Fabrication of ZnO and its enhancement of charge injection and transport in hybrid organic/inorganic light emitting devices

ZnO nanocrystals were synthesized by hydrolysis in methanol. X-ray diffraction and photoluminescence spectra confirm that good crystallized ZnO nanoparticles were formed. Utilizing those ZnO nanoparticles and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), light emitting devices with indium tin oxide (ITO)/poly(3,4-oxyethyleneoxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS)/ZnO:MDMO-PPV/Al and ITO/PEDOT:PSS/MDMO-PPV/Al structures were fabricated. Electroluminescence (EL) spectra reveal that EL yield of hybrid MDMO-PPV and ZnO nanocrystals devices increased greatly as compared with pristine MDMO-PPV devices. The current-voltage characteristics indicate that addition of ZnO nanocrystals can facilitate electrical injection and charge transport. The decreased energy barrier to electron injection is responsible for the increased efficiency of electron injection.     

电极处势垒对新结构器件电子输运的影响

在新结构薄膜电致发光器件中,电极处的势垒的高度决定电子的注入数量.在电极界面处插入不同的薄膜材料,可以改变势垒的高度,并对电子注入数量和器件的发光亮度产生影响.通过拟合计算得到ZnO/SiO,ITO/SiO的界面势垒高度分别为0.51和1.87eV. 关键词：     

掺杂PEDOT ∶PSS对聚合物太阳能电池性能影响的研究

研究了掺杂后poly(3,4-ethylene dioxythiophene):poly(styrenesulphonic acid)(PEDOT ∶PSS)电导率的变化以及掺杂PEDOT ∶PSS薄膜对聚合物太阳能电池器件性能的影响. 实验发现,向PEDOT ∶PSS中掺入极性溶剂二甲基亚砜(DMSO)明显提高了薄膜的电导率,掺杂后的电导率最大值达到1.25 S/cm,比未掺杂时提高了3个数量级. 将掺杂的PEDOT ∶PSS薄膜作为缓冲层应用于聚合物电池 (ITO/PEDOT ∶PSS/P3HT ∶PCBM/LiF/Al) 中,发现高电导率的PEDOT ∶PSS降低了器件的串联电阻,增加了器件的短路电流,从而提高了器件的性能. 最好的聚合物太阳能电池在100 mW/cm²的光照下,开路电压(V_oc)为0.63 V,短路电流密度(J_sc)为11.09 mA·cm^-2,填充因子(FF)为63.7%,能量转换效率(η)达到4.45%. 关键词： PEDOT ∶PSS 电导率 聚合物太阳能电池 能量转换效率     

Effect of dimethyl sulfoxide on the electrical properties of PEDOT:PSS/n-Si heterojunction diodes

Effect of dimethyl sulfoxide (DMSO) on the electrical properties of PEDOT:PSS/n-Si heterojunction diodes has been studied. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was deposited on n-type Si wafer using facile process of spin coating. The DMSO content was varied from 0 to 8 vol%. Electrical characterization of these heterojunction diodes as performed using both current–voltage (I–V) and capacitance–voltage (C–V) measurements. All diodes showed rectifying behavior. AFM measurement revealed that the surface became more rough after the DMSO treatment of PEDOT:PSS films. The RMS values were found in the range of 4–6 nm. The resistivity of the PEDOT:PSS films decreased with increase in temperature. The addition of DMSO into PEDOT:PSS solution results in a decrease in resistivity of films by approximately two orders of magnitude. PEDOT:PSS films showed high transmission more than 85% in the entire visible region. Raman spectroscopy indicated effect of the DMSO treatment on the chemical structure of PEDOT chains, suggesting a conformational change of PEDOT chain in the film. An optimal value of DMSO was obtained with 5 vol% content, and it showed the best PEDOT:PSS films properties and good quality heterojunction diodes characteristics with ideality factor of 2.4 and barrier height 0.80 eV.     

