Hole Mobility Enhancement Research Articles

The role of hydrogen plasma power on surface roughness and carrier transport in transfer-doped H-diamond

Surface transfer doping of diamond fundamentally requires termination of the diamond surface with a species such as hydrogen to allow the interfacial charge exchange required to establish surface conductivity. Here we show the effects of varied hydrogen plasma power on the roughness and conductivity of the (100) diamond surface. Prior to hydrogen termination, substrates were etched using tailored Cl2 + Ar and O2 + Ar chemistries to produce a very smooth surface of ~0.2 nm roughness average while also removing ~3.4 μm from the top surface as measured by Atomic Force Microscopy (AFM). Use of etching post polishing provides an effective means of producing smoother diamond surfaces with reduced crystal damage as opposed to scaife polishing alone. By producing nominally identical etched surfaces, a relationship between surface conductivity and hydrogen termination plasma power was observed. Using MoO3 as a surface acceptor material, Hall measurements were performed to examine sheet resistance, carrier density and mobility within the diamond. Increased surface conductivity due to enhanced hole mobility was observed at higher hydrogen plasma power conditions, despite an associated increase in roughness of the diamond surface.

Enhancement of photoluminescence and hole mobility in 1- to 5-layer InSe due to the top valence-band inversion: strain effect.

Recently, two-dimensional (2D) few-layer InSe nanosheets have become one of the most interesting materials due to their excellent electron transport, wide bandgap tunability and good metal contact. However, their low photoluminescence (PL) efficiency and hole mobility seriously restrict their application in 2D InSe-based nano-devices. Here, by exerting a suitable compressive strain, a remarkable modification for the electronic structure and the optical and transport properties of 1- to 5-layer InSe has been confirmed by powerful GW-BSE calculations. Both top valence band inversion and indirect-to-direct bandgap transition are induced; the light polarization is reversed from E||c to E⊥c; and the PL intensity and hole mobility are enhanced greatly. Surprisingly, under 6% compressive strain, the light emission of monolayer InSe with E⊥c is allowed at 2.58 eV, which has never been observed previously. Meanwhile, for the 2D few-layer InSe, the PL with E⊥c polarization increases over 10 times in intensity and has a blue-shift at about 0.6-0.7 eV, and the hole mobility increases two orders of magnitude up to 103 cm2 V-1 s-1, as high as electron mobility. The strained few-layer InSe are thus a promising candidate for future 2D electronic and optoelectronic nano-devices.

Contrasting Effects of Energy Transfer in Determining Efficiency Improvements in Ternary Polymer Solar Cells

AbstractCrystallizable, high‐mobility conjugated polymers have been employed as secondary donor materials in ternary polymer solar cells in order to improve device efficiency by broadening their spectral response range and enhancing charge dissociation and transport. Here, contrasting effects of two crystallizable polymers, namely, PffBT4T‐2OD and PDPP2TBT, in determining the efficiency improvements in PTB7‐Th:PC71BM host blends are demonstrated. A notable power conversion efficiency of 11% can be obtained by introducing 10% PffBT4T‐2OD (relative to PTB7‐Th), while the efficiency of PDPP2TBT‐incorporated ternary devices decreases dramatically despite an enhancement in hole mobility and light absorption. Blend morphology studies suggest that both PffBT4T‐2OD and PDPP2TBT are well dissolved within the host PTB7‐Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains. While negligible charge transfer is determined in binary blends of each polymer mixture, effective energy transfer is identified from PffBT4T‐2OD to PTB7‐Th that contributes to an improvement in ternary blend device efficiency. In contrast, energy transfer from PTB7‐Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC71BM.

Metal-Ion-Modified Black Phosphorus with Enhanced Stability and Transistor Performance.

