Large Work Function Research Articles

Interfacial Charge Modulation: An Efficient Strategy for Boosting Spatial Charge Separation on Semiconductor Photocatalysts

AbstractSurface modulation via injection or extraction of charge carriers in microelectric devices has been used to tune the energy band alignment for desired electrical and optical properties, yet not well recognized in photocatalysis field. Here, taking semiconductor bismuth tantalum oxyhalides (Bi4TaO8X) as examples, chemically inactive molybdenum oxide (MoO3) with a large work function is introduced to qualitatively tune the properties of interfacial charges, achieving an evidently enhanced upward band bending and intensive built‐in electric field. Such a simple charge modulation exhibits a remarkable improvement in photocatalytic water oxidation, reaching an apparent quantum efficiency of 25% at the input wavelength of 420 nm. The validity and generality of surface charge modulating strategy are further demonstrated using other semiconductors (e.g., C3N4) and decorators (e.g., V2O5). The findings not only provide a promising strategy for rationally manipulating the interfacial built‐in electric field in photocatalysis but also pave the way to learn from microelectronic technologies to construct artificial photosynthesis systems for solar energy conversion.

Revealing Carbon Phenomena at Palladium Nanoparticles by Analyzing the Work Function

Carbon at metal nanoparticles (NPs) plays a fundamental role in heterogeneous catalysis. However, as experimental detection of small amounts of carbon is difficult, in particular when occupying subsurface sites, reaction mechanisms involving absorbed carbon are highly debated. Here, we show that the work function (WF) of metal NPs can be used as a measure of carbon adsorption and absorption, which we demonstrate by Kelvin probe force microscopy and density functional theory calculations for (111)-faceted palladium NPs (PdNPs) on graphite. Growth of PdNPs between 150 and 480 °C leads to carbon etching of the graphite steps and carbon absorption into the first subsurface layer below the NP's facets. This strongly reduces the WF of Pd(111) by up to -1 eV. During a 1 h long postannealing at 650 °C, more carbon is etched from the graphite steps, leading to a carbon precursor structure adsorbed on the NP's facets, as verified by scanning tunneling microscopy. The carbonaceous structures are replaced by graphene upon further annealing (1 to 2 h), followed by a decrease in the WF by ∼-1.4 eV. Similar phenomena are observed after short-time ethylene decomposition at PdNPs at 650 °C. Apart from subsurface carbon, we suggest that the large WF shifts observed experimentally could be attributed to structural defects on NP's facets.

Improved power conversion efficiency of silicon nanowire solar cells based on transition metal oxides

An n-type Si nanowire (SiNW) solar cell based on transition metal oxides V2O5 and TiO2 has been fabricated. The cell is demonstrated in the ITO/V2O5/n-SiNWs/TiO2/Al heterojunction structure, in which the V2O5 and TiO2 work as the hole- and the electron-selective layers, respectively. Parameters of cells with different lengths of nanowires have been investigated. Owing to the excellent light trapping effect of nanowire arrays and superior carrier transporting properties of thin V2O5 and TiO2 layers, an improved power conversion efficiency of 12.7% has been achieved in a cell with 1.45-μm-long nanowires. Due to the large work function difference between V2O5 and n-Si, a build-in potential of 0.75 V was obtained from the capacitance-voltage measurement, which implies the possibility of achieving a large open circuit voltage for these nanowire solar cells. The efficiency loss is mainly caused by the recombination via the defects on the unpassivated surface of SiNWs. By solving this problem, the cell efficiency may be improved further and realize the application in PV industries.

Impact of electrodes on the extraction of shift current from a ferroelectric semiconductor SbSI

Noncentrosymmetric bulk crystals generate photocurrent without any bias voltage. One of the dominant mechanisms, shift current, comes from the quantum interference of electron wave functions being distinct from the classical current caused by electrons' drift or diffusion. The dissipation-less nature of shift current, however, has not been fully verified, presumably due to the premature understanding of the role of electrodes. Here, we show that the photocurrent dramatically enhanced by choosing electrodes with large work functions for a p-type ferroelectric semiconductor SbSI. An optimized device shows a nearly constant zero-bias photocurrent despite the significant dependence of photocarrier mobility on temperature, which could be a clear hallmark for the dissipation-less nature of shift current. Distinct from conventional photovoltaic devices, the shift current generator operates as a majority carrier device. The present study provides fundamental design principles for energy-harvesting and photo-detecting devices with novel architectures optimal for the shift current photovoltaic effect.