Synthesis of zinc oxide porous structures by anodization with water as an electrolyte

We report a simple, reliable and one-step method of synthesizing ZnO porous structures at room temperature by anodization of zinc (Zn) sheet with water as an electrolyte and graphite as a counter electrode. We observed that the de-ionized (DI) water used in the experiment is slightly acidic (pH=5.8), which is due to the dissolution of carbon dioxide from the atmosphere forming carbonic acid. Porous ZnO is characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and photoluminescence (PL) studies. The current-transient measurement is carried out using a Gamry Instruments Reference 3000 and the thickness of the deposited films is measured using a Dektak surface profilometer. The PL, Raman and X-ray photoelectron spectroscopy are used to confirm the presence of ZnO phase. We have demonstrated that the hybrid structures of ZnO and poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) exhibit good rectifying characteristics. The evaluated barrier height and the ideality factor are 0.45 eV and 3.6, respectively.     

In-situ characterization of electrochromism based on ITO/PEDOT:PSS towards preparation of high performance device

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) is usually sandwiched between indium tin oxide(ITO) and a functional polymer in order to improve the performance of the device. However, because of the strong acidic nature of PEDOT:PSS, the instability of the ITO/PEDOT:PSS interface is also observed. The mechanism of degradation of the device remains is unclear and needs to be further studied. In this article, we investigate the in-situ electrochromism of PEDOT:PSS to disclose the cause of the degradation. X-ray photoelectron spectroscopy(XPS) was used to characterize the PEDOT:PSS films, as well as the PEDOT:PSS plus polyethylene glycol(PEG) films with and without indium ions. The electrochromic devices(ECD) based on PEDOT:PSS and PEG with and without indium ions are carried out by in-situ micro-Raman and laser reflective measurement(LRM). For comparison, ECD based on PEDOT:PSS and PEG films with LiCl, KCl, NaCl or InCl_3 are also investigated by LRM. The results show that PEDOT:PSS is further reduced when negatively biased, and oxidized when positively biased. This could identify that PEDOT:PSS with indium ions from PEDOT:PSS etching ITO will lose dopants when negatively biased. The LRM shows that the device with indium ions has a stronger effect on the reduction property of PEDOT:PSS-PEG film than the device without indium ions. The contrast of the former device is 44%, that of the latter device is about 3%. The LRM also shows that the contrasts of the device based on PEDOT:PSS+PEG with LiCl, KCl, NaCl, InCl_3 are 30%, 27%, 15%, and 18%, respectively.     

Band bending at the conducting polymer/indium tin oxide interfaces with and without ultraviolet treatment

In this study, the effect of ultraviolet treatment on the band bending at the poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate)/indium tin oxide (PEDOT:PSS/ITO) interfaces were researched. The authors suggested that ultraviolet treatment could lead to a reduction in the band bending at the PEDOT:PSS/ITO interface, owing to the removal of carbon contamination at the ITO surfaces and a decrease in the number of the trap-states at the PEDOT:PSS/ITO interface.     

Synthesis and characterization of p-GaSe thin films and the analyses of I–V and C–V measurements of p-GaSe/p-Si heterojunction under electron irradiation

Gallium Selenide (GaSe) thin films were grown by the electrochemical deposition (ECD) technique on Indium tin oxide (ITO) and p-Si (100) substrates. The Electron paramagnetic resonance (EPR) spectrum of GaSe thin films’ growth on ITO was recorded at room temperature. According to EPR results, the g value of an EPR signal obtained for GaSe deposited on ITO is 2.0012?±?0.0005. In/GaSe/p-Si heterojunction was irradiated with high-energy (6?MeV) and low-dose (1.53?×?10¹⁰?e^??cm^?2) electrons. The ideality factor of the In/GaSe/p-Si device was calculated as 1.24 and barrier height was determined as 0.82?eV from I–V measurements before irradiation. Acceptor concentration, built-in potential and barrier height of the In/GaSe/p-Si device were also obtained as 0.72?×?10^14?cm^?3, 0.65?eV and 0.97?eV from C–V measurements, respectively. After irradiation, the ideality factor n and barrier height Φ_b values of the In/GaSe/p-Si device were calculated as 1.55 and 0.781?eV, respectively. Acceptor concentration, the built-in potential and barrier height values of the In/GaSe/p-Si device have also shown a decrease after 6?MeV electron irradiation. This change in heterojunction device parameters shows that current transport does not obey thermionic emission, and thus tunneling could be active due to the defects formed by irradiation at the In–GaSe interface.     

