Transparent Cathode Research Articles

Electron transport layer-free polymer solar cells show 40% higher efficiency than using ZnO transparent cathode

The poor compatibility of inorganic materials (electron transport layer) with the active layer and an ultrathin film of conjugated polymers becomes the great obstacle to producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel electron transport layer-free polymer solar cells have been constructed by integrating the conjugated aminoalkyl-functionalized polymer, poly[3-(5-(9,9-bis(3-(dimethylamino)propyl)-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-2,5-bis(2-butyloctyl)-6-(5-methylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PDTFN) into the active layer. PDTFN was synthesized by different Suzuki reaction conditions to gain different molecular weights- small molecular weight (PDTFN-L) and high molecular weight (PDTFN-H). It is noteworthy that PDTFN-H can significantly enhance the power conversion efficiency of the solar cells that incorporate a photoactive layer composed of poly[(3-hexylthiophene)-2,5-diyl] (P3HT) and the fullerene acceptor [6,6-phenyl-C71-butyric acid methyl ester] (PC71BM). The power conversion efficiency varies from 2.5% for ZnO transparent cathode based solar cells to 3.6% for PDTFN-H based electron transport layer-free solar cells. This improved performance can be attributed to the following reasons: the PDTFN-H can enhance cathode electron transporting by introducing the PDTFN-H into active layer without extra coating the cathode interfacial material; besides, a favorable vertical phase separation of active layer was formed due to the PDTFN-H excellent compatibility with the bulk-heterojunction. These experimental results revealed that the electron transport layer-free polymer solar cells based on PDTFN-H can be a promising novel effective fabrication with simplified manufacturing process and lower cost.

P‐168: Top Emission WOLED for High Resolution OLED TV

In this article, two top emission WOLED (TE‐WOLED) devices with different transparent cathode was developed and evaluated. The two devices show different performances due to their different cathode. The improved cathode can largely reduce the IR‐drop and was successfully applied in 31‐inch UHD display panel.

P‐13.4: Enhancement of top emission white organic light‐emitting diodes with a buffer layer

The top‐emitting white organic light‐emitting diodes (TE‐WOLEDs) containing transparent cathode with various structures of buffer layers were developed. The HATCN effectively protects the underlying organic layers from plasma damage during deposition by sputtering, and the optimized thickness was determined by the UV—Vis absorption spectroscopy. The device with 23 nm HATCN showed higher luminance than other devices. A simple and effective method to determine the buffer layer thickness was discovered. The device using the hybrid buffer layer (HATCN /Mg : Ag) showed the lower voltage and longer lifetime than single buffer layer. The reason is that the hybrid buffer layer have excellent contact and plasma protection performance.

Highly Deformable and See-Through Polymer Light-Emitting Diodes with All-Conducting-Polymer Electrodes.

Despite the high expectation of deformable and see-through displays for future ubiquitous society, current light-emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see-through polymer LEDs (PLEDs) are demonstrated. The CP-based PLEDs exhibit outstanding dual-side light-outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP-based PLEDs fabricated on 4 µm thick plastic foils with all-solution processing have extremely deformable and foldable light-emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.

Low absorption semi-transparent cathode for micro-cavity top-emitting organic light emitting diodes

Abstract We present a low absorption semi-transparent cathode for top-emitting organic lighting diodes (TEOLEDs). To reduce the light loss by electrode absorption, transparent cathode materials with a high refractive index and low extinction coefficient characteristics at the visible light range are investigated for the highly efficient TEOLEDs. Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) are considered as the transparent materials. Due to the low conductivity of MoO3 and WO3, a small amount of silver (Ag) is blended in order to enhance the electrical property as electrodes. Optimized MoO3:Ag mixed film shows the reduced extinction coefficient by half compared to Mg:Ag. With a general organic material (refractive index∼1.8) as a capping layer, the average absorption, transmittance and omnidirectional reflectance of 18.2, 65.9, and 25.6% are observed, respectively, with a sheet resistance of 12.7 Ω/□. The current efficiencies of fabricated red, green and blue TEOLEDs with optimized MoO3:Ag cathode are 32.4, 76.8, and 2.8 cd/A, respectively. The performance enhancement ratio is 9.8, 13.6, and 86.7% for red, green and blue devices compared with Mg:Ag cathode devices.

