Film morphology effects on the electrical and optical properties of bulk heterojunction organic solar cells based on MEH-PPV/C60 composite

The influence of film morphology on the electrical behaviour of an MEH-PPV/C₆₀ organic solar cells has been investigated. The dissociation of photogenerated charge pairs in composites of buckminsterfullerenes (C₆₀) in a conjugated polymer matrix (MEH-PPV) forming dispersed heterojunctions was studied at low C₆₀ acceptor concentrations to separate electron transfer from charge transport effects. The motivation of this study was to analyse the strong dependence of organic solar cell efficiencies on the morphology of the composite. Two effects controlling film morphology have been investigated; the first one being the influence of the fullerene concentration and the second one is the effect of the organic solvent used to deposit the photoactive layer. The sample morphology was studied using atomic force microscopy (AFM). Photoluminescence (PL) experiments and current–voltage (I–V) measurements were performed on the deposited photovoltaic film to investigate the influence of dispersion on the charge transfer process between MEH-PPV and C₆₀. An attempt to explain all the results will be presented.     

Preparation and characterization of (100 − x) TiO<Subscript>2</Subscript> + (x)ZnO nanocomposites for dye-sensitized solar cells using <Emphasis Type="Italic">Beta vulgaris</Emphasis> and <Emphasis Type="Italic">Syzygium cumini</Emphasis> natural dye extract

The present paper attempts to report the preparation of TiO₂–ZnO nanocomposite photoanode materials for dye-sensitized solar cells (DSSC) and analyse the efficiency of DSSC with natural dyes. The structural and optical characteristics of the composites were studied by transmission electron microscopy, X-ray diffraction, field effective scanning electron microscopy, energy dispersive spectrometry, photoluminescence and absorption spectroscopy. The synthesized nanocomposites formed on FTO substrates are applied as photoanode in a dye-sensitized solar cell (DSC). The natural dyes extracted from Beta vulgaris (Beetroot) and Syzygium cumini (black plum) were used in the fabrication of DSSC. The solar cells’ photovoltaic performance in terms of short-circuit current, open circuit voltage, fill factor and energy conversion efficiency was tested with photocurrent density–voltage measurements. The evolution of the solar cells parameters is explored as a function of the photoanode and type of dye used in DSSC fabrication.The obtained results show that the efficiency of DSSC significantly changes with the addition of ZnO to TiO_2, while the Beta vulgaris dye has evidently shown higher photo sensitized performance compared to Syzygium cumini in the preparation of DSSC.     

Probing mechanism of dye double layer formation from dye-cocktail solution for dye-sensitized solar cells

Absorption of photon in wide wavelength region is an important requirement for the enhancement of photoconversion efficiency of dye-sensitized solar cells (DSSC). Lack of photon absorption from visible to NIR wavelength region by a single dye requires the use of plural dyes for the panchromatic sensitization of nanoporous TiO₂. To our incredible surprise, when a dye cocktail of organic dye NK3705 and inorganic ruthenium based dye Z907 was implied for the dye adsorption, it led to the formation of dye double layer in spite of random arrangement of two dyes as evidenced from confocal laser microscopic investigations. Investigation pertaining to the evaluation of rate of dye adsorption and dye desorption for different organic and inorganic sensitizing dyes suggests that a combination of one dye with faster diffusion along with weak binding on TiO₂ surface and another dye with slow diffusion along with strong binding leads to the formation of dye double layer from a dye mixture by a simple dipping process.     

Novel D-π-A dye sensitizers of polymeric metal complexes with phenylethyl or carbazole derivatives as donors for dye-sensitized solar cells: synthesis,characterization, and photovoltaic application

Four novel polymeric metal complexes containing D-π-A type structures were synthesized, characterized and applied as dye sensitizers in dye-sensitized solar cells (DSSCs). The alkoxy benzene or carbazole (CZ) derivative acts as an electron donor (D), C=C acts as π-bridge (π) and the 8-hydroxyquinoline derivative complex acts as electron acceptor. Bipyridine derivative was ancillary ligand as well as providing anchoring group. FT-IR, gel permeation chromatography, thermogravimetric analyses, differential scanning calorimetry, UV–Vis absorption spectroscopy, Elemental analysis, cyclic voltammetry, J-V curves and input photon to converted current efficiency plots were introduced to investigate the four dyes. These dyes exhibit good thermal stability with 5 % weight loss at temperatures (T_d) of around 300 °C. For the DSSCs devices using dyes with CZ derivatives as electron D (P2, P4) exhibited higher power conversion efficiency (PCE) than that with alkoxy benzene (P1, P3). The DSSC based on P4 exhibited the highest PCE value of 2.42 % (J _sc  = 4.93 mA/cm², V _oc  = 0.73 V, FF = 67.2 %) under AM 1.5G solar irradiation. This indicates a new way to design dye sensitizers for DSSCs.     

