Simple metal-free organic D-pi-A dyes with alkoxy- or fluorine substitutions: application in dye sensitized solar cells

Two new metal-free organic sensitizers with simplest structural variations have been synthesized for application in nanocrystalline TiO2 sensitized solar cells. The donor-pi-bridge-acceptor (D-pi-A) structure dyes, Y2 and Y3 each designed with three parts, an electron donor unit (substituted phenyl), a linker unit (thiophene), and an anchor unit (cyanoacrylic acid) showed maximal monochromatic incident photon to current conversion efficiencies (IPCE) in a device reaching upto 67% and 82% respectively. The organic sensitizers with 3,4,5-trimethoxy phenyl (Y3) as donor moieties obtained better solar light to electrical energy conversion efficiencies of 3.30% where as the organic sensitizer with 2,4-difluoro phenyl as donor (Y2) showed comparatively lower efficiency of 1.02%. The efficiency obtained with the reference sensitizer N719 under similar fabrication and evaluation conditions was 5.84%.     

Fabrication and characterization of photoelectrode thin films with different morphologies of TiO2 nanoparticles for dye-sensitized solar cells

This study deals with the fabrication of three different morphologies of TiO2 nanoparticles to fabricate two-layer photoelectrode thin film for dye-sensitized solar cells (DSSC). The four different TiO2 morphologies are titania nanotubes (Tnt), TiO2 nanoparticles (H220), TiO2 nanoparticle (SP) and commercial DP-25 nanoparticles (P-25). To prepare the thin films of the photoelectrodes, the first layer is coated by H220 TiO2 nanoparticles, and the second is coated by 3 kinds of materials optimally proportionally mixed - P25, SP and Tnt. The photoelectric conversion efficiency of DSSCs with photoelectrodes fabricated using H220 reached 6.31%. Finally, the TiO2 nanaomaterials with four different morphologies were used to prepare a two layer photoelectrode with the structure of H220/P25-Tnt-SP which was combined with a Pt counter electrode to assemble DSSCs. These DSSCs had photoelectric conversion efficiencies of as high as 7.47%.     

Understanding polycarbazole-based polymer:CdSe hybrid solar cells

We report for the first time the fabrication and characterization of organic-inorganic bulk heterojunction (BHJ) hybrid solar cells made of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and pyridine-capped CdSe nanorods. By optimizing both CdSe loading and active layer film thickness, the power conversion efficiencies (PCEs) of PCDTBT:CdSe hybrid solar cells were able to reach 2%, with PCDTBT:CdSe devices displaying an open-circuit voltage (V(OC )) that is 35% higher than P3HT:CdSe devices due to the deeper HOMO level of PCDTBT polymer. The performance of PCDTBT:CdSe devices is limited by its morphology and also its lower LUMO energy offset compared to P3HT:CdSe devices. Hence, the performance of PCDTBT:CdSe solar cells could be further improved by modifying the morphology of the films and also by including an interlayer to generate a built-in voltage to encourage exciton dissociation. Our results suggest that PCDTBT could be a viable alternative to P3HT as an electron donor in hybrid BHJ solar cells for high photovoltage application.     

An unconventional route to high-efficiency dye-sensitized solar cells via embedding graphitic thin films into TiO2 nanoparticle photoanode

Graphitic thin films embedded with highly dispersed titanium dioxide (TiO(2)) nanoparticles were incorporated for the first time into the conventional dye-sensitized solar cells (DSSCs), resulting in a remarkably improved cell efficiency due to its superior electron conductivity. Massively ordered arrays of TiO(2) dots embedded in carbon matrix were fabricated via UV-stabilization of polystyrene-block-poly(4-vinylpyridine) films containing TiO(2) precursors followed by direct carbonization. For dye-sensitized TiO(2) based solar cells containing carbon/TiO(2) thin layers at both sides of pristine TiO(2) layer, an increase of 62.3% [corrected] in overall power conversion efficiency was achieved compared with neat TiO(2)-based DSSCs. Such a remarkably improved cell efficiency was ascribed to the superior electron conductivity and extended electron lifetime elucidated by cyclic voltammetry and impedance spectroscopy.     

