Novel iminocoumarin dyes as photosensitizers for dye-sensitized solar cell

Novel iminocoumarin dyes (2a-c and 3a-c) having carboxyl and hydroxyl anchoring groups onto the dyes skeletons have been designed and synthesized for the application of dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). The photophysical and electrochemical studies showed that these iminocoumarin dyes are suitable as light harvesting sensitizers in DSSC application. The dyes having carboxyl and hydroxyl anchoring groups (2a-c) showed better efficiency when compared to the dyes having carboxyl group (3a-c) alone. The cell consisted of dye 2a generated the highest solar-to-electricity conversion efficiency (η) of 0.767% (open circuit voltage (V_oc) = 0.491 V, short circuit photocurrent density (J_sc) = 2.461 mA cm⁻², fill factor (ff) = 0.635) under simulated AM 1.5 irradiation (1000 W m⁻²) with a total semiconductor area of 0.25 cm². The corresponding incident photon-to-current conversion efficiency (IPCE) of the above cell was 21.38%. The overall low efficiency of the dyes is ascribed to the lack of light harvesting ability at longer wavelength region.     

Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells

This study employs chlorophyll extract from pomegranate leaf and anthocyanin extract from mulberry fruit as the natural dyes for a dye-sensitized solar cell (DSSC). A self-developed nanofluid synthesis system is employed to prepare TiO₂ nanofluid with an average particle size of 25 nm. Electrophoresis deposition was performed to deposit TiO₂ nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO₂ thin film with the thickness of 11 μm. Furthermore, this TiO₂ thin film was sintered at 450 °C to enhance the thin film compactness. Sputtering was used to prepare counter electrode by depositing Pt thin film on FTO glass at a thickness of 20 nm. The electrodes, electrolyte (), and dyes were assembled into a cell module and illuminated by a light source simulating AM 1.5 with a light strength of 100 mW/cm² to measure the photoelectric conversion efficiency of the prepared DSSCs. According to experimental results, the conversion efficiency of the DSSCs prepared by chlorophyll dyes from pomegranate leaf extract is 0.597%, with open-circuit voltage (V_OC) of 0.56 V, short-circuit current density (J_SC) of 2.05 mA/cm², and fill factor (FF) of 0.52. The conversion efficiency of the DSSCs prepared by anthocyanin dyes from mulberry extract is 0.548%, with V_OC of 0.555 V and J_SC of 1.89 mA/cm² and FF of 0.53. The conversion efficiency is 0.722% for chlorophyll and anthocyanin as the dye mixture, with V_OC of 0.53 V, J_SC of 2.8 mA/cm², and FF of 0.49.     

Synthesis and characterization of trivalent metal porphyrin with NCS ligand for application in dye-sensitized solar cells

Two novel trivalent metal porphyrin dyes, P_Mn-HT-SCN and P_Ga-HT-SCN, were designed, synthesized and firstly applied in dye-sensitized solar cells (DSSCs). These two dyes possess porphyrin donor modified with manganese (III) and gallium (III) as coordination metal and NCS as the second ligand, cyanoacrylic acid as electron-accepting moiety and 4-hexylthiophene as π-spacers. Each of the porphyrin showed different adsorption behavior and saturated coverage on the TiO₂ surface. Between the two dyes, the P_Mn-HT-SCN-based DSSCs afforded the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 4.32 mA/cm², an open-circuit photovoltage (V_oc) of 0.61 V and a fill factor (FF) of 0.58, corresponding to a solar-to-electricity conversion efficiency of 1.53% under 100 mW/cm² irradiation.     

Triphenylamine- and benzothiadiazole-based dyes with multiple acceptors for application in dye-sensitized solar cells

Two novel dyes TPAR3 and BTDR2 based on triphenylamine and benzothiadiazole, respectively, with multiple electron acceptors were synthesized and characterized by FT-IR, ¹H NMR, TGA and thermomechanical analysis (TMA). They carried terminal cyanoacrylic acid electron acceptors/anchoring moieties, which were connected with the central unit through a thiophene ring. The absorption bands of the dyes were extended up to ∼570 nm with long-wave absorption maximum at 425-455 nm and optical band gap of 2.10-2.17 eV. The dyes emitted yellow-orange light with photoluminescence maximum at 547-615 nm. We have investigated the photovoltaic properties of quasi solid state dye sensitized solar cells (DSSCs) based on these metal free organic dyes. It has been found that the power conversion efficiency of the DSSCs based on composite zinc titanium oxide (ZTO) nanocrystalline photoelectrode is higher than that for TiO₂ based DSSCs. This has been attributed to the longer electron lifetime and more negative conduction band edge of ZTO. The overall power conversion efficiency of the DSSCs based on TPAR3 and BTDR2 employing ZTO photoelectrode is 6.3% and 3.6%, respectively. These results indicate that both the acceptor moiety of metal free organic dyes and ZTO photoelectrode have an effect on the photovoltaic performance of DSSCs.     

