Long-wavelength Absorption Maximum Research Articles

Significant Improvement of Optoelectronic and Photovoltaic Properties by Incorporating Thiophene in a Solution-Processable D-A-D Modular Chromophore.

Through the incorporation of a thiophene functionality, a novel solution-processable small organic chromophore was designed, synthesized and characterized for application in bulk-heterojunction solar cells. The new chromophore, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)acrylonitrile) (coded as AS2), was based on a donor–acceptor–donor (D–A–D) module where a simple triphenylamine unit served as an electron donor, 1,4-phenylenediacetonitrile as an electron acceptor, and a thiophene ring as the π-bridge embedded between the donor and acceptor functionalities. AS2 was isolated as brick-red, needle-shaped crystals, and was fully characterized by 1H- and 13C-NMR, IR, mass spectrometry and single crystal X-ray diffraction. The optoelectronic and photovoltaic properties of AS2 were compared with those of a structural analogue, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-(diphenylamino)phenyl)-acrylonitrile) (AS1). Benefiting from the covalent thiophene bridges, compared to AS1 thin solid film, the AS2 film showed: (1) an enhancement of light-harvesting ability by 20%; (2) an increase in wavelength of the longest wavelength absorption maximum (497 nm vs. 470 nm) and (3) a narrower optical band-gap (1.93 eV vs. 2.17 eV). Studies on the photovoltaic properties revealed that the best AS2-[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based device showed an impressive enhanced power conversion efficiency of 4.10%, an approx. 3-fold increase with respect to the efficiency of the best AS1-based device (1.23%). These results clearly indicated that embodiment of thiophene functionality extended the molecular conjugation, thus enhancing the light-harvesting ability and short-circuit current density, while further improving the bulk-heterojunction device performance. To our knowledge, AS2 is the first example in the literature where a thiophene unit has been used in conjunction with a 1,4-phenylenediacetonitrile accepting functionality to extend the π-conjugation in a given D–A–D motif for bulk-heterojunction solar cell applications.

Development of the 8-aza-3-bromo-7-hydroxycoumarin-4-ylmethyl group as a new entry of photolabile protecting groups

A significant substitution effect of the position of the bromo group on the photosensitivity of the 8-azacoumarin chromophore leads to the development of a highly photosensitive 8-aza-3-bromo-7-hydroxycoumarin-4-ylmethyl (aza-3-Bhc) group that shows excellent photolytic efficiency and hydrophilicity with long-wavelength absorption maxima. The newly identified aza-3-Bhc group can be applied to caged glutamates for ester-type and carbamate-type protections of carboxyl and amino functionalities.

Symmetrical and unsymmetrical donor–acceptor–donor organic dyes: Design, synthesis and characterization. Engineering panchromic absorbance

A series of four novel organic semiconductor materials based on a donor–acceptor–donor (D–A–D) motif have been designed, synthesized and characterized to understand the effects of symmetry/asymmetry and acceptor strength. In this strategy, two symmetrical dyes with 2-(1,3,3-trimethylindolin-2-ylidene)acetaldehyde derived donor at both ends of the central acceptor and two unsymmetrical based on a 2-(1,3,3-trimethylindolin-2-ylidene)acetaldehyde derived donor on one end and an oligothiophene donor on the other end of the central acceptor were made. The central acceptors were dicyanovinylidene and 1,3-diethylthiobarbituric acid derivatives of 2-indanone in the set of symmetrical dyes and dicyanovinylidene in unsymmetrical dyes. The different acceptors allowed the tuning of optoelectronic properties. All the new materials were soluble in variety of organic solvents, such as chloroform, toluene and chlorobenzene. With the increasing acceptor strength, the wavelength of the longest wavelength absorption maximum as well as respective extinction coefficients were enhanced, which resulted in band gap reduction. The symmetrical molecules gave relatively narrow strong primary absorbances while asymmetric molecules gave weaker but double or very broad absorption peaks spanning the entire visible region to provide panchromatic absorption.

Molecular engineering for panchromatic absorbing oligothiophene donor–π–acceptor organic semiconductors

A series of three novel organic semiconductor molecules based on the donor–π-bridge–acceptor (D–π–A) modular design have been designed, synthesised and characterised. These small organic molecules have a common donor (triphenylamine), π-bridge (an alkylated tetrathiophene) and various acceptors to provide tuning of optical and electronic properties. The examined acceptors were dicyanovinylidene, cyanopyridone and oxoindenemalononitrile groups. These compounds were highly soluble in common organic solvents, such as chloroform, toluene and chlorobenzene. As acceptor strength increased from dicyanovinylidene to cyanopyridone to oxoindenemalononitrile groups, the wavelength of the longest wavelength absorption maximum increased (518, 587, 619nm (solution)), the respective extinction coefficients increased (4.9×104, 6.3×104, 7.4×104M−1cm−1) and with increasing acceptor strength the band gap narrowed (1.63, 1.42 1.38eV). Panchromatic absorbance was observed for the compounds comprising the cyanopyridone and oxoindenemalononitrile acceptors.

