Reduction Of Work Function Research Articles

A Self-Organized Poly(vinylpyrrolidone)-Based Cathode Interlayer in Inverted Fullerene-Free Organic Solar Cells.

Herein, poly(vinylpyrrolidone) (PVP) is used as the cathode interlayer (CIL) through the self-organization method in inverted organic solar cells (OSCs). By coating a solution of PVP and active layer materials onto a glass/indium tin oxide (ITO) substrate, the PVP can segregate to the near ITO side due to its high surface energy and strong intermolecular interaction with the ITO electrode. The power conversion efficiency (PCE) of the obtained OSC device reaches 13.3%, much higher than that of the control device with a PCE of only 10.1%. The improvement results from the increased exciton dissociation efficiency and the depressed trap-assisted recombination, which can be attributed to the reduced work function of the cathode by the self-organized PVP. Additionally, the molecular weight of the PVP has almost no influence on the device performance, and the PVP-modified device presents superior stability. This method can also be applied in other highly efficient fullerene-free OSCs, and with a fine selection of the active layer, a high PCE of 14.0% is obtained. Overall, this work demonstrates the great potential of the PVP-based CIL in inverted OSCs fabricated via the self-organization method.

Efficient organic-inorganic hybrid cathode interfacial layer enabled by polymeric dopant and its application in large-area polymer solar cells

An organic-inorganic hybrid cathode interfacial layer (CIL) was developed by doping ZnO with the naphthalene-diimide based derivative NDI-PFNBr. It was found the resulting organic-inorganic hybrid CIL showed apparently improved conductivity and could act as an effective cathode interlayer to modify indium tin oxide (ITO) transparent electrodes. As a result, by employing the blend of PTB7-Th:PC71BM as the photoactive layer, the inverted polymer solar cells (PSCs) exhibited a remarkable enhancement of power conversion efficiency (PCE) from 8.52% for the control device to 10.04% for the device fabricated with the hybrid CIL. Moreover, all device parameters were simultaneously improved by using this hybrid CIL. The improved open-circuit voltage ( V OC) was attributed to the reduced work function of the ITO cathode, whereas the enhancements in fill factor (FF) and short-circuit current density ( J SC) were assigned to the increased conductivity and more effective charge extraction and collection at interface. Encouragingly, when the thickness of the hybrid CIL was increased to 80 nm, the resulting device could still keep a PCE of 8.81%, exhibiting less thickness dependence. Considering these advantages, 16 and 93 cm2 large-area PSCs modules were successfully fabricated from the hybrid CIL by using doctor-blade coating techniques and yielded a remarkable PCE of 8.05% and 4.49%, respectively. These results indicated that the hybrid CIL could be a promising candidate to serve as the cathode interlayer for high-performance large-area inverted PSCs.

Conductivity and field emission enhancement of C60-encapsulated single-walled carbon nanotubes

C60 molecules are encapsulated inside single-walled carbon nanotubes (SWNTs) and resulting charge (electron) transfer from the SWNTs to the C60s is observed. A Raman upshift in the G-band for the C60-encapsulated SWNTs (C60@SWNTs) is indicative of a charge transfer occurred in this system. The observed conductivity enhancements strongly support the p-doping of the SWNTs by C60 encapsulation, implying the electron concentration of the SWNTs is reduced by the insertion of C60. We also observe the significant field emission (FE) enhancement of C60@SWNTs, which is not expected since the reduced electron concentration of SWNTs is typically to induce a Fermi level downshift. However, ultraviolet photoelectron spectroscopy results show that C60 encapsulation induce a Fermi level upshift (i.e. reduced work function). Although the p-doping of SWNT is achieved by C60 encapsulation, the reduced work function of C60@SWNTs provide the electrons a higher probability for tunneling, resulting in both FE and conductivity enhancement.

