The effect of small-molecule electron transporting materials on the performance of polymer solar cells

It has been studied that the effect of different electron transporting materials, including 2-(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4-oxidiazole (PBD), tris-8-hydroxy-quinolinato aluminum and Bis[2-(2-benzothiazoly)phenolato]zinc(II) on the properties of solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-buytyric acid methyl ester composites. The results suggest that the insertion of electron transporting layers (ETL) can improve the open circuit voltage (Voc) and power conversion efficiency (PCE) of the polymer solar cells. And the effect of thickness of the three ETL has also been investigated. The cell with a PBD layer exhibits an enhanced Voc by 0.16 V and the highest PCE, which is 1.6 times that of the cell without ETL. The improved performance is due to the increased built-in potential in the interface between the active layer and Al electrode.     

Influence of sodium compounds at the Cu(In,Ga)Se2/(PVD)In2S3 interface on solar cell properties

The present contribution deals with indium sulfide buffer layers grown by thermal co-evaporation of elemental indium and sulfur. It has been found necessary to deposit these buffer layers at low substrate temperatures in order to reach V_oc values similar to those with (CBD)CdS. However, such deposition conditions lead to the formation of a highly recombinative Cu(In,Ga)Se₂/indium sulfide interface. This behaviour may be associated to the presence of sodium carbonates/oxides at the interface even though the Cu(In,Ga)Se₂ surface was cleaned in NH₃ (1 M, room temperature) prior to the indium sulfide deposition. An explanation is that, despite the chemical etch, sodium carbonates/oxides remain in the air exposed Cu(In,Ga)Se₂ grain boundaries and can migrate towards the surface when the Cu(In,Ga)Se₂ is heated under vacuum. These polluted interface areas act as recombination zones and thus inferior devices. A possibility to improve the device performance (i.e. improve the interface quality) is to sulfurize the remaining sodium carbonates/oxides. The resulting Na₂S can then leave the interface by formation of a solid solution with the indium sulfide. By adapting the buffer layer deposition process, 13.3% efficiency devices with co-evaporated indium sulfide are realized, performance which is close to that reached with (CBD)CdS.     

Effect of solution processed salt layers on the device performances of polymer solar cells

We report the solution processed Li salt layers (i.e. LiBF₄, and LiTFSI) in poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methylester (PCBM) bulk heterojunction solar cells, which facilitate electron injection at the interface between active layer and Al electrode. The Li salt layers are deposited on top of P3HT:PCBM active layer by simple drop-casting combined with controlled evaporation process. The solar cells employing Li salt layers exhibit the increase of short-circuit current (J_SC) and fill factor (FF) by 10% when compared with devices without such an injection layer, resulting in about 28% increase of power conversion efficiency. The effect of Li salt layers on the device performances is investigated with current–voltage (J–V) characteristics and impedance spectroscopy measurements.     

Characterization of a-SiC(x):H thin films as an encapsulation material for integrated silicon based neural interface devices

A fully integrated, wireless neural interface device is being developed to free patients from the restriction and risk of infection associated with a transcutaneous wired connection. This device requires a hermetic, biocompatible encapsulation layer at the interface between the device and the neural tissue to maintain long-term recording/stimulating performance of the device. Hydrogenated amorphous silicon carbide (a-SiC_x:H) films deposited by a plasma enhanced chemical vapor deposition using SiH₄, CH₄, and H₂ precursors were investigated as the encapsulation layer for such device. Si-C bond density, measured by Fourier transform infrared absorption spectrometer, suggests that deposition conditions with increased hydrogen dilution, increased temperature, and low silane flow typically result in increase of Si-C bond density. From the variable angle spectroscopic ellipsometry measurement, no dissolution of a-SiC_x:H was observed during soaking tests in 90 °C phosphate buffered saline. Conformal coating of the a-SiC_x:H in Utah electrode array was observed by scanning electron microscope. Electrical properties were studied by impedance spectroscopy to investigate the performance of a-SiC_x:H as an encapsulation layer, and the results showed long term stability of the material.     

