Aluminum‐doped Zn1 − xMgxO as transparent conductive oxide of Cu(In,Ga)(S,Se)2‐based solar cell for minimizing surface carrier recombination

A 19.5%‐efficient Cu(In,Ga)(S,Se)₂ (CIGSSe)‐based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O buffer layers for increasing short‐circuit current density because band‐gap energies of Cd_0.75Zn_0.25S and Zn_0.79Mg_0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E _C) between Zn_1 − x Mg_x O:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg] + [Zn]), x . It is revealed that Zn_1 − x Mg_x O:Al films with [Mg]/([Mg] + [Zn]) in a range of 0.10 to 0.12, enhancing E _g from 3.72 to 3.76 eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn_0.88Mg_0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open‐circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E _C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency (η ) of CIGSSe solar cell with alternative Cd_0.75Zn_0.25S/Zn_0.79Mg_0.21O/Zn_0.88Mg_0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E _C alignment, which is higher η up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.     

Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells

Fabrication of Zn_1−xMg_xO films by atomic layer deposition (ALD) has been studied for use as buffer layers in Cu(In,Ga)Se₂ (CIGS)‐based solar cell devices. The Zn_1−xMg_xO films were grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in the temperature range from 105 to 180°C. Single‐phase ZnO‐like films were obtained for x < 0·2, followed by a two phase region of ZnO‐ and MgO‐like structures for higher Mg concentrations. Increasing optical band gaps of up to above 3·8 eV were obtained for Zn_1−xMg_xO with increasing x. It was found that the composition of the Zn_1−xMg_xO films varied as an effect of deposition temperature as well as by increasing the relative amount of magnesium precursor pulses during film growth. Completely Cd‐free CIGS‐based solar cells devices with ALD‐Zn_1−xMg_xO buffer layers were fabricated and showed efficiencies of up to 14·1%, which was higher than that of the CdS references. Copyright © 2006 John Wiley & Sons, Ltd.     

Thin‐film Cu(In,Ga)(Se,S)2‐based solar cell with (Cd,Zn)S buffer layer and Zn1−xMgxO window layer

(Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer were investigated to replace the traditional CdS buffer layer and ZnO window layer in Cu(In,Ga)(Se,S)₂ (CIGSSe)‐based solar cell. (Cd,Zn)S with band‐gap energy (E _g) of approximately 2.6 eV was prepared by chemical bath deposition, and Zn_1−x Mg_x O films with different [Mg]/([Mg] + [Zn]) ratios, x , were deposited by radio frequency magnetron co‐sputtering of ZnO and MgO. The estimated optical E _g of Zn_1−x Mg_x O films is linearly enhanced from 3.3 eV for pure ZnO (x  = 0) to 4.1 eV for Zn_0.6Mg_0.4O (x  = 0.4). The quality of the Zn_1−x Mg_x O films, implied by Urbach energy, is severely deteriorated when x is above 0.211. Moreover, the temperature‐dependent current density‐voltage characteristics of the CIGSSe solar cells were conducted for the investigation of the heterointerface recombination mechanism. The external quantum efficiency of the CIGSSe solar cell with the (Cd,Zn)S buffer layer/Zn_1−x Mg_x O window layer is improved in the wavelength range of 320–520 nm. Therefore, a gain in short‐circuit current density up to about 5.7% was obtained, which is higher conversion efficiency of up to around 5.4% relative as compared with the solar cell with the traditional CdS buffer layer/ZnO window layer. The peak efficiency of 19.6% was demonstrated in CIGSSe solar cell with (Cd,Zn)S buffer layer and Zn_1−x Mg_x O window layer, where x is optimized at 0.211. Copyright © 2017 John Wiley & Sons, Ltd.     

Temperature‐dependent current‐voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD‐Zn1‐xMgxO buffer layers

In this paper, lightsoaking and temperature‐dependent current‐voltage (JVT) measurements on Cu(In,Ga)Se₂ solar cells with atomic layer deposited Zn_1‐xMg_xO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT‐measurements to enable fitting of these to the one‐diode model. The most prominent effect of lightsoaking cells with Mg‐rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase V_oc·. The activation energy is extracted from JVT‐measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature‐dependence. A buffer layer consisting either of ZnO or Zn_1‐xMg_xO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling‐enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The J_sc values of the Zn_1‐xMg_xO cells surpass that of the reference due to the larger bandgap of Zn_1‐xMg_xO compared to CdS. Copyright © 2009 John Wiley & Sons, Ltd.     

