Interfacial Dipole Layer Research Articles

Interface analysis and intrinsic thermal stability of MoOx based hole-selective contacts for silicon heterojunction solar cells

A possible research path to increase the photo-generated current in silicon heterojunction (SHJ) solar cells is to replace doped layers on the front-side of the cell, which result in significant parasitic light absorption losses. MoOx is one candidate to replace the p-doped a-Si:H layer in such devices, although it is claimed to be relatively unstable to thermal treatments. We found that a MoOx film with a thickness of 6 nm is sufficient to achieve a JSC of 36 mA/cm2, which is 0.5 mA/cm2 on average higher than that of our classical SHJ reference cell. We also established a contact sintering condition for printed Ag at 160 °C after MoOx deposition, without degrading the cell performance. The champion MoOx-contacted cell yielded VOC of 724 mV and FF of 74.1%, resulting in an efficiency of 19.3%. From a detailed analysis of the interfaces of the hole contact, an interfacial a-SiOx of 1.6–2 nm was observed between a-Si:H and MoOx irrespective of the MoOx thickness (6–10 nm) before and after contact sinter annealing at 160 °C. We postulate that this a-SiOx layer acts as an interfacial dipole layer and also increases the contact resistivity at this contact. The intrinsic stability of the optimised MoOx-contacted cell is studied using a one-cell mini-module under standard damp-heat testing (85 °C/85% humidity/1000 h). More than 97 %rel of the original efficiency is maintained after 1010 h of testing, which is comparable to the behavior observed in a classical SHJ reference one-cell mini-module that was similarly tested.

Energy level alignment and interfacial dipole layer formation at the P3HT:PCBM-Electrolyte interface in organic photoelectrochemical cells

The influence of PCBM on the electronic energy levels of P3HT:PCBM photocathodes was investigated in a non-aqueous electrolyte using a benzoquinone redox couple. The presence of PCBM introduces a density of interface states at the semiconductor-electrolyte interface, consequently leading to the formation of an interfacial dipole layer and decrease in the built-in potential developed at the semiconductor-electrolyte interface. Photoelectrochemical measurements reveal a decrease in photovoltage from 0.26 V to 0.16 V and cathodic shift in onset potential from -0.27 V to -0.34 V (vs. Ag/Ag+) as PCBM content of the P3HT:PCBM photocathode is increased from 0 through 50 wt%. Additionally, depletion layer widths decreased from 90 nm to 50 nm. When the thickness of the active layer was controlled to coincide with the depletion layer thickness (e.g., 90 nm), photocurrent densities at pristine P3HT photocathodes were found to be higher than for P3HT:PCBM photocathodes. This suggests that the built-in electric field within the active layer is seemingly capable of effective exciton dissociation in the absence of a fullerene acceptor.

Influence of interface dipole layers on the performance of graphene field effect transistors

The linear band dispersion of graphene's bands near the Fermi level gives rise to its unique electronic properties, such as a giant carrier mobility, and this has triggered extensive research in applications, such as graphene field-effect transistors (GFETs). However, GFETs generally exhibit a device performance much inferior compared to the expected one. This has been attributed to a strong dependence of the electronic properties of graphene on the surrounding interfaces. Here we study the interface between a graphene channel and SiO2, and by means of photoelectron spectromicroscopy achieve a detailed determination of the course of band alignment at the interface. Our results show that the electronic properties of graphene are modulated by a hydrophilic SiO2 surface, but not by a hydrophobic one. By combining photoelectron spectromicroscopy with GFET transport property characterization, we demonstrate that the presence of electrical dipoles in the interface, which reflects the SiO2 surface electrochemistry, determines the GFET device performance. A hysteresis in the resistance vs. gate voltage as a function of polarity is ascribed to a reversal of the dipole layer by the gate voltage. These data pave the way for GFET device optimization.

Polarized Ferroelectric Polymers for High-Performance Perovskite Solar Cells.