Enhanced electron transport enables over 12% efficiency by interface engineering of non-fullerene organic solar cells

Organic solar cells have attracted much attention in the recent years due to their many intrinsic advantages, such as, light weight, flexibility, low-cost, solution processing, and facile device fabrication. In this study, effective interface engineering was employed to improve all the photovoltaic parameters of an organic solar cell simultaneously by incorporating ZnO nanoparticle (ZnO-NP) interlayer in between the indium tin oxide cathode and sol-gel processed ZnO electron transport layer. The significance of incorporating a ZnO-NP/ZnO bilayer as the electron transport layer in the bulk heterojunction organic solar cells was demonstrated via systematic study, employing a high efficiency photoactive layer system (PBDB-T:ITIC). The bilayer electron transport layer demonstrated reduced work function compared to the sol-gel ZnO, which enabled effective electron transfer from the active layer to the electron transport layer. In addition, improved bilayer surface morphology, via reduction of ZnO-NP layer roughness, and better crystallinity compared to sol-gel ZnO facilitated charge separation and transmission between electron transport layer and active layer. Consequently, the devices with bilayer interlayer exhibited an enhancement of > 13% in power conversion efficiency compared to the control devices with sol-gel only ZnO as electron transport layer. The mechanisms behind the improvement in device performance were analysed using the ultraviolet and X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The champion bilayer device exhibited 12.24% efficiency which is much higher than the efficiency of 10.69% for the control device.

Electrosprayed Polymer-Hybridized Multidoped ZnO Mesoscopic Nanocrystals Yield Highly Efficient and Stable Perovskite Solar Cells.

Solid-state perovskite solar cells have been expeditiously developed since the past few years. However, there are a number of open questions and issues related to the perovskite devices, such as their long-term ambient stability and hysteresis in current density–voltage curves. We developed highly efficient and hysteresis-less perovskite devices by changing the frequently used TiO2 mesoscopic layer with polymer-hybridized multidoped ZnO nanocrystals in a common n–i–p structure for the first time. The gradual adjustment of ZnO conduction band position using single- and multidopant atoms will likely enhance the power conversion efficiency (PCE) from 8.26 to 13.54%, with PCEmax = 15.09%. The highest PCEavg of 13.54% was demonstrated by 2 atom % boron and 6 atom % fluorine co-doped (B, F:ZnO) nanolayers (using optimized film thickness of 160 nm) owing to their highest conductivity, carrier concentration, optical transmittance, and band-gap energy compared to other doped films. We also successfully apply a fine polyethylenimine thin layer on the doped ZnO nanolayers, causing the reduction in work function and overall demonstrating the enhancement in PCE from ∼10.86% up to 16.20%. A polymer-mixed electron-transporting layer demonstrates the remarkable PCEmax of 20.74% by decreasing the trap sites in the oxide layer that probably reduces the chances of carrier interfacial recombination originated from traps and thus improves the device performance. Particularly, we produce these electron-rich multidoped ZnO nanolayers via electrospray technique, which is highly suitable for the future development of perovskite solar cells.

Indium-doped ZnO mesoporous nanofibers as efficient electron transporting materials for perovskite solar cells

We introduce a viable electrospinning route for the development of highly porous indium-doped (In-doped) ZnO nanofibers as electron transporting materials (ETMs) in perovskite solar cells (PSCs) for the first time ever. The nanofibers ETMs with optimal thickness leads to highly efficient and hysteresis-free PSCs with an average power conversion efficiency (PCEavg) of 16.03% and a best power conversion efficiency (PCEbest) of 17.18%, thanks to high porosity and high crystallinity of the nanofibers, better infiltration of the absorber and rapid charge transport characteristics due to indium (In) doping. Furthermore, the modification of In-doped ZnO nanofibers ETMs, by coating a thin layer of a polymer polyethyleneimine (PEI) further enhances the PCEavg to 16.86% (PCEbest = 18.69%) by effectively reducing the energy barrier for electron extraction due to a reduced work function. This study reveals that In-doped ZnO nanofibers are the best ETMs for generating the inexpensive and high efficiency PSCs with long-term stability.

