Suppression Of Electron Recombination Research Articles

Experimental investigation of spin coating acceleration effect on the DSSC performance

The optimization of the TiO2 mesoporous structure plays significant role in dye-sensitized solar cell (DSSC) to produce efficient devices. In this study, the TiO2 mesoporous layer was coated by using a spin coating equipment with different spin accelerations. As a consequence of this investigation, the impacts of the spin coating acceleration on the optoelectronic and electrical performance characteristics of the DSSC were investigated. It has been shown that altering the spin coating acceleration has a direct impact on the mesoporous layer, which in turn influences the absorption ability of dye. The light absorbance of the sample A5 (coated at 2000 rpm s−1) ascended drastically in accordance to other samples. Thanks to this augmentation in absorbance, the current density (J SC ) and power conversion efficiency (PCE) values also improved. According to electrochemical impedance spectroscopy analysis, it was attained that recombination resistance values increases with the rising spin coating acceleration rates after 500 rpm s−1 and reaches up to highest value at 2000 rpm s−1. A relatively longer electron lifetime of 40.36 ms and recombination resistance of 12.22 Ω were obtained for the device coated at the rate of 2000 rpm s−1. The device coated at a rate of 2000 rpm s−1 had a PCE (5.51%) that was superior than other devices because of its improved light collecting ability, quick electron transport, suppressed electron recombination, and having longer electron life time. As a starting point for future investigations and applications, results of present study provide an insight into the optimal spin coating parameters for DSSC applications.

Semicrystalline Cathode Interlayer Based on Morphology Control Additives Using Nonconjugated Small‐Molecule Zwitterions for Efficient Organic Solar Cells

The cathode interlayer (CIL) has significant impact on the performance of organic solar cells (OSCs), including the abilities to align energy levels, form ohmic contacts between the active layer and the electrode, and promote the electron extraction and transportation. However, the developments of CILs are still far behind the rapid explosion of active layer materials, especially for these nonfullerene acceptors (NFAs). This research provides a brand‐new CIL optimization approach by applying the nonconjugated small‐molecule zwitterion (NSMZ) as the morphology control additives (MCAs). Two novel NSMZs, 4‐(1‐methyl piperidin‐1‐ium‐1‐yl)butane‐1‐sulfonate (MPBS) and 3‐(1‐methylpiperidin‐1‐ium‐1‐yl)‐propane‐1‐sulfonate, are developed as MCAs for the improvements of short‐circuit current, fill factor, and power conversion efficiency (PCE) in various solar cell systems of the classic active layers and CILs. The interaction mechanism is systematically investigated. With the reduced energy barrier and suppressed electron recombination, a champion PCE of 18.65% on binary NFA–OSCs is achieved by incorporating the zwitterion of MPBS into the cathode interlayer as the additive, which is among the highest efficiency of the reported binary OSCs. The application of the zwitterion as MCAs for the cathode interlayer opens a novel avenue for highly efficient OSCs.

Photocatalytic Degradation of Organic Dyes with Magnetically Separable PANI/Fe3O4 Composite under Both UV and Visible‐light Irradiation

AbstractThe creation of a novel composite photocatalyst has gained great attention in the field of organic dye remediation. Herein, the PANI/Fe3O4 photocatalyst has been prepared using the chemical oxidative polymerization of aniline. The as‐prepared photocatalyst represents a denser and uniform morphology along with strong magnetic properties. The photocatalytic activity of the PANI/Fe3O4 was investigated for the degradation of Malachite Green (MG), Jakazol Kala (JK) and Levafix Red (LR). In the presence of PANI/Fe3O4 composite, the reaction rate constants were found to be 0.1997, 0.0521, and 0.1993 min−1 for MG, JK and LR, under UV‐light irradiation and 0.0522, 0.0491, 0.0544 min−1 under visible light irradiation, respectively. The nanocomposite can be easily separated from the treated water and reused several times. The suppression of electron recombination, the reduced bandgap and the broad light‐harvesting in both UV and visible‐light regions contribute to the advanced photocatalytic performance of the PANI/Fe3O4.

