Flexible Photoanodes Research Articles

Chemical sintering by chlorinated carbon compounds for flexible photoanodes of dye-sensitized photovoltaic cells

Novel binary solvent mixtures of chlorinated carbon compounds (CCs = CCl4, CHCl3, and C2Cl4) and 1-octanol are used to prepare pastes of TiO2 nanoparticles (NPs), facilitating the formation of TiO2 mesoporous films for flexible photoanodes.

ELECTRICAL ANALYSIS OF SEMI-FLEXIBLE AND FLEXIBLE DSSCS: COMPARISON STUDY

In this study, semi-flexible dye-sensitized solar cell (DSSC) and flexible DSSC are examined comparatively. First, TiO2 paste with HCl was prepared and flexible photoanodes were produced by coating this paste on the indium tin oxide-coated poly(ethylene terephthalate) (ITO/PET) flexible substrates under the same conditions. These flexible photoanodes have been sensitized with Ruthenizer N719 dye at room temperature for 48[Formula: see text]h. The Pt was used as the back-electrode. The first back-electrode was fabricated for semi-flexible DSSC by spraying Pt on the fluorine-doped tin oxide (FTO). The second back-electrode was fabricated for flexible DSSC by coating Pt on ITO/PET using the chemical reduction method. The flexible photoanodes and Pt counter-electrodes have been combined. The photovoltaic performance of the semi-flexible DSSC was compared to the flexible DSSC.

Heat sink assisted elevated temperature sintering process of TiO2 on polymer substrates for producing high performance flexible dye-sensitized solar cells

We report a plate fin heat sink assisted elevated temperature sintering of TiO2 pastes doctor-bladed onto the indium tin oxide coated polyethylene terephthalate (ITO PET) substrates to function as photoanodes in the fabrications of flexible dye sensitized solar cells (DSSCs). In this process, sintering through a convective heat transfer has been employed to heat the pastes of TiO2 nanoparticles and its composite coated onto the ITO PET substrates at an elevated temperature up to 250 °C without the thermal decomposition and loss of integrity of PET polymer and then used in the preparation of flexible DSSC photoanodes. The TiO2 composite flexible photoanode in the DSSCs has produced a maximum power conversion efficiency (PCE) of 4.11 ± 0.32 % with an open circuit voltage (Voc) of 0.77 V. These values are found higher than the PCE and Voc values reported for the ITO PET based flexible DSSCs processed by various high pressure compression methods. Besides, the mechanical stability studies of flexible DSSC photoanodes prepared by this method have shown only less than 9 % decrease in the PCE values after 30 bending cycles, indicating good mechanical stability and adhesion between the TiO2 films and ITO PET substrates.

Plasmon-enhanced hierarchical photoelectrodes with mechanical flexibility for hydrogen generation from urea solution and human urine

We have demonstrated plasmon-enhanced flexible and hierarchical photoanodes for hydrogen production from human urine in a photoelectrochemical cell. The photoanodes consist of cobalt-doped α-Fe2O3 nanorod arrays functionalized with Au nanoparticles and Ni(OH)2. The Au nanoparticles and Ni(OH)2 work as plasmonic nanostructures and urea oxidation catalyst, respectively. Benefiting from the plasmonic and catalytic effects, the photoanodes exhibit an AM 1.5 photocurrent of 5.0 ± 0.1 mA cm−2 (urea solution) and 7.5 ± 0.1 mA cm−2 (human urine) at 0.3 V versus Ag/AgCl. At a Pt counter electrode, continuous hydrogen gas evolution is achieved at a small bias. With their high performance and mechanical flexibility that facilitates the large-scale transportation and implementation in the field, the photoanodes are paving a sustainable way towards hydrogen production and urine treatment.

