Increase In Open-circuit Voltage Research Articles

A Study of the Efficiency Enhancement of the Gel Electrolyte-based SnO2 Dye-sensitized Solar Cells Through the Use of Thin Insulating Layers

Even though SnO2 has promising electronic properties such as higher election mobility among the high band gap semiconductor nanoparticles that are commonly used in Dye-sensitized Solar Cells (DSCs), the cell efficiency of SnO2-based DSCs is considerably low due to high electron recombination; especially through the nanoparticle surface. There have been a few successful attempts in reducing this recombination through the use of very thin insulating layers to cover the surfaces of the nanoparticles in liquid electrolyte-based DSCs. There is enough evidence that this process enhances the efficiency of SnO2-based DSCs. Some of the practical limitations associated with the liquid electrolyte-based DSCs’ such as electrolyte leakage, solvent evaporation and sealing imperfections can be suppressed through the use of gel polymer electrolytes. In this study, we have investigated the effect of different coating materials: CaCO3, MgO and Al2O3 on the cell performance of DSCs based on gel electrolytes. In all cases, there was a considerable increase in Open Circuit Voltage (VOC) and Short-Circuit Current Density (JSC). The enhancement was found to depend on the insulator/SnO2 molar ratio in all cases. CaCO3-coated DSCs gave the best enhancement compared to other systems. The composite system having a molar ratio of 0.12:1 of CaCO3 to SnO2 showed 46% increase of VOC, 50% increase of JSC and 233% increase of efficiency compared to the pure SnO2-based devices. We have employed several experimental techniques and theories to explain the observed performance of the DSC devices fabricated with the above insulating layers and two different gel electrolytes based on Pr4N+I− and LiI salts.

Understanding the effects of TCO work function on the performance of organic solar cells by numerical simulation

The influences of the work function of transparent conducting oxides (TCO) on the performance of organic solar cells, including open circuit voltage, conversion efficiency and fill factor, has been simulated. It is found that for non-ohmic contact the open circuit voltage and conversion efficiency increase monotonically with the TCO work function but remain constant for ohmic contact. The fill factor decreases and increases with the electrode work function when the electrode work function is below and above a critical value (4.2 eV for TCO and 4.5 eV for back-contact), respectively. The results of this simulation are significant in the choice of TCO contacts to optimize organic planar heterojunction solar cells.

Solution-Processed 8-Hydroquinolatolithium as Effective Cathode Interlayer for High-Performance Polymer Solar Cells.

Solution-processed 8-hydroxyquinolinatolithium (s-Liq) was successfully applied as an efficient cathode interlayer in bulk heterojunction polymer solar cells (PSCs), giving rise to enhancement in device performance. The ultraviolet photoelectron spectra results revealed that the presence of s-Liq could lower work function of Al cathode, allowing for the ohmic contacts with the fullerene acceptor for better electron extraction and also a larger work function difference between the two electrodes, which leads to an increase in open-circuit voltage (V(oc)). Scanning Kelvin probe microscopy study on the surface potential of active layers suggested that an interfacial dipole was formed in the s-Liq interlayer between the active layer and the Al cathode, which enhanced the intrinsic built-in potential in the device for better charge transportation and extraction. Consequently, the V(oc), fill factor, and current density of the device can be improved by the introduction of s-Liq interlayer, leading to a power conversion efficiency (PCE) improvement. With PTB7 (or PTB7-Th) as the donor and PC71BM as the acceptor, the s-Liq-based PSC devices exhibited a PCE of 8.37% (or 9.04%), much higher than those of devices with the evaporated Liq (7.62%) or commonly used PFN (8.14%) as the cathode interlayer. Moreover, the s-Liq-based devices showed good stability, maintaining 75% (in N2) and 45% (in air) of the initial PCE after 7 days, respectively. These results suggest the great potential of s-Liq as cathode interlayer material for high-performance solar cells application.

Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH3NH3PbI3 Perovskite

The influence of monovalent cation halide additives on the optical, excitonic, and electrical properties of CH3NH3PbI3 perovskite is reported. Monovalent cation halide with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+, have been added to explore the possibility of doping. Significant reduction of sub‐bandgap optical absorption and lower energetic disorder along with a shift in the Fermi level of the perovskite in the presence of these cations has been observed. The bulk hole mobility of the additive‐based perovskites as estimated using the space charge limited current method exhibits an increase of up to an order of magnitude compared to the pristine perovskites with a significant decrease in the activation energy. Consequentially, enhancement in the photovoltaic parameters of additive‐based solar cells is achieved. An increase in open circuit voltage for AgI (≈1.02 vs 0.95 V for the pristine) and photocurrent density for NaI‐ and CuBr‐based solar cells (≈23 vs 21 mA cm−2 for the pristine) has been observed. This enhanced photovoltaic performance can be attributed to the formation of uniform and continuous perovskite film, better conversion, and loading of perovskite, as well as the enhancement in the bulk charge transport along with a minimization of disorder, pointing towards possible surface passivation.

Effects of humidity during formation of zinc oxide electron contact layers from a diethylzinc precursor solution

This work focuses on the role of humidity in the formation of ZnO thin films from a reactive diethylzinc precursor solution for use as the electron contact layer (ECL) in organic photovoltaic (OPV) devices. This method is well suited for flexible devices because the films are annealed at 120 °C, making the process compatible with polymer substrates. ZnO films were prepared by spin coating and annealing at different relative humidity (RH) levels. It is found that RH during coating and annealing affects the chemical and physical properties of the ZnO films. Using x-ray photoelectron spectroscopy it is found that increasing RH during the formation steps produces a more stoichiometric oxide and a higher Zn/O ratio. Spectroscopic ellipsometry data shows a small decrease in the optical band gap with increased humidity, consistent with a more stoichiometric oxide. Kelvin probe measurements show that increased RH during formation results in a larger work function (i.e. further from vacuum). Consistent with these data, but counter to what might be expected, when these ZnO films are used as ECLs in OPV devices those with ZnO ECLs processed in low RH (less stoichiometric) had higher power conversion efficiency than those with high-RH processed ZnO due to improved open-circuit voltage. The increase in open-circuit voltage with decreasing humidity was observed with two different donor polymers and fullerene acceptors, which shows the trend is due to changes in ZnO. The observed changes in open-circuit voltage follow the same trend as the ZnO work function indicating that the increase in open-circuit voltage with decreasing humidity is the result of improved energetics at the interface between the bulk-heterojunction and the ZnO layer due to a vacuum level shift.

Enhancement of DSSC by Using Gold Nanoparticles

In this research, AuNPs prepared by laser ablation in liquid (PLAL) at 750 mJ energy and 90 pulses. They have been added to N719-dye to form (Au-N719) mixture. TiO2 paste was deposited on FTO substrates and immersion in a mixture dye and AuNPs. DSSC was fabricated and enhancement of the DSSC conversion efficiency was achieved. Scanning electron microscope, X-ray diffraction and UV-Visible spectrophotometer were used to characterize and study the DSSC components. The UV-Vis data show high absorbance of AuNPs+N719 dye compared to N719 dye only; that means the immersion of AuNPs in the N719-dye solution tend to increase the total absorbance of the dye. The XRD pattern of AuNPs has diffraction peaks at 38.2° and 44.4° which can be indexed by (111), (200) plane of Au in the cubic phase. The shape and size distribution shows spherical AuNPs with particle size about (50-60) nm. The relative increase of short circuit current density after the adding gold nanoparticles was about 76%. In the same way the relative increase of open circuit voltage after adding gold nanoparticles was about 6.7%. The total photon-to-current energy conversion efficiency for the standard DSSC is 1.75 while its 2.8 of the enhanced DSSC with gold NPs.The maximum enhancement is about 60 % under illumination (105 mW/cm2, (AM1.5).

