Photovoltaic effect in arylenevinylene-co-pyrrolenevinylene (AVPV)

Arylenevinylene-co-pyrrolenevinylene (AVPV) is a promising candidate amongst the group of new photovoltaic materials. It is a low band gap organic material with a band gap of 1.84 eV and absorbs sunlight in 300-700 nm range. In this paper, we demonstrate the photovoltaic effect in an organic bulk heterojunction photovoltaic device based on the blend of AVPV as an electron donor and [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) as the acceptor. The short-circuit current density of the device was of the order of 0.55 μA cm⁻² with an open-circuit voltage of 0.7 V, measured under 1 sun illumination of AM 1.5 through a calibrated solar simulator. Fill factor was estimated to be 12%. Further, the tests conducted after 2 weeks showed that short-circuit current was 0.21 μA cm⁻² and open-circuit voltage was 0.5 V with a fill factor of 9.8%, suggesting the possibility of stable AVPV-based organic solar cell (OSC).     

A study of the factors influencing the performance of ternary MEH-PPV:porphyrin:PCBM heterojunction devices: A steric approach to controlling charge recombination

A series of three porphyrins varying only in the steric bulk of their peripheral groups have been synthesised and integrated into the active layer of bulk heterojunction solar cells. The porphyrins broaden the spectral response of the device and contribute to the total photocurrent generated. More importantly, the device characteristics change systematically with increasing steric bulk on the peripheral meso phenyl groups of the tetra-phenyl porphyrin. Optical, scanning probe and scanning transmission X-ray microscopy are used to demonstrate that the observed changes do not arise from morphological differences in film structure. Cyclic voltammetry, UV-vis spectroscopy and DFT calculations are used to establish that the porphyrin LUMO, HOMO and bandgap are independent of side group. We conclude that the variations in open-circuit voltage with side group are the result of the porphyrin acting as a bimolecular recombination centre, with an efficiency that is dependent on the side group type. The possibility of designing optimised macromolecules for OPV devices based on an understanding the effect of porphyrin steric bulk upon device performance is discussed.     

Anthraquinone dyes as photosensitizers for dye-sensitized solar cells

Three anthraquinone dyes with carboxylic acid as anchoring group are designed and synthesized as sensitizers for dye-sensitized solar cells (DSSCs). Preliminary photophysical and photoelectrochemical measurements show that these anthraquinone dyes have very low performance on DSSC applications, although they have broad and intense absorption spectra in the visible region (up to 800 nm). Transient absorption kinetics, fluorescence lifetime measurements and density functional theory (DFT) calculations are conducted to investigate the cause of such low DSSC performance for these dyes. The results show that the strong electron-withdrawing character of the two carbonyl groups on anthraquinone framework may lie behind the low performance by suppressing the efficient electron injection from the dye to the conduction band of TiO₂.     

Fabrication and characterization of 4-tricyanovinyl-N,N-diethylaniline/p-silicon hybrid organic–inorganic solar cells

Hybrid organic–inorganic solar cell was fabricated by thin film of 4-tricyanovinyl-N,N-diethylaniline deposited on p-Si substrates. The capacitance–voltage characteristics indicated that the junction is of abrupt nature. The dark forward current density-voltage characteristics indicated a tunneling conduction at relatively low voltages followed by a space-charge-limited-conduction mechanism at relatively high voltages. Under illumination, the cell exhibits photovoltaic characteristics with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.15 mA cm⁻², and a power conversion efficiency of 3.10%. The effect of γ-rays irradiation (100 kGy absorbed dose) on the characteristics of the cell was also investigated. The fill factor and the power conversion efficiency decrease by 20.9% and 39% of the original value, respectively.     

