Open Circuit Voltage Decay Measurements Research Articles

Performance improvement of dye-sensitized solar cells by introducing a hierarchical compact layer involving ZnO and TiO2 blocking films

The novel hierarchical compact layer involving ZnO and TiO2 blocking films is used to improve the overall performance of dye-sensitized solar cells. The ZnO compact films are fabricated as a bottom layer to block the electrons injection to the conductive glass and improve the open-circuit voltage. The TiO2 compact layer is employed to suppress back electron transfer and avoid ZnO contacting the acid dyes directly. The short-circuit current density of the DSSC containing the hierarchical compact layer is improved by 30% compared to that of one with only ZnO as the compact layer. The open-circuit voltage is increased by 4% after introducing the additional TiO2 blocking layer between the ZnO compact layer and mesoporous TiO2 layer. Overall, the short-circuit current density, open-circuit voltage and solar energy conversion efficiency are enhanced by 15.3%, 3.7% and 29.6%, respectively, relative to the traditional DSSC without any blocking layers. The ZnO–TiO2 blocking layer has been characterized by electrochemical impedance spectroscopy, open-circuit voltage decay and dark current measurement to reveal the mechanism of short-circuit current density and open-circuit voltage improvement.

Persistent photovoltage in methylammonium lead iodide perovskite solar cells

We herein perform open circuit voltage decay (OCVD) measurements on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to increase the understanding of the charge carrier recombination dynamics in this emerging technology. Optically pulsed OCVD measurements are conducted on CH3NH3PbI3 solar cells and compared to results from another type of thin-film photovoltaics, namely, the two reference polymer–fullerene bulk heterojunction solar cell devices based on P3HT:PC60BM and PTB7:PC70BM blends. We observe two very different time domains of the voltage transient in the perovskite solar cell with a first drop on a short time scale that is similar to the decay in the studied organic solar cells. However, 65%–70% of the maximum photovoltage persists on much longer timescales in the perovskite solar cell than in the organic devices. In addition, we find that the recombination dynamics in all time regimes are dependent on the starting illumination intensity, which is also not observed in the organic devices. We then discuss the potential origins of these unique behaviors.

Open circuit voltage decay characteristics of 4H-SiC p–i–n diode with carbon implantation

The open circuit voltage decay (OCVD) characteristics of 4H-SiC p–i–n diodes fabricated with the carbon implantation process are investigated. The bulk carrier lifetime in the fabricated devices can be estimated using OCVD measurements. The carrier lifetime at a high injection level (τHL) of the fabricated diode with carbon implantation is 10.5 µs, which is extremely long as compared with that of a diode fabricated with the standard process (1.3 µs).

Improved performance of dye-sensitized solar cell with a specially tailored TiO2 compact layer prepared by RF magnetron sputtering

We demonstrate that a graded index TiO2 antireflective compact layer (arc-TiO2) employed by RF sputtering can modulate the optical transmittance and reduce the electron recombination in dye-sensitized solar cell (DSSC). The spectral response of the DSSC is improved, due to higher and red-shifted transmittance spectra in a specific region that favours the sensitization effect of the dye. The effects of arc-TiO2 prepared at various RF sputtering powers to the performances of DSSC were investigated by means of the incident photo to current efficiency (IPCE), open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS). The slow decay of the photo-voltage attributed to the desirable merits of the arc-TiO2 compact layer has been evidenced by the OCVD measurement. Meanwhile, the improvement of adhesion between an arc-TiO2 film and porous-TiO2 has decreased the interfacial-charge resistance, R1 in the EIS measurement. This lower R1 then facilitates the charge-transfer process of the electron in the DSSC. At 100W of RF power, these blended effects improved the overall conversion efficiency of the DSSC by an increase of 42% compared to the cell without the compact layer.

Application of ZnO micro-flowers as scattering layer for ZnO-based dye-sensitized solar cells with enhanced conversion efficiency

ZnO micro-flowers were synthesized by a simple solution deposition method without the need of seed or keeping fresh deposition solution. The synthesized ZnO micro-flowers were used as scattering layer for dye-sensitized solar cells (DSSCs) fabricated with ZnO nanoparticles photoanode. The UV–vis diffused reflectance absorption spectra (DRS), electrochemical impedance spectroscopy (EIS) and photoinduced open-circuit voltage decay (OCVD) measurements show that the ZnO micro-flowers/nanoparticles bilayer film-based solar cell has much higher light harvesting efficiency, lower resistance and longer electron lifetimes as compared with the ZnO nanoparticles film-based one, and consequently resulting in an improved conversion efficiency due to the scattering effect of ZnO micro-flowers layer. The simple methods of the fabrication of both the ZnO micro-flowers and the ZnO bilayer film-based solar cell, which was fabricated with photoanode consisting of ZnO nanoparticles as underlayer and ZnO micro-flowers as overlayer, are promising for the development of the low-cost, eco-friendly, and durable devices with high light-to-electricity conversion efficiency.