Organic dyes containing fluorenylidene functionalized phenothiazine donors as sensitizers for dye sensitized solar cells

Two new organic sensitizers featuring fluorenylidene decoration on the phenothiazine donor have been synthesized and characterized as sensitizers for nanocrystalline TiO₂-based dye sensitized solar cells (DSSCs). The dyes possess cyanoacrylic acid as acceptor/anchoring group and a conjugation pathway composed of fluorene and thiophene. Introduction of the fluorenylidene moiety on phenothiazine enhances the optical density of the dyes while the extension of conjugation by thiophene insertion red-shifts the absorption peak originating from the intramolecular charge transfer from phenothiazine to cyanoacrylic acid. The ground and excited state oxidation potentials of the fluorenylidene-containing dyes are upwardly shifted when compared to the parent dyes indicating a π-delocalized donor segment. The electronic properties were supported by density functional theoretical computations. Among the DSSCs fabricated, the dye (5a) having fluorene and thiophene in the spacer resulted higher power conversion efficiency (3.31 %) than the corresponding bithiophene analog (5b, 2.83 %) attributable to the relatively high electron life time and enhanced resistance for recombination in the former.     

Enhanced performance of P3HT/TiO2 bilayer heterojunction photovoltaic device having gold nanoparticles in the donor layer

The photovoltaic effects of blending gold nanoparticles (AuNPs) into the donor layer of a poly(3-hexylthiophene) (P3HT)/TiO2 bilayer heterojunction device have been studied. P3HT was synthesized via the modified Gragnard metathesis method and AuNPs with sizes ranging from 12 to 15 nm were formed via a reduction of HAuCl4. The blending of AuNPs into P3HT caused a lower photoluminescence (PL) intensities and a decreased energy level of the highest occupied molecular orbital (HOMO) than the pristine P3HT owing to the good electron-accepting nature of AuNPs. Upon the use of P3HT-AuNPs as the donor layer, the decreased HOMO(donor) resulted in an increased open circuit voltage (V(OC)) and thus enabled the fabricated (P3HT-AuNPs)/TiO2 bilayer heterojunction photovoltaic device to have an improved power conversion efficiency of solar energy. V(OC) as well as the overall power conversion efficiency increased with an increase in the AuNP content as a result of additional interfaces which facilitated the charge separation of excitons and percolation pathways which enhanced the electron transfer to the TiO2 acceptor. Furthermore, unannealed P3HT-AuNPs exhibited nanoholes and provided photovoltaic devices a power conversion efficiency nearly two time higher than annealed P3HT-AuNPs.     

Effect of coadsorbent on the photovoltaic performance of squaraine sensitized nanocrystalline solar cells

The effect of chenodeoxycholic acid as the coadsorbent with a squaraine sensitizer on TiO(2) nanocrystalline solar cells was investigated, and it was found that the coadsorbent prevents the squaraine sensitizer from aggregating on the TiO(2) nanoparticles but reduces dye loading leading to an interdependent photovoltaic performance. Analysis of the absorption spectra, and incident monochromatic photon-to-current conversion efficiency data showed that the load of squaraine sensitizer as well as the appearance of H-aggregates is strongly dependent on the molar concentration of chenodeoxycholic acid coadsorbent. The open circuit voltage of the solar cells with chenodeoxycholic acid increases due to the enhanced electron lifetime in the TiO(2) nanoparticles coupled with the band edge shift of TiO(2) to negative potentials.     

Hierarchical structured TiO2 photoanodes for dye-sensitized solar cells

A novel approach has been developed to fabricate hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs). The appropriately aggregated TiO2 clusters in the photoanode layer could cause stronger light scattering and higher dye loading that increases the efficiency of photovoltaic device. For detailed light-harvesting study, different molecular weights of polyvinyl alcohol (PVA) were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. A series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices, were measured and investigated. An optimized device had energy conversion efficiency of 4.47% having a higher dye loading and good light harvesting, achieving a 23% increase of short-circuit current J(sc) in DSSCs.     