Mitigation of multipacting in 113 MHz superconducting rf photoinjector

Superconducting RF (SRF) photo-injectors are one of the most promising devices for generating continuous wave (CW) electron beams with record high brightness. Ultra-high vacuum of SRF guns provides for long lifetime of the high quantum efficiency (QE) photocathodes, while SRF technology provides for high accelerating gradients exceeding 10 MV/m. It is especially true for low frequency SRF guns where electrons are generated at photocathodes at the crest of accelerating voltage. Two main physics challenges of SRF guns are their compatibility with high QE photocathodes and multipacting. The first is related to a possibility of deposition of photocathode materials (such as Cs) on the walls of the SRF cavity, which can result in increased dark current via reduction of the bulk Nb work function and in enhancing of a secondary electron emission yield (SEY). SEY plays critical role in multipacting, which could both spoil the gun vacuum and speed up the deposition of the cathode material on the walls of the SRF cavity. In short, the multipactor behavior in superconducting accelerating units must be well understood for successful operation of an SRF photo-injector. In this paper we present our studies of 1.2 MV 113 MHz quarter-wave SRF photo-injector serving as a source of electron beam for the Coherent electron Cooling experiment (CeC) at BNL. During three years of operating our SRF gun we encountered a number of multipacting zones. We also observed that presence of $\textrm{CsK}_{2}\textrm{Sb}$ photocathode in the gun could create additional multipacting barriers. We had conducted a comprehensive numerical and experimental study of the multipactor discharge in our SRF gun, and had developed a process of crossing the multipacting barriers from zero to the operational voltage without affecting the lifetime of our photocathode and enhancing the strength of multipacting barriers.

Electron-induced secondary electron emission of Zn-doped MgO/Au composite film

An MgO/Au composite film with a 1.8 at% Zn-doped MgO surface layer prepared by reactive magnetron sputtering exhibited a superior electron-induced secondary electron emission (SEE) performance, and it had a SEE coefficient of 5.4 with an increase of 9.3% and a similar SEE degradation rate in comparison with an undoped composite film under primary electron bombardment of 200 eV. The mechanism for this SEE coefficient increase is tightly related to the reduced work function of Zn-doped MgO crystal analyzed by first-principles calculations and the enlarged MgO grains as well as the decreased surface roughness of Zn-doped MgO/Au film observed by morphological characterizations. In addition, for the Zn-doped MgO/Au film its improved electrical conductivity induced by Zn doping results in a similar SEE degradation rate even though it emits secondary electrons with the higher coefficient all the time under continuous electron bombardment compared with the undoped film.

Strategic Design of Vacancy-Enriched Fe1- xS Nanoparticles Anchored on Fe3C-Encapsulated and N-Doped Carbon Nanotube Hybrids for High-Efficiency Triiodide Reduction in Dye-Sensitized Solar Cells.

A new class of hybrids with the unique electrocatalytic nanoarchitecture of Fe1- xS anchored on Fe3C-encapsulated and N-doped carbon nanotubes (Fe1- xS/Fe3C-NCNTs) is innovatively synthesized through a facile one-step carbonization-sulfurization strategy. The efficient synthetic protocols on phase structure evolution and dynamic decomposition behavior enable the production of the Fe1- xS/Fe3C-NCNT hybrid with advanced structural and electronic properties, in which the Fe vacancy-contained Fe1- xS showed the 3d metallic state electrons and an electroactive Fe in +2/+3 valence, and the electronic structure of the CNT was effectively modulated by the incorporated Fe3C and N, with the work function decreased from 4.85 to 4.63 eV. The meticulous structural, electronic, and compositional control unveils the unusual synergetic catalytic properties for the Fe1- xS/Fe3C-NCNT hybrid when developed as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs), in which the Fe3C- and N-incorporated CNTs with reduced work function and increased charge density provide a highway for electron transport and facilitate the electron migration from Fe3C-NCNTs to ultrahigh active Fe1- xS with the electron-donating effect, and the Fe vacancy-enriched Fe1- xS nanoparticles exhibit ultrahigh I3- adsorption and charge-transfer ability. As a consequence, the DSSC based on the Fe1- xS/Fe3C-NCNT CE delivers a high power conversion efficiency of 8.67% and good long-term stability with a remnant efficiency of 8.00% after 168 h of illumination, superior to those of traditional Pt. Furthermore, the possible catalytic mechanism toward I3- reduction is creatively proposed based on the structure-activity correlation. In this work, the structure engineering, electronic modulation, and composition control opens up new possibilities in constructing the novel electrocatalytic nanoarchitecture for highly efficient CEs in DSSCs.

