Open Circuit Voltage Decay Research Articles

Effect of defects in TiO2 nanotube thin film on the photovoltaic properties of quantum dot-sensitized solar cells

In the liquid-phase-deposition (LPD) method, the deposition temperature is considered to be one of the most important factors in TiO2 nanotube crystal growth. We investigated the effects of the deposition temperature on the surface morphology and defects in TiO2 nanotube (NT–TiO2) thin film electrodes utilizing scanning-electron-microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL), together with the effects of these on the photovoltaic characteristics of CdSe quantum dot (QD)-sensitized NT–TiO2 solar cells. In addition, we studied the effect of these defects on the physical properties, such as the carrier recombination and electron transport at the TiO2 and TiO2/QD interface. NT–TiO2 electrodes prepared at low temperatures have a more uniform surface and lower defects than those prepared at high temperatures. From the PL measurements and the photovoltaic characterization such as shunt resistance (Rsh) and open circuit voltage decay (OCVD), these defects can act as carrier recombination centers. The defect density increases with increasing deposition temperature, leading to an increase in carrier recombination. Series resistances (Rs) of the solar cells with NT–TiO2 electrodes prepared at high temperatures were larger than those of the solar cells with NT–TiO2 electrodes prepared at low temperatures, suggesting that the defects can also affect the carrier transport characteristics. Eventually, CdSe QD-sensitized NT–TiO2 solar cells employing NT–TiO2 prepared at low temperatures showed higher conversion efficiencies than those prepared at high temperatures.

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.

Morphology-controllable polycrystalline TiO2 nanorod arrays for efficient charge collection in dye-sensitized solar cells

In this work, rutile TiO2 nanorod arrays (NRAs) are prepared on TiO2 seeding/FTO substrates by a hydrothermal process. The synthetic recipe is systematically studied to probe the morphology-synthesis interactions. Results indicate that increasing the seeding thickness greatly improves the uniformity of the top NRAs, and the initial growth for nanorods can be classified into three steps, i.e., nucleation, plant-shaped growth and array-appearance formation. The structural study suggests that each TiO2 nanorod is in fact closely-stacked by many tiny secondary nanorods parallel to each other, which could be opened by a chemical etching process. Upon etching, the performance of dye-sensitized solar cells (DSSCs) based on TiO2 NRAs is substantially improved. The device efficiency achieves a highest value of 4.46% using a 14μm NRAs etched for 3h. The charge dynamics of DSSCs was further investigated by intensity-modulated photocurrent/photovoltage spectroscopy, electrochemical impedance spectroscopy and open-circuit voltage decay. Results show that the electron collection efficiency in the TiO2 NRAs is greatly enhanced after etching, mainly due to the retarded charge recombination, which is probably induced by the morphology change of the etched NRAs. The present study reveals some important issues for growth of rutile TiO2 NRAs on seeded FTO substrates, introduces a novel etching way for improving charge collection in semiconductor NRAs, and would advance the applications of TiO2 NRAs in various areas, such as solar cells, sensors and batteries.

Electron Transport, Trapping and Recombination in Anodic TiO<sub>2</sub> Nanotube Arrays

Anodically synthesized TiO2 nanotube arrays (TNTAs) constitute an exciting ordered large bandgap semiconductor nanoarchitecture for use as scaffolds and active layers for solutionprocessable devices including but not limited to, optoelectronic sensors, photovoltaics, photodetectors, photocatalysts and photoelectrochemical cells. Charge transport, trapping and recombination are key attributes of the material architecture that significantly influence the properties and performance of the resulting optoelectronic devices, thus motivating this review article. Since nanocrystalline mesoporous TiO2 films (np-TiO2) are actively researched for the same applications, in many cases, TNTAs and np-TiO2 are direct competitors and it is therefore meaningful to compare the optoelectronic properties of the two architectures head-to-head. In addition, there exists a whole host of TNTA-specific applications such as bottom-up fabricated photonic crystals, bulk heterojunction organic solar cells and metallodielectric metamaterials that leverage the ordered channel architecture. Recent studies have established the order of magnitude superior recombination lifetimes in sensitized TNTAs as compared to sensitized np-TiO2 as well as the salutary effect of lower structural disorder in TNTAs resulting in trap-free electron diffusion coefficients approaching those of single crystals and two orders of magnitude larger than np-TiO2. Photoconductivity measurements using bandgap illumination in both single nanotubes and nanotube ensembles have resulted in similar values of the mobility-lifetime product (10-5-10-4 cm2V-1), which are four to six orders of magnitude higher than in nanoparticle electrodes. At the same time, TiO2 nanotubes have a larger trap density and a greater average trap-depth than nanoparticulate Ti2 films, pointing to the importance of synthesis modification to improve material quality and post-synthesis techniques for trap passivation. Keywords: Cyclic voltammetry, electrochemical impedance spectroscopy, IMPS/IMVS studies, open-circuit voltage decay, photoconductivity, photoluminescence, resistivity, TiO2 nanotubes arrays.

