High-performance Dye-sensitized Solar Cells Research Articles

Exploration on Charge Transfer and Absorption Spectra of Spiro[fluorene-9,90-xanthene]-based Polyoxometalate Hybrids Toward High Performance Dye-sensitized Solar Cell

Based on spiro[fluorene-9,90-xanthene](SFX, dye 1), the Lindqvist-type polyoxometalate(POM) functio-nalized with SFX and its derivatives(dyes 2―4) used in dye-sensitized solar cells(DSSCs) were designed and inves-tigated with the density functional theory(DFT) and time-dependent DFT(TD-DFT) calculations. The results indicate that Lindqvist-type POM is the main contribution to the lowest unoccupied molecular orbital(LUMO) and affects the LUMO energies of dyes 2―4. The maximum absorptions of the designed dyes containing POM(dyes 2―4) are red shifted comparing with that of dye 1. The introduction of electron-donating group onto SFX segment is helpful to red shift the absorption spectra. The major factors affecting the performance of DSSCs, including light harvesting and electron injection were evaluated. Considering the absorption spectra and photovoltaic parameters, dyes 3 and 4 are promising high performance dye sensitizers in n-type DSSCs.

High-Performance Dye-Sensitized Solar Cells through Graded Electron Transport in Band-Engineered W-TiO2 Cascade Layer

The efficiency of TiO2-based dye-sensitized solar cells (DSSCs) has been limited because charge transfer is not fully achieved, and the injected electrons always recombine with the acceptor species before the electrode can collect them. The novel device architecture of offering graded bands into the TiO2 photoanode network of DSSCs is presented to enhance the power conversion efficiency (PCE). Mono (x %) as well as graded (w–x–y–z %) W-doped TiO2 films are prepared to form intermediate bands in the bandgap of TiO2. A significant enhancement in the PCE is achieved when mono x % W-TiO2 films are tested as the photoanode of the DSSCs. Furthermore, DSSCs with band-engineered W-TiO2 cascade layer exhibit significant photovoltaic performance showing graded carrier transport to current collector. In particular, a graded 1–5–7–9 W cascade film exhibits the highest photovoltaic performance of 11.30% without decreasing the open-circuit voltage, which is 48% higher than pristine DSSCs without a blocking layer. This ...

One-Step Fiber Laser Fabrication of Mesoporous and Compact TiO2 Layers for Enhanced Performance of Dye-Sensitized Solar Cells

Mesoporous TiO2 structure (mp-TiO2) with a compact TiO2 blocking layer (bl-TiO2) has been recognized as an effective photoanode for high-performance dye-sensitized solar cells (DSSCs). However, producing both mesoporous and compact TiO2 layers by conventional method requires a high-temperature furnace process via a two-step manufacturing route. Here, a one-step laser technique has been successfully developed to generate both mesoporous and compact TiO2 layers on tin-doped indium oxide (ITO) coated glass by a millisecond pulsed fiber laser with a wavelength of 1070 nm in ambient atmosphere. Compared with the conventional furnace process, the laser process exhibits a significant reduction of overall fabrication time from 5 h 20 min to 2 min. More importantly, the DSSCs fabricated by the laser process show a remarkable improvement with the maximum power conversion efficiency (PCE) by 34% compared with the DSSCs fabricated by the furnace. The short-circuit current density (Jsc) reaches 16.5 mA cm–2 for the DSSCs fabricated by the laser process while 10.6 mA cm–2 is obtained for those fabricated by the furnace process. The improvement is associated with the increased dye adsorption, decreased charge transfer resistance, and increased electron lifetime of the TiO2 photoanode fabricated by the laser process.

