Influence of alkylpyridine additives in electrolyte solution on the performance of dye-sensitized solar cell

The influence of alkylpyridines additive to an I⁻/I₃⁻ redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (J_sc), but most of the alkylpyridines increased the open-circuit photovoltage (V_oc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the J_sc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the V_oc of the cell. Under AM 1.5 (100 mW/cm²), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine.     

Influence of pyrazole derivatives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized TiO2 solar cell performance

The influence of pyrazole additives in an I⁻/I₃⁻ redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (J_sc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the V_oc, but the smaller the J_sc values. As the dipole moment of the pyrazole derivatives increased, the V_oc value increased, but the J_sc value decreased. The V_oc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO₂ photoelectrode, the I⁻/I₃⁻ electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance.     

Influence of alkylaminopyridine additives in electrolytes on dye-sensitized solar cell performance

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm²). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (J_sc), but increased the open-circuit photovoltage (V_oc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the J_sc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the V_oc of the cell. The highest V_oc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum V_oc of 0.9 V for a cell consisting of TiO₂ electrode and I⁻/I₃⁻ redox system.     

Influence of aminothiazole additives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized nanocrystalline TiO2 solar cell performance

The influence of aminothiazole additives in acetonitrile solution of an I⁻/I₃⁻ redox electrolyte on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′- bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics were investigated under AM 1.5 (100 mW/cm²) for nine different aminothiazole compounds. The aminothiazole additives tested had varying influences on the solar cell performance. Most of the additives enhanced the open-circuit photovoltage (V_oc), but reduced the short circuit photocurrent density (J_sc) of the solar cell. Both the physical and chemical properties of the aminothiazoles were computationally calculated in order to determine the reasons that the additive influenced solar cell performance. The larger the calculated partial charge of the nitrogen atom in the thiazole, the higher the V_oc value. The V_oc value increased as the dipole moment of aminothiazoles in acetonitrile increased. Moreover, the V_oc of the solar cell also increased as the size of the aminothiazole molecules decreased. These results suggest that the electron donicity of the aminothiazole additives influenced the interaction with the TiO₂ photoelectrode, which altered the dye-sensitized solar cell performance.     

Effects of pH of a hybrid gel incorporated with 3-aminopropyltrimethoxysilane on the performance of a quasi-solid state dye-sensitized solar cell

Network hybrid gel prepared with tetraethyl orthosilicate, 3-aminopropyltrimethoxysilane (APS) and poly(ethylene glycol) was used as an electrolyte matrix in a quasi-solid state dye-sensitized solar cell (DSSC). Change in pH of this hybrid gel by varying the composition of APS was found to have remarkable effects on the photoelectrochemical performance of the cell. The hybrid gel matrix, having silane polymer backbones with free amine functionality, was characterized by FT-IR spectra and FE-SEM images, and the assembled DSSC by photocurrent-voltage and incident photon to current conversion efficiency curves. The unsealed, quasi-solid state DSSC with the hybrid gel has shown an increase in the open-circuit voltage (V_oc) and a steady decrease in the short-circuit photocurrent (J_sc), with increase in the content of APS. A maximum conversion efficiency of 4.5% was obtained for a DSSC by using 20% of APS in its hybrid gel at a light intensity of 100 mW cm⁻². Upon replacing the amino group of APS by bulkier aniline and benzophenoaniline groups, conversion efficiencies of the corresponding DSSCs were reduced. Variations in the V_oc and J_sc are explained in terms of shift of the flat band potential of TiO₂ and a complex formation between I₃⁻ and −NH₂ of APS of the electrolyte.     

Influence of electrolytes on the photovoltaic performance of organic dye-sensitized nanocrystalline TiO2 solar cells

We have studied the influence of electrolytes on the photovoltaic performance of mercurochrome-sensitized nanocrystalline TiO₂ solar cells using LiI, LiBr, and tetraalkylammonium iodides as the electrolyte. Short-circuit photocurrent density (J_sc) and open-circuit photovoltage (V_oc) depended strongly on the electrolyte. J_sc of 3.42 mA cm⁻² and V_oc of 0.52 V were obtained for the LiI electrolyte and J_sc of 2.10 mA cm⁻² and V_oc of 0.86 V were obtained for the Pr₄NI electrolyte. This difference in photovoltaic performance was due to the change in the conduction band level of the TiO₂ electrode. Large V_oc of 0.99 V was obtained for the LiBr electrolyte due to the large energy gap between the conduction band level of TiO₂ and the Br/Br₂ redox potential. Solar cell performance also depended strongly on organic solvent, suggesting that the physical properties of solvents such as Li ion conductivity and donor number affect photovoltaic performance.     