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...

Long Vertically Aligned TiO2 Nanotube Electrodes Prepared via Two‐Step Anodization for Highly Efficient Photovoltaics

AbstractVertically aligned long TiO2 nanotube (NT) electrodes with optimized structures for photovoltaic application were prepared by a two‐step anodization method. The structures of the conventional TiO2 NT electrodes prepared by one‐step anodization became severely bundled and disordered for tube lengths exceeding 20 μm. On the other hand, two‐step anodization resulted in uniform and bundle‐free TiO2 NT electrodes, with tube lengths of up to 38 μm. When used in dye‐sensitized solar cells (DSSCs) as photoanodes, the two‐step TiO2 NT electrodes exhibited enhanced short‐circuit current density (Jsc) with increasing tube length, owing to an increased roughness factor, which resulted in high dye loading. However, the dark current and open‐circuit voltage decay (OCVD) curves confirmed that the electron recombination rate increased, and the electron lifetime decreased, with increasing tube length. This reduced electron lifetime resulted in sharp reductions in the open‐circuit voltage (Voc) with increasing tube length. Owing to the trade‐off between the enhanced current density and the reduced electron lifetime, an optimal conversion efficiency of 4.56 % was obtained for a tube length of 29 μm.

Highly-ordered arrays of TiO2 thin film for dye-sensitized solar cells fabricated by anodic oxidation process

Highly ordered arrays of TiO2 photoelectrode thin films of dye-sensitized solar cells (DSSCs) were fabricated by anodic oxidation method. This study analyzed the effects of electrolytes under similar NH4F concentrations but different reaction times in the anodic oxidation process on the photoelectric conversion efficiency of DSSC, open-circuit voltage decay, electron lifetime, and incident photon-to-electron conversion efficiency (IPCE) of the prepared DSSC. Experimental results show that the TiO2 nanotube thin film prepared by anodic oxidation with an electrolyte with a NH4F concentration at 0.5 wt% and with a reaction time of 5 hr can be fabricated with a length of 17 μm and a photoelectric conversion efficiency of 4.87%, open-circuit voltage of 0.74 V, and short-circuit current density of 13.2 mA/cm2. Additionally, the same NH4F concentration but with a reaction time of 10 hr achieves a length of 22 μm and a photoelectric conversion efficiency of 6.58%, open-circuit voltage of 0.81 V, and short-circuit current density of 16.1 mA/cm2.

Photoelectrochemical properties and photodegradation of organic pollutants using hematite hybrids modified by gold nanoparticles and graphitic carbon nitride

In this study, heterojunctions of gold (Au) nanoparticles with three different average sizes (26, 20, and 12nm in diameter), graphitic carbon nitride (g-C3N4) sheets, and rhombohedral hematite (Fe2O3) hybrid photocatalysts were fabricated via sonication at room temperature and used in the degradation of rhodamine B (RhB) dye under visible irradiation. The Au sensitized g-C3N4/Fe2O3 hybrid photocatalyst using 12nm-sized Au particles, exhibited apparent rate constant of 31.6×10−3min−1, nearly 19-fold higher than that of pure hematite (Fe2O3=1.7×10−3min−1), and higher than those of the other hybrid samples. The superior performance of the Au/g-C3N4/Fe2O3 hybrid was attributed to a potential energy level difference, high optical absorbance, and the effective separation of photogenerated charge carriers. Additionally, the Au sensitized g-C3N4/Fe2O3 hybrid using 12nm-sized Au particles, showed the highest specific surface area (46.5m2g−1), compared with that of pure Fe2O3 (5.5m2g−1) and g-C3N4/Fe2O3 (29.7m2g−1) hybrid photocatalysts, showing numerous available sites for dye adsorption and degradation. Moreover, the transient photoresponse and open-circuit voltage decay clearly showed a high photocurrent and longer charge carrier lifetime, respectively. Reusability tests confirmed that the Au/g-C3N4/Fe2O3 hybrid photocatalyst was highly stable and recyclable. Hence, the work presented demonstrates the efficient, stable Au/g-C3N4/Fe2O3 hybrid photocatalyst for degradation of toxic pollutants under visible irradiation.