Self Encapsulated Poly(3‐hexylthiophene)‐poly(fluorinated alkyl methacrylate) Rod‐Coil Block Copolymers with High Field Effect Mobilities on Bare SiO2

Conjugated rod‐coil block copolymers provide an interesting route towards enhancing the properties of the conjugated block due to self‐assembly and the interplay of rod‐rod and rod‐coil interactions. Here, we demonstrate the ability of an attached semi‐fluorinated block to significantly improve upon the charge carrier properties of regioregular poly(3‐hexyl thiophene) (rr‐P3HT) materials on bare SiO₂. The thin film hole mobilities on bare SiO₂ dielectric surfaces of poly (3‐hexyl thiophene)‐block‐polyfluoromethacrylates (P3HT‐b‐PFMAs) can approach up to 0.12 cm² V^?1 s^?1 with only 33 wt% of the P3HT block incorporated in the copolymer, as compared to rr‐P3HT alone which typically has mobilities averaging 0.03 cm² V^?1 s^?1. To our knowledge, this is the highest mobility reported in literature for block copolymers containing a P3HT. More importantly, these high hole mobilities are achieved without multistep OTS treatments, argon protection, or post‐annealing conditions. Grazing incidence wide‐angle x‐ray scattering (GIWAX) data revealed that in the P3HT‐b‐PFMA copolymers, the P3HT rod block self‐assembles into highly ordered lamellar structures, similar to that of the rr‐P3HT homopolymer. Grazing incidence small‐angle x‐ray scattering (GISAXS) data revealed that lamellar structures are only observed in perpendicular direction with short PFMA blocks, while lamellae in both perpendicular and parallel directions are observed in polymers with longer PFMA blocks. AFM, GIWAXS, and contact angle measurements also indicate that PFMA block assembles at the polymer thin film surface and forms an encapsulation layer. The high charge carrier mobilities and the hydrophobic surface of the block copolymer films clearly demonstrates the influence of the coil block segment on device performance by balancing the crystallization and microphase separation in the bulk morphological structure.     

Enhanced Electron Mobility Due to Dopant‐Defect Pairing in Conductive ZnMgO

The increase of the band gap in Zn_1‐xMg_xO alloys with added Mg facilitates tunable control of the conduction band alignment and the Fermi‐level position in oxide‐heterostructures. However, the maximal conductivity achievable by doping decreases considerably at higher Mg compositions, which limits practical application as a wide‐gap transparent conductive oxide. In this work, first‐principles calculations and material synthesis and characterization are combined to show that the leading cause of the conductivity decrease is the increased formation of acceptor‐like compensating intrinsic defects, such as zinc vacancies (V_Zn), which reduce the free electron concentration and decrease the mobility through ionized impurity scattering. Following the expectation that non‐equilibrium deposition techniques should create a more random distribution of oppositely charged dopants and defects compared to the thermodynamic limit, the paring between dopant Ga_Zn and intrinsic defects V_Zn is studied as a means to reduce the ionized impurity scattering. Indeed, the post‐deposition annealing of Ga‐doped Zn_0.7Mg_0.3O films grown by pulsed laser deposition increases the mobility by 50% resulting in a conductivity as high as σ = 475 S cm^‐1.     

The effect of Zn1‐xMgxO buffer layer deposition temperature on Cu(In,Ga)Se2 solar cells: A study of the buffer/absorber interface

The effect of atomic layer deposition temperature of Zn_1‐xMg_xO buffer layers for Cu(In,Ga)Se₂ (CIGS) based solar cell devices is evaluated. The Zn_1‐xMg_xO films are grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in a temperature range of 105 to 180°C. High efficiency devices are produced in the region from 105 up to 135°C. At a Zn_1‐xMg_xO deposition temperature of 120°C, a maximum cell efficiency of 15·5% is reached by using a Zn_1‐xMg_xO layer with an x‐value of 0·2 and a thickness of 140 nm. A significant drop in cell efficiency due to large losses in open circuit voltage and fill factor is observed for devices grown at temperatures above 150°C. No differences in chemical composition, structure and morphology of the samples are observed, except for the samples prepared at 105 and 120°C that show elemental selenium present at the buffer/absorber interface. The selenium at the interface does not lead to major degradation of the solar cell device efficiency. Instead, a decrease in Zn_1‐xMg_xO resistivity by more than one order of magnitude at growth temperatures above 150°C may explain the degradation in solar cell performance. From energy filtered transmission electron microscopy, the width of the CIGS/Zn_1‐xMg_xO chemical interface is found to be thinner than 10 nm without any areas of depletion for Cu, Se, Zn and O. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural and optical properties of Zn<Subscript>1−x</Subscript>Mg<Subscript>x</Subscript>O thin films synthesized with metal organic chemical vapor deposition

