Origin of enhancement in open-circuit voltage by adding ZnO to nanocrystalline SnO2 in dye-sensitized solar cells

SnO2 + ZnO working electrodes for dye-sensitized solar cells were made by mixing a nanocrystalline SnO2 colloidal dispersion with ZnO or Zn(CH3COO)2. Addition of ZnO or Zn(CH3COO)2 enhanced the open-circuit voltage (V(oc)) of the cells with respect to cells containing only SnO2. Dependence of the electron lifetime in the electrodes on short-circuit photocurrent density (J(sc)) gave evidence against the assumption that the suppression of back electron transfer to the electrolyte is the origin for the V(oc) enhancement by addition of Zn. V(oc) dependence on temperatures indicated a decrease in the combined capacitance of the mixed electrode. The slope of the V(oc) dependence versus the logarithm of J(sc) indicated that the contribution of unpinning of the band to the enhancement of V(oc) could be neglected. From the cyclic voltammograms of the electrodes, the combined capacitance of the mixed electrode was 1 order of magnitude smaller than that of SnO2. The decrease in the combined capacitance in the mixed electrode could be explained by the decrease in the chemical capacitance of SnO2, thus the shift of the conduction band position toward the vacuum level. X-ray photoelectron spectra of Sn 3d(5/2) peaks showed a shift toward lower binding energy with an increasing amount of added Zn. This was attributed to an increase in the surface potential toward the negative direction, which might have resulted from a dipole moment formed by Zn on the surface of SnO2.     

A 3D star-shaped non-fullerene acceptor for solution-processed organic solar cells with a high open-circuit voltage of 1.18 V

A novel 3D star-shaped acceptor based on triphenylamine as a core and diketopyrrolopyrrole as arms (S(TPA-DPP)) was synthesized. S(TPA-DPP) exhibited excellent thermal stability, strong absorption, and very high open-circuit voltage (1.18 V) in solution-processed organic solar cells based on P3HT:S(TPA-DPP).     

High open-circuit voltage solution-processed organic solar cells based on a star-shaped small molecule end-capped with a new rhodanine derivative

A new star-shaped small molecule named TCNR3TTPA,with a triphenylamine(TPA)unit as the central building block and2-(1,1-dicyanomethylene)-3-octyl rhodanine(CNR)as the end-capped group,has been designed and synthesized.TCNR3TTPA showed a deep highest occupied molecular orbital(HOMO)energy level( 5.60 e V)and broad absorption.The solution-processed bulk heterojunction(BHJ)solar cells based on TCNR3TTPA:PC61BM(1:1,w/w)exhibited a high open-circuit voltage(Voc)of 0.99 V,a short-circuit current density(Jsc)of 5.76 m A/cm2,and a power conversion efficiency(PCE)of 2.50%under the illumination of AM 1.5 G,100 m W/cm2.The high Voc is ascribed to the strong electron-with-drawing ability of the end-capped 2-(1,1-dicyanomethylene)-3-octyl rhodanine group.These results demonstrated that the Voc of small-molecule organic solar cells could be increased by introducing a strong electron-withdrawing end-capped block,and that this is an effective strategy to design high-performance small molecules for organic solar cells.     

High-performance Pt-NiO nanosheet-based counter electrodes for dye-sensitized solar cells

High-performance counter electrodes for dye-sensitized solar cells (DSSCs) are fabricated with platinum-nickel oxide (Pt-NiO) nanosheets as catalytic materials. Firstly, the Pt-Ni nanosheets are synthesized via galvanic replacement reaction between pre-synthesized Ni nanosheets and an aqueous H₂PtCl₆ solution. Secondly, after thermal treatment in air, the Pt-Ni alloys are turned to Pt-NiO nanosheets. The related data of cyclic voltammetry, electrochemical impedance spectroscopy, and Tafel polarization reveal that Pt-NiO counter electrodes show highly catalytic activity and low charge transfer resistance. The DSSC with Pt-NiO counter electrode exhibits power conversion efficiency (PCE) of 8.40 %, which is lower than that of the DSSC containing commercial available Pt counter electrode (9.15 %) under full sunlight illumination (100 mW cm^?2, AM1.5G). However, owing to the extremely high transparency of Pt-NiO counter electrode, when putting an Ag mirror behind the back side of the DSSC, the reflected light can bring great enhanced PCE (11.27 %).     

Non-precious transition metal-based counter electrodes for dye-sensitized solar cells

Journal of Solid State Electrochemistry - A non-platinum metal catalyst, TiNx-C, was synthesized through the high-pressure pyrolysis method, which was characterized using a transmission electron...     

