High-performance Dye-sensitized Solar Cells Research Articles

Novel CoS2 embedded carbon nanocages by direct sulfurizing metal-organic frameworks for dye-sensitized solar cells.

Owing to its excellent electrocatalytic properties, cobalt disulfide (CoS2) is regarded as a promising counter electrode (CE) material for dye-sensitized solar cells (DSSCs). However, hindered by its relatively poor electrical conductivity and chemical instability, it remains a challenge to apply it into high-performance DSSCs. In this work, we have developed novel CoS2 embedded carbon nanocages as a CE in DSSCs, using ZIF-67 (zeolitic imidazolate framework 67, Co(mim)2, mim = 2-methylimidolate) as a template. The CoS2 samples sulfurized for different time lengths are prepared through a facile solution process. It is found that the sulfurization time can be optimized to maximize the DSSC efficiency and the DSSC based on the CoS2 embedded carbon nanocages sulfurized for 4 hours exhibits the highest photovoltaic conversion efficiency (PCE) of 8.20%, higher than those of DSSCs consisting of other CoS2 CEs and Pt-based DSSC (7.88%). The significantly improved DSSC PCE is contributed by the synergic effect of inner CoS2 nanoparticles and an amorphous carbon matrix, leading to a CE with high catalytic activity, good electrical conductivity and excellent durability. This study demonstrates that the CE based on inexpensive CoS2 embedded carbon nanocages is a prospective substitute to expensive platinum and provides a new approach for commercializing high-efficiency DSSCs.

Recent Development of Graphene-Based Cathode Materials for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted extensive attention for serving as potential low-cost alternatives to silicon-based solar cells. As a vital role of a typical DSSC, the counter electrode (CE) is generally employed to collect electrons via the external circuit and speed up the reduction reaction ofI3-toI-in the redox electrolyte. The noble Pt is usually deposited on a conductive glass substrate as CE material due to its excellent electrical conductivity, electrocatalytic activity, and electrochemical stability. To achieve cost-efficient DSSCs, reasonable efforts have been made to explore Pt-free alternatives. Recently, the graphene-based CEs have been intensively investigated to replace the high-cost noble Pt CE. In this paper, we provided an overview of studies on the electrochemical and photovoltaic characteristics of graphene-based CEs, including graphene, graphene/Pt, graphene/carbon materials, graphene/conducting polymers, and graphene/inorganic compounds. We also summarize the design and advantages of each graphene-based material and provide the possible directions for designing new graphene-based catalysts in future research for high-performance and low-cost DSSCs.

Organic sensitizers with a rigid dithienobenzotriazole-based spacer for high-performance dye-sensitized solar cells

Sensitizers with a rigidified spacer show J-aggregation enhanced photon-to-electron conversion of DSSCs and cell efficiencies up to 9.1% (1 sun) and 10.6% (0.25 sun), respectively.

TiO2 cement for high-performance dye-sensitized solar cells

An energy conversion efficiency of 8.31% is reached by using a cemented photoanode for dye-sensitized solar cells, attaining a 31.1% improvement over the standard Degussa P25 sample.

Robust High-performance Dye-sensitized Solar Cells Based on Ionic Liquid-sulfolane Composite Electrolytes.

Novel ionic liquid-sulfolane composite electrolytes based on the 1,2,3-triazolium family of ionic liquids were developed for dye-sensitized solar cells. The best performing device exhibited a short-circuit current density of 13.4 mA cm−2, an open-circuit voltage of 713 mV and a fill factor of 0.65, corresponding to an overall power conversion efficiency (PCE) of 6.3%. In addition, these devices are highly stable, retaining more than 95% of the initial device PCE after 1000 hours of light- and heat-stress. These composite electrolytes show great promise for industrial application as they allow for a 14.5% improvement in PCE, compared to the solvent-free eutectic ionic liquid electrolyte system, without compromising device stability.