Effectiveness of tris-(8-Hydroxyquinoline)-aluminum doped with cesium-derivatives in organic light-emitting diodes

The effectiveness of Cs-derivatives, including cesium carbonate (Cs₂CO₃), cesium fluoride (CsF), and cesium nitrite (CsNO₃), as dopants of tris-(8-Hydroxyquinoline)-aluminum (Alq₃) in organic light-emitting diodes (OLEDs) were investigated via photoemission spectroscopy and current-voltage characteristics. The n-type doping of Cs₂CO₃ in Alq₃ is most effective. In current injection efficiency, the performance of OLEDs with CsF in cathode structures is similar to that with Cs₂CO₃, working well with both aluminum and silver in cathodes. As for CsNO₃, it is effective only with aluminum cathode due to the reaction between aluminum and CsNO₃.     

Organometallic tris(8-hydroxyquinoline)aluminum complexes as buffer layers and dopants in inverted organic solar cells

Tris(8-hydroxyquinoline)aluminum (Alq₃) is a frequently used material for organic light emitting diodes. The electronic properties and solubility can be tuned by chemical tailoring of the quinoline part, which makes it an interesting candidate for organic solar cells. Steady-state absorption and fluorescence, as well as time-resolved fluorescence properties of the parent Alq₃ and a series of complexes consisting of derivatives, such as 4-substituted pyrazol, methylpyrazol, arylvinyl, and pyridinoanthrene moieties, of the quinoline ligand, were studied in solutions and in thin films. Suitability of the complexes as anodic buffer layers or dopants in inverted organic solar cells based on the well known bulk heterojunction of poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) was tested. The devices equipped with the derivatives showed higher power conversion efficiency (η) compared to the photocells containing the parent Alq₃. Open circuit voltage (V_oc) was increased when the derivatives were utilized as the anodic buffer layer. Doping of the P3HT:PCBM with a small amount of Alq₃ or its derivative improved short circuit current density, V_oc, fill factor, and η, while the series resistance decreased. In addition, the devices were stable in air over several weeks without encapsulation. Possible mechanisms leading to the improvements in the photovoltaic performance by using the parent Alq₃ or its derivative as buffer layer or dopant are discussed.     

SnO2: A Wonderful Electron Transport Layer for Perovskite Solar Cells

The highest power conversion efficiency of perovskite solar cells is beyond 22%. Charge transport layers are found to be critical for device performance and stability. A traditional electron transport layer (ETL), such as TiO₂, is not very efficient for charge extraction at the interface, especially in planar structure. In addition, the devices using TiO₂ suffer from serious degradation under ultraviolet illumination. SnO₂ owns a better band alignment with the perovskite absorption layer and high electron mobility, which is helpful for electron extraction. In this Review, recent progresses in efficient and stable perovskite solar cells using SnO₂ as ETL are summarized.     

Electroluminescence and impedance analyses of organic light emitting diodes using anhydride materials as cathode interfacial layers

Pyromellitic dianhydride (PMDA) and trimellitic anhydride (TMA) were tried as cathode interfacial layers between tris-(8-hydroxyquinoline) aluminum (Alq₃) and Al in organic light emitting diodes (OLEDs). Both ultra-thin anhydride cathode interfacial layers improved the electroluminescence characteristics of OLEDs compared to those without any interfacial layer, and the PMDA interfacial layer showed the most significant enhancement of the device performance. According to impedance measurements and equivalent circuit analysis, the PMDA interfacial layer decreased the impedance, probably due to the increase in the injection efficiency of electrons from the Al cathode.     

