the excimer (τ2) of the naphthyl moieties to the manganese center.

The growth of highly conductive nanodomains in regioregular poly(3-hexylthiophene) (P3HT) thin films was visualized with a resolution of 20 nm by conductive atomic force microscopy (C-AFM). The current distribution images by C-AFM indicated that P3HT nanocrystallites with sizes smaller than 20 nm were dispersed in a matrix of disordered phase in the film spin-cast from chloroform; with thermal annealing, highly conductive spots appeared in the image. In contrast, the film spin-cast from o-dichlorobenzene showed a large number of conductive domains with dimensions from 20 to 200 nm. The conductive domains occupied 52% of the entire surface, and the current carried through the domains accounted for 66% of the total current, demonstrating that these conductive networks are essential for obtaining a sufficient macroscopic conductivity. The increase in macroscopic conductivity of the P3HT films was well explained from the viewpoint of nanoscale charge transport through the development of the conductive nanodomains.

The charge generation and recombination dynamics in amorphous blend films of poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5′,8′-di-2-thienyl-2′,3′-diphenylquinoxaline)] (N-P7) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were comprehensively studied in order to address the origin of the relatively high performance for amorphous polymer-based solar cells. Upon photoexcitation, N-P7 singlet excitons are promptly converted to the interfacial charge transfer (CT) state, which is a Coulombically bound pair of the N-P7 polaron and PCBM radical anion, with 100% efficiency in a picosecond. More than half of the N-P7 polarons in the CT state are dissociated into free carriers, and the rest of them recombine to the ground state in a nanosecond. The dissociation efficiency ηCD is estimated to be 0.65 under the open-circuit condition and is slightly enhanced up to 0.7–0.8 under the short-circuit condition. Such highly efficient dissociation is well explained by considering charge delocalization. The charge collec...

The dynamics of singlet fission in poly(9,9′-di-n-octylfluorene) (PFO) films was studied by transient absorption spectroscopy. Under a high excitation intensity, triplet excitons were rapidly interconverted from singlet excitons on a picosecond time stage. From the excitation intensity dependence of the generation yield, the rapid triplet exciton formation is attributed to the singlet fission from a higher singlet excited state Sn formed by the singlet–singlet annihilation (singlet fusion followed by singlet fission). The singlet fission from the Sn state was about 2 times more efficient in amorphous PFO films than in ordered β-PFO films, in which the conjugated main chains are planarly extended and aligned. On the other hand, about a half of triplet pairs were deactivated in the amorphous PFO film through the triplet–triplet annihilation with a time constant of 1.7 ns, while no triplet pair decay was observed in the β-PFO film. As a result, the overall triplet generation yield was comparable in these fil...

The dye sensitization method for bulk heterojunction polymer solar cells is applied to polymer/polymer solar cells composed of conjugated donor and acceptor polymers. The power conversion efficiency is successfully improved by addition of a near-infrared (NIR) dye. Spectroscopic analysis and J-V characteristics indicate that the dye molecules effectively contribute to the charge generataion through direct absorption of NIR light and also through the energy transfer from the donor polymers, according to the cascaded HOMO and LUMO potentials. These results demonstrate the versatility of dye sensitization as a useful method for improving the conversion efficiency of photovoltaics.

The charge carrier dynamics in blend films of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was studied by transient absorption spectroscopy in order to address the origin of limited external quantum efficiency (EQE) of this solar cell compared to that of a benchmark solar cell composed of regioregular poly(3-hexythiphene) (RR-P3HT) and PCBM. Upon photoexcitation, PCPDTBT singlet excitons promptly convert to the interfacial charge transfer (CT) state that is a Coulombically bound charge pair of PCPDTBT polaron and PCBM anion at the heterojunction with almost 100% efficiency in a picosecond. In other words, the exciton diffusion efficiency ηED and charge transfer efficiency ηCT are 100% in this blend, which are higher than and comparable to those of the RR-P3HT/PCBM solar cell, respectively. On a time scale of nanoseconds, 70% of the PCPDTBT bound polarons are dissociated into free ch...