Highly Ordered Nanotube Arrays as Photo-Anodes for Dye-Sensitized Solar Cells by Electrochemical Method

Symmetrically packed TiO2 nanotubes (TNTs) were fabricated in fluorine free perchloric acid medium by electrochemical anodization method. During anodization interconnected pores developed by forming amorphous/ hydroxides of titanium film. On increasing anodization, thickness of film increased and pores tend to become aligned providing surface layer of nanotubes on the electrode material. The 1-D charge carrier transport property of the tubular geometry have attracted scientists attention in using TNTs for TiO2 photovoltaic and photo catalysis applications. By controlling the anodization process, the length, diameter and wall thickness of TNTs can be tailored. The iron and chromium doped highly ordered 1-D TNTs obtained during anodization are superior photoanodes for dye-sensitized solar cells because of its reduced inter tube connections, vertical electron transport, suppressed electron recombination and enhanced current density, efficiency in power conversion and light scattering. The morphological characteristics and microstructure of TNTs were investigated by SEM and X-ray diffraction analysis. Diffusion electron spectroscopy has been used to analyze the amount absorption of the dye on the surface of the various TNT, which acknowledges the direct co relation between the dye absorption and morphology of the sample.

Improving the optoelectronic efficiency of novel meta-azo dye-sensitized TiO2 semiconductor for DSSCs

A series of novel azo dyes possessing varying conjugation lengths and different donor moieties, based on 5-amino isophthalic acid were designed and synthesized. Azobenzene unites were utilized as the π-spacer part to extend the conjugation range and connect the donor to acceptor unit. When the dialkylamino substituent was changed from dimethyl to diethanol, a red shift in the absorption spectra and λonset was observed. The photophysical and electrochemical properties of the straightforward-synthesized dyes were investigated in solution and on photoanode surface which promised the suitability of the dyes as photosensitizers for dye sensitized solar cells (DSSCs). Increased dye adsorption strength on the TiO2 surface as well as light harvesting capability was expected due to bearing two anchoring-electron accepting groups which could lead to enhanced electron transfer (ET). The ATR absorption spectra clearly showed that these dyes were adsorbed on the TiO2 surface. It was realized that increasing π-conjugation length as well as hydroxyl containing donor group gave rise to improved photovoltaic performance of DSSCs. Reduced band gap along with suppressed electron recombination and amended dye regeneration were recognized to play an important role in enhancing performance parameters. DSSCs based on these dyes exhibited higher solar conversion efficiency in comparison with efficiency of other meta azo dyes that were previously synthesized. Theoretical calculations (DFT/TDDFT) expressed that among the dyes, members 3a and 3b possessed localized and non-continuous electron distribution in their frontier orbitals as well as maximum amount of oscillator strength.

Investigation on Photoanode Modified With TiO2–ZnO–Ag Nanofibers in Dye-Sensitized Solar Cell Under Different Intensities of Illuminations

A photoanode modified with silver (Ag)–zinc oxide (ZnO)–titanium dioxide (TiO2) composite nanofibers (NFs) was prepared and used in dye-sensitized solar cells (DSSCs). TiO2–ZnO–Ag NFs were added as an additional layer between the TiO2 layer and the electrolyte of the DSSC to improve photovoltaic performance. Photovoltaic parameters of DSSC were measured under air mass 1.5 (AM 1.5, 100 mW/cm2). The impedance between the interface of photoanode/dye/electrolyte was identified by electrochemical impedance spectroscopy (EIS). We also investigated power conversion efficiencies under various intensities of the incident light through filters. The results of the photovoltaic measurements show that the power conversion efficiency ( $\eta $ ) of the DSSC modified with Ag–ZnO–TiO2 composite NFs is 6.43%. Compared with the power conversion efficiency of the DSSC without modification (5.44%), it is increased by 18%. The incorporation of ZnO and Ag into TiO2 NFs can enable the rapid transport of electrons and suppresses the opportunity for electron recombination. From Nyquist plot, the DSSC modified with Ag–ZnO–TiO2 composite NFs shows higher impedance than the DSSC without modification. It means that the photoanode modified with Ag–ZnO–TiO2 composite NFs can successfully suppress electron recombination. Under the intensity of incident light at 30 W/cm2, the DSSC modified with Ag–ZnO–TiO2 composite NFs has the highest power conversion efficiency of 9.16%.