Black phosphorus (BP), a burgeoning elemental 2D semiconductor, has aroused increasing scientific and technological interest, especially as a channel material in field-effect transistors (FETs). However, the intrinsic instability of BP causes practical concern and the transistor performance must also be improved. Here, the use of metal-ion modification to enhance both the stability and transistor performance of BP sheets is described. Ag+ spontaneously adsorbed on the BP surface via cation-π interactions passivates the lone-pair electrons of P thereby rendering BP more stable in air. Consequently, the Ag+ -modified BP FET shows greatly enhanced hole mobility from 796 to 1666 cm2 V-1 s-1 and ON/OFF ratio from 5.9 × 104 to 2.6 × 106 . The mechanisms pertaining to the enhanced stability and transistor performance are discussed and the strategy can be extended to other metal ions such as Fe3+ , Mg2+ , and Hg2+ . Such stable and high-performance BP transistors are crucial to electronic and optoelectronic devices. The stability and semiconducting properties of BP sheets can be enhanced tremendously by this novel strategy.

Effect of CdS nanocrystals on charge transport mechanism in poly(3-hexylthiophene)

The present manuscript demonstrates the optical and electrical characteristics of poly(3-hexylthiophene) (P3HT) and cadmium sulphide (CdS) hybrid nanocomposites. Optical results suggest that there is a formation of charge transfer complex (CTC) between host P3HT and guest CdS nanocrystals (NCs). Electrical properties of P3HT and P3HT-CdS thin films have been studied in hole only device configurations at different temperatures (290 K–150 K), and results were analysed by the space charge limited conduction mechanism. Density of traps and characteristic trap energy increase on incorporation of inorganic NCs in the polymer matrix, which might be due to the additional favourable energy states created by CdS NCs in the band gap of P3HT. These additional trap states assist charge carriers to move quicker which results in enhancement of hole mobility from 7 × 10−6 to 5.5 × 10−5 cm2/V s in nanocomposites. These results suggest that the P3HT-CdS hybrid system has desirable optical and electrical properties for its applications to photovoltaics devices.

Performance improvement of organic thin film transistors by using active layer with sandwich structure

We report organic thin film transistors (OTFTs) with pentacene/fluorinated copper phthalo-cyanine (F 16 CuPc)/pentacene (PFP) sandwich configuration as active layers. The sandwich devices not only show hole mobility enhancement but also present a well control about threshold voltage and off-state current. By investigating various characteristics, including current-voltage hysteresis, organic film morphology, capacitance-voltage curve and resistance variation of active layers carefully, it has been found the performance improvement is mainly attributed to the low carrier traps and the higher conductivity of the sandwich active layer due to the additional induced carriers in F 16 CuPc/pentacene. Therefore, using proper multiple active layer is an effective way to gain high performance OTFTs.

Thiophene Rings Improve the Device Performance of Conjugated Polymers in Polymer Solar Cells with Thick Active Layers

AbstractDeveloping novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large‐scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick‐film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor–acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron‐rich benzo[1,2‐b:4,5‐b′]dithiophene units and electron‐deficient 5,6‐difluorobenzo[2,1,3]thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation.

Three-Dimensional Morphology Control Yielding Enhanced Hole Mobility in Air-Processed Organic Photovoltaics: Demonstration with Grazing-Incidence Wide-Angle X-ray Scattering.

Polymer organic photovoltaic (OPV) device performance is defined by the three-dimensional morphology of the phase-separated domains in the active layer. Here, we determine the evolution of morphology through different stages of tailored solvent vapor and thermal annealing techniques in air-processed poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric acid methyl ester-based OPV blends. A comparative evaluation of the effect of solvent type used for vapor annealing was performed using grazing-incidence wide-angle X-ray scattering, atomic force microscopy, and UV-vis spectroscopy to probe the active-layer morphology. A nonhalogenated orthogonal solvent was found to impart controlled morphological features within the exciton diffusion length scales, enhanced absorbance, greater crystallinity, increased paracrystalline disorder, and improved charge-carrier mobility. Low-boiling, fast-diffusing isopropanol allowed the greatest control over the nanoscale structure of the solvents evaluated and yielded a cocontinuous morphology with narrowed domains and enhanced paths for the charge carrier to reach the anode.