The highest power conversion efficiency (PCE) of 2.7% has been achieved for all-polymer solar cells made with a blend of poly(3-hexylthiophene) (P3HT, electron donor) and poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thienyl)-2',1',3'-benzothiadiazole)] (PF12TBT, electron acceptor). The PCE of the P3HT/PF12TBT solar cells increases from 1.9% to 2.7% with an increase in the molecular weight (Mw) of PF12TBT from 8500 to 78 000 g mol(-1). In a device with high-molecular-weight PF12TBT, efficient charge generation is maintained even at high annealing temperatures because of the small phase separation on the length scale of exciton diffusion due to an increase in the glass transition temperature (Tg) and a reduced diffusional mobility of the PF12TBT chains above Tg. On the other hand, efficient charge transport is also achieved through the formation of interconnected networks of PF12TBT-rich domains, which is facilitated by the high molecular weight of PF12TBT, and the ordering of P3HT chains in P3HT-rich domains, which is a result of high-temperature annealing. Thus, when high-molecular-weight PF12TBT is used, an optimal blend morphology that supports efficient charge generation as well as charge transport can be obtained by thermal annealing, and consequently, the highest PCE reported so far for an all-polymer solar cell is achieved.

Polymer solar cells with a low-bandgap dye molecule are demonstrated by employing ternary blends of poly(3-hexylthiophene) (P3HT), fullerene derivative (PCBM), and silicon phthalocyanine derivative (SiPc) under different annealing conditions. The solvent annealing induces selective allocation of SiPc molecules at amorphous P3HT domains that would be formed at the interface between P3HT and PCBM crystalline domains, and furthermore effectively suppresses diffusion and aggregation of low molecular weight components which degrade the performance of the thermally annealed devices. As a result, the solvent-annealed ternary blend solar cells exhibited an enhanced power conversion efficiency of 4.5%.

The charge carrier dynamics in blend films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and conjugated polymers with different ionization potentials are measured using transient absorption spectroscopy to study the formation mechanism of PCBM radical cation, which was previously discovered for blend films of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and PCBM. On a nanosecond time scale after photoexcitation, polymer hole polaron and PCBM radical anion are observed but no PCBM radical cation is found in the blends. Subsequently, the fraction of polymer hole polarons decreases and that of PCBM radical cations increases with time. Finally, the fraction of PCBM radical cations becomes constant on a microsecond time scale. The final fraction of PCBM radical cation is dependent on the ionization potential of polymers but independent of the excitation wavelength. These findings show that the formation of PCBM radical cation is due to hole injection from polymer to PCBM domains. Furthermore, the energetic conditions for such hole injection in polymer/PCBM blend films are discussed on the basis of Monte Carlo analysis for hole hopping in a disordered donor/acceptor heterojunction with varying energetic parameters.

The origin of open-circuit voltage (VOC) was studied for polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and seven fullerene derivatives with different LUMO energy levels and side chains. The temperature dependence of J–V characteristics was analyzed by an equivalent circuit model. As a result, VOC increased with the decrease in the saturation current density J0 of the device. Furthermore, J0 was dependent on the activation energy EA for J0, which is related to the HOMO–LUMO energy gap between P3HT and fullerene. Interestingly, the pre-exponential term J00 for J0 was larger for pristine fullerenes than for substituted fullerene derivatives, suggesting that the electronic coupling between molecules also has substantial impact on VOC. This is probably because the recombination is non-diffusion-lmilited reaction depending on electron transfer at the P3HT/fullerene interface. In summary, the origin of VOC is ascribed not only to the relative HOMO–LUMO energy gap but also to the electronic couplings between fullerene/fullerene and polymer/fullerene.

The light-harvesting mechanism in polymer:fullerene:dye ternary blends was comprehensively studied by transient absorption spectroscopy. The ternary blend films consist of poly(3-hexylthiophene) (P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light-harvesting dye. Upon dye excitation of P3HT:PCBM:SiPc blends, P3HT polarons and SiPc anions were generated from SiPc excitons, and then electrons were shifted from SiPc to PCBM, suggesting that SiPc molecules are selectively located in a disordered P3HT matrix mixed with PCBM molecules at the P3HT/PCBM interface. Upon polymer excitation, P3HT excitons decayed more rapidly in ternary blends than in binary blends with an identically rapid formation of SiPc photobleaching, suggesting efficient energy transfer from P3HT to SiPc because of the large spectral overlap between P3HT emission and SiPc absorption, followed by slightly delayed generation of P3HT polarons and SiPc anions from SiPc excitons. Subsequently, the photob...

