Space Charge Limited Current Mobility Research Articles

Anisotropic mobilities with different packing motifs of the thin films of fused-selenophenes measuring from the SCLC method

In this article, we attempt to establish a correlation between the structural property of three polycyclic arene-fused selenophenes with different crystal packing such as naphthoselenophene ( Nap-Se ), phenanthroselenophene ( Phe-Se ), and naphtho-bis(selenophene) ( Nap-Se2 ) and their mobility measured via the space charge limited current (SCLC) method. The energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the fused selenophenes was a governing factor of their hole mobilities. We also correlate their theoretically calculated anisotropic mobility to obtain some initial idea about their performance as a donor in solar cell devices. The naphtho-bis(selenophene) ( Nap-Se2 ) exhibit the highest mobility by maintaining superiority with all the factors. • The mobility of a semiconductor depends on its HOMO-LUMO energy gap. • Naphtho-bis(selenophene) with 1-D lamellar crystal packing showed the highest mobility. • Vertically measured SCLC mobility provides a lower limit of mobility. • Anisotropic mobility is dictated by the effective charge transfer integral between the dimers.

Thermal Annealing Effect on the Thermal and Electrical Properties of Organic Semiconductor Thin Films

ABSTRACTThe thermal and electrical properties of organic semiconductor are playing critical roles in the device applications especially on the devices with large area. Although the effect may be minor in a single device like field effect transistors, the unwanted waste heat would cause much more severe problems in large-scale devices as the power density will go up significantly. The waste heat would lead to performance degradation or even failure of the devices, and thus a more detailed study on the thermal conductivity and carrier mobility of the organic thin film would be beneficial to predict the limits of the device or design a thermally stable device. Here we explore the thermal annealing effect on the thermal and electrical properties of the small molecule organic semiconductor, dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT). After the post deposition thermal annealing, the grain size of the film increases and in-plane crystallinity improves while cross-plane crystallinity keeps relatively constant. We demonstrated the cross-plane thermal conductivity is independent of the thermal annealing temperature and high annealing temperature will reduce the space-charge-limited current (SCLC) mobility. When the annealing temperature increase from 24 °C to 140 °C, the field effect mobility shows a gradual increase while the threshold voltage shifts from positive to negative. The different dependence of the SCLC mobility and field effect mobility on the annealing temperature suggest the improvement of the film crystallinity after thermal annealing is not the only dominating effect. Our investigation provides the constructive information to tune the thermal and electrical properties of organic semiconductors.

EDOT‐diketopyrrolopyrrole copolymers for high bulk hole mobility and near infrared absorption

ABSTRACTTo obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T]2‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10−4 cm2 V−1 s−1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 639–648

Improved Crystallinity of Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61 Butyric Acid Methyl Ester Film by Pulsed Electrospray Deposition

We investigated crystallinity parameters and ordering domains of poly(3-hexylthiophene-2,5-diyl) blended in [6,6]-phenyl-C61-butyric acid methyl ester films, which were fabricated by pulsed electrospray deposition methods. The crystallinity parameter and the ordering domain were estimated from Raman and optical absorption spectra, respectively. As a result, they were improved with decreasing the off time of pulse voltage, corresponding to the slow evaporation speed of solvent. In addition, both the space-charge limited current mobility and the photoconversion efficiency showed same trend. A highest photoconversion efficiency of 1.11 % was achieved without the thermal annealing process after depositing the active layer.

Conjugated foldamers with unusually high space-charge-limited current hole mobilities.

