Quantum Efficiency Of Generation Research Articles

Dynamic, Charge Photogeneration and Excitons Distribution Function in Organic Bulk Heterojunction Solar Cells

In this work, the excitons distribution function in organic bulk hetero junction solar cells, at a depth z has been determined from solving the charge continuity equation, by exploiting the Laplace transform with appropriate boundary conditions. Next, the influence of the electron-hole pair separation distance on the excitons dissociation probability, the internal quantum efficiency and the binding energy, has been studied. The simulated results show that the probability density of the carriers photo generated depends on the generation rate, excitons dissociation and the charge carriers in the cells. The potential improvement of the internal quantum efficiency of charge generation depends on electron-hole pair separation distance, the excitons dissociation probability into free charges and depends strongly on the optical absorption of the photons in the active layer.

Graphene as transparent and current spreading electrode in silicon solar cell

Fabricated bi-layer graphene (BLG) has been studied as transparent and current spreading electrode (TCSE) for silicon solar cell, using TCAD-Silvaco 2D simulation. We have carried out comparative study using both Ag grids and BLG as current spreading electrode (CSE) and TCSE, respectively. Our study reveals that BLG based solar cell shows better efficiency of 24.85% than Ag-based cell (21.44%), in all of the critical aspects, including generation rate, recombination rate, electric field, potential and quantum efficiency. Further BLG based cell exhibits pronounce rectifying behavior, low saturation current, and good turn-on voltage while studying in dark.

Effect of molecular weight and protection ratio on line edge roughness and stochastic defect generation in chemically amplified resist processes of extreme ultraviolet lithography

The suppression of stochastic effects such as line edge roughness (LER) and stochastic defect generation is required for the high-volume production of semiconductor devices with 11 nm critical dimension using extreme ultraviolet (EUV) lithography. In this study, the effects of the molecular weight and protection ratio of a resist polymer on LER and stochastic defect generation were investigated, assuming line-and-space patterns with 11 nm half-pitch. By increasing the molecular weight, LER and the probability of stochastic defect generation were decreased. The negative effect caused by the increase of molecular weight is the increase of the minimum dissolution block size in the development process. By increasing the protection ratio, a similar effect was expected. However, LER and the probability of stochastic defect generation were not significantly decreased because of the decrease of the quantum efficiency of acid generation caused by the protection of proton sources. It is important to increase the protection ratio without decreasing the quantum efficiency of acid generation for the suppression of LER and stochastic defect generation.

Dependence of stochastic defect generation on quantum efficiency of acid generation and effective reaction radius for deprotection in chemically amplified resist process using extreme ultraviolet lithography

The suppression of stochastic defect generation is an urgent task for realizing the high-volume production of semiconductor devices with the 11 nm critical dimension using extreme ultraviolet (EUV) lithography. In this study, the dependence of stochastic defect generation on the quantum efficiency of acid generation and the effective reaction radius for deprotection was investigated. The formation of line-and-space patterns was simulated using a Monte Carlo method on the basis of the sensitization mechanisms of EUV resists. By increasing the quantum efficiency for acid generation and the effective reaction radius for deprotection, the probability of stochastic defect generation can be decreased. For the suppression of stochastic defect generation, increasing the quantum efficiency is more efficient than increasing the effective reaction radius for deprotection.

Hole removal rate limits photodriven H2 generation efficiency in CdS-Pt and CdSe/CdS-Pt semiconductor nanorod-metal tip heterostructures.

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

Polarised two-photon excitation of quantum well excitons for manipulation of optically pumped terahertz lasers

Optical pumping of excited exciton states in a semiconductor quantum well embedded in a microcavity is a tool for realisation of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited (2p) and ground (1s) exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarisation of both pumping photons. Five-fold variation of the threshold power for terahertz lasing by switching from circular to co-linear pumping is predicted. We identify photon polarisation configurations for achieving maximum THz photon generation quantum efficiency.

Twin-induced one-dimensional homojunctions yield high quantum efficiency for solar hydrogen generation

Efficient charge separation is of crucial importance for the improvement of photocatalytic activity for solar hydrogen evolution. Here we report efficient photo-generated charge separation by twin-induced one-dimensional homojunctions with type-II staggered band alignment, using a ternary chalcogenate, i.e. Cd0.5Zn0.5S nanorod as a model material. The quantum efficiency of solar hydrogen evolution over this photocatalyst, without noble metal loading, reaches 62%. Unlike traditional heterojunctions, doping or combination of additional elements are not needed for the formation of this junction, which permits us to tune the band structures of semiconductors to the specific application in a more precise way. Our results highlight the power of forming long-range ordered homojunctions at the nanoscale for photocatalytic and photoelectrochemical applications.

