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.     

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.     

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.     

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 BF₂-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%.     

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.     

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.     

Novel symmetric squaraine chromophore containing triphenylamine for solution processed small molecule bulk heterojunction solar cells

New symmetrical low band-gap small molecule materials, SQ-bis[HP-HT2-TPA] and SQ-bis[HP-HT2-BT] incorporating as novel derivative of squaraine-pyrrole framework and π-extended thiophene with triphenyamine (TPA) and benzothiophene (BT) end group, have been synthesized and characterized. The effects of TPA moiety were investigated. Compared to SQ-bis[HP-HT2-BT], SQ-bis[HP-HT2-TPA] exhibited three times improved transporting property of hole carrier and four times enhanced absorptivity by more efficient intermolecular π−π interaction for high-efficiency bulk heterojunction (BHJ) device, suggesting that TPA contributes to a better hole mobility. The bulk-heterojunction photovoltaic devices fabricated with SQ-bis[HP-HT2-TPA]/C₇₁-PCBM BHJ film had an average power-conversion efficiency of 1.83%(±0.12) under 100 mW/cm², with a short-circuit current (J_sc) of 9.32 mA/cm², fill factor (FF) of 0.30, and open-circuit voltage (V_oc) of 0.65 V, which has ∼42% higher efficiency compared to SQ-bis[HP-HT2-BT]/C₇₁-PCBM BHJ films.     

Two-dimensional regioregular polythiophenes with conjugated side chains for use in organic solar cells

We have prepared two two-dimensional polythiophenes (2D-PTs; P1 and P2) possessing alkyl-thiophene side chains by Stille coupling reactions. Optical measurements indicate that the bandgaps of P1 and P2 being 1.98 and 1.77 eV, respectively. P2 displayed a red-shift in its absorption spectrum because of the longer length of its conjugated side chains. Desirable highest occupied molecular orbital (HUMO) and lowest unoccupied molecular orbital (LUMO) energy levels were obtained from electrochemical studies, which suggested that these systems would exhibit high open-circuit voltages when blended with fullerene as electron acceptors. The hole mobility (thin film transistor (TFT) measurement) of P1 and P2 are 3.5×10⁻⁴ and 4.6×10⁻³ cm² V⁻¹ s⁻¹, respectively. A power conversion efficiency of 2.5% is obtained under simulated solar illumination (AM 1.5G, 100 mW cm⁻²) from a polymer solar cell comprising an active layer containing 25 wt% P1 and 75 wt% [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM).     

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.     

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.     

Synthesis and organic photovoltaic (OPV) properties of triphenylamine derivatives based on a hexafluorocyclopentene “core”

A series of new “D-A-D” chromophores containing hexafluorocyclopentene thiophene as an acceptor and a triphenylamine unit as a donor, called TP-G1, TP-G2, TPB-G1 and TPB-G2, were designed and synthesized. Heterojunction organic photovoltaic (OPV) cells containing these chromophores were fabricated, and device 1, with the structure of ITO/PEDOT:PSS/TP-G1:P3HT/LiF/Al, displayed an open-circuit voltage (V_oc), short-circuit current (J_sc) and power-conversion efficiency (η) of 0.74 V, 1.178 mA/cm² and 0.22%, respectively. The triphenylamine group could effectively induce the open-ring isomer to close because the 4- and 4′- positions of the benzene rings were substituted by an electron-donating group and the value of the quantum yields of the closed-ring isomers increased. As a result, the closed-ring isomer facilitated intramolecular π-electron delocalization and exhibited a broad absorption band ranging from 200 to 850 nm. Due to the fluorine substitution of hexafluorocylopentene at the molecular center and the hole-transport characteristics of the triphenylamine moiety on the periphery, our chromophores showed obvious dual semiconductor properties, i.e., n- and p-type, which demonstrated a potential application for OPV devices.     

Bulk heterojunction organic photovoltaic devices based on low band gap small molecule BTD-TNP and perylene-anthracene diimide

Bulk heterojunction (BHJ) photovoltaic devices were fabricated from the blends of compounds BTD-TNP (electron donor) and P-A (electron acceptor) and characterized through current-voltage measurements under illumination. Compound BTD-TNP contains dithyenyl-benzothiadiazole central unit and cyanovinylene-p-nitrophenyl terminal moieties. Compound P-A is a symmetrical perylene-anthracene diimide with tert-butylphenoxy side groups at the 1,7-bay positions. Both the absorption spectra and the incident photon to the current conversion efficiency (IPCE) spectra of the device were extended up to 800 nm. A power conversion efficiency of 2.85% with short-circuit current density of 6.8 mA/cm², open-circuit voltage of 0.88 V and fill factor of 0.47 were obtained. It was found that the hole and electron mobility in the active layer were about 4.6×10⁻⁵ and 8.8×10⁻⁴ cm²/Vs, respectively, which indicates the fairly balanced charge transport in the device.     

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%.     

Syntheses and photovoltaic properties of polymeric sensitizers using thiophene-based copolymer derivatives for dye-sensitized solar cells

We synthesized the thiophene-based copolymers (P(3TAF-co-3TAa)-A-n and P(3TAF-co-3TAa)-B-n) using two different kinds of thiophene monomers, (N-(3-thienylmethylene)-2-aminofluorene and 3-thiophene acetic acid), as sensitizers on the DSSCs. P(3TAF-co-3TAa)-A-n (n=1, 2, 3) was synthesized with different molar ratios (3TAF:3TAa=1:5, 1:10, 1:20) of monomers at room temperature, respectively. Also, P(3TAF-co-3TAa)-B-n (n=1, 2, 3) was synthesized with above molar ratios of monomers at 0 °C, respectively. The DSSCs devices were fabricated using the thiophene-based copolymers as sensitizers and their photovoltaic performances were measured by using a solar simulator under AM 1.5. In the DSSCs devices using polymeric sensitizers, V_oc is 0.53-0.60 V, J_sc is 1.9-4.5 mA/cm², FF is 0.51-0.63 and the power conversion efficiency is 0.63-1.53%, respectively.     

