Improved power conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic devices using small molecule additive

We report that the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic device based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) blend was improved by incorporating a small molecule SM having absorption band in the longer wavelength region. SM is a small molecule containing thienothiadiazole central unit with terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit via a thiophene ring. The combination of SM with P3HT and PCBM allows not only a broad band absorption up to longer wavelength, but also tuning the inter-energy level leading to a higher short circuit current (J_sc) and open circuit voltage (V_oc). The device based on the as cast P3HT:PCBM:SM exhibits a PCE of 3.69%, which is higher than the device based on P3HT:PCBM and SM:PCBM blends. The overall PCE of the device based on thermally annealed blend is further improved up to 4.1%. The improvement of the PCE has been attributed to a better charge transport in the device, due to the increased crystallinity of the blend through thermal annealing.     

Synthesis, photophysical and photovoltaic properties of star-shaped molecules with triphenylamine as core and phenylethenylthiophene or dithienylethylene as arms

Two solution-processable star-shaped compounds, P and T ,that contain triphenylamine as core and phenylethenylthiophene or dithienylethylene, respectively, as arms were synthesized. They carry also a cyano group on the vinylene bond in the arms. These compounds showed an excellent thermal stability and relatively low (66-72 °C) glass transition temperatures. Their UV-vis spectra showed maximum at 431-459 nm with optical band gaps of 2.22-2.41 eV. They behaved as yellow-orange light emitters with photoluminescence (PL) maximum at 521-610 nm. The PL maximum of T was red shifted relative to P. Photovoltaic devices with active layers based on P (or T) and PCBM were prepared and the thin film composition and the thermal annealing treatment were screened in order to optimize the performance of the devices. The morphology of the blend films and the hole mobilities of P and T were also investigated. Photovoltaic performances of the devices with blend film containing 50 wt%/PCBM in P and T showed highest power conversion efficiencies (PCEs) 0.29% and 0.41%, respectively.     

Understanding the performance and loss-mechanisms in donor-acceptor polymer based solar cells: Photocurrent generation, charge separation and carrier transport

Advances in bandgap and molecular spectral engineering have led to color-tunable donor-acceptor (DA) polymers for use in esthetically colored bulk-heterojunction (BHJ) photovoltaic devices where control and optimization of the physical processes governing their operation is necessary to realize high efficiency. We detail a study of the processes of photogeneration of charge carriers, their separation at the DA heterojunctions and their transport in solar cells fabricated from a series of dioxythiophene-benzothiadiazole (DOT-BTD) copolymers (PG1-PG3) as the electron donor and methanofullerene (PCBM) derivatives as the electron acceptor. We also examine the effect of the blend composition on carrier transport and photoresponse. The photogeneration of carriers and their dissociation increases with increase in concentration of the acceptor and peaks at ∼88% of PCBM. Measurements on single-carrier devices show that hole mobility in the polymer phase of the blends is enhanced by up to two orders at compositions having >80% of PCBM by weight. However, even at the optimized composition, the electron mobility in the blend is two orders of magnitude higher than the hole mobility. This largely imbalanced carrier transport results in the build-up of space-charges, limiting the efficiency of charge separation at short-circuit (∼53%) and the fill factor (<45%). To highlight the importance of attaining charge balance in BHJ DA blends for photovoltaics, we further investigate and compare solar cells from two model polymers, a low bandgap DA polymer (PSBTBT) and poly-3-hexylthiophene (P3HT), blended with PCBM. Our findings indicate that, in contrast to the DOT-BTD copolymers, carrier transport is significantly more balanced in these model systems, preventing space-charge build-up and reducing recombination, which results in a markedly improved charge separation efficiency and fill factor.     

On the spectral bands measurements for combustion monitoring

In this work, spatial–spectral experimental issues affecting the detection of radical emissions in a natural gas flame are discussed and studied by a radiometric analysis of the flame spectral emission. As results of this analysis, Local and Global Spectral Radiation Measurements (LSRM and GSRM respectively) techniques are proposed, and guidelines for selecting the radical emission bands and spatial location of photodetectors are given. Two types of experiments have been performed in order to demonstrate the reliability of the GSRM technique for combustion characterization. In the first experiment, the LSRM and the GSRM have been implemented by using a home made sensor array, based on silicon photodiodes, for sensing the excited CH^∗ and radicals in a natural gas flame. It has been experimentally demonstrated that by using the GSRM, the signal’s dispersion can be reduced to about 86% for the CH^∗ and 76% for the with respect to the obtained values with LSRM methodology. In the second experiment, the GSRM technique has been applied for sensing the CH^∗ and radicals, where it has been found that the signals emissions ratio /CH^∗ provides a good indicator of the thermal combustion efficiency and the CO pollutants emissions, with small dispersion. Thus, the GSRM technique has corroborated the usefulness of that ratio for combustion monitoring.     