New organic photosensitizers incorporating carbazole and dimethylarylamine moieties for dye-sensitized solar cells

Four organic dyes (XS10–13) employing carbazole unit as electron donor and N,N-dimethylarylamine moieties as electron-donating groups were designed and synthesized for nanocrystalline TiO₂ dye-sensitized solar cells. The electron-donating groups of dimethylarylamine increase the electron density of donor moiety and enhance the molar extinction coefficient of dyes. For a typical device the maximum IPCE value could reach 86%, with a short-circuit photocurrent density (J_sc) of 9.8 mA cm^?2, an open-circuit photovoltage (V_oc) of 642 mV, and fill factor (FF) of 0.63, which corresponds to an overall conversion efficiency (η) of 4.0%. For a comparison, the N719-sensitized TiO₂ solar cell showed an efficiency of 6.4%.     

Pyridinium molten salts as co-adsorbents in dye-sensitized solar cells

The influence of using pyridinium molten salts as co-adsorbents to modify the monolayer of a TiO₂ semiconductor on the performance of a dye-sensitized solar cell is studied. The current-voltage characteristics are measured under AM 1.5 (100 mW cm⁻²). The pyridinium molten salts significantly enhance the open-circuit photovoltage (V_oc), the short circuit photocurrent density (J_sc) as well as the solar energy conversion efficiency (η). 1-Ethyl-3-carboxypyridinium iodide ([ECP][I]) is applied successfully to prepare an insulating molecular layer with N719, and achieve high energy conversion efficiency as high as 4.49% at 100 mW cm⁻² and AM 1.5. The resulting efficiency is 20% higher than that of a non-additive device. This enhancement of conversion efficiency is attributed to the negative shift of the conduction band (CB) edge and the abundant concentration of I⁻ on the surface of the electrode when using [ECP][I] as the co-adsorbent.     

A study of vapor CdCl2 treatment by CSS in CdS/CdTe solar cells

We report the effect of CdCl₂ vapor treatment on the photovoltaic parameters of CdS/CdTe solar cells. Vapor treatment allows combining CdCl₂ exposure time and annealing in one step. In this alternative treatment, the CdS/CdTe substrates were treated with CdCl₂ vapor in a close spaced sublimation (CSS) configuration. The substrate temperature and CdCl₂ powder source temperature were 400 °C. The treatment was done by varying the treatment time (t) from 15 to 90 min. Such solar cells are examined by measuring their current density versus voltage (J-V) characteristics. The open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) of our best cell, fabricated and normalized to the area of 1 cm², were V_oc = 663 mV, J_sc = 18.5 mA/cm² and FF = 40%, respectively, corresponding to a total area conversion efficiency of η = 5%. In cells of minor area (0.1 cm²) efficiencies of 8% have been obtained.     

High-voltage (1.8 V) tandem solar cell system using a GaAs/AlXGa(1−X)As graded solar cell and dye-sensitised solar cells with organic dyes having different absorption spectra

Stacked multijunction (tandem) solar cells have been prepared by mechanically stacking dye-sensitised solar cells (DSCs) and a GaAs/Al_XGa_(1−X)As graded solar cell (GGC) as the top and bottom cells, respectively. Three organic dyes with different absorption spectra (D131, D102 and D205) were used in the DSCs, in order to match the photocurrent density between the DSC and the GGC. Tuning the absorption range of the DSC by choosing an appropriate dye, increased the overall photovoltaic conversion efficiency due to the optimal utilisation of the solar spectrum in the individual cells. The open circuit photovoltages (V_OC) of the GGC and the DSC with D131 were 1.11 V and 0.76 V, respectively, resulting in a V_OC of 1.85 V and a photovoltaic conversion efficiency of 7.63% for the tandem cell. Although the overall conversion efficiency has not exceeded that of the GGC (7.66%), these tandem cells provide adequate V_OC values for water splitting applications.     