Spiro‐Phenylpyrazole‐9,9′‐Thioxanthene Analogues as Hole‐Transporting Materials for Efficient Planar Perovskite Solar Cells

AbstractPerovskite solar cells have emerged as a promising technique for low‐cost, light weight, and highly efficient photovoltaics. However, they still largely rely on 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) to serve as hole‐transporting materials (HTMs). Here, a series of HTMs with small molecular weight is designed, which are constructed on a spiro core involving phenylpyrazole and a second heteroaromatics, i.e., xanthene (O atom), thioxanthene (S atom), and acridine (N atom). Through varying from phenylpyrazole substituted xanthene (PPyra‐XA), thioxanthene (PPyra‐TXA), to acridine (PPyra‐ACD), their optical and electrochemical properties, hole mobilities, and the photovoltaic performance are optimized. As a consequence, PPyra‐TXA based device exhibits the highest power conversion efficiency (PCE) of 18.06%, outperforming that of Spiro‐OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene. In addition, the dopant‐free device shows PCE of 11.7%. These results open a new direction for designing spiro‐HTMs by simple modification of chemical structures.

Selenium-Containing Medium Bandgap Copolymer for Bulk Heterojunction Polymer Solar Cells with High Efficiency of 9.8%

In this work, a new D–A copolymer based on m-alkoxyphenyl substituted benzodithiophene (BDT-m-OP) as donor unit and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) as acceptor unit was designed and synthesized, in which selenophene unit as π-conjugated spacer was incorporated into the polymer backbone to broaden the absorption spectrum, enhance the charge transport properties, and even improve the photovoltaic properties. Compared with PBPD-Th with thiophene as π-conjugated spacer, PBPD-Se exhibits an evidently extended absorption spectrum and an enhanced hole mobility with a slightly raised HOMO energy level. The PBPD-Se:PC71BM-based PSCs exhibits a significantly improved PCE of 9.8% with an enhanced Jsc of 14.9 mA cm–2 and a slightly lower Voc of 0.90 V in comparison with a PCE of 8.4% with a Voc of 0.95 V and a Jsc of 12.4 mA cm–2 for PBPD-Th:PC71BM-based devices. These results indicate that the rational selection of π-conjugated spacer in the D–A copolymer backbone is very essential to achieve high eff...

Monovalent Cation Doping of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Thick‐Film High‐Performance Bulk‐Heterojunction Solar Cells Retaining 90% PCEs of the Optimized Thin Film Cells

Most optimized, high performance, bulk‐heterojunction (BHJ) polymer solar cells have an active layer thickness of about 100 nm. The thin active layer is unfavorable for optical absorption and film coating. This contribution employs a ternary cell to address this problem. In this paper, thick BHJ cells can be fabricated that retain 90% power conversion efficiencies (PCEs) of the optimized thin‐film cells. The BHJs under investigations are PTB7:PC71BM, PTB7‐Th:PC71BM, and P3HT:PCBM. Into these BHJs, a ternary component, p‐DTS(fbtth2)2 (DTS) is introduced. Without DTS, the binary thick‐film devices (≈200, ≈200, and ≈400 nm) have PCEs of 6.3%, 7.4%, and 3.2%. With DTS, the corresponding BHJs have markedly improved PCEs of 7.6%, 8.3%, and 3.9%, respectively. The results are more than 90% the PCEs of the optimized binary BHJs. The origins of the improvement are investigated. Addition of the ternary component DTS enhances hole mobility and reduces trap states. Both observations are well correlated with improved fill factors of the ternary BHJ cells. Photothermal deflection spectroscopy and 1H nuclear magnetic resonance are used to trace the electronic and the nanoscale interactions of the DTS with the polymer and fullerene. The results suggest the DTS behaves as a conducting bridge in between two neighboring polymer segments.