Mineralization of flexible mesoporous TiO2 photoanodes using two low‐temperature dielectric barrier discharges in ambient air

Two types of dielectric barrier discharges (DBDs), volume DBD (called Industrial Corona) and coplanar DBD, were used for low temperature (70 °C) atmospheric pressure plasma mineralization of mesoporous methyl‐silica/titanium dioxide nanocomposite photoanodes. The photoanodes with a thickness of approx. 300 nm were inkjet‐printed on flexible polyethylene terephthalate (PET) foils. Plasma treatments of both DBDs led to changes in the chemical stoichiometry and morphology of the mesoporous photoanodes, resulting in a significant increase of the work function from approx. 4.0 to 4.3 eV and 4.8 eV, after plasma mineralization with volume DBD and coplanar DBD, respectively. We also studied the effect of plasma mineralization on the photoelectrochemical properties of the flexible mesoporous TiO2 photoanodes. Plasma mineralization with volume DBD and coplanar DBD showed different effects on the generated photocurrent in the photoanodes. Although the plasma mineralization with volume DBD showed only a minor effect on the photocurrent, plasma mineralization with coplanar DBD led to significantly higher photocurrents. We found that the enhancement of the photoelectrochemical properties was related to the homogeneity of the plasma‐treated surfaces—arising from different spatial properties of the plasma between volume and coplanar DBDs. Furthermore, the results showed that plasma mineralization using coplanar DBD can effectively change the energy levels of the surface. This resulted in the enhancement of the work function and the photoelectrochemical properties of the mesoporous TiO2 photoanodes. This contribution shows that coplanar arrangement of electrodes in DBDs generates plasma of higher efficacy compared with standard volume DBD that is currently often used in industrial processes.

Scale-Up of the Electrodeposition of ZnO/Eosin Y Hybrid Thin Films for the Fabrication of Flexible Dye-Sensitized Solar Cell Modules.

The low-temperature fabrication of flexible ZnO photo-anodes for dye-sensitized solar cells (DSSCs) by templated electrochemical deposition of films was performed in an enlarged and technical simplified deposition setup to demonstrate the feasibility of the scale-up of the deposition process. After extraction of eosin Y (EY) from the initially deposited ZnO/EY hybrid films, mesoporous ZnO films with an area of about 40 cm2 were reproducibly obtained on fluorine doped tin oxide (FTO)-glass as well as flexible indium tin oxide (ITO)–polyethylenterephthalate (PET) substrates. With a film thickness of up to 9 µm and a high specific surface area of up to about 77 m2·cm−3 the ZnO films on the flexible substrates show suitable properties for DSSCs. Operative flexible DSSC modules proved the suitability of the ZnO films for use as DSSC photo-anodes. Under a low light intensity of about 0.007 sun these modules achieved decent performance parameters with conversion efficiencies of up to 2.58%. With rising light intensity the performance parameters deteriorated, leading to conversion efficiencies below 1% at light intensities above 0.5 sun. The poor performance of the modules under high light intensities can be attributed to their high series resistances.

Low-temperature (70 °C) ambient air plasma-fabrication of inkjet-printed mesoporous TiO2 flexible photoanodes

Titania/silica electron-generating and -transporting nanocomposite 300 nm layers of high porosity were deposited onto ITO/PET flexible foils using inkjet printing. Prior to printing, the ITO surface had been modified by novel low-temperature ambient air roll-to-roll plasma in order to enhance its surface properties by removing carbon and oxygen contaminants, a process that led to rapid improvement of surface energy. Consequently the ITO work function, an important parameter involving charge injection efficiency in energy harvesting systems, increased by 1 eV. Afterwards, the TiO2/methyl-silica ink exhibited excellent wetting on a 2 s plasma-treated ITO surface. The coating was further processed/mineralized by an additional low-temperature ambient air plasma treatment step. The plasma processing of raw photoanodes led to the mineralization of the methyl-silica binder which resulted in the formation of a fully inorganic TiO2/SiO2 mesoporous structure and significantly increased electrophotocatalytic activity, leading to increased photocurrents. The entire two-step plasma process was performed at low-temperature (70 °C) and high speeds, enabling practical applications of such a procedure for large-area fabrication of flexible photoanodes.