Fluorescent Penta- and Hexaene Fatty Acids by a Wittig–Horner/Elimination Strategy

Molecular fluorescent probes have revolutionized biochemical and biophysical studies in the last decades, but with regard to lipids there has been a lack of combining the slim shape of saturated acyl chains with fluorescent properties. Our strategy to pentaene and hexaene fatty acids builds upon commercially available 4-(E)-decenal, which is subjected to a Wittig-Horner reaction after chlorination in α-position. DBU-mediated β-elimination of HCl proceeding the olefination establishes a highly conjugated system to which a salt-free Wittig reaction adds a final double bond leading to a good (Z)-selectivity of 83-86%. The double bond geometry can be optionally isomerized with I(2) to furnish the all-(E)-species. The five conjugated alkene moieties result in a longest-wavelength absorption maximum of about 350 nm. A red-shift to 380 nm was realized by addition of another double bond employing a common Wittig-Horner prolongation sequence. Stokes shifts of about 7300 and 7800 cm(-1), respectively, were observed.

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 3: The Distinctive Impact of Auxochromes at the 7‐ versus 3‐Positions

Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7-substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3-substituent (vinyl versus formyl), yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13(1)-oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3-, 7- and/or 13-positions. Substituent effects on the absorption spectra are well accounted for using Gouterman's four-orbital model. Key findings are that (1) the dramatic difference in auxochromic effects of a given substituent at the 7- versus 3- or 13-positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7- or 8-position effectively "porphyrinizes" the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle.

Benzothiadiazole-Dithienopyrrole Donor–Acceptor–Donor and Acceptor–Donor–Acceptor Triads: Synthesis and Optical, Electrochemical, and Charge-Transport Properties

2,2′-(Benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-dialkyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) (DTP-BTD-DTP) donor–acceptor–donor (D-A-D) and 4-alkyl-2,6-bis(benzo[c][1,2,5]thiadiazol-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (BTD-DTP-BTD) acceptor–donor–acceptor (A-D-A) triads, with or without additional alkylation in the DTP 6- or BTD 7-positions, respectively, have been synthesized using Stille coupling reactions, characterized using UV–vis absorption spectroscopy and electrochemistry, modeled using density functional theory calculations, and used as charge-transport materials in field-effect transistors. The choice of alkyl substitution pattern has only minor effects on the optical and redox behavior but can be used to modify the thermal properties and solubility of these compounds. The D-A-D and A-D-A triads show long-wavelength absorption maxima at 566–588 and 517–521 nm, respectively, in solution. These transitions are attributed to excitation from a delocalized HOMO to a BTD-localized LUMO and, accordingly, are bathochromically shifted from those of analogous compounds in which the BDT moieties are replaced by benzene rings (393–417 nm). The triads are oxidized at potentials of +0.01 to +0.37 V vs ferrocenium/ferrocene and are reduced at potentials of −1.95 to −1.74 V, with the D-A-D species being both the most easily oxidized and most easily reduced. Field-effect transistors based on solution-processed films of some of the triads showed p-channel behavior; the highest average hole mobility value measured was 5.9 × 10–3 cm2 V–1 s–1 for 2,2′-(benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-di-n-dodecyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) and was accompanied by an on/off current ratio of ca. 103.

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.

New photosensitizer with phenylenebisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units for dye-sensitized solar cells

A new low band gap photosensitizer, D, which contains 2,2′-(1,4-phenylene) bisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units at both sides was synthesized. The two carboxyls attached to the 2,5-positions of the phenylene ring act as anchoring groups. Dye D was soluble in common organic solvents, showed long-wavelength absorption maximum at 620–636 nm and optical band gap of 1.72 eV. The electrochemical parameters, i.e. the highest occupied molecular orbital (HOMO) (−5.1 eV) and the lowest unoccupied molecular orbital (LUMO) (−3.3 eV) energy levels of D show that this dye is suitable as molecular sensitizer. The quasi solid state dye-sensitized solar cell (DSSC) based on D shows a short circuit current ( J sc) of 9.95 mA/cm 2, an open circuit voltage ( V oc) of 0.70 V, and a fill factor (FF) of 0.64 corresponding to an overall power conversion efficiency (PCE) of 4.40% under 100 mW/cm 2 irradiation. The overall PCE has been further improved to 5.52% when diphenylphosphinic acid (DPPA) coadsorbent is incorporated into the D solution. This increased PCE has been attributed to the enhancement in the electron lifetime and reduced recombination of injected electrons with the iodide ions present in the electrolyte with the use of DPPA as coadsorbant. The electrochemical impedance spectroscopy (EIS) results indicated that the augment ascribes to inhibited interfacial charge recombination between the conduction band electrons and the tri-iodide ions in the electrolyte.