In hybrid organic-inorganic lead halide perovskite solar cells, the energy loss is strongly associated with nonradiative recombination in the perovskite layer and at the cell interfaces. Here, a simple but effective strategy is developed to improve the cell performance of perovskite solar cells via the combination of internal doping by a ferroelectric polymer and external control by an electric field. A group of polarized ferroelectric (PFE) polymers are doped into the methylammonium lead iodide (MAPbI3 ) layer and/or inserted between the perovskite and the hole-transporting layers to enhance the build-in field (BIF), improve the crystallization of MAPbI3 , and regulate the nonradiative recombination in perovskite solar cells. The PFE polymer-doped MAPbI3 shows an orderly arrangement of MA+ cations, resulting in a preferred growth orientation of polycrystalline perovskite films with reduced trap states. In addition, the BIF is enhanced by the widened depletion region in the device. As an interfacial dipole layer, the PFE polymer plays a critical role in increasing the BIF. This combined effect leads to a substantial reduction in voltage loss of 0.14 V due to the efficient suppression of nonradiative recombination. Consequently, the resulting perovskite solar cells present a power conversion efficiency of 21.38% with a high open-circuit voltage of 1.14 V.

Polymer Electrolyte Blend Gate Dielectrics for High-Performance Ultrathin Organic Transistors: Toward Favorable Polymer Blend Miscibility and Reliability.

We report on systematic mobility enhancements in electrolyte-gated organic field-effect transistors (OFETs) by thinning down the active layer and exploiting polymer solid-state electrolyte gate insulators (SEGIs). The SEGI is composed of homogeneous poly(vinylidene fluoride- co-hexafluoropropylene) [P(VDF-HFP)] polymer solution-ion gel blends of high areal capacitance of >10 μF cm-2 at 1 Hz. By scaling up the poly(3-hexylthiophene) (P3HT) semiconducting layer by 1 order of magnitude (5-50 nm), an ultraviolet photoelectron spectroscopy examination reveals a downward vacuum-level shift generating a substantial hole injection barrier that originates from different interfacial dipole layer formations. The ultrathin (5.1 nm) P3HT FETs outperformed the other devices, exhibiting stable device characteristics with a highest field-effect mobility of >2 cm2 V-1 s-1 (effective mobility of 0.83 ± 0.05 cm2 V-1 s-1), on/off ratio of ∼106, low threshold voltage of <-0.6 V, and low gate-leakage current levels of ∼105 below the on-current levels in 10 μm channel length devices. We observed a positive threshold voltage shift in the P3HT/SEGI FETs with decreasing semiconductor thickness. The aforementioned mobility is at least 10 times greater than that of neat P(VDF-HFP) devices. The significant FET performance is attributed to a better insulator/semiconductor interface, efficient hole injection from the Au electrode resulting in a low contact resistance of <500 Ω cm, and boosted charge-carrier densities in the transistor channel. This work demonstrates an excellent approach for carrier mobility enhancement and reliability assessment in low-voltage-operated electrolyte-gated OFETs.

Anomalous temperature dependence of Al2O3/SiO2 and Y2O3/SiO2 interface dipole layer strengths

The temperature dependences of the dipole layer strength at Al2O3/SiO2 and Y2O3/SiO2 interfaces were investigated. The dipole layer strength at each temperature was quantified from the flatband voltage (Vfb) shift of metal-oxide-semiconductor capacitors with Al2O3/SiO2 and Y2O3/SiO2 bilayer gate dielectrics on Si substrates. In order to accurately extract the dipole layer strength, the effects of the difference between the gate metal work function and the Si Fermi level, the fixed charges at the SiO2/Si interface, and the fixed charges at the high-k/SiO2 interface on the Vfb shift were excluded. It was found that both interface dipole layer strengths increased but in the opposite direction at the approximated rates of +2.2 mV K−1 and −0.7 mV K−1 for Al2O3/SiO2 and Y2O3/SiO2 interfaces, respectively, when the temperature was raised from 100 K to 300 K. The temperature dependences are larger than expected if only the thermal expansion of the oxides was considered as the factor which changes the effective dipole length of the dipole layers.