Improved photoanode structure based on anodic titania nanotube array covered by TiO2-NPs/nanographite composite layer for ETA-cells

This work is devoted to the morphology and electrical properties optimization of flexible photoanodes based on anodic titanium oxide nanotubular arrays (TiO2-NTAs) for solar cells with extremely thin absorbing layer (ETA-cells) by TiO2-nanographite thin composite layer formation on the TiO2-NTAs surface. First, the carbon doped TiO2-NTAs were synthesized by annealing of the as-anodized TiO2-NTAs in argon without foreign carbonaceous precursor. The residual ethylene glycol absorbed on the nanotube wall during anodization serves as the carbon source and the C species are uniformly distributed along the entire nanotube to form the C-TiO2 NTAs. Further decorating of C-TiO2-NTAs surface by TiO2 nanoparticles to form the TiO2-nanographite (NG) composite layer with high conductivity and increased photoanode effective area showed improved ETA-cells performance.

Optimization of Selective Electrophoretic Deposition and Isostatic Compression of Titania Nanoparticles for Flexible Dye-Sensitized Solar Cells

Flexible photoanodes on PEN/PET based substrates for dye-sensitized solar cells (DSSC) by using electrophoretic deposition (EPD) method for the formation of titania nanoporous layer with titania nanoparticles (P-25) were produced. Considering commercially available titania nanoparticles containing both anatase and rutile phases, the undesired rutile nanoparticles, for DSSC applications, were almost eliminated from the coatings by selective pH adjustment of the EPD solutions without any binders. The coating characteristics and the selective coating capabilities of prepared ethanol and isopropanol based EPD solutions and their acetylacetone additive (as surfactant) alternatives were investigated using zeta potential measurements and mean colloid size analysis. As a subsequent complementary process, deposited films were compressed using Cold Isostatic Pressing (CIP) with an alternative clamping-effect pressure profile in order to increase the interparticular coherence and the overall adherence of the film onto substrates. Having obtained a 14–16μm film thickness values, cells were assembled with active area values of 0.5×0.5cm2, 1.0×1.0cm2 and 2.5×2.5cm2 and solar measurements were conducted, resulting in up to 4.22%, 4.13% and 4.00% efficiencies, relatively, under AM 1.5 conditions.

Production of flexible polymeric photoanodes using binder‐free electrophoretic deposition and compression for dye‐sensitized solar cells

AbstractElectrophoretic deposition (EPD) of titania nanoparticles have been utilized in order to form the nanoporous photoanodes of dye‐sensitized solar cells (DSSCs) on flexible polymeric (PEN/PET) substrates. Highly uniform, adherent nanoporous thick films were produced by uniformly dispersing the titania nanoparticles in alcoholic suspensions without any binders. Suspension performances of each type of isopropanol based mixtures have been examined. Additionally, the effect of acetylacetone addition in suspensions on zeta potential and effective particle size of the colloids is investigated. EPD process has been conducted in single step while the uniformity of the morphology and the thickness of the films were maintained preventing the damage on ITO layer due to excessive voltage exposure. Subsequently, the mechanical compression is applied via cold isostatic pressing (CIP) with a pressure profile in order to obtain 12‐14 µm film thickness and photon to current conversion efficiency of 2.16% was obtained under AM1.5 conditions. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of the solvent ratio on the electron transport for non-sintering flexible TiO2 photoanodes

Chemically modified TiO2 colloidal paste with a mixed solvent of water and t-butanol are developed for preparation of dye-sensitised photoanodes on plastic indium-tin oxide (ITO)-coated polyethylene-naphthalate (PEN) substrates. The effects of the solvent ratio on the rheological behaviour of the binder-free paste and the contact angle between the paste and the substrate for TiO2 photoanodes are scrutinised. First, tests of the rheological behaviour revealed that the increasing content of water leads to a decrease in the viscosity of the paste. Second, the dependence of the contact angle on the content of t-butanol is attributed to the trends of surface tension. Third, both the rheological properties and the contact angle influence the microstructure and morphology of the TiO2 coating on the ITO-PEN surface. Finally, the EIS results confirm that electron transport and recombination are significantly influenced for the mixed case. All of these results enable the optimisation of the connection in the photoanodes, achieving the highest energy conversion efficiency of 5.30%.