On the validity of a design method for a solar-assisted ejector cooling system

A solar-assisted ejector cooling system is simulated in order to investigate the validity of a design methodology. Hourly simulation results allow for computing the solar fraction, in cases when the cooling capacity of the ejector cycle is kept constant during daily periods. The computed solar fraction is compared with estimates obtained from the method based on the utilizability concept. An equivalent minimum temperature for the utilizability of the solar system is found, which proves to be different, but close to, the vapor generator temperature of the ejector cycle. It is shown that the solar fraction derived from the utilizability concept based on the monthly means of the global solar radiation is applicable to solar-assisted ejector cooling cycles, in cases when the minimum temperature at which solar heat is supplied to the load is determined. Good agreement is found between the solar fraction results obtained from the simulations and those obtained by the method.     

Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells

A new class of triphenylamine substituted methanofullerene derivatives, bis(4'-(diphenylamino)biphenyl-4-yl)methanofullerene (1) and the bisadduct (2), were synthesized. The incident photon to current efficiency (IPCE) studies revealed that the diphenylamino components have contribution to the photocurrent that expands the light harvesting window around 400 nm. When being blended with poly (3-hexylthiophene) (P3HT) to fabricate the solar cell, the device of P3HT:1 (1:0.7) shows high open circuit voltage (V_oc) of 0.69 V under the illumination of AM 1.5, 100 mW/cm² with high power conversion efficiency (PCE) of 3.16%, which is about 0.1 V higher than that of the corresponding [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) devices. This indicates that the arylamine substituents on 1 have played some special roles on the high V_oc performance. Similar effects are also observed for 2. The device of P3HT:2 (1:1) shows even higher V_oc of 0.87 V with the PCE of 1.83%. These results indicate that 1 and 2 are alternative high performance acceptors.     

Low band gap poly(1,4-arylene-2,5-thienylene)s with benzothiadiazole units: Synthesis, characterization and application in polymer solar cells

We describe the synthesis of two novel poly(1,4-arylene-2,5-thienylene)s P1 and P2 containing benzo[c][2,1,3]thiadiazole monomeric units via Suzuki-Miyaura polymerization of a thiophene diboronic ester with aryl diiodides. The use of a catalyst complex consisting of Pd(OAc)₂ in combination with the electron-rich biaryl phosphine S-Phos resulted in efficient polymerization reactions. The polymers synthesized, P1 and P2, were characterized by UV-vis spectroscopy and cyclic voltammetry. Theoretical calculations and electrochemical measurements on P1 suggested a favorable position of the molecular orbitals for employment in polymer solar cells in combination with PCBM. Devices containing P1:PCBM 1:2 in the active layer showed an efficiency of 1.2% by simple spin casting from chloroform.     

Efficient bulk heterojunction solar cells using an alternating phenylenevinylene copolymer with dithenyl(thienothiadiazole) segments as donor and PCBM or modified PCBM as acceptor

A novel low band gap alternating phenylenevinylene copolymer, P, with dithenyl (thienothiadiazole) segments was synthesized by Heck coupling. It was soluble in common organic solvents, showed broad absorption curve with long-wavelength absorption maximum at 580–598 nm and optical band gap of 1.74 eV, which is comparable with the electrochemical band gap of about 1.80 eV. P (electron donor) was blended with PCBM or modified PCBM i.e. F (electron acceptor) to fabricate bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of the devices based on P:PCBM and P:F cast from 1.2-dichlorobenzene (DCB) was found to be 1.40% and 2.32%, respectively. The higher value of PCE for the device with P:F as compared to P:PCBM is attributed to the increase in both short circuit current (J_sc) and open circuit voltage (V_oc). The increase in the J_sc is due to the stronger light absorption of F in visible region as compared to PCBM, i.e. more exciton generation in the blend. On the other hand, the higher difference between the LUMO levels of P and F, as compared to P and PCBM is responsible for the enhancement in the V_oc. A maximum overall PCE of 4.20% was obtained for the BHJ polymer cell based on the active layer (P:F) deposited from mixed solvents 1-chloronapthalene/1,2-dichlorobenzene (CN/DCB) and subsequent thermal annealing at 120 °C. This improvement in the PCE has been attributed to the enhanced crystallinity of the blend and more balanced charge transport in the device due to the thermal treatment.     

Preparation of thin films by sulfurizing sol–gel deposited precursors

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing precursors deposited by the sol–gel method. Copper (II) acetate monohydrate, zinc (II) acetate dihydrate and tin (II) chloride dihydrate were used as the starting materials of the sol–gel method, and 2-methoxyethanol and monoethanolamine were used as the solvent and the stabilizer, respectively. The solution was spin coated on soda lime glass substrates and dried at . The coated glasses were sulfurized by annealing at in a hydrogen sulfide-containing atmosphere. The annealed thin films showed X-ray diffraction peaks attributed to the single phase CZTS. The chemical composition of the films was almost stoichiometric and the band gap energy was at room temperature.     