Origin of increased efficiency and decreased hysteresis of perovskite solar cells by using 4-tert-butyl pyridine as interfacial modifier for TiO2

With close to 100% internal quantum efficiency over the visible light region, photocurrents in perovskite solar cells (PSCs) are at their practical limits. Achieving high open-circuit voltage (Voc) and fill factor (FF) are imperative to further improve the power conversion efficiency (PCE) of PSCs. Here, we demonstrate that Voc and FF of PSCs can be significantly improved from 1.03 to 1.09 V and from 0.71 to 0.75, respectively, by modifying TiO2 with 4-tert-butyl pyridine (t-BP), resulting in 9% improvement in PCE from 16.14% to 17.58% for the best performing devices. To illuminate origin of increased PCE by t-BP modification, UPS, PL and EIS are performed. The results reveal that t-BP modification improves efficiency through two separated mechanisms. First, t-BP interfacial dipole layer can tune Fermi energy level and work function of TiO2, thus increasing built-in potential and improving band alignment at TiO2/CH3NH3PbI3 interface. Second, reduced defect states of TiO2 by specific binding of t-BP at defect sites, resulting in reduction of charge recombination are found to be another mechanism for increased efficiency. Meanwhile, the lower defect states on TiO2 further make contribution to reduce hysteresis of PSCs. These results will pave the way for high efficiency and low hysteresis PSCs.

Strontium Fluoride and Zinc Oxide Stacked Structure as an Interlayer in High-Performance Inverted Polymer Solar Cells.

Enhanced power conversion efficiency is reported in inverted polymer solar cells when an ultrathin layer of strontium fluoride (SrF2) is evaporated on the surface of the solution-processed zinc oxide (ZnO) electron transport layer. The photoactive layer is made up of bulk heterojunction composites of poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b']dithiopheneco-3-fluorothieno[3,4- b]-thiophene-2-carboxylate] and [6,6]-phenyl-C71-butyric acid methyl ester. The ZnO film acts as an effective electron transport layer, whereas the ultrathin SrF2 layer improves the energy level alignment and enhances the built-in potential via the formation of an interfacial dipole layer at the interfaces between the ZnO film and the photoactive layer, resulting in an enhanced electron extraction efficiency and a decreased carrier recombination loss. Furthermore, the SrF2 layer reduces the inherent incompatibility between the hydrophilic ZnO film and the hydrophobic photoactive layer. As a result, all the photovoltaic performance parameters are remarkably improved, leading to a high efficiency of up to 10.46% (with a fill factor of 71.38%), corresponding to a ca. 21% improvement over the reference device performance (8.64%). The use of a ZnO/SrF2 stacked interlayer provides a simple, but effective, approach to obtain high-efficiency inverted PSCs.

Catalytic mechanism of graphene-nickel interface dipole layer for binder free electrochemical sensor applications

The combination of graphene with metal nanoparticles can produce enhanced catalytic properties because of synergistic effects, and has been used to develop highly active catalysts for different applications. However, the mechanism of the synergistic effect between graphene and metal is poorly understood. Here we demonstrate that graphene-coated nickel foam shows a significant catalytic effect on electrodeless metal (gold, platinum, silver, and copper) deposition without any external reducing agent. This is attributed to the formation of an interface dipole layer, induced by the interaction between graphene and nickel. The interface dipole layer catalytic mechanism accelerates metal reduction reaction and explains the simultaneous formation of nickel hydroxide. The nickel hydroxide-wrapped silver hybrid self-assembly developed on the graphene-coated nickel foam serves as an efficient binder-free electrochemical sensor owing to its hierarchical structure.

2-Methyl-9,10-bis(naphthalen-2-yl)anthracene doped rubidium carbonate as an effective electron injecting interlayer on indium-tin oxide cathode in inverted bottom-emission organic light-emitting diodes

