Surface Recombination Of Charge Carriers Research Articles

Improving the Performance of Si/PEDOT:PSS Hybrid Solar Cells with More Economical and Environmentally Friendly Alcohol Ether Solvents.

The photoelectric characteristics of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films significantly affect the power conversion efficiency and stability of Si/PEDOT:PSS hybrid solar cells. In this paper, we investigated PEDOT:PSS modification with alcohol ether solvents (dipropylene glycol methyl ether (DPM) and propylene glycol phenyl ether (PPH)). The reduction of PSS content and the transformation of the PEDOT chain from benzene to a quinone structure in PEDOT:PSS induced by doping with DPM or PPH are the reasons for the improved conductivity of PEDOT:PSS films. DPM and PPH doping improves the quality of silicon with the PEDOT:PSS heterojunction and silicon surface passivation, thereby reducing the surface recombination of charge carriers, which improves the photovoltaic performance of Si/PEDOT:PSS solar cells. Comparing the power conversion performance (PCE) and air stability of Si/PEDOT:PSS solar cells with DPM (13.24%), DPH (13.51%), ethylene glycol (EG, 13.07%), and dimethyl sulfoxide (DMSO, 12.62%), it is suggested that doping with DPM and DPH can replace DMSO and EG to enhance the performance of Si/PEDOT:PSS solar cells. The EG and DMSO solvents not only have a certain toxicity to the human body but also are not environmentally friendly. In comparison to DMSO and EG, DPM and DPH are more economical and environmentally friendly, helping to reduce the manufacturing cost of Si/PEDOT:PSS solar cells and making them more conducive to their commercial applications.

Passivation of perovskite layer surface states with pyridine in flexible and printed perovskite solar cells

Perovskite solar cells (PSCs), prepared by using solution-processed printing techniques, gained much attention over the past few years and a considerable progress has been achieved in improving the power conversion efficiencies of these devices. Nevertheless, there are still some advancements that can be implemented, especially in terms of passivation of surface defects in the perovskite photoactive layer. Passivation can afford considerable reduction in surface recombination of charge carriers in the photoactive layer and help to obtain devices with better performance. In this work, poly(3-hexylthiophene-2,5-diyl)-based inks with small amount of pyridine as an additive are used to deposit the hole transport layer and simultaneously passivate the surface defects of the perovskite layer in flexible and printed PSCs. The devices are fabricated on flexible conductive plastic substrates using a slot-die coating method. It is found that 2.5 wt.% pyridine-containing inks for preparing hole transport layer have a positive effect on the performance of resulting PSCs. On average, around 13% improvement in the power conversion efficiency is observed for the devices with passivation as opposed to the reference devices without passivation. The effect of pyridine passivation on the structural and electronic properties of the perovskite layer on a flexible substrate is studied using experimental and analytical techniques, whereas the computer simulation methods are employed to rule out the possible mechanisms for the performance improvements in the devices with passivation. The approach presented here can be useful for developing simplified protocols for printing of flexible PSCs with the passivated perovskite layer and improved device efficiency.

Composition-Dependent Passivation Efficiency at the CdS/CuIn1- x Gax Se2 Interface.

The bandgap of CuIn1- x Gax Se2 (CIGS) chalcopyrite semiconductors can be tuned between ≈1.0 and ≈1.7 eV for Ga contents ranging between x = 0 and x = 1. While an optimum bandgap of 1.34 eV is desirable for achieving maximum solar energy conversion in solar cells, state-of-the-art CIGS-based devices experience a drop in efficiency for Ga contents x > 0.3 (i.e., for bandgaps >1.2 eV), an aspect that is limiting the full potential of these devices. The mechanism underlying the limited performance as a function of CIGS composition has remained elusive: both surface and bulk recombination effects are proposed. Here, the disentanglement between surface and bulk effects in CIGS absorbers as a function of Ga content is achieved by comparing photogenerated charge carrier dynamics in air/CIGS and surface-passivated ZnO/CdS/CIGS samples. While surface passivation prevents surface recombination of charge carriers for low Ga content (x < 0.3; up to 1.2 eV bandgap), surface recombination dominates for higher-bandgap materials. The results thus demonstrate that surface, rather than bulk effects, is responsible for the drop in efficiency for Ga contents larger than x ≈ 0.3.

Effects of surface recombination on the charge collection in h-BN neutron detectors

Hexagonal boron nitride (h-BN) epilayers have been recognized as a promising material for applications in solid-state neutron detectors. However, the highest detection efficiency of 58% attained so far for 1 mm2 detectors fabricated from 50 μm thick B-10 enriched h-BN films still falls short of the expected theoretical value of 64%. This is due to the less than perfect charge collection efficiency. In this work, we have fabricated and analyzed the photocurrent-voltage characteristics of 11 h-BN neutron detectors. The dependence of the charge collection efficiency (ηc) on the charge carrier mobility-lifetime (μτ) product of the bulk trapping parameter reveals that ηc is nearly perfect at a bias voltage of 200 V if we neglect the effects of surface recombination. Our results have clearly demonstrated that the surface recombination of charge carriers is the dominant factor that prevents a further enhancement in the charge collection efficiency in our current detectors. The surface recombination field ES (=s/μ), defined as the ratio of the surface recombination velocity (s) to the carrier mobility (μ) of holes, was found to have a linear relationship with that of electrons and has a magnitude of the order of 104 V/cm. The present study indicates that it is critical to reduce ES in h-BN in order to further push the charge collection and hence the total detection efficiency of h-BN neutron detectors to 100%.

