Matched Band Gap Research Articles

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175°C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N2O), hydrogen (H2), silane (SiH4), and diborane (B2H6) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71eV to 2.0eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7×10−5S/cm to 5.1×10−7S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (Voc)=800mV, short circuit current density (Jsc)=16.3mA/cm2, fill-factor (FF)=72%, and photovoltaic conversion efficiency (η)=9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. Jsc, FF and Voc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.

Binary Nanoparticle Superlattices in the Semiconductor−Semiconductor System: CdTe and CdSe

We report binary nanoparticle superlattices obtained by self-assembly of two different semiconductor quantum dots. Such a system is a means to include two discretized, quantum-confined, and complimentary semiconductor units in close proximity, for purposes of band gap matching and/or energy transfer. From a range of possible structures predicted, we observe an exclusive preference for the formation of Cuboctahedral AB13 and AB5 (isostructural with CaCu5) obtained in the system of 8.1 nm CdTe and 4.4 nm CdSe nanoparticles. For this system, a possible ionic origin for the formation of structures with lower packing densities was ruled out on the basis of electrophoretic mobility measurements. To understand further the principles of superlattice formation, we constructed space-filling curves for binary component hard spheres over the full range of radius ratio. In addition, the pair interaction energies due to core-core and ligand-ligand van der Waals (VDW) forces are estimated. The real structures are believed to form under a combined influence of entropic driving forces (following hard-sphere space filling principles) and the surface (due to ligand-ligand VDW).

Modeling the atomic and electronic structure of diamond nanocrystals containing [NV]− centers by the density functional method

UDC 535.33:548.0 We have used the density functional method to model the atomic and electronic structure of diamond nanocrystals passivated by hydrogen atoms and either not containing defects or containing a single [NV]– center. We have shown that in all cases, after relaxation the nanocrystals are formed as diamond-like structures. We have studied the features of the electronic structure of the nanocrystals. We have analyzed in detail the mechanism for the formation of energy levels in the bandgap due to [NV] – centers. We have established that the optical absorption and fluorescence spectra for the [NV] – centers are mainly associated with transitions of electrons between the highest occupied β orbitals (projection of the electron spin equal to +1/2) and lower unoccupied α orbitals (projection of the electron spin equal to –1/2). The results on the localization and energy position of the states in the bandgap match data obtained for the [NV] – center in bulk diamond.

Optimizing OLED Efficacy of 2,7‐Diconjugated 9,9‐Dialkylfluorenes by Variation of Periphery Substitution and Conjugation Length

AbstractThe photoluminescence (PL) and electroluminescence (EL) of four 2,7‐bis(phenylethenyl)fluorenes (OFPVs) and two 2,7‐diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460–480 nm emission maxima but show large variation in the organic light‐emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9‐diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A–1 and greater (9,9‐diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT:PSS/(emitter)/Ca–Al (ITO: indium tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9‐diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m–2 at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400–410 nm emission maxima but also an undesired green emission component at 500–550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long‐wavelength component when the emitter load is 10–25 %, but the OFPh luminance efficiencies, turn‐on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid‐state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute.

Analysis of graphene nanoribbons as a channel material for field-effect transistors

Electronic properties of graphene (carbon) nanoribbons are studied and compared to those of carbon nanotubes. The nanoribbons are found to have qualitatively similar electron band structure which depends on chirality but with a significantly narrower band gap. The low- and high-field mobilities of the nanoribbons are evaluated and found to be higher than those of carbon nanotubes for the same unit cell but lower at matched band gap or carrier concentration. Due to the inverse relationship between mobility and band gap, it is concluded that graphene nanoribbons operated as field-effect transistors must have band gaps <0.5eV to achieve mobilities significantly higher than those of silicon and thus may be better suited for low power applications.

Measurement of built-in electrical potential in III–V solar cells by scanning Kelvin probe microscopy

We report on direct measurements of the built-in electrical potential in III–V semiconductor-based solar cell devices by using scanning Kelvin probe microscopy. Potential profiles on cross sections of the devices were measured quantitatively and spatially resolved in open and short circuits, under and without illuminations, with selective photon energies matching band gaps of the junctions. The measurements provide valuable information about the electrical properties of the devices, and are useful for understanding the performance of solar cells. On a GaInP2-single junction cell, two potential features were measured and were assigned to the p-n junction and the potential barrier at the interface between the GaInP2 base layer and the GaAs substrate. The potential on the p-n junction is photoactive, and that on the GaInP2/GaAs interface is photoinactive. On a GaInP2/GaAs tandem cell, two potential features were measured near the top and the bottom p-n junctions. When the sample was illuminated by light with photon energies larger than the band gap of the top junction, in the condition of short circuit, a potential peak appeared on the boundary between the bottom cell and tunnel junction due to charge accumulations on the bottom junction.

