Open Circuit Voltage Decay Measurements Research Articles

Inhibition of charge recombination for enhanced dye-sensitized solar cells and self-powered UV sensors by surface modification

The surface modification to inhibit charge recombination was utilized in dye-sensitized solar cells (DSSCs) and self-powered ultraviolet (UV) sensors based on SnO2 hierarchical microspheres by TiO2 modification. For DSSCs with SnO2 photoelectrodes modified by TiO2, the power conversion efficiency (PCE) was improved from 1.40% to 4.15% under standard AM 1.5G illumination (100mW/cm2). The electrochemical impedance spectroscopy and open-circuit voltage decay measurements indicated that the charge recombination was effectively inhibited, resulting in long electron lifetime. For UV sensors with SnO2 photoelectrodes modified by TiO2 layer, the self-powered property was more obvious, and the sensitivity and response time were enhanced from 91 to 6229 and 0.15s to 0.055s, respectively. The surface modification can engineer the interface energy to inhibit charge recombination, which is a desirable approach to improve the performance of photoelectric nanodevice.

Revealing and reducing the possible recombination loss within TiO2 compact layer by incorporating MgO layer in perovskite solar cells

Here, we report an origin of loss in open circuit voltage (Voc) in perovskite solar cells due to the possible trap-assisted recombination within the widely used TiO2 compact layer (CL) and at its interface with FTO substrate. A thin layer (TL) of MgO was employed to investigate the recombination mechanism. MgO in place of TiO2 CL (50–60nm) enhances Voc of the cell significantly while the MgO layer coated on TiO2 CL (TiO2-MgO bilayer) does not change the Voc much. In combination with open-circuit voltage decay (OCVD) measurements, we reveal that recombination at FTO/TiO2 interface is a major factor regulating the voltage and efficiency of TiO2-based perovskite solar cells.

P-Type dye-sensitized solar cells: Enhanced performance with a NiO compact blocking layer

A photoactive NiO electrode for p-type dye-sensitized solar cells (p-DSCs) has been equipped by the introduction of different coatings of a compact NiO blocking layer. This study examined the facile process for the fabrication of a compact NiO blocking layer by decomposing an ethanolic precursor solution of nickel acetate tetrahydrate. The P1-dye used as the photosensitizer for the NiO electrode in the p-DSCs device and their performances have been analyzed. The enhancement of both short-circuit photocurrent (Jsc) and fill factor (FF) resulted in a twofold increase in the power conversion efficiency (η). The electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) measurements demonstrated that charge recombination was suppressed when a compact NiO blocking layer was applied. The best p-DSC was achieved by employing spin-coated process for twice. The carrier lifetime of all the devices with a compact NiO blocking layer was improved compared to the bare cell.

Improving performance of copper(I)-based dye sensitized solar cells through I3−/I− electrolyte manipulation

The performances of dye-sensitized solar cells (DSSCs) containing a heteroleptic bis(diimine)copper(I) dye and an I3−/I− liquid electrolyte have been investigated as a function of the electrolyte composition. Starting from a standard electrolyte with LiI (0.1 M), I2 (0.05 M), 1-butyl-3-methylimidazolium (BMII) ionic liquid (0.6 M), 1-methylimidazole additive (0.5 M) in 3-methoxypropionitrile (MPN), a series of ten electrolytes was initially screened; the solvent, additives, concentration of I2, and sources (LiI and/or BMII) and concentrations of I− were varied. The highest short-circuit current densities (JSC = 7.85 and 7.60 mA cm−2) and photoconversion efficiencies (η = 2.64 and 2.70% relative to 6.56 and 6.11% for two N719 references) were observed for DSSCs with an electrolyte comprising I2 (0.03 M), BMII (0.6 M), 4-tert-butylpyridine (TBP, 0.4 M) and guanidinium thiocyanate (GNCS, 0.1 M) in MPN solvent; no LiI was added. This composition was further optimized by varying the amounts of TBP and GNCS. A low concentration of I2 (0.015 M) is optimal. The presence or absence of GNCS has little effect on overall DSSC performance, but increasing the concentration of TBP is detrimental to DSSC performance, and an absence of TBP is beneficial. Open-circuit voltage decay (OCVD) measurements confirm that the addition of GNCS and/or TBP reduces recombination rates at the TiO2-dye interface. The best DSC parameters obtained for the copper(I)-based dye were JSC = 7.80 mA cm−2, open-circuit voltage (VOC) = 501 mV and η = 2.76% (relative to 6.72% for N719).