2-Methyl-9,10-bis(naphthalen-2-yl)anthracene doped rubidium carbonate (MADN:Rb2CO3) is used as an effective electron injecting interlayer on an indium-tin oxide (ITO) cathode for inverted bottom-emission organic light-emitting diodes (IBOLEDs). At a Rb2CO3 doping concentration of 20% in MADN, the device exhibits enhanced characteristics, some of which are turn-on voltage, luminance at a given current density, and current efficiency. The attained performance is better than that of the device using lithium fluoride (LiF) as an n-type dopant. Space-charge-limited current acknowledges improved electrical properties of Rb2CO3 doped MADN. Ultraviolet and X-ray photoelectron spectroscopy investigation unveils an interfacial dipole layer induced by charge transfer between Rb2CO3 and ITO, leading to a lowered ITO work function and an electron injection barrier. The improved electron injection and transport capabilities contribute to better charge balance in IBOLED, thus resulting in advanced luminance efficiency. In addition, the morphology stability of organic films is also amended, which benefits long-term reliability under operationally induced thermal stress. Moreover, the effectiveness of using Rb2CO3:MADN as an electron injecting layer for IBOLEDs is superior to many of its alkali-based counterparts demonstrated in the literature, with high compatibility with different types of sophisticated ITO-based IBOLEDs.

Control of dipole properties in high-k and SiO2 stacks on Si substrates with tricolor superstructure

The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (σ) to that with lower σ. When the two high-k materials are selected with the order of σHK1 &amp;gt; σSiO2 &amp;gt; σHK2 in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of σAl2O3 &amp;gt; σSiO2 &amp;gt; σY2O3. The two stacking sequence samples composed of SiO2/Y2O3/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems.

Demonstration of Large Flatband Voltage Shift by Designing Al2O3/SiO2 Laminated Structures with Multiple Interface Dipole Layers

The presence of the dipole layer often results in a flatband voltage (VFB) shift in metal/high-k/SiO2/Si gate stacks typically in the order of a few hundreds of mV [1]. Enlargement of the attainable range of VFB shift caused by those interface dipole layers would be advantageous in some applications including power MOSFETs, which need high VTH for the stable operation of the devices. However, the amount of positive VFB shift attainable by a single interface dipole layer formation is limited within ~0.6 V using Al2O3-on-SiO2 stacks. One of the simple solutions to overcome this limitation would be adding up the VFB shifts induced by multiple interface dipole layers in a laminated dielectric stack. In this study we investigated the VFB shifts of MOS capacitors employing laminated stacks with Al2O3 and SiO2 layers ((Al2O3/SiO2)n) as the gate dielectric [2], to demonstrate a possible design of a stack with a large positive shift of VFB (>1 V).Silicon wafers with a resistivity of 1-5 Ωcm were thermally oxidized to grow 5 nm-thick-SiO2 followed by the fabrication of three kinds of samples (i)-(iii). The sample (i) is Au/Al2O3(2-8 nm)/SiO2/Si stack obtained by Al2O3 deposition by rf-sputtering on thermally grown oxide. The sample (ii) is Au/SiO2(2-7 nm)/Al2O3(3.0 nm)/SiO2/Si stacks, prepared by a SiO2 growth by the oxidation of metallic-Si with different thicknesses on the identical structure with the sample (i), to investigate the effects of the dipole layer formation by the deposition of top-SiO2. The metallic-Si was deposited by electron beam evaporation method followed by the oxidation conducted at 800°C in O2 ambient. The sample (iii) is Au/Al2O3(2-6 nm)/SiO2(3.0 nm)/Al2O3(3.0 nm)/SiO2/Si stack fabricated by the deposition of Al2O3 on the identical structure with the sample (ii), to evaluate the effects of dipole layer formation at top-Al2O3-on-SiO2 interface. The VFB shift caused by each dipole layer was determined from capacitance-voltage characteristics by excluding the effect of fixed charges. From the first-Al2O3 thickness-dependence of VFB for Al2O3/SiO2/Si structure of sample (i), it was found that a dipole layer inducing +0.47±0.05 V VFB shift was formed. For SiO2/Al2O3/SiO2/Si structure of sample (ii), one might expect a formation of the dipole layer at the top SiO2-on-Al2O3 interface should induce VFB shift with the same magnitude but the opposite direction to the one observed for the sample (i), because both are the interfaces between Al2O3 and SiO2 but with the opposite stacking sequences. However, the experimentally observed VFB shift due to dipole layer at the top SiO2-on-Al2O3 was determined to be only 0~+0.2V in our experimental condition. We speculate that the dipole effects for this interface would be suppressed because the thermal oxidation process of metallic-Si on Al2O3 results in a reduction of the stress at this interface, taking account of the model [1] that the dipole layer would be triggered by structural relaxation by an adjustment of oxygen density at the interface. For Al2O3/SiO2/Al2O3/SiO2/Si structure of sample (iii), the VFB shift caused by the dipole layer at the top Al2O3-on-SiO2 interface was estimated as approximately +0.8±0.2 V to the positive direction. As a result, a large VFB shift ~+1.2 V in total was demonstrated for this stack due to the additivity of multiple dipole layers in the laminated stack [2], thanks to the selective formation of dipole layers at Al2O3-on-SiO2 interfaces and the effective suppression of those at SiO2-on-Al2O3 interfaces. From these results we can conclude that by designing the number of laminated layers and tuning the stacking conditions, a very large VFB shift (>1V) is attainable with ((Al2O3/SiO2)n) laminated dielectric layers.[1] K. Kita and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009).[2] H. Kamata and K. Kita, Appl. Phys. Lett. 110, 102106 (2017).

