Novel D‑π‑A Organic Dyes with Thieno[3,2-<i>b</i>]thiophene-3,4-ethylenedioxythiophene Unit as a π‑Bridge for Highly Efficient Dye-Sensitized Solar Cells with Long-Term Stability

This paper reports on new D-π-A organic dyes for application in dye-sensitized solar cells (DSSCs), which were developed by incorporating thieno­[3,2-b]­thiophene-thiophene (M9) and thieno­[3,2-b]­thiophene-EDOT (M10) as π-bridges. These dyes exhibited relatively small highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps in spite of the short π-conjugation lengths, resulting in broad spectral responses. As photosensitizers in DSSCs, M10 showed a broader spectral response than M9, leading to a greater short-circuit photocurrent (Jsc). In addition, M10 exhibited higher open-circuit voltage (Voc) compared to M9, because of the greater electron lifetime of the photoanode. The impedance analysis revealed that the greater electron lifetime of the photoanode with M10 was attributed to the lower electron recombination rate caused by the blocking effect of the bulky EDOT unit. As a result, M10 showed much higher conversion efficiency (η = 7.00%) than M9 (η = 5.67%) under one sun condition (AM 1.5 G, 100 mW/cm2). This conversion efficiency was comparable to that of the conventional Ru-based dye N719 (η = 7.24%) under the same condition. In addition, M10 exhibited a remarkable long-term stability, i.e., 95% of the initial conversion efficiency was maintained after light soaking for 45 days (1080 h)

Importance of 4-<i>tert</i>-Butylpyridine in Electrolyte for Dye-Sensitized Solar Cells Employing SnO₂ Electrode

The photovoltaic performance of dye-sensitized solar cells (DSSCs) employing SnO2 electrodes was investigated while increasing the content of 4-tert-butylpyridine (TBP) in the conventional liquid-type electrolyte. As the added TBP content increased, the open circuit voltage (Voc) and conversion efficiency were highly enhanced while the short circuit current (Jsc) was not much affected. With the electrolyte of 2.0 M TBP, the Voc and conversion efficiency were increased by 26 and 33%, respectively, compared with the conventional electrolyte (0.5 M TBP). The electrochemical impedance spectra revealed that the enhancement of Voc resulted from the negative shift of the SnO2 conduction band potential and the increase in resistance of electron recombination by 1 order of magnitude. It is noteworthy that the optimized concentration of TBP for the SnO2 electrode is greatly larger than that for the TiO2 electrode. This may be due to the much faster electron recombination rate and more positive conduction band potential of the SnO2 electrode. The SnO2 electrode modified with TiO2 shell showed only slightly enhanced performance due to the similar effects of shell layer and those of the TBP. In contrast to the SnO2, TiO2 electrodes did not show performance enhancement with the electrolyte of TBP concentration larger than 0.5 M. The impedance spectra of symmetric dummy cells employing Pt counter electrodes indicated that the catalytic effect of Pt was deteriorated, and the resistance of electrolyte diffusion was increased by the higher concentration of TBP. This brings up the need for development of a counter electrode that TBP is not easily adsorbed on, and alternative additives to TBP which are not highly viscous

Surface Modification of Stretched TiO₂ Nanotubes for Solid-State Dye-Sensitized Solar Cells

Straight-stranded anatase TiO2 nanotubes were produced by anodic oxidation on a pure titanium substrate in an aqueous solution containing a 0.45 wt % NaF electrolyte (pH 4.3 fixed). The average length of the TiO2 nanotubes was approximately 3 μm, which had an effect on the level of dye adsorption in the dye-sensitized solar cells. The anodic TiO2 nanotubes were applied as a working electrode in a solid-state dye-sensitized solar cell. An approximately 1 nm ZnO shell was coated on the TiO2 nanotube to improve the open-circuit voltage (Voc) and conversion efficiency of the solar cell, and to retard any back reaction. Although the Voc and short-circuit current (Jsc) of the cell were improved, there was a low fill factor as a result of the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate. A parameter on the degradation of fill factor (37%) is related to the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate interface. A hydrogen peroxide treatment was performed in an attempt to narrow the TiO2 barrier layer. This treatment was found to influence not only fill factor (37−49%) but also the conversion efficiency (0.704−0.906%) of the cell by eliminating the remnant after anodic reaction and barrier narrowing through an etching effect. This result was confirmed by X-ray photoelectron spectroscopy (XPS) and photocurrent-voltage measurements. The longer electron lifetime on the ZnO coated TiO2 film was measured by the open-circuit voltage decay. The improvement in the electron lifetime from the thin ZnO coating affects the number of electrons collected on the Ti substrate and the retardation of charge recombination. Therefore, the ZnO coating on the TiO2 nanotube film improves the efficiency of dye-sensitized TiO2 solar cells from the extended Voc from ZnO coating confirmed by the Mott−Schottky plots and the increased Jsc through the inhibition of charge recombination confirmed by IPCE measurements