A high-performance counter electrode based on poly(3,4-alkylenedioxythiophene) for dye-sensitized solar cells

A poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et₂) counter electrode prepared by electrochemical polymerization on a fluorine-doped tin oxide (FTO) glass substrate was incorporated in a platinum-free dye-sensitized solar cell (DSSC). The surface roughness and I⁻/I₃⁻ redox reaction behaviors based on PProDOT-Et₂, poly(3,4-propylenedioxythiophene) (PProDOT), poly(3,4-ethylenedioxythiophene) (PEDOT), and sputtered-Pt electrodes were characterized, and their performances as counter electrodes in DSSCs were compared. Cells fabricated with a PProDOT-Et₂ counter electrode showed a higher conversion efficiency of 7.88% compared to cells fabricated with PEDOT (3.93%), PProDOT (7.08%), and sputtered-Pt (7.77%) electrodes. This enhancement was attributed to increases in the effective surface area and good catalytic properties for I₃⁻ reduction. In terms of the film thickness effect, the fill factor was strongly dependent on the deposition charge capacity of the PProDOT-Et₂ layer, but the aggregation of PProDOT-Et₂ in thicker layers (>80 mC cm⁻²) resulted in decreases in J_SC and the cell conversion efficiency. The charge transfer resistances (R_ct1) of the PProDOT-Et₂ counter electrodes had the lowest value of ∼18 Ω at a deposition charge capacity of 40 mC cm⁻². These results indicate that films with high conductivity, high active surface area, and good catalytic properties for I₃⁻ reduction can potentially be used as the counter electrode in a high-performance DSSC.     

Low molecular mass organogelator based gel electrolyte gelated by a quaternary ammonium halide salt for quasi-solid-state dye-sensitized solar cells

Quasi-solid-state dye-sensitized solar cells (DSC) are fabricated using tetradodecylammonium bromide as a low molecular mass organogelator (LMOG) to form gel electrolyte with a high solution-to-gel transition temperature (T_SG) of 75 °C to hinder flow and volatilization of the liquid. The steady-state voltammograms reveal that the diffusion of the I₃⁻ and I⁻ in the gel electrolyte is hindered by the self-assembled network of the gel. An increased interfacial exchange current density (j₀) of 4.95 × 10⁻⁸ A cm⁻² and a decreased electron recombination lifetime (τ) of 117 ms reveal an increased electron recombination at the dyed TiO₂ photoelectrode/electrolyte interface in the DSC after gelation. The results of the accelerated aging tests show that the gel electrolyte based dye-sensitized solar cell can retain over 93% of its initial photoelectric conversion efficiency value after successive heating at 60 °C for 1000 h, and device degradation is negligible after one sun light soaking with UV cutoff filter for 1000 h.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

The effect of irradiance growing on hydrogen photoevolution and on the kinetic growth in Rhodopseudomonas palustris, strain 42OL

The relationship between hydrogen generation and the age of culture was investigated under fed-batch growth conditions. The specific growth rate (μ_e) was determined during the log phase of the growth curve and the μ_eMax was 0.02643 h⁻¹. Boltzmann's sigmoidal regression model was used to determine the specific rate of hydrogen evolution (μH): the maximum was 0.04440 h⁻¹. At low irradiance (36–75 W m⁻²), an inverse relationship was found between μH and I; after increasing the irradiance further, μH reached a plateau (0.00916 h⁻¹). The maximum reactor yield of cumulative hydrogen (4.5 l) was obtained at an irradiance of 320 W m⁻², but the highest hydrogen evolution rate (17.217 ml h⁻¹) was achieved at 500 W m⁻². The light conversion efficiency reached its maximum (6.91%) at the lowest irradiance investigated (36 W m⁻²); when the irradiance increased further, it decreased progressively down to 0.36%.     

p-Cu2O/n-ZnO heterojunction applied to visible light Orange II degradation

The p-Cu₂O/n-ZnO system is studied for its potential use as a photoactive heterojunction able to highly perform under visible light. The main application deals with the degradation of organic dyes such as Orange II and the effects of Cu₂O amount, Orange II concentration and light intensity are investigated. Results show that the kinetics of degradation follows a pseudo-first order and the optimum sensitization effect is obtained using a 70% concentration of Cu₂O. The degradation rate reaches its maximum (R_initial = 22.45 × 10⁻² mg l⁻¹ min⁻¹) at 15 mg l⁻¹ of Orange II. The effect of the irradiation intensity is also investigated taking the electrical energy consumption per order of magnitude (EE/O) as a figure of merit. The highest efficiency is obtained at an irradiation intensity of ∼122 × 10⁻⁵ kW with k_OBS = 14.97 × 10⁻³ min⁻¹ and EE/O, which corresponds to 12.54 kW h m⁻³ of energy consumption. This heterojunction allows a ∼25% saving of electrical energy in comparison to the p-Cu₂O/n-TiO₂ system, demonstrating the important role of the collector.     