Polarization selection rules in exciton-based terahertz lasers

Optical pumping of excited exciton states in semiconductor quantum wells is a tool for the realization of ultracompact terahertz (THz) lasers based on stimulated optical transition between excited ($2p$) and ground ($1s$) exciton states. We show that the probability of two-photon absorption by a $2p$ exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.

Two Channels of Charge Generation in Perylene Monoimide Solid‐State Dye‐Sensitized Solar Cells

The mechanism of charge generation in solid‐state dye‐sensitized solar cells using triarylamine‐substituted perylene monoimide dyes is studied by vis‐NIR broadband pump‐probe transient absorption spectroscopy. The experiments demonstrate that photoinduced electron injection into the TiO2 can only occur in regions where Li+, from the commonly used Li‐TFSI additive salt, is present on the TiO2 surface. Incomplete surface coverage by Li+ means that some dye excitons cannot inject their electron into the TiO2. However it is observed in the solar cell structure that some of the dye excitons that cannot directly inject an electron still contribute to free charge generation by the previously hypothesized reductive quenching mechanism (hole transfer to the solid‐state hole transporter followed by electron injection from the dye anion into the TiO2). The contribution of reductive quenching to the quantum efficiency of charge generation is significant, raising it from 68% to over 80%. Optimization of this reductive quenching pathway could be exploited to maintain high quantum efficiency in dyes with greater NIR absorption to achieve overall enhancements in device performance. It is demonstrated that broadband NIR transient spectroscopy is necessary to obtain population kinetics in these systems, as strong Stark effects distort the population kinetics in the visible region.

Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold: Influence of Orbital Energy Level Alignment on Short-Circuit Current Generation

Photoinduced current generation at metal–organic monolayer interfaces is observed upon photoexcitation of a monolayer of hemicyanine molecules chemically adsorbed onto a gold electrode. A series of hemicyanines is investigated that bind to the gold via a thiol moiety, in an orientation such that the acceptor moiety of the hemicyanine is closer to the metal than its donor part. The quantum yield of short-circuit photocurrent generation in a diode using a liquid electrolyte as second contact, correlates with the strength of the donor moiety of the dyes. Modeling of the photocurrent generation using Marcus theory indicates that the net photocurrent results from asymmetry in the electron transfer rates of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the electrodes. The quantum efficiency of short-circuit photocurrent generation decreases for the HOMO levels of the hemicyanine going deeper below the Fermi-level of the metal. The deeper HOMO level provides a ...

Spectroscopic properties and singlet oxygen production by the compound ethyl 3,12-dioxopyran[3,2-a]xanthone-2-carboxylate

The steady state and time resolved experiments together with absorption and emission spectroscopies and quantum chemical calculations have been employed to investigate spectroscopic properties of a xanthone-type compound (ethyl 3,12-dioxopyran[3,2-a]xanthone-2-carboxylate). The spectroscopic data show good agreement with results obtained from quantum chemical calculations. Additionally, this compound shows expressive quantum efficiency for triplet population and a quantum efficiency of singlet oxygen generation very close to unity. Correlations between the nature of singlet and triplet excited states and spectroscopic properties were performed in order to understand the high quantum efficiency of singlet oxygen generation by this compound.

Photoelectric and photorefractive properties of polyvinylcarbazole composites with graphene in the visible spectral range

The photoelectric and photorefractive characteristics of polyvinylcarbazole (PVK) composites with 0.15 wt % graphene at a wavelength of 532 nm have been measured. The dependence of the quantum efficiency of generation of mobile charge carriers as determined from the photocurrent is well approximated by the Onsager equation calculated accurate to E 0 3 for the quantum yield of thermalized electron-hole pairs of φ0 = 1 and their initial separation radius of r 0 = 10.9 Å. The long-wavelength edge of optical absorption for PVK lies at 365 nm. Thus, the photogeneration of mobile carriers during illumination of the composite at 532 nm is due to photoexcitation of graphene. The measurement of the photorefractive properties has revealed that the two-beam coupling gain coefficient is Γ = 50 cm−1 at E 0 = 150 V/μm and equal intensities of incident beams. It has been found that at E 0 = 83.3 V/μm, the two-beam coupling gain coefficient increases from 8 to 14 cm−1 as the ratio of incident beam intensities I 1(0)/I 2(0) increases from 1 to 2.4.