Control of the recombination rate by changing the polarity of the electrolyte in dye-sensitized solar cells.

Recombination in Dye-sensitized Solar Cells (DSCs) is an electron transfer process critical for high efficiency. The chemical nature of the electron acceptor is known to have an important impact on recombination and, hence, limits the choice of hole conductors in DSCs and related solar cells. In this respect, Room Temperature Ionic liquids (RTILs) have been recognized as an alternative to volatile organic solvents due to their negligible vapor pressure, which offers the chance for long-term stability. However, RTIL-based electrolytes lead to lower performance, a feature that has been attributed to the high viscosity of ionic liquids and the mass-transport limitation associated with it. In this work we show that the origin of the lower performance is also related to an increase in the recombination loss due to the polar nature of the RTIL and the influence of the reorganization energy of the electron acceptor in a polar environment. To investigate this chemical effect, different mixing ratios of RTILs and an organic solvent (acetonitrile) have been considered. The fabricated devices have been characterized by small-perturbation techniques (Impedance Electrochemical Spectroscopy and Intensity-Modulated Photovoltage and Photocurrent Spectroscopies) and Open-Circuit Voltage Decay measurements, which have been used to extract electron lifetimes at different applied voltages. Two different ruthenium dyes (hydrophilic N719 and hydrophobic Z907) and two different cations in the RTIL (imidazolium- and pyrrolidinium-based) have been considered. The results obtained show that for pure ionic liquids the lifetime-voltage curve is exponential, which is a signature of large reorganization energies for electron transfer. In contrast, pure acetonitrile exhibits a non-exponential behavior, which is consistent with relatively low reorganization energy. Interestingly, and as a general rule, we find that recombination is faster in systems with higher reorganization energies. This is interpreted as a consequence of the availability of more acceptor states for electron transfer. In addition, it is found that mixing RTILs and acetonitrile is an interesting strategy to increase the stability of DSCs without significant recombination losses, provided that the right dye and RTIL, in particular, a pyrrolidinium component, are used.

Tin-Doped Indium Oxide-Titania Core-Shell Nanostructures for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) hold great promise in the pursuit of reliable and cheap renewable energy. In this work, tin-doped indium oxide (ITO)-TiO2core-shell nanostructures are used as the photoanode for DSSCs. High-density, vertically aligned ITO nanowires are grown via a thermal evaporation method and TiO2is coated on nanowire surfaces via TiCl4treatment. It is found that high TiO2annealing temperatures increase the crystallinity of TiO2shell and suppress electron recombination in the core-shell nanostructures. High annealing temperatures also decrease dye loading. The highest efficiency of 3.39% is achieved at a TiO2annealing temperature of 500°C. When HfO2blocking layers are inserted between the core and shell of the nanowire, device efficiency is further increased to 5.83%, which is attributed to further suppression of electron recombination from ITO to the electrolyte. Open-circuit voltage decay (OCVD) measurements show that the electron lifetime increases by more than an order of magnitude upon HfO2insertion. ITO-TiO2core-shell nanostructures with HfO2blocking layers are promising photoanodes for DSSCs.

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.

Electrospun Hierarchical TiO2 Nanorods with High Porosity for Efficient Dye-Sensitized Solar Cells

Ultraporous anatase TiO2 nanorods with a composite structure of mesopores and macropores fabricated via a simple microemulsion electrospinning approach were first used as photoanode materials for high-efficiency dye-sensitized solar cells (DSSCs). The special multiscale porous structure was formed by using low-cost paraffin oil microemulsion droplets as the soft template, which can not only provide enhanced adsorption sites for dye molecules but also facilitate the electrolyte diffusion. The morphology, porosity, and photovoltaic and electron dynamic characteristics of the porous TiO2 nanorod based DSSCs were investigated in detail by scanning electron microscopy (SEM), N2 sorption measurements, current density-voltage (J-V) curves, UV-vis diffuse reflectance spectra, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and open-circuit voltage decay (OCVD) measurements. The results revealed that, although fewer amounts of dyes were anchored on the porous TiO2 nanorod films, they exhibited stronger light scattering ability, fast electrolyte diffusion, and extended electron lifetime compared to the commercial P25 nanoparticles. A power conversion efficiency of 6.07% was obtained for the porous TiO2 nanorod based DSSCs. Moreover, this value can be further improved to 8.53% when bilayer structured photoanode with porous TiO2 nanorods acting as the light scattering layer was constructed. This study demonstrated that the porous TiO2 nanorods can work as promising photoanode materials for DSSCs.