Detection of intra-band gap defects states in spin-coated sol-gel SnOx nanolayers by photoelectron spectroscopies

The presence of occupied intra-band gap states in oxygen-deficient tin dioxide (SnOx; 1 < x < 2) is crucial for efficient manufacturing of multipurpose electronic devices based on transparent conducting oxides. Former experimental determination of these states was conducted for well-defined, usually thick tin oxides obtained under highly controlled vacuum conditions. In this work, we present precise specification of gap defects states for ultra-thin SnOx layers prepared by sol-gel synthesis followed with spin-coat deposition. Post-deposition drying and annealing processing changed layers’ surface morphology and bulk crystalline structure as monitored by scanning electron microscopy, atomic force microscopy and x-ray diffraction. An x-ray photoemission spectroscopy (XPS) analysis of chemical composition revealed the presence of both Sn2+ and Sn4+ species in layers with and without post-drying annealing step. A stronger contribution of SnO was found for dried SnOx. In the valence band region, XPS studies revealed pronounced O 2p and hybridised Sn 5p/5s–O 2p states as well as deep, overlapping with the O 2p, band gap states resulting from Sn 5s orbitals. These states—attributed to defect states—indicated enhanced presence of Sn2+ cations, and were assigned to ‘bridging’ oxygen vacancies. Complementary photoemission yield spectroscopy (PYS) studies of the SnOx band gap region revealed an increased effective density of occupied electronic states below the Fermi level EF for annealed layers. The consequence was a work function reduction by 0.15 eV after the annealing process. PYS results allowed a precise detection of SnOx shallow band gap states close to EF. These states were attributed to surface oxygen vacancies, which was confirmed by computer modelling. Finally, the annealed layers exhibited higher calculated charge carrier concentration, hence the increased n-type character.

Overcoming the quantum efficiency-lifetime tradeoff of photocathodes by coating with atomically thin two-dimensional nanomaterials

Photocathodes are key components of electron injectors for X-ray free electron laser and X-ray energy recovery linacs, which generate brilliant, ultrafast, and coherent X-rays for the exploration of matter with ultrahigh resolutions in both space and time. Whereas alkali-based semiconducting photocathodes display a higher quantum efficiency (QE) in the visible light spectrum than their metallic counterparts, their lifetimes are much shorter due to the high reactivity of alkali-based surfaces to the residual gases in the vacuum chamber. Overcoming the tradeoff between QE and lifetimes has been a great challenge in the accelerator community. Herein, based on ab initio density functional calculations, we propose an approach to overcome this tradeoff by coating with atomically thin two-dimensional (2D) nanomaterials. On one hand, the 2D coating layers can enhance the lifetimes of photocathodes by preventing the chemical reactions with the residual gases. On the other hand, the 2D coating layers can effectively engineer the work function of photocathodes, thus controlling their QE. A monolayer of insulating BN reduces the work function, whereas a monolayer of semi-metallic graphene or semiconducting molybdenum disulfide (MoS2) increases the work function. This phenomenon originates from the induced interfacial dipoles. The reduction of work function by BN implies that it is capable of maintaining the high QE of semiconducting photocathodes in addition to enhance their lifetimes. This study advances our understandings on the surface chemistry of coated photocathodes and opens new technological avenues to fabricate photocathodes with high QE and longer lifetimes.

Alcohol soluble porphyrin for the cathode buffer layers of fullerene/perovskite planar heterojunction solar cells

Abstract A π-conjugated ionized porphyrin 5,15-bis(3-acetenyl-pyridine ethyl ammonium bromide)-10,20-bis(5-(2-butyloctyl)-thienyl)-porphyrin zinc (II) (m-PYBrZnPor) is developed as cathode buffer layers (CBLs) to modify the interfaces between Ag cathode and PCBM ([6,6]-phenyl C61-butyric acid methyl ester) of fullerene/perovskite (CH3NH3PbI3) planar heterojunction solar cells (PSCs). m-PYBrZnPor is soluble in ethanol, and therefore can be spin-coated on the PCBM layer conveniently via solution processing as CBLs without corroding the under-layers of PCBM when we fabricate the devices. Furthermore, this simple modification of the cathode/PCBM interfaces by m-PYBrZnPor can remarkably enhance the power conversion efficiency (PCE) up to 17.5% with simultaneously enhanced open circuit voltage, short circuit current and fill factor values even compared with those of the devices with bathocuproine (BCP) CBLs, which are contributed by the suppressed leakage current, reduced recombination losses and reduced work function induced by the porphyrin CBLs.

