Solution processable PEDOT:PSS based hybrid electrodes for organic field effect transistors

We report high performance solution processed conductive inks used as contact electrodes for printed organic field effect transistors (OFETs). Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electrodes show highly improved very low sheet resistance of 65.8 ± 6.5 Ω/square (Ω/□) by addition of dimethyl sulfoxide (DMSO) and post treatment with methanol (MeOH) solvent. Sheet resistance was further improved to 33.8 ± 8.6 Ω/□ by blending silver nanowire (AgNW) with DMSO doped PEDOT:PSS. Printed OFETs with state of the art diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT) semiconducting polymer were demonstrated with various solution processable conductive inks, including bare, MeOH treated PEDOT:PSS, single wall carbon nanotubes, and hybrid PEDOT:PSS-AgNW, as the source and drain (S/D) electrode by spray printing using a metal shadow mask. The highest field effect mobility, 0.49 ± 0.03 cm² V⁻¹ s⁻¹ for DPPT-TT OFETs, was obtained using blended AgNW with DMSO doped PEDOT:PSS S/D electrode.     

Oligothiophene-based small molecules with 3,3′-difluoro-2,2′-bithiophene central unit for solution-processed organic solar cells

Two new oligothiophene-based small molecules, namely DRCN6T-F and DRCN8T-F, with 3,3′-difluoro-2,2′-bithiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as end groups, were designed and synthesized. Compared to their non-fluorinated counterparts DRCN6T and DRCN8T, DRCN6T-F and DRCN8T-F exhibit enhanced intermolecular interactions and lower HOMO energy levels. However, PCEs of 2.26% and 5.07% were obtained for DRCN6T-F and DRCN8T-F based optimized devices, respectively, lower than those of non-fluorinated molecules DRCN6T and DRCN8T. The relatively poor performance for the DRCN6T-F and DRCN8T-F were mainly caused by their low short-circuit current densities, due to their unfavorable morphologies and low charge carrier mobilities.     

An accurate method to determine the through-plane electrical conductivity and to study transport properties in film samples

The through-plane conductivity of a film sample is critically important because it largely affects the performance of batteries, capacitors, and thermoelectric devices. In this study, we developed a modified four-probe through-plane electrical conductivity measurement method using a coaxial structure. This method is general and works for free-standing film samples. We studied different samples including a steel sheet, highly oriented pyrolytic graphite, and conducting polymers. We confirmed metallic transportation in the steel sheet and hopping transportation in graphite in the through-plane direction by conducting low temperature measurements at 100 K. In the case of a conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate, the conductivity anisotropic ratio decreases with increasing in-plane conductivity. Temperature dependent measurements show two distinct activation energy regimes in the through-plane direction in PEDOT/PSS but almost no change in the in-plane electrical conductivity activation energy. This could be due to additional carrier paths that occur through the more disordered region (the PSS-rich region) in the through-plane direction. We also examined the Meyer–Neldel rule in PEDOT/PSS and concluded that PEDOT/PSS follows the anti-Meyer–Neldel rule, likely due to the high carrier density in the film.     

Controlling the interface energetics of PCPDTBT by p-doping

The doping of organic semiconductors is a promising way for both improving the charge carrier transport and tuning the energy level alignment at interfaces. We study the influence of p-doping of the low band gap polymer PCPDTBT with F4-TCNQ on the energy level alignment in a prototype organic solar cell structure with ITO as an electrode material and the fullerene C₆₀ as electron acceptor material using Ultraviolet and X-ray photoelectron spectroscopy. As a consequence of the doping, a Quasi-Ohmic contact at the interface to ITO is formed, whereas the energy level alignment to C₆₀ is almost not affected. In contrast to a related system, we observe a depletion of the dopant at the polymer surface. The change of the energy level alignment only at the electrode interface might be advantageous for the application in organic solar cell devices.     