Efficient Bulk Heterojunction Solar Cells Based on a Broadly Absorbing Phenylenevinylene Copolymer Containing Thiophene and Pyrrole Rings

A novel low band gap alternating phenylenevinylene copolymer, P, containing thiophene and pyrrole rings was synthesized by Heck polycondensation. It was soluble in common organic solvents and showed a long-wavelength absorption maximum at 596−635 nm and an optical band gap of 1.65 eV. The electrochemical properties and electronic energy levels of P were studied by cyclic voltammetry. The highest occupied molecular orbital (HOMO) level of P is deeper than that of poly(3-hexylthiophene) (P3HT). The polymer solar cells (PSCs) based on P with a modified 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM) (i.e., F) as an electron acceptor shows higher open circuit voltage (Voc) than that for the device based on P3HT. The power conversion efficiency (PCE) for the devices P−F and P−PCBM processed from tetrahydrofuran (THF) solvent is approximately 2.42% and 1.50%, respectively. The enhanced PCE is attributed to the higher value of both Voc and short circuit current (Jsc). The PCE of the devices based on P−F p...

Low band-gap phenylenevinylene and fluorenevinylene small molecules containing triphenylamine segments: Synthesis and application in bulk heterojunction solar cells

Starting from triphenylamine, two low band gap small molecules, PH and FL, based on phenylenevinylene and fluorenevinylene, respectively were synthesized. They were soluble in common organic solvents such as tetrahydrofuran, chloroform and dichloromethane. Their long-wavelength absorption maximum was at 605–643 nm with optical band gap of 1.64–1.66 eV. These small molecules showed a band gap lower than that of P3HT and also have deeper HOMO level, which is beneficial for improvement of the open circuit voltage. The photovoltaic properties have been investigated using the bulk heterojunction (BHJ) active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power conversion efficiency (PCE) (1.42%) than the device based on PH:PCBM (1.02%), when the blend films were cast from chloroform solvent. Moreover, various casting solvents were used for the BHJ solar cells based on FL:PCBM blend and their effect on the photovoltaic performance was investigated. The results indicate that high boiling point solvents lead to an enhanced self-organization of FL in the active layer, which causes an increased charge transport. Finally, we have used a modified PCBM, i.e. F as electron acceptor along with FL as electron donor, to increase the light harvesting in the wavelength region below 500 nm and the PCE is about 4.38% when the BHJ ( FL: F blend) device was spin casted from mixed 1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed to the increased light absorption and higher hole mobility in the active layer, which resulted in more balanced charge transport.

Synthesis and low-temperature dehydrating imidation polymerization of 1,4-dihydro-1,4-diarsininetetracarboxylic acid dianhydride

cis-1,4-Dihydro-1,4-dimethyl-1,4-diarsinine-2,3,5,6-tetracarboxylic acid dianhydride (cis-DHDADA) was quantitatively formed from cis-1,4-dihydro-1,4-dimethyl-2,3,5,6-tetrakis(t-butoxycarbonyl)-1,4-diarsinine (cis-DHDAtBu) treated with formic acid as a Brønsted acid or with a cationic gold(I) catalyst as a Lewis acid. X-ray crystallography suggested that the six-membered ring in cis-DHDADA is in a boat conformation and is nearly flat, with the As–C=C bond angle of cis-DHDADA extending to 133°. The ultraviolet–visible absorption spectra of cis-DHDADA in CH2Cl2 showed a long-wavelength absorption maximum at 321 nm, which was redshifted compared with that of cis-DHDAtBu. Polymerization of cis-DHDADA with 1,3-bis(4-aminophenoxy)benzene at 80 °C in N,N-dimethylacetoamide (DMAc) for 12 h yielded a soluble polyimide, with number-average and weight-average molecular weights estimated at 23 400 and 60 300, respectively. The polyimide was obtained even when the polymerization was conducted at 60 °C for 24 h.