Improved performance of photoconductive gain hybrid UV detector by trap state engineering of ZnO nanoparticles

With the purpose of examining the impact of donor polymer on the performance of nanocomposite photodetectors (PDs) and to better understand the underlying physics, different wide-bandgap semiconducting polymers, poly(N-vinylcarbazole), poly(9, 9-di-n-octylfluorenyl-2, 7-diyl) , and [9,9′-dioctyl-fluorene-2,7-diyl]-copoly[diphenyl-p-tolyl-amine-4,4′-diyl] (BFE), are mixed with ZnO nanoparticles (NPs) to fabricate hybrid UV PDs. Three different polymer matrix nanocomposites were investigated that differ in the electron-trap depth in the nanocomposite and also the carrier tunneling energy at the interface. All the fabricated PDs exhibit strong photoconductive gain characteristics which can be attributed to trapped electron accumulation and band bending at the cathode interface. Experimental results show that the manipulation of the photoactive nanocomposite improves the PD properties simultaneously, namely, the external quantum efficiency (EQE, ∼104%), the maximum detectivity (D*, ∼1013 Jones), and the linear dynamic range (LDR, ∼85 dB). In addition, the gain bandwidth product of the device improves more than 50 times. Furthermore, the effect of the photogenerated carrier profile within the active layer is investigated experimentally by changing the direction of the incident light using a transparent cathode. Interestingly, under illumination through the Al cathode, faster photocurrent response, wider spectral range toward the deep UV region, and higher EQE in relatively low voltages are observed. These considerations might provide a general strategy to fabricate low-cost photoconductive PDs with a reasonably good combination of gain, response speed, LDR, and selectivity.

Enhancing the Performance of Inverted Organic Photovoltaics Using Cathode Interlayers Based on Solution‐Processable Tetrabutylammonium Halides

High‐performance inverted organic photovoltaics (OPVs) require the work function of transparent bottom cathodes to be low, which can be efficiently achieved by inserting an ultrathin interlayer to form an interface dipole. Herein, solution‐processable tetrabutylammonium halides (TBAXs, X = Cl, Br, and I) are employed as a cathode interlayer in inverted OPVs, leading to the disappearance of an S‐shaped kink in their current density–voltage curves and increasing their open‐circuit voltages and fill factors, which resulted in significantly enhanced power conversion efficiencies. Ultraviolet photoelectron spectroscopy measurements revealed that the work function of indium tin oxide (ITO) was considerably lowered in the presence of TBAX interlayers, resulting in efficient electron extraction from the C60 acceptor to the ITO bottom cathode and increasing the built‐in potential across devices. Thus, TBAXs are demonstrated to be promising materials for efficient cathode interlayer formation in inverted OPVs.

Efficient Inverted Organic Solar Cells Based on a Fullerene Derivative-Modified Transparent Cathode.

Indium tin oxide (ITO) is a transparent conductive material which is extensively used in organic solar cells (OSCs) as electrodes. In inverted OSCs, ITO is usually employed as a cathode, which should be modified by cathode buffer layers (CBLs) to achieve better contact with the active layers. In this paper, an amine group functionalized fullerene derivative (DMAPA-C60) is used as a CBL to modify the transparent cathode ITO in inverted OSCs based on PTB7 as a donor and PC71BM as an acceptor. Compared with traditional ZnO CBL, DMAPA-C60 exhibited comparable transmittance. OSCs based on DMAPA-C60 show much better device performance compared with their ZnO counterparts (power conversion efficiencies (PCEs) improved from 6.24 to 7.43%). This is mainly because a better contact between the DMAPA-C60 modified ITO and the active layer is formed, which leads to better electron transport and collection. Nanoscale morphologies also demonstrate that the surface of DMAPA-C60-modified ITO is plainer than the ZnO counterparts, which also leads to the better device performance.