Reduced graphene oxide as efficient and stable hole transporting material in mesoscopic perovskite solar cells

Perovskite solar cells (PSCs) are evolving with an impressive pace, emerging as the most promising next-generation photovoltaic devices. Nevertheless, beside these enormous progresses, the stability of PSCs remains one of the major problems to be challenged, because the most commonly used hole transporting material (HTM), the 2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)−9,9′-spirobifluorene (Spiro-OMeTAD), suffers long-term stability. Here, we tackle this long-lasting issue exploiting reduced graphene oxide (RGO), prepared by an easy and efficient reduction method, as HTM in PSCs. We carried out endurance tests over 1987h of shelf life and 120h of light soaking. The PSCs based on RGO have shown power conversion efficiency (PCE) of 6.6%, outperforming the ones achieved with the Spiro-OMeTAD (PCE=6.5%), after the shelf life test. The light soaking test confirmed the better durability of RGO-based PSCs with respect to the Spiro-OMeTAD ones, the latter showing a reduction in PCE 2.7 times higher than the RGO-based cells. Moreover, we demonstrate by Open Circuit Voltage Decay measurements that RGO, beside the HTM function, acts also as an anti-recombination layer, reducing the charge carriers recombination pathways and, hence, increasing their lifetime.

Novel CdS Hole-Blocking Layer for Photostable Perovskite Solar Cells.

Currently, the stability issue of the perovskite solar cells (PSCs) is one of the most critical obstacles in the commercialization of PSCs. Although incredible advances in the photovoltaic efficiencies of PSCs have been achieved in the past few years, research on the stability of PSCs has been relatively less explored. In this study, a new kind of CdS hole-blocking layer replacing the traditional compact TiO2 layer is developed to improve the photostability of PSCs because the intrinsic oxygen vacancies of the TiO2 surface are suspected to be the main cause for the photoinduced degradation of PSCs. As a result, PSCs with the CdS layer exhibit considerably improved photostability, maintaining over 90% of the initial efficiency after continuous sunlight illumination for 12 h, while the TiO2 PSC retains only 18% of the initial efficiency under the same conditions. Charge-transfer characteristics related to photodegradation are investigated by various analyses including electrochemical impedance spectroscopy and open-circuit voltage decay and time-resolved photoluminescence decay measurements. the CdS PSC exhibits negligible degradation in the charge-carrier dynamics, while the TiO2 PSC suffers from severely damaged characteristics like increased charge recombination rate, charge-transfer resistance, and reduced charge extraction rate.

Effect of TiO 2 blocking layer synthesised by a sol–gel method in performances of fluorine‐doped tin oxide/TiO 2 /dyed‐TiO 2 /electrolyte/pt/fluorine‐doped tin oxide solar cells based on natural mallow dye

Titanium dioxide (TiO2) thin film prepared by sol–gel technique and deposed by spin coating was used as the blocking layer between fluorine doped tin oxide and dyed TiO2 photoanode deposed by doctor blade in dye-sensitised solar cell (DSSC). The synthesised films show a polycrystalline structure at annealing temperature of 400°C with small amount of amorphous phase observed at 300°C. The values of the electrical conductivity varies from 0.11 to 1.17 Ω−1 cm−1 for one and two layers thin films, respectively, and the corresponding activation energies decreased as a function of the thickness, it passes from 109 meV for one layer to 55 meV for two layers. The photoanode of the DSSC was sensitised by the mallow diluted in ethanol having absorption spectra with two peaks, one around 480 nm (blue light) and the second at 665 nm (red light) attributable to the chlorophyll mixture. Improvement of the electrical performance is due to the increased electron lifetime and decreased of the recombination process showed by the open circuit voltage decay measurement. The lifetime values increase from 0.80 ms for the cell without blocking layer to 1.24 ms for the cell based on the blocking layer.