Corrosion inhibition, hydrogen evolution and antibacterial properties of newly synthesized organic inhibitors on 316L stainless steel alloy in acid medium

Electrochemical corrosion behavior and hydrogen evolution reaction of 316L stainless steel has been investigated, in 0.5 M sulfuric acid solution containing four novel organic inhibitors as derivatives from one family, using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) measurements and surface examination via scanning electron microscope (SEM) technique. The effect of corrosion inhibitors on the hydrogen evolution reaction was related to the chemical composition, concentration and structure of the inhibitor. The inhibition efficiency, for active centers of the four used compounds, was found to increase in the order: -Cl < -Br < -CH₃ < -OCH₃. The corrosion rate and hydrogen evolution using the compound with methoxy group as a novel compound was found to increase with either increasing temperature or decreasing its concentration as observed by polarization technique and confirmed by EIS measurements. The compound with methoxy group (newly synthesized) has very good inhibition efficiency (IE) in 0.5 M sulfuric acid (98.3% for 1.0 mM concentration). EIS results were confirmed by surface examination. Also, antibacterial activity of these organic inhibitors was studied. The results showed that the highest inhibition efficiency was observed for the compound that posses the highest antibacterial activity.     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.     

On the use of Woods metal for fabricating and testing polymeric organic solar cells: An easy and fast method

In this work, we propose the use of Woods metal, which is an eutectic alloy of Pb/Bi/Cd/Sn (25%, 50%, 12.5% and 12.5%, respectively), as a convenient substitute to evaporated aluminum for testing polymeric organic photovoltaic (OPVs) devices. The Woods metal, which melts at 75 °C, was used as cathode and it permits an easy and quick device implementation that can be performed through a vacuum free process. As anode, the commercial and transparent indium tin oxide (ITO) deposited on glass slides was used. OPVs cells were prepared based on 6-nitro-3-(E)-3-(4-dimethylaminophenyl)allylidene)-2,3-dihydrobenzo[d]-[1,3,2]-oxazaborole (M1) and (E)-3-(4-dimethylaminophenyl)allylidene)-2,3-dihydrobenzo[d]-[1,3,2]-oxazaborole (M2), which are conjugated organo-boron molecules, the well known photoconductor polymer MEH-PPV, and the fullerene PC₆₁BM as the sensitizer. M1 and M2 were mixed with MEH-PPV to enhance the absorption of the OPVs devices. The OPVs cell electrical performance is acceptable considering the fast evaluation of promising materials.     

Sulfides organic polymer: Novel cathode active material for rechargeable lithium batteries

Two novel sulfide polymers, poly(2-phenyl-1,3-dithiolane) and poly[1,4-di(1,3-dithiolan-2-yl)benzene], were prepared via facile oxidative-coupling polymerization under ambient conditions, characterized by FT-IR, XRD, TGA and elemental analysis, and were tested as cathode materials in rechargeable lithium battery. The charge–discharge tests showed that the specific capacity of poly[1,4-di(1,3-dithiolan-2-yl)benzene)] was 378 mAh g⁻¹ at the third cycle, and retained at 300 mAh g⁻¹ after 20 cycles. The specific capacity of poly(2-phenyl-1,3-dithiolane) was 117 mAh g⁻¹ at the second cycle, and retained at 100 mAh g⁻¹ after 20 cycles. The results indicated that thiolane group could be used as cathode active function group for lithium secondary batteries and the novel electrode reaction is proposed tentatively.     

Efficiency enhancement in large-area organic photovoltaic module using theoretical power loss model

For efficiency enhancement of a large-area monolithic organic photovoltaic (OPV) module, we studied the influence of the OPV cell geometry parameters using theoretical and experimental methods. For this work, a unit OPV cell as a reference device and four types of monolithic OPV module with different active cell lengths were fabricated together on a glass substrate. The characteristics of the fabricated unit OPV cell were measured and the voltage (V_mp) and current density (J_mp) at the maximum power point were extracted. The parasitic power losses were calculated from the extracted parameters and the material parameters using a theoretical power loss model, taking into consideration the series resistance, contact resistance, and shading (or dead area) losses at the calculated maximum power of the monolithic OPV module. To analyze the influence of OPV cell layout on efficiency of the large-area monolithic OPV module, the power conversion efficiency of the four type monolithic OPV modules with different active cell lengths was measured and compared with the calculated power conversion efficiency. The calculated PCE ratio of the monolithic OPV module with three cells was approximately 78%, and the measured PCE ratio of the fabricated monolithic OPV module with three cells was also approximately 78%. The measured PCE ratio of fabricated monolithic OPV modules with two, four, and five cells also exhibited this tendency for the calculated PCE ratio. Thus, a large-area monolithic OPV module with optimum electrical power loss and an appropriate number of OPV cells can be designed by extracting the parameters of the unit OPV cell and calculating the electrical power loss using the proposed theoretical power loss model.     