Compound formation and energy levels at the interface betweeb amorphous Se and Cu,Ag and Al electrodes

The formation of selenide layers at the interface between Se and Cu, Ag and Al electrodes has been studied by vacuum photoemission. Cu and Ag react rapidly to form thick selenide layers while Al rectts to produce a layer protective against further reaction. The valence band maximum in the thick Se layer lies 1.26 eV below E_F with the copper electrode, 1.2 eV below E_F with an Ag electrode and 1.0 eV with an aluminum electrode.     

Influence of anode roughness and buffer layer nature on organic solar cells performance

Fluorine doped transparent conductive tin oxide thin films (FTO) of different surface roughness have been deposited by chemical vapor deposition (FTO_SOL), classical chemical spray pyrolysis (FTO_CSP), and spray pyrolysis onto heated substrates using infra red irradiation (FTO_IRSP); the three deposition methods inducing different surface roughness. It was found that the different FTOs presented similar electrical properties while their structural, morphological and optical properties were related to surface properties. These FTO films have been used as anode in multilayer organic solar cells, based on coupled donor/acceptor-copper phthalocyanine/fullerene. To improve solar cell performance, buffer layers of different natures have been tried at the anode/organic material interface. Deposition of a thin molybdenum oxide film onto FTO smooth films afforded reproducible devices with performance similar to those obtained with indium tin oxide anodes. However, cell efficiency decreased as FTO surface roughness increased. The degree of degradation depended on the nature of the buffer layer. We show that it is necessary to use buffer layer material that allows consistency and completeness of the electrode coverage.     

Admittance spectroscopic analysis of organic light emitting diodes with the CFX plasma treatment on the surface of indium tin oxide anodes

Admittance spectroscopic analysis was used to examine the effect of a CF_X plasma surface treatment on indium tin oxide (ITO) anodes using CF₄ gas and model the equivalent circuit for organic light emitting diodes (OLEDs) with the of ITO anode surface treated with CF_X plasma. This device with the ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum/lithium fluoride/Al structure was modeled as a simple combination of two resistors and a capacitor. The ITO anode surface treated with the CF_X plasma showed a shift in the vacuum level of the ITO, which resulted in a decrease in the barrier height for hole injection at the ITO/organic interface. Admittance spectroscopy measurements of the devices with the CF_X plasma treatment on the surface of the ITO anodes showed a change in the contact resistance, bulk resistance and bulk capacitance.     

Transition Metal Oxides for Organic Electronics: Energetics,Device Physics and Applications

During the last few years, transition metal oxides (TMO) such as molybdenum tri‐oxide (MoO₃), vanadium pent‐oxide (V₂O₅) or tungsten tri‐oxide (WO₃) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long‐term stable p‐type doping of wide band gap organic materials, charge‐generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi‐transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO‐based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution‐based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.     

Organic field-effect transistors using perylene

Organic thin-film field-effect transistors using organic semiconductor, perylene are fabricated, and electrical measurements are performed. The field-effect mobility of the device using perylene shows only p-type behavior while the electron and hole mobilities of its single crystal form are 5.5 and 0.5 cm²/V s, respectively. Stacked layers of perlyene (a layer fabricated with low deposition rate followed by another layer with high deposition rate) are formed for the active layer. Furthermore, hexadecafluorocopperphthalocyanine (F₁₆CuPc) and pentacene buffer layers are also used to modify the interface. For all of these devices, perylene layers acts as p-type. Electron trapping at grain boundaries and interface is thought to be a crucial factor. Hole mobility of 3.9×10⁻⁴ cm²/V s is obtained for the perylene film field-effect transistor device.     

Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell

This paper presents an implementation of our recent theory on the suspension of electron-hole recombination via electronic- and micro-structure optimization to study the influence of Zr-doping on the efficiency (η) of TiO₂-based dye-sensitized solar cells (DSSCs). We developed a four-layered working electrode, in which the size of particles increased from the bottom layer of TiO₂ (P-25) through three successive layers of Zr-doped TiO₂, which were calcined at 450, 600, and 850 °C respectively. The enhancement in open-circuit photovoltage (V_oc) and short-circuit photocurrent density (J_sc) can be attributed to the electronic- and micro-structures in the working electrode. The former is related to band bending, whereas the latter is related to light-scattering within multiple layers. Simulation results (FactSage) demonstrate that Zr doping in TiO₂ can suspend or delay the formation of oxygen vacancies and thereby reduce the number of electron scattering centers, which helps to suspend electron-hole recombination by strengthening Ti-O bonds. The proposed four-layered working electrode produced an 80.2% increase in η, compared with DSSCs using a TiO₂ (P-25) electrode. This study demonstrated a novel metal doping strategy for the manipulation of electronic structure and photoelectron conversion efficiency. The proposed methodology could also be used to guide the design of photo-catalysts in general.     

Spatial organization of peptide nanotubes for electrochemical devices

A novel biosensor for hydrogen peroxide was developed by combining the known properties of microperoxidase-11 (MP11) as an oxidation catalyst, and the interesting properties of diphenylalanine peptide nanotubes (PNTs) as a supporting matrix to allow a good bioelectrochemical interface. In this case, the synthesized MP11/PNTs were immobilized onto the ITO electrode surface via layer-by-layer (LBL) deposition, using poly(allylamine hydrochloride) (PAH) as positively charged polyelectrolyte layers. The PNTs provide a favorable microenvironment for MP11 to perform direct electron transfer to the electrode surface. The resulting electrodes showed a pair of well-defined redox peaks with formal potential at about −343 mV (versus SCE) in phosphate buffer solution (pH 7). The experimental results also demonstrated that the resulting biosensor exhibited good electrocatalytic activity to the reduction of H₂O₂ with a sensitivity of 9.43 μA cm⁻² mmol⁻¹ L, and a detection limit of 6 μmol L⁻¹ at the signal-to-noise ratio of 3. Moreover, we also observed that the peptides self-assembly can be influenced upon changing the pH of the solution. Alkaline solution appears to favor the packing of diphenylalanine nanotubes being closer than acidic or neutral conditions. The study proved that the combination of PNTs with MP11 is able to open new opportunities for the design of enzymatic biosensors with potential applications in practice.     

Organic light emitting diodes based on dipyridamole drug

The dipyridamole drug [DIP: 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido(5,4-d)pyrimidine] is widely used in treatment of coronary heart disease for its antiplatelet and vasodilating activities, and its high intensity photoluminescence (PL) has been widely reported. In this work, the fabrication and the characterization of a new OLED using the DIP molecule as an emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) or the 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD) as hole-transporting layer, the DIP layer as an emitting layer and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited in a high vacuum by thermal evaporation onto indium tin oxide substrates and without breaking vacuum. Continuous electroluminescence emission was obtained in all configurations upon varying the applied bias voltage from 4 to 30 V, the observed wide emission band was centered at 493 nm. The luminance of the devices was about 1500 (cd)/m² with 4.5 cd/A of efficiency for the best device. The charge transport behavior in the OLED is also discussed as a function of different carrier injection levels.     

Efficiency enhancement in DIBSQ:PC<Subscript>71</Subscript>BM organic photovoltaic cells by using Liq-doped Bphen as a cathode buffer layer

We have improved the photovoltaic performance of 2,4-bis[4-(N,Ndiisobutylamino)- 2,6-dihydroxyphenyl] squaraine:[6,6]-phenyl C71-butyric acid methyl ester (DIBSQ:PC₇₁BM) organic photovoltaic (OPV) cells via incorporating Liq-doped Bphen (Bphen-Liq) as a cathode buffer layer (CBL). Based on the Bphen-Liq CBL, a DIBSQ:PC₇₁BM OPV cell possessed an optimal power conversion efficiency of 4.90%, which was 13% and 60% higher than those of the devices with neat Bphen as CBL and without CBL, respectively. The enhancement of the device performance could be attributed to the enhanced electron mobility and improved electrode/active layer contact and thus the improved photocurrent extraction by incorporating the Bphen-Liq CBL. Light-intensity dependent device performance analysis indicates that the incorporating of the Bphen-Liq CBL can remarkably improve the charge transport of the DIBSQ:PC₇₁BM OPV cell and thus decrease the recombination losses of the device, resulting in enhanced device performance. Our finding indicates that the doped Bphen-Liq CBL has great potential for high-performance solution-processed small-molecule OPVs.     