(Invited) Molecular Dynamics of Dipole Layer Formation at High-k/SiO2 Interface

The metal/high-k gate stack added new degrees of freedom for controlling the threshold voltage (VTH) of CMOS devices. The VTH is altered depending on the materials of high-k oxide, which is attributed to an electric dipole layer formed at the interface between high-k oxide and underling interfacial SiO2 layer [1]. Several mechanisms were proposed to explain the origin of the interface dipole layer, but a unified view has not yet been reached. The proposed mechanisms is roughly classified into two groups: (1) electronic redistribution around the interface and (2) redistribution of ions across the interface. The second one, proposed by Kita and Toriumi [2] has gained popular support because of its applicability to a wide range of high-k materials. The model states that the interface dipole is formed by the movement of negatively charged O ions from higher oxygen-density side to a lower side. This model is referred to as “oxygen-density difference accommodation,” hereafter. The picture of oxygen-density difference accommodation model was well reproduced in our MD simulation (Fig.1) of Al2O3/SiO2 system [3-5], which employs a simple two-body ionic interaction model of the Born-Mayer-Huggins potential. A dipole layer is formed by the migration of O ions from the Al2O3 side to the SiO2 side (Fig.2). The direction and magnitude of the dipole coincides well with experimental results. The driving force of the O- ions migration is found to be the short-range repulsion between ionic cores. The repulsive force becomes larger in the higher oxygen density materials, so that the O- ions is pushed into the lower oxygen density side (Fig.3). In my previous talk in this conference held in 2014 [4], I discussed that an imbalance of multipole potentials between both oxide layer is a possible origin of the O- ion migration. However, the subsequent analysis [6] clarified that the dominant force acting on O- ions in near the interface is not originated from the charge-to-charge interaction, so that my previous hypothesis was denied nowadays. Furthermore, the author’s group succeeded in reproducing the direction of dipoles at MgO/SiO2 and SrO/SiO2 interfaces (Fig.2) [5], which is opposite to that of the Al2O3/SiO2 system. The MgO/SiO2 system is an irregular case of the oxygen-density difference accommodation model, because the oxygen density of MgO is higher than that of SiO2, but the experimentally observed dipole is opposed to the direction of the density gradient of O ions. In spite of the fact, our MD simulation can correctly reproduce the direction of the dipole at the MgO/SiO2 interface. The dipoles at the MgO/SiO2 and SrO/SiO2 interfaces are formed by a preferential migration of metal cations from the high-k oxide toward the SiO2 layer, and silicate layers are formed at these interfaces. In conclusion, the dipole formation at high-k/SiO2 interfaces can be explained by the ion redistribution model, as shown in Fig.4. The negative dipoles at the MgO/SiO2 and SrO/SiO2 interfaces are formed by a preferential migration of metal cations from the high-k oxide toward the SiO2 layer during the formation of a stable silicate phase. Thus, the migration of metal cations as well as the migration of oxygen ions must be taken into account when considering the mechanics of the dipole layer formation. Acknowledgement This work was supported by JST-CREST and a Grant-in-Aid for Scientific Research (B) (15H03979) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Demonstration of Large Flatband Voltage Shift by Designing Al2O3/SiO2 Laminated Structures with Multiple Interface Dipole Layers