Selective dye loading at the polymer/fullerene interface was studied for ternary blend bulk heterojunction solar cells, consisting of regioregular poly(3-hexylthiophene) (RR-P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light-harvesting dye. The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located in the disordered P3HT domains at the RR-P3HT/PCBM interface rather than in the PCBM and crystal P3HT domains. From the peak wavelength of SiPc absorption, the local concentration of SiPc ([SiPc]Local) was estimated for the RR-P3HT:PCBM:SiPc ternary blends. Even for amorphous films of regiorandom P3HT (RRa-P3HT) blended with PCBM and SiPc, [SiPc]Local was higher than the original content, suggesting dye segregation into the RRa-P3HT/PCBM interface. For RR-P3HT:PCBM:SiPc blends, [SiPc]Local increased with the increase in the P3HT crystallinity. Such interfacial segregation of dye molecules in ternary blend films can be rationally explained in terms of the surface energy of each component and the crystallization of P3HT being enhanced by annealing. Notably, the solvent annealing effectively segregated dye molecules into the interface without the formation of PCBM clusters.

Titania-based ultrathin films were fabricated by layer-by-layer (LbL) assembly of titanium (IV) bis(ammonium lactato) dihydroxide (TALH) and polyelectrolytes. The thickness of the titania monolayer in the LbL films was 4–5 nm, which was consistent with the particle size of TALH in water. The conductivity and the electron mobility of the LbL films reached 10− 8 S cm− 1 and 10− 5 cm2 V− 1 s− 1, respectively. These results suggest that there exist percolation paths for the electron transport in the LbL film. Furthermore, it is demonstrated that photovoltaic properties of the LbL film can serve as an effective electron-transporting and accepting layer.

CH3NH3PbI3 thin films were formed with various deposition rates of PbI2 first layer using a sequential vacuum evaporation method (SVE) to understand the formation behavior. Under low PbI2 deposition rates with 1 and 3 A s−1, the thin films did not form the hybrid perovskite structure. Over the 6 A s−1 deposition rate, the perovskite structure is observed with the remaining PbI2 and an intermediate phase together. In the 10 A s−1, the intermediate phase is increased, and the CH3NH2 molecular defect is observed. The hybrid perovskite thin film formed by SVE is confirmed with tiny grains and CH3NH2-incorporated intermediate state.

The bimolecular recombination dynamics in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) has been studied by transient absorption spectroscopy. On a microsecond time scale, two polaron bands were observed at 700 and 1000 nm and exhibited different bimolecular recombination dynamics. The 700-nm band decayed with a time-independent bimolecular recombination rate of 10(-12) cm(3) s(-1). The activation energy was as small as approximately 0.078 eV independently of the carrier density. On the other hand, the 1000-nm band decayed with a time-dependent bimolecular recombination rate, which varied from 10(-12) to 10(-13) cm(3) s(-1), depending on time or carrier density. The activation energy decreased exponentially from 0.178 to 0.097 eV with the increase in the carrier density. Therefore, we assigned the 700-nm band to freely mobile delocalized polarons in crystalline P3HT domains and the 1000-nm band to localized polarons trapped in relatively disordered P3HT domains. At a charge density of 10(17) cm(-3), which corresponds to 1 sun open-circuit condition, some localized polarons exhibited trap-free bimolecular recombination due to trap-filling. These findings suggest that not only delocalized polarons but also some localized polarons play a crucial role in the efficient hole transport in P3HT:PCBM solar cells.

Fullerene ultrathin films were fabricated by layer-by-layer (LbL) assembly of an anionic fullerene C(61)(COO(-))(2) (FDCA) and poly(diallyldimethylammonium chloride) (PDDA) or a cationic fullerene C(60)C(2)H(4)N(CH(3))(2)(+) (FMAC) and poly(sodium 4-styrenesulfonate) (PSS). The dynamic light scattering and zeta-potential measurements revealed that both water-soluble fullerenes are stably dispersed as polyelectrolytes with a diameter of 20-70 nm for FDCA and 60-180 nm for FMAC in aqueous solutions. In spite of such large fullerene aggregates, the thickness of fullerene LbL films increased regularly by a few nanometers with each deposition, and the resultant LbL films were homogeneous. For FMAC/PSS LbL films, the monolayer thickness was evaluated to be 4 nm for FMAC and 0.5 nm for the PSS layer. In other words, the volume fraction of the fullerene moiety is as high as approximately 80 vol %. The conductivity of fullerene LbL films was comparable to that of a C(60)-dispersed polystyrene film with a similar fullerene fraction, suggesting that there exist effective percolating networks due to the high fullerene fraction in the LbL films. The electron mobility of FMAC/PSS LbL films was as high as 3 x 10(-5) cm(2) V(-1) s(-1), which is comparable to the hole mobility of poly(p-phenylenevinylene) LbL films reported previously.