Highly interconnected porous TiO2-Ni-MOF composite aerogel photoanodes for high power conversion efficiency in quasi-solid dye-sensitized solar cells

Three-dimensional mesoporous TiO2-Ni-MOF composite aerogels were synthesized via sol–gel route and used as photoanode materials for quasi-solid dye-sensitized solar cell (QSDSC) applications. The assessment of their photovoltaic performance revealed an enhancement compared to pure aerogel-based QSDSCs; the maximum photo-conversion efficiency of the fabricated photoanode was 8.846% for 0.5% composite aerogel, which is ~30% higher than that of pure aerogel-based ones (6.805%). The achievement of other key factors required for efficiency enhancement such as increased photocurrent density, reduced charge-transfer resistance, and suppressed electron recombination was confirmed by photocurrent density–applied voltage curves and electrochemical impedance measurements. The surface area of the composite aerogels ranged between 269 and 233 m2 g−1, which make them promising candidates for high-efficiency QSDSCs.

Engineering of the Back Contact between PCBM and Metal Electrode for Planar Perovskite Solar Cells with Enhanced Efficiency and Stability

AbstractThe cathode interface plays a critical role in achieving high‐performance fullerene/perovskite planar solar cells. Herein, the simple molecule Isatin and its derivatives are introduced at the back contact [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/Al as a cathode modification interlayer. It is revealed that the Isatin interlayers facilitate electron transport/extraction and suppress electron recombination, attributed to the formation of negative dipole potential steps and the passivation of the interfacial trap density. The average power conversion efficiencies of the resulting devices are significantly improved by 11% from 17.68% to 19.74%, with an enhancement in all device parameters including short‐circuit current, open‐circuit voltage, and fill factor. The hysteresis index is found to disappear. In addition, such interlayer enhances device stability under ambient conditions compared to the control devices due to suppression of moisture‐induced degradation of the perovskite films. These findings provide a comprehensive understanding of the engineering of the back contact between PCBM and the metal electrode to improve efficiency and stability of perovskite solar cells.

Performance of Perovskite Solar Cell Using Compact Layer by Sputter Deposition

A uniform compact layer is essential for obtaining high-efficiency perovskite solar cells (PSCs). This is because direct contact between the FTO glass substrate and the perovskite layer should be blocked to suppress electron recombination and to collect electrons. In the present study, PSCs were fabricated using sputtered TiO₂ compact layers prepared at different deposition times. The structure, morphology, and electrical properties of the sputter deposited TiO₂ compact layer were compared to those of a spin coated TiO₂ film. The TiO₂ compact layer prepared by the sputtering process was very smooth and inhibited electron recombination at the FTO interface, resulting in a higher open circuit voltage and a higher short circuit current density. A photoelectric conversion efficiency of 13.37% was obtained using the TiO₂ compact layer deposited for 30 min.

Organic Dyes based on Tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles for Photovoltaic and Photocatalysis Applications with the Suppressed Electron Recombination.

A tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles (TAPP) moiety with the combination of two pyrrole rings and four phenyl moieties demonstrated strong electron-donating ability and nonplanar configuration simultaneously. Once incorporated into the organic dyes as a novel electron donor, it was beneficial for the enhancement of light-harvesting ability and suppression of electron recombination in the photovoltaic and photocatalysis systems. With the linkage of tunable conjugated bridges and electron acceptor, the corresponding organic dyes exhibited improved photovoltaic performance in dye-sensitized solar cells and facilitated photocatalytic hydrogen generation with a highest turnover number (TON) of 4337. Through the detailed investigation of relationship between molecular structures and photovoltaic/photocatalysis property, the connection and difference in molecular design for these two systems are well explained, with the aim to promote the application of dye-sensitized technology in various fields.