All‐Solution‐Processed Pure Formamidinium‐Based Perovskite Light‐Emitting Diodes

All‐solution‐processed pure formamidinium‐based perovskite light‐emitting diodes (PeLEDs) with record performance are successfully realized. It is found that the FAPbBr₃ device is hole dominant. To achieve charge carrier balance, on the anode side, PEDOT:PSS 8000 is employed as the hole injection layer, replacing PEDOT:PSS 4083 to suppress the hole current. On the cathode side, the solution‐processed ZnO nanoparticle (NP) is used as the electron injection layer in regular PeLEDs to improve the electron current. With the smallest ZnO NPs (2.9 nm) as electron injection layer (EIL), the solution‐processed PeLED exhibits a highest forward viewing power efficiency of 22.3 lm W^?1, a peak current efficiency of 21.3 cd A^?1, and an external quantum efficiency of 4.66%. The maximum brightness reaches a record 1.09 × 10⁵ cd m^?2. A record lifetime T₅₀ of 436 s is achieved at the initial brightness of 10 000 cd m^?2. Not only do PEDOT:PSS 8000 HIL and ZnO NPs EIL modulate the injected charge carriers to reach charge balance, but also they prevent the exciton quenching at the interface between the charge injection layer and the light emission layer. The subbandgap turn‐on voltage is attributed to Auger‐assisted energy up‐conversion process.     

Insulating oxide buffer layer formed by sol–gel method for planarization of stainless steel substrate of a-Si:H thin film solar cell

The planarization of flexible stainless steel (SS) foils was investigated for the application of flexible solar cells. The sol–gel SiO₂ film containing nanoparticles was evaluated for a buffer layer on SS foils, and methods to improve the adhesion of SiO₂ film to SS foil were studied. The improvement of adhesion by adding Al₂O₃ matrix was discussed by analyzing the interface between Al₂O₃–SiO₂ film and SS foil. The usefulness of sol–gel buffer layer was also verified by comparing the performance of single junction a-Si:H thin film solar cells fabricated on bare SS foil and buffer layer-coated SS foil. The cell characteristics such as V_oc, J_sc, fill factor, and efficiency were all improved by adopting the buffer layer. The efficiency of the cell on buffer layer-coated and non-textured SS foil was 6.1% whereas the efficiency was 4.9% on bare SS foil.     

Aqueous Metal Oxide Inks for Modifying Electrode Work Function in Polymer Solar Cells

A method to prepare aqueous metal oxide inks for tuning the work function (WF) of electrodes is demonstrated. Thin films prepared from the metal oxide ink based on vanadium oxide (V₂O₅) nanoparticles are found to increase the WF of an indium‐tin‐oxide (ITO) electrode. ITO substrates modified with V₂O₅ films are applied as a hole selective layer (HSL) in polymer solar cells (PSCs) using a poly(3‐hexylthiophene) and [6,6]‐phenyl‐C61 butyric acid methyl ester blend as a photoactive layer. The PSCs prepared with V₂O₅‐modified ITO show better device performance, achieving a power conversion efficiency of 3.6%, demonstrating 15% enhancement compared to conventional ITO/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT‐PSS) based devices. Furthermore, ITO/V₂O₅‐modified devices exhibit better ambient stability with 60% improvement in device lifetime than those using PEDOT:PSS as an HSL. This solution‐processable and highly stable WF‐modifying metal oxide film can be a potential alternative material for engineering interfaces in optoelectronic devices.