Synthesis of novel tetracationic asymmetric monomeric monomethine cyanine dyes – highly fluorescent dsDNA probes

Quaternisation of monomethine cyanine dyes bearing haloalkyl chains, with the appropriate monoquaternised 1,4‐diazabicyclo[2.2.2]octane derivatives, yielded novel tetracationic asymmetric monomeric monomethine cyanine dyes belonging to the thiazole orange family. The optimal conditions for this reaction were investigated. The longest wavelength absorption maxima of the studied dyes are in the region 504–507 nm. The molar absorptivities of the target dyes are high, with values between 70 500 and 99 500 l mol−1 cm−1. The fluorescence intensity and the fluorescence quantum yields of the free dyes in solution or in the presence of double‐stranded DNA were determined using thiazole orange for comparison. The dyes are suitable for nucleic acid detection and have a high binding affinity, attributable to their four positive charges.

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 TiO 2 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 × 10 6. This improvement has been attributed to the enhancement in iodide ions diffusion due to the increase in free volume of polymer gel electrolyte.

Fluorescent monolayer protected gold nanoparticles – Preparation and structure elucidation

A novel N-substituted 4-methoxy-1,8-naphthalimide (NAFTA 8) especially designed for fluorescent labeling of gold nanoparticles has been synthesized. NAFTA 8 bears a long methylene chain at the imide N atom and has a terminal SH group, which enables its chemical binding to gold nanostructures. The longest wavelength absorption maximum of NAFTA 8 in chloroform is at 370 nm, the fluorescent maximum is at 430 nm and the fluorescent quantum yield is 0.95. The newly synthesized fluorophore is applied for functionalization of gold nanoparticles with diameter 1.5 ± 0.5 nm prepared through chemical reduction. The obtained Monolayer Protected Clusters are characterized by elemental analysis, TEM, XPS, FT-IR, absorption and fluorescence spectroscopy. The performed investigations provide evidence for the formation of chemical bond between the thiol ligand and the gold surface. They also show that the obtained metal/dielectric 3D structures are highly fluorescent.

High efficiency dye laser with low fluorescence yield pyrromethene dyes: experimental and theoretical studies

A combined experimental and theoretical study of the photo-physical, laser properties and molecular structures of three relatively recent Pyrromethene (PM) class dyes, PM597, PM580 and PM567, have been carried out. Laser characteristics of these three PM dyes were compared with three other widely used Rhodamine (RH) class dyes, RH6G, RHB and KRS, using a narrow-band dye laser setup, transversely pumped by the second harmonic (532 nm) of a Q-switched Nd-YAG laser. In addition to generating comparative data of these dyes for optimal use in dye lasers, we observed that unlike the RH dyes, the PM dyes show high efficiencies and wide tunability, despite the low fluorescence yield and high rate of non-radiative decay. Particularly, PM597 dye, in spite of a very low quantum yield of fluorescence (Φ=0.42), high non-radiative decay rate, and a large distortion from planarity in its excited state, when used in a laser cavity it exhibited similar laser efficiency and a beneficially wider tuning curve in comparison to other two PM dyes. Theoretical studies were carried out applying density functional theory and time-dependent density functional theory (DFT/TDDFT) to obtain new information on ground and the first excited state geometrical parameters of the PM dyes. Good correlation between calculated molecular properties and experimental results was observed for the evolution of the longest wavelength absorption maximum.

Bulk Heterojunction Photovoltaics Using Broadly Absorbing Small Molecules Based on 2-Styryl-5-phenylazo-pyrrole

Three new soluble small molecules (B, B6, and A) with a low band gap based on 2-styryl-5-phenylazo-pyrrole were synthesized. Molecules B and B6 contained pyrrole and N-hexylpyrrole, respectively, as the central unit, which was connected to N,N-dimethylphenyl-4-azo on one side of the pyrrole molecule. Molecule A contained N-hexylpyrrole as the central unit, which was connected to anthracenyl-9-azo on one side of the pyrrole molecule. The other side of the pyrrole molecule was connected to cyanovinylene 4-nitrophenyl for all molecules. The long-wavelength absorption maximum of the molecules was located at 601-637 nm, and their optical band gap was 1.62-1.67 eV. The photovoltaic properties have been investigated using blends of B, B6, or A with PCBM, and it was found that the device based on A:PCBM had a higher power conversion efficiency (PCE) (2.06%) than the devices based on B:PCBM (1.33%) and B6:PCBM (1.36%). This has been attributed to the higher hole mobility, the lower band gap of A relative to that of B or B6, and the higher energy difference between the LUMO of A and PCBM. The effect of solvent annealing and thermal-solvent annealing on the photovoltaic response of the device based on the A:PCBM blend has been investigated, and it was found that the devices based on solvent-treated and subsequent thermally annealed blends have PCEs of 2.56 and 2.83%, respectively. The increase in the PCE has been attributed to the enhanced crystallinity of the blend and the improvement in the charge transport due to a reduction in the difference between the electron and hole mobility in the blend.