ZnO@TiO2 core–shell nanorod arrays with enhanced photoelectrochemical performance

ZnO@TiO2 core–shell nanorod arrays (NRAs) were fabricated by radio frequency magnetron sputtering of a Ti layer on ZnO NRAs followed by post-oxidation treatment. Both the photoluminescence spectra and open circuit voltage decay measurement show that the thin TiO2 shell can effectively passivate the surface trap states of ZnO NRAs. Compared with the bare ZnO, ZnO@TiO2 core–shell nanostructure sensitized with CdS quantum dots (QDs) using as the photoelectrodes for photoelectrochemical (PEC) cells shows enhanced PEC performance. It is supposed that this enhanced PEC performance is attributed primarily to two effects. On one hand, the TiO2 shell layer suppresses the recombination of photogenerated carriers by passivating the surface recombination sites in ZnO NRAs and forming an energy barrier between the ZnO core and TiO2 shell. On the other hand, the rough surface of the TiO2 shell on ZnO NRAs facilitates the deposition of CdS QDs, which contributes to enhance light absorption in visible light region. However, the experiment results suggest that an optimal thickness of about 15nm for the TiO2 (or pre-deposited Ti) layer is highly desired, the further incremental thickness of the layer will restrain the PEC performance of photoelectrode due to several shortages, such as reduced light harvesting due to the shadow effect of the incomplete oxidized Ti layer, the introduction of additional leakage current, and the blocking effect for the penetration of the redox couples ions (S2−/Sn2−) into ZnO NRAs.

RF sputtered tri-functional antireflective TiO2 (arc-TiO2) compact layer for performance enhancement in dye-sensitised solar cell

A novel antireflective TiO2 compact layer (arc-TiO2) that can reduce electron recombination and improve transmittance was deposited by RF magnetron sputtering at the interface between indium tin oxide and porous-TiO2layer of a dye-sensitised solar cell (DSSC). Effects of the arc-TiO2 thicknesses on the performance of the arc-TiO2-based DSSC were investigated by means of incident photon-to-current efficiency (IPCE), open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS). The sensitisation effect of N719 dye was remarkably improved due to relatively higher and red-shifted transmittance spectra of the ITO/arc-TiO2 electrode in a specific region, and evidenced by the IPCE measurement. Meanwhile, the slow decay behaviours of the photo-voltage owed to the compact layer were verified by the OCVD measurement. The adhesion enhancement between the arc-TiO2 film and porous-TiO2 layer decreases the interfacial resistance R1 in the EIS measurement, hence facilitating the charge transfer process of the electrons in the DSSC. A significant improvement in the overall solar energy-to-electrical conversion efficiency, which constituted almost 50% higher compared with the uncoated ITO cell, is mainly attributed to the higher transmittance and reduced recombination of the arc-TiO2 film employed as the tri-functional compact layer in the DSSC.

Enhancement of electron lifetime in dye-sensitized solar cells using anodically grown TiO2 nanotube/nanoparticle composite photoanodes

Dye-sensitized solar cells (DSCs) based on TiO2 nanotube/nanoparticle (NT/NP) composite photoanodes were fabricated including different NT content into the NP network. The NPs expose large surface area for the dye anchoring, while the incorporated nanotubes can improve the electron lifetime and scatter the incident photons enhancing the light harvesting. TiO2 NTs were fabricated by fast anodic oxidation of Ti foil and mixed into NP dispersion. Morphology of the as-grown NTs and of the prepared paste was investigated by means of Field Emission Scanning Electron Microscopy. DSCs were assembled and characterized by current–voltage electrical measurements and Incident Photon-to-electron Conversion Efficiency. Electrochemical Impedance Spectroscopy and Open Circuit Voltage Decay measurements were employed in order to study the recombination and to evaluate the electron lifetime into the anode materials. The overall photo-conversion efficiency of the NT/NP composite photoanode was found to be a 10% higher with respect to pure NP electrode evidencing an improvement of the charge lifetime.