Molecular design enabled reduction of interface trap density affords highly efficient and stable perovskite solar cells with over 83% fill factor

Advancing perovskite solar cell (PVSC) technologies toward its theoretical power conversion efficiency (PCE) and optimum stability requires stringent control over interfacial trap densities in the devices. By introducing a new macromolecular interlayer material of PPDIN6 at the PCBM/Ag interface for planar p-i-n PVSCs, we succeeded in significantly reducing the trap density, thereby facilitating electron extraction and suppressing electron recombination at the interface. Consequently, a PCE of 20.43% was achieved with a record fill factor of 83.4%, which is one of the highest values for planar p-i-n PVSCs. Moreover, the amine groups in PPDIN6 could neutralize the migrating iodide ions and thus inhibit the formation of the insulating Ag–I bonds on the surface of the Ag electrode. As a result, we realized long-term stability for PPDIN6 based PVSCs, which maintained 85% of their initial performance after continuous operation at their maximum power point for 200 h under 1-sun illumination in air with a relative humidity of 30–50%.

Molecular design towards suppressing electron recombination and enhancing the light-absorbing ability of dyes for use in sensitized solar cells: a theoretical investigation

We report a molecular design strategy with a comprehensive consideration of both inhibiting electron recombination and enhancing the light-harvesting ability.

High-Performance Perovskite Solar Cells with a Weak Covalent TiO2:Eu3+ Mesoporous Structure

Mesoporous TiO2 nanocrystals with attached Eu3+ ions in an amorphous state are employed as structures for highly efficient perovskite solar cells. The down-conversion TiO2:Eu3+ nanocrystal shows a good photoluminescence characteristic to convert the incident high-energy photon into a lower-energy photon, which improves the UV light utilization capability of perovskite solar cells (PSCs). The device within 1.0 wt % Eu3+ ions shows an improvement of 29.2% from 12.22% to 15.79% in PCE compared with the typical devices without Eu3+ in TiO2, which is attributed to the increased utilization of incident UV light and the dramatically suppressed electron recombination process. To summarize, the combination of a high UV light-harvesting capability and a slow electron recombination process with TiO2:Eu3+ mesoporous structure could successfully develop more efficient and stable PSCs.

Enhancing the performance of dye-sensitized solar cells by ZnO nanorods/ZnO nanoparticles composite photoanode

In this paper, ZnO nanorods (NRs) were prepared by a two-step solution phase reaction. A composite photoanode architecture is fabricated by adding 0–0.20 at.% ZnO NRs into ZnO nanoparticles (NPs). The scanning electron microscopy image shows that the average diameter and length of the ZnO NRs are about 50 nm and 2–5 µm, respectively, and the ZnO NRs are uniformly embedded into the ZnO NPs photoanode. The UV–vis spectrum analysis reveals that the amount of dye adsorption of the composite photoanode decreases with increasing ZnO NRs content. Meanwhile, the influence of ZnO NRs contents on the dye-sensitized solar cells (DSSCs) performance is systematically investigated. The photocurrent density–voltage (J–V) characteristics reveal that the device performance of DSSCs can be significantly enhanced by the composite photoanode. Typically, the DSSC with 0.15 at.% ZnO NRs obtains the optimal energy conversion efficiency of 3.8%, which is 28.4% higher than that of the pristine ZnO DSSCs. The electrochemical impedance spectroscopy (EIS) analysis shows that ZnO NRs can provide a direct pathway for accelerating electron transport, extending the electron lifetime, suppressing electron recombination and improving electron collection efficiency. These results indicate that the incorporation of ZnO NRs in the photoanode is an effective way to improve the performance of DSSCs.