Manufacturing method for transparent electric windows using dye-sensitized TiO2 solar cells

The transparent electric windows based on dye-sensitized nanocrystalline TiO₂ solar cells have been prepared. The solar cell consists of dye-sensitized TiO₂ electrode with a TiO₂ layer of an about 8 μm thickness and of a 80×80 mm² active area, Pt counter electrode and redox electrolyte. The solar cell shows a transmittance of approximately 60% in the visible range and an open-circuit voltage (V_oc) of 0.64 V and a short-circuit photocurrent (J_sc) of 250 mA. A moderately transparent electric window composed of nine unit solar cells in series generates V_oc of 5.7 V and J_sc of 220 mA at one sun light intensity.     

Electrical and photovoltaic response of bulk hetero-junction device made from poly (3-phenyl azo methine thiophene) (PPAT) and 1, 1′-diallyl substituted 4, 4′-dipyridine (DADP)

Electrical and photovoltaic properties of donor–acceptor composite system comprised of poly (3-phenyl azo methine thiophene) (PPAT) and 1, 1′–diallyl substituted 4, 4′-dipyridine (DADP) were investigated. A significant enhancement of photocurrent was observed when PPAT was blended with DADP. The increase in photocurrent has been explained in terms of efficient charge separation that resulted from the transfer of photo-excited electrons from PPAT to DADP. The strong quenching of fluorescence of PPAT was caused by the presence of DADP that indicates the photo-induced charge transfer from PPAT to DADP. The open circuit voltage (V_oc) generated in the device is independent of the variation of work function of negative metal electrode that has been explained in terms of Fermi level pinning between DADP and metal via surface charges. The electrical characteristics of ITO/PPAT: DADP/Al photovoltaic device were determined by analyzing the dependence of short circuit photocurrent density (J_sc) and V_oc under illumination at different temperatures. The V_oc decreases almost linearly with increasing temperature, while short-circuit photocurrent increases logarithmically with temperature and saturates at higher temperature above 330 K. This dependence of J_sc and V_oc on temperature has been discussed in terms of possible mechanism that involves the photovoltage generation and charge carrier transport in the device under thermally activated state. The photovoltaic device made from PPAT: DADP blend has shown three times higher photosensitivity than that of made from pure PPAT.     

Influence of alkyl dihalide gelators on solidification of dye-sensitized solar cells

Quasi-dye-sensitized solar cells were prepared by using ionic liquid-type electrolytes and gelators consisting of polyvinylpyridine and alkyl dihalides. Gelation occurred by the reaction of polyvinylpyridine and alkyl dihalides. When the chain length of the dihalides was varied, the short-circuit current (J_sc) increased with an increase in the chain length. However, the open-circuit voltage (V_oc) and fill factor (ff) slightly decreased. The increase in J_sc was brought about by the decrease in the interfacial resistances between the gel electrolyte and the counter electrode. In addition, the increase in the J_sc was explained by increases in the apparent diffusion coefficient of I⁻/I₃⁻ when the chain length increased. Decreases in V_oc and ff were explained by back-electron transfers from TiO₂ to iodine in the electrolytes. V_oc of the cells solidified by alkyldiiodide was lower than that solidified by alkyldichloride or alkyldibromide. It was explained by negatively shifted redox potential of I⁻/I₃⁻, compared with those for Cl⁻/Cl₂ or Br⁻/Br₂.     

Photosensitization of nanocrystalline TiO2 films by a polymer with two carboxylic groups, poly (3-thiophenemalonic acid)

Photovoltaic devices were assembled using a conducting polymer; poly (3-thiophenemalonic acid) sensitized TiO₂ electrodes and an electrolyte containing I₃⁻/I⁻ redox couple. This cell exhibited a short-circuit photocurrent (J_sc) of 6.65 mA cm⁻², an open circuit voltage (V_oc) of 355 mV and an efficiency of 1.5% under the illumination of 100 mW cm⁻² (AM 1.5). Addition of an ionic liquid, 1-methyl 3-n-hexylimidazolium iodide, into the electrolyte led to an improvement in the cell performances, achieving an overall efficiency of 1.8% under the same illumination. The average cell characteristics of the later devices are , with a fill factor of 0.65.     