Improved charge extraction with N-doped carbon quantum dots in dye-sensitized solar cells

Dye-sensitized solar cells (DSSC) have narrow spectral response and serious electron-hole recombination, which are two burdens for photovoltaic performance enhancement. In order to make high-performance DSSCs featured by wide-spectral absorption and fast charge extraction, we present here experimental realization of co-sensitization of nitrogen (N)-doped carbon quantum dots (N-CQDs) with N719 dye. Arising from up-conversion and hole extraction behaviors of N-CQDs, a maximized power conversion efficiency (PCE) as high as 9.29% under one sun illumination is achieved for N300-CQDs/N719 co-sensitized DSSC in comparison with 8.09% for N300-CQDs-free device. Due to light storing and emitting characteristics of photofluorescent long persistence phosphors in mesoscopic titanium dioxide/long persistence phosphor (m-TiO2/LPP) photoanode, electricity is persistently generated when ceasing sunlight illumination. This work is far from optimization, but the physical proof-of-concept co-sensitization and fast charge extraction may remarkably widen light-response windows and promote electron-hole separation for advanced photovoltaic devices.

Interfacial charge transport studies and fabrication of high performance DSSC with ethylene cored unsymmetrical dendrimers as quasi electrolytes

In this paper, authors focus the synthesis of conjugated unsymmetrical stilbenoid dendrimers by Heck and Horner-Wadsworth-Emmons coupling. In UV–visible absorption spectrum, the intensity of the absorption increases with increase in the generation of dendrimers. Further, bathochromic shift is observed on increasing the generation of the dendrimer from zero to first due to the greater widening of the energy gap between π-π* orbitals of the dendrimer system. Interfacial charge transport kinetics such as resistance, chemical capacitance and relaxation lifetime of the fabricated dye-sensitized solar cells (DSSC) are investigated using Nyquist and Bode phase plots by electrochemical impedance spectroscopy. Reduced electron relaxation lifetime (τe) of 1.83 ms (LiI + 7) and 1.04 ms (LiI + 8) provides efficient charge injection and thus reducing recombination process in the device. The performance of DSSC fabricated using unsymmetrical conjugated dendrimers with iodide electrolyte shows higher power conversion efficiency (PCE) than standard LiI based device. Two fold increments are achieved in PCE with first generation unsymmetrical dendrimers compared to their zeroth counterpart. The first generation unsymmetrical dendrimer 8 shows better PCE of 9.037% than all other synthesized dendrimers in the newly fabricated DSSC.

Tandem organic dye-sensitized solar cells: Looking for higher performance and durability

High-performance dye-sensitized solar cells (DSSCs) are the result of state-of-the-art blend of knowledge and experience; but finding accurate sensitizers from one side and explicit demonstration of the effects of material parameters on the power conversion efficiency (PCE) from the other side are direct challenges to the manufacture of DSSCs. Tandem DSSCs (T-DSSCs) is a young member of such family which allows one to attain higher power conversion efficiencies (PCE), but complications beyond T-DSSC manufacture has urged researchers to make use of Edisonian approach. Response Surface Methodology (RSM) and Desirability Function (DF) approaches are used to optimize T-DSSCs manufacture. Thickness of TiO2 layer, time required for TiCl4 treatment, and the concentration of anti-aggregation agent are explanatory variables changed to manufacture T-DSSCs with optimized photocurrent (Jsc), photovoltage (Voc) and Durability. The concentration of anti-aggregation agent was the key factor in attaining high-performance T-DSSCs. By the combined use of RSM and DF and through multi-objective optimization, it was found that under anti-aggregation agent concentration of 0.012 mM, thickness of TiO2 of 24.2 μm, and TiCl4 treatment time of ca. 78 min, T-DSSC having the highest possible performance with Jsc (13.37 mA/cm2), Voc (1.054 V), and Durability (1388 h) can be achieved.