Flexible photoanodes of TiO2 particles and metallic single-walled carbon nanotubes for flexible dye-sensitized solar cells

Recent advance in flexible electronics demands development of flexible energy sources. Of particular interests are flexible dye-sensitized solar cells (DSCs). However, a brittle nature of TiO2 materials is one of hurdles to realize flexible DSCs. Here we synthesized flexible photoanodes of TiO2 particles and single-walled carbon nanotubes (SWNTs). Metallic SWNTs provided a greater photovoltaic conversion efficiency than semiconducting SWNTs due to the more efficient electron transport. The metallic SWNTs also constructed effective mechanical network among TiO2 particles providing flexibility and durability. The photoanode was transferred on an indium tin oxide (ITO)-coated polyethylene terephthalate film and characterized for front-illuminated DSCs under the AM 1.5 simulated sunlight. There was only a small decrease in photovoltaic conversion efficiency with bending which was primarily caused by cracking of the ITO layer. Due to this limitation, the TiO2–metallic SWNT photoanode was transferred on a Ti foil and went through up to 1000 bending cycles. The cycled photoanode was assembled for back-illuminated DSCs due to the non-transparent Ti foil. There was no decrease in photovoltaic conversion efficiency even after 1000 bending cycles demonstrating excellent flexibility and durability.

Effect of titanium isopropoxide addition in low-temperature cured TiO2 photoanode for a flexible DSSC

In this study, titanium isopropoxide (TIPP) was employed as a binder for low-temperature processed photoanode of a flexible dye sensitized solar cell. The coating paste for dip-coating method was prepared comprising commercially available TiO2 nanoparticles, TIPP and small amount of water. Effect of binder addition was compared at three different TIPP to TiO2 nanoparticle molar ratios; 0.05, 0.1 and 0.3. By adding TIPP molecules directly to the coating paste, flexible photoanodes were successfully prepared. An optimum TIPP to TiO2 particle ratio was selected considering the particle dispersion stability in the paste and the microstructure of prepared photoanode. The photoanode prepared using the paste with optimized composition showed highest photoconversion efficiency of 1.96 % from a unit cell.

Hydrothermal synthesis of oriented ZnO nanorod–nanosheets hierarchical architecture on zinc foil as flexible photoanodes for dye-sensitized solar cells

ZnO nanorods–nanosheets (NR–NS) hierarchical architecture consisted of long nanorod backbones and thin nanosheet branches have been synthesized on zinc foil substrate via a two-step hydrothermal growth process for dye-sensitized solar cells application. The SEM observation showed that the smooth surfaces of nanorods were surrounded by nanosheets, resulting in the enhancement of internal surface area. The photovoltaic results indicated that short-circuit current density of the ZnO NR–NS hierarchical architecture based DSSC significantly increased as compared to that of nanorods based DSSC, which was ascribed to higher dye loading and superior light scattering within the NR–NS hierarchical architecture photoanode. As a result, the conversion efficiency (η) of the ZnO NR–NS hierarchical architecture based DSSC increased by 90% compared with that of the nanorods based DSSC. The electron transport properties were further investigated by means of open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS), showing higher electron recombination in NR–NS hierarchical photoanode.