Metal-electrode-free inverted organic photovoltaics using electrospray-deposited PEDOT:PSS and spin-coated HAT-CN exciton blocking layer

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) films were fabricated using an electrospray deposition (ESD) method. The ESD PEDOT:PSS films exhibited higher PSS content on the surface than spin-coated PEDOT:PSS films, which results in a higher work function. Based on this result, metal-electrode-free inverted organic photovoltaics (OPVs) were fabricated. The ESD PEDOT:PSS was used as the top electrode on the poly(3-hexythiophene-2,5-diyl) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) light-absorbing layer. The power conversion efficiency (PCE) of OPVs was significantly increased with the 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile layer. The improved PCE would be attributed to the suppression of exciton quenching at the P3HT:PCBM and PEDOT:PSS interface.     

The insert of zinc oxide thin film in indium tin oxide anode for organic electroluminescence devices

Zinc oxide films were prepared by rf magnetron sputtering on glass substrates with designed ZnO target using high-purity of zinc oxide (99.99%) powder. Systematic study on dependence of target-to-substrate distance (D_ts) on structural, electrical and optical properties of the as-grown ZnO films was mainly investigated in this work. XRD showed that highly preferred ZnO crystal in the [0 0 1] direction was grown in parallel to the substrate, while the D_ts did not effect to the peak position of XRD. With decreasing D_ts, the growth rate is increased while the electrical resistivity as well as crystal size in the ZnO films was decreased. The XPS data showed that the O/Zn ratio in ZnO films was increased with increasing D_ts in the films. The as-grown ZnO films have an average transmittance of above 85% at the visible region. The optical band gap of the as-grown ZnO films was changed from 3.18 to 3.36 eV with D_ts. With decreasing D_ts, the electrical resistivity was decreased, while the growth rate was increased.A bilayer is used as an anode electrode for organic electroluminescent devices. The bilayer consists of an ultrathin ZnO layer adjacent to a hole-transporting layer and an Indium tin oxide (ITO) outerlayer. We tried to bring low the barrier between the devices as deposited ZnO films on ITO substrates. We fabricated the organic EL structure consisted of Al as a cathode, Al₂O₃ as an electro transport layer, Alq₃ as a luminously layer, TPD as a hole transport layer and ZnO (1 nm)/ITO (150 nm) as an anode. The result of this experiment was not good compared with the case of using ITO, nevertheless, at this structure we obtained the lowest turn-on voltage as the value of 19 V and the good brightness (6200 cd/m²) of the emission light from the devices. Then the quantum efficiency was to be 1.0%.     

A prerequisite to achieving high performance polymer/inorganic thin film diodes

An alternative approach to high performance polymeric rectifiers based on p-type polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and an n-type zinc oxide (ZnO) films is demonstrated. It is evident that nanoscale grain compatibility at an interface does not ensure that the device has the highest performance, but only a prerequisite for the two materials to exhibit the interaction. Being a non-invasive probe, Raman spectroscopy is used to monitor the degree of interaction between PEDOT:PSS and ZnO films. High performance of devices is achieved by the control of grain matching and more essentially, by the strong interaction of molecules at the interface. The developed PEDOT:PSS/ZnO diode can rectify an incoming a.c. voltage signal at frequencies up to 18 MHz. Operating with a low turn-on voltage, the diode has a current density of 220 mA/cm² and a rectification ratio of 4000 in the open atmosphere which are higher than previously reported polymer/inorganic vertical diodes.     

Improvement in performance of organic light-emitting devices by inclusion of multi-wall carbon nanotubes

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was modified by different concentrations of multi-wall carbon nanotubes (MWNTs), and the nanocomposites of PEDOT:PSS and MWNTs were firstly used as hole-injection layer in fabrication of organic light-emitting devices (OLEDs) by using a double-layer structure with hole-injection layer of doped PEDOT:PSS and emitting/electron transport layer of tris(8-hydroxyquinolinato) aluminum (Alq₃). PEDOT:PSS solution doped with MWNTs was spin-coated on clean polyethylene terephthalate (PET) substrate with indium tin oxide (ITO). It was found that the electroluminescence (EL) intensity of the OLEDs were greatly improved by using PEDOT:PSS doped with MWNTs as hole-injection layer which might have resulted from the hole-injection ability improvement of the nanocomposites. Higher luminescence intensity and lower turn-on voltage were obtained by these devices and the luminance intensity obtained from the device with the hole-injection layer of PEDOT:PSS doped by 0.4 wt.% MWNTs was almost threefolds of that without doping.     