We present the structural and optical properties of Zn_1?xMg_xO thin films studied using x-ray diffraction (XRD), extended x-ray absorption fine structure (EXAFS), and photoluminescence (PL) measurements. The Zn_1?xMg_xO films on sapphire [0001] substrates were fabricated with metal organic chemical vapor deposition (MOCVD). The XRD measurements showed that the Zn_1?xMg_xO films (x≤0.05) had a wurtzite structure without any MgO phase and were epitaxially grown along the c-axis of the Al₂O₃ substrate. The lattice constant of the Zn_0.95Mg_0.05O film shrank by 0.023 Å, compared with that of ZnO crystals. From the EXAFS measurements on the Zn_1?xMg_xO films at Zn K-edge, we found a substantial amount of distortion in the bond length of Zn-Zn pairs with a small amount of Mg substitution on the Zn site. The PL measurements showed a gradual increment of the main exciton transitions from 3.36 eV (x=0.0) to 3.57 eV (x=0.05) at 10 K. We also observed a strong deep-level emission near 2.3 eV from the specimen with x=0.05.     

One‐Step Self‐Assembly Fabrication of High Quality NixMg1‐xO Bowl‐Shaped Array Film and Its Enhanced Photocurrent by Mg,2+ Doping

A series of high quality Ni_xMg_1‐xO bowl‐shaped array films are successfully prepared by a simple one‐step assembly of polystyrene colloidal spheres and metal oxide precursors at oil–water interface, and further used to fabricate nanodevices. The doping of Mg²⁺ can greatly enhance the current and spectrum responsivity of NiO film‐based nanodevice. The maximum R_λ value of these bowl‐shaped Ni_xMg_1‐xO film‐based devices measured in the study shows 4–5 orders of enhancement than the previously reported Ni_xMg_1‐xO film at equal doping.     

Entanglement of Conjugated Polymer Chains Influences Molecular Self‐Assembly and Carrier Transport

The influence of polymer entanglement on the self‐assembly, molecular packing structure, and microstructure of low‐M_w (lightly entangled) and high‐M_w (highly entangled) poly (3‐hexylthiophene) (P3HT), and the carrier transport in thin‐film transistors, are investigated. The polymer chains are gradually disentangled in a marginal solvent via ultrasonication of the polymer solution, and demonstrate improved diffusivity of precursor species (coils, aggregates, and microcrystallites), enhanced nucleation and crystallization of P3HT in solution, and self‐assembly of well‐ordered and highly textured fibrils at the solid–liquid interface. In low‐M_w P3HT, reducing chain entanglement enhances interchain and intrachain ordering, but reduces the interconnectivity of ordered domains (tie molecules) due to the presence of short chains, thus deteriorating carrier transport even in the face of improving crystallinity. Reducing chain entanglement in high‐M_w P3HT solutions increases carrier mobility up to ≈20‐fold, by enhancing interchain and intrachain ordering while maintaining a sufficiently large number of tie molecules between ordered domains. These results indicate that charge carrier mobility is strongly governed by the balancing of intrachain and interchain ordering, on the one hand, and interconnectivity of ordered domains, on the other hand. In high‐M_w P3HT, intrachain and interchain ordering appear to be the key bottlenecks to charge transport, whereas in low‐M_w P3HT, the limited interconnectivity of the ordered domains acts as the primary bottleneck to charge transport.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

“Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3‐hexylthiophene) and Methanofullerenes

The self‐organization of the polymer in solar cells based on regioregular poly(3‐hexylthiophene) (RR‐P3HT):[6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) is studied systematically as a function of the spin‐coating time t_s (varied from 20–80 s), which controls the solvent annealing time t_a, the time taken by the solvent to dry after the spin‐coating process. These blend films are characterized by photoluminescence spectroscopy, UV‐vis absorption spectroscopy, atomic force microscopy, and grazing incidence X‐ray diffraction (GIXRD) measurements. The results indicate that the π‐conjugated structure of RR‐P3HT in the films is optimally developed when t_a is greater than 1 min (t_s ～ 50 s). For t _s < 50 s, both the short‐circuit current (J_SC) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < t_s < 55 s, the J_SC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered π‐conjugated structure of RR‐P3HT to 1.2 % for a less‐ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon‐to‐electron conversion efficiency spectrum of the less‐ordered solar cell shows a clear loss in both the overall magnitude and the long‐wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open‐circuit voltage (V_OC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.     

Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: M_n = 7 200 g mol^?1; high‐MW: M_n = 27 000 g mol^?1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10^?2 cm² V^?1 s^?1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.     

Effect of Mesoscale Crystalline Structure on the Field‐Effect Mobility of Regioregular Poly(3‐hexyl thiophene) in Thin‐Film Transistors

Regioregular poly(3‐hexyl thiophene) (RR P3HT) is drop‐cast to fabricate field‐effect transistor (FET) devices from different solvents with different boiling points and solubilities for RR P3HT, such as methylene chloride, toluene, tetrahydrofuran, and chloroform. A Petri dish is used to cover the solution, and it takes less than 30 min for the solvents to evaporate at room temperature. The mesoscale crystalline morphology of RR P3HT thin films can be manipulated from well‐dispersed nanofibrils to well‐developed spherulites by changing solution processing conditions. The morphological correlation with the charge‐carrier mobility in RR P3HT thin‐film transistor (TFT) devices is investigated. The TFT devices show charge‐carrier mobilities in the range of 10^–4 ～ 10^–2 cm² V^–1 s^–1 depending on the solvent used, although grazing‐incidence X‐ray diffraction (GIXD) reveals that all films develop the same π–π‐stacking orientation, where the <100>‐axis is normal to the polymer films. By combining results from atomic force microscopy (AFM) and GIXD, it is found that the morphological connectivity of crystalline nanofibrils and the <100>‐axis orientation distribution of the π–π‐stacking plane with respect to the film normal play important roles on the charge‐carrier mobility of RR P3HT for TFT applications.     

The Zn(S,O,OH)/ZnMgO buffer in thin‐film Cu(In,Ga)(Se,S)2‐based solar cells part II: Magnetron sputtering of the ZnMgO buffer layer for in‐line co‐evaporated Cu(In,Ga)Se2 solar cells

A ZnS/Zn_1‐xMg_xO buffer combination was developed to replace the CdS/i‐ZnO layers in in‐line co‐evaporated Cu(In,Ga)Se₂(CIGS)‐based solar cells. The ZnS was deposited by the chemical bath deposition (CBD) technique and the Zn_1‐xMg_xO layer by RF magnetron sputtering from ceramic targets. The [Mg]/([Mg] + [Zn]) ratio in the target was varied between x = 0·0 and 0·4. The composition, the crystal structure, and the optical properties of the resulting layers were analyzed. Small laboratory cells and 10 × 10 cm² modules were realized with high reproducibility and enhanced stability. The transmission is improved in the wavelength region between 330 and 550 nm for the ZnS/Zn_1‐xMg_xO layers. Therefore, a large gain in the short‐circuit current density up to 12% was obtained, which resulted in higher conversion efficiencies up to 9% relative as compared to cells with the CdS/i‐ZnO buffer system. Peak efficiencies of 18% with small laboratory cells and 15·2% with 10 × 10 cm² mini‐modules were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Origin of Reduced Bimolecular Recombination in Blends of Conjugated Polymers and Fullerenes

Bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative ((6,6)‐phenyl‐C₇₁‐butyric acidmethyl ester, PC₇₁BM) is studied by two complementary techniques. TRMC (time‐resolved microwave conductance) monitors the conductance of photogenerated mobile charge carriers locally on a timescale of nanoseconds, while using photo‐CELIV (charge extraction by linearly increasing voltage) charge carrier dynamics are monitored on a macroscopic scale and over tens of microseconds. Despite these significant differences in the length and time scales, both techniques show a reduced Langevin recombination with a prefactor ζ close to 0.05. For TQ1:PC₇₁BM blends, the ζ value is independent of temperature. On comparing TRMC data with electroluminescence measurements it is concluded that the encounter complex and the charge transfer state have very similar energetic properties. The ζ value for annealed poly(3‐hexylthiophene) (P3HT):(6,6)‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) is approximately 10^?4, while for blend systems containing an amorphous polymer ζ values are close to 1. These large differences can be related to the extent of charge delocalization of opposite charges in an encounter complex. Insight is provided into factors governing the bimolecular recombination process, which forms a major loss mechanism limiting the efficiency of polymer solar cells.     