ZnO hierarchical structures for efficient quasi-solid dye-sensitized solar cells

We report a direct precipitation method for mass production of ZnO microflowers (MFs) containing hierarchical structures. The ZnO MFs are constructed by interlaced single crystalline and porous nanosheets which are ideal photoanode material for dye-sensitized solar cells (DSCs) because the MFs can largely improve the energy harvesting performance and the efficiency of DSCs. Compared with other forms of nano-sized structures, the novel hierarchical structures show obvious advantages in DSC application because of their large surface area for dye-loading, good light scattering efficiency and excellent electrical transport property. The quasi-solid state DSCs fabricated with the MF hierarchical structures exhibited an efficiency of 4.12%, much higher than that of ZnO nanoparticle-based DSCs, indicating a great potential for the development of highly-efficient quasi-solid DSCs.     

Enhanced open-circuit voltage in methoxyl substituted benzodithiophene-based polymer solar cells

The open-circuit voltage (V _oc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high V _oc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)-T1) exhibited a HOMO value of–5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)-T1). As a result, POBDT(S)-T1-based solar cells show a high V _oc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)-T1-based devices show a relatively lower V _oc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.     

Economical Pt-free catalysts for counter electrodes of dye-sensitized solar cells

Three classes (carbides, nitrides and oxides) of nanoscaled early-transition-metal catalysts have been proposed to replace the expensive Pt catalyst as counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Of these catalysts, Cr(3)C(2), CrN, VC(N), VN, TiC, TiC(N), TiN, and V(2)O(3) all showed excellent catalytic activity for the reduction of I(3)(-) to I(-) in the electrolyte. Further, VC embedded in mesoporous carbon (VC-MC) was prepared through in situ synthesis. The I(3)(-)/I(-) DSC based on the VC-MC CE reached a high power conversion efficiency (PCE) of 7.63%, comparable to the photovoltaic performance of the DSC using a Pt CE (7.50%). In addition, the carbide catalysts demonstrated catalytic activity higher than that of Pt for the regeneration of a new organic redox couple of T(2)/T(-). The T(2)/T(-) DSCs using TiC and VC-MC CEs showed PCEs of 4.96 and 5.15%, much higher than that of the DSC using a Pt CE (3.66%). This work expands the list of potential CE catalysts, which can help reduce the cost of DSCs and thereby encourage their fundamental research and commercial application.     

Dye molecular structure device open-circuit voltage correlation in Ru(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells

Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.     

Electrophoretic deposition of ZnO photoanode for plastic dye-sensitized solar cells

Plastic dye-sensitized solar cells have been fabricated based on an organic dye (D 149) and ZnO photoanode prepared via room temperature electrophoretic deposition (EPD) to yield a conversion efficiency of 4.17% under 100 mW cm^?2 AM 1.5 illumination. Intensity modulated photocurrent spectroscopy analyses reveal that the fabricated ZnO electrodes have adequate interparticle connection, even in the absence of any post-treatment. This study demonstrates that EPD is a convenient method for photoanode fabrication and ZnO photoelectrodes obtained via EPD are promising for efficient plastic solar cells.     

11.2% Efficiency all-polymer solar cells with high open-circuit voltage

Herein,we fabricated all-polymer solar cells(all-PSCs)based on a fluorinated wide-bandgap p-type conjugated polymer PM6 as the donor,and a narrow bandgap n-type conjugated polymer PZ1 as the acceptor.In addition to the complementary absorption and matching energy levels,the optimized blend films possess high cystallinity,predominantly face-on stacking,and a suitable phase separated morphology.With this active layer,the devices exhibited a high V_(oc)of 0.96 V,a superior J_(sc)of 17.1 mA cm~(-2),a fine fill factor(FF)of 68.2%,and thus an excellent power conversion efficiency(PCE)of 11.2%,which is the highest value reported to date for single-junction all-PSCs.Furthermore,the devices showed good storage stability.After 80 d of storage in the N_2-filled glovebox,the PCE still remained over 90%of the original value.Large-area devices(1.1 cm~2)also demonstrated an outstanding performance with a PCE of 9.2%,among the highest values for the reported large-area all-PSCs.These results indicate that the PM6:PZ1 blend is a promising candidate for scale-up production of large area high-performance all-PSCs.     