Tailoring the benzotriazole (BTZ) auxiliary acceptor in a D-A′-π-A type sensitizer for high performance dye-sensitized solar cells (DSSCs)

Molecular engineering toward decreasing the band gap and thus extending the absorption spectrum of benzotriazole-based dye is a challenge. Herein, the benzotriazole structure is finely tuned by fusing another electron deficient ring to the benzo unit leading to a series of benzotriazole-based electron-withdrawing heterocyclic rings including benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole. These together with benzotriazole are incorporated into the π-bridge, respectively, as an auxiliary acceptor between the triphenylamine donor and the cyanoacrylic acid anchoring group for the construction of donor-auxiliary acceptor-π-acceptor type sensitizers (LC-2, LC-3, LC-4 and LC-5). The benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole units show greatly increased electron-withdrawing capability and the corresponding sensitizers (LC-3, LC-4 and LC-5) exhibit broadened spectral response range compared to the reference benzotriazole-based dye LC-2 due to the decreased energy gap. Thus, the LC-3 and LC-4 cells exhibit significantly enhanced photocurrent density (Jsc) values due to the greater light-harvesting abilities compared to LC-2. However, the electrochemical impedance analysis suggests the electron lifetime and charge recombination resistance are decreased due to the introduction of benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole, resulting in a slight loss of open-circuit voltage (Voc). The findings provide a guidance for the future molecular design of highly efficient organic sensitizers for use in dye-sensitized solar cells.

Fabrication of large Pt nanoparticles-decorated rGO counter electrode for highly efficient DSSCs

This study describes the radiolytic fabrication of large Pt nanoparticles (NPs)-decorated reduced graphene oxide (L-Pt/rGO) and its use as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). A homogenous aqueous solution of GO in 10wt% isopropyl alcohol (IPA) aqueous solution containing Pt precursor was irradiated with γ-ray at room temperature to prepare the L-Pt/rGO. The analytic results from TEM, EDX, XPS, and Raman revealed that the rGO decorated with 100nm Pt NPs was successfully formed by the γ-ray irradiation-induced reduction of both the GO and Pt precursor in aqueous IPA solution. Based on the results of the DSSCs performance test, the energy conversion efficiency of the DSSCs with the L-Pt/rGO-based CE outperformed that with the rGO-based one due to the lower sheet resistance, and even was comparable to that of the Pt-based CE. This L-Pt/rGO fabricated by a simple, room temperature and scalable radiolytic method can be used as a promising CE material for low-cost and high-performance DSSCs.

Rational design of cost-effective dyes for high performance dye-sensitized cells in indoor light environments

One of the important features of dye-sensitized solar cells (DSCs) is that their performance is relatively unaffected by lighting conditions, e.g., lighting angles and lighting intensity, which makes them ideal for application in indoor light environments. While most of past research on DSCs has focused on their performance under one-sun conditions, the effect of the structure of the dye on device performance under indoor lights has not been studied before. For this study, we designed a series of organic dyes with donor-π-bridge-acceptor (D-π-A) structure that can be synthesized in three reaction steps from inexpensive raw materials. The photophysical and electrochemical properties of the dyes and the device characteristics of the DSCs employing these dyes under indoor light (LED and T5 lamp, 600 lux) were investigated. In our research, the best PCE value of DSCs under LED lamps was 12.21% when TKU-6 was used as the sensitizer (70.8% of N719). Under T5 lamps, it was 13.43% when TKU-4 was used as the sensitizer (68.5% of N719). Moreover, we proposed the structural design strategies for dyes for high performance DSCs under indoor lights based on the type of π-bridge, the number of alkyl chains and the absorption spectrum of the dye.

Development of electrospun PAN/CoS nanocomposite membrane electrolyte for high-performance DSSC

In the present work, we prepared electrospun polyacrylonitrile (PAN)/ cobalt sulfide (CoS) nanocomposite membranes by encapsulating different weight percentage (1, 2, and 3 wt%) of CoS nanoparticles in PAN nanofibers for dye-sensitized solar cells (DSSCs). The electrospun PAN/CoS nanocomposite membranes (esCPMs) were activated by immersing in an ionic liquid electrolyte containing 0.5 M LiI, 0.05 M I2, 0.5 M 4-tert butylpyridine and 0.5 M 1-butyl-3-methylimidazolium iodide in acetonitrile to obtain their corresponding nanocomposite membrane electrolytes (esCPMEs). The influences of CoS content on the physical and electrochemical properties of esPM were investigated. The addition of CoS nanoparticles reduced the degree of partial crystallinity of esPM to an amorphous state. An electrochemical impedance and Tafel polarization measurements revealed that the esCPME has high electrocatalytic activity with enhanced ionic conductivity. The 2 wt% CoS encapsulated esCPME ensured the effective charge transfer at the electrode/electrolyte interface with the lower electron recombination rate in DSSC, which ultimately increased the photoconversion efficiency (7.41%) at an incident light intensity of 100 mW cm−2.