The efficient acetoxy-group-based additives in protecting of anode in the rechargeable aluminium-air batteries

The inhibitive effects of functional groups originating from the presence of additives on corrosion are linked to the physicochemical and electronic properties of these surficial chemical species. In this work, the acetoxy group (i.e. weak bases) with different orbital characters is integrated with donating/accepting molecules which reveals dissimilar inhibitive behaviour. Introducing cations on the acetoxy groups (-Ac) as additives leads to mitigation of undesirable self-corrosion of the anode. The Al-air battery corresponded to a maximum capacity of 2817 mAhg^?1 at 10 mAcm^?2 with the use of barium-Ac/KOH electrolyte, demonstrates nearly 95% of the theoretical capacity. However, a capacity of only 2392 mAhg^?1 is observed for free additive. In addition, the maximum power density (i.e. 91.32 mWcm^?2) increased by nearly 50% with the employment of barium-Ac/KOH. The rechargeable Al–air battery based on barium-Ac/KOH presents a low voltage gap of 0.83 V with a remarkable cyclic stability of 25,000 s. The results show that the acetoxy group integrated with metal ions can be a key player in reducing H₂ evolution rate in the aluminium-air batteries. XPS analysis confirmed an Al-complex (i.e. Al-acetoxy) is formed using barium-ac. DFT calculations confirmed that barium-Ac/KOH provides the minimum value of ΔE (about 5.4 eV), indicating the remarkable ability for reacting with Al⁺³ ions to adsorb on the anode surface leading to complex (Ba-Ac-Al³⁺) formation. This study highlights the parasitic corrosion reaction leading to low coulombic efficiency in rechargeable Al- air batteries.     

Synthesis and photovoltaic properties of a star-shaped molecule with triphenylamine as core and benzo[1,2,5]thiadiazol vinylene as arms

A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D-A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D-A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.     

Stable dye-sensitized solar cells based on organic chromophores and ionic liquid electrolyte

A series of polyene-diphenylaniline based organic dyes (coded as D5, D7, D9 and D11) have been reported for the application in ionic liquid electrolyte based dye-sensitized solar cells. The effects of substitution of organic dyes on the photovoltaic performance have been investigated, which show addition of methoxy groups on the triphenylamine donor group increases short-circuit current, open-circuit voltage and photovoltaic performance. A power conversion efficiency of 6.5% under AM 1.5 sunlight at 100 mW/cm² has been obtained with D11 dye in combination with a binary ionic liquid electrolyte, which when subjected to accelerated testing under one sun light soaking at 60 °C, the efficiency remained 90% of initial efficiency.     

Surfactant-free CdTe nanoparticles mixed MEH-PPV hybrid solar cell deposited by spin coating technique

Hybrid absorber layers have been deposited using spin coating technique (using surfactant-free CdTe nanoparticles mixed with poly (2-methoxy, 5-(2-ethyl-hexyloxy)-p-phenyl vinylene) (MEH-PPV). The blend solution is heated at ∼50 °C prior to the thin film deposition to achieve proper dispersion of CdTe nanoparticles in MEH-PPV. ¹HNMR spectra confirm the chemical attachment of CdTe nanoparticles to alkoxy group of MEH-PPV. For the film deposited with weight ratio CdTe:MEH-PPV (40:3), the absorbance extends from 350 nm to near-IR region and the effect of charge transfer complex (CTC) is also seen. Photoluminescence (PL) measurement confirms the PL quenching for the hybrid layer which is a measure of the degree of dispersion of nanoparticles in the MEH-PPV matrix and also confirms that the required dissociation of excitons is taking place. The solar cell prepared with the CdTe mixed MEH-PPV hybrid absorbing layer shows a power conversion efficiency of ∼0.06%.     

Improvement of the photovoltaic properties of polythiophene-based cells

The photovoltaic properties of organic cells based on a polythiophene film have been studied. An external power efficiency of 10⁻⁴% has been obtained in the pristine polymer diode. In order to improve this result, three complementary routes have been investigated. Optical sensitization of the polythiophene with a dye of the indane group leads to a 75 increase in power efficiency. A hybrid molecular/polymeric p–n type junction instead of the original Schottky type provides a 0.15% efficiency. Molecular rectification in oriented polymers incorporating diode-like chromophores increases the PV efficiency by a factor of 40.     

