Effects of anchoring groups in multi-anchoring organic dyes with thiophene bridge for dye-sensitized solar cells

Organic photo-sensitizers were designed and synthesized based on a phenothiazine framework containing single- and double-electron acceptors that were bridged with thiophene for the dye sensitized solar cells (DSSCs). The optimized geometries were determined with density functional theory (DFT) calculations to estimate the photovoltaic properties of the dyes in the design stage. The organic dye with the double electron acceptors exhibited a better light absorption at long wavelength and an effective electron extraction pathway from the electron donor to the TiO₂ surface, leading to an improved short-circuit current (11.6 mA cm^?2), compared with that of the dye with the single electron acceptor (10.2 mA cm^?2) and the conventional N3 Ru-dye (10.4 mA cm^?2). Contrarily, the open-circuit voltage of the organic dye with the double electron acceptors decreased because the additional protonated carboxylic groups caused a positive shift in the Fermi level of TiO₂.     

Phenothiazine conjugated bipyridine as ancillary ligand in Ru(II)-complexes for application in dye sensitized solar cell

A new high molar extinction coefficient ruthenium(II)-bipyridine complex “cis-Ru(4,4′-bis((E)-2-(10-decyl-10H-phenothiazin-3-yl)vinyl)-2,2′-bipyridine)(4,4′-dicarboxylic acid-2,2′-bipyridine)(NCS)₂ PTZ1″ was synthesized through conjugation of phenothiazine unit with bipyridine and characterized by FT-IR, ¹H-NMR and ESI-MASS spectroscopes. Absorption measurements and time dependent-density functional theory (TD-DFT) calculations show increased spectral response for the ancillary ligand and the corresponding complex. The dye upon anchoring onto mesoporous nanocrystalline TiO₂ solar cells exhibited solar-to-electric energy conversion efficiency (η) of 3.77% short-circuit photocurrent density (J_SC) = 7.79 mA/cm², open-circuit voltage (V_OC) = 640 mV, fill factor = 0.750) under air mass 1.5 sunlight, the reference Z907 and HRS1sensitized solar cells, fabricated and evaluated under identical conditions exhibited η-value of 7.02% (J_SC = 15.25 mA/cm², V_OC = 650 mV, fill factor = 0.705) and 3.05% (J_SC = 8.20 mA/cm², V_OC = 610 mV, fill factor = 0.620) respectively. The lower film absorption of PTZ1on TiO₂ surface could be probably due to larger molecular diameter and planarity of phenothiazine prone to aggregate in solution as well as on TiO₂ surface. The DFT calculations show that the first three HOMOs of PTZ1 have t2g character as observed in case of Z907, while HOMO-4 and HOMO-5 have π-orbitals with major component on phenothiazine moieties of L1.     

New fluoranthene-based cyanine dye for dye-sensitized solar cells

In this paper, a new fluoranthene-based unsymmetrical organic cyanine dye I and the corresponding cyanine dye II containing ethynyl unit for the purpose of comparison were designed and synthesized as sensitizers for the application in dye-sensitized solar cells (DSSCs). The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. The DSSCs based on the fluoranthene dye I showed the better photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 67%, a short-circuit photocurrent density (J_sc) of 7.83 mA cm^?2, an open-circuit photovoltage (V_oc) of 0.476 V, and a fill factor (ff) of 0.63, corresponding to an overall conversion efficiency of 2.34% under simulated AM 1.5G solar light condition. Also, the effects of chenodeoxycholic acid (CDCA) in a solution as a co-adsorbate on the photovoltaic performance of DSSCs based on cyanine dyes were also investigated. The presence of CDCA for 0.5 h, increases both the photovoltage and photocurrent of the DSSC incorporating I, in which the photovoltage and photocurrent increase 9.3% and 20%, respectively. The above photovoltaic results indicate that coadsorption of appropriate amount CDCA is effective to improve solar cell performance.     

