Open-circuit Voltage Decay Curves Research Articles

English

In this manuscript, we present a study on the effects of magnetic field on charge carrier effective lifetime within the base of a polycrystalline PV solar cell p-n junction. This study is elaborated in two forms as a theoretical study and an experimental study. Indeed, under various magnetic field values, PV solar cell is illuminated by using the light from stroboscopic. From a theoretical approach mainly use the open circuit voltage decay method.  Green’s functions and boundaries conditions are used to solve charge carrier’s diffusion equation. Open circuit transient voltage decay expression is found. The charge carriers’ effective lifetime values inversely proportional to the slope of open circuit voltage decay curve are deducted. A simple experimental set-up used open circuit voltage decay method mainly, these experimental measurements of charge carriers’ effective lifetime are done and the effects of magnetic field are then outlined. Globally, magnetic effects on effective charge carrier lifetime are then analyzed. These two types of results obtained present qualitatively the decay of charge carriers’ effective lifetime when magnetic field increase. In the first time, theoretical results and experimental results are in good agreement and in the second time these results are in good agreement with literature results. Key words: Effective lifetime, open circuit voltage decay, magnetic field.

Down-conversion Ce-doped TiO2 nanorod arrays and commercial available carbon based perovskite solar cells: Improved performance and UV photostability

Ce-doped TiO2 nanorod arrays as the mesoporous supporting layer and TiO2 compacted electron blocking layer was simultaneously fabricated by one-pot hydrothermal method. Ce-doped TiO2 nanorod was utilized as down-conversion luminescence centers and commercial available carbon was used in hole-transport-free perovskite solar cells. SEM indicated that the doping of Ce reduced the diameter of TiO2 nanorods. XRD and EDS spectra demonstrate the successful incorporation of Ce ions. UV–Vis absorption spectra and PL emission spectra show that the down-conversion center Ce4+ can effectively utilize ultraviolet light and extend the absorption into the ultraviolet region. The carbon-based device using the optimized Ce doping concentration achieved power conversion efficiency (PCE) of 10.1%, which was 24.6% higher than that of undoped device. Ce doping changes the band gap of TiO2 and the Fermi energy level, and thus improve the open circuit voltage. The open circuit voltage decay curve show that Ce-doped TiO2 prolongs electron lifetime. EIS analysis hints that more carriers are generated. The UV stability has also been significantly improved., PCE of the optimized Ce-doped device still retains 90% of the initial value, however, the undoped device decreases to 55% of the initial value under light irradiation at ambient atmosphere for 12 h.

Polyoxometalate-modified TiO2 nanotube arrays photoanode materials for enhanced dye-sensitized solar cells

In this work, we prepared for the first time the TiO2 nanotube arrays (TNAs) photoanode with polyoxometalate(POMs)-modified TiO2 electron-transport layer for improving the performance of zinc phthalocyanine(ZnPc)-sensitized solar cells. The as-prepared POMs/TNAs/ZnPc composite photoanode exhibited higher photovoltaic performances than the TNAs/ZnPc photoanode, so that the power conversion efficiency of the solar cell device based on the POMs/TNAs/ZnPc photoanode displayed a notable improvement of 45%. These results indicated that the POMs play a key role in reducing charge recombination in phthalocyanine-sensitized solar cells, together with TiO2 nanotube arrays being helpful for electron transport. The mechanism of the performance improvement was demonstrated by the measurements of electrochemical impedance spectra and open-circuit voltage decay curves. Although the resulting performance is still below that of the state-of-the-art dye-sensitized solar cells, this study presents a new insight into improving the power conversion efficiency of phthalocyanine-sensitized solar cells via polyoxometalate-modified TiO2 nanotube arrays photoanode.

A TiO2@{Mo368} composite: synthesis, characterization, and application in dye-sensitized solar cells

The high-nuclear cluster compound Na48[HxMo368O1032(H2O)240(SO4)48]·ca.1000H2O (denoted as {Mo368}) represents the known nanoscale hedgehog-type cluster anion with the diameter of approximately 6 nm. Herein, a TiO2@{Mo368} composite was prepared through a sol–gel process for the first time. SEM, XPS, and UV–vis spectra were employed to characterize their chemical composition and structures. Meanwhile, the as-obtained composite was further mixed with P25 (Degussa P25 titania photocatalyst) then applied as the photoanodes of dye-sensitized solar cells (DSSCs); the results showed that DSSCs with the P25-TiO2@{Mo368}-based photoanodes exhibited better performance than that with pure P25-based photoanodes, which was due to less carrier recombination and longer electron lifetime in the former DSSCs by the results from analysis of dark current measurement, electrochemical impedance spectroscopy, and open-circuit voltage decay curve.

Synthesis of Flower-Like ZnO Films and Their Photovoltaic Properties for Dye-Sensitized Solar Cells

Flower-like ZnO films were synthesized by chemical bath deposition, and their performance as photoanodes for dye-sensitized solar cells (DSSCs) was investigated. The physical and chemical properties of the grown layers as a function of Zn2+ concentration were investigated by scanning electron microscopy, energy-dispersive spectroscopy, and ultraviolet (UV) –visible spectroscopy. Electrochemical impedance spectroscopy and open-circuit voltage decay curve measurements were used to determine the kinetic processes of the ZnO photoanodes of the DSSCs. The results show that the bandgap of the ZnO film was smaller than that of the pure one. DSSCs with flower-like ZnO film with 0.05 M Zn2+ exhibited the smallest electron-transfer resistance in the ZnO/dye/electrolyte interface, indicating the highest light conversion efficiency. DSSCs based on 0.075 M Zn2+ showed the highest open-circuit voltage of 0.74 V.

Nanoporous TiO2 aerogel blocking layer with enhanced efficiency for dye-sensitized solar cells

TiO2 aerogel film possessing nanoporous feature was used as the blocking layer between fluorine-doped tin oxide (FTO) substrate and mesoporous TiO2 layer in dye-sensitized solar cell (DSSC). TiO2 aerogel film was deposited on FTO glass via sol–gel and ambient drying processes and sintered at 500 °C, which exhibited the crystallite size of 5–25 nm, the pore size of 3–10 nm, and the thickness of ∼320 nm. Brunauer–Emmett–Teller (BET) analysis confirmed its nanoporous feature and average pore size of 3.8 nm after annealing at 500 °C. In addition, TiO2 aerogel film showed good interfacial adhesion with FTO and mesoporous TiO2 layer, high transmittance over 70% in the visible-near infrared band, and large band gap of 3.67 eV. The DSSC device based on this aerogel blocking layer showed obviously enhanced photocurrent density (Jsc) and open-circuit voltage (Voc) compared with the cell without blocking layer. The highest conversion efficiency of 6.0% was achieved for P25 TiO2 mesoporous photoanode based on the aerogel blocking layer, improved by 36% than the control cell. The much decreased recombination frequency (aerogel cell: 11.9 Hz; control cell: 31.5 Hz) observed in Nyquist plots, together with the much prolonged electron lifetime (aerogel cell: ∼1 s; control cell: 0.02–0.1 s at 0.2–0.55 V) obtained from open-circuit voltage decay curves, indicated that the increased electron life and retarded recombination at FTO/electrolyte interface were primarily responsible for the efficiency improvement in the aerogel blocking layer based DSSC.

TiO2 photoanodes prepared by cathodic electrophoretic deposition in 2-propanol: effect of the electric field and deposition time

This paper evaluates the influence of electric field and deposition time applied on cathodic electrophoretic formation of TiO2 films in organic medium (2-propanol). The film morphology was tracked by measuring the deposited mass and film thickness. The variation in film porosity was correlated with the apparition of surface states distribution in the cyclic voltammetric characterization in the dark, due to grain boundaries defects generated in the contact of the TiO2 particles. The open-circuit voltage decay curves showed that there is no formation of deep energy states inside the band gap of the TiO2. The photopotential of the films increased until a critical thickness but the photocurrents showed to be dependent on operational variables, due to the fact that anodic polarization in thin films increases the electric field generated by the illumination at the ITO/TiO2 interface, favoring the transport of the photogenerated electrons to the rear contact.

Effects of rare earth ion modifications on the photoelectrochemical properties of ZnO-based dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) were fabricated by using porous ZnO film electrodes derived from home-made ZnO nanoparticles. Electrochemical impedance spectroscopy and open-circuit voltage decay curve measurements were performed to investigate the photoelectrochemical characteristics of the ZnO films modified with different rare earth (La, Ce, Nd, Sm, and Gd) ions. The experimental results indicate that the rare earth oxides covered on the electrode surfaces can form energy barrier and maintain a lower charge recombination probability, and some rare earth ion modifications can passivate the surface states of ZnO electrode. Among these rare earth ions tested, the Nd-, Sm- and Gd-ion modifications can obviously enhance the open-circuit photovoltage and fill factor of the ZnO-based solar cell; whereas the La-, Ce-, Nd-, and Sm-ion modificaions lead to a decreased short-circuit photocurrent. The optimal conversion efficiency is obtained from the Gd-ion modified ZnO-based cells, with a 44.5% improvement in the efficiency as compared to the unmodified one, indicating this rare earth ion modification is promising in the ZnO-based solar cell.

Improved Electron Diffusion Coefficient in Electrospun TiO2 Nanowires

TiO2 nanowires with a diameter of ∼150 nm, length of ∼3−4 μm, and aspect ratio of 10:1, were prepared by ultrasonically dispersing electrospun continuous nanofibers in monocarboxylic acids. The resulting pastes were used for making nanowire films on conducting glass substrates with thicknesses in the range of 500 nm to 100 μm, good adhesion, and high nanowire packing. These films were used to fabricate dye-sensitized solar cells using the D131 dye and the iodide/triiodide electrolyte. Transient photocurrent measurements showed a high electron diffusion coefficient in those nanowire films. The measured diffusion coefficient in those TiO2 nanowires was orders of magnitude higher than that observed in nanoparticles under similar experimental conditions. The charge-transport mechanism in the nanowire sample is discussed in support with the measured open-circuit voltage decay curves.

Theoretical investigations of experimentally-observed Open-Circuit Voltage-Decay (OCVD) curves

The study of the open-circuit voltage-decay (OCVD) curve has significant importance for determining the minority carrier lifetime in the quasineutral base of a semiconductor diode. Different models, proposed earlier, have provided a basis for explaining the experimental results. However, the observed peculiar nature of the curves cannot be fully explained in terms of phenomena associated with base and depletion regions. In the present work, a theoretical analysis has been carried out to investigate the possible mechanisms responsible for the observed peculiarities in the OCVD curves, by including the transition carrier capacitance of the depletion region. This capacitance has been used in addition to the well-known and widely discussed effects, such as the space-charge capacitance due to ionized impurities, recombination current, and base region effects. The inclusion of the transition carrier capacitance has been done for the first time.

Charge injection and transport in high voltage water/glycol capacitors

A water/ethylene glycol mixture to be operated at temperatures below 0 °C is being investigated for use as the high permittivity, low loss dielectric in pulse forming lines for pulsed power applications. High voltage open circuit decay curves show a dielectric relaxation time that decreases with initial voltage at room temperature, while at −10 °C, the open circuit decay has a negative second derivative with time in contrast to the expected exponential decay which always has a positive second derivative. It is hypothesized that this anomalous behavior is due to space-charge injection which increases the ohmic conductivity σ to σ+q μ, where q is the time and space dependent net charge density with mobility μ. A drift dominated conduction model is used to solve for the electric field and charge density distributions and the terminal voltage-current behavior of a lossy capacitor where one electrode injects charge proportional to the instantaneous local electric field. The specific case studied is of a parallel plate capacitor rapidly charged to an initial voltage and quickly open circuited. The calculated open circuit voltage decay curves show good agreement with measurements and yield experimental values for the charge injection parameter and the high field mobility of the injected charge. The value of mobility for the best fit of experimental data to the theory agrees with the value for the hydronium ion. Preliminary Kerr electro-optic field mapping measurements in highly purified water without glycol, confirm positive charge injection.