Highly durable inverted-type organic solar cell using amorphous titanium oxide as electron collection electrode inserted between ITO and organic layer

An indium tin oxide/titanium oxide/[6,6]-phenyl C₆₁ butyric acid methyl ester:regioregular poly(3-hexylthiophene)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au type organic solar cell (ITO/TiO_x/PCBM:P3HT/PEDOT:PSS/Au) with 1 cm² active area, which is called “inverted-type solar cell”, was developed using an ITO/amorphous titanium oxide (TiO_x) electrode prepared by a sol-gel technique instead of a low functional electrode such as Al. The power conversion efficiency (η) of 2.47% was obtained by irradiating AM 1.5G-100 mW cm⁻² simulated sunlight. We found that a photoconduction of TiO_x by irradiating UV light containing slightly in the simulated sunlight was required to drive this solar cell. The device durability in an ambient atmosphere was maintained for more than 20 h under continuous light irradiation. Further, when the air-stable device was covered by a glass plate with a water getter sheet which was coated by an epoxy-UV resin as sealing material, the durability was still higher and over 96% of relative efficiency was observed even after continuous light irradiation for 120 h.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Relaxor ferroelectric and photocatalytic behaviour of Ba0.785Bi0.127Y0.017TiO3 composition

The composition of Ba_0.785Bi_0.127Y_0.017TiO₃ (BaBiYTiO) belongs to the BaTiO₃-Bi₂O₃-Y₂O₃ system. A dielectric study on ceramics performed at temperatures in the range (77 K-500 K) and frequency (10²-2 × 10⁵ Hz) revealed ferroelectric relaxor behaviour with a phase transition close to room temperature. The conductivity of BaBiYTiO obeys to an Arrhenius-type law with activation energy equals to 0.21 eV, in conformity with a small polaron hopping where most electrons are localized. The oxide is lightly doped leading to a wide space charge region (480 nm) in which photoeffect occurs. The presence of domains promotes the separation of the charge carriers through conversion of light into chemical energy. The electrode acquired n-type behaviour, evidenced from the negative thermopower and anodic photocurrent. The flat band potential V_fb (−0.46V_SCE) and the electron density N_D (2.82 × 10¹⁵ cm⁻³) were determined in KOH solutions (0.5 M). The Nyquist plot exhibits two well defined time constants characteristic of bulk and grains boundaries contributions and an equivalent electrical circuit has been proposed according to the Randles model. The energy band diagram shows the potentiality of the oxide for the solar-energy conversion. BaBiYTiO has been tested successfully for H₂ production upon visible light when combined to the delafossite CuFeO₂ as sensitizer. An evolution rate of 24 μmol mn⁻¹ and a quantum yield of 0.4% under polychromatic light were obtained.     

Dye-sensitized, nano-porous TiO2 solar cell with poly(acrylonitrile): MgI2 plasticized electrolyte

Dye-sensitized solar cells are promising candidates as supplementary power sources; the dominance in the photovoltaic field of inorganic solid-state junction devices is in fact now being challenged by the third generation of solar cells based on dye-sensitized, nano-porous photo-electrodes and polymer electrolytes. Polymer electrolytes are actually very favorable for photo-electrochemical solar cells and in this study poly(acrylonitrile)-MgI₂ based complexes are used. As ambient temperature conductivity of poly(acrylonitrile)-salt complexes are in general low, a conductivity enhancement is attained by blending with the plasticizers ethylene carbonate and propylene carbonate. At 20 °C the optimum ionic conductivity of 1.9 × 10⁻³ S cm⁻¹ is obtained for the (PAN)₁₀(MgI₂)_n(I₂)_n/10(EC)₂₀(PC)₂₀ electrolyte where n = 1.5. The predominantly ionic nature of the electrolyte is seen from the DC polarization data. Differential scanning calorimetric thermograms of electrolyte samples with different MgI₂ concentrations were studied and glass transition temperatures were determined. Further, in this study, a dye-sensitized solar cell structure was fabricated with the configuration Glass/FTO/TiO₂/Dye/Electrolyte/Pt/FTO/Glass and an overall energy conversion efficiency of 2.5% was achieved under solar irradiation of 600 W m⁻². The I-V characteristics curves revealed that the short-circuit current, open-circuit voltage and fill factor of the cell are 3.87 mA, 659 mV and 59.0%, respectively.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.