Effect of Solvent, Dye-Loading Time, and Dye Choice on the Performance of Dye-Sensitized Solar Cells

Anatase titania films with a thickness of up to 20 μm and deposited over a fluorine-doped tin-oxide substrate are impregnated with ruthenium dyes N-719 and N-749 using Dip and supercritical-fluid methods for the purpose of fabricating dye-sensitized solar cell devices. The dyes are dissolved in different solvent mixtures, including supercritical carbon dioxide, as well as combinations of more traditional solvents including mixtures of acetonitrile, and t-butanol. Analytical studies included thin-film analyzing and scanning electron microscopy to measure titania film thickness and porosity, UV-Vis spectroscopy to quantify dye concentration, and current-voltage device characterizations to assess energy conversion efficiency, as well as open-circuit voltage decay measurements and quantum efficiency to examine electron collection efficiency. A significant result is that using the dye N-749 in a solvent that includes supercritical carbon dioxide leads to energy conversion efficiencies that are higher for devices with a thick 20 μm semiconductor film than for the case of devices with thinner films, including the 10 μm film thickness that is traditionally considered an upper threshold. The supercritical-fluid method for the N-719 dye also enabled shorter impregnation duration than more conventional classical Dip Methods.

Enhanced electron lifetime in dye-sensitised solar cells via suppression of electron-hole recombination

Electron-hole recombination is the major problem that reduces the light-electric energy conversion efficiency of photovoltaic devices. We fabricated oxide passivation layers on the surface of transparent conducting oxides (TCO) and photoanodes by using atomic layer deposition (ALD) with the optimised thickness. The intensity-modulated spectroscopy and voltage decay measurements show that the oxide layers, which are considered as blocking layers, effectively reduce the recombination and enhance electron lifetime. Enhanced electron lifetime affects the fill factor and open circuit voltage. With the optimised thickness of the blocking layers, the fill factor and the open circuit voltage were increased by 6% and 14%, respectively. Furthermore, we separated roles of passivation layers on the photoanode and the TCO with open circuit voltage decay (OCVD) measurement. A comparison of the electron recombination characteristics with the oxide layers on the surfaces of NP and the TCO exhibits quite different behaviour, in that the recombination of electrons with higher potential mostly occurs at the surface of NPs, while others with lower potential occur at the surface of TCO.

Control of morphology and defect density in zinc oxide for improved dye-sensitized solar cells.

While zinc oxide (ZnO) with a mesoporous network has long been explored for adsorption of dyes and as an electron-transporting medium in dye-sensitized solar cells (DSSCs), the performance of ZnO-based DSSCs remains unsatisfactory. Despite the importance of understanding the surface characteristics of ZnO in DSSC applications, most of the studies relevant to ZnO-based DSSCs are focused on the synthesis of unique nanostructures of ZnO. In this study, we not only introduce a novel disk-shaped ZnO nanostructure, but also provide insight into the distinctive surface properties of ZnO and its influence on DSSC performance. When utilized in DSSCs, the mesoporous ZnO nanodisk yields 60% better power conversion efficiency (PCE) compared to commercial ZnO nanoparticles, which is attributed to less surface and bulk trap densities as concluded by an in-depth open-circuit voltage decay (OCVD) analysis and electrochemical impedance spectroscopy (EIS). Another aspect that contributes to the higher PCE is the better connectivity of primary particles that join together to form secondary disk-shaped particles. Furthermore, a 40% improvement in the PCE was observed by coating the mesoporous ZnO nanodisk with TiO2, which results from the passivation of the surface defects that aid in suppressing the interfacial charge recombination.