Highly efficient interfacial layer using SILAR-derived Ag2S quantum dots for solid-state bifacial dye-sensitized solar cells

Ag2S quantum dots (QDs) were employed as an interfacial layer between polymer electrolyte and transparent polyaniline counter electrode (PANI CE) in bifacial dye-sensitized solar cells (DSSCs). The Ag2S QDs film, which was prepared by successive ionic layer adsorption and reaction (SILAR) could deliver an extra visible light harvesting and enhance the catalytic performance of PANI CE, leading to increased photo-generated electrons and suppressed interfacial recombination. The device performances have been significantly improved as compared to the controlled DSSC without Ag2S QDs interfacial layer (QDIL). An optimal power conversion efficiency of 6.50% was achieved in the Ag2S QDILs based solid-state bifacial DSSC with 2 SILAR cycles, which was comparable to the highest reported efficiency of 7–8% for solid-state DSSCs, and was almost 130% of the conventional structured DSSC without Ag2S QDIL (5.02%). Electrochemical and electron transport/recombination performances have also been demonstrated for this type of device. The results also highlight the general importance of light-active QDs interfacial layer in transparent cathode for other kind of devices.

Measurement of contact surface photo-voltage from forward bias C-V characteristics of P3HT:PCBM based BHJ solar cells

The surface photovoltage (SPV) and carrier accumulation in open circuit conditions are sensitive indicators of the contact quality and band bending. However, these are not accessible directly in experiments. We analyze the capacitance-voltage (C-V) characteristics in P3HT:PCBM based bulk heterojunction to investigate SPV for both conventional (transparent anode) and inverted (transparent cathode) architectures. The C-V characteristics under dark conditions are analyzed using drift-diffusion model to extract built-in voltage, while under white light illumination it yields SPV. The SPV and the corresponding carrier density at the anode are quantified as a function of light intensity for conventional structure and compared with those of inverted structure to distinguish between the mechanisms in the two cases. The inverted structure with higher open circuit voltage is marked with less SPV compared to the conventional one. The technique of obtaining SPV as described here and the underlying mechanism can be used to suggest modifications in device architecture.

Synergic Interface and Optical Engineering for High‐Performance Semitransparent Polymer Solar Cells

AbstractIn this study the thickness of the PTB7‐Th:PC71BM bulk heterojunction (BHJ) film and the PF3N‐2TNDI electron transport layer (ETL) is systematically tuned to achieve polymer solar cells (PSCs) with optimized power conversion efficiency (PCE) of over 9% when an ultrathin BHJ of 50 nm is used. Optical modeling suggests that the high PCE is attributed to the optical spacer effect from the ETL, which not only maximizes the optical field within the BHJ film but also facilitates the formation of a more homogeneously distributed charge generation profile across the BHJ film. Experimentally it is further proved that the extra photocurrent produced at the PTB7‐Th/PF3N‐2TNDI interface also contributes to the improved performance. Taking advantage of this high performance thin film device structure, one step further is taken to fabricate semitransparent PSCs (ST‐PSCs) by using an ultrathin transparent Ag cathode to replace the thick Ag mirror cathode, yielding a series of high performance ST‐PSCs with PCEs over 6% and average visible transmittance between 20% and 30%. These ST‐PSCs also possess remarkable transparency color perception and rendering properties, which are state‐of‐the‐art and fulfill the performance criteria for potential use as power‐generating windows in near future.

P‐202: A Flexible Transparent OLED Display with FlexUPTM Technology

A flexible segment transparent OLED display (T‐OLED) with FlexUPTM technology was demonstrated in this paper. Our monochromatic T‐OLED with transparent cathode structure Mg:Ag/HATCN/IZO can reach transmittance of 64.2%. Also, a flexible electrochromic (EC) shutter as a light‐blocking layer was placed behind the T‐OLED to enhance ambient contrast ratio. Total transmittance of T‐OLED/EC system is 54.2%. This T OLED/EC system can be used as information displays in curved windows like windshield even in a high brightness environment.

Experimental Demonstration of a High-Efficiency Relativistic Magnetron With Diffraction Output With Spherical Cathode Endcap