Impedance Spectroscopic Investigation of the Degraded Dye-Sensitized Solar Cell due to Ageing

This paper investigates the effect of ageing on the performance of dye-sensitized solar cells (DSCs). The electrical characterization of fresh and degraded DSCs is done under AM1.5G spectrum and the current density-voltage (J-V) characteristics are analyzed. Short circuit current density (JSC) decreases significantly whereas a noticeable increase in open circuit voltage is observed. These results have been further investigated electroanalytically using electrochemical impedance spectroscopy (EIS). An increase in net resistance results in a lowerJSCfor the degraded DSC. This decrease in current is mainly due to degradation of TiO2-dye interface, which is observed from light and darkJ-Vcharacteristics and is further confirmed by EIS measurements. A reduction in the chemical capacitance of the degraded DSC is observed, which is responsible for the shifting of Fermi level with respect to conduction band edge that further results in an increase of open circuit voltage for the degraded DSC. It is also confirmed from EIS that the degradation leads to a better contact formation between the electrolyte and Pt electrode, which improves the fill factor of the DSC. But the recombination throughout the DSC is found to increase along with degradation. This study suggests that the DSC should be used under low illumination conditions and around room temperature for a longer life.

Inhibiting Interfacial Recombination Events in Dye-Sensitized Solar Cells using Self-Assembled Bilayers.

The rate and efficiency of electron transfer events at the semiconductor-dye-electrolyte interface is of critical importance to the overall performance of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells. In this work, we introduce self-assembled bilayers composed of a metal oxide electrode, bridging molecules, linking ions, and dye as an effective strategy to manipulate interfacial electron transfer events at the photoanode of DSSCs. Spectroelectrochemical measurements including current-voltage, incident photon-to-current efficiency, and electrochemical impedance spectroscopy are used to quantify interfacial electron transfer and transport events with respect to the length of the bridging molecules. The general trend in increased lifetime and diffusion length in TiO2 as well as an increase in open circuit voltage with bridge length indicate that the bilayer is an effective strategy in inhibiting the TiO2(e(-)) to redox mediator recombination events. However, the increased separation between the dye and the semiconductor also reduces the electron injection rate resulting in a decrease in photocurrent as the bridge length increases. The observed enhancement in open circuit voltages are far outweighed by the significant decrease in photocurrent and thus overall device performance decreases with increasing bridge length.

Self-assembled oleamide layer applied for cathode buffer layer of bulk heterojunction solar cells based on PTB7:PC71BM

In this work, bulk heterojunction solar cells based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] and phenyl-C71-butyric-acid-methyl-ester were fabricated using 1,2-dichlolobenzene solutions containing different weight ratios of oleamide. The oleamide layers were self-assembled on the active layer surfaces during the solidification of the active layer after spin coating. A significant increase in open-circuit voltage was observed after the introduction of oleamide at the expense of short-circuit current density. The optimal performance of the solar cell was obtained by spin coating the active layer at 1000 rpm for 60 s using a 1,2-dichlolobenzene solution containing 3% oleamide. The solar cell exhibited a short-circuit current density, an open circuit voltage, a fill factor, and a power conversion efficiency of 13.95 mA/cm2, 0.79 V, 0.47, and 5.22%, respectively. These solar cell behaviors are discussed on the basis of results of morphological analysis by optical microscopy, atomic force microscopy, and surface energy analysis.

Improvement of hydrogenated amorphous silicon germanium thin film solar cells by different p-type contact layer

In this study, we report an appreciably increased efficiency from 6% up to 9.1% of hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells by using a combination of different p-doped window layers, such as boron doped hydrogenated amorphous silicon (p-a-Si:H), amorphous silicon oxide (p-a-SiOx:H), microcrystalline silicon (p-µc-Si:H), and microcrystalline silicon oxide (p-µc-SiOx:H). Optoelectronic properties and the role of these p-layers in the enhancement of a-SiGe:H cell efficiency were also examined and discussed. An improvement of 1.62mA/cm2 in the short-circuit current density (Jsc) is attributed to the higher band gap of p-type silicon oxide layers. In addition, an increase in open-circuit voltage (Voc) by 150mV and fill factor (FF) by 6.93% is ascribed to significantly improved front TCO/p-layer interface contact.

Effects of Sodium Dodecyl Sulfate as a Co-Adsorbate on the Performance of Dye-Sensitized Solar Cells.

As a surface modifier, sodium dodecyl sulfate (SDS), was applied to dye-sensitized solar cells (DSSCs), and its effects on the photovoltaic performance of the solar cells were investigated. When the SDS was co-adsorbed with dye (N719) onto TiO2 surface, the DSSCs with SDS showed an increase in short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor, leading to a considerable improvement of over 23% in power conversion efficiency, compared to the reference cell without SDS. Incorporation of SDS on TiO2 surface induced longer lifetime of electrons injected from excited dyes to conduction band of TiO2, leading to an increase in the electron collection efficiency and thus an enhancement in Jsc. The longer lifetime by a suppression of the interfacial charge recombination could also contribute to an increment in Voc.