Understanding Catalytic Behavior of Co-Sn Alloy/Graphene Counter Electrode Electrocatalysts in Liquid-Junction Photovoltaic Devices

Understanding the electrocatalytic activity of counter electrode (CE) favors to tune the sluggish triiodide reduction kinetics and to compare their catalytic activities. We present here the synthesis of CoXSn1-X alloys (0 ≤ X ≤ 1) on reduced graphene oxide (RGO) under atmospheric pressure plasma without using any toxic chemical reagents. The developed materials are then applied as catalysts for the reduction of triiodide ions to iodide ions in CE of dye-sensitized solar cells (DSCs). It should be noted that the triiodide reduction reaction is one of the utmost significances in the advanced electrochemical energy conversion of DSCs. Therefore, insights into the relationship between alloy composition, electronic structure and catalytic activity should be carefully determined in this study. As the TEM measurements, CoSn nanoparticles with size in the range of 1-5 nm is successfully immobilized on the surface of RGO. Electrochemical catalytic activity performances indicate that the Co0.9Sn0.1/RGO gave the highest catalytic activity compared other electrodes which were determined by the lowest charge-transfer resistance and diffusion impedance values. Thus, the highest power conversion efficiency was obtained for the device using Co0.9Sn0.1/RGO CE. The strategy has a great promise in developing high-performance DSCs and the launched reduction rate constant having commonality in comparing the catalytic activity of CE electrocatalysts.

High-Performance Dye-Sensitized Solar Cells Based on Colloid-Solution Deposition Planarized Fluorine-Doped Tin Oxide Substrates.

The transmittance and conductivity of fluorine-doped tin oxide (FTO) conductive glasses are the critical factors limiting the performance of dye-sensitized solar cells (DSSCs). Here, the transmittance and conductivity of commercial FTO glasses were improved via a colloid-solution deposition planarization (CSDP) process. The process includes two steps. First, the FTO nanocrystal colloid was deposited on the FTO glasses by spin-coating. Secondly, the coated glasses were treated by FTO precursor solution. Compared to the bare FTO glasses, the modified FTO glasses by the CSDP process achieved 4% increase in transmittance (at 550 nm) and 11% decrease in sheet resistance, respectively. In addition, the modified FTO glasses can reduce the aggregation of Pt nanoparticles and improve the electrocatalytic activity of Pt counter electrodes. When the modified FTO glasses were used to assemble DSSCs, the cells got a photoelectric conversion efficiency as high as 9.37%. In contrast, the efficiency of reference cells using bare FTO substrates was about 8.24%.

Poly(3,4-ethylenedioxythiophene)/reduced graphene oxide composites as counter electrodes for high efficiency dye-sensitized solar cells

A facile, low cost, easy-controllable method to prepare Poly(3,4-ethylenedioxythiophene) (PEDOT)/reduced graphene oxide (rGO) composites by electrochemical deposition onto fluorinated tin oxide (FTO) as counter electrodes (CEs) in high performance dye-sensitized solar cells (DSSCs) is reported. The electro-deposition process was accomplished by electro-polymerization of graphene oxide (GO)/PEDOT composites onto FTO substrates followed by electrochemical reduction of the GO component. Electrochemical measurements show that the I−/I3− catalytic activity of the as-prepared PEDOT/rGO CE is improved compared with that of the pure PEDOT and PEDOT/GO electrode. Through the analysis of photoelectric properties, the performance of the electrodes fabricated with different polymerization times are compared, and the optimal preparation condition is determined. The photoelectric conversion efficiency (PCE) of the DSSC assembled with PEDOT/rGO electrode reaches 7.79%, close to 8.33% of the cell with Platinum (Pt) electrode, and increases by 13.2% compared with 6.88% of the device with the PEDOT electrode.