Synthesis and characterization of aniline copolymers containing carboxylic groups and their application as sensitizer and hole conductor in solar cells

Copolymers of m-aminobenzoic acid and o-anisidine doped with p-toluenesulphonic acid in different proportions were successfully synthesized by oxidative polymerization. The copolymers were characterized by FT-IR, UV–vis, ¹H NMR and EPR spectroscopies, cyclic voltammetry, conductivity and SEM. The copolymer with equivalent amounts of the monomers o-anisidine and m-aminobenzoic acid presented the highest conductivity, The EPR analyses and SEM images show that this copolymer provides more homogeneous films with particle size distribution of approximately 1–2 μm. The copolymer with a high fraction of o-anisidine gives rises to films with larger particle sizes and a more defined electrochemical process. The m-aminobenzoic acid monomer was intentionally chosen in order to promote a better electronic coupling between the conducting copolymer and the TiO₂ surface. The copolymers were tested as both sensitizers and hole conducting materials in dye-sensitized solar cells. The device assembled using the copolymer with the highest proportion of m-aminobenzoic acid units as sensitizer produced the highest photocurrent (I_sc = 0.254 mA cm^?2) and photovoltage (V_oc = 0.252 V) at 100 mW cm^?2. The energy diagram shows that although the electronic injection from the conducting polymer excited state is an allowed process the regeneration of the positive charges created after the electron transfer process is forbidden, thus explaining the low efficiency of solar energy conversion. When this copolymer was applied as a hole conducting material, an improvement in the V_oc to 0.4 V, was observed, indicating that this material is more suitable for charge transport when applied in this type of solar cells.     

Polypyridyl Ru(II)-sensitizers with extended π-system enhances the performance of dye sensitized solar cells

New polypyridyl ruthenium(II) complexes “cis-Ru(4,4′-dimesityl-2,2′-bipyridine) (Ln) (NCS)₂ H102” and “cis-Ru(4,4′-bis(2,3,6-tri-isopropylphenyl)-2,2′-bipyridine) (Ln) (NCS)₂ H105”, where Ln = 4,4′-dicarboxylic acid-2,2′-bipyridine; were synthesized and successfully applied to sensitization of nano-crystalline TiO₂ based solar cells (DSSCs). The DSSCs of H102 and H105 fabricated from 0.16 cm² TiO₂ electrodes exhibited broader comparable photocurrent action spectra with almost identical solar-to-electrical energy conversion efficiency (η) as compared to N719 sensitizer. The incident photon-to-current conversion efficiency (IPCE) values of 98% and 95% were obtained for H102 and H105 sensitizers respectively. Under 1 sun condition, η-values of 8.39% (short-circuit photocurrent (J_SC) = 16.4 mA/cm², open-circuit photo voltage (V_OC) = 692 mV, fill factor = 0.734), 8.76% (J_SC = 16.3 mA/cm², V_OC = 735 mV, fill factor = 0.734) and 9.12% (J_SC = 16.1 mA/cm², V_OC = 745 mV, fill factor = 0.753) were obtained for H102, H105 and N719 sensitizers respectively.     

Molecular design of donor–acceptor-type cruciform dyes for efficient dyes-sensitized solar cells

Novel donor–acceptor (D–A) type cruciform dyes were synthesized for use in a dye-sensitized solar cell. The geometries and electronic structures of the dyes were designed by the results of density functional theory (DFT) calculations. Two phenothiazine units in these dyes act as electron donors, while two cyanoacrylic acid groups act as electron acceptors. The solubility of these compounds is increased because of the presence of the hexyl groups in the phenothiazine rings. The photovoltaic properties of 14D-25A and 12A-45D were measured to identify the effects of the dye having two electron donors and two electron acceptors on the DSSC performance. The 12A-45D dye showed a power conversion efficiency of 4.5% under AM 1.5 illumination (100 mW cm^?2) in an photoactive area of 0.42 cm², short circuit current density of 10.8 mA cm^?2, open circuit photovoltage of 0.65 V and a fill factor of 64%.     

Effect of UV–ozone treatment on ITO and post-annealing on the performance of organic solar cells

In this paper, the influence of the ultraviolet (UV)–ozone treatment of indium tin oxide (ITO) surface and the active layer post-annealing treatment on the performance of organic solar cells were investigated. Bulk heterojunction organic solar cells based on the blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) were fabricated. It is found that the devices with the UV–ozone treatment for 5 min on ITO substrates show the better performance, compared with the devices without this treatment. The results demonstrate that the short-circuit current density (J_sc) and fill factor (FF) could be improved by the post-annealing treatment. The devices with both treatments together show the best performance, with the increase of J_sc from 2.68 mA/cm² to 4.13 mA/cm² and the enhancement of FF from 32.2% to 38.8%. Therefore, the power conversion efficiency is improved from 0.62% to 1.08%. The morphology of the active layers with and without the post-annealing treatment was characterized by atomic force microscopy.     