Novel phenothiazine-based organic dyes with a heteroleptic dual-electron-acceptor for dye-sensitized solar cells

We designed a novel organic dye with a heteroleptic dual-electron acceptor (cyanocrylic acid and rhodanine-acetic acid) on each side of a phenothiazine-based organic dye as a photosensitizer for dye-sensitized solar cells (DSSCs). Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were used to estimate the photovoltaic properties of the dyes, with the findings showing that the organic dyes when used in the heteroleptic dual-electron-acceptor type resulted in higher performance than their single electron-acceptor and homoleptic dual-electron-acceptor counterparts due to the higher molar extinction coefficients and the orientation of the adsorbed dye. It was attributed to relatively broad and intense absorption spectra in the visible region with the rhodanine-acetic acid moiety and abundant electronic coupling with TiO2 of the cyanoacrylic acid anchoring group.     

Diketopyrrolopyrrole-containing oligothiophene-fullerene triads and their use in organic solar cells

We report the characterization of a series of oligothiophene-diketopyrrolopyrrole-fullerene triads and their use as active materials for solution processed organic solar cells (OSCs). By incorporating the diketopyrrolopyrrole (DPP) core with electron rich oligothiophene units and electron withdrawing fullerene units, multifunctional electronic molecules have been prepared; these molecules show high solubility in common organic solvents, excellent photophysical properties with high extinction coefficients (1 × 10(4) to 1 × 10(5) M(-1) cm(-1)) and broad absorption spectra coverage (250-800 nm), as well as suitable molecular orbital energy levels (HOMO of approximately -5.1 eV, LUMO of approximately -3.7 eV). Solution-processed thin-film organic field effect transistors (OFETs) from these triads revealed good n-type characteristics with electron mobilities up to 1.5 × 10(-3) cm(2) V(-1) s(-1). With these multifunctional triads, single-component OSCs have been fabricated, exhibiting power conversion efficiencies (PCEs) of up to 0.5 % under AM 1.5 G simulated 1 sun solar illumination. Blending these molecules with poly(3-hexylthiophene) (P3HT) afforded bulk heterojunction OSCs with PCEs reaching as high as 2.41%.     

TiO2 nanotubes infiltrated with nanoparticles for dye sensitized solar cells

We present a detailed study of the infiltration of titanium dioxide (TiO(2)) nanotubes (NTs) with TiO(2) nanoparticles (NPs) for dye sensitized solar cells (DSSCs). The aim is to combine the merits of the NP's high dye loading and high light harvesting capability with the NT's straight carrier transport path and high electron collection efficiency to improve the DSSC performance. On infiltrating NTs with TiCl(4) solution followed by hydrothermal synthesis, 10 nm size NPs were observed to form a conformal and dense layer on the NT walls. Compared with the bare NT structure, dye loading of this mixed NT and NP structure is more than doubled. The overall photon conversion efficiencies of the fabricated DSSCs are improved by 152%, 107%, and 49% for 8, 13, and 20 μm long NTs, respectively. Electron transport and recombination parameters were extracted based on electrochemical impedance spectroscopy measurements. Although a slight reduction of electron lifetime was observed in the mixed structures due to enhanced recombination with a larger surface area, the diffusion length is still significantly longer than the NT length used, suggesting that most electrons are collected. In addition to dye loading and hence photocurrent increment, the photovoltage and filling factor were also improved in the mixed structure due to a low serial resistance, leading to the enhancement of the overall efficiency.     

High efficiency dye-sensitized solar cells based on hierarchically structured nanotubes

Dye-sensitized solar cells (DSSCs) based on hierarchically structured TiO(2) nanotubes prepared by a facile combination of two-step electrochemical anodization with a hydrothermal process exhibited remarkable performance. Vertically oriented, smooth TiO(2) nanotube arrays fabricated by a two-step anodic oxidation were subjected to hydrothermal treatment, thereby creating advantageous roughness on the TiO(2) nanotube surface (i.e., forming hierarchically structured nanotube arrays-nanoscopic tubes composed of a large number of nanoparticles on the surface) that led to an increased dye loading. Subsequently, these nanotubes were exploited to produce DSSCs in a backside illumination mode, yielding a significantly high power conversion efficiency, of 7.12%, which was further increased to 7.75% upon exposure to O(2) plasma.     

Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells

Bulk heterojunction solar cells have been extensively studied owing to their great potential for cost-effective photovoltaic devices. Although recent advances resulted in the fabrication of poly(3-hexylthiophene) (P3HT)/fullerene derivative based solar cells with efficiencies in the range 4.4-5.0%, theoretical calculations predict that the development of novel donor materials with a lower bandgap is required to exceed the power-conversion efficiency of 10%. However, all of the lower bandgap polymers developed so far have failed to reach the efficiency of P3HT-based cells. To address this issue, we synthesized a soluble, intensely coloured platinum metallopolyyne with a low bandgap of 1.85 eV. The solar cells, containing metallopolyyne/fullerene derivative blends as the photoactive material, showed a power-conversion efficiency with an average of 4.1%, without annealing or the use of spacer layers needed to achieve comparable efficiency with P3HT. This clearly demonstrates the potential of metallated conjugated polymers for efficient photovoltaic devices.     

Enhancement of photovoltaic performance in dye-sensitized solar cells with the spin-coated TiO2 blocking layer

The TiO2 thin film layers were introduced with the spin-coating method between FTO electrode and TiO2 photoanode in dye sensitized solar cell (DSSC) to prevent electron back migration from the FTO electrode to electrolyte. The DSSC containg different thickness of TiO2 thin film (10-30, 40-60 and 120-150 nm) were prepared and photovoltaic performances were analysed with /-Vcurves and electrochemical impedance spectroscopy. The maximum cell performance was observed in DSSC with 10-30 nm of TiO2 thin film thickness (11.92 mA/cm2, 0.74 V, 64%, and 5.62%) to compare with that of pristine DSSC (11.09 mA/cm2, 0.65 V, 62%, and 4.43%). The variation of photoelectric conversion efficiency of the DSSCs with different TiO2 thin film thickness was discussed with the analysis of crystallographic and microstructural properties of TiO2 thin films.     

13.9%‐Efficiency and Eco‐Friendly Nonfullerene Polymer Solar Cells Obtained by Balancing Molecular Weight and Solubility in Chlorinated Thiophene‐Based Polymer Backbones

To industrialize nonfullerene polymer solar cells (NFPSCs), the molecular design of the donor polymers must feature low‐cost materials and a high overall yield. Two chlorinated thiophene‐based polymers, P(F–Cl) and P(Cl–Cl), are synthesized by introducing halogen effects like fluorine (F) and chlorine (Cl) to the previously reported P(Cl), which exhibits low complexity. However, the molecular weights of these polymers are insufficient owing to their low solubility, which in turn is caused by introducing rigid halogen atoms during the polymerization. Thus, they show relatively low power conversion efficiencies (PCEs) of 11.8% and 10.3%, respectively. To overcome these shortcomings, two new terpolymers are designed and synthesized by introducing a small amount of 1,3‐bis(5‐bromothiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) unit into each backbone, namely, P(F–Cl)(BDD = 0.2) and P(Cl–Cl)(BDD = 0.2). As a result, both polymers remain inexpensive and show a better molecular weight–solubility balance, achieving high PCEs of 12.7% and 13.9%, respectively, in NFPSCs processed using eco‐friendly solvents.     

Influence of cell fabrication procedure on the performance of the dye sensitized solar cell

The recent technological advancements of the Dye Sensitized Solar Cells (DSSCs) fabrication technology is gaining momentum as a low cost and simple fabrication technology to convert solar energy into electric energy. A systematic study of the DSSC fabrication procedure and its influence on the cell efficiency are presented in this paper. Preparation of the titanium dioxide (TiO2) layer on the working electrode was the most significant process improvement made to enhance cell efficiency. The Coatema tool was used to develop an automated TiO2 coating process, which yielded layer thicknesses with minimum micro cracks and repeatable TiO2 weight loading in the range of 8-13 microm. Secondary process improvements implemented were: vacuum drying step for the TiO2 layer, dilution ratio of the sensitized dye and sealant thickness. These optimized cell fabrication steps enhanced cell efficiencies over 200% and reduced total process time. The work in progress demonstrated higher cell efficiency slightly greater than 9% by reducing the cell size using the optimized fabrication process described in this paper. We are confident that higher efficiency cells can be fabricated with this optimized fabrication process illustrated in this paper.     