TiO2 Nanowires/ZnO Nanosheets Hierarchically Structure on Ti Foil as Flexible Photoanodes for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted much attention of researchers since it was reported first in 19911. The highest power conversion efficiency of DSSCs has reached over 12% until now2. The grain boundaries of nanocrystals in the TiO2 nanoparticle photoanode usually used could cause slow electron mobility. To overcome the slow electron transport of nanocrystalline TiO2, researches focused on the ordered one-dimensional (1D) structures. These 1D structures provided direct pathways for injected electrons and then improved electron transport3.However, DSSCs based on 1-D structured photoanode have not obtained expected extremely high power conversion due to the much smaller surface area, which resulted in smaller dye-loading compared with nanoparticles. To further improve the photovoltaic performance of DSSCs, constructing hierarchical morphologies based on 1-D structures and nanoparticles or nanosheets turns out to be an effective way4. The hierarchical structure could enhance the dye-loading and maintain the fast electron transport at the same time.5 In this work,a novel TiO2 Nanowires/ZnO Nanosheets (TiO2 NW/ZnO NS) hierarchical structure has been fabricated on Ti foil substrate through a two-step process of hydrothermal and homogenous precipitation, which is much easier and lower-power consumption than commonly two-step hydrothermal method6.As shown in Fig. 1 A and B, the obtained TiO2 NWs are well ordered 1-D structure, forming TiO2 NWs arrays on Ti foil substrate. The length of TiO2 NWs is about 12 μm, and the diameter of TiO2 NWs is about 100 nm. After the homogenous precipitation process, a TiO2 NW/ZnO NS hierarchical structure has been fabricated. The diameter of NWs increased clearly with ZnO NS attaching, enhancing to about 400 nm, which is about 4 times of TiO2NWs, which could increase the surface area and dye-loading significantly.As shown in Fig. 1 E, DSSCs device based on such structured flexible photoanode showed a significantly improvement of power conversion efficiency compared to that based on TiO2 NWs, which increased from 0.80% to 1.41% under 100 mW cm-2 illumination, enhancing 76.2% relatively. The improvement of conversion efficiency is mainly due to the much larger surface area and resulting increased dye-loading amount and photocurrent of TiO2 NW/ZnO NS hierarchical structure. References 1 B. O' Regan, M. Grätzel, Nature , 353, 737, (1991).2 A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Grätzel. Science, 334, 629, (2011).3 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, C hem. R ev., 110, 6595, (2010).4 S. Yodyingyong, Q. Zhang, K. Park, C. S. Dandeneau, X. Zhou, D. Triampo and G. Cao, A ppl. P hys. L ett., 96, 073115, (2010).5 R. Gao, J. Tian, Z. Liang, Q. Zhang, L. Wang and G. Cao, Nanoscale, 5, 1894, (2013). 6 J. Y. Liao, B. X. Lei, H.-Y. Chen, D.-B. Kuang and C.-Y. Su, Energy Environ. Sci., 5, 5750, (2012).

TiO2 Nanowires/ZnO Nanosheets Hierarchically Structure on Ti Foil as Flexible Photoanodes for Dye-Sensitized Solar Cells

In this paper, a novel TiO2 Nanowires/ZnO Nanosheets (TiO2 NW/ZnO NS) hierarchical structure has been fabricated on Ti foil substrate through a two-step process of hydrothermal and homogenous precipitation. DSSCs device based on such structured flexible photoanode showed a significantly improvement of power conversion efficiency compared to that based on TiO2 NWs, which increased from 0.80% to 1.41% under 100 mW cm-2 illumination, enhancing 76.2% relatively. The improvement of conversion efficiency is mainly due to the much larger surface area and resulting increased dye-loading amount and photocurrent of TiO2 NW/ZnO NS hierarchical structure.

A ZnO nanorod/nanoparticle hierarchical structure synthesized through a facile in situ method for dye-sensitized solar cells

Nanorods/nanoparticles hierarchically structured on a ZnO foil were fabricated by a facile method as flexible photoanodes with a significantly improved performance.