First-principles investigation of metal-doped cubic BaTiO3

The electronic properties of metal-doped cubic BaTiO3 have been investigated using first-principles calculations. Through systematic analysis on effects of various metals doping through substituting Ti with the main group, 3d and 4d transitions metal atoms, we show that most transition metal doping can effectively reduce the band gap of BaTiO3. The partial density of states analysis indicates that different doping elements have different effects on the band-gap engineering. The effective mass of BaTiO3 before and after doping was further analyzed. And we found that the hole effective mass apparently decrease after Fe, Cu, Zn, Mo, and Ag doping, which could lead to increase of hole mobility and charge transfer enhancement. The calculation results showed that substituting Ti with Fe or Mo would be more effective for improving the photoelectric property of BaTiO3. Our theoretical calculations provide useful insights and beneficial guidance into experimental study of BaTiO3 in the photoelectric applications.

Readily synthesized dopant-free hole transport materials with phenol core for stabilized mixed perovskite solar cells

Abstract With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PVSCs), device lifetime has become one of the extensive research interests and concerns. To enhance the device durability, developing high performance dopant-free hole transport materials (HTMs) is a promising strategy. Herein, two new C3 -symmetric HTMs with phenol core, TCP-OH and TCP-OC 8 are readily prepared and show ultra-wide energy band-gap and excellent film-formation property. PCEs of 16.97% and 15.28% are achieved with pristine TCP-OH and TCP-OC 8 film as HTMs, respectively, even though their hole mobilities are as low as 10 −6 cm 2 V −1 s −1 . Phenol acts as hole trap in traditional concept, however, TCP-OH shows higher hole mobility than that of TCP-OC 8 . Moreover, TCP-OH shows higher glass transition temperature and better matching band alignment than those of TCP-OC 8 . Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs. Moreover, our study demonstrates an interesting viewpoint to design HTMs with the balance of hole mobility and electron blocking effect.

Charge transport in thin films of MDMO PPV dispersed with lead sulfide nanoparticles

Enhanced hole mobility is observed in thin films of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO PPV) dispersed with Lead Sulfide nanoparticles. Changes in the polymer film morphology upon the dispersion of nanoparticles and its influence on the enhancement of carrier mobility is investigated using Photoluminescence (PL) and Raman spectroscopic studies. PL and Raman studies show that at low concentrations of nanoparticles there is an enhancement in polymer interchain interaction. At higher concentrations, the interhain interaction decreases. Raman studies show that the observed enhancement of polymer interchain interaction at low nanoparticle concentration is due to the tighter packing of polymer chains. Thus, at low nanoparticle concentration, the observed increase in mobility is assigned to both the enhancement in interchain interaction and tunneling of carriers through nanoparticles. At higher nanoparticle concentrations the observed enhancement of mobility can be totally attributed to the tunneling of carriers through nanoparticles.

Enhancement of hole mobility in InSe monolayer via an InSe and black phosphorus heterostructure.

To enhance the low hole mobility (∼40 cm2 V-1 s-1) of InSe monolayer, a novel two-dimensional (2D) van der Waals heterostructure made of InSe and black phosphorus (BP) monolayers with high hole mobility (∼103 cm2 V-1 s-1) has been constructed and its structural and electronic properties are investigated using first-principles calculations. We find that the InSe/BP heterostructure exhibits a direct band gap of 1.39 eV and type-II band alignment with electrons (holes) located in the InSe (BP) layer. The band offsets of InSe and BP are 0.78 eV for the conduction band minimum and 0.86 eV for the valence band maximum, respectively. Surprisingly, the hole mobility in the InSe/BP heterostructure exceeds 104 cm2 V-1 s-1, which is one order of magnitude larger than the hole mobility of BP and three orders larger than that of the InSe monolayer. The electron mobility is also increased to 3 × 103 cm2 V-1 s-1. The physical reason has been analyzed deeply, and a universal method is proposed to improve the carrier mobility of 2D materials by forming heterostructures with them and other 2D materials with complementary properties. The InSe/BP heterostructure can thus be widely used in nanoscale InSe-based field-effect transistors, photodetectors and photovoltaic devices due to its type-II band alignment and high carrier mobility.