Plasmon-enhanced perovskite solar cells using ultra-thin LiF spacer isolating AgAl and Au composite nanoparticles from metal electrode

Abstract Plasmon enhanced perovskite solar cells (PSCs) incorporated with AgAl and Au composite nanoparticles (NPs) isolating from AgAl metal electrode with ultra-thin LiF spacer were investigated. PSCs with AgAl-Au composite NPs got 17.02% of the power conversion efficiency (PCE), which is 11.10% higher than PCE (15.32%) of the control PSC with only AgAl electrode. Furthermore, the hysteresis effect of PSCs with LiF modified AgAl and Au composite NPs can be neglected compared to the control cells without NPs and LiF spacer. In the meanwhile, carrier transport and collection probabilities of PSCs with AgAl and Au composite NPs were increased to 96.86% from 89.51% in control cells, indicating the improved electron transport and extraction ability. The enhanced PCEs of PSCs with AgAl(3 nm)/Au(2 nm)/LiF(1 nm) are attributed to the improved light absorption of perovskite layer and the reduced contact barrier at metal electrode, which can be supported with its enhanced photon-to-electron conversion efficiency (△IPCE) and the disappeared negative capacitive behavior. Furthermore, the obviously reduced work function values of AgAl-Au composite NPs and AgAl metal electrode after LiF modification were observed with ultraviolet photoelectron spectroscopy spectra. Our results indicate that thermally evaporating a ultra-thin LiF spacer can well inhibit the energy dissipation of plasmon effects of AgAl-Au composite NPs and reduce the contact barrier between PCBM and AgAl metal electrode, which is an efficient strategy to achieve efficient plasmon-enhanced perovskite solar cells.

Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices.

Electron injection layers (EILs) based on a simple polymer blend of polyethyleneimine ethoxylated (PEIE) and poly[(9,9-bis(3'-(( N, N-dimethyl)- N-ethylammonium)-propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) can suppress the dependence of organic light-emitting device (OLED) performance on thickness variation compared with single PEIE or PFN-Br EILs. PEIE and PFN-Br were compatible with each other and PFN-Br uniformly mixed in the PEIE matrix. PFN-Br in PEIE formed more fluorene-fluorene pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the work function (WF) substantially compared with single PEIE or PFN-Br polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower driving voltages and smaller dependences of device performance on EIL thickness than single PEIE or PFN-Br-based devices. These improvements were attributed to electron-transporting fluorene moieties, increased fluorene-fluorene pairs working as channels of electron transport, and the large WF reduction effect of PEIE:PFN-Br blends.

Solution‐Processed In2O3/ZnO Heterojunction Electron Transport Layers for Efficient Organic Bulk Heterojunction and Inorganic Colloidal Quantum‐Dot Solar Cells

We report the development of a solution‐processed In2O3/ZnO heterojunction electron transport layer (ETL) and its application in high efficiency organic bulk‐heterojunction (BHJ) and inorganic colloidal quantum dot (CQD) solar cells. Study of the electrical properties of this low‐dimensional oxide heterostructure via field‐effect measurements reveals that electron transport along the heterointerface is enhanced by more than a tenfold when compared to the individual single‐layer oxides. Use of the heterojunction as the ETL in organic BHJ photovoltaics is found to consistently improve the cell's performance due to the smoothening of the ZnO surface, increased electron mobility and a noticeable reduction in the cathode's work function, leading to a decrease in the cells’ series resistance and a higher fill factor (FF). Specifically, non‐fullerene based organic BHJ solar cells based on In2O3/ZnO ETLs exhibit very high power conversion efficiencies (PCE) of up to 12.8%, and high FFs of over 70%. The bilayer ETL concept is further extended to inorganic lead‐sulphide CQD solar cells. Resulting devices exhibit excellent performance with a maximum PCE of 8.2% and a FF of 56.8%. The present results highlight the potential of multilayer oxides as novel ETL systems and lay the foundation for future developments.