Tuning the energy band gap of ternary alloyed Cd1-xPbxS quantum dots for photovoltaic applications

Tuning the energy band gap of ternary alloyed Cd1-xPbxS (x: 0, 0.33, 0.5, 0.67 and 1) quantum dots (QDs) for photovoltaic applications is studied. Alloyed Cd1-xPbxS QDs were adsorbed onto TiO2 nanoparticles (NPs) using ssuccessive ionic layer adsorption and reaction (SILAR) methode. EDX measurements ensure the success adsorption of alloyed Cd1-xPbxS QDs onto the TiO2 electrode. At 100 mW/cm2 (AM 1.5) sun illumination, the photovoltaic performance of alloyed Cd1-xPbxS QDs sensitized solar cells (QDSSCs) was measured. The maximum values of Jsc (1.92 mA/cm2) and η (0.36%) for the alloyed Cd1-xPbxS QDSSCs were obtained when the molar ratio of Cd/Pb is 0.33/0.67. the open circuit voltage (Voc) is equal 0.61 ± 0.01 V for all alloyed Cd1-xPbxS QDSSCs. The electron back recombination rates decrease considerably for alloyed Cd1-xPbxS QDSSCs as x value increases, peaking at 0.67. The electron lifetime (τ) for Cd0.33Pb0.67S QDSSCs is one order of magnitude larger than that of the other alloyed Cd1-xPbxS QDSSCs with different x value. Under ON–OFF cycles to solar illumination, the open circuit voltage decay measurements show the high sensitivity and reproducibility of alloyed Cd1-xPbxS QDSSCs.

Single step synthesized 1D TiO2 vertically aligned nanorod arrays for CdS sensitized quantum dot sensitized solar cells

In this article, single step synthesized few micrometer vertically aligned 1D rutile TiO2 nanorods (TNR) were deposited directly onto FTO substrate by controlled hydrothermal route. Simple and cost-effective non-aqueous successive a ionic layer adsorption and reaction (SILAR) technique was adopted for the synthesis of cadmium sulfide (CdS) quantum dots (QDs) sensitization onto TNR. These CdS–TNR samples were further used for their quantum dot sensitization (QDSSC) performance in 0.5M Na2S electrolyte. The CdS–TNR photoelectrode deposited at 40 SILAR cycles exhibits a photovoltaic performance with an open-circuit photovoltage (VOC) (0.606V) and short-circuit photocurrent (JSC) (6.19mA/cm2). The highest power conversion efficiency of QDSCs is 1.87% has been achieved at relatively low nanorod length. Further, the transient and interface resistance properties were studied by open circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) measurements.

Temperature-Dependent Characteristics of GaN Homojunction Rectifiers

We report a homojunction gallium nitride (GaN) p-i-n rectifier fabricated on free-standing GaN substrates with the breakdown voltage $>800$ V and low specific on-resistance ( $R_{\mathrm{{\scriptscriptstyle ON}}}A$ ). At 298 K, $R_{\mathrm{{\scriptscriptstyle ON}}}A$ is 0.28 $\text{m}\Omega $ -cm $^{2}$ at the current density ( $J$ ) of 2.5 kA/cm $^{\vphantom {R^{R^{}}}2}$ and the corresponding Baliga’s figure of merit is $>2.5$ GW/cm $^{2}$ . At a given temperature, $R_{\mathrm{{\scriptscriptstyle ON}}}A$ values decrease with $J$ due to conductivity modulation in the drift region. The ambipolar lifetime ( $\tau _{a})$ is also determined by open-circuit voltage decay measurement. The value for $\tau _{a }$ is 9.6 ns at 298 K and it monotonically increases to 22 ns at 448 K. The reverse $I$ – $V$ measurement reveals the reverse leakage current mechanism is mainly attributed to a field-assisted ionization process from deep-level centers in the space-charge region. The analysis of $T$ – $I$ – $V$ curve yields the Poole-Frenkel coefficient ( $\sim 3.1\times 10^{-4} ~\text {eV} \cdot \text {V}^{-1/2}\cdot {\rm cm}^{-1/2})$ and a deep-level trap ( $\sim 0.7$ eV) at zero bias.