We studied the dipole induced flatband voltage (VFB) shifts of Si MOS capacitors with Al2O3/SiO2/Al2O3/SiO2/Si laminated stacks ((Al2O3/SiO2) n /Si, n=2) designed for a large positive shift of VFB. The VFB shift caused by each dipole layer was determined from capacitance-voltage characteristics by excluding the effect of fixed charges. Due to the additivity of multiple dipole layers in the laminated stack, a large VFB shift (>1 V) was observed. In our experimental condition, the dipole layers at Al2O3-on-SiO2 interfaces were selectively formed, while those at SiO2-on-Al2O3 interfaces were effectively suppressed. The employed SiO2 deposition process was suggested to be one of the key parameters to determine the formation or suppression of interface dipole layer at SiO2-on-Al2O3 interface.

N-Type Superconductivity in an Organic Mott Insulator Induced by Light-Driven Electron-Doping.

The presence of interface dipoles in self-assembled monolayers (SAMs) gives rise to electric-field effects at the device interfaces. SAMs of spiropyran derivatives can be used as photoactive interface dipole layer in field-effect transistors because the photochromism of spiropyrans involves a large dipole moment switching. Recently, light-induced p-type superconductivity in an organic Mott insulator, κ-(BEDT-TTF)2 Cu[N(CN)2 ]Br (κ-Br: BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) has been realized, thanks to the hole carriers induced by significant interface dipole variation in the spiropyran-SAM. This report explores the converse situation by designing a new type of spiropyran monolayer in which light-induced electron-doping into κ-Br and accompanying n-type superconducting transition have been observed. These results open new possibilities for novel electronics utilizing a photoactive SAMs, which can design not only the magnitude but also the direction of photoinduced electric-fields at the device interfaces.

Improved hydrogenated amorphous silicon thin-film solar cells realized by replacing n-type Si layer with PFN interfacial layer

Improvement in the device performance of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells (TFSCs) without hazardous doping gases and complex processes has been a long-standing aim for many researchers. In this work, we replaced the n-type Si layer in an a-Si:H TFSC with an interfacial dipole layer of conjugated polymer electrolyte material, poly [(9,9-bis(3′-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (PFN), while keeping the conventional layer scheme. The addition of PFN eliminated the process complexity, improved the device performance, and generated a built-in potential (Vbi) across the p-type Si layer. The power conversion efficiency of the optimized device reached a maximum of 7.17%, which is significant when using a toxicant-free layer. The open-circuit voltage was improved to 0.80V from 0.47V in comparison to a reference a-Si:H TFSC without PFN, and the stability in light and dark conditions were greatly enhanced. The fill factor was increased from 0.45 to 0.59 because of the enhancement in shunt/series resistance. The improvement in device performance is mainly due to the creation of an interfacial dipole by the PFN layer, which generated the Vbi across the p-type Si layer, decreased the potential barrier between the i-Si layer and aluminum cathode, and consequently reduced the defects resulting from the coating of the i-Si layer and enhanced electron extraction.

Heterogeneous Pyrolysis: A Route for Epitaxial Growth of hBN Atomic Layers on Copper Using Separate Boron and Nitrogen Precursors.

Growth of hBN on metal substrates is often performed via chemical vapor deposition from a single precursor (e.g., borazine) and results in hBN monolayers limited by the substrates catalyzing effect. Departing from this paradigm, we demonstrate close control over the growth of mono-, bi-, and trilayers of hBN on copper using triethylborane and ammonia as independent sources of boron and nitrogen. Using density functional theory (DFT) calculations and reactive force field molecular dynamics, we show that the key factor enabling the growth beyond the first layer is the activation of ammonia through heterogeneous pyrolysis with boron-based radicals at the surface. The hBN layers grown are in registry with each other and assume a perfect or near perfect epitaxial relation with the substrate. From atomic force microscopy (AFM) characterization, we observe a moiré superstructure in the first hBN layer with an apparent height modulation and lateral periodicity of ∼10 nm. While this is unexpected given that the moiré pattern of hBN/Cu(111) does not have a significant morphological corrugation, our DFT calculations reveal a spatially modulated interface dipole layer which determines the unusual AFM response. These findings have improved our understanding of the mechanisms involved in growth of hBN and may help generate new growth methods for applications in which control over the number of layers and their alignment is crucial (such as tunneling barriers, ultrathin capacitors, and graphene-based devices).