Rational design of anatase TiO2 architecture with hierarchical nanotubes and hollow microspheres for high-performance dye-sensitized solar cells

Large surface area, sufficient light-harvesting and superior electron transport property are the major factors for an ideal photoanode of dye-sensitized solar cells (DSSCs), which requires rational design of the nanoarchitectures and smart integration of state-of-the-art technologies. In this work, a 3D anatase TiO2 architecture consisting of vertically aligned 1D hierarchical TiO2 nanotubes (NTs) with ultra-dense branches (HTNTs, bottom layer) and 0D hollow TiO2 microspheres with rough surface (HTS, top layer) is first successfully constructed on transparent conductive fluorine-doped tin oxide glass through a series of facile processes. When used as photoanodes, the DSSCs achieve a very large short-current density of 19.46 mA cm−2 and a high overall power conversion efficiency of 8.38%. The remarkable photovoltaic performance is predominantly ascribed to the enhanced charge transport capacity of the NTs (function as the electron highway), the large surface area of the branches (act as the electron branch lines), the pronounced light harvesting efficiency of the HTS (serve as the light scattering centers), and the engineered intimate interfaces between all of them (minimize the recombination effect). Our work demonstrates a possibility of fabricating superior photoanodes for high-performance DSSCs by rational design of nanoarchitectures and smart integration of multi-functional components.

Preparation of anatase TiO2 nanorods with high aspect ratio for high-performance dye-sensitized solar cells

Due to offering a direct conduction pathway and fast electron transport, 1D nanostructures play an important role in improving charge collection efficiency in dye-sensitized solar cells (DSSCs). The anatase TiO2 nanorods with different aspect ratios between 3.2 and 6.3 were obtained by controlling reaction time for DSSCs. As their aspect ratios increased, more dye was adsorbed on the anatase TiO2 nanorods film. A promising power conversion efficiency of 7.51% was obtained for the anatase TiO2 nanorods with the biggest aspect ratio of 6.3.

Sr, Zn co-doped TiO2 xerogel film made of uniform spheres for high-performance dye-sensitized solar cells

This study comes up with the facile preparation of Sr,Zn co-doped TiO2 xerogel film for boosting the short circuit current density of dye-sensitized solar cells (DSCs). The film contains 2.5-μm-diameter spheres assembled from 60 nm nanoparticles. X-ray photoelectron spectroscopy (XPS) shows that Sr2+ and Zn2+ ions to be well incorporated into the TiO2 crystal lattice without forming specific strontium and zinc compositions. The crystallite size, phase composition, and band structure of the spheres depend on the dopants concentration. Isolated energy levels near valence band as a result of the foreign ions introduction improve the photocatalytic activity of the prepared TiO2 spheres, enhancing the short circuit current density of the cells. The DSC co-doped with 0.075 at.% Sr and 0.4 at.% Zn showed the highest power conversion efficiency of 7.87 % and short circuit current density of 18.75 mA cm−2 thanks to lower charge transfer resistance (2.16 Ω cm2), lower electron transit time (1.19 ms), and higher electron diffusion coefficient (18.1 × 104 cm2 S−1) compared to the other cells, demonstrated by electrochemical impedance spectroscopy (EIS). The concept of the simultaneous introduction of alkaline earth ions and transition ions into TiO2 xerogel films will open up a new insight into the fabrication of high performance DSCs.

Theoretical design of organic-inorganic hybrids based on hexamolybdate toward high performance dye-sensitized solar cells

The electronic structures, absorption spectra and photovoltaic (PV) performance of four dyes based on triphenylamine and polyoxometalate (POM) organic–inorganic hybrids for p-type dye-sensitised solar cells have been discussed by density functional theory (DFT) and time-dependent DFT calculations. In the four designed dyes, the triarylamine and carbazole take the role of the electron donor and the POMs act as the electron acceptor. It was found that introduction of electron donating groups (diphenylamine group and carbazole) into the triarylamine unit enabled better PV performance. This work is expected to be helpful for designing triarylamine–POM hybrid dyes with target properties.

Effect of conjugated side groups on the photovoltaic performances of triphenylamine-based dyes sensitized solar cells

Three novel organic dyes (LD1, LD2 and LD3) with conjugated side groups on the thiophene π-bridge are designed and synthesized for dye sensitized solar cells (DSSCs). The photophysical, electrochemical, and photovoltaic properties of the dyes have been successfully tuned by incorporating different conjugated side groups into the thiophene π-bridges moieties of dyes. Compared with the analogous dyes with non-conjugated side groups, the side group-conjugated dyes exhibited wider absorption spectra, higher molar extinction coefficients, and better photovoltaic performances. For the LD2-based DSSCs, the maximum power conversion efficiency (PCE) of 7.71% is obtained under AM 1.5 G irradiation (100 mW cm−2). The result demonstrates that high-performance DSSCs can be acquired by choosing suitable conjugated side groups into the π-bridges moieties of organic dyes.

Fine Tuning of Nanocrystal and Pore Sizes of TiO2 Submicrospheres toward High Performance Dye-Sensitized Solar Cells.