Photovoltaic devices based on polythiophenes and substituted polythiophenes

In recent years there has been considerable interest in the fabrication of photovoltaic devices using polymeric and organic materials. This paper presents work carried out using a range of polythiophenes, including some substituted with porphyrin moieties as light harvesters. Homopolymers and copolymers were investigated for their performance in photovoltaic devices, and the use of both solid polymer electrolyte and liquid electrolyte was examined. Both photoelectrochemical cells and Schottky devices were investigated. The best photoelectrochemical cell was fabricated using polyterthiophene which had V_oc=139 mV, I_sc=123.4 μA cm⁻², fill factor=0.38, and energy conversion efficiency=0.02% at a halogen lamp intensity of 317 W m⁻². The Schottky device gave a V_oc=0.5 V and I_sc of 0.98 μA cm⁻² at a halogen lamp intensity of 500 W m⁻².     

Blue organic light-emitting materials based on diphenylaminofluorene and N-phenylcarbazole derivatives

A series of diphenylaminofluorene- and phenylcarbazole-derived, blue fluorescent molecules have been synthesized via the Hornor–Wadsworth–Emmons and Suzuki-cross coupling reactions. To explore the electroluminescent properties of these molecules, multilayer devices were fabricated with a structure of ITO/NPB/(1–6) doped in MADN/Bphen/Liq/Al, yielding a device that exhibited highly efficient sky-blue emissions with the luminous efficiency of 11.2 cd/A at 20 mA/cm², a power efficiency of 7.35 lm/W at 20 mA/cm², and CIE_x,y coordinates of (x = 0.16, y = 0.26) at 8 V. Also, a deep blue OLED with CIE_x,y coordinates of (x = 0.16, y = 0.13) at 8 V showed a luminous efficiency of 2.13 cd/A and power efficiency of 1.20 lm/W at 20 mA/cm², respectively.     

Composition and annealing effects in solution-processable functionalized graphene oxide/P3HT based solar cells

Characterization of a solution-processable functionalized graphene oxide (SPFGraphene oxide) was investigated by FT-IR spectroscopy and the result of elemental analysis showed that the isocyanate treatment results in the functionalization groups in SPFGraphene oxide. Doping SPFGraphene oxide to P3HT based solar cells induces absorbing spectra more strongly and a great quenching of the photoluminescence of the P3HT. With an increase in the SPFGraphene oxide content, the overall performances of the hybrid devices increases first, reaching the peak efficiency for the 10 wt% SPFGraphene oxide content, and then decreases. After annealing at 160 °C for different time durations, the device containing 10 wt% of SPFGraphene oxide for 10 min shows the best performance with a power conversion efficiency of 1.046%, an open-circuit voltage of 0.73 V, a short-circuit current density of 3.98 mA cm^?2 and a fill factor of 0.36 under simulated AM1.5G conditions at 100 mW cm^?2; The similar content one for 20 min shows η value of 1.013%, which is lower than the former one to a small extent for longer annealing duration. The graphene has the potential to act as the next-generation material in the photovoltaic devices and other applications for ease of preparation, low price, large surface area, high conductivity and excellent transparency.     

Synthesis and photovoltaic application of donor–acceptor type copolymers containing 9-cyanomethylenefluorene derivatives with chemically tunable electronic properties

Four 9-cyanomethylenefluorene derivatives were synthesized as a new class of electron-deficient building blocks for donor–acceptor (D–A) type alternating copolymers. UV–vis absorption spectroscopy, cyclic voltammetry and density functional theory calculations show that the substituent X at the cyanomethylene unit, depending on its electron-withdrawing ability (X = –H, –CONMe₂, –COOiBu and –CN), can continuously control the LUMO energy level of the monomer. D–A type copolymers were synthesized using the derivatives and an oligothiophene as the electron-donating block. The photovoltaic performance of D–A copolymer:PCBM bulk heterojunction devices shows the importance of LUMO level alignment to achieve efficient charge separation at the donor/acceptor interface. After optimization, the power conversion efficiency of the device with X = –H reached 1.14% with V_OC of 0.61 V, J_SC of 3.24 mA cm^?2 and FF of 0.58 under the irradiation of AM1.5 simulated solar light. The electronic properties of the copolymers correlated well with the electron-withdrawing ability of the monomers while the structure of the π-conjugated backbone was preserved, demonstrating the advantage of the chemically tunable monomer design.     