Facile synthesis of chrysanthemum flowers-like TiO2 hierarchical microstructures assembled by nanotube for high performance dye-sensitized solar cells

Abstract Chrysanthemum flowers-like TiO2 hierarchical microstructures assembled by nanotube are synthesized by a two-step hydrothermal route. The final TiO2 hierarchical microstructures are applied as a scattering layer on top of a transparent nanocrystalline TiO2 film, serving as the photoelectrode of dye-sensitized solar cells (DSSCs). In addition to efficient light scattering, the hierarchical microstructures provide a large surface area (105.4 m2/g) for higher dye loading and nanotube primary structure also provide a straight path for the electron fast transport. This multi functioning suggests that chrysanthemum flowers-like TiO2 hierarchical microstructures may be good scatter materials in DSSCs. A high efficiency of 7.71% is achieved for using the as-synthesized hierarchical microstructures as scattering layer, which is 21.8% higher than that of commercial P25 photoanode (6.33%) with the similar thickness.

Bifacial dye-sensitized solar cells from transparent RuSe counter electrodes

Bifacial solar cells that can generate electricity from either front or rear side are regarded as advanced photovoltaics for markedly increased photoelectric conversion efficiency. We present here the fabrication of transparent RuSe counter electrodes by an alternating electrodeposition method for bifacial dye-sensitized solar cells (DSSCs). The catalytic and photovoltaics performances are maximized by tuning stoichiometric Ru/Se ratio and bilayer number. Upon irradiation by AM1.5 (100 mW cm−2), the device yields a maximized front efficiency of 8.72% and a rear efficiency of 5.9%, arising from the >80%-transparency of RuSe electrode in visible light region. This strategy provides new opportunities for fabricating high-performance DSSCs.

The TiO2 Hierarchical Structure with Nanosheet Spheres for Improved Photoelectric Performance in Dye-Sensitized Solar Cells

A bi-layer photoanode is successfully fabricated for dye-sensitized solar cells (DSSCs) composed of P25/TiO2 nanorod (P25/TNR) as the underlayer and TiO2 nanosheet spheres (TNSs) as the light-scattering layer. Notably, the P25-TNR provides multiple functions, including more dye loading, more efficient charge transport and a lower electron recombination rate for the photoanode. Besides, the unique structure of TNS can significantly improve the light-harvesting capacity, boosting the light-harvesting efficiency. Therefore, an enhanced short-circuit current and power conversion efficiency of 18.04 mA cm−2 and 5.99%, respectively, were achieved for the P25/TNR-TNS-based DSSC, which was better than that of the P25-TNS-based (15.17 mA cm−2, 5.36%) and bare TNS-based (11.43 mA cm−2, 4.14%) DSSCs. This indicates that this bi-layer structure effectively combines the advantages of the one-dimensional (1D) nanostructure and three-dimensional (3D) hierarchical structure. In short, this work demonstrates the possibility of fabricating desirable photoanodes for high-performance DSSCs by rational design of nanostructures and effective combination of multi-functional components.

Coaxial heterojunction carbon nanofibers with charge transport and electrocatalytic reduction phases for high performance dye-sensitized solar cells.

Novel coaxial heterojunction carbon nanofibers, fabricated by electro-spinning a mixture of hydro-pitch and polyacrylonitrile, served as the counter electrode for dye-sensitized solar cells. Their high power conversion efficiency, being comparable to that of Pt CE, was achieved due to their good conductivity and high heteroatom content.

High-performance dye-sensitized solar cells using Ag-doped CoS counter electrodes.