Surface thulium-doped TiO2 nanoparticles used as photoelectrodes in dye-sensitized solar cells: improving the open-circuit voltage

This study presents the incorporation of thulium oxide onto the surface of TiO2 nanoparticles, which were used as the photoelectrode in dye-sensitized solar cells. The Tm-TiO2-based semiconductors were widely characterized using techniques such as atomic emission spectroscopy, X-ray diffraction, Raman spectroscopy, UV–Vis spectroscopy, and X-ray photoelectron spectroscopy. The presence of a Tm oxide on the surface was confirmed, and neither the crystalline phases present nor the band gap of TiO2 were affected. In turn, the presence of Tm3+ resulted in new electronic transitions, which led to luminescence processes in the Tm-TiO2 semiconductors. Furthermore, the use of these semiconductors as photoelectrodes in DSSCs led to an increase in open-circuit voltage of up to 6 %. This increase can be reasonably explained by the negative shift of the flat-band potential of the photoelectrodes.

Influence of pressure on solid oxide electrolysis cells investigated by experimental and modeling approach

For many industrial applications, hydrogen must be pressurized before being used or stored. Because compressing liquid water is less energetic than the first levels of gaseous H2 compression, High Temperature Steam Electrolysis (HTSE) performed under pressure might represent an advantageous way for hydrogen production. With the goal of improving the electrolysis efficiency, an experimental and modeling approach has been adopted in order to better understand the basic underlying mechanisms of pressurized electrolysis operation. Experiments were carried on two different single commercial solid oxide cells at 800 °C in the pressure range of 1–10 bar. As a first result, according to the i–V curves, two main pressure effects have been observed. First, as expected, the Open Circuit Voltage is higher under pressure. Then, the limiting current density is increased with increasing the pressure, meaning that the hydrogen production can be improved. The electrochemical model, which has been adjusted on the experimental i–V curves obtained at atmospheric conditions, has been validated for the pressurized operation. Simulations have shown that the improvement of the limiting current is related to the decrease of cathode's concentration overpotential with pressure. Moreover, an optimal pressure can be defined depending on the cell polarization. Higher pressures than the optimal one lead to slightly decrease the hydrogen production rate, mainly due to Open Circuit Voltage increase that cannot be balanced enough by the cathode's concentration overpotential decrease. Finally, this study demonstrates that cell performances under pressure are less sensitive to the variation of cermet-support microstructural properties.

Increased Power Conversion Efficiency of Dye-Sensitized Solar Cells with Surface-Modified Photoelectrode

A nanoporous TiO2 film was modified with an aqueous potassium chloride (KCl), and the resulting film was applied to the photoelectrode of dye-sensitizes solar cell (DSSC). The DSSC with KCl-modified photoelctrode showed an increase in short circuit current (Jsc) and open circuit voltage (Voc), resulting in a power conversion efficiency (PCE) of 7.04%, compared to that (6.36%) of reference device with bare TiO2. The suppression of charge recombination between photo-injected electrons and triiodide (I3−) ions was found to increase both Jsc and Voc of the device with KCl-modified photoelctrode.

Efficiency enhancement of organic photovoltaic modules prepared via wash processing

To enhance OPV performance, a novel module-fabrication process was developed. Chlorobenzene and acetone were employed to wash away unnecessary residual high-resistance materials after full modules were fabricated. Monolithic interconnecting electrode lines between the anode and the cathode were formed using a dispenser after wash process. The wash process allowed for minimization of interconnection resistance and enhancement of power conversion efficiency. Module efficiency changes were also studied by varying the coating conditions of PEIE. The power conversion efficiency increased as a result of the wash-off process compared with modules fabricated via conventional processes. The power conversion efficiency enhancement was attributed to increases in open circuit voltage and fill factor. Applying this wash-off process to larger-area OPV modules is expected to be effective for further enhancing power conversion efficiency.