Effect of graphene/ZnO hybrid transparent electrode on characteristics of GaN light-emitting diodes**Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA034801), the Foundation of the State Key Laboratory of Mechanical Transmission of Chongqing University, China (Grant Nos. SKLMT-ZZKT-2017M15 and SKLM-ZZKT-2015Z16), the National Natural Science Foundation of China (Grant Nos. 11544010, 11374359, 11304405, and 1155305),

In order to reduce the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), conductive transparent thin films with large work function are required to be inserted between Gr and p-GaN layers. In the present work, three kinds of transparent conductive oxide (TCO) zinc oxide (ZnO) films, Al-, Ga-, and In-doped ZnO (AZO, GZO, and IZO), are introduced as a bridge layer between Gr and p-GaN, respectively. The influence of different combinations of Gr/ZnO hybrid transparent conducting layers (TCLs) on the optical and thermal characteristics of the GaN-LED was investigated by the finite element method through COMSOL software. It is found that both the TCL transmittance and the surface temperature of the LED chip reduce with the increase in Gr and ZnO thickness. In order to get the transmittance of the Gr/ZnO hybrid TCL higher than 80%, the appropriate combination of Gr/ZnO compound electrode should be a single layer of Gr with ZnO no thicker than 400 nm (1L Gr/400-nm ZnO), 2L Gr/300-nm ZnO, 3L Gr/200-nm ZnO, or 4L Gr/100-nm ZnO. The LEDs with hybrid TCLs consisting of 1L Gr/300-nm AZO, 2L Gr/300-nm GZO, and 2L Gr/300-nm IZO have good performance, among which the one with 1L Gr/300-nm GZO has the best thermal property. Typically, the temperature of LEDs with 1L Gr/300-nm GZO hybrid TCLs will drop by about 7 K compared with that of the LEDs with a TCL without ZnO film.

Unveiling Conversion Reaction on Intercalation‐Based Transition Metal Oxides for High Power, High Energy Aqueous Lithium Battery

AbstractAqueous lithium batteries are reaching their energy and power limits partly due to limited capacity from intercalation chemistry. Alternatively, conversion reactions bring added capacity that can significantly increase the capacity ceiling of the cell. However, such reactions can only be realized in organic electrolytes, and similar redox chemistry in aqueous lithium batteries is not reported yet. In this work, it is discovered that a large work function difference (up to 1.78 eV) between crystalline LiMn2O4 (cLMO) and semicrystalline MnO2 (scMO) makes energy bands of MnO2 bend downward toward their interface, resulting in the accumulation of free electrons on scMO and inducing a reversible conversion reaction between scMO and Li+ in aqueous media. Based on this mechanism, a rechargeable hybrid aqueous battery employing hierarchical porous cLMO–scMO composite as a cathode delivers an extremely high cell‐level energy (171 Wh kg−1) and power density (5118 W kg−1) with excellent cycling stability (up to 3000 cycles) in a wide working temperature range (25 to 60 °C). More significantly, similar conversion reactions can also be achieved on LiCoO2, which provides a general design principle to obtain unprecedented energy and power density in commonly used transition metal oxides.

Azobenzene Adsorption on the MoS2(0001) Surface: A Density Functional Investigation within van der Waals Corrections

The interest in light-sensitive organic molecules, such as azobenzene, has increased because of their ability to functionalize two-dimensional layered systems and engineer their electronic structure. In this work, we explore the azobenzene trans and cis isomer adsorption on a molybdenum disulfide MoS2(0001) layer employing the density functional theory (DFT) within van der Waals (vdW) corrections to the semilocal exchange–correlation functional. We found that the aromatic rings of azobenzene lay parallel to the surface (two in the trans isomer and one in the cis isomer), which contributes to increasing the configuration stability by vdW interactions. Furthermore, we found a relatively large work function change upon adsorption because of the electron density rearrangement, and hence, it might affect the electronic transport properties within the single MoS2(0001) layer. We observed an increase in the relative DFT + vdW total energy among the azobenzene isomers (trans and cis) from 0.51 eV in the gas-phase...