A highly efficient deep blue fluorescent OLED based on diphenylaminofluorenylstyrene-containing emitting materials

We have designed and synthesized five blue emitters based on diphenylaminofluorenylstyrene emitting core groups. Multilayered OLEDs were fabricated using these materials as dopants in a 2-methyl-9,10-di(naphthen-2-yl)anthracene (MADN) host. One of them in particular a deep blue OLED using dopant 9-[4-(2-diphenylamino-9,9-diethylfluoren-7-yl)phenyl]-9-phenylfluorene (3) at 15% doping concentration exhibited a maximum luminance of 4720 cd m^?2 at 9.0 V, a luminous efficiency of 5.3 cd A^?1 at 20 mA cm^?2, a power efficiency of 2.9 lm W^?1 at 20 mA cm^?2, an external quantum efficiency of 4.8% at 20 mA cm^?2, and CIE coordinates (x = 0.15, y = 0.13) at 8.0 V. Furthermore, this deep blue device had very stable CIE coordinates of (x = 0.15, y = 0.13) that did not vary with doping concentration from 5% to 15%.     

Ionic liquid doped poly(N-methyl 4-vinylpyridine iodide) solid polymer electrolyte for dye-sensitized solar cell

Ion conducting polymer electrolyte, poly(N-methyl 4-vinylpyridine iodide) (PVPI) is synthesized for dye-sensitized solar cell (DSSC) application. A new solid polymer electrolyte composite containing low viscosity ionic liquid (IL) 1-ethyl 3-methylimidazolium dicyanamide (EMImDCN) doped PVPI is developed and its structural, electrical and photoelectrochemical studies are presented in detail. Fourier transform infrared (FTIR) spectroscopy, proton NMR and atomic force microscopy (AFM) affirms the modified polymer and its composite nature with porous surface morphology. The developed solid polymer electrolyte shows enhancement in ionic conductivity (σ) due to IL doping. The maximum σ value of 9.12 × 10^?6 S cm^?1 was obtained at 40 wt% IL concentration. The redox behavior of the electrolyte has been verified by the cyclic voltammetry studies. For device application, we have fabricated a DSSC using this solid polymer–IL electrolyte system which shows energy conversion efficiency of the solid-state cell as 0.65% under irradiation of simulated sunlight (AM 1.5, 100 mW cm^?2).     

Synthesis and characterization of triphenylamine-based organic dyes for dye-sensitized solar cells

We synthesized three organic dyes (DYE 1, DYE 2, and DYE 3) containing triphenylamine (TPA) moieties as electron donors and cyanoacrylic acid moieties as electron acceptors, designed at the molecular level, and developed them for use in dye-sensitized solar cells (DSSCs). Among all the dyes, DYE 2 exhibited the highest overall solar-energy-to-electricity conversion efficiency of 4.06% with a short-circuit photocurrent density of 11.15 mA cm^?2, an open-circuit voltage of 0.64 V, and a fill factor of 0.57 under AM 1.5 irradiation with 100 mW cm^?2 simulated sunlight. The results indicate that the organic dyes are promising in the development of DSSC.     

Crystal structure and physical properties of a magnetic molecular conductor (EDO-TTFVODS)2FeCl4

Crystal structure, and electrical conducting and magnetic properties of a radical cation salt of EDO-TTFVODS with magnetic FeCl₄^? ion, (EDO-TTFVODS)₂FeCl₄ (EDO-TTFVODS = ethylenedioxytetrathiafulvalenoquinone-1,3-diselenolemethide) are reported. In this salt, there are two independent donor molecules formed two different layers A and B, and the counter FeCl₄^? ions layer is sandwiched between two donor layers A and B along the b-axis. The donor molecules form the one-dimensional columns along the a-axis in both donor layers. This salt shows high conductivity at room temperature (σ_RT = 25 S cm^?1) and a metallic behavior down to ca. 80 K, where a metal–insulator transition however occurs. The magnetic susceptibility obeys a Curie–Weiss law (Curie constant C = 4.42 emu K mol^?1 and Weiss temperature Θ = ?1.5 K), without any magnetic ordering down to 1.8 K. This result suggests the weak antiferromagnetic interaction between the d spins of FeCl₄^? ions.     