Novel ITO/arc-TiO2 antireflective conductive substrate for transmittance enhanced properties in dye-sensitized solar cells

Graphical abstractDisplay Omitted HighlightsWe fabricated ITO/arc-TiO2 antireflective TCO with blocking layer capability for DSSCs.Two maximum transmittance peaks around 410nm and 750nm could be seen.We fabricated DSSC based on the novel substrate and the uncoated ITO.IPCE spectra show a quench mimics the transmittance spectra.Overall device performance up to 50% was achieved with the use of novel substrate. A novel antireflective TiO2 compact layer (arc-TiO2) that can improve transmittance is employed by radio frequency sputtering at the interface between ITO and porous-TiO2. Effects of the absence and presence of the TiO2 layer were mainly investigated by means of incident photon-to-current efficiency (IPCE), open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS). The sensitization effect of N719 dye has been improved due to the right-shifted transmittance spectra as shown by IPCE measurement. The slow decay behaviour of the photo-voltage attributed to the merits brought by the compact layer has been evidenced by the OCVD measurement. The improvement of adhesion between the arc-TiO2 film and porous-TiO2 has decreased the interfacial resistance R1 in the EIS measurement, and this facilitated the charge transfer process of the electron in the dye-sensitized solar cells (DSSC) later on. A remarkable improvement in the overall conversion efficiency of 3.444%, representing almost 50% increment compared to the cell without the compact layer, is mainly responsible for the higher transmittance and fewer recombination effects of the arc-TiO2 compact layer employed in the DSSC.

TiO2Nanotube Array as Efficient Transparent Photoanode in Dye-Sensitized Solar Cell with High Electron Lifetime

In the present work, the fabrication and characterization of non-curling, free-standing TiO2 nanotube membranes and their integration in front-side illuminated dye-sensitized solar cells are reported. Vertically oriented TiO2 nanotube arrays were fabricated by anodic oxidation of a titanium foil. Nanotube membranes were detached from the metallic foil, transferred and bonded on transparent fluorine-doped tin oxide/glass substrates employing a TiO2 sol as a binder. Crystalline phase and morphology of the film were investigated, evidencing the formation of a highly ordered 1D nanotubes carpet, with a pure anatase crystalline structure. TiO2 nanotube-based DSCs were fabricated using reversible microfuidic architecture. The cell performances were studied by I-V electrical characterization, incident-photon-to-electron conversion efficiency, electrochemical impedance spectroscopy and open circuit voltage decay measurements, showing an increase in electron lifetime compared to nanoparticle-based dye-sensitized solar cells.

Single-step synthesis of 3D nanostructured TiO2 as a scattering layer for vertically aligned 1D nanorod photoanodes and their dye-sensitized solar cell properties

In the present investigation we have successfully synthesized 1D vertically aligned rutile TiO2 nanorods (TNR) and 3D TiO2 nanostars (TNS) as a scattering layer by a single step hydrothermal route. The synthesized nanostructures were characterized by X-ray diffraction, scanning-and transmission electron microscopy and X-ray photoelectron spectroscopy. The 1D TiO2 nanorod and 3D TiO2 nanostar samples were further used for N-719 dye sensitized solar cells (DSSC) application. Compared to a nanorod based cell, the photovoltaic performance of the nanostars/nanorods TiO2 cell exhibits excellent DSSC performance, including superior light scattering, rapid electron transport and lower electron recombination rate. The 3D/1D TNS/TNR based DSSC cell exhibits 5.39% power conversion efficiency, which is remarkably higher than that of the bare 1D nanorod based (3.74%) photoelectrode. The detailed interface and transient properties of these nanorod and nanostar based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopy measurements and open circuit voltage decay measurements in order to understand the critical factors contributing to such high power conversion efficiency. The enhancement of the efficiency for the 3D/1D TNS/TNR photoanode based cell compared to the 1D TNR is mainly attributed to better light scattering capability, faster electron transport and lower electron recombination.

Coupled analysis of steady-state and dynamic characteristics of dye-sensitized solar cells for determination of conduction band movement and recombination parameters

A new research strategy for determining the conduction band movement of TiO(2) films and charge recombination between electrons in the TiO(2) film and electron acceptors in the electrolyte was proposed. Steady-state short-circuit current density versus open-circuit voltage was employed to attain the exchange current density and recombination reaction order. Transient photovoltage decay and open-circuit voltage decay measurements were carried out to obtain the energetic distribution of trapped electrons. Reduced voltage-dependent trapped electron concentration and trapped electron concentration-dependent recombination current density were used to analyze influence factors of open-circuit voltage, including contributions from conduction band movement and charge recombination. The simulated and measured electron concentration were in agreement and confirmed the validity of this method for extracting conduction band movement and recombination parameters. This new approach provides a physical insight which could help us to more conveniently and efficiently understand the operation of DSCs.