Performance Characterization of Solid Oxide Cells Under High Pressure

AbstractIn this work, recent pressurized test results of a planar Ni‐YSZ (YSZ: Yttria stabilized Zirconia) supported solid oxide cell are presented. Measurements were performed at 800 °C in both fuel cell and electrolysis mode at different pressures. A comparison of the electrochemical performance of the cell at 1 and 3 bar shows a significant and equal performance gain at higher pressure in both fuel cell mode and electrolysis mode. Electrochemical impedance spectroscopy revealed that the serial resistance was not affected by the operation pressure; all the other processes that are dependent on partial pressures (oxygen, steam and hydrogen) were affected by increasing the pressure. In electrolysis mode at low current density, the performance improvement was counteracted by the increase in open circuit voltage, but it has to be borne in mind that the pressurized gas contains higher molar free energy.

First-principles investigation of organic photovoltaic materialsC60, C70, [C60]PCBM, and bis-[C60]PCBMusing a many-bodyG0W0-Lanczos approach

We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C$_{60}$, C$_{70}$, [6,6]-phenyl-C$_{61}$-butyric-acid-methyl-ester ([C$_{60}$]PCBM), and bis-[C$_{60}$]PCBM using many-body perturbation theory within the Hedin's $G_0W_0$ approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C$_{60}$ and C$_{70}$ agree very well with experimental values measured in gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin-films, exhibiting a close agreement - a rigid energy-gap renormalization owing to intermolecular interactions in the thin-films. In addition, it is shown that the low-lying unoccupied states of [C$_{60}$]PCBM are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene C$_{60}$. The functional side group in [C$_{60}$]PCBM introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene C$_{60}$ with [C$_{60}$]PCBM as the electron acceptor in bulk heterojunction polymer solar cells.

Simple Preparation of Fluorine-Doped TiO2 Photoanode for High Performance Dye Sensitized Solar Cells

TiO2 nanocrystallites with titanium (IV) fluoride treatment were fabricated and studied as dye sensitized solar cells photoanode. F doping into pre-existing mesoporous TiO2 is achieved by a simple hydrolysis method. Overall photo-to-current conversion-efficiency of the device could be enhanced up to 23% due to the increase of photocurrent density, fill factor and open-circuit voltage, compared with undoped TiO2. From electrochemical impedance measurements, we deduce that the presence of F dopants enhances cell performance by improving the charge transport and retarding the recombination process in the photoanode.

Synthesis and modeling of uniform complex metal oxides by close-proximity atmospheric pressure chemical vapor deposition.

A close-proximity atmospheric pressure chemical vapor deposition (AP-CVD) reactor is developed for synthesizing high quality multicomponent metal oxides for electronics. This combines the advantages of a mechanically controllable substrate-manifold spacing and vertical gas flows. As a result, our AP-CVD reactor can rapidly grow uniform crystalline films on a variety of substrate types at low temperatures without requiring plasma enhancements or low pressures. To demonstrate this, we take the zinc magnesium oxide (Zn(1-x)Mg(x)O) system as an example. By introducing the precursor gases vertically and uniformly to the substrate across the gas manifold, we show that films can be produced with only 3% variation in thickness over a 375 mm(2) deposition area. These thicknesses are significantly more uniform than for films from previous AP-CVD reactors. Our films are also compact, pinhole-free, and have a thickness that is linearly controllable by the number of oscillations of the substrate beneath the gas manifold. Using photoluminescence and X-ray diffraction measurements, we show that for Mg contents below 46 at. %, single phase Zn(1-x)Mg(x)O was produced. To further optimize the growth conditions, we developed a model relating the composition of a ternary oxide with the bubbling rates through the metal precursors. We fitted this model to the X-ray photoelectron spectroscopy measured compositions with an error of Δx = 0.0005. This model showed that the incorporation of Mg into ZnO can be maximized by using the maximum bubbling rate through the Mg precursor for each bubbling rate ratio. When applied to poly(3-hexylthiophene-2,5-diyl) hybrid solar cells, our films yielded an open-circuit voltage increase of over 100% by controlling the Mg content. Such films were deposited in short times (under 2 min over 4 cm(2)).

Symmetry-breaking charge transfer in a zinc chlorodipyrrin acceptor for high open circuit voltage organic photovoltaics.

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.