Investigating the performance of P3HT:PCBM organic solar cells involving gamma-irradiated PEDOT:PSS layer

In this work, we investigated for the first time the characteristics of (poly (3-hexylthiopene) and [6, 6]-phenyl C61-butyric acid methyl ester) (P3HT:PCBM) blends-based organic solar cell with 1.25?mg/mL boric-acid (H₃BO₃)-doped poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer which is irradiated under the 40 Gray (Gy) dose of gamma (γ) ray. Experimental results showed that the parameters of solar cell improved with exposure to low-dose gamma radiation. In particular, it has provided a significant improvement in short-circuit current density (J_sc) and power conversion efficiency (PCE). About 49% increase in PCE to 1.22% and 40% increase in J_sc to 6.28?mA/cm² was obtained between the bare device and the device containing irradiated PEDOT:PSS:H₃BO₃. Also, it was determined that the H₃BO₃-doped PEDOT:PSS is more stable to temperature. More importantly, solar cell containing gamma-irradiated PEDOT:PSS:H₃BO₃ showed best performance comparing to conventional PEDOT:PSS-based cell.     

电化学合成聚噻吩薄膜提高光伏电池的开路电压

以铟锡氧化物(ITO)/聚(3,4-亚乙二氧基噻吩)-聚(苯乙烯磺酸)(PEDOT:PSS)为工作电极,采用电化学沉积法,直接在其上形成聚3-己基噻吩(P3HT)薄膜。其紫外可见吸收光谱的峰值约位于410nm处,吸收边延至610nm处,禁带宽度为2.04eV。测得其最高占有分子轨道(HOMO)能级为-5.21eV,而化学合成P3HT的HOMO能级为-5.02eV,这可能源于电化学合成聚噻吩的规整度比化学合成的要高。原子力显微镜AFM形貌结果表明电化学合成的P3HT中噻吩分子排列紧密,循环伏安扫描表明此P3HT薄膜的电化学性质稳定。采用该电化学合成的聚噻吩与富勒烯衍生物[6,6]-苯基-C61-丁酸甲酯(PCBM)复合而成的光伏电池的开路电压高达0.76V,这主要源于电化学合成聚噻吩HOMO能级的降低,因而揭示了提高光伏电池开路电压的新途径。     

紫外臭氧处理超薄银修饰ITO电极的高效柔性有机太阳能电池

以紫外臭氧处理超薄Ag复合MoO₃或PEDOT：PSS修饰ITO电极的高效柔性有机太阳能电池。通过优化紫外臭氧处理Ag薄膜的时间,提高了以P3HT：PCBM为有源层的器件的功率转换效率,从1.68%（未经过紫外臭氧处理）提高到2.57%（紫外臭氧处理Ag 1 min）。提高的原因推测是紫外臭氧处理形成了AgO_x薄膜,提高了电荷提取并使器件具有高光学透明度、低串联电阻和优异的表面功函数等一些性能。并且,紫外臭氧处理Ag薄膜与MoO₃或者PEDOT：PSS复合修饰ITO的器件效率分别得到提高,Ag薄膜与MoO₃复合修饰ITO的器件效率从2.02%（PET/ITO/MoO₃）提高到2.97%（PET/ITO/AgO_x/MoO₃）,Ag薄膜与PEDOT：PSS复合修饰ITO的器件效率从2.01%（PET/ITO/PEDOT：PSS）提高到2.93%（PET/ITO/AgO_x/PEDOT：PSS）。此外,以PBDTTT-EFT：PC₇₁BM为有源层的柔性聚合物太阳能电池效率可达6.21%。基于ITO的柔性光电器件效率的提高主要归于ITO被Ag/PEDOT：PSS或Ag/MoO₃修饰后功函数的提高。     

Degradation process in organic thin film devices fabricated using P3HT

The stability of regioregular poly(3-hexylthiophene 2,5-diyl) (P3HT) thin films sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes have been investigated under normal environmental conditions (25°C and RH∼45–50%). Electrical and optical properties of ITO/P3HT/Al devices have been studied over a period of 30 days. Mobility μ of the order of 10⁻⁴ cm²/V-s has been obtained from the V ² law in the as-deposited P3HT films. Scanning electron microscopy (SEM) investigations show blistering of Al contacts in devices with a poly(3,4-ethylenedioxythiophene) (PEDOT) interlayer on application of voltage whereas no blistering is seen in devices without PEDOT. The results have been explained in terms of trap generation and propagation and the moisture-absorbing nature of PEDOT.      