Enhanced Photovoltaic Properties of a Cobalt Bipyridyl Redox Electrolyte in Dye-Sensitized Solar Cells Employing Vertically Aligned TiO₂ Nanotube Electrodes

Photovoltaic performances of TiO2 nanoparticle (NP) electrodes and TiO2 nanotube (NT) electrodes in dye-sensitized solar cells (DSSCs) employing a cobalt bipyridyl redox electrolyte were compared. The TiO2 NP electrodes had pore sizes ranging from 15 to 20 nm while the NT electrodes had a lager pore size of 80 nm. Highly ordered and vertically oriented TiO2 NT electrodes were prepared by a two-step anodization method. In application to DSSCs employing the cobalt redox electrolyte, the 11-μm-thick NP electrode exhibited an efficiency of 1.60% with a Jsc of 3.96 mA/cm2. Meanwhile, despite nearly half of the amount of adsorbed dye molecules, the 11-μm-thick NT electrode exhibited a slightly enhanced efficiency of 1.84% with a Jsc of 5.86 mA/cm2. In addition, the 35-μm-thick NT electrode showed an efficiency of 2.38% with a Jsc of 9.80 mA/cm2. Compared to the 11-μm-thick NP electrode, the 35-μm-thick NT electrode exhibited a 1.5 times higher efficiency with a 2.5 times higher Jsc in spite of having a similar amount of adsorbed dye molecules. Photocurrent transient measurements revealed that the mass transport limitation of the cobalt redox electrolyte within the conventional NP electrodes was greatly alleviated within the NT electrodes. In addition, the electrochemical impedance spectra indicated that the interfacial contact between the cobalt redox electrolyte and TiO2 electrode was prominently enhanced in the NT electrodes. Furthermore, the electron lifetime and electron diffusion length were all greatly longer within the NT electrodes. These superior photovoltaic properties may be attributed to the large pore size and vertically oriented structures of the NT electrodes

Highly Efficient Photoelectrochemical Hydrogen Production Using Nontoxic CuIn_1.5Se₃ Quantum Dots with ZnS/SiO₂ Double Overlayers

Quantum dots (QDs) are a promising material for photoelectrochemical (PEC) hydrogen (H2) production because of their attractive optical properties including high optical absorption coefficient, band-gap tunability, and potential multiple exciton generation. To date, QDs containing toxic elements such as Cd or Pb have been mainly investigated for PEC H2 production, which cannot be utilized in practice because of the environmental issue. Here, we demonstrate a highly efficient type II heterojunction photoanode of nontoxic CuIn1.5Se3 (CISe) QDs and a mesoporous TiO2 film. In addition, ZnS/SiO2 double overlayers are deposited on the photoanodes to passivate surface defect sites on the CISe QDs, leading to the enhancement of both photocurrent density and photostability. Due to a combination of a wide light absorption range of the CISe QDs and the reduced interfacial charge recombination by the overlayers, a remarkable photocurrent density of 8.5 mA cm–2 (at 0.5 VRHE) is obtained under 1 sun illumination, which is a record for the PEC sulfite oxidation based on nontoxic QD photoanodes

Water-Based Thixotropic Polymer Gel Electrolyte for Dye-Sensitized Solar Cells

For the practical application of dye-sensitized solar cells (DSSCs), it is important to replace the conventional organic solvents based electrolyte with environmentally friendly and stable ones, due to the toxicity and leakage problems. Here we report a noble water-based thixotropic polymer gel electrolyte containing xanthan gum, which satisfies both the environmentally friendliness and stability against leakage and water intrusion. For application in DSSCs, it was possible to infiltrate the prepared electrolyte into the mesoporous TiO₂ electrode at the fluidic state, resulting in sufficient penetration. As a result, this electrolyte exhibited similar conversion efficiency (4.78% at 100 mW cm^–2) and an enhanced long-term stability compared to a water-based liquid electrolyte. The effects of water on the photovoltaic properties were examined elaborately from the cyclic voltammetry curves and impedance spectra. Despite the positive shift in the conduction band potential of the TiO₂ electrode, the open-circuit voltage was enhanced by addition of water in the electrolyte due to the greater positive shift in the I^–/I₃^– redox potential. However, due to the dye desorption and decreased diffusion coefficient caused by the water content, the short-circuit photocurrent density was reduced. These results will provide great insight into the development of efficient and stable water-based electrolytes