Conjugated small molecules with broad absorption containing pyridine and pyran units: Synthesis and application for bulk heterojunction solar cells

Two new conjugated small molecules (SMs), M1 and M2, of low band gap with pyridine and pyran units, respectively, were synthesized by a convenient two-step reaction. They were soluble in common organic solvents and showed broad absorption ranging from 300 to 750 nm. Their long-wavelength absorption maximum was at 601–640 nm and the optical band gap was 1.60–1.67 eV. These SMs and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were used as donor and acceptor respectively to fabricate bulk heterojunction organic photovoltaic (OPV) devices through solution processing. The power conversion efficiencies (PCEs) for the OPV devices based on the as cast M1:PCBM and M2:PCBM are 1.42% and 1.80%, respectively. The higher PCE for the M2:PCBM based devices is attributed to the higher hole mobility, broader absorption spectra and lower band gap of M2 as compared to M1. The PCEs have been improved up to 1.75% and 2.18% for the solvent treated M1:PCBM and M2:PCBM blends, respectively. Finally, they have been further enhanced to 2.03% and 2.77% for the thermally annealed solvent treated M1:PCBM and M2:PCBM blends, respectively. This improvement has been attributed to the increase in the crystallinity of the blend and balance charge transport of the devices based on the solvent treated and thermally annealed solvent treated blends.

Synthesis and photovoltaic properties of an alternating phenylenevinylene copolymer with substituted-triphenylamine units along the backbone for bulk heterojunction and dye-sensitized solar cells

We have fabricated bulk heterojunction (BHJ) photovoltaic devices based on the as cast and thermally annealed P:[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) blends and found that these devices gave power conversion efficiency (PCE) of about 1.15 and 1.60% respectively. P is a novel alternating phenylenevinylene copolymer which contains 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid units along the backbone and was synthesized by Heck coupling. This copolymer was soluble in common organic solvents and showed long-wavelength absorption maximum at 390–420 nm with optical band gap of 1.94 eV. The improvement of PCE after thermal annealing of the device based on the P:PCBM blend was attributed to the increase in hole mobility due to the enhanced crystallinity of P induced by thermal treatment. In addition, we have fabricated BHJ photovoltaic devices based on the as cast and thermally annealed PB: P:PCBM ternary blend. PB is a low band gap alternating phenylenevinylene copolymer with BF 2–azopyrrole complex units, which has been previously synthesized in our laboratory. We found that the device based on this ternary blend exhibited higher PCE (2.56%) as compared to either P:PCBM (1.15%) or PB:PCBM (1.57%) blend. This feature was associated with the well energy level alignment of P, PB and PCBM, the higher donor–acceptor interfaces for the exciton dissociation and the improved light harvesting property of the ternary blend. The further increase in the PCE with thermally annealed ternary blend (3.48%) has been correlated with the increase in the crystallinity of both P and PB. Finally, we used copolymer P as sensitizer for quasi solid state dye-sensitized solar cell and we achieved PCE of approximately 3.78%.

Symmetrical molecules of low band gap with a central spacer connected via ether bond with terminal 4-nitro-α-cyanostilbene units: Synthesis and application for bulk heterojunction solar cells

Two new soluble and symmetrical molecules M1 and M2 of low band gap with a central spacer which was connected at both sides via ether bond with terminal 4-nitro-α-cyanostilbene units were synthesized. The spacer of M1 and M2 consisted of dihexyloxyphenylene and n-hexylene, respectively. Their long-wavelength absorption maximum was at 590–640 nm. The thin film absorption onset for both molecules was located at 742 nm which corresponds to an optical band gap of 1.67 eV. We have fabricated bulk heterojunction (BHJ) photovoltaic devices using these molecules as donor and PCBM as acceptor. We have investigated the solvent vapor treatment effect of these blends on their morphology and photovoltaic properties. We found that the overall power conversion efficiency (PCE) for the devices based on the solvent vapor treated M1:PCBM and M2:PCBM blends is 3.05% and 1.90%, respectively, higher than those of pristine blends. This increase of PCE has been attributed to the increase in surface roughness of the blend and better balance in charge transport. The PCE has been further increased up to 3.57% and 2.42% with the thermally annealed solvent treated M1:PCBM and M2:PCBM blend, respectively. This increase of PCE may be attributed to the enhanced crystallinity of the blend and reduction of the space charge effect, improving the charge transport and collection efficiency.