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.

Characterization of the copper iodide hole-selective contact for silicon solar cell application

Hole-selective contacts have been extensively studied in recent years due to their low-cost by low temperature processing (<250 °C). The γ-copper (I) iodide (CuI) behaves as a p-type semiconductor with a zinc blende structure (cubic) and it has previously been studied as a hole-selective layer in organic solar cells, solid-state dye-sensitized solar cells. In this paper, CuI thin films with wide band gaps and large work functions were fabricated on n-type silicon substrates via a thermal evaporation technique. Changes to the structural, morphological and optical properties of CuI thin films based on different thicknesses (11 to 58 nm) were analyzed. X-ray diffraction patterns reveal that poly crystalline CuI thin films a have γ-phase with preferential growth in the (111) direction. Increases in the peak intensity for the (111) diffraction plane were observed in the thicker films. Generally, deposited CuI films exhibit triangular shapes with azimuthal orientation but these shapes disappeared as the film thickness was increased. Excellent electrical and optical properties were obtained at a CuI film thickness of 11 nm; measured values were 642 mV implied voltage, ~3.00 eV band gap energy, 5.68 eV work function value and 97.5% transmittance.

Internal Photoemission Spectroscopy Measurements of the Energy Barrier Heights between ALD Dielectrics and Ta-Based Amorphous Metals

High speed metal/insulator/metal (MIM) diodes show promise for rectenna based energy harvesting of IR radiation, for IR sensing, and as building blocks for beyond CMOS hot-electron (MIMIM) transistors. Operation of these devices is ideally based on Fowler-Nordheim tunneling, which is exponentially dependent both on electric field and the height of the energy barriers between the metal electrodes and the insulator. Smooth interfaces are thus necessary to control electric fields. The amorphous metal ZrCuAlNi (ZCAN) has been shown to function well as an ultra-smooth bottom electrode for MIM devices,1,2 but suffers from an interfacial oxide layer and thermal instability above ~300 °C. Recently, Ta based amorphous metals such as TaWSi and TaNiSi have been shown to have larger work functions and stability approaching 900 °C and above.3 Precise knowledge of metal/insulator barrier heights is critical for predicting, understanding, and optimizing MIM diode device operation. Although the Schottky-Mott model may be used to predict ideal barrier heights (φBn = ΦM - χi) from the metal vacuum work function (ΦM) and insulator electron affinity (χi), actual barrier heights depend strongly on deposition method and interfacial properties and often deviate substantially. Internal photoemission (IPE) spectroscopy may be used to directly measure barrier heights at buried interfaces. To date, there have been few reports of IPE measurements of MIM structures and none on these amorphous metals. In this work, IPE is utilized to measure the metal/insulator barrier heights in MIM stacks with Ta-based amorphous metal bottom electrodes with TaN as a reference. Results are compared with current-voltage measurements. TaWSi and TaNiSi bottom electrodes were sputtered onto 100 nm of thermally grown SiO2 on silicon followed by deposition of various high-k insulators via atomic layer deposition. Al and Au dots were evaporated through a shadow mask to serve as semitransparent (~10 nm thick) top gates. For IPE, current was monitored under a fixed applied field while photon energy was swept from 2 to 5 eV. A range of negative and positive applied fields were used to characterize both bottom and top interfaces. Barrier heights will be reported for all electrode/insulator combinations. Schottky plots of bias dependent barrier heights vs. square root of electric field for Al2O3 are shown in Fig. 1 for (a) positive bias (Au interface) and (b) negative bias (TaXN interface). A number of conclusions may be drawn from the results of this measurement array. First, metal/Al2O3 barrier heights are qualitatively consistent with the expected TaXN metal work function as determined as a weighted average of the constituent metals. Second, Au/insulator barrier heights are not affected by the bottom electrode. Third, the asymmetry in the current-voltage response (I(-V)/I(V)) is qualitatively consistent with the IPE determined barrier heights whereas ideal theory predictions are not always. TaWSi and TaNiSi electrodes showed consistently higher barrier heights than ZCAN electrodes and comparable performance to high-quality TaN, indicating promise as a thermally stable bottom electrode for MIM tunnel diodes. Support from NSF Center for Sustainable Materials Chemistry, CHE-1606982. 1 N. Alimardani, S.W. King, B.L. French, C. Tan, B.P. Lampert, and J.F. Conley Jr., J. Appl. Phys. 116, 24508 (2014). 2 N. Alimardani, E.W. Cowell, J.F. Wager, J.F. Conley Jr., D.R. Evans, M. Chin, S.J. Kilpatrick, and M. Dubey, J. Vac. Sci. Technol. A 30, 01A113 (2012). 3 J.M. McGlone, K.R. Olsen, W.F. Stickle, J.E. Abbott, R.A. Pugliese, G.S. Long, D.A. Keszler, and J.F. Wager, MRS Commun. 7, 715 (2017). Figure 1