Highly Efficient Copper–Indium–Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper–indium–selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO₂ electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date

Highly Efficient Bifacial Dye-Sensitized Solar Cells Employing Polymeric Counter Electrodes

Dye-sensitized solar cells (DSCs) are promising solar energy conversion devices with aesthetically favorable properties such as being colorful and having transparent features. They are also well-known for high and reliable performance even under ambient lighting, and these advantages distinguish DSCs for applications in window-type building-integrated photovoltaics (BIPVs) that utilize photons from both lamplight and sunlight. Therefore, investigations on bifacial DSCs have been done intensively, but further enhancement in performance under back-illumination is essential for practical window-BIPV applications. In this research, highly efficient bifacial DSCs were prepared by a combination of electropolymerized poly­(3,4-ethylenedioxythiphene) (PEDOT) counter electrodes (CEs) and cobalt bipyridine redox ([Co­(bpy)₃]^3+/2+) electrolyte, both of which manifested superior transparency when compared with conventional Pt and iodide counterparts, respectively. Keen electrochemical analyses of PEDOT films verified that superior electrical properties were achievable when the thickness of the film was reduced, while their high electrocatalytic activities were unchanged. The combination of the PEDOT thin film and [Co­(bpy)₃]^3+/2+ electrolyte led to an unprecedented power conversion efficiency among bifacial DSCs under back-illumination, which was also over 85% of that obtained under front-illumination. Furthermore, the advantage of the electropolymerization process, which does not require an elevation of temperature, was demonstrated by flexible bifacial DSC applications

Enhanced Photovoltaic Properties and Long-Term Stability in Plasmonic Dye-Sensitized Solar Cells via Noncorrosive Redox Mediator

We demonstrate the localized surface plasmon resonance (LSPR) effect, which can enhance the photovoltaic properties of dye-sensitized solar cells (DSSCs), and the long-term stability of size-controlled plasmonic structures using a noncorrosive redox mediator. Gold nanoparticles (Au NPs) were synthesized with a phase transfer method based on ligand exchange. This synthetic method is advantageous because the uniformly sized Au NPs, can be mass produced and easily applied to DSSC photoanodes. The plasmonic DSSCs showed an 11% improvement of power conversion efficiency due to the incorporation of 0.07 wt % Au NPs, compared to the reference DSSCs without Au NPs. The improved efficiency was primarily due to the enhanced photocurrent generation by LSPR effect. With the cobalt redox mediator, the long-term stability of the plasmonic structures also significantly increased. The plasmonic DSSCs with cobalt­(II/III) tris­(2,2′-bipyridine) ([Co­(bpy)₃]^2+/3+) redox mediator maintained the LSPR effect with stable photovoltaic performance for 1000 h. This is, to our knowledge, the first demonstration of the long-term stability of plasmonic nanostructures in plasmonic DSSCs based on liquid electrolytes. As a result, the enhanced long-term stability of plasmonic NPs via a noncorrosive redox mediator will increase the feasibility of plasmonic DSSCs

Rapid Dye Adsorption via Surface Modification of TiO₂ Photoanodes for Dye-Sensitized Solar Cells

A facile method for increasing the reaction rate of dye adsorption, which is the most time-consuming step in the production of dye-sensitized solar cells (DSSCs), was developed. Treatment of a TiO₂ photoanode with aqueous nitric acid solution (pH 1) remarkably reduced the reaction time required to anchor a carboxylate anion of the dye onto the TiO₂ nanoparticle surface. After optimization of the reaction conditions, the dye adsorption process became 18 times faster than that of the conventional adsorption method. We studied the influence of the nitric acid treatment on the properties of TiO₂ nanostructures, binding modes of the dye, and adsorption kinetics, and found that the reaction rate improved via the synergistic effects of the following: (1) electrostatic attraction between the positively charged TiO₂ surface and ruthenium anion increases the collision frequency between the adsorbent and the anchoring group of the dye; (2) the weak anchoring affinity of NO₃^– in nitric acid with metal oxides enables the rapid coordination of an anionic dye with the metal oxide; and (3) sufficient acidity of the nitric acid solution effectively increases the positive charge density on the TiO₂ surface without degrading or transforming the TiO₂ nanostructure. These results demonstrate the developed method is effective for reducing the overall fabrication time without sacrificing the performance and long-term stability of DSSCs