Photoelectrochemical characterization of ITO/TiO2 electrodes obtained by cathodic electrodeposition from aqueous solution

In the present work, the photoelectrochemical characterization of ITO/TiO2 electrodes electrosynthesized at two distinct TiO2 film charges (0.35 and 1.00 C) was performed. Scanning electron microscopy presented a globular-like nanostructure and a typical morphology that are dependent on the growing charge, where the photoelectrode synthesized at 0.35 C presented a more homogeneous morphology. Such dependence was also observed at the photoelectrochemical response, once the photoactivity for the photoelectrode synthesized at 0.35 C was better than the photoelectrode synthesized at 1.00 C, which was explained by the surface recombination process and the electron lifetime. In order to explore the charge-transfer process and the displacement of the quasi-Fermi level upon illumination, electrochemical impedance spectroscopy (EIS) was performed at distinct applied potentials. EIS results corroborate the previous results, presenting a higher charge-transfer resistance and a lower chemical capacitance for the 1.00 C electrode film, the last one in accordance with the open-circuit voltage decay.

Light induced structural changes in CH3NH3PbI3 Perovskite Solar Cells

Perovskite solar cells have recently become one of the most promising alternative to the current solar cell technologies by achieving efficiencies superior to 20%. Owing to their distortable structure these devices have proven to show unusually slow relaxation processes upon illumination. We have identified this phenomenon in the frequency domain by impedance spectroscopy (IS) and in the time domain by transient photovoltage decay (TPD) and open circuit voltage decay (OCVD). However the origin of this process is still unclear although three main hypotheses have been proposed in the literature: i) high internal electric field, ii) ferroelectric domains and iii) ion/defect diffusion. In this conference paper we first present our works on light induced structural changes in perovskite and then discuss the main hypotheses previously mentioned with the insight of first principle calculations.

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.

Electrospun ZnO Nanofibers as a Photoelectrode in Dye-Sensitized Solar Cells.

A nanoparticle-based DSSC shows limited efficiency levels due to its disordered geometrical structure and interfacial interference during electron transport, whereas the use of nanofibers in a DSSC can increase the electron mobility at the interfacial area of the materials due to the reduced recombination of electrons before reaching the collecting electrode. In this study, we describe the fabrication and characteristics of a ZnO nanofiber electrode for DSSC. From the results of a thermogravimetric analysis, a stepped heat treatment was developed for the calcinations of the ZnO electrodes. The ZnO electrode morphology and crystalline structure were confirmed by scanning electron microscopy and the X-ray diffraction patterns, respectively. The DSSC with the ZnO nanofiber photoelectrode (wire shaped) created by electrospinning showed an enhanced short-circuit current density (37% enhancement) compared to that of a ZnO sphere particle-shaped photoelectrode under irradiation of AM 1.5 simulated sunlight (100 mW/cm2). Moreover, we have investigated the origin of the improved performance through electrochemical impedance spectroscopic (EIS) and open-circuit voltage-decay (OCVD) measurements.

Performance enhancement of dye-sensitized solar cell with a TiCl4-treated TiO2 compact layer

We here show that an effective blocking layer for dye-sensitized solar cells (DSSCs) can be formed by spin coating a commercial TiO2 paste onto a conducting glass substrate. The spin-coated TiO2 layer was made more compact than the main absorption layer by TiCl4 treatment. DSSCs employing a compact layer exhibited an average current density and an efficiency of 19.09 mA/cm2 and 9.10%, respectively, while 16.91 mA/cm2 and 8.33% were obtained from unblocked reference cells. The enhanced DSSC performance is attributed to the increased electron lifetime. Intensity-modulated photovoltage spectroscopy and open-circuit voltage decay analysis showed that a TiCl4-treated compact layer substantially suppresses the charge recombination at the TiO2/substrate interface, thereby increasing the electron lifetime.

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.