Nanorods and nanolipsticks structured ZnO photoelectrode for dye-sensitized solar cells

Nanostructured ZnO photoelectrodes were synthesized on SnO₂:F (FTO) glass substrate coated with sol–gel based ZnO seed layer via hydrothermal route at various deposition times: 30, 60, 90 and 180 min. Ruthenium based dye and carbon counter electrode were used for the fabrication of dye-sensitized solar cells (DSCs). Interestingly, nanolipsticks structures were found with low deposition times, where two dissimilar nanorods (111 and 165 nm) attached together. The number of nanolipsticks structures decreased and nanorods increased with increase of deposition time. The DSCs based on ZnO nanorods for 180 min, shows the maximum efficiency, 1.04% at 100 mW/cm² light intensity.     

Hierarchical twin-scale inverse opal TiO2 electrodes for dye-sensitized solar cells

We describe the preparation of three-dimensional hierarchical twin-scale inverse opal (ts-IO) electrodes for dye-sensitized solar cells (DSSCs). The ts-IO TiO(2) structure was obtained from a template fabricated via the assembly of mesoscale colloidal particles (40-80 nm in diameter) in the confined geometry of a macroporous IO structure. The photovoltaic properties of ts-IO electrodes were optimized by varying the layer thickness or the size of mesopores in the mesoscale colloidal assembly. Electron transport was investigated using impedance spectroscopy. The result showed that due to the competing effects of recombination and dye adsorption, the maximum efficiency was observed at an electrode thickness of 12 μm. The electrodes of smaller mesopores diameters yielded the higher photocurrent density due to the decrease in the electron transport resistance at the TiO(2)/dye interface. A maximum efficiency of 6.90% was obtained using an electrode 12 μm thick and a mesopore diameter of 35 nm.     

Efficient ternary blend bulk heterojunction solar cells with tunable open-circuit voltage

To explore the potential of ternary blend bulk heterojunction (BHJ) photovoltaics as a general platform for increasing the attainable performance of organic solar cells, a model system based on poly(3-hexylthiophene) (P3HT) as the donor and two soluble fullerene acceptors, phenyl-C(61)-butyric acid methyl ester (PC(61)BM) and indene-C(60) bisadduct (ICBA), was examined. In all of the solar cells, the overall ratio of polymer to fullerene was maintained at 1:1, while the composition of the fullerene component (PC(61)BM:ICBA ratio) was varied. Photovoltaic devices showed high short-circuit current densities (J(sc)) and fill factors (FF) (>0.57) at all fullerene ratios, while the open-circuit voltage (V(oc)) was found to vary from 0.61 to 0.84 V as the fraction of ICBA was increased. These results indicate that the V(oc) in ternary blend BHJ solar cells is not limited to the smallest V(oc) of the corresponding binary blend solar cells but can be varied between the extreme V(oc) values without significant effect on the J(sc) or FF. By extension, this result suggests that ternary blends provide a potentially effective route toward maximizing the attainable J(sc)V(oc) product (which is directly proportional to the solar cell efficiency) in BHJ solar cells and that with judicious selection of donor and acceptor components, solar cells with efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active-layer processing step.     

Effect of solvent and additives on the open-circuit voltage of ZnO-based dye-sensitized solar cells: a combined theoretical and experimental study

We have investigated the role of electrolyte composition, in terms of solvent and additive, on the open-circuit voltage (V(oc)) of ZnO-based dye-sensitized solar cells (DSSCs) using a combined experimental and theoretical approach. Calculations based on density functional theory (DFT) have been performed in order to describe the geometries and adsorption energies of various adsorbed solvents (nitromethane, acetonitrile and dimethylformamide) and p-tert-butylpyridine (TBP) (modeled by methylpyridine) on the ZnO (100) surface using a periodic approach. The densities of states (DOS) have been calculated and the energy position of the conduction band edge (CBE) has been evaluated for the different molecules adsorbed. The effect of the electrolyte composition on the standard redox potential of the iodide/triiodide redox couple has been experimentally determined. These two data values (CBE and standard redox potential) allowed us to determine the dependence of V(oc) on the electrolyte composition. The variations determined using this method were in good agreement with the measured V(oc) for cells made of electrodeposited ZnO films sensitized using D149 (indoline) dye. As in the case of TiO(2)-based cells, a correlation of V(oc) with the donor number of the adsorbed species was found. The present study clearly points out that both the CBE energy and the redox potential variation are important for explaining the experimentally observed changes in the V(oc) of DSSCs.     

ZnO hierarchial structure prepared via direct precipitation for dye-sensitized solar cells

Research on Chemical Intermediates - A ZnO hierarchical structure was prepared via simple aqueous method at room temperature using zinc nitrate solution as precursor. Time-dependent trails...