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)-N,N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN)₂, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN)₂ was found to emit a yellow light at λ_max = 590 nm (with the CIE coordinates x = 0.51; y = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN)₂-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence efficiency η_EL = 1.3 cd/A, an external quantum efficiency η_ext = 0.4 % and a luminance L = 127 cd/m² (at 10 mA/cm²) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x = 0.40; y = 0.43), η_EL = 2.0 cd/A, η_ext = 0.7%, L = 197 cd/m² (at 10 mA/cm²) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.     

Growth and optical spectroscopy of KPb2Cl5 crystal containing Mn2+

KPb₂Cl₅:Mn²⁺ crystal with sizes of centimeters has been successfully grown by a modified Bridgman method. The electronic spectra of Mn²⁺ in the title compound have been measured at different temperatures and compared, revealing that the red emission observed at 10 and 77 K in the spectral range from 500 to 800 nm, which vanishes at room temperature, is associated with the radiation from Pb²⁺ ions with adjacent defects whereas the orange-red emission in the spectral range from 530 to 680 nm can be undoubtedly attributed to the spin forbidden transition ⁴T_1g (⁴G) → ⁶A_1g(⁶S) of Mn²⁺. The electronic excitation spectra are interpreted in a strong crystal field scheme with the use of the Tanabe–Sugano diagram. Good agreement with the experimental spectroscopic data is obtained with the use of crystal field splitting parameter 10Dq = 7902 cm^?1, Racah parameters B = 618 cm^?1, and C = 3549 cm^?1.     

Enhanced efficiency of polymer:fullerene bulk-heterojunction solar cells with the insertion of thin CdS layer near the Al electrode

The insertion layer of cadmium sulfide (CdS) between polymer–fullerene blend and Al electrode is used to enhance the short-circuit current (I_sc) and the power conversion efficiency (PCE). The solar cells based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C₆₀ with the function layer of CdS (∼10 nm) shows the open-circuit voltage (V_oc) of ∼0.7 V, short-circuit current (I_sc) of ∼4.6 mA/cm², filling factor (FF) of ∼0.28, and the power conversion efficiency (PCE) of ∼5.3% under monochromatic light (532 nm) photoexcitation of about 16.7 mW/cm². Compared to cells without the CdS layer, the power conversion efficiency increases about an order of magnitude. The thickness of CdS layer was varied from 10 to 40 nm using e-beam deposition, and we obtained optimum current density–voltage characteristics for 10 nm thick CdS layer.     

Charge transport properties of an organic solar cell

The electronic properties of ITO/PEDOT-PSS/P3HT-PCBM/LiF/Al organic solar cell have been investigated using current-voltage and capacitance-voltage techniques. The electrical parameters like ideality factor n, series resistance R_s and barrier height ?_b, have been extracted using current-voltage and capacitance-voltage techniques. The photovoltaic parameters such as open circuit voltage V_oc, short circuit current I_sc and fill factor FF were determined from current-voltage characteristics of the solar cell under illumination. The ideal value than unity for the organic solar cell indicates the presence of non-ideal behaviour. The barrier height of the solar cell is found to be dependent both temperature and applied voltage and the model of Gaussian distribution of the barrier height was presented for explaining their anomalous behaviour. The standard deviation of the barrier height distribution σ_o indicates the presence of interface inhomogeneities. The ideality factor n, open circuit voltage V_oc, short circuit current density J_sc, fill factor FF and Richardson constant A* values for the organic solar cell were found to be 2.29, 0.58 V, 5.84 mA/cm², 0.31 and 10.41 A/cm² K², respectively.     

Synthesis and characterization of a novel fullerene derivative containing carbazole group for use in organic solar cells

A novel carbazole-group-containing fullerene derivative (CBZ-C₆₀) with good solubility in common organic solvents was synthesized. This derivative was analyzed by using many techniques such as NMR, FAB-MS, FTIR and UV–vis absorption spectroscopy. Further, bulk heterojunction photovoltaic devices were fabricated. Since the LUMO energy level of CBZ-C₆₀ was higher than that of fullerene, the open-circuit voltage (V_oc) of devices based on CBZ-C₆₀ was higher than that of devices based on fullerene. The power-conversion efficiency was highest for devices with composite thin films that have P3HT/CBZ-C₆₀ composition ratios of 1:1 and were annealed at 150 °C for 10 min. The maximum V_oc, short-circuit current density, and PCE of the best device were 0.64 V, 2.32 mA/cm², and 0.48%, respectively.