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

Composition‐ and Shape‐Controlled Synthesis and Optical Properties of ZnxCd1–xS Alloyed Nanocrystals

Composition‐tunable Zn_xCd_1–xS alloyed nanocrystals have been synthesized by a new approach consisting of thermolyzing a mixture of cadmium ethylxanthate (Cd(exan)₂) and zinc ethylxanthate (Zn(exan)₂) precursors in hot, coordinating solvents at relatively low temperatures (180–210 °C). The composition of the alloyed nanocrystals was accurately adjusted by controlling the molar ratio of Cd(exan)₂ to Zn(exan)₂ in the mixed reactants. The alloyed Zn_xCd_1–xS nanocrystals prepared in HDA/TOP (HDA: hexadecylamine; TOP: trioctylphosphine) solution exhibit composition‐dependent shape and phase structures as well as composition‐dependent optical properties. The shape of the Zn_xCd_1–xS nanocrystals changed from dot to single‐armed rod then to multi‐armed rod with a decrease of Zn content in the ternary nanoparticles. The alloying nature of the Zn_xCd_1–xS nanocrystals was consistently confirmed by the results of high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), and UV‐vis absorption and photoluminescence (PL) spectroscopy. Further, the shape‐controlled synthesis of the ternary alloyed nanocrystals was realized by selecting appropriate solvents. Uniform nanodots in the whole composition range were obtained from TOPO/TOP solution, (TOPO: trioctylphosphine oxide) and uniform nanorods in the whole composition range were prepared from HDA/OA solution (OA: octylamine). The effect of the reaction conditions, such as solvent, reaction temperature, and reaction time, on the PL spectra of the alloyed Zn_xCd_1–xS nanocrystals was also systematically studied, and the reaction conditions were optimized for improving the PL properties of the nanocrystals.     

High Carrier Density and Metallic Conductivity in Poly(3‐hexylthiophene) Achieved by Electrostatic Charge Injection

The use of electrostatic charge injection (i.e., the transverse field effect) to induce both very large two‐dimensional hole densities (～ 10¹⁵ charges cm^–2) and metallic conductivities in poly(3‐hexylthiophene) (P3HT) is reported. Films of P3HT are electrostatically gated by a solution‐deposited polymer‐electrolyte gate dielectric in a field‐effect‐transistor configuration. Exceptionally high hole field‐effect mobilities (up to 0.7 cm² V^–1 s^–1) are measured concurrently with large hole densities, resulting in an extremely large sheet conductance of 200 μS sq.^–1. The large room‐temperature conductivity of 1000 S cm^–1 together with the very low measured activation energies (0.7–4 meV) suggest that the metal–insulator transition in P3HT is achieved. A maximum in sheet conductance versus charge density is also observed, which may result from near‐filling of the valence band or from charge correlations that lower the carrier mobility. Importantly, the large hole densities in P3HT are achieved using capacitive coupling between the polymer‐electrolyte gate dielectric and P3HT (i.e., the field effect) and not via chemical or electrochemical doping. Electrostatic control of carrier density up to 10¹⁵ charges cm^–2 (～ 10²² charges cm^–3) opens opportunities to explore systematically the importance of charge‐correlation effects on transport in conjugated polymers without the structural rearrangement associated with chemical or electrochemical doping.     

Plasmonic Heterostructure TiO2‐MCs/WO3−x‐NWs with Continuous Photoelectron Injection Boosting Hot Electron for Methane Generation

Nonmetallic plasmonic heterostructure TiO₂‐mesocrystals/WO_3?x‐nanowires (TiO₂‐MCs/WO_3?x‐NWs) are constructed by coupling mesoporous crystal TiO₂ and plasmonic WO_3?x through a solvothermal procedure. The continuous photoelectron injection from TiO₂ stabilizes the free carrier density and leads to strong surface plasmon resonance (SPR) of WO_3?x, resulting in strong light absorption in the visible and near‐infrared region. Photocatalytic hydrogen generation of TiO₂‐MCs/WO_3?x‐NWs is attributed to plasmonic hot electrons excited on WO_3?x‐NWs under visible light irradiation. However, utilization of injected photoelectrons on WO_3?x‐NWs has low efficiency for hydrogen generation and a co‐catalyst (Pt) is necessary. TiO₂‐MCs/WO_3?x‐NWs are used as co‐catalyst free plasmonic photocatalysts for CO₂ reduction, which exhibit much higher activity (16.3 µmol g^?1 h^?1) and selectivity (83%) than TiO₂‐MCs (3.5 µmol g^?1 h^?1, 42%) and WO_3?x‐NWs (8.0 µmol g^?1 h^?1, 64%) for methane generation under UV–vis light irradiation. A photoluminescence study demonstrates the photoelectron injection from TiO₂ to WO_3?x, and the nonmetallic SPR of WO_3?x plays a great role in the highly selective methane generation during CO₂ photoreduction.