Electrospray preparation of hierarchically-structured mesoporous TiO₂ spheres for use in highly efficient dye-sensitized solar cells

We report a simple method to prepare hierarchically structured TiO(2) spheres (HS-TiO(2)), using an electrostatic spray technique, that are utilized for photoelectrodes of highly efficient dye-sensitized solar cells (DSSCs). This method has an advantage to remove the synthesis steps in conventional sol-gel method to form nano-sized spheres of TiO(2) nanoclusters. The fine dispersion of commercially available nanocrystalline TiO(2) particles (P25, Degussa) in EtOH without surfactants and additives is electro-sprayed directly onto a fluorine-dopoed tin-oxide (FTO) substrate for DSSC photoelectrodes. The DSSCs of HS-TiO(2) photoelectrodes show high energy conversion efficiency over 10% under illumination of light at 100 mW cm(-2), AM1.5 global. It is concluded from frequency-dependent measurements that the faster electron diffusion coefficient and longer lifetime of HS-TiO(2) than those in nonstructured TiO(2) contribute to the enhanced efficiency in DSSCs.     

上转换发光材料La(OH)3∶Er3+/Yb3+的制备及在染料敏化太阳能电池中的应用

针对N719染料仅可吸收可见光这一局限,本研究旨在通过引入上转换发光材料并将其应用于染料敏化太阳能电池的光阳极来拓宽光谱吸收范围,提高光的捕获率,进而达到提高电池光电转换效率的目的。首先,采用水热合成法以不同pH值的先驱体溶液,成功制备了Yb~(3+)/Er~(3+)双掺杂La(OH)_3粉末,然后将适量合成的稀土发光粉掺入TiO_2纳米浆料中,采用刮涂法成膜制备光阳极,并将其组装成染料敏化太阳能电池。研究结果表明,稀土发光粉的加入拓宽了光谱吸收范围,在其掺杂量达到3%时,电池的短路电流密度Jsc提高到17.72mA·cm-2,最终获得了8.3%的光电转换效率。     

Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays

We report on the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells (DSSCs) incorporating oriented TiO2 nanotube (NT) arrays. The morphology of the NT arrays, which were prepared from electrochemically anodized Ti foils, were characterized by scanning and transmission electron microscopies. The arrays were found to consist of closely packed NTs, several micrometers in length, with typical wall thicknesses and intertube spacings of 8-10 nm and pore diameters of about 30 nm. The calcined material was fully crystalline with individual NTs consisting of about 30 nm sized crystallites. The transport and recombination properties of the NT and nanoparticle (NP) films used in DSSCs were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. While both morphologies display comparable transport times, recombination was much slower in the NT films, indicating that the NT-based DSSCs have significantly higher charge-collection efficiencies than their NP-based counterparts. Dye molecules were shown to cover both the interior and exterior walls of the NTs. Analysis of photocurrent measurements indicates that the light-harvesting efficiencies of NT-based DSSCs were higher than those found for DSSCs incorporating NPs owing to stronger internal light-scattering effects.     

Fabrication of dye-sensitized solar cell (DSSC) using different particle sizes of TiO2 deposited via nano-particle deposition system (NPDS)

TiO2 layers were fabricated using a nano-particle deposition system (NPDS) on transparent conductive oxide (TCO) glass for dye sensitized solar cells (DSSCs). Conventionally, TiO2 paste for working electrodes has been fabricated using paste type methods. The fabricated paste composed of a mixture of nano-sized TiO2 powders, binders and solutions is then painted on TCO glass. After drying, the TiO2 layer on TCO glass is sintered to make a path for electron transfer. TiO2 layers formed by this paste type method require numerous steps, which can be time consuming. In this study, TiO2 powders were sprayed directly on TCO glass using NPDS in order to simplify the fabrication steps. To improve porosity and produce scattering layers, commercial nanocrystalline TiO, powders with different sizes were alternately deposited. Moreover, powders with different sizes were mixed and deposited on the TCO glass. The results indicate that the DSSCs with a TiO2 layer composed of different particle sizes had better cell performance than the cells assembled with single-sized TiO2 particles. Therefore, this study shows that a dry TiO2 coating process is possible for DSSC fabrication to improve its cell efficiencies, and this method can easily be applied on flexible substrates since NPDS is a room-temperature deposition process.