电容耦合氧等离子体处理ITO基片对OLED的影响

在不同条件下采用电容耦合氧等离子体处理用于有机电致发光（OLED）的ITO基片,使用接触电势法测量了基片表面功函数的改变。研究发现,氧等离子体处理可以有效地提高ITO表面的功函数。X射线光电子能谱的测量揭示了其本质：氧等离子体处理可以提高表面氧原子的含量,同时降低ITO表面锡／铟原子的比例,由此导致了ITO表面功函数的提高。高功函数的ITO可降低空穴由ITO向OLED空穴传输层中注入空穴的势垒,从而提高OLED器件的性能。进一步的基于联苯二胺衍生物NPB／8-羟基喹啉铝（Alq3）的标准器件的研究证明了这一点。研究同时发现,在相同的真空和氧压条件下,保持处理时间不变,随着射频激发功率的升高,ITO表面功函数会逐渐降低。这个功函数的降低,使得OLED器件的驱动电压升高且电流效率减小。因此使用电容耦合氧等离子体处理的ITO来制备OLED器件,需要在优化的条件下进行,以达最佳效果。在本实验系统下处理条件为射频功率100W、时间25s。     

A Morphological Model for the Solvent‐Enhanced Conductivity of PEDOT:PSS Thin Films

The well‐known enhanced conductivity of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films that is obtained by addition of high‐boiling solvents like sorbitol to the aqueous dispersion used for film deposition is shown to be associated with a rearrangement of PEDOT‐rich clusters into elongated domains, as evidenced from STM and AFM. Consistently, temperature dependent conductivity measurements for sorbitol‐treated films reveal that charge transport occurs via quasi 1D variable range hopping (VRH), in contrast to 3D VRH for untreated PEDOT:PSS films. The typical hopping distance of 60–90 nm, extracted from the conductivity measurements is consistent with hopping between the 30–40 nm sized grains observed with scanning probe microscopy.     

An ideal host-guest system to accomplish high-performance greenish yellow and hybrid white organic light-emitting diodes

Greenish yellow organic light-emitting diodes (GYOLEDs) have steadily attracted researcher's attention since they are important to our life. However, their performance significantly lags behind compared with the three primary colors based OLEDs. Herein, for the first time, an ideal host-guest system has been demonstrated to accomplish high-performance phosphorescent GYOLEDs, where the guest concentration is as low as 2%. The GYOLED exhibits a forward-viewing power efficiency of 57.0 lm/W at 1000 cd/m², which is the highest among GYOLEDs. Besides, extremely low efficiency roll-off and voltages are achieved. The origin of the high performance is unveiled and it is found that the combined mechanisms of host-guest energy transfer and direct exciton formation on the guest are effective to furnish the greenish yellow emission. Then, by dint of this ideal host-guest system, a simplified but high-performance hybrid white OLED (WOLED) has been developed. The WOLED can exhibit an ultrahigh color rendering index (CRI) of 92, a maximum total efficiency of 27.5 lm/W and a low turn-on voltage of 2.5 V (1 cd/m²), unlocking a novel avenue to simultaneously achieve simplified structure, ultrahigh CRI (>90), high efficiency and low voltage.     

Investigation of voltage reduction in nanostructure-embedded organic light-emitting diodes

We investigated the reduction of the operating voltage in organic light-emitting diodes containing WO₃ nanoislands. The thickness of the organic layer and the periodicity of the nanoislands were varied in order to quantitatively analyze the electrical changes. The thickness of the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) layer was varied from 150 nm to 600 nm, and various periodic nanoislands of 300 nm, 330 nm, and 370 nm were fabricated. Two geometric factors, which are the effective length and effective area, influence the operating voltage. The effective length is determined by the relative thickness of the nanoislands compared with the organic thickness, and the reduction of the operating voltage is linearly proportional to the relative thickness. The effective area is a nonlinear function of periodicity, and the voltage is reduced as the periodicity decreases.     

Cesium carbonate-doped 1,4,5,8-naphthalene-tetracarboxylic-dianhydride used as efficient electron transport material in polymer solar cells

The use of appropriate charge carrier transport materials in organic solar cells strongly influences the device performance. In this work, we focused on the molecular electron transport material 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) doped by cesium carbonate (Cs₂CO₃). We first investigated the electrical properties of such n-type doped material as a function of the doping concentration before using it as electron transport layer (ETL) in polymer solar cells. The doped transparent ETL reduces the series resistance leading to an increased open circuit voltage. A power conversion efficiency of 3.8% was finally achieved in a device with a blend of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer and a 5 nm-thick NTCDA:Cs₂CO₃ film with a molar ratio of 30% as ETL.