Asymmetric All-Metal-Oxide Supercapacitor with Superb Cycle Performance.

Metal oxides have great potential for developing high-performance supercapacitors due to their high specific capacitances. However, achieving high energy densities while maintaining good rate capability and long cycle life has proved to be challenging. We propose herein a strategy for constructing all-metal-oxide asymmetric supercapacitors (ASCs), in which both the cathode and anode are based on metal oxides, and demonstrate their outstanding electrochemical performance. We anchored SnO2 nanoparticles on the surface of reduced graphene oxide (RGO) through Sn-O-C bonds (as the cathode of ACSs), and employed low-crystalline RGO/MoO3 nanosheets as the anode, based on the large work function difference between SnO2 and MoO3 . The resulting ASC can operate stably at 1.8 V in neutral aqueous electrolyte and deliver an energy density of up to 33 W h kg-1 , which remains at 13.8 W h kg-1 even at 37.5 kW kg-1 . Moreover, the ASC exhibits a good cycling stability of 92.5 % capacitance retention after 20 000 cycles.

Selective Chemical Epitaxial Growth of TiO2 Islands on Ferroelectric PbTiO3 Crystals to Boost Photocatalytic Activity

Summary Integrating a semiconducting light absorber with an appropriate co-catalyst appears almost indispensable for photocatalytic solar fuel generation. Although ferroelectric materials with spontaneous electrical polarization are considered promising light absorbers with the ability to induce oppositely directed transport of photogenerated electrons and holes in the bulk, their applications are intrinsically restricted by the large Schottky barrier at the interface of the ferroelectric material and the co-catalyst, which has a larger work function. Here, we demonstrate that, by selective chemical epitaxial growth of anatase TiO 2 islands on the positively poled (00-1) facet of PbTiO 3 single-crystal particles to form an atomically smooth interface with a small potential difference, the material shows significantly improved photocatalytic hydrogen and oxygen generation under both UV-visible and visible light, while the island-free PbTiO 3 is inactive in visible light. This strategy may be applicable to various ferroelectric materials to produce unusual asymmetric micro-nano structures for excellent performance.

Significant Effects of Electrode Metal Work Function on Resistive Memory Devices with Gelatin Biodielectric Layer

Biomaterials gelatin is used as the dielectric layer in this work to develop metal/gelatin/Tungsten (W) RRAM devices, with silver (Ag), copper (Cu), W, and gold (Au) as the top electrode (TE) to study the effects of electrode metals on device characteristics. All types of the RRAM devices exhibit bipolar resistive switching characteristics with diverse performances. The work function difference between the TE and bottom electrode (BE) metals is found to play a profound role in determining the characteristics of the devices and even reversing the set and reset processes. The devices with larger work function difference between the TE and BE metals have higher and more stable on/off resistance ratio. The Ag/gelatin/W devices exhibit a relatively stable switching behavior with an on/off resistance ratio of >102, while that with Au TE exhibits a reversed set and reset processes with a high on/off resistance ratio of >105 owing to the large positive work function difference between Au and W. Metallic conductive filaments are confirmed to be responsible for the switching behaviors of the devices experimentally, and what metal element in the conductive filaments is determined by the work function difference of the metals used for the top and bottom electrodes.