Surface Grafting of Conjugated Polymers onto Self-assembled Monolayer Modified Conducting Substrates by Electrochemistry

A new method of grafting conjugated polymers onto solid substrates by self-assembled monolayers (SAM) and electrochemistry is described. The polyfluorene was designed to have the carbazole group as the pendant group to offer electropolymerizability. As has been studied, carbazole is easily electropolymerized by cyclic voltammetry at low oxidation potential. In our study, this copolymer was deposited and cross-linked on the carbazole-SAM modified ITO substrate through electrochemistry. Consistent with the oxidation potential differences between polyfluorene and the carbazole unit, electropolymerization of the pendant carbazole groups has no effect on the polymer backbone. The carbazole group was chosen because of its better band gap match between the anode and the polyfluorene and at the same time it gives better adhesion contact between ITO and the polymer layer. In addition, the electrochemically deposited polymer becomes cross-linked, rendering it insoluble in any solvent.

Fabrication of Solar Cells Having SiH2Cl2 Based I-Layer Materials

Intrinsic amorphous silicon films were fabricated using electron cyclotron resonance (ECR) assisted chemical vapor deposition and SiH2Cl2 source gas. Intrinsic layers were used for material characterization and also for the absorber layer of solar cells. The highly reducing atmosphere produced by the high energy ECR hydrogen plasma used to deposit these intrinsic films caused some degradation and/or etching of the previously deposited solar cell doped layers as well as the SnO2-coated glass substrates. The p-layer etching rates were greater than those of the n-layer when these layers were exposed to ECR hydrogen plasma. Optimum photovoltaic performance was achieved when an optimized n/i interfacial buffer layer was used for a solar cell deposited in the n-i-p sequence. Better solar cell performances were obtained when the solar cells were measured under n-side illumination. In part, the buffer layer optimization involved careful consideration of band gap matching to the relatively wide band gap (1.85 eV) intrinsic layers prepared from SiH2Cl2. Further performance gains were possible through transparent conductive oxide/substrate optimization. For example, the open circuit voltage (Voc) increased to ∼0.89 V when gallium-doped zinc oxide/glass substrates were used compared to ∼0.63 V when tin oxide/glass substrates were used. Interface recombination and minority carrier diffusion lengths were probed by n- and p-side illuminated quantum efficiency measurement and analysis. The electron and hole µτ products were estimated to be 4.4×10-8 cm2/V and 3.5×10-8 cm2/V, respectively. The stability of the solar cells was also examined.

Series resistance in n-MoSe 2 (TMDC)-based PEC solar cells

Transition metal dichalcogenides (TMDCs) have been noticed as potentials for the PEC solar cells because they are inherently stable against the electrolytic environment. Since MoSe 2—a member of group VI TMDCs—possesses an optically matching band gap of around 1.4 eV, it holds relatively more promise as a better material for such devices. In this article, the authors report their investigations on PEC solar cells fabricated using n-MoSe 2 crystals grown by a direct vapour transport technique. The photoconversion characteristics of n-MoSe 2/ I 2/ I −/pt PEC solar cells were investigated under polychromatic illumination from an incandescent lamp at various intensities. Since the series resistance of TMDC-based PEC solar cells is expected to be high, it may be one of the major parameters blocking the available power on photoconversion from such devices. Efforts have been made here to estimate its value. In addition, the effect of thermal treatment of the photoelectrode on the series resistance was also investigated. It has been found that the series resistance decreases from 4.01 K ohms to 1.93 K ohmson controlled thermal treatment of the photoelectrode. This is accompanied by a marked increase in the photoconversion efficiency (from 3% to around 12%). Thus, it can be concluded that the contribution of the series resistance in TMDC-based PEC solar cells is quite significant and can be reduced by giving controlled thermal treatment to the photoelectrodes.

Stacked solar cells of amorphous silicon

Multiple-junction (stacked) solar cells of hydrogenated amorphous silicon (a-Si:H) have been made, consisting of two and three p+in+ cells. Three kinds of tunnel junction cell interconnections have been tested including n+p+ (a-Si:H), n+/cermet/p+, and n+/metal/cermet/p+ layers. The quality of the tunnel junctions improved in that order. The open circuit voltage (Voc) for a single cell was 0.76 V, for a two-cell stack 1.57 V, and for a three-cell stack 1.92 V. The stacked cell has the potential of increasing the efficiency of a-Si:H solar cells in at least two ways. One is by means of bandgap matching to the solar spectrum. The other is by means of a more complete utilization of a given range of wavelengths which cannot be fully absorbed within the limited depletion width of a single cell. In the present device the a-Si:H materials had the same optical bandgap throughout, thereby precluding bandgap matching, but allowing greater light absorption. Thus far, a modest 2% efficiency has been realized in the two-cell stack and 2.5% in a three-cell stack.