Anchorage of N3 dye-linked polyacrylic acid to TiO2/electrolyte interface for improvement in the performance of a dye-sensitized solar cell

A synthetic route was developed to link N3 dye to polyacrylic acid (PAA) using ethylenediamine (en) as the linker. The resulting complex, PAA–en–N3, was then coated onto a TiO₂ film. The modified TiO₂ film electrode (hereafter PAA–en–N3/TiO₂), when used as the photoanode in a dye-sensitized solar cell (DSSC), exhibited enhanced solar energy conversion efficiency compared with that of the usual DSSC with the N3/TiO₂ film electrode. The increase in efficiency was attributed to the increased open-circuit voltage (V_oc) and short-circuit photocurrent (J_sc). The increase in V_oc was attributed to the formation of a hydrophobic PAA–en–N3 layer on the TiO₂/electrolyte interface, while the increase in J_sc was attributed to the additional dye acquired by the TiO₂ film from the PAA–en–N3 complex.     

On the use of triethylamine hydroiodide as a supporting electrolyte in dye-sensitized solar cells

For the first time, the application of a molten salt, triethylamine hydroiodide (THI), as a supporting electrolyte was investigated for the dye-sensitized solar cells (DSSCs). Titanium dioxide (TiO₂) electrode was modified by incorporation of high- and low-molecular weight poly(ethylene glycol) along with TiO₂ nanoparticles of two different sizes (300 nm (30 wt%) and 20 nm (70 wt%)). The highest apparent diffusion coefficient (D) of 8.12×10⁻⁶ cm² s⁻¹ was obtained for I⁻ (0.5 M of THI) from linear sweep voltammetry (LSV). Short-circuit current density (J_sc) increases with the concentration of THI whereas open-circuit potential (V_oc) remains the same. Optimum J_sc (19.28 mA cm⁻²) and V_oc (0.7 V) with a highest conversion efficiency (η) of 8.45% were obtained for the DSSC containing 0.5 M of THI/0.05 M I₂/0.5 M TBP in CH₃CN. It is also observed that the J_sc and η of the DSSC mainly relates with the D values of I⁻ and charge-transfer resistances such as R_ct1 and R_ct2 operating along Pt/TiO₂ electrolyte interface, obtained from LSV and electrochemical impedance spectroscopy (EIS). For comparison, tetraethylammonium iodide (TEAI) and LiI were also selected as supporting electrolytes. Though both the THI and TEAI have similar structures, replacement of one methyl group by hydrogen improves the efficiency of the DSSC containing the former electrolyte. Further, the DSSC containing THI exhibits higher J_sc and η than LiI (7.70%), from which it is concluded that THI may be used as an efficient and alternative candidate to replace LiI in the current research of DSSCs.     

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO₂, Nb₂O₅, ZnO, SnO₂, and In₂O₃ with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I⁻/I₃⁻ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm⁻²) reached 2.5% with a short-circuit photocurrent density (J_sc) of 7.44 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.52 V, and a fill factor (ff) of 0.64. The J_sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V_oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.     

Enhancement in the performance of dye-sensitized solar cells containing ZnO-covered TiO2 electrodes prepared by thermal chemical vapor deposition

A ZnO-covered TiO₂ (denoted as ZnO/TiO₂) film was prepared by incorporating a small quantity of particulate ZnO in a TiO₂ matrix by thermal chemical vapor deposition. When used in a dye-sensitized solar cell, an enhancement was observed in both short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc) by 12% and 17%, respectively, relative to those of a cell containing a bare TiO₂ film. The observed J_sc enhancement is attributed to the increase in the surface area of the ZnO/TiO₂ film, and the V_oc enhancement to the formation of a potential barrier by ZnO at TiO₂/electrolyte interface. The films were characterized by FE-SEM, EDX, and XRD.     

Density functional study of alkylpyridine–iodine interaction and its implications in the open-circuit photovoltage of dye-sensitized solar cell

A density functional theory (DFT) method was used to study the monomer and intermolecular charge-transfer complexes of 22 different alkylpyridines with diiodine. DFT calculations revealed that the σ* orbital of iodine interacts with the nitrogen lone pair in pyridines. The open-circuit photovoltage (V_oc) values of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized nanocrystalline TiO₂ solar cell with an I⁻/I₃⁻ redox electrolyte in acetonitrile using alkylpyridines additive were compared to computational calculations on the interaction between pyridines and I₂ by a DFT method. The optimized geometries, frequency analyses, Mulliken population analyses, and interaction energies suggest that the V_oc value of the solar cell is higher, the more alkylpyridine complexes with I₂.     