New application of AIEgens realized in photodetectors: reduced work function of transparent electrodes and much improved performance

Highly sensitive organic photodetectors were achieved by the work function reduction of transparent electrodes with AIE molecules.

Spitzer shaped ZnO nanostructures for enhancement of field electron emission behaviors.

We observed enhanced field emission (FE) behavior for spitzer shaped ZnO nanowires synthesized via a hydrothermal approach. The spitzer shaped and pointed tipped 1D ZnO nanowires of average diameter 120 nm and length ∼5–6 μm were randomly grown over an ITO coated glass substrate. The turn-on field (Eon) of 1.56 V μm−1 required to draw a current density of 10 μA cm−2 from these spitzer shaped ZnO nanowires is significantly lower than that of pristine and doped ZnO nanostructures, and MoS2@TiO2 heterostructure based FE devices. The orthodoxy test that was performed confirms the feasibility of a field enhancement factor (βFE) of 3924 for ZnO/ITO emitters. The enhancement in FE behavior can be attributed to the spitzer shaped nanotips, sharply pointed nanotips and individual dispersion of the ZnO nanowires. The ZnO/ITO emitters exhibited very stable electron emission with average current fluctuations of ±5%. Our investigations suggest that the spitzer shaped ZnO nanowires have potential for further improving in electron emission and other functionalities after forming tunable nano-hetero-architectures with metal or conducting materials.

The electronic structures and work functions of (100) surface of typical binary and doped REB6 single crystals

The density function theory been used to calculate the electronic structures of binary and doped rare earth hexaborides (REB6), which exhibits the large density of states (DOS) near Fermi level. The d orbital elections of RE element contribute the electronic states of election emission near the Fermi level, which imply that the REB6 (RE=La, Ce, Gd) with wide distribution of high density d orbital electrons could provide a lower work function and excellent emission properties. Doping RE elements into binary REB6 can adjust DOS and the position of the Fermi energy level. The calculated work functions of considered REB6 (100) surface show that the REB6 (RE=La, Ce, Gd) have lower work function and doping RE elements with active d orbital electrons can significantly reduce work function of binary REB6. The thermionic emission test results are basically accordant with the calculated value, proving the first principles calculation could provide a good theoretical guidance for the study of electron emission properties of REB6.

Low pressure hydrogen sensing based on carbon nanotube field emission: Mechanism of atomic adsorption induced work function effects

The mechanism of the innovative adsorption based field emission hydrogen sensing technique is investigated from experimental and first-principles investigations. Hydrogen pressure from 10−3 to 10−7 Pa range could be detected. High carbon nanotube (CNT) crystallinity plays key role for the reduction of the effective work function from the dissociated hydrogen adsorption, leading to the field emission current increase. For CNT emitters with high dangling-bond (DB) type of defects/structures, the dissociated H adsorption occurs preferentially on the DB site with negligible work function variation. Joule heating & temperature rising is a core factor to assist the dissociated hydrogen adsorption with enhanced sensing effect. For work function sensitive CNT devices, operations under hydrogen & high temperature conditions may produce performance stability issue.

Simply modified indium tin oxides by ultrathin aluminum and sodium chloride composite interlayer for high performance inverted polymer solar cells

We report the fabrication of high performance inverted polymer solar cells with simply modified indium tin oxide (ITO) by an ultrathin aluminum (Al) and sodium chloride (NaCl) composite layer. The device efficiency and stability were both improved. The optimized device with poly(3-hexylthiophene) as the donor and [6,6]-phenyl-C61-butyric acid methylester as the acceptor under AM 1.5 (100 mw cm−2) radiation achieved a high power conversion efficiency of 3.88% with an open-circuit voltage of 0.60 V and a fill factor of 0.61, which is significantly higher than those of the inverted devices with only Al or NaCl as modification interlayer, respectively. Moreover, the stability is enhanced by about 70% more than that of the conventional device. The significant enhancement is attributed to the reduced work function of ITO electrode from 4.75 to 3.90 eV by modification as well as the improvement of the electrode interface.