Significance of a Noble Metal Nanolayer on the UV and Visible Light Photocatalytic Activity of Anatase TiO2 Thin Films Grown from a Scalable PECVD/PVD Approach.

UV and visible light photocatalytic composite Pt and Au-TiO2 coatings have been deposited on silicon and glass substrates at low temperature using a hybrid ECWR-PECVD/MS-PVD process. Methylene blue, stearic acid, and sulfamethoxazole were used as dye, organic, and antibiotic model pollutants, respectively, to demonstrate the efficiency of these nanocomposite coatings for water decontamination or self-cleaning surfaces applications. Raman investigations revealed the formation of anatase polymorph of TiO2 in all synthesized coatings with a shifting of the main vibrational mode peak to higher wavenumber in the case of Au-TiO2 coating, indicating an increase number of crystalline defects within this coating. Because of the difference of the chemical potentials of each of the investigated noble metals, the sputtered metal layers exhibit different morphology. Pt sputtered atoms, with high surface adhesion, promote formation of a smooth 2D layer. On the other hand, Au sputtered atoms with higher cohesive forces promote the formation of 5-10 nm nanoparticles. As a result, the surface plasmon resonance phenomenon was observed in the Au-TiO2 coatings. UV photoactivity of the nanocomposite coatings was enhanced 1.5-3 times and 1.3 times for methylene blue and stearic acid, respectively, thanks to the enhancement of electron trapping in the noble metal layer. This electron trapping phenomenon is higher in the Pt-TiO2 coating because of its larger work function. On the other hand, the enhancement of the visible photoactivity was more pronounced (3 and 7 times for methylene blue and stearic acid, respectively) in the case of Au-TiO2 thanks to the surface plasmon resonance. Finally, these nanocomposite TiO2 coatings exhibited also a good ability for the degradation of antibiotics usually found in wastewater such as sulfamethoxazole. However, a complementary test have showed an increase of the toxicity of the liquid medium after photocatalysis, which could be due the presence of sulfamethoxazole's transformation byproducts.

Role of projectile energy and surface work function on charge transfer of negative ions grazing scattering on dissociated H2O-covered Cu(110)

Abstract Shifted Fermi sphere model and parallel velocity effects on clean metals have been verified by a number of previous studies. However, for fast F − and Cl − ions scattering on dissociated water covered Cu(110) at grazing incidence, the negative-ion fraction is projectile dependent and monotonically increases with the increase of impact velocity, which cannot be explained by the conventional parallel velocity effect. The positive-ion fraction increases with the increase of perpendicular velocity, which can be well described by the exponential scaling. Positive ions efficiently formed at large perpendicular velocities strongly suppress the negative ion yield at short distances from the surface, so that negative ions are only formed at large distances. In addition, the large surface work function further promotes a transition between two kinds of parallel velocity effects. The calculation taking into account these contributions is in rough agreement with the experimental data.

Simulation on effect of metal/graphene hybrid transparent electrode on characteristics of GaN light emitting diodes**Project supported by the National High-Technology Research and Development Program of China (Grant No. 2015AA034801), the Foundation of the State Key Laboratory of Mechanical Transmission of Chongqing University (Grant Nos. SKLMT-ZZKT-2017M15, SKLM-ZZKT-2015Z16, and SKLMT-KFKT-201419), the National Natural Science Foundation of China (Grant Nos. 11374359, 11304405, and

In order to decrease the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), some transparent thin films with good conductivity and large work function are essential to insert into Gr and p-GaN layers. In this work, the ultra-thin films of four metals (silver (Ag), golden (Au), nickel (Ni), platinum (Pt)) are explored to introduce as a bridge layer into Gr and p-GaN, respectively. The effect of a different combination of Gr/metal transparent conductive layers (TCLs) on the electrical, optical, and thermal characteristics of LED was investigated by the finite element methods. It is found that both the TCLs transmittance and the surface temperature of the LED chip reduces with the increase of the metal thickness, and the transmittance decreases to about 80% with the metal thickness increasing to 2 nm. The surface temperature distribution, operation voltage, and optical output power of the LED chips with different metal/Gr combination were calculated and analyzed. Based on the electrical, optical, and thermal performance of LEDs, it is found that 1.5-nm Ag or Ni or Pt, but 1-nm Au combined with 3 layered (L) Gr is the optimal Gr/metal hybrid transparent and current spreading electrode for ultra-violet (UV) or near-UV LEDs.