Growth and Characterization of Thick Multi-Layer 4H-SiC Epiwafer for Very High-Voltage p-Channel IGBTs

Thick multi-layer 4H-SiC epitaxial growth was investigated for very high-voltage Si-face p-channel insulated gate bipolar transistors (p-IGBTs). The multi-layer included n+ buffer, p+ field stop, and thick p- drift layers. Two processes were employed to enhance the carrier lifetime of the p- drift layer: carbon ion implantation/annealing and hydrogen annealing, and the enhanced carrier lifetime was confirmed by the open-circuit voltage decay measurement. Using the grown thick multi-layer 4H-SiC, simple pin diodes were fabricated instead of p-IGBTs to demonstrate efficient conductivity modulation in the thick p- drift layer. While the on-state voltage was high at room temperature, it decreased significantly at elevated temperatures, and attained 3.5 V at 100 A/cm2 at 200°C for the diode with the carrier lifetime enhancement processes, indicating sufficient conductivity modulation.

Dynamics of Electron Recombination and Transport in Water-Splitting Dye-Sensitized Photoanodes

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO2 electrodes. Despite a high quantum yield of charge injection and low requirement for the catalytic turnover rate, the quantum yield of water splitting in WS-DSPECs is typically low (<1%). Here we examine the charge separation and recombination processes in WS-DSPECs photoanodes functionalized with varying amounts of IrO2 nanoparticle catalyst. Charge extraction and transient open-circuit voltage decay measurements provide insight into the relationship between light intensity, conduction band electron density, open-circuit photovoltage, and recombination time scale. We correlate these results with electrochemical impedance spectroscopy and present the first complete equivalent circuit model for a WS-DSPEC. The data show quantitatively that recombination of photoinjected electrons with oxidized sensitizer molecules a...

Evaluation of various diameters of titanium oxide nanofibers for efficient dye sensitized solar cells synthesized by electrospinning technique: A systematic study and their application

In the present investigation, anatase TiO2 nanofibers with various diameters have been synthesized by electrospinning technique. The electrospinning feeding rate has been varied from 1.0, 1.5, 2.0 and 2.5ml.h−1 in order to control the diameter of TiO2 nanofibers. The synthesized nanofibers are characterized by X-ray diffraction (XRD), field emission scanning-and high resolution transmission electron microscopy (FESEM-HRTEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) measurements. From FESEM and XRD results it is clear that, the nanofibers diameter and crystallinity increases with respect to increasing feed rate. The synthesized nanofibers having various diameters further used for dye sensitized solar cells (DSSCs) application. Our controlled results show that, the nanofibers deposited at 1.5ml.h−1 feeding rate exhibits promising 5.39% power conversion efficiency (PCE) with short circuit current density (JSC) 13.32mAcm−2 and open circuit voltage (Voc)=0.698V. The detailed interface and transient properties of these nanofibers based DSSCs devices have been analyzed by electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/voltage spectroscopy (IMPS/IMVS) and open circuit voltage decay (OCVD) measurements.

Insights into Planar CH3NH3PbI3 Perovskite Solar Cells Using Impedance Spectroscopy

Impedance spectroscopy (IS) is emerging as a valuable tool for the characterization of perovskite-based solar cells (PSCs). However, earlier reports of the IS response of mesoscopic PSCs have revealed notable discrepancies, with the interpretation of their spectra having been generalized to planar PSC devices despite fundamental differences in the device operation. The present study analyzes the impedance response of planar PSC devices through the characterization of cells employing a variety of constituent layers. Distinctive high-frequency and low-frequency features are observed in IS measurements and are attributed to the charge recombination across the perovskite/contact interfaces and the dielectric relaxation in the interfacial regions of the perovskite layer, respectively. Comparison of the characteristic IS time constants with time-resolved photoluminescence (TRPL) and open-circuit voltage decay (OCVD) measurements further substantiates the proposed impedance model. This work provides an empirical...