Design of Al2O3/SiO2 laminated stacks with multiple interface dipole layers to achieve large flatband voltage shifts of MOS capacitors

We studied the dipole induced flatband voltage (VFB) shifts of Si MOS capacitors with Al2O3/SiO2/Al2O3/SiO2/Si laminated stacks ((Al2O3/SiO2)n/Si, n = 2) designed for a large positive shift of VFB. The VFB shift caused by each dipole layer was determined from capacitance-voltage characteristics by excluding the effect of fixed charges. Due to the additivity of multiple dipole layers in the laminated stack, a large VFB shift (&amp;gt;1 V) was observed. In our experimental condition, the dipole layers at Al2O3-on-SiO2 interfaces were selectively formed, while those at SiO2-on-Al2O3 interfaces were effectively suppressed. The validity of such additivity of VFB shifts induced by selectively formed dipole layers was also experimentally demonstrated for n ≥ 3 laminated stacks. An introduction of multiple dipole layers is applicable for a threshold voltage tuning in a wider range than the tuning with a single dipole layer.

&gt;10% Efficiency Polymer:Fullerene Solar Cells with Polyacetylene‐Based Polyelectrolyte Interlayers

Polymer solar cells have gained great attention due to their tremendous potential for applications in light‐weight, large‐area, and flexible photovoltaic modules fabricated via continuous roll‐to‐roll processes. Despite the significant progress, however, their efficiency and operating stability are still inadequate for commercial applications. Interfacial engineering of the electron‐collecting buffer layer and the organic photoactive layer through the use of organic dipole interlayers, has been proposed as a simple and scalable way to improve the overall solar cell performance. Here, highly efficient inverted polymer:fullerene solar cells have been successfully developed with a power conversion efficiency of over 10%. The bulk heterojunction layer consists of the poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b]dithiophene‐alt‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th) and the [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM), as the electron donor and electron acceptor, respectively. Key to this success is the insertion of the ionic polyacetylene‐based conjugated polymer, poly(N‐dodecyl‐2‐ethynylpyridinium bromide), as an interfacial dipole layer. The latter is shown to lower the work function of the electron transporting zinc oxide layer and increase the built‐in potential, consequently facilitating efficient charge transport/extraction. Optimized solar cells exhibit power conversion efficiency values exceeding 10% while their operating stability under continuous solar‐simulated illumination is significantly enhanced when ultraviolet light is effectively blocked using a suitable optical filter.

Interfacial Energy Alignment at the ITO/Ultra-Thin Electron Selective Dielectric Layer Interface and Its Effect on the Efficiency of Bulk-Heterojunction Organic Solar Cells.

We have investigated the photovoltaic properties of an inverted bulk heterojunction (BHJ) cell in a device with an indium-tin-oxide (ITO)/electron selective layer (ESL)/P3HT:PCBM active layer/MoOx/Ag multilayered structure. The insertion of only single layer of poly(diallyl-dimethyl-ammonium chloride) (PDDA) cationic polymer film (or poly(ethyleneimine) (PEI) polymeric interfacial dipole layer) and titanium oxide nanosheet (TN) films as an ESL effectively improved cell performance. Abnormal S-shaped curves were observed in the inverted BHJ cells owing to the contact resistance across the ITO/active layer interface and the ITO/PDDA/TN/active layer interface. The series resistance across the ITO/ESL interface in the inverted BHJ cell was successfully reduced using an interfacial layer with a positively charged surface potential with respect to ITO base electrode. The positive dipole in PEI and the electronic charge phenomena at the electrophoretic deposited TN (ED-TN) films on ITO contributed to the reduction of the contact resistance at the electrode interface. The surface potential measurement revealed that the energy alignment by the transfer of electronic charges from the ED-TN to the base electrodes. The insertion of the ESL with a large positive surface potential reduced the potential barrier for the electron injection at ITO/TN interface and it improved the photovoltaic properties of the inverted cell with an ITO/TN/active layer/MoOx/Ag structure.