A Low-Cost NiO Hole Transfer Layer for Ohmic Back Contact to Cu2 O for Photoelectrochemical Water Splitting.

Cuprous oxide (Cu2 O) photocathode is reported as a promising candidate for photoelectrochemical water splitting. The p-type Cu2 O usually forms a Schottky junction with the conductive substrate due to its large work function, which blocks the collection of photogenerated holes. NiO is considered as one of the most promising hole transfer layers (HTL) for its high hole mobility, good stability, and easy processability to form a film by spin coating. The utilization of NiO HTL to form an Ohmic back contact to Cu2 O is described, thus achieving a positive onset potential of 0.61 V versus reversible hydrogen electrode and a twofold increase of solar conversion efficiency.

A flexible asymmetric fibered-supercapacitor based on unique Co3O4@PPy core-shell nanorod arrays electrode

Flexible and asymmetric fibered-supercapacitor (AFS) has recently attracted increasing interests, due to versatilities of the device design and its potential applications in wearable electronics. However, its relatively poor performances are still limited practical applications. Herein, due to high capacitance resulting form the redox character of the unique Co3O4@PPy core-shell nanorod arrays, an AFS is fabricated using these materials as the positive electrode and active carbon fiber (ACF) as the negative electrode. Especially, the large work function difference between Co3O4@PPy core-shell nanorod and ACF help the AFS to exhibit a wide potential window of 1.5V and a high areal capacitance of 1.02Fcm−2. As a result, the AFS exhibits excellent supercapacitive performances with a high energy density of 1.151mWhcm−2 at a power density of 1.15mWcm−2 and a perfect stability after 10,000 cycles at a current density of 50mAcm−2, meanwhile, the AFS shows good flexibility and could withstand the bending test. The remarkable electrochemical and flexible properties of AFS can be extended to the fabrication of flexible wearable micro energy storage devices.

Triggering surface ferroelectric order in Pb(Zr,Ti)O3(001) by deposition of platinum

By platinum deposition on a 150nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface.

Modification of the Highly Conductive PEDOT:PSS Layer for Use in Silver Nanogrid Electrodes for Flexible Inverted Polymer Solar Cells.

Silver nanogrid based flexible transparent electrode is recognized as the most promising alternative to ITO electrode for organic electronics, owing to its low production cost and excellent flexibility. Typically, a highly conductive thin film coating layer, such as highly conductive PEDOT:PSS (HC-PEDOT:PSS) is usually deposited onto the Ag-grid electrode to smooth the surface and to minimize the sheet resistance. In this paper, we found that inverted flexible polymer solar cells with structure of Ag-grid/HC-PEDOT:PSS/ZnO/photoactive layer/MoO3/Al generally exhibits strong S-shaped J-V curves, which could be eliminated by light-soaking treatment. Kelvin probe force microscope (KPFM) measurement proved that a large work function (WF) difference (0.70 eV) between HC-PEDOT:PSS and ZnO is the main reason for the formation of S-shape. White light soaking of the Ag-grid/HC-PEDOT:PSS gradually decreased the WF of HC-PEDOT:PSS from 5.10 to 4.60 eV, leading to a reduced WF difference between HC-PEDOT:PSS and ZnO from 0.70 to 0.38 eV. Such a WF difference decrease was believed to be the working mechanism for the light-soaking effect in this flexible device. Based on this finding, the HC-PEDOT:PSS solution was then modified by doping with polyethylenimine (PEI) and aqueous ammonia. The modified PEDOT:PSS film is characteristic of adjusting WF through varying PEI doping concentrations. By using such a modified PEDOT:PSS layer, light-soaking-free flexible inverted polymer solar cell with a power conversion efficiency of 6.58% was achieved for PTB7-Th:PC71BM cells. The current work provides a useful guideline for interfacial modification for Ag-grid based flexible electrode.