Novel zinc porphyrin with phenylenevinylene meso-substituents: Synthesis and application in dye-sensitized solar cells

A novel zinc porphyrin, P, with phenylenevinylene segments at two opposite meso-positions and carboxyphenyl at the other two meso-positions of the porphyrin ring, was synthesized and characterized. The phenylenevinylene substituents were terminated with electron-accepting 4-nitro-α-cyanostilbene units. Elongation of the π-conjugation enhanced the solubility of P as well as broadened and strengthened the absorption spectrum. We have investigated the application of P in quasi solid state dye-sensitized solar cells (DSSCs). Under illumination intensity of 100 mW cm⁻², a power conversion efficiency of 2.90% was obtained for the DSSC based on P as sensitizer, which was significantly improved to 4.22% upon addition of deoxycholic acid (DCA) into the P solution for TiO₂ sensitization. Coadsorption of DCA decreased the dye adsorption, but significantly improved both short circuit current (J_sc) and open circuit voltage (V_oc). The breakup of π stacked aggregates might improve the electron injection yield and thus J_sc. The electrochemical impedance data indicate that the electron lifetime was improved by the coadsorption of DCA, which was attributed to the improvement in both V_oc and J_sc. The increase in J_sc has also been attributed to the reduction of the back reaction i.e., the recombination of electrons with tri-iodide ions.     

Effects of a perfluorinated compound as an additive on the power conversion efficiencies of polymer solar cells

A perfluorinated compound, 4-amino-2-(trifluoromethyl)benzonitrile (ATMB), was applied as an additive to polymer solar cells (PSCs) with P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl-C₆₁-butyric acid methyl ester] blend films. The addition of 6 wt% ATMB to a P3HT:PCBM layer led to an increased power conversion efficiency of 5.03% due to the enhanced short circuit current and fill factor when compared with that of the reference cell without an additive. On the other hand, the devices with 4-aminobenzonitrile as an additive, not containing fluorine atoms in the molecule, displayed lower PCEs than that of the reference cell. The UV-visible absorption spectra, X-ray measurements and carrier mobility studies revealed that ATMB facilitated ordering of the P3HT chains, resulting in higher absorbance, larger crystal size of P3HT and enhanced hole mobility. XPS depth profiling measurements also showed that the additive molecules were predominantly positioned in the range of 25 nm under the surface of the P3HT:PCBM film, leading to improved fill factor.     

Dimethyl-2H-benzimidazole based small molecules as donor materials for organic photovoltaics

Utilization of 2,2-dimethyl-2H-benzimidazole has received strong attention as the electron-deficient unit for the generation of electron donor material for organic photovoltaic cells (OPVs). This paper reports the first small organic molecules based on dimethyl-2H-benzimidazole, which can produce intramolecular charge transfer. Two soluble small organic molecules, MMM and OMO, with dimethyl-2H-benzimidazole unit were synthesized by Suzuki coupling reaction with Pd(0)-catalyst. The spectra of MMM and OMO in the solid thin films show absorption bands with maximum peaks at 374, 598 and 373, 588 nm, and the absorption onsets at 678 and 673 nm, corresponding to band gaps of 1.83 and 1.84 eV, respectively. The devices comprising MMM with PC61BM (1:3) showed a V_OC of 0.66 V, a J_SC of 2.03 mA/cm², and a fill factor (FF) of 0.27, giving a power-conversion efficiency of 0.37%.     

Low band gap dyes based on 2-styryl-5-phenylazo-pyrrole: Synthesis and application for efficient dye-sensitized solar cells

A new series of low band gap dyes, C1, C2 and S, based on 2-styryl-5-phenylazo-pyrrole was synthesized. These dyes contain one carboxy, two carboxy and one sulfonic acid anchoring groups, respectively. They were soluble in common organic solvents, showed long-wavelength absorption maximum at ∼620 nm and optical band gap of 1.66-1.68 eV. The photophysical and electrochemical properties of these dyes were investigated and found to be suitable as photosensitizers for dye sensitized solar cells (DSSCs). The quasi solid state DSSCs with dye S showed a maximum monochromatic incident photon to current efficiency (IPCE) of 78% and an overall power conversion efficiency (PCE) of 4.17% under illumination intensity of 100 mW cm⁻² (1.5 AM), which is higher than the other dyes (3.26% for C2 and 2.59% for C1). Even though dye S contains one sulfonic acid anchoring group, the higher PCE for the DSSCs based on this dye has been attributed to the higher dye loading at the TiO₂ surface and enhanced electron lifetime in the device, as indicated by absorption spectra and electrochemical impedance spectra measurements. Finally, by increasing the molecular weight of poly(ethylene oxide) (PEO) in electrolyte, the PCE also increases up to 4.8% for the electrolyte with PEO molecular weight of 2.0 × 10⁶. This improvement has been attributed to the enhancement in iodide ions diffusion due to the increase in free volume of polymer gel electrolyte.     