A previous computational study of a relativistic magnetron with diffractive output at the University of New Mexico using the particle-in-cell code MAGIC achieved an efficiency of 70%. The research described in this paper aimed to achieve an experimental verification of 70% efficient operation of this design. Changes to the topology were necessary, such as the introduction of a Helmholtz-like magnetic coil pair and the use of a polished stainless steel endcap on the downstream end of the transparent cathode. These structures maintain a high degree of magnetic field uniformity in the anode–cathode gap interaction space and intercept leakage electrons that might otherwise impact the dielectric output window. The use of these structures stands in contrast to the modified magnetic fields used in the work of others to protect the window. An ethanol calorimeter and S-band waveguide detector were placed in the near-field to obtain radio frequency pulse shape and frequency and to infer peak power through a measurement of total energy radiated. A coaxial D-dot probe and self-integrating Rogowski coil were used to measure pulser voltage and total current, respectively, and showed that the tube operated at an efficiency of 63.5% until the endcap started emitting electrons, after which the efficiency was 40.4% in $\pi $ -mode. Images of radiated field pattern on a neon-bulb grid and of air breakdown outside of the 21-cm diameter output window were captured to confirm conversion of the $\pi $ -mode of operation to a TE31 circular radiated output mode.

Generation and stability of freestanding aqueous microbubbles

A new type of aqueous bubble, composed of a spherical liquid film with inner and outer gas/liquid interfaces, is generated during electrolysis of acidified water at a transparent platinum cathode. These quasi-stable freestanding hydrogen-filled microbubbles are studied by high-speed photography in an inverted microscope. Their radii are in the range 2–10μm and the film thickness is about 5% of the radius. Extrapolated lifetimes at the cathode are 3–10s, although the microbubbles collapse prematurely into sessile droplets. Their stability is attributed to interfacial diffusion barriers related to coherent scattering of hydrogen gas by a surface layer of correlated water molecules.

Efficient Inverted ITO-Free Organic Solar Cells Based on Transparent Silver Electrode with Aqueous Solution-Processed ZnO Interlayer

Efficient inverted organic solar cells (OSCs) with the MoO3(2 nm)/Ag (12 nm) transparent cathode and an aqueous solution ZnO electron extraction layer processed at low temperature are investigated in this work. The blend of low bandgap poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) is employed as the photoactive layer here. A power conversion efficiency (PCE) of 5.55% is achieved for such indium tin oxide- (ITO-) free OSCs under AM 1.5G simulated illumination, comparable to that of ITO-based reference OSCs (PCE of 6.11%). It is found that this ZnO interlayer not only slightly enhances the transparency of MoO3/Ag cathode but also obtains a lower root-mean-square (RMS) roughness on the MoO3/Ag surface. Meanwhile, ITO-free OSCs also show a good stability. The PCE of the devices still remains above 85% of the original values after 30 days, which is slightly superior to ITO-based reference OSCs where the 16% degradation in PCE is observed after 30 days. It may be instructive for further research of OSCs based on metal thin film electrodes.

Long lifetime stable and efficient semitransparent organic solar cells using a ZnMgO-modified cathode combined with a thin MoO3/Ag anode

Long-term stable and efficient semitransparent organic solar cells with good color perception are demonstrated by combining a transparent ZnMgO-modified cathode with thin MoO3/Ag anodes.

Investigation of the operating characteristics of a 12 stepped-cavity relativistic magnetron with axial extraction driven by an “F” transparent cathode using particle-in-cell simulations

We explore the performance of a 12 stepped-cavity relativistic magnetron with axial extraction (12 stepped-cavity RMDO) driven by an “F” transparent cathode (the “F” transparent cathode is a coaxial transparent cathode with two azimuthal periods of increased thickness and which looks like the letter “F,” so we call it “F” transparent cathode) through particle-in-cell (PIC) simulations. It is shown that using the “F” transparent cathode, an electronic efficiency of 70% with gigawatt output power is obtained while reducing the axial leakage current by about 50% compared to using the usual transparent cathode. Further PIC simulations demonstrate that frequency bifurcation occurs and mode switching can be achieved using several hundred kilowatts input RF power in the 12 stepped-cavity RMDO driven by an “F” transparent cathode. For example, it was found that using an applied driver power of 180 kW for 10 ns, the operating TE31 mode can be switched to the TE41 mode. It is also found that the secondary electron and backscattered electron emission and axial leakage current were two disturbing factors for the 12 stepped-cavity RMDO when it works at a stable operation mode but when the 12 stepped-cavity RMDO works near the critical magnetic field at the boundary between two modes, these two factors would lead to the operation modes changing.