Solution processable quinacridone based materials as acceptor for organic heterojunction solar cells

Two series of novel quinacridone (QA) based materials that combined a strong absorption over a broad range in visible region with good electrical characteristics, which were used as the new electron-accepting materials for organic solar cells, are explored. Unique cyclic compounds 1-6 are synthesized by incorporating electron withdrawing groups (CN, COOH) at carbonyl position of alkyl substituted quinacridones, which lead to the compounds possessing the characteristics of solution-processed and being suitable for photovoltaic applications. Heterojunction solar cells with simple device configuration using these soluble materials as acceptor and effective donor poly (3-hexyl thiophene) (P3HT) were fabricated. The maximum power conversion efficiency (PCE) achieved in the solar cell based on compound 5 is 0.42% under simulated AM 1.5 solar irradiation with J_sc=1.80 mA cm⁻², V_oc=0.50 V and FF=47%. Although the aimed devices just exhibit moderate PCE, our results clearly suggest that the new-type electron-accepting materials different from fullerene have great potential as acceptor in heterojunction solar cell due to many advantages of the QA derivatives such as relatively inexpensive, good electrochemical stability and could be readily modified.     

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.     

New compounds in the potassium-aluminium-hydrogen system observed during release and uptake of hydrogen

Three new compounds are observed in the potassium-aluminium-hydrogen system, which are characterised using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis (TG and DSC) and Siverts measurements (PCT). All three new compounds (denoted 1, 2 and 3) are observed during release and uptake of hydrogen in the potassium-aluminium-hydrogen system and may be new intermediates. All three compounds were indexed and the following unit cells were found, 1: cubic, a = 17.0248(9) Å, 2: cubic, a = 14.2746(4) Å and 3: orthorhombic, a = 10.46(1), b = 6.661(6) and c = 6.173(5) Å. Formation and observation of 1, 2 and 3 depends on the mechano-chemical sample preparation (ball milling), temperature, (heating rate), and hydrogen pressure (and temperature for pressure change). Compound 1 is often observed in the temperature range 55-150 °C for a medium ball-milled sample heated under a constant hydrogen pressure of 50 bar 1 is clearly an intermediate for formation of KAlH₄ and may have the composition K_yAlH_x with 1 ≤ y ≤ 3 and 4 ≤ x ≤ 6. Applying hydrogen pressure abruptly at elevated temperatures leads to faster hydrogenation, which can then be performed at lower hydrogen pressures. In the latter case, 1 is only observed shortly in a few PXD patterns. Compounds 2 and 3 are mainly observed during dehydrogenation of KAlH₄ in the temperature ranges of ca. 200 to 350 °C and 200 to 390 °C respectively, as relatively weak Bragg diffraction peaks.     

Effects of hydrogen addition on cellular instabilities of the spherically expanding propane flames

An experimental study on the effects of hydrogen addition on the instabilities of spherically expanding propane–air flames was conducted in a constant volume combustion vessel over a wide range of mixture compositions and initial temperatures and pressures. The measured laminar burning velocities were compared with those calculatedvalues by using one dimensional freely propagating flames and a recently developed detailed kinetic mechanism. Goodagreementwas obtained between the experiment and calculation. The schlieren images show that for lean mixture combustion, hydrogen addition willincrease the hydrodynamic instability due to the decreased flame thickness and increase the diffusional-thermal instability due to the decreased Lewis number. While forrich mixture combustion, the flame front is initially destabilized and later tends to the stabilized with the increase of hydrogen fraction. This is due to the competing effects of the hydrodynamic instability and the diffusional-thermal instability.     

Electrochemical and chemical enrichment methods of a sodic–saline inoculum for microbial fuel cells

In microbial fuel cells (MFCs) efficient extracellular electron transfer microbes, also known as anode-respiring bacteria, play an important role on cell performance. This type of microbes can be developed by application of enrichment procedures. The objective of this study was to compare a chemical (only C, final terminal electron acceptor Fe(III)), an electrochemical (only E), and a hybrid method (H, i.e., E followed by 3 serial transfers in iron (III) citrate medium) enrichment methods departing from a saline–sodic soil inoculum. In the electrochemical enrichment procedure in an electrolysis cell, the inoculum was subjected to a continuous electrical stress continually by posing the cell at −150 mV/SCE (+94 mV/SHE). The only C enrichment method delivered powers superior to the only E one (higher values of P_An,max = 49 mW m⁻² and P_V,max = 558 mW m⁻³ of C compared to 33 and 379 of only E). Interestingly, overall resistance as determined by EIS was lower for only E (1240 Ω) than for only C (1632 Ω). Yet, the hybrid H method, showed electrochemical characteristics consistently superior to both only C and only E methods (higher P_An,max and P_V,max, lower internal resistance). Further detailed electrochemical studies of only E-method showed that the anodic resistance decreased with the time of operation of the electrolysis cell that would be consistent with the adaptability/enrichment purpose of the method. Also, Cyclic voltammetry peaks with values close to those reported for bacterial cytochromes appeared with time of cell operation.