Charge carrier mobility and its optimization play a critical role in the development of cutting-edge organic electronic and optoelectronic devices. Even though space-charge-limited current (SCLC) hole mobilities as high as 1.4 cm(2) V(-1) s(-1) have been reported for microscopically sized highly ordered liquid-crystalline conjugated small molecules, the SCLC hole mobility of device-sized thin films of conjugated polymers is still much lower, ranging from 10(-6) to 10(-3) cm(2) V(-1) s(-1). Herein, we report the synthesis, characterizations, and thin-film SCLC mobility of three discotic conjugated polymers, INDT-TT, INDT-BT, and INDT-NDT. Optical studies indicate that polymer INDT-NDT adopts a folded conformation in solutions of good or poor solvents, whereas polymer INDT-TT stays as random monomeric chains in good solvents and interchain aggregates in poor solvents. INDT-BT polymer chains, however, stay as foldamers in dilute solutions of good solvents but interchain aggregates in concentrated solutions or poor solvents. Circular dichroism spectroscopy provides clear evidence for the helical folding of INDT-NDT in solutions. Thin films spin-coated from 1,2-dichlorobenzene solutions of the polymers show SCLC hole mobility of 2.20 × 10(-6), 8.79 × 10(-5), and 2.77 × 10(-2) cm(2) V(-1) s(-1) for INDT-TT, INDT-BT, and INDT-NDT, respectively. HRTEM and powder XRD measurements show that INDT-NDT pristine thin films contain nanocrystalline domains, whereas the INDT-TT and INDT-BT films are amorphous. Thin films of INDT-NDT:PC71BM blends show increased crystallinity and further improved SCLC hole mobility up to 1.29 × 10(-1) cm(2) V(-1) s(-1), one of the highest SCLC mobility values ever recorded on solution-processed organic semiconducting thin films. The persistent folding conformation of INDT-NDT is believed to be responsible for the high crystallinity of its thin films and its high SCLC mobilities.

Stereochemistry of spiro-acetalized [60]fullerenes: how the exo and endo stereoisomers influence organic solar cell performance.

Exploiting bis-addition products of fullerenes is a rational way to improve the efficiency of bulk heterojunction-type organic photovoltaic cells (OPV); however, this design inherently produces regio- and stereoisomers that may impair the ultimate performance and fabrication reproducibility. Here, we report unprecedented exo and endo stereoisomers of the spiro-acetalized [60]fullerene monoadduct with methyl- or phenyl-substituted 1,3-dioxane (SAF6). Although there is no chiral carbon in either the reagent or the fullerene, equatorial (eq) rather than axial (ax) isomers are selectively produced at an exo-eq:endo-eq ratio of approximately 1:1 and can be easily separated using silica gel column chromatography. Nuclear Overhauser effect measurements identified the conformations of the straight exo isomer and bent endo isomer. We discuss the origin of stereoselectivity, the anomeric effect, intermolecular ordering in the film state, and the performance of poly(3-hexylthiophene):substituted SAF6 OPV devices. Despite their identical optical and electrochemical properties, their solubilities and space-charge limited current mobilities are largely influenced by the stereoisomers, which leads to variation in the OPV efficiency. This study emphasizes the importance of fullerene stereochemistry for understanding the relationship between stereochemical structures and device output.

Alkoxyphenylthiophene Linked Benzodithiophene Based Medium Band Gap Polymers for Organic Photovoltaics: Efficiency Improvement upon Methanol Treatment Depends on the Planarity of Backbone

Two donor–acceptor (D–A) medium band gap polymers, P1 and P2, alkoxyphenylthiophene (APTh) linked benzodithiophene (BDT) as an electron-rich unit and 1,3-di(2′-bromothien-5′-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) (A1) or [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (BT) (A2) as an electron-deficient unit, have successfully been synthesized via microwave-assisted Stille polymerization and utilized for bulk heterojunction (BHJ) polymer solar cells (PSCs). P1 shows a well-distinguished absorption shoulder between 590 and 620 nm attributed to the π–π stacking of a polymer backbone; such kind of absorption shoulder is not observed in P2, indicating that the P1 has more planar structure than that of P2. This is due to the fact that the sulfur atom of thiophene spacer and the oxygen atom of carbonyl groups in TPD have more pronounced intramolecular noncovalent interactions (INCI) in P1 than that of the sulfur atom of thiophene spacer and the oxygen atom of alkoxy groups of BT in P2. The bulk heterojunction polymer solar cells (BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer (P1 or P2):PC71BM/LiF/Al. The P1 device shows better photovoltaic performance with open-circuit voltage (Voc) of 0.91 V and the power conversion efficiency (PCE) of 4.19% than the P2 device (Voc: 0.71 V; PCE: 1.88%) in neat blend films under the illumination of AM 1.5G (100 mW/cm2). Upon treating the active layers containing P1 and P2 with methanol, the PCE of the P1 device is increased from 4.19 to 7.14%. In contrast, the PCE of the P2 device is decreased from 1.88 to 1.82%. Space charge limited current mobility, atomic force microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and impedance spectroscopy studies strongly support the enhanced PCE for the P1 device is attributed to the increased mobility, nanoscale morphology, and reduced resistance upon methanol treatment; these favorable properties for the P1 polymer are highly correlated with the planarity of the backbone.