Optimisation of the I–V measurement scan time through dynamic modelling of solar cells

High-performance solar cells and photovoltaic modules exhibit high internal capacitance, limiting the speed of their transient responses including the current-voltage characteristics scans. This study proffers a model-based method to obtain optimal scan time during the current-voltage performance characterisation of a solar cell or module while preserving a pre-set accuracy. Static model parameters are extracted from the quasi-static current-voltage characteristic, whereas the capacitive character, modelled by two bias voltage dependent capacitances, is determined from the open-circuit voltage decay measurement. The obtained model is used to calculate the optimal current-voltage curve scan time. Efficacy of the proposed method is demonstrated through test results obtained on three wafer-based solar cells. I-V curve errors determined by the proposed method at different scan times are in good agreement with the measurements. Results show that in order to achieve <; 0.5<; error in curve fitting, determined scan times of tested crystalline silicon solar cells lie within the range of 3.6<;45 ms for constant angle step semiconductor curve tracer. Use of a capacitive-based curve tracer, however, requires approximately twice that time to retain a comparable error.

Template-free solvothermal fabrication of hierarchical TiO2 hollow microspheres for efficient dye-sensitized solar cells

Hierarchical TiO2 hollow spheres consisting of anatase nanoparticles are successfully prepared via a simple one-pot solvothermal method by using titanium n-butoxide and oxalic acid dihydrate as the precursors. This detailed investigation reveals that apart from the reaction time, the molar ratio of oxalic acid dihydrate to titanium n-butoxide in the precursor reagents plays a key role in the crystallization of TiO2 as well as the formation of the hollow architecture. As confirmed by UV-vis diffuse reflectance spectroscopy, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) and open-circuit voltage decay (OCVD) measurements, the as-prepared hierarchical TiO2 hollow microspheres exhibit a stronger light scattering ability and extended electron lifetime compared to commercial P25 nanoparticles, which makes it a promising photoanode material for dye-sensitized solar cells (DSSCs). As a result, a power conversion efficiency of 6.33% is obtained for the hierarchical TiO2 hollow sphere-based DSSCs, which is lower than the value of P25 nanoparticle-based cells (7.69%) due to the significantly lower dye adsorption. However, the efficiency of the DSSCs can be enhanced to 8.76% when a bi-layer structured photoelectrode composed of P25 nanoparticles (dye adsorption layer) and hierarchical TiO2 hollow spheres (light scattering layer) is used. The great improvement in photovoltaic performance is mainly due to the excellent light scattering properties, relatively high dye adsorption and the slower recombination rate of the bi-layer structure.

Polyoxometalate–anatase TiO2 composites are introduced into the photoanode of dye-sensitized solar cells to retard the recombination and increase the electron lifetime

Polyoxometalate-TiO(2) composites have been successfully introduced into the photoanode of dye-sensitized solar cells to reduce the recombination of the electrons, which results in a longer electron lifetime. The performance of the cells with the polyoxometalate-modified photoanode is better than the cell with a pure P25 photoanode. The effect of the polyoxometalate was studied by electrochemical impedance spectroscopy and open-circuit voltage decay measurement. The results show that the electron lifetime becomes longer following an increase in the amount of the polyoxometalate.

Magnesia nanoparticles in liquid electrolyte for dye sensitized solar cells: An effective recombination suppressant?

Recombination reactions at the photoanode/electrolyte interface reduce the photovoltaic conversion efficiency of dye sensitized solar cells (DSSCs). Unlike modification of titania photoanode by coating with MgO which act as a barrier layer toward recombination, addition of MgO nanopowder to electrolyte prevents recombination through adsorption of anions (triiodide/iodide) from electrolyte. In the present study, the surface charge of MgO has been utilized to adsorb anions from electrolyte. This anionic adsorption onto the MgO nanopowders in electrolyte has been confirmed by zeta potential measurements. MgO retards the recombination reaction as efficiently as 4-tert-butylpyridine (TBP) which is the most widely used additive in the electrolyte. Higher photocurrent and conversion efficiency is achieved by using MgO loaded electrolyte as compared to TBP added electrolyte. Dark current measurements show that recombination reactions are effectively retarded by use of MgO loaded electrolytes. Open circuit voltage decay measurements also confirm higher electron lifetime at the titania/electrolyte interface in MgO loaded electrolyte based cell as compared to additive free electrolyte based cell.