Enhancement of hole mobility in hybrid titanium dioxide/poly(3-hexylthiophene) nanocomposites by employing an oligothiophene dye as an interface modifier

Hole-mobility in nanocrystalline TiO2/P3HT composites is increased by over an order of magnitude when the TiO2 surface is treated with a 3-hexylthiophene oligomer bearing a cyanoacrylic acid group (4T).

Parallel Nanoshaping of Brittle Semiconductor Nanowires for Strained Electronics.

Semiconductor nanowires (SCNWs) provide a unique tunability of electro-optical property than their bulk counterparts (e.g., polycrystalline thin films) due to size effects. Nanoscale straining of SCNWs is desirable to enable new ways to tune the properties of SCNWs, such as electronic transport, band structure, and quantum properties. However, there are two bottlenecks to prevent the real applications of straining engineering of SCNWs: strainability and scalability. Unlike metallic nanowires which are highly flexible and mechanically robust for parallel shaping, SCNWs are brittle in nature and could easily break at strains slightly higher than their elastic limits. In addition, the ability to generate nanoshaping in large scale is limited with the current technologies, such as the straining of nanowires with sophisticated manipulators, nanocombing NWs with U-shaped trenches, or buckling NWs with prestretched elastic substrates, which are incompatible with semiconductor technology. Here we present a top-down fabrication methodology to achieve large scale nanoshaping of SCNWs in parallel with tunable elastic strains. This method utilizes nanosecond pulsed laser to generate shock pressure and conformably deform the SCNWs onto 3D-nanostructured silicon substrates in a scalable and ultrafast manner. A polymer dielectric nanolayer is integrated in the process for cushioning the high strain-rate deformation, suppressing the generation of dislocations or cracks, and providing self-preserving mechanism for elastic strain storage in SCNWs. The elastic strain limits have been studied as functions of laser intensity, dimensions of nanowires, and the geometry of nanomolds. As a result of 3D straining, the inhomogeneous elastic strains in GeNWs result in notable Raman peak shifts and broadening, which bring more tunability of the electrical-optical property in SCNWs than traditional strain engineering. We have achieved the first 3D nanostraining enhanced germanium field-effect transistors from GeNWs. Due to laser shock induced straining effect, a more than 2-fold hole mobility enhancement and a 120% transconductance enhancement are obtained from the fabricated back-gated field effect transistors. The presented nanoshaping of SCNWs provide new ways to manipulate nanomaterials with tunable electrical-optical properties and open up many opportunities for nanoelectronics, the nanoelectrical-mechanical system, and quantum devices.

High-Mobility P-Type MOSFETs with Integrated Strained-Si0.73Ge0.27 Channels and High-κ/Metal Gates**Supported by the National Basic Research Program of China under Grant No 2011CBA00605, and the National Natural Science Foundation of China under Grant No 61404165.

Strained-Si0.73Ge0.27 channels are successfully integrated with high-κ/metal gates in p-type metal-oxide- semiconductor field effect transistors (pMOSFETs) using the replacement post-gate process. A silicon cap and oxide inter layers are inserted between Si0.73Ge0.27 and high-κ dielectric to improve the interface. The fabricated Si0.73Ge0.27 pMOSFETs with gate length of 30 nm exhibit good performance with high drive current (∼428 μA/μm at VDD = 1 V) and suppressed short-channel effects (DIBL∼77 mV/V and SS∼90 mV/decade). It is found that the enhancement of effective hole mobility is up to 200% in long-gate-length Si0.73Ge0.27-channel pMOSFETs compared with the corresponding silicon transistors. The improvement of device performance is reduced due to strain relaxation as the gate length decreases, while 26% increase of the drive current is still obtained for 30-nm-gate-length Si0.73Ge0.27 devices.