Mixed P25 nanoparticles and large rutile particles as a top scattering layer to enhance performance of nanocrystalline TiO2 based dye-sensitized solar cells

Herein, we report a bilayer TiO2 photoanode composed of nanocrystalline TiO2 (NCT) bottom layer and mixed P25 nanoparticles and large rutile particles (PR) top scattering layer. The present structure performs well in solar light harvesting which is mainly attributed to the fact that the top scattering layer exhibits superior light scattering effect and meanwhile the NCT bottom layer with large dye-loading capacity can make better use of the back-scattered light. Moreover, electrochemical impedance spectroscopy and open circuit voltage decay measurements demonstrate that DSSC based on bilayer photoanode shows faster interfacial electron transfer and slower charge recombination process than that based on NCT monolayer photoanode. These advantages render the DSSCs based on NCT-PR bilayer photoanode exhibiting superior performance under AM1.5G simulated solar irradiation. As an example, by tuning mass ratio between P25 nanoparticles and large rutile particles in the top scattering layer, the DSSC based on NCT-PR bilayer photoanode exhibits an optimum solar energy conversion efficiency of 9.0%, which is about 1.25 times higher than that of monolayer NCT device (7.2%) with the same film thickness.

Conical islands of TiO2 nanotube arrays in the photoelectrode of dye-sensitized solar cells.

Ti conical island structures were fabricated using photolithography and the reactive ion etching method. The resulting conical island structures were anodized in ethylene glycol solution containing 0.25 wt% NH4F and 2 vol% H2O, and conical islands composed of TiO2 nanotubes were successfully formed on the Ti foils. The conical islands composed of TiO2 nanotubes were employed in photoelectrodes for dye-sensitized solar cells (DSCs). DSC photoelectrodes based on planar Ti structures covered with TiO2 nanotubes were also fabricated as a reference. The short-circuit current (Jsc) and efficiency of DSCs based on the conical island structures were higher than those of the reference samples. The efficiency of DSCs based on the conical island structures reached up to 1.866%. From electrochemical impedance spectroscopy and open-circuit voltage (Voc) decay measurements, DSCs based on the conical island structures exhibited a lower charge transfer resistance at the counter cathode and a longer electron lifetime at the interface of the photoelectrode and electrolyte compared to the reference samples. The conical island structure was very effective at improving performances of DSCs based on TiO2 nanotubes.Graphical Conical islands of TiO2 nanotube arrays are fabricated by an anodizing process with Ti protruding dots which have a conical shape. The conical islands are applied for use in DSC photoelectrodes. DSCs based on the conical islands of TiO2 nanotube arrays have the potential to achieve higher efficiency levels compared to DSCs based on normal TiO2 nanotubes and TiO2 nanoparticles because the conical islands of TiO2 nanotube arrays enlarge the surface area for dye adsorption.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0737-2) contains supplementary material, which is available to authorized users.

Comparison of Solid-State Quantum-Dot-Sensitized Solar Cells with ex Situ and in Situ Grown PbS Quantum Dots

Differences in the solar cell performance of solid-state PbS quantum-dot-sensitized solar cells fabricated with ex situ and in situ grown PbS quantum dots were investigated. The PbS quantum dots were either anchored on mesoporous TiO2 via l-glutathione (GSH) linker exchange or prepared in situ by the successive ionic layer adsorption/reaction (SILAR) method to create quantum-dot-sensitized solar cells. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The performance of the cells was evaluated with current–voltage, external quantum efficiency, and impedance spectroscopy (IS) measurements, and electron lifetimes were measured with open-circuit voltage decay (OCVD), intensity-modulated photovoltage spectroscopy (IMVS) techniques as well as with IS measurements. Analysis of the experimental data indicates that the SILAR route provides more intimate contacts at both TiO2/PbS and PbS/HTM interfaces, which results in more efficient charge injection and separation and thus higher photocurrents. However, the results of the OCVD, IMVS, and IS measurements demonstrate that the cells sensitized with PbS quantum dots prepared ex situ have longer electron lifetimes, indicating that back-transfer of electrons to the HTM is slower, probably as a consequence of passivation of surface states by the GSH ligands.