Optimizing the Parameters of a 12-Cavity Rising-Sun Relativistic Magnetron With Single-Stepped Cavities for $\pi $ -Mode Operation

We investigated the use of single-stepped cavities instead of typical tapered cavities in an A6 magnetron with diffraction output (MDO) and a 12-cavity MDO through particle-in-cell (PIC) simulations using MAGIC [1] and UNIPIC [2] in our earlier publications [3] – [5] . The interaction space, where the charged particles interact with the induced RF waves, was increased by replacing tapered cavities with single-stepped cavities in a 12-cavity rising-sun relativistic magnetron with diffraction output (RMDO) that the electronic efficiency of $\pi $ -mode generation in a 12-cavity rising-sun RMDO with single-stepped cavities driven by a transparent cathode [6] with gigawatt output power level can be as high as 83% for $\alpha = 12.5^{\circ }$ , 79% for $\alpha = 17.5^{\circ }$ , and 82% for $\alpha = 18.2^{\circ }$ at $\beta = 32^{\circ }$ , where $\alpha $ is the angle between the inner wall and the $z$ -axis, and $\beta $ is the angle between the outer wall and the $z$ -axis. When $\alpha $ , the angle between the inner wall and the $z$ -axis, is changed, the depth of single-stepped cavities was changed. This would lead to the coupling between deep cavities and shallow cavities in the 12-cavity rising-sun MDO changing too, and which finally results in different frequencies of magnetron operation in the $\pi $ -mode. As we know, gigawatt nanosecond output power pulse is useful for the nanoradar system, and our PIC simulations found that when a 400-kV voltage pulse of 10-ns duration is applied to a 12-cavity rising-sun MDO driven by a transparent cathode or a solid cathode, the output power of the $\pi $ -mode can be as high as 1.5 GW. Without loss of generality, for $\alpha = 12.5^{\circ }$ and $\beta = 32^{\circ }$ , the peak efficiency of $\pi $ -mode generation in a 12-cavity rising-sun MDO with single-stepped cavities design is around 82% and occurs for voltage $V\sim 400$ kV ± 50 kV. This paper describes optimizing the parameters in a 12-cavity rising-sun RMDO for the desired $\pi $ -mode operation.

Relation Between Current Density Distribution in Land-Channel Direction and Presence of Liquid Water in Proton Exchange Membrane Fuel Cell Flow Channel

Since most of the PEMFCs have land-channel geometry in their flow field, the distance between the flow channel to the catalyst layer is not uniform. The distance is longer from the flow channel to the catalyst layer under the land area than to the catalyst layer under the channel area. This difference causes the non-uniform current density in land-channel direction, and the distribution largely depends on the operating conditions. As the inlet gas humidity in the operating condition decreases, the trend in the current density distribution changes from high local current under the channel to relatively uniform, and to higher local current under the land (Figure 1). This trend is due to the combined effects of (1) non-uniform oxygen transport resistance, (2) non-uniform liquid water transport distance causing the local flooding under the land area at wet condition, and (3) the tendency of faster membrane dryout under the channel area at dry condition. In order to investigate the distributions of various parameters in the land-channel direction, a small-scale segmented cell with about 300-micron resolution was developed by the authors’ group, and successfully measured the current and high-frequency resistance distribution in the land-channel direction. Distribution of oxygen transport resistance from the flow channel to the catalyst layer was also measured using limiting current technique. In this study a new segmented cell is developed. Using the transparent cathode plate the behavior of liquid water in cathode flow channel can be observed, and simultaneously the current density distribution is measured in land-channel direction. With this cell the relation between the current density distribution in the land-channel direction and the presence of liquid water is investigated. First, it is studied that at what gas humidity and at what overall current density the liquid water starts emerging from the gas diffusion layer surface to the flow channel. This provides us the information about the dependency of the liquid water emergence location on the current density distribution is studied. Then, how the presence of liquid water in the flow channel changes the current distribution is studied. Figure 1

Efficient Magnetron With a Virtual Cathode

We use particle-in-cell (PIC) computer simulations to demonstrate high-efficiency operation of a magnetron with diffraction output (MDO) that uses a virtual cathode in the interaction space instead of a physical cathode. The cathode generating the electron beam is external to the MDO interaction space and, therefore, is immune from electron bombardment, pulse shortening, decreasing electron efficiency, and frequency shift attributed to expanding cathode plasma. Electronic efficiencies comparable to those achieved with the MDO with a transparent cathode have been achieved in the simulations.