Resonance running hologram velocity nonlinearity dependence upon light intensity in photorefractive crystals

We report on the nonlinear relation between the resonance hologram velocity and the recording light irradiance in fringe-locked running hologram experiments carried out on a nominally undoped photorefractive Bi12TiO20 crystal using 532 nm wavelength laser light. Such nonlinearity is due to the dependence of the material diffusion length on the light irradiance. Experimental data show a good agreement with the theoretical equations thus supporting the mathematical model here reported and allowing to compute the quantum efficiency for photoelectron generation, the dependence of the diffusion length upon the recorded light irradiance, and the far from saturation diffusion length.

Polarization Control of Optically Pumped Terahertz Lasers

ABSTRACTTwo-photon pumping of excited exciton states in semiconductor quantum wells is a tool for realization of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited 2p and ground 1s exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.

Backaction limits on self-sustained optomechanical oscillations

The maximum amplitude of mechanical oscillators coupled to optical cavities is studied both analytically and numerically. The optical backaction on the resonator enables self-sustained oscillations whose limit cycle is set by the dynamic range of the cavity. The maximum attainable amplitude and the phonon generation quantum efficiency of the backaction process are studied for both unresolved and resolved cavities. Quantum efficiencies far exceeding one are found in the resolved sideband regime where the amplitude is low. On the other hand, the maximum amplitude is found in the unresolved system. Finally, the role of mechanical nonlinearities is addressed.

Size-Selected Subnanometer Cluster Catalysts on Semiconductor Nanocrystal Films for Atomic Scale Insight into Photocatalysis

We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size.

Parallel Pool Analysis of Transient Spectroscopy Reveals Origins of and Perspectives for ZnO Hybrid Solar Cell Performance Enhancement Using Semiconducting Surfactants.

Recently, the performance of ZnO nanocrystals as an electron acceptor in a solar cell device was significantly increased by a semiconducting surfactant. Here we show, using transient absorption spectroscopy and a parallel pool analysis, that changes in the quantum efficiency of charge generation account for the performance variation among semiconducting-surfactant-coated, surfactant-coated, and uncoated ZnO nanoparticles. We demonstrate that even better surfactant design to suppress fast recombination could still lead to a further doubling of device efficiency.

Enhancing extraction of photogenerated excitons from semiconducting carbon nanotube films as photocurrent

We study the effect of residual polymer on exciton transport and the external quantum efficiency (EQE) of photocurrent generation in thin film semiconducting single walled carbon nanotube (s-SWCNT)/C60 heterojunction diodes. Specifically, increasing the s-SWCNT film content from 22% to 43% increases peak EQE from absorption by s-SWCNTs from 15% to 23%. We monitor intertube exciton energy transfer via steady state photoluminescence spectroscopy and determine the length scale for exciton migration via s-SWCNT film thickness dependence of EQE. We observe increased intertube exciton transfer in photoluminescence spectra with increased polymer removal, and EQE-thickness dependence suggests increased intratube exciton transport along isolated pathways. Our results extend the state of the art with respect to the use of s-SWCNT thin films as photoabsorbers in photovoltaics, describe exciton migration in s-SWCNT films, and provide a framework for the design of high efficiency s-SWCNT photovoltaic and photodetector devices.

Photocurrent Generation by Two-Step Photon Absorption With Quantum-Well Superlattice Cell

We have observed photocurrent due to two-step photon absorption using an InGaAs/GaAsP strain-balanced quantum-well (QW) superlattice cell, with barrier thickness of 3 nm. Upon infrared irradiation with a filtered air mass 1.5 light source (λ > 1.4 μm), quantum efficiency was increased by 0.8% at the wavelength range corresponding to the absorption of the quantum wells. No efficiency enhancement was observed either for a conventional QW solar cell with thick (11 nm) barriers or for a GaAs pin cell, suggesting that efficient separation of photogenerated electrons and holes in the superlattice is essential for this achievement of two-step photon absorption. The quantum efficiency of such two-step photocurrent generation can be enhanced by specific tailoring of a superlattice layer structure, by which the QW superlattice will be a possible active region for an intermediate-band solar cell.

Hole scavenger redox potentials determine quantum efficiency and stability of Pt-decorated CdS nanorods for photocatalytic hydrogen generation

We use Pt-decorated CdS nanorods for photocatalytic hydrogen generation in the presence of sacrificial hole scavengers. Both the quantum efficiency for hydrogen generation and the stability of the colloidal nanocrystals in solution improve with increasing redox potential of the hole scavenger. The higher redox potential leads to faster hole scavenging, which increases quantum efficiency and stability since electron hole recombination and oxidation of the CdS become less important. The quantum efficiencies can be tuned over more than an order of magnitude. This finding is important for choosing hole scavengers and for comparing efficiencies and stabilities for different photocatalytic nanosystems.