In general, the properties and performance of mesoporous TiO2 are greatly dependent on its crystal size, crystallinity, porosity, surface area, and morphology; in this regard, design and fine-tuning the crystal and pore sizes of the TiO2 submicrospheres and investigating the effect of these factors on the properties and photoelectric performance of dye-sensitized solar cells (DSSCs) is essential. In this work, uniform TiO2 submicrospheres were synthesized by a two-step procedure containing hydrolysis and solvothermal process. The crystal and pore sizes of the TiO2 submicrospheres were fine-tuned and controlled in a narrow range by adjusting the quantity of NH4OH during the solvothermal process. The effect of crystal and pore size of TiO2 submicrosphere on the performance of the DSSCs and their properties including dye-loading capacity, light scattering effect, power conversion efficiency (PCE), incident photon-to-electron conversion efficiencies (IPCEs), and electron recombination were compared and analyzed. The results show that increasing pore size plays a more significant role in improving the dye-loading capacity and PCE than increasing surface area, and an overall PCE value of 8.62% was obtained for the device with a 7.0 μm film thickness based on the TiO2 submicrospheres treated with 0.6 mL of NH4OH. Finally, the best TiO2 submicrosphere based photoanode film was optimized by TiCl4 treatment, and increasing film thickness and a remarkable PCE up to 11.11% were achieved.

Branched hierarchical titanium dioxide nanoflower on carbon nanofiber with efficient scattering layer for high performance dye-sensitized solar cells

A new bilayered photoanode is fabricated for dye-sensitized solar cells (DSSCs) composed of the carbon nanofiber coated by hierarchical TiO2 nanoflower (CNF-TNF) as underlayer and the urchin-like TiO2 hollow sphere (UTHS) as light-scattering layer. The CNF-TNF with a large specific surface area of 336.5m2g−1 provides dual-functions of more dye loading and efficient charge transport, leading to the enhanced short-circuit photocurrent density of 21.40mA cm−2 and prolonged electron lifetime of 189.2ms. The incorporated carbon nanofiber not only reduces the interfacial resistance and the charge recombination rate, but also improves the light harvesting, resulting in a high photoelectric conversion efficiency (PCE) up to 8.57% for the single-layered CNF-TNF based DSSC. In addition, by optimizing the surface morphology of TiO2 hollow sphere, we obtain the unique 3D UTHS, exhibiting strong light-scattering effect and low charge recombination due to the favorable connection among spheres provided by modified porous shell. As a consequence, an overall PCE of 9.21% is achieved for CNF-TNF+UTHS based DSSC, which is far superior to that of P25 based DSSC (5.10%). To the best of our knowledge, it is the first time that the hierarchically CNF-TNF composite is synthesized and utilized for enhanced efficiency DSSCs.

High performance dye-sensitized solar cells with inkjet printed ionic liquid electrolyte

We successfully inkjet printed an ionic liquid electrolyte in the dye-sensitized solar cell (DSSC) as a part of a new cell fabrication sequence that eliminates drilling holes in the DSSC substrates. The inkjet printing of ionic liquid electrolyte does not only reduce the overall fabrication cost since no additional thermoplastic sealant and glass cover are required to seal the cells, but also removes one extra cell sealing step present in the traditional cell sealing process. The dye-sensitized DSSCs fabricated with printed electrolyte exhibited 6% enhancement in overall cell conversion efficiency compare to the reference cells. More significantly, the printed electrolyte based solar cells maintained 100% of the initial performance in an accelerated ageing test for 1120h that was performed under full sun light intensity at 35°C. These results provide a solid base to develop this process further for the production of cheaper, more robust, large area DSSC solar panels.

Enhanced performance of dye-sensitized solar cells aided by Sr,Cr co-doped TiO2 xerogel films made of uniform spheres

One-pot preparation of Sr,Cr co-doped TiO2 xerogel film for boosting the short circuit current density of dye-sensitized solar cells (DSCs) is reported. The 2.5-μm-diameter spheres are assembled from 60nm nanoparticles by a modified sol–gel method. X-ray photoelectron spectroscopy (XPS) shows that Sr2+ and Cr3+ ions to be well incorporated into the titania crystal lattice without forming specific strontium and chromium compositions. The crystallite size, phase composition, and band structure of the particles depend on the dopants concentration. Isolated energy levels near valence band as a result of the transition ion (i.e., Cr) introduction, in conjunction with the local lattice distortions owing to the alkaline earth ion (i.e., Sr) insertion, improves the photocatalytic activity of the prepared TiO2 spheres, enhancing the short circuit current density of the cells. The DSC co-doped with 0.075 at.% Sr and 2.5 at.% Cr (i.e., S7C25 solar cell) showed the highest power conversion efficiency of 7.89% and short circuit current density of 18.58mA/cm2 thanks to lower charge transfer resistance (2.35Ωcm2), lower electron transit time (1.26ms), and higher electron diffusion coefficient (17.1×104cm2S−1) compared to the other cells, demonstrated by electrochemical impedance spectroscopy (EIS). The concept of simultaneously introduction of alkaline earth ions and transition ions into TiO2 lattice will open up a new insight into the fabrication of high performance DSCs.