ITO-free flexible organic solar cells with printed current collecting grids

The presence of a transparent conductive electrode such as indium tin oxide (ITO) limits the reliability and cost price of organic photovoltaic devices as it is brittle and expensive. Moreover, the relative high sheet resistance of an ITO electrode on flexible substrates limits the maximum width of a single cell. We have developed an alternative ITO-free transparent anode, based on solution processed high conductive PEDOT:PSS in combination with a printed current collecting grid. The screen printed silver grid demonstrates a typical sheet resistance of 1 Ω/□ with 6.4-8% surface coverage. The efficiency of a flexible device with an active area of 4 cm² with such a grid is much higher than a similar device based on ITO. Furthermore, as this composite anode is solution-processed, it is a step forward towards low-cost large area processing.     

New organic discotic materials for photovoltaic conversion

Two-layer organic photovoltaic cells have been fabricated using a triphenylene ether as a hole transporting material, and perylene derivatives as electron transporting materials. Three devices were studied and showed external quantum efficiencies of around 3%. These results are interpreted in the context of electrochemical measurements that provide the ionization potential and electron affinity. Furthermore, the high-exciton diffusion lengths and absorption coefficients contribute to the high-observed photocurrents. The organic/organic interface was found to be the main origin of the photocurrent generation. However, the photovoltaic parameters were found to be dependent also on the ITO/organic interface. In particular, we show that ITO treatments with argon plasma and UV–ozone modify the open-circuit voltage.     

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.     

Enhancement of photovoltaic characteristics using a PEDOT interlayer in TiO2/MEHPPV heterojunction devices

The effect of inserting a PEDOT interlayer between the MEHPPV layer and the Au electrode of a nanocrystalline ITO/TiO₂/MEHPPV/Au heterojunction device on the photovoltaic characteristics of the device has been studied. The MEHPPV layer has both a light-sensitizing role and a hole-transporting function. The overall conversion efficiency of the device with a PEDOT layer is better by more than 80% than that obtainable without a PEDOT layer. The modified device shows improved photocurrent density–voltage (J–V) characteristics, in that there is a strong reduction of the roll-over behavior in the forward bias region, and an increase in the fill factor. These improvements are due to the reduction of junction resistance across the MEHPPV/Au interface in the presence of the PEDOT interlayer, which results in improved hole injection.     

Organic solar cells based on the spin-coated blend films of TPA-th-TPA and PCBM

In this paper, the optical and photo-induced electron transfer properties of a new conjugated molecule, 4,7-bis{5′-[4″,4″-N,N-diphenylamino-styryl]thiphen-2′-yl} -benzo[1,2,5-thiadiazole] (simplified as TPA-th-TPA), were investigated. Using TPA-th-TPA as a photoactive layer, organic solar cells with three different architectures were fabricated by spin-coating method. The photosensitive layers of these architectures comprise pure TPA-th-TPA layer, heterojunction of bi-layered TPA-th-TPA and C₆₀, and bulk-heterojunction of TPA-th-TPA and [6, 6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend. Furthermore, towards the bulk-heterojunction devices, the effect of the cathode materials (Mg, Ca, LiF/Al, Ba) on the performance of the devices was studied. The power conversion efficiency reached 0.26% for the device based on the blend of TPA-th-TPA and PCBM with Ba/Al as the cathode.     

Progress in stability of organic solar cells exposed to air

In this paper, the stability of small-molecule organic solar cells based on copper phthalocyanine (CuPc) and fullerene (C₆₀) is investigated. The use of silver instead of aluminum as the metal electrode in these solar cells, with smaller grain size and grain boundaries as well as with more uniform grain size distribution in the film, results in significant improvement in the lifetime of the devices. The substantial role of silver in the protection of the cells against permeation of oxygen and/or water molecules into the organic thin films is confirmed. Substitution of a thin buffer layer (70 Å) of bathophenanthroline (Bphen) for bathocuproine (BCP), sandwiched between C₆₀ and the cathode, makes considerable progress in the lifetime of the device.