Surface Recombination in Ultra-Fast Carrier Dynamics of Perovskite Oxide La0.7Sr0.3MnO3 Thin Films

Aspects of the optoelectronic performance of thin-film ferromagnetic materials are evaluated for application in ultrafast devices. Dynamics of photocarriers and their associated spin polarization are measured using transient reflectivity (TR) measurements in cross linear and circular polarization configurations for La0.7Sr0.3MnO3 films with a range of thicknesses. Three spin-related recombination mechanisms have been observed for thicker films (thickness of d ≥ 20 nm) at different time regimes (τ), which are attributed to the electron-phonon recombination (τ < 1 ps), phonon-assisted spin-lattice recombination (τ ∼ 100 ps), and thermal diffusion and radiative recombination (τ > 1 ns). Density functional theory (DFT+U) based first-principles calculations provide information about the nature of the optical transitions and their probabilities for the majority and the minority spin channels. These transitions are partly responsible for the aforementioned recombination mechanisms, identified through the comparison of linear and circular TR measurements. The same sets of measurements for thinner films (4.4 nm ≤ d < 20 nm) revealed an additional relaxation dynamic (τ ∼ 10 ps), which is attributed to the enhanced surface recombination of charge carriers. Our DFT+U calculations further corroborate this observation, indicating an increase in the surface density of states with decreasing film thickness which results in higher amplitude and smaller time constant for surface recombination as the film thickness decreases.

Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence

In the quest for better solar panels, we continue to refine our understanding of the physics of photovoltaic absorbers, such as hybrid metal-organic perovskites. By scrutinizing photoluminescence transients, the authors explain how to determine fundamental semiconductor properties and estimate the potential open-circuit voltage of perovskite thin films. Additionally, they highlight the impacts of photon recycling, doping, and bulk and surface recombination of charge carriers on device performance.

Nanoarchitectured Semiconducting Photoelectrodes for Enhanced Stability and Photon Conversion Efficiency

The characteristics of two dimensional (graphene), one dimensional (single walled carbon nanotube incorporated poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, composite) and zero dimensional (ZnO quantum dots) nanostructures have been systematically analyzed to elucidate the possibility of employing such nanostructures (transparent conducting corrosion resistant (TCCR)) for the passivation of the surface of a photoelectrode. Thin layer of graphene, carbon nanotubes embedded on a conductive polymer matrix (PEDOT:PSS) and ZnO quantum dots are incorporated on the surface of photoelectrode namely ‘In’ rich CuInS2 (ICIS) to act as corrosion resistant layers. The TCCR layers are found to be effective in inhibiting the surface recombination of charge carriers. Graphene exhibits a higher tendency towards the suppression of surface recombination across electrode-electrolyte interface, where an enhancement by a factor of 104 s is observed in the electron recombination life time. SWCNT/PEDOT:PSS composite films when employed as transparent conducting corrosion resistant layers enabled an enhancement of the cell stability by 95% when compared to unprotected ‘In’ rich CuInS2 electrode. The TCCR layers are found have a positive impact on the conversion efficiency of the photoelectrode and the maximum enhancement in the conversion efficiency achieved is 71% with ZnO quantum dot sensitized electrodes.

Transparent Nanoparticulate FeOOH Improves the Performance of a WO3 Photoanode in a Tandem Water-Splitting Device

Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O2 Faradaic efficiency from ∼70 to ∼100%. The data from the photovoltammetry, electrochemical impedance, and gas evolution measurements show that these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated WO3 in a solar-powered, tandem water-splitting device by com...

Boundary Conditions for Thermoelectric Cooling in p–n Junction

The article is devoted to analysis of the boundary conditions for the Peltier effect in semiconductors containing potential barriers (p–n junction). The full system of boundary conditions, taking into consideration the presence of nonequilibrium charge carriers, is offered. The surface recombination of charge carriers is taken into account for both the electric current and the propagation of heat.

Using Design of Experiments Approach to Optimize Custom Emitter Clean Process used in PV Manufacturing

Abstract We describe a ‘Design of Experiment’ (DOE) technique used to optimize the process parameters of a new commercially available emitter clean solution aimed to enhance the output efficiency of the cell. Emitter etch solutions are known to substantially improve efficiency by reducing surface recombination of charge carriers and increasing the short circuit current of the cell. We have used a full factorial design of experiment with three variables kept at two levels. Appropriate randomization of the run order and three centre points were chosen in order to reduce experimental errors. Adjacent multicrystalline wafers chosen for this experiment were subjected to acid texturization, phosphorus doping, diffusion and phosphosilicate glass etching before the emitter clean step. We chose mean emitter sheet resistance (Rsheet) (average over 25 positions on each wafer) as the response of this experiment. The aim was to achieve an increment of 10% on Rsheet after emitter clean. Voc scan of the wafers were done to validate the Rsheet increments observed. The results showed that H2O2 concentration and temperature had statistically significant effects on change of Rsheet. Regression analysis was used to predict the optimized parameters that may achieve a 10% increase of Rsheet. A validation run was carried out with this recipe in an automated production line and solar cells fabricated from this run showed a 0.18% (absolute) increase in efficiency and 15% (relative) reduction of recombination current density (Jo2) compared to standard reference cells processed without emitter clean.

Silicon-on-insulator nanotransistors with two independent gates

The results of numerical simulation of electrical characteristics of silicon-on-insulator MOSFET nanotransistors with two independent gates are reported. The cases of grounded and floating bases with the surface recombination of charge carriers either disregarded or taken into account are considered. It is shown that, at specified design parameters (gate length 50–100 nm, thickness of silicon layer 25–50 nm), one can vary the transistor’s threshold voltage by 0.45 V, reduce the transistor current in the off state by seven orders of magnitude, and decrease the subthreshold slope of the gate characteristics to 60 mV/decade by varying the voltage applied to the second gate. Suppression of the short-channel effects in the transistors under consideration depends on a number of parameters (listed in order of decreasing effect): the gate material, the lifetime of charge carriers (for floating or grounded base), the thickness of the top silicon layer, the voltage applied to the additional gate, and the channel length.

Silicon Passivated by Insulating Erbium Oxide Films

The recombination properties of silicon passivated by insulating films of erbium oxide (Er2O3) have been studied. After deposition of an oxide film, the effective lifetime of nonequilibrium charge carriers, measured by method of nonstationary photoconductivity relaxation, increased by a factor of two to three as compared to the initial value. The rates of the surface recombination of charge carriers at the silicon-oxide interface have been determined. The insulating films of erbium oxide are promising passivating coatings for siliconbased devices and integrations.

Silicon passivated by two-layer insulating films of ytterbium oxide and dysprosium oxide

We have studied the recombination properties of silicon passivated by two-layer insulating films of ytterbium oxide and dysprosium oxide. After deposition of such a two-layer coating of rare earth element (REE) oxides, the effective lifetime of nonequilibrium charge carriers, measured by the method of nonstationary photoconductivity relaxation, is two to three times the initial value. The rates of the surface recombination of charge carriers at the silicon-REE oxide interface have been determined. Two-layer insulating films of REE oxides are promising passivating coatings for semiconductor devices and integrations.

Laser modulated optical reflectance of thin semiconductor films on glass

Semiconductor films, deposited by reactive magnetron sputtering on glass substrates, have been analyzed with the help of laser modulated optical reflectance. The results are discussed with respect to the thermal and charge carrier transport properties. Charge carrier surface recombination at the front surface and at the interface between semiconductor film and glass substrate have been identified both for microcrystalline and amorphous films.

Ultrafast Dynamics at Semiconductor and Metal Surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.

The Ternary Cd0.7Zn0.3Se Compound, a Novel Room Temperature X-Ray Detector

A novel ternary semiconductor compound Cd0.7Zn0.3Se has been developed as a nuclear radiation detector. A higher energy band gap comparatively to the binary CdSe enabled the preparation of detectors with a lower leakage current and consequently a lower electronic noise. As a result lower noise threshold could be obtained allowing nuclear spectrometry down to the energy of 6KeV. The charge collection efficiency has been studied as a function of voltage applied to a thin Cd0.7Zn0.3Se platelet. The limitation of transport properties is discussed in terms of bulk trapping versus surface recombination of charge carriers.

Temperature gradient solution zoning growth and characterization of Zn xCd 1− xSe single crystals

Single crystals of Zn x Cd 1- x Se with 0 ⩽ x ⩽ 0.3 have been grown from a high temperature Se solution using the TGSZ technique. The Zn/Cd distribution has been measured using an original XRF analysis system and found uniform throughout any particular crystal. The dark resistivity of Zn 0.3Cd 0.7Se crystals (5 × 10 7 ohm cm) is higher than that of CdSe crystals (10 6 ohm cm) grown by the same method, due to the larger energy band gap of the ternary compound. A discrete electron trapping level located 0.49 eV below the conduction band with a trap density of 1.9 × 10 11 cm −3 has been attributed to a compensated donor formed by an interstitial Cu impurity and a nearby Cd vacancy. The charge collection efficiency has been studied as a function of voltage applied to a thin Zn 0.3Cd 0.7Se platelet. It is not limited, however, by the electron traps, but rather by the surface recombination of charge carriers. It has been suggested that an improved charge collection needed for the fabrication of (Zn,Cd) Se nuclear radiation detectors can be obtained by reduction of surface defects generated during the crystalline platelet processing.