A Strategy to Enhance the Efficiency of Quantum Dot-Sensitized Solar Cells by Decreasing Electron Recombination with Polyoxometalate/TiO2 as the Electronic Interface Layer.

Electron recombination occurring at the TiO2 /quantum dot sensitizer/electrolyte interface is the key reason for hindering further efficiency improvements to quantum dot sensitized solar cells (QDSCs). Polyoxometalate (POM) can act as an electron-transfer medium to decrease electron recombination in a photoelectric device owing to its excellent oxidation/reduction properties and thermostability. A POM/TiO2 electronic interface layer prepared by a simple layer-by-layer self-assembly method was added between fluorine-doped tin oxide (FTO) and mesoporous TiO2 in the photoanode of QDSCs, and the effect on the photovoltaic performance was systematically investigated. Photovoltaic experimental results and the electron transmission mechanism show that the POM/TiO2 electronic interface layer in the QDSCs can clearly suppress electron recombination, increase the electron lifetime, and result in smoother electron transmission. In summary, the best conversion efficiency of QDSCs with POM/TiO2 electronic interface layers increases to 8.02 %, which is an improvement of 25.1 % compared with QDSCs without POM/TiO2 . This work first builds an electron-transfer bridge between FTO and the quantum dot sensitizer and paves the way for further improved efficiency of QDSCs.

Titanium mesh supported TiO2 nanowire arrays/upconversion luminescence Er3+-Yb3+ codoped TiO2 nanoparticles novel composites for flexible dye-sensitized solar cells

Ti-mesh supported TiO2 nanowire arrays (NWAs)/upconversion luminescence Er3+-Yb3+ codoped TiO2 nanoparticles (UC-EY-TiO2 NPs) composite structured photoanodes for fully flexible dye sensitized solar cells (DSSCs) were firstly constructed via a hydrothermal and spin coating process. UV–vis-NIR absorption spectra of the TiO2 NWAs/UC-EY-TiO2 NPs composites exhibited strong absorption around near infrared (NIR) 980nm. The composites excited by 980nm NIR laser could emit upconversion fluorescence at 489, 526, 549 and 658nm, which expanded the spectral response range and sunlight capturing capability of formed flexible DSSCs. Moreover, the TiO2 NWAs/UC-EY-TiO2 NPs was coated with an Nb2O5 thin layer to further suppress electron recombination losses. The complete flexible DSSCs based on Nb2O5 coated TiO2 NWAs/2.0mol% Er3+-1.0mol% Yb3+ codoped TiO2 NPs photoanode and Pt/ITO-PEN counter electrode exhibited an enhanced photon to current conversion efficiency of 8.10%, a 68% improvement compared to TiO2 NWAs/undoped TiO2 NPs based DSSCs (4.82%).

Ordered Single-Crystalline Anatase TiO2 Nanorod Clusters Planted on Graphene for Fast Charge Transfer in Photoelectrochemical Solar Cells.

Achieving efficient charge transport is a great challenge in nanostructured TiO2 -electrode-based photoelectrochemical cells. Inspired by excellent directional charge transport and the well-known electroconductibility of 1D anatase TiO2 nanostructured materials and graphene, respectively, planting ordered, single-crystalline anatase TiO2 nanorod clusters on graphene sheets (rGO/ATRCs) via a facial one-pot solvothermal method is reported. The hierarchical rGO/ATRCs nanostructure can serve as an efficient light-harvesting electrode for dye-sensitized solar cells. In addition, the obtained high-crystallinity anatase TiO2 nanorods in rGO/ATRCs possess a lower density of trap states, thus facilitating diffusion-driven charge transport and suppressing electron recombination. Moreover, the novel architecture significantly enhances the trap-free charge diffusion coefficient, which contributes to superior electron mobility properties. By virtue of more efficient charge transport and higher energy conversion efficiency, the rGO/ATRCs developed in this work show significant advantages over conventional rGO-TiO2 nanoparticle counterparts in photoelectrochemical cells.