Dye-sensitized solar cell using natural dyes extracted from rosella and blue pea flowers

Dye-sensitized solar cells (DSSCs) were fabricated using natural dyes extracted from rosella, blue pea and a mixture of the extracts. The light absorption spectrum of the mixed extract contained peaks corresponding to the contributions from both rosella and blue pea extracts. However, the mixed extract adsorbed on TiO₂ does not show synergistic light absorption and photosensitization compared to the individual extracts. Instead, the cell sensitized by the rosella extract alone showed the best sensitization, which was in agreement with the broadest spectrum of the extract adsorbed on TiO₂ film. In case that the dyes were extracted at , using water as extracting solvent, the energy conversion efficiency (η) of the cells consisting of rosella extract alone, blue pea extract alone and mixed extract was 0.37%, 0.05% and 0.15%, respectively. The sensitization performance related to interaction between the dye and TiO₂ surface is discussed. The explanations are supported by the light absorption of the extract solution compared to extracts adsorbed on TiO₂ and also dye structures. The effects of changing extracting temperature, extracting solvent and pH of the extract solution are also reported. The efficiency of rosella extract sensitized DSSC was improved from 0.37% to 0.70% when the aqueous dye was extracted at instead of and pH of the dye was adjusted from 3.2 to 1.0. Moreover, DSSC stability was also improved by the changes in conditions. However, the efficiency of a DSSC using ethanol as extracting solvent was found to be diminished after being exposed to the simulated sunlight for a short period.     

A regioregular head to tail thiophene based “double-cable” polymer with pendant anthraquinone functional groups: Preparation, spectroscopy and photovoltaic properties

We present an investigation into the electronic, photophysical and photovoltaic properties of a π-conjugated “double-cable” polymer based on regioregular head to tail poly(3-functionalized thiophene) with pendant anthraquinone acceptor groups (PT-AQ). This material was compared with model blends composed of a mixture of regioregular head to tail poly(3-hexylthiophene) (P3HT) as the polymer donor and anthraquinone-2-carboxylic acid octyl ester (AQ) as a small-molecule acceptor. We find that PT-AQ shows complete quenching of fluorescence whilst the donor/acceptor blend shows residual fluorescence emission and both phase separation and crystallization of the acceptor component. The operation of the double-cable based devices is however, dominated by geminate recombination and poor charge transport and extraction. Nevertheless, we show that the PT-AQ devices have a higher efficiency than devices based on representative bulk-heterojunction AQ:P3HT blends as the very coarse phase separation and crystallization is suppressed.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

Activation characteristics and microstructure of composite hydrogen storage alloys prepared by two-step re-melting

In the present work, novel composites (x=0,5,10,30) for hydrogen storage were prepared by two-step re-melting and their activation characteristic and microstructure were investigated. The influence of Mg₂Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg₂Ni, activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg₂Ni has the least cycle number for activation and the highest discharge capacity. It is activated after only 6 cycles (C_n=6) at room temperature and its maximum discharge capacity (C_max) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg₂Ni is difficult to be activated at room temperature. It is also found that it is easier to be activated for the composites at and than that at and , but their discharge capacity decay slightly at the condition of and . The XRD and SEM analysis show that, with the increasing Mg₂Ni content, the microstructure of the composites varies gradually from lamellar (x=5), acicular (x=10) to massive (x=30), and the activity of the composite declines as a result of the grain size of phase Mg₂Ni grows up.     

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.     

Carrier collection in Cu(In,Ga)Se2 solar cells with graded band gaps and transparent ZnO:Al back contacts

Cu(In,Ga)Se₂ Solar cells with graded band gap and efficiencies up to 13% have been fabricated on transparent ZnO:Al back contacts. The back contact structure includes a transparent 10 nm thin Mo interlayer with NaF precursor between the ZnO:Al and the Cu(In,Ga)Se₂ absorber that transforms the blocking ZnO:Al/Cu(In,Ga)Se₂ interface into an Ohmic back contact. To investigate the electronic quality of the back contact, the cells are analyzed by internal quantum efficiency measurements under illumination from front and back side. A new semianalytical model for the quantum efficiency of graded band gap absorbers yields quantitative information about the back contact recombination velocity as well as optical and electronic material parameters of the absorber layer. Band gap grading significantly increases carrier collection. However, in the immediate vicinity of the back contact carrier collection is limited by a high ratio of back contact recombination velocity and diffusion constant .