Interfacial engineering and configuration design of bilayered photoanode consisting of macroporous tin dioxide/titanium dioxide for high performance dye-sensitized solar cells

Two kinds of tin oxide (SnO2)/titanium oxide (TiO2) composite materials have been synthesized and applied as electrodes in dye-sensitized solar cells (DSSCs) with high performance. The porous SnO2/TiO2 material (PSTM) consisting of SnO2 nanosheets cluster and TiO2 nanoparticles (P25) shows a superior dye adsorption ability due to its large specific surface area (151.1m2g−1). The PSTM based cell exhibits the slowest electron recombination rate among the cells tested from electrochemical impedance spectra measurements, and obtains final power conversion efficiency (PCE) up to 6.80%. Another novel structure of SnO2@TiO2 hollow sphere (STHS) prepared via a facile water bath process is designed to improve the light utilization efficiency with its excellent light scattering ability. Though the charge recombination resistance of STHS (25.6Ω) is smaller than that of P25 (30.6Ω), the PCE of the DSSCs based on the former is 5.82%, showing over 9.2% increment than the latter (5.33%). This can be mainly ascribed to the enhanced light-harvesting ability and charge collection efficiency of the macroporous hollow sphere structure, both of which contribute to a higher short current density and hence for the better photovoltaic performance. Furthermore, we demonstrate a bilayered film of PSTM (charge conduction layer) and STHS (light scattering layer) as photoanode aiming to further improve the efficiency of DSSC by engineered integration of different promising materials. The results indicate that the PSTM+STHS based cell shows an obvious 14.6% increase of PCE (7.79% with a Jsc of 17.49mA cm−2, Voc of 0.73V and FF of 0.61) as compared to the single layered PSTM photoelectrode with the same thickness of ∼6.9μm, providing the specific evidence for taking full advantages of the superior dye adsorption, fast charge collection as well as strong light harvesting simultaneously. Fundamentally, this study not only provides a scheme for the guidance of effective materials surface and interfacial modification, but also highlights the significance of the ideal photoanode configuration for high-efficiency solar cells application.

Boosting Photovoltaic Performance of Dye-Sensitized Solar Cells Using Silver Nanoparticle-Decorated N,S-Co-Doped-TiO2 Photoanode.

A silver nanoparticle-decorated N,S-co-doped TiO2 nanocomposite was successfully prepared and used as an efficient photoanode in high-performance dye-sensitized solar cells (DSSCs) with N719 dye. The DSSCs assembled with the N,S-TiO2@Ag-modified photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency of 8.22%, which was better than that of a DSSC photoanode composed of unmodified TiO2 (2.57%) under full sunlight illumination (100 mWcm−2, AM 1.5 G). This enhanced efficiency was mainly attributed to the reduced band gap energy, improved interfacial charge transfer, and retarded charge recombination process. The influence of the Ag content on the overall efficiency was also investigated, and the optimum Ag content with N,S-TiO2 was found to be 20 wt%. Because of the enhanced solar energy conversion efficiency of the N,S-TiO2@Ag nanocomposite, it should be considered as a potential photoanode for high-performance DSSCs.

Enhanced Dye-Sensitized Solar Cells with Catalytic Carbon Aerogel Counter Electrodes

Carbon aerogel (CA) with ultra-low density and three dimensional network nanostructures is investigated as the electrocatalyst for tri-iodide (I3−) reduction in dye-sensitized solar cells (DSSCs). The catalytic properties of CA and platinum (Pt) counter electrodes are analyzed and compared by electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements. The devices applying CA counter electrode exhibit a lower charge-transfer resistance (1.60Ωcm2) and larger diffusion coefficient (3.39×10−6cm2s−1) compared to the devices with conventional Pt counter electrode. As a new kind of porous carbon materials, CA counter electrode has high active surface area and conductance. These factors improve the power conversion efficiency (η) up to 8.04%, which is better than that of the reference Pt based DSSC (6.79%). This paper indicates that carbon aerogel with good catalytic activity can potentially be used as a promising alternative to the expensive Pt counter electrode in a high-performance DSSC device design.