(Invited) A Versatile Plasmonic Film for Biosensing and Photocatalytic Applications

Novel Au@TiO2 plasmonic films were fabricated by individually placing Au nanoparticles into TiO2 nanocavity arrays through a sputtering and dewetting process. These discrete Au nanoparticles in TiO2 nanocavities showed strong visible-light absorption due to the plasmonic resonance. Surface-enhanced Raman spectra (SERS) of this plasmonic film showed a high sensityvity to organic molecule at low concentration (down to 0.1 micro molar). Photoelectrochemical studies demonstrated that the developed Au@TiO2 plasmonic films exhibited significantly enhanced catalytic activities towards oxygen reduction reactions with an onset potential of 0.92 V (vs reversible hydrogen electrode), electron transfer number of 3.94, and limiting current density of 5.2 mA cm-2. The deposited Au film thickness remarkably affected the catalytic activities of Au@TiO2, and TiO2 coated with 5 nm Au (Au5@TiO2) exhibited the best catalytic function to reduce oxygen. The plasmon-enhanced reductive activity is attributed to the surface plasmonic resonance of embedded Au nanoparticles in TiO2 nanocavities and suppressed electron recombination.

Simultaneously composition and interface control for ZnO-based dye-sensitized solar cells with highly enhanced efficiency

In this work, we concentrate on the investigation of the fundamental effects of annealing temperatures on the synthesized polydisperse of ZnO aggregates for the exploration of effective electrode material for dye-sensitized solar cells (DSCs). Systematic characterizations including TEM, SEM, BET analysis, XRD, and r-PL were performed for the ZnO aggregates annealed at various temperatures, and the results revealed that the morphology of aggregates, crystal size of primary particle, crystallinity, specific surface areas and the surface states largely depends on the annealing temperature. The photoelectrical efficiency tests demonstrated that the ZnO aggregates annealed at 450 °C exhibited the highest efficiency of 5.10%. The excellent performance could mainly originate from the more efficient light harvesting property of the resulted ZnO photoanode indicated by the incident photon-to-current efficiency (IPCE) and the accelerated electron transport within the ZnO film testified by electrochemical impedance spectroscopy (EIS). Moreover, the combination of the composition control with the introduction of both a ZnO compact layer (CL) and a TiO2 protecting layer (PL) was simultaneously integrated into one photoanode film for the first time. As a consequence, an impressive power conversion efficiency of 5.83% was achieved with the new photoanode architecture, an improvement of 39.5% over the unmodified DSC. The synergistically reduced charge-transport resistance and suppressed electron recombination are regarded to be the key element for outstanding performance.

Enhanced Photoelectrocatalytic Reduction of Oxygen Using Au@TiO2 Plasmonic Film.

Novel Au@TiO2 plasmonic films were fabricated by individually placing Au nanoparticles into TiO2 nanocavity arrays through a sputtering and dewetting process. These discrete Au nanoparticles in TiO2 nanocavities showed strong visible-light absorption due to the plasmonic resonance. Photoelectrochemical studies demonstrated that the developed Au@TiO2 plasmonic films exhibited significantly enhanced catalytic activities toward oxygen reduction reactions with an onset potential of 0.92 V (vs reversible hydrogen electrode), electron transfer number of 3.94, and limiting current density of 5.2 mA cm-2. A superior ORR activity of 310 mA mg-1 is achieved using low Au loading mass. The isolated Au nanoparticle size remarkably affected the catalytic activities of Au@TiO2, and TiO2 coated with 5 nm Au (Au5@TiO2) exhibited the best catalytic function to reduce oxygen. The plasmon-enhanced reductive activity is attributed to the surface plasmonic resonance of isolated Au nanoparticles in TiO2 nanocavities and suppressed electron recombination. This work provides comprehensive understanding of a novel plasmonic system using isolated noble metals into nanostructured semiconductor films as a potential alternative catalyst for oxygen reduction reaction.