Influence of ZnO nanoparticles on Coumarin-503 and Coumarin-540 dye mixture for energy transfer distributed feedback dye lasers

Abstract

Using organic dyes Coumarin-503 (C503) and Coumarin-540 (C540) as donor and acceptor dyes, respectively, and Nd-YAG as pumping source (355 nm), an energy transfer-distributed feedback dye laser (ETDFDL) was constructed and its characteristics studied. Theoretical studies such as critical transfer radius (R_o), critical concentration (C_o), and half quenching concentration (C_1/2) were carried out using the absorption and fluorescence spectra of donor and acceptor dyes. On varying the input pump energy to the nanoparticle-incorporated ETDFDL and keeping the acceptor and donor dye concentrations constant, the lasing output obtained was found to be higher than that without the use of nanoparticles. This enhancement was due to the size, shape, and coupling between nanoparticles with the dye mixture. Tunability in the range of 435–553 nm was obtained for both donor (C503) and acceptor (C540) DFDL as a function of the angle of interfering beams of the pump laser.     

Synthesis and characterization of mesoporous composite silica monolith

Coumarin4 (Cou4) was loaded into mesoporous silicon SBA-15 monolith channels by a simple postgrafting method at room temperature. The obtained Cou4/SBA-15 composite monolith was compared with the blank SBA-15 monolith. The investigation of X-ray diffraction, transmission electron microscopy (TEM), UV–vis and photoluminescence (PL) reveals the existence of cou4 in the channels of SBA-15 monolith. In addition, a red-shift and strong fluorescence in PL spectrum was observed from the composite monolith compared with that of Cou4 in C₂H₅OH solution. This is good for the new application of optical materials.     

Comparison of photostability, optical and structural properties of TiO2/conjugated polymer hybrid composites prepared via different methods

In this work, we have synthesized high quality TiO₂ nanocrystallites by sol-gel method (TiO₂ white (w)) and compared its properties with the ones synthesized by the simple hydrolysis method in aqueous solution (TiO₂ transparent (t)). The TiO₂/MEH-PPV nanocomposites are formed mainly by two ways: (i) Prepared in the form of the colloidal solution by adding the known concentration of the TiO₂ in MEH-PPV and then sonicate it well; (ii) In the thin film form by depositing the above solution over a glass substrate by spin coating. The properties of the resulting dispersions could be tailored by varying the composition and concentration of TiO₂ nanoparticles in CP's. The TiO₂ nanoparticles prepared by both methods show anatase character of TiO₂ as elucidated by X-ray diffraction (XRD) studies. Transmission electron microscopic (TEM) studies reveal that the transparent colloidal suspension of TiO₂ exhibits agglomeration of TiO₂ nanoparticles (size ~ 150-300 nm) and this trend is maintained in the MEH-PPV matrix for TiO₂/MEH-PPV composites as well. However, the composite obtained by mixing MEH-PPV with sol-gel prepared TiO₂(w) shows uniform nanoscale dispersion of TiO₂ (size ~ 20 nm) in MEH-PPV matrix. The UV-VIS absorption, photoluminescence (PL) and lifetime studies confirm the presence of dynamic quenching effect indicating efficient photoinduced charge transfer in TiO₂/MEH-PPV hybrid composites particularly with white TiO₂. It is conjectured that the devices containing TiO₂/MEH-PPV composites for TiO₂ prepared by the sol-gel method should lead to significant improvement in the photovoltaic performance of TiO₂/MEH-PPV hybrid solar cells.     

Rose bengal-sensitized nanocrystalline ceria photoanode for dye-sensitized solar cell application

For efficient charge injection and transportation, wide bandgap nanostructured metal oxide semiconductors with dye adsorption surface and higher electron mobility are essential properties for photoanode in dye-sensitized solar cells (DSSCs). TiO₂-based DSSCs are well established and so far have demonstrated maximum power conversion efficiency when sensitized with ruthenium-based dyes. Quest for new materials and/or methods is continuous process in scientific investigation, for getting desired comparative results. The conduction band (CB) position of CeO₂ photoanode lies below lowest unoccupied molecular orