Open-circuit Voltage Decay Technique Research Articles

Superior ZnS deposition for augmenting the photostability and photovoltaic performance of PbS quantum-dot sensitized solar cells

Simply by elevating the successive ionic layer adsorption and reaction temperature for ZnS deposition, we successfully demonstrated improved photostability of PbS-based quantum dot-sensitized solar cells (QDSCs) which was credited to efficient surface coverage of ZnS over PbS QDs and TiO2. Using open-circuit voltage decay and cyclic voltammetry techniques distinctly, it was affirmed that the back electron transfer at TiO2/PbS/electrolyte interface was reduced. Superior deposition of ZnS in terms of surface coverage not only exhibited increase in photocurrent density (from 20.7 mA/cm2 to 22.3 mA/cm2) and power conversion efficiency (from 3.1% to 3.4%), it also stabilized the normalized current significantly.

Effective photo-harvesting by dye sensitized solar cell based on dihydrothieno [3,4-b][1,4] dioxine bridge based metal free organic dye

Abstract Here in design and synthesized new dimer organic metal free dyes based on various donor moieties along with dihydrothieno [3,4-b][1,4] dioxine and cynoacrylic acid are bridge and acceptor respectively. The influence of dye consist various donor moieties like phenothiazine (PTZ), phenoxazine (POZ) and carbazole (CBZ) studied carefully. The same donor and acceptor groups optical and electrochemical properties change when we change donor groups. The all dyes structure geometry optimization and energy levels (HOMO-LUMO) was calculated by density functional theory (DFT) with B3LYP/6-31G(d.p) basis set. It is evident from the studies that the strong electron withdrawing donor moiety based dye, POZ-102 exhibit highest short circuit current density (Jsc), open circuit voltage (Voc) and efficiency (η = 6.4%). The inside device electron recombination and electron lifetime was calculated by impedance spectroscopy and open circuit voltage decay techniques. In last the highest efficiency achieved POZ-102 dye based device checked long term stability up to 500hr under 100 mW/cm2 light illumination.

Electrical and optical properties of Si microwire solar cells

A record-high efficiency of 16.92% was realized by using microscale-patterned Si solar cells. Si microwire (SiMW) solar cells were designed to compare their performances with the standard flat-Si solar cell. The diameter and period of SiMWs were tuned to investigate the effects on the operation and the electrical properties of the solar cells. The highest efficiency SiMW solar cell has a width of 1.57µm to provide the significantly enhanced efficiency of 16.92% from 10.15% of the flat-Si solar cell, due to the improved values of open circuit voltage (608mV), short circuit current density (36.47mA/cm2) and fill factor (76.29%). Impedance spectroscopy and capacitance-voltage studies were carried out to obtain series and shunt resistances, built-in potential, acceptor carrier density, depletion width and flat band potential. Optical properties of SiMW solar cells were analyzed by external and internal quantum efficiencies. The carrier lifetime was measured by using open circuit voltage decay technique. Systematic analyses were performed to reveal the optical and electrical features of SiMW solar cells. These findings may provide efficient design schemes for high-performing solar cells using structured platforms.

Improved performance of nanoporous TiO2 film in dye-sensitized solar cells via ZrCl4 and TiCl4 surface co-modifications

In this study, nanoporous TiO2 films were modified by a dip-coating process using a mixture aqueous solution of ZrCl4 and TiCl4 followed by calcination to prepare a photoanode for dye-sensitized solar cells. Compared with the control film modified with 0.04molL−1 TiCl4, the power conversion efficiency of the TiO2 film modified with a mixed solution of 0.05molL−1 ZrCl4 and 0.04molL−1 TiCl4, was 18.67% higher because of the improved short circuit current (Jsc) and open circuit voltage (Voc). The improvement in Jsc was due to the suppression of charge recombination, which was demonstrated by a series of measurements, including electrochemical impedance spectroscopy, monochromatic incident photon-to-electron conversion efficiency spectroscopy, and the open-circuit voltage decay technique. The Mott-Schottky measurement results indicated that the negative shift of a flat band led to the increased Voc.

Uncovering the charge transfer and recombination mechanism in ZnS-coated PbS quantum dot sensitized solar cells

In this work, the charge transfer and recombination mechanism is uncovered for the PbS/ZnS quantum dot sensitized solar cells (QDSSCs) based on nanoporous TiO2 electrodes. PbS quantum dots (QDs) were in-situ grown on TiO2 nanoparticles through the successive ionic absorption and reaction (SILAR) method, followed by the surface passivation of ZnS for the sensitized electrodes. It was observed that the ZnS coating cycles play a significant role in determining the photovoltaic parameters. The highest power conversion efficiency of 1.4% was achieved by coating 13 cycles of ZnS on TiO2/PbS electrode. It is essential to understand why and how ZnS passivation layers improve the photovoltaic performance of PbS QDSSCs. All obtained solar cells were characterized thoroughly by optical and electrical techniques. The open-circuit voltage decay technique and electrochemical impedance measurements indicated that the ZnS passivation layers significantly suppressed the charge recombination at the TiO2/electrolyte and TiO2/QD interfaces. The transient grating measurements suggested that the electron injection from PbS QDs to TiO2 was obviously enhanced by the ZnS coating layers. This could be attributed to the reduction of carrier trapping and recombination in PbS QDs after surface passivation. These beneficial effects of ZnS layers, therefore, resulted in the improved photovoltaic performances of PbS QDSSCs. This work provides better understanding on the passivation effect of ZnS layers in PbS QDSSCs, which would be beneficial for the further improvement of QDSSCs.

Highly ordered and vertically oriented TiO2/Al2O3 nanotube electrodes for application in dye-sensitized solar cells

The surface of long TiO2 nanotube (NT) electrodes in dye-sensitized solar cells (DSSCs) was modified without post-annealing by using atomic layer deposition (ALD) for the enhancement of photovoltage. Vertically oriented TiO2 NT electrodes with highly ordered and crack-free surface structures over large areas were prepared by a two-step anodization method. The prepared TiO2 NTs had a pore size of 80 nm, and a length of 23 μm. Onto these TiO2 NTs, an Al2O3 shell of a precisely controlled thickness was deposited by ALD. The conformally coated shell layer was confirmed by high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The open-circuit voltage (Voc) of the DSSCs was gradually enhanced as the thickness of the Al2O3 shell of the TiO2/Al2O3 NT electrodes was increased, which resulted from the enhanced electron lifetime. The enhanced electron lifetime caused by the energy barrier effect of the shell layer was measured quantitatively by the open-circuit voltage decay technique. As a result, 1- and 2-cycle-coated samples showed enhanced conversion efficiencies compared to the bare sample.

Enhanced electron collection in photoanode based on ultrafine TiO2 nanotubes by a rapid anodization process

The separated and ultrafine TiO2 nanotubes are fabricated by a modified rapid anodization method, which cannot be achieved through conventional anodization. Then, model dye-sensitized solar cells based on the prepared TiO2 nanotubes and commercial TiO2 nanoparticles (P25) are investigated, and a discrepancy is discovered between the light-harvesting capability and the power conversion efficiency. The charge transport and recombination are studied by the electrochemical impedance spectroscopy and the open-circuit voltage decay technique. Results show that the nanotube photoanode owns a longer electron diffusion length and a larger electron lifetime than the nanoparticle one, which can compensate for the loss of light absorption. The enhanced electron collection efficiency observed is attributed to the facilitated charge carrier pathways in the photoanode composed by the separated TiO2 nanotubes fabricated in this work. Therefore, the TiO2 nanotubes synthesized by this method are verified to have good electronic properties, which might find applications not only in photovoltaic, but also in catalysis, sensors, and other areas.

Experimental considerations for temperature controlled measurements of fast charge recombination times in dye sensitized solar cells using open circuit voltage decay and impedance spectroscopy

Charge recombination at the electrolyte–photoanode interface of a dye sensitized solar cell (DSSC) is a major efficiency-limiting factor of the cell. To mitigate this recombination effect it is necessary to ensure that the effective electron lifetime in the DSSC is longer than the electron’s transit time across the photoanode of mesoporous TiO2. While the efforts aimed at accomplishing this goal are often based on new materials/designs of photoanodes, a quantitative evaluation of these designs relies on the precision of the benchmarking measurements of electron lifetimes. The open circuit voltage decay (OCVD) technique offers an effective yet straightforward method for such measurements. The present work focuses on certain experimental criteria for ensuring the accuracy of these experiments, and probes the associated effects of temperature variations in the solar cell. The results demonstrate that, a high rate of data sampling is essential for adequately resolving the fast initial stages of charge recombination. The results also show the effects of nonlinear recombination where second order OCV variations are operative. The findings of the OCVD experiments are compared with a parallel set of tests carried out using impedance spectroscopy. The relative roles of the two sets of analytical measurements are examined.

Photovoltaic performance improvement of dye-sensitized solar cells through introducing In-doped TiO2 film at conducting glass and mesoporous TiO2 interface as an efficient compact layer

In-doped TiO2 thin film was introduced at the interface of fluorine-doped tin oxide (FTO) substrate and mesoporous TiO2 film by spin-coating method, and its application as a new compact layer material for dye-sensitized solar cells (DSSCs) was investigated. The scanning electron microscopy (SEM), UV-visible spectroscopy, current-voltage characteristics, Mott-Schottky analysis, electrochemical impedance spectroscopy (EIS) analysis and open-circuit voltage decay (OCVD) technique are used to characterize the morphology, optical transmittance and flat-band potentials (Vfb) of In-doped titania compact film and its effect to the photoelectron conversion process. It was found that In-doping increased the transmittance of TiO2 compact layer, the interfacial resistance between FTO substrate and porous TiO2 film and the flat-band potential of TiO2 film. The In-doped TiO2 compact layer effectively suppressed the charge recombination from FTO to the electrolyte, increased the optical absorption of dye and then increased the short-circuit photocurrent density (Jsc). Furthermore, In-doped TiO2 compact layer acted as a weak energy barrier, which increased the electron density in the mesoporous TiO2 film, thus improved open-circuit photovoltage (Voc). As a result, the overall energy conversion efficiency of the DSSC with In-doped TiO2 compact layer was enhanced by 11.9% and 6.9% compared to the DSSC without compact layer and with pure TiO2 compact layer, respectively. It indicated that In-doped TiO2 is a promising compact layer material for dye-sensitized solar cells.

Temperature-dependent photovoltaic characterization of a CdTe/CdSe nanocrystal’s solar cell

An all-inorganic thin film solar cell was fabricated with colloidal CdTe and CdSe nanocrystals. Its temperaturedependent photovoltaic characterization was measured, and an open circuit voltage decay technique was used to determine the electron lifetime. The photovoltaic parameters, such as the exact carrier lifetime, were remarkably different between under low-temperature region and under temperature above 267 K. It is suggested that the temperature-dependent carrier conductivity results from a thermally-activated hopping process and this nanocrystals device is a kind of donor-acceptor solar cell.

Influence of defect density on the ZnO nanostructures of dye-sensitized solar cells

The relationship between bilayer nanostructure, defect density and dye-sensitized solar cell (DSCC) performances was investigated. By adjusting bilayer nanostructures, defect density of ZnO nanodendrite-nanorods structure was decreased comparing to that of nanoflower-nanorods structure. The performances of DSCC based on ZnO nanodendrites-nanorods structure and nanoflower-nanorods structure were studied by Raman spectrum, room temperature photoluminescence, dye loading, photocurrent density-voltage characteristic and open-circuit voltage decay (OCVD) technique. The device with nanodendrite-nanorods structure has lower charge recombination rate and prolonged electron lifetime due to its microstructure feature.

A Comparison of the Microwave Photoconductivity Decay and Open-Circuit Voltage Decay Lifetime Measurement Techniques for Lifetime-Enhanced 4H-SiC Epilayers

This work compares the optical microwave photoconductivity decay (μPCD) and electrical open-circuit voltage decay (OCVD) techniques for measuring the ambipolar carrier lifetime in 4H-silicon carbide (4H-SiC) epitaxial layers. Lifetime measurements were carried out by fabricating P+/intrinsic/N+ (PiN) diodes on 100-μm-thick, 1 × 1014 cm−3 to 4.5 × 1014 cm−3 doped N-type 4 H-SiC epilayers, and measuring the lifetime optically using μPCD prior to metallization, then electrically using OCVD after contact deposition. Both as-grown epilayers as well as epilayers with improved lifetime (via thermal oxidation) were measured using both techniques. The observed ambipolar lifetime was improved from 1.4 μs on an unenhanced wafer to 4 μs on a wafer enhanced through the oxidation process as measured by μPCD. Little difference was observed between the μPCD and OCVD measurements on the unenhanced wafer; the ambipolar lifetime on the enhanced wafer measured by OCVD was approximately 5.5 μs, or 1.5 μs higher than the μPCD measurement. Continuous evaluation of the OCVD transient waveform was necessary due to the high lifetime in the enhanced wafer; shunt resistances included to discharge the P+/N junction capacitance were found to damp the OCVD response and yield low values for the measured lifetime. Simulation of the μPCD measurement including various surface recombination conditions yielded a good match to experimentally observed μPCD measurements for high values of the surface recombination velocity. The OCVD lifetime measurement technique is expected to yield measured lifetime values closer to the physical value due to its independence from surface conditions, provided that the experimental conditions are appropriately chosen.

Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode

The influence of temperature on the intensity of light emitted by as well as the carrier life time τ of a standard AlGaAs based light emitting diode has been investigated in the temperature range from 345 to 136 K. The open-circuit voltage decay (OCVD) technique has been used for measured the carrier lifetime. Our experimental results reveal a 16% average increase in intensity and a 163.482–19.765 ns variation in carrier lifetime in the above temperature range. Further, theoretical and experimental analysis show that for negligible carrier density the intensity is inversely proportional to carrier lifetime for this sample.

Electrophoretically self-assembled mixed metal oxide-TiO2 nano-composite film structures for photoelectrochemical energy conversion: Probing of charge recombination and electron transport resistances

Nanotitania/metal oxide composite films are prepared via the coupling of low DC voltage Electrophoretic Deposition (EPD) of P25 TiO2 nanoparticles with simultaneous electrolytic deposition of hydrous metal oxides, namely ZnO, MgO and Al2O3. The nanocomposite films are built into dye-sensitized solar cell photoanodes and their interfacial charge recombination and electronic resistances are investigated. The nano hydrous oxides were found to co-deposit (1–3 wt%) uniformly within the TiO2 film (forming island-like nanodeposits) significantly increasing film adhesion. Analysis via Electrochemical Impedance Spectroscopy and Open Circuit Voltage Decay techniques found, among the three composite films, the TiO2–Al2O3 electrode to exhibit the highest charge recombination resistance at the TiO2/electrolyte interface (Rrec) and as consequence an increase in Voc. However, its conversion efficiency (4.14%) was the lowest because it suffered from very high electron transport resistance (Rt) in the TiO2 network. By comparison, the TiO2–MgO film resulted in 5.40% efficiency and the TiO2–ZnO film in 5.85% efficiency-both exhibiting significantly lower Rt resistance. The obtained results point to the need for simultaneous optimization of the nanocomposite TiO2/metal oxide film structure in terms of high interfacial charge recombination resistance and low overall electron transport resistance.

ZnO–ionic liquid hybrid films: electrochemical synthesis and application in dye-sensitized solar cells

An innovative ionic liquid (IL)-based synthesis route was developed to obtain ZnO-based hybrid nanostructured films with a modified surface. In particular, ZnO–IL hybrid films, with thickness from 1.5 to 4.5 μm and an appealing sponge-like morphology, were obtained from the electrochemical reduction of NO3− in ionic liquid-based electrolytes containing Zn2+. The presence of the ionic liquid moieties and/or derivatives in the as-deposited films was demonstrated by Fourier transform infra-red spectroscopy and energy dispersive X-ray spectroscopy. However, a relatively soft thermal annealing (i.e. 1 hour in air at 350 °C) was proved to be an effective way to remove the ionic liquid content from the samples, leading to porous ZnO films with high specific surface area. In comparison to ionic liquid-free samples, a blue shift of ∼40 meV in the absorption onset is detected for the ZnO-PYR14TFSI hybrid films, which suggests a modification of the bandgap. Both kind of films (i.e. ZnO–IL and ZnO) were sensitized with an indoline dye, coded D358, and evaluated as photoanodes in Dye-sensitized Solar Cells (DSCs). Although less D358 was adsorbed onto ZnO–IL samples (suggesting co-adsorption of the IL and D358), a clear enhancement (by a factor of ca. 2) of the power conversion efficiency was detected in DSC based on ZnO–IL hybrid films. This improvement was mainly due to a huge increase (∼180 mV) in the photovoltage, which reaches values of up to 780 mV. The DSC characterization, by electrochemical impedance spectroscopy and open circuit voltage decay techniques, indicates that the photovoltage improvement is likely due to a negative displacement of the conduction band in hybrid films. Therefore, the use of metal oxide–IL hybrid anodes appears to be a promising strategy to increase the open circuit voltage of the DSCs.

Charge transport and recombination in dye-sensitized solar cells based on hybrid films of TiO 2 particles/TiO 2 nanotubes

In this paper, anodic TiO 2 nanotubes are blended into the TiO 2 mesoporous films based on P25 nanoparticles to assemble a list of dye-sensitized solar cells (DSSCs) with different nanotube concentrations. The electron properties of transport and recombination in the fabricated DSSCs are studied by using electrochemical impedance spectroscopy and the open-circuit voltage decay technique under AM 1.5 illumination. Results indicate that the electron lifetime increases with increasing the concentration of the anodic TiO 2 nanotubes, the electron transport time at a blending level of 10 wt% TiO 2 nanotubes is short as compared to that at 0 wt%, and above 10 wt%, the electron transport time has a trend of becoming large. Due to the combining effects of the electron transport and recombination, the electron collecting efficiency and the electron diffusion length obtain maxima at a blending level of 10 wt% nanotubes, which results in a highest short circuit current and a maximum energy conversion efficiency at this point in the DSSCs. This study gives a clear explanation for the performance enhancement of TiO 2 particle-based DSSCs at a blending level of 10 wt% anodic TiO 2 nanotubes and for the performance decrease at a blending level over 10 wt% anodic TiO 2 nanotubes from the angle of the electron transport and recombination. This study also supplies a feasible and easy way to improve the performance of particle-based DSSCs by restraining electron recombination and accelerating electron transportation.

Efficiency enhancement in dye-sensitized solar cells by interfacial modification of conducting glass/mesoporous TiO 2 using a novel ZnO compact blocking film

A novel and thin ZnO compact blocking film is employed at the interface of fluorine-doped tin oxide (FTO) substrate and mesoporous TiO 2, and its influence on dye-sensitized solar cells (DSSCs) is investigated. The ZnO film prepared by spin-coating method on FTO is characterized by energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and UV–vis spectrophotometer. The ZnO film is firstly employed as an energy barrier between FTO and mesoporous TiO 2 film in DSSCs, which improves open-circuit photovoltage ( V oc) and fill factor (FF) with compensation of J sc decrease, finally increasing energy conversion efficiency from 5.85% to 6.70%. Electrochemical impedance spectra (EIS) analysis and open-circuit voltage decay (OCVD) technique reveal that the existence of the energy barrier not only resulted in the effect of suppressing back electrons transfer from FTO to electrolyte but also blocking the electrons injection from the conductive band of TiO 2 to FTO. The former effect effectively reduces the recombination which occurs in the region of FTO substrate, and the latter leads to remarkable increment of electron density in the TiO 2, thus resulting in enhanced V oc and FF. These results suggest that the methodology of introducing the semiconductor with a more negative conduction band edge than TiO 2 as the compact blocking film in DSSCs may be feasible.

Performance Enhancement and Limitations of Cobalt Bipyridyl Redox Shuttles in Dye-Sensitized Solar Cells

A series of one-electron outersphere cobalt bipyridyl redox couples were used as redox shuttles in dye-sensitized solar cells (DSSCs). Atomic layer deposition was used to deposit an ultrathin coating of alumina on nanoparticle-based TiO2 DSSC photoanodes, which results in significantly improved quantum yields for all of the DSSCs containing outersphere redox systems. However, a significant discrepancy in performance remains between DSSCs containing the different cobalt redox shuttles. Variation of the driving force for regeneration by ∼500 mV, by employing [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 as a dye, combined with concentration dependence studies indicates that the cobalt redox couples are not limited by dye regeneration; however, in certain cases the iodide electrolyte was, one of the very few systems where alternate redox couples perform significantly better than triiodide/iodide. Electron lifetimes were measured with the open circuit voltage decay technique. The differences in the lifetimes (recombin...

The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell

The open circuit voltage decay (OCVD) technique has been used to determine the minority carrier lifetime. In this study, an experimental and analytical method is described for determination of minority carrier lifetime at porous Si based solar cell by photo induced OCVD technique. The cell is illuminated by a monochromatic light source ( λ = 658 nm) in the open circuit configuration, and the decay of voltage is measured after abruptly terminating the excitation. For the analysis of the OCVD characteristic of solar cell device, equivalent electrical circuit has been proposed in which the diffusion capacitance is connected in series with the contribution of the solar cell interface. Exact minority carrier lifetimes at low (50–170 K) and high (190–330 K) temperature regions have been obtained as 28.9 and 2.65 μs from the temperature dependent OCVD measurements by using an alternative extraction technique.

Effect of anodic TiO2 powder as additive on electron transport properties in nanocrystalline TiO2 dye-sensitized solar cells

Highly crystalline anodic TiO2 (AO-TiO2) powder was obtained by grinding of annealed TiO2 nanotube arrays grown using anodization of Ti foil. The influence of AO-TiO2 as additive in nanocrystalline TiO2 film on electron transport properties affecting the performance of these dye-sensitized solar cell was investigated using electrochemical impedance spectroscopy and the open-circuit voltage decay technique under AM 1.5 illumination. As a result of the enhanced charge-collection and light harvesting efficiencies, an increase of above 20% in photocurrent density was observed after the addition of AO-TiO2.