Transition metal doping for enhancing quantum dot sensitized solar cells performance

In this paper we show that efficiency of quantum dot sensitized solar cells (QDSCs) can be systematically enhanced by simply doping the semiconductor quantum dots with various transition metals. A general study is conducted on the effect of various doping materials like Zn, Co, and Mn on the performance of QDSCs. For the first reported CdS:Zn/CdSe, and CdS:Co/CdSe sensitized cells, the cell performance with 2.82% and 3.42 % (Voc = 609 mV, Jsc = 11.25 mA cm−2, FF = 0.5) efficiency is obtained respectively which is enhanced compared with the photovoltaic properties of the cells with conventional CdS/CdSe sensitized cells (η = 2.24%, Voc = 545 mV, Jsc = 9.12 mA cm−2, FF = 0.45). Impedance spectroscopy (IS) and open circuit voltage decay is performed on the doped QDSCs in order to understand the origin of the performance increment. Results indicate that the electron recombination loss is reduced in the case of doped QDs, while reduced electron conductivity in the cells is a limiting factor that should be considered for further efficiency improvements. Mechanism of charge transport in the doped cells is explained based on electron trapping/detrapping through midgap trap states in the doped QDs. This general study indicates that QDs doping could be introduced as a simple effective method for increasing the performance of QDSCs.

Experimental Determination of Effective Minority Carrier Lifetime in HgCdTe Photovoltaic Detectors Using Optical and Electrical Methods

This paper presents experiment measurements of minority carrier lifetime using three different methods including modified open-circuit voltage decay (PIOCVD) method, small parallel resistance (SPR) method, and pulse recovery technique (PRT) onpnjunction photodiode of the HgCdTe photodetector array. The measurements are done at the temperature of operation near 77 K. A saturation constant background light and a small resistance paralleled with the photodiode are used to minimize the influence of the effect of junction capacitance and resistance on the minority carrier lifetime extraction in the PIOCVD and SPR measurements, respectively. The minority carrier lifetime obtained using the two methods is distributed from 18 to 407 ns and from 0.7 to 110 ns for the different Cd compositions. The minority carrier lifetime extracted from the traditional PRT measurement is found in the range of 4 to 20 ns forx=0.231–0.4186. From the results, it can be concluded that the minority carrier lifetime becomes longer with the increase of Cd composition and the pixels dimensional area.

Copper selenide as a new counter electrode for zinc oxide nanorod based quantum dot solar cells

This work investigates the improvement in efficiency of quantum dot sensitized solar cells (QDSSCs) using copper selenide (Cu2Se) as a new counter electrode. Cu2Se counter-based QDSSC exhibits power conversion efficiency (η) of 2.28%, which is higher than the 1.17% efficiency of copper sulfide (CuS) counter electrode. Electrochemical impedance spectroscopy results show that by the changing counter electrode from CuS to Cu2Se, the charge transfer resistance at the counter electrode/electrolyte interfaces is reduced and, consequently, causes current density rise from 6.02mA/cm2 to 11.26mA/cm2. Also, open circuit voltage decay and dark current density–voltage results show that the lifetime of electron in the Cu2Se counter-based QDSSC is higher than that of QDSSC with CuS counter electrode. Therefore, Cu2Se counter electrode can be regarded as a promising alternative to the noble CuS counter electrode in QDSSCs.

Effects of acetyl acetone-typed co-adsorbents on the interface charge recombination in dye-sensitized solar cell photoanodes

Acetyl acetone and its derivatives, 3-butyl-2,4-pentanedione and 3-phenyl-2, 4-pentanedione are investigated as co-adsorbents of photoelectrodes for dye-sensitized solar cells (DSSC). The DSSC based on 3-phenyl-2,4-pentanedione co-adsorbent shows the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 67.3%, a short-circuit photocurrent density (JSC) of 15.73mAcm2, an open-circuit photo-voltage (VOC) of 0.74V and a fill factor (FF) of 0.69, corresponding to an overall conversion efficiency of 8.07% under the standard global AM 1.5 solar light condition. Electrochemical impedance (EIS) data and open-circuit voltage decay (OCVD) data indicate that the electron lifetime is improved by co-adsorption of 3-phenyl-2,4-pentanedione, it is associated with the 3-phenyl-2,4-pentanedione co-adsorbent forms a protect layer of TiO2 which can inhibit the electron recombination efficiently. The results confirm that the acetyl acetone-typed co-adsorbents with less effect on the adsorption of dye, more hydrophobic structure and stronger electron donating ability, such as 3-phenyl-2,4-pentanedione, can improve the performance of DSSC.