Linear and star-shaped pyrazine-containing acene dicarboximides with high electron-affinity

A series of linear and star-shaped pyrazine-containing acene molecules 1a-b, 2a-b and 3 substituted by dicarboximide groups are synthesized via condensation reactions between o-diamine and dione. High electron affinity (up to 4.01 eV) is achieved due to the introduction of an electron-deficient pyrazine moiety and the attachment of electron-withdrawing dicarboximide groups. Some of these molecules show thermal liquid crystalline behavior and their space-charge limited current mobilities are measured. The high electron affinity and liquid crystalline property qualify them as promising electron transporting materials in organic optoelectronic devices.

Microphase separation-promoted crystallization in all-conjugated poly(3-alkylthiophene) diblock copolymers with high crystallinity and carrier mobility

The crystallinity of all-conjugated diblock copolymer, poly(3-butylthiophene)-b-poly(3-dodecylthiophene) (P3BDDT), with varied block ratios was significantly enhanced by a “two-step” thermal annealing treatment. The resulting P3BDDT exhibited an attractively high crystallinity of ∼35%, which is a 3-fold enhancement over those of its homopolymer counterparts. The space-charge limited current (SCLC) mobility measurement revealed that the carrier mobility of the highly crystalline P3BDDT film was increased to as high as ∼8.4 × 10−3 cm2 V−1 s−1, exceeding the highest SCLC mobility of poly(3-alkylthiophene) homopolymer films reported in previous work (i.e., ∼1.6 × 10−3 cm2 V−1 s−1). DSC, XRD, AFM and SAXS characterizations demonstrated that the interplay of crystallization and microphase separation during the “two-step” thermal annealing treatment plays a key role in the improvement of P3BDDT crystallinity.

2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-based donor–acceptor type copolymers for photovoltaic cells

Alternating donor–acceptor type copolymers, poly[{5,11-di(9′-heptadecanyl)indolo[3,2-b]carbazole}-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] ( PIC-TZ) and poly[{6,6′,12,12′-tetraoctylindeno[1,2-b]fluorene}-alt-{2,5-di(thiophen-2-yl)thiazolo[5,4-d]thiazole-5,5′-diyl}] ( PIF-TZ), were synthesized and examined for applications in polymeric photovoltaic cells. The polymers have a fused coplanar main backbone with good planarity for intermolecular packing and high charge mobility. The indolocarbazole and indenofluorene units contain two or four binding sites for alkyl substituents that have pronounced solution processiblity compared to the carbazole and fluorene moieties. The number-average molecular weights ( M n) of the synthesized polymers were determined to be 11,000 g/mol (PDI = 2.27) for PIC-TZ, and 17,000 g/mol (PDI = 1.77) for PIF-TZ. The optical band gap of PIC-TZ and PIF-TZ in film was determined to be 2.14 eV and 2.21 eV, respectively, and an electrochemical study confirmed the desirable HOMO/LUMO levels of the copolymers, which enabled efficient electron transfer and a high open circuit voltage ( V OC) when blending them with fullerene derivatives. The space charge limited current mobility measurements showed a hole mobility of 10 −3 cm 2 V −1 s −1 for the copolymers. When the polymers were blended with [6,6]phenyl-C 61-butyric acid methyl ester (PCBM), PIC-TZ showed the best performance with V OC, short-circuit current and power conversion efficiency of 0.86 V, 4.16 mA/cm 2 and 1.64%, respectively, under AM 1.5G illumination conditions (100 mW cm −2).