Electro-optical characterization and analysis of CuPc-based solar cells with high photovoltage

Organic solar cells using the CuPc and PTCBI semiconductor layers were studied. A high open circuit voltage of 1.15 V was obtained in a device with ITO/PEDOT:PSS/CuPc (15 nm)/PTCBI (7 nm)/Al structure. Results were interpreted in terms of a modified CuPc-Al Schottky diode for the thin PTCBI case and a CuPc-PTCBI heterojunction for the thick PTCBI case. Also, the formation of a thin aluminum oxide layer under the aluminum electrode was postulated. This layer has a beneficial aspect wherein shunting losses are reduced and a high photovoltage is enabled. However, it adds greatly to the series resistance to a point where the short circuit current density is reduced. CuPc Schottky diodes with an ITO/PEDOT:PSS/CuPc/Al structure yielded a high V _oc of 900 mV for a CuPc layer of thickness 140 nm. The V _oc increased with increase in CuPc layer thickness.     

UV/ozone‐treated WS2 hole‐extraction layer in organic photovoltaic cells

WS₂ nanosheets obtained through a simple sonication exfoliation method are employed as a hole‐extraction layer to improve the efficiency of organic photovoltaic cells (OPVs). A reduction in the wavenumber difference in the Raman spectra, the appearance of a UV absorption peak, and atomic force microscopy images indicate that WS₂ nanosheets are formed through the sonication method. The power conversion efficiency (PCE) values of OPVs with and without untreated WS₂ layers are both 1.84%. After performing a UV‐ozone (UVO) treatment on the WS₂ surface for 15 min, the PCE increases to 2.4%. Synchrotron radiation photoelectron spectroscopy data show that the work function of WS₂ increases from 4.9 eV to 5.1 eV upon UVO treat‐ ment, suggesting that the increase in the PCE value is caused by the band alignment. Upon inserting poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT: PSS) between the WS₂ and the active layer, the PCE value of the OPV increases to 3.07%, which is superior to that of the device employing only PEDOT:PSS (2.87%). Therefore, it is considered that the use of UVO‐treated WS₂ is able to improve the performance of OPV cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and electroluminescent properties of a number of new derivatives of divinyl dibenzothiophene sulfone

Photoluminescent and electroluminescent properties of four new bipolar linear derivatives of divinyl dibenzothiophene sulfone are studied. It is found that amorphous films of solutions, as well as films of the compounds under study in the poly(N-vinylcarbazole) matrix, have a rather high quantum yield of photoluminescence in the blue and blue-green spectrum regions. Bright blue electroluminescence is obtained in the samples with a structure of ITO/PEDOT:PSS/TPD/OC/LiF/Al using vacuum deposition of the compounds under study and in the single-layer ITO/PEDOT:PSS/PVK:OC/LiF/Al structure when applied from the solution with a threshold voltage of 2.5–3.5 V. The influence of a molecule structure on the spectra and quantum yield of fluorescence as well as on the electroluminescent properties of the compounds is shown. Results of quantum-chemical calculations in the context of the density functional theory of the structure and characteristics of main molecular orbitals are presented.     

Increased work function in PEDOT:PSS film under ultraviolet irradiation

An increase of work function （0.3 eV） is achieved by irradiating poly（3,4-ethylenedioxythiophene）：poly（styrene sul- fonate） （PEDOT：PSS） film in vacuum with 254-nm ultraviolet （UV） light. The mechanism for such an improvement is investigated by photoelectron yield spectroscopy, X-ray photo electron energy spectrum, and field emission technique. Sur- face oxidation and composition change are found as the reasons for work function increase. The UV-treated PEDOT：PSS film is used as the hole injection layer in a hole-only device. Hole injection is improved by UV-treated PEDOT：PSS film without baring the enlargement of film resistance. Our result demonstrates that UV treatment is more suitable for modifying the injection barrier than UV ozone exposure.