First application of diethyl oxalate as efficient additive in high performance dye-sensitized solar cells based on iodide/triiodide electrolyte

In this study, diethyl oxalate (DEOX) is applied as an effective inexpensive additive based iodide/triiodide electrolyte in the dye-sensitized solar cells (DSSCs). Addition the suitable amount of DEOX as 1M into the electrolyte shows dramatically improvement in the short circuit current (Jsc) and consequently, in the total conversion efficiency (η). The fabricated devices based on N719 and 2-cyano-3-(4-(diphenylamino) phenyl) acrylic acid (TPA) sensitizers with modified electrolyte show the efficiency of 7.33% and 2.63% at an irradiation of AM1.5, and an 37% and 22% energy conversion efficiency increments, respectively. The boost in the photocurrent density mainly is due to the molecular complex formation between DEOX and redox species in the electrolyte solution that promotes the electrochemical properties of electrolyte. Also, electrochemical impedance measurements indicate adsorbing of DEOX on the semiconductor surface leads to an incensement in the lifetime (τ) and the electron density (ns) in the conduction band (CB) of TiO2 that shifts the Fermi level (EF) which leads to small enhancement in the Voc. Adsorbing of DEOX on the titania surface retards the interfacial charge recombination that has a beneficial effect on the Voc and Jsc. Furthermore, we compares the effect mechanisms of DEOX and 4-tert-butylpyridine (TBP) additives on the cell performance by applying different electrolytes containing the additives. These results show that TBP increases Voc while DEOX additive has effect on the Jsc and using of combination of additives leads to a remarkable improvement in η. As a result, DEOX is a new promising co-additive which can be used for high efficient and low cost DSSCs.

Peanut shaped ZnO microstructures: controlled synthesis and nucleation growth toward low-cost dye sensitized solar cells

This paper describes a simple, low-temperature and cost effective chemical precipitation method in aqueous media to synthesis uniformly distributed zinc oxide (ZnO) microstructures for the fabrication of dye-sensitized solar cells (DSSCs). The size and morphology of the ZnO microstructures are systematically controlled by adjusting the concentration of the precursors, zinc acetate dihydrate and ammonium hydroxide. X-ray diffraction (XRD) and scanning electron microscopy are used for the structural characterizations and photoluminescence and Fourier transform infrared spectroscopy are used to characterize the optical properties of the ZnO, respectively. The results reveal that ZnO crystallites exhibit hexagonal wurtzite structure with preferential orientation along c-axis. The effect of ammonia concentration on the crystallinity, morphology and optical properties of ZnO microstructures and the concomitant effect on the efficiency of DSSCs is also quantified. The peanut-shaped ZnO microstructure, which was found to increase DSSCs performance over other microstructure, is studied in detail in order to develop a formation mechanism. A sandwich type eosin yellow sensitized solar cell is prepared using peanut-shaped ZnO microstructures, which showed an efficiency of 0.37%. Ammonia was found to play a crucial role in the evolution of ZnO morphologies. These results are promising and provide a path towards low-cost high-performance DSSCs based on peanut-shaped ZnO microstructures and produced with only relatively simple wet chemistry synthesis.

Nitrogen-doped porous carbon prepared by a facile soft-templating process as low-cost counter electrode for High-performance dye-sensitized solar cells

Nitrogen-doping porous carbon (NPC) with high surface area is prepared by a simple soft-templating method and explored as a low-cost counter electrode for dye-sensitized solar cells. Nitrogen adsorption analysis shows that the as-prepared NPC has a high surface area and mesoporous structure. Electrochemical impedance spectroscopy test shows a low charge-transfer resistance of 0.77Ωcm2 for NPC electrode in iodide/triiodide redox electrolyte. Such excellent electrocatalytic activity can be attributed not only to good surface properties including high surface area and mesoporous structure but also to nitrogen doping in NPC framework. Among various nitrogen species in the NPC framework, pyridinic and quaternary nitrogen are considered to contribute significantly to the electrocatalytic activity. A conversion efficiency of 7.09% is achieved for dye-sensitized solar cells with NPC counter electrode at one sun illumination, which is comparable to that of the cell with Pt counter electrode. These results reveal that the NPC electrode is a promising counter electrode candidate for dye-sensitized solar cells.