Facile one-step synthesis of highly branched ZnO nanostructures on titanium foil for flexible dye-sensitized solar cells

Highly branched ZnO (HBZ) nanostructures were prepared on titanium (Ti) foil using a facile, one-step vapor confined chemical vapor deposition technique. The as-prepared ZnO layer showed a good connection with the Ti foil even after 50 bending cycles, and the resultant HBZ/Ti electrode possessed high bendability. The HBZ/Ti electrode was composed of four different layers, including a highly branched ZnO layer, a ZnO compact layer, a Ti-Zn alloy layer and Ti foil. The good adhesion of the as-prepared ZnO layer to Ti foil was ascribed to the formation of a Ti-Zn alloy layer and a ZnO compact layer during the growth process. A flexible dye-sensitized solar cell was assembled using the D149-sensitized HBZ/Ti as a photoanode, and a power conversion efficiency (PCE) of 3.3% was achieved with an open-circuit photovoltage of 0.664 V, a short-circuit current density of 7.53 mA cm(-2), and a fill factor of 0.66 measured under rear-side illumination (AM 1.5, 100 mW cm(-2)). The power conversion efficiency of the device remained at 92% of the initial value even after 50 bending cycles. These results indicate that the vapor confined chemical vapor deposition method which does not necessarily use any catalyst or seed is a facile, one-step approach to obtain highly branched ZnO nanostructures with high bendability on Ti foil. The tight bonding between the highly branched ZnO layer and Ti substrate by a Ti-Zn alloy layer and a ZnO compact layer makes the vapor confined CVD method very attractive for the preparation of high-performance flexible photoanodes.

Nanorod–nanosheet hierarchically structured ZnO crystals on zinc foil as flexible photoanodes for dye-sensitized solar cells

In this paper, ZnO nanorod-nanosheet hierarchical structures were fabricated using a facile method on zinc foil and used as flexible photoanodes in dye-sensitized solar cells (DSCs). Compared to nanorods (NRs) obtained by the dissolution-precipitation method, the nanorod-nanosheet (NR-NS) hierarchical structures obtained by a second-step homogeneous precipitation improved the performance of DSCs by increasing photocurrent density significantly. As a result, the power conversion efficiency of the devices based on such a NR-NS hierarchical structure reached up to 2.4% under 100 mW cm(-2) illumination condition. This represents an enhancement by 108% as compared to DSCs based on NR assembled nanoflowers, for which the efficiency was 1.1%. The enhancement of the photocurrent was due mainly to the much larger specific surface area and resulting dye-loading amount. The electron transport properties in this structure were also investigated by means of electrochemical impedance spectroscopy (EIS). Furthermore, the formation mechanism of the NR-NS hierarchical structures are discussed.

One-Step Electrophoretic Deposition of Magnesium Oxide-Coated Tin Oxide Film and Its Application to Flexible Dye Sensitized Solar Cells

Magnesium oxide-coated tin oxide (MgO-coated SnO2) powder consisted of aggregates was prepared by a facile method. The MgO-coated SnO2 powder was characterized by X-ray diffractometer, field emission scanning electron microscope, energy dispersive spectroscopy and Raman spectroscopy. The aggregates were composed of nanoparticles, which promised a large surface area of 74.27 m2 g−1. MgO-coated SnO2 porous films utilized in dye-sensitized solar cells (DSSCs) were prepared using one-step electrophoretic deposition (EPD) method. The as-deposited films showed no cracks when the film thickness was 49.9 μm. The deposited films are hydrolysed at 413 K and irradiated under UV light. The post-treated MgO/SnO2 films are used as flexible photoanodes in dye-sensitized solar cells (DSSCs). Electrochemical impedance spectroscopy (EIS) is employed to quantify the electron lifetime (τe) of the MgO/SnO2 films. The amount of MgO coating and film thickness are optimized to achieve high energy conversion efficiency. The optimized MgO/SnO2 photoanodes based on transparent conductive glass substrates show an excellent photovoltaic response, with overall power conversion efficiency reaching 5.48% under AM 1.5 G illumination (100 mW cm−2), having a short circuit current density, open circuit voltage, and fill factor of 11.48 mA cm−2, 0.656 V and 0.73, respectively. All-plastic DSSCs are also assembled, which show a light-to-electricity conversion efficiency of 4.01%.