CoS has been emerging as a promising Pt-free counter electrode (CE) material for dye-sensitized solar cells (DSSCs) due to its satisfactory electrocatalytic properties for redox reactions. However, its low electronic and ionic conductivities have limited its use in DSSCs. The doping of Ag with appropriate amount significantly improved the properties of CoS for application as a CE. Ag-doped CoS samples with various doping amounts were prepared by a facile one-step hydrothermal approach. There were very sharp changes of morphologies and particle sizes after doping different amounts of Ag. It is found that the DSSC fabricated with the 5% Ag-doped CoS CE achieved an impressive power-conversion efficiency (PCE) of 8.35% which was higher than that of the DSSC with a Pt CE (8.17%) by 2.2%, while the DSSC consisting of undoped CoS only exhibited a PCE of 6.93%. Such an enhanced PCE could be attributed to the significantly improved electrochemical activity and mixed conductivity resulting from the Ag dopant. Therefore, the excellent electrocatalytic activity, facile preparation and low material cost of the Ag-doped CoS electrode provide it with promising potential for large-scale production of new-generation DSSCs.

Highly conductive and stable graphene/PEDOT:PSS composite as a metal free cathode for organic dye-sensitized solar cells.

Platinum (Pt) as the counter electrode (CE) in dye-sensitized solar cells (DSSCs) is expensive and non-optimal for cobalt(ii/iii) redox couples, which can tune and improve the performance of DSSCs, thus motivating the search for replacements of the Pt CE. Graphene nanoplatelets (GnPs) are possible alternatives to Pt CEs but they are mechanically unstable as CEs due to their poor substrate adhesion. Here we report a new type of PEDOT:PSS (PP)/GnP (PPG) composite that maintains the catalytic performance of GnPs with enhanced adhesion to the substrate via a conductive PEDOT matrix. The resultant PPG exhibited extremely low charge-transfer resistance (Rct) compared to Pt in its role as an electrocatalyst toward a Co(bpy)32+/3+ (bpy = 2,2′-bipyridine) redox couple, and displayed extremely high electrochemical stability for Co(bpy)33+ reduction even after 1000 cycles. The inter-stacking of GnP layers between PEDOT and PSS was confirmed by XPS and Raman spectra. It helps to delocalize charges in the PEDOT backbone and rapidly transfers electrons from the external circuit to Co(bpy)33+. This reduces the Rct and ultimately improves the photovoltaic performance. The DSSC based on Y123 sensitizer and PPG-CE showed a higher photovoltaic performance of 8.33% than its Pt counterpart does (7.99%) under the optimized conditions.

Electrochemical Exfoliation of Graphene Flake Embedded in SiNWs as Counter Electrode for Dye-Sensitized Solar Cells

In this paper, a three-dimensional graphene wrapped silicon nanowire (SiNWs) architecture was synthesized by electrochemical exfoliation method and proposed to be the counter electrode for dye-sensitized solar cells (DSSCs). The results show that the few high-quality layers of graphene sheets have been obtained by electrochemical exfoliation method. The size of graphene is distributed from 550[Formula: see text]nm to 650[Formula: see text]nm, appropriately dispersed onto the SiNW supporter. The graphene/SiNW nanostructure still vertically aligned to the substrate finely. The channel between each wire is clear. The fabricated graphene/SiNWs electrode is a promising structure for future applications. The performance of the graphene/SiNWs as the counter electrode is found to be dependent on its dispersion in the whole backbone, with better dispersion offering more surface areas for the catalytic reduction reaction. Electrochemical tests reveal that the DSSC with graphene/SiNWs exhibits higher electro-catalytic activity and lower charge transfer resistance, suggesting that the 3D structure presents a potential way to fabricate low-cost, integrated and metal-free counter electrode for high-performance DSSCs. The DSSC based on graphene/SiNWs has an open-circuit voltage ([Formula: see text]) of 738.11[Formula: see text]mV, short-circuit current density ([Formula: see text]) of 15.48[Formula: see text]mA cm[Formula: see text], fill factor (FF) of 0.67 and conversion efficiency ([Formula: see text]) of 7.66%.