Solar Cells Based on Block Copolymer Semiconductor Nanowires: Effects of Nanowire Aspect Ratio

The solution-phase self-assembly of nanowires (NWs) from diblock copolymer semiconductors, poly(3-butylthiophene)-block-poly(3-octylthiophene), of different block compositions gave crystalline NWs of similar width (13-16 nm) but a tunable average aspect ratio (length/width) of 50-260. The power conversion efficiency of bulk heterojunction solar cells comprising the diblock copolythiophene NWs and PC(71)BM was found to increase with increasing aspect ratio, reaching 3.4% at the highest average aspect ratio of 260. The space charge limited current mobility of holes in neat films of the copolymer NWs and in copolymer NWs/PC(71)BM films (∼1.0 × 10(-4) cm(2)/(V s)) was invariant with aspect ratio, reflecting the parallel orientation of the NWs to the substrate. The enhancement of photovoltaic efficiency with increasing aspect ratio of NWs was explained in terms of increased exciton and charge photogeneration and collection in the bulk heterojunction solar cells.

Effects of Intercalation on the Hole Mobility of Amorphous Semiconducting Polymer Blends

Fullerenes have been shown to intercalate between the side chains of many crystalline and semicrystalline polymers and to affect the properties of polymer:fullerene bulk heterojunction solar cells. Here we present the first in-depth study of intercalation in an amorphous polymer. We study blends of the widely studied amorphous polymer poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) with a variety of molecules using photoluminescence measurements, scanning electron microscopy, and space-charge limited current mobility measurements. The blends with elevated hole mobilities exhibit complete photoluminescence quenching and show no phase separation in a scanning electron microscope. We conclude that intercalation occurs in MDMO-PPV:fullerene blends and is responsible for the increase in the MDMO-PPV hole mobility by several orders of magnitude when it is blended with fullerenes, despite the dilution of the hole-conducting polymer with an electron acceptor.

Crystalline Random Conjugated Copolymers with Multiple Side Chains: Tunable Intermolecular Interactions and Enhanced Charge Transport and Photovoltaic Properties

Random poly(3-butylthiophene-co-3-octylthiophene) of several compositions were synthesized and found to be highly crystalline in spite of the statistical arrangement of the different-sized side chains. The melting (Tm) and recrystallization (Tc) temperatures of the random copolythiophenes varied continuously with composition. The interlayer d100 stacking distance in the lamellar crystalline structure of the random copolymers, determined from X-ray diffraction of films, varied linearly with copolymer composition over the entire range. Fullerene-based bulk heterojunction solar cells made from the random copolythiophenes had power conversion efficiencies of up to 3.0% under 100 mW/cm2 AM1.5 illumination in air, which is significantly enhanced compared to the homopolymers and their physical blends. The space charge limited current mobility of holes in the random copolythiophenes was enhanced compared to poly(3-butylthiophene) and physical blends of the homopolymers. Transmission electron microscopy images of ...

Enhanced Performance of Bulk Heterojunction Solar Cells Using Block Copoly(3-alkylthiophene)s

The photovoltaic properties, charge transport, and morphology of a series of diblock conjugated copolymers, poly(3-butylthiophene)-b-poly(3-octylthiophene) (P3BT-b-P3OT), were investigated as a function of block composition. Bulk heterojunction solar cells comprising blends of P3BT-b-P3OT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were found to have power conversion efficiencies as high as 3.0%, which represents factors of 1.6−9 enhancements compared to those of the homopolymers made under similar conditions. Imaging of P3BT-b-P3OT/PC71BM blends by atomic force microscopy and transmission electron microscopy revealed interpenetrating network with 11−18 nm crystalline polymer domains. The zero-field space charge limited current mobility of holes (∼(1−3) × 10−4 cm2 V−1 s−1) was similarly enhanced in the diblock copolymer solar cells compared to the homopolymers. These results demonstrate that block conjugated copolymers offer a promising approach to advanced materials for polymer solar cells and that the block composition is an attractive means to optimize the materials.