Radiation effects on fabricated Cu2S/CdS heterojunction photovoltaic cells

The photovoltaic properties including I–V characteristics, junction capacitance (C–V), short-circuit current (I_sc), open-circuit voltage (V_oc), fill factor (ff), efficiency (η), and spectral response of Cu₂S/CdS heterojunction cells have been examined before and after exposure to nuclear radiation. This included γ-rays of Co-60, and electron beams (at 1.5 MeV energy).The short-circuit current (I_sc) decreased, while the open-circuit voltage (V_oc), the fill factor (ff) and the efficiency (η) increased after heat treatment (at 260°C in air for 20 min). The I_sc effect during exposure to γ-rays was studied. It was found that I_sc increases as the dose rate increases. The sensitivity dependence of the I_sc density on dose rate was observed to be linear, and hence a universal constant for its sensitivity is found to be 45 (nA/cm²) (rad/s).No permanent damage was shown until about 300 Mrad for γ rays and 380 Mrad for electron beams. After these doses, the I_sc and V_oc slightly decreased on increasing the absorbed dose.After heat treatment, the spectral response was modulated. It was found that the wavelength response against the photocurrent decreased from 1000 to 800 nm and the photocurrent also slightly decreased in the range of wavelengths from 800 to 450 nm and increased from 350 to 540 nm. Heat treatment before irradiation improved the photovoltaic cells. After irradiation by γ-rays and electron beams, the photocurrent went back to its original value by annealing (for 2 h at 500°C). The capacitance–voltage behavior decreased after irradiation and hence the doping decreased.     

Blue sensitizers for solar cells: Natural dyes from Calafate and Jaboticaba

Blue-violet anthocyanins from Jaboticaba (Myrtus cauliflora Mart) and Calafate (Berberies buxifolia Lam) were employed as TiO₂ dye-sensitizers. Solar cells sensitized by Jaboticaba extracts achieved up to J_sc=9.0 mA cm⁻², V_oc=0.59 V, P_max=1.9 mW cm⁻² and ff=0.54, while for Calafate sensitized cells the values determined were up to J_sc=6.2 mA cm⁻², V_oc=0.47 V, P_max=1.1 mW cm⁻² and ff=0.36. Other natural dyes were evaluated without significant photocurrent, demonstrating that only selected extracts are capable of converting sunlight in electricity. The results obtained with extracts of Jaboticaba and Calafate show a successful conversion of visible light into electricity by using natural dyes as wide band-gap semiconductor sensitizers in dye-sensitized solar cells. It also represents an environmentally friendly alternative for dye-sensitized solar cells with low cost production and an excellent system for educational purposes.     

DFT investigation of the TiO2 band shift by nitrogen-containing heterocycle adsorption and implications on dye-sensitized solar cell performance

A density functional theory (DFT) method (periodic DMol³) with full geometry optimization was used to investigate the adsorption of nitrogen-containing heterocycles such as 4-t-butylpyridine (TBP) and imidazole on a TiO₂ anatase (1 0 1) surface. Negative shifts of the TiO₂ Fermi level by N-containing heterocycle adsorption were observed. Imidazole adsorption shifted the Fermi level of TiO₂ more negatively than TBP. This shift corresponded to the enhancement of the open-circuit photovoltage (V_oc) and the reduction of the short-circuit photocurrent density (J_sc) in a dye-sensitized TiO₂ solar cell. We are the first to theoretically discover a TiO₂ band shift upon N-containing heterocycles adsorption, and have successfully related this shift to the effect as an additive in an electrolyte solution on dye-sensitized solar cell performance.     

Photovoltaic effect in single-layer organic solar cell devices fabricated with two new imidazolin-5-one molecules

Organic solar cells were fabricated with two new imidazolin-5-one molecules as active layers. The use of imidazolin-5-ones, derivatives of a biomolecule chromophore, for photovoltaic applications is particularly attractive due to its biodegradable nature and tunable properties. Single-layer devices with two analogues of imidazolin-5-ones were prepared and characterized. Devices fabricated with one of the molecules as the active layer showed a maximum J_sc of 0.52 μA cm⁻² and V_oc of 0.68 V at an incident power of 20.32 mW cm⁻², while the other set of devices showed a maximum J_sc of 0.63 μA cm⁻² and V_oc of 0.57 V at the same incident power.     

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (J_sc) of 0.93 μ Acm⁻², open-circuit photovoltage (V_oc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm⁻² intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a J_sc value of 2.36 μ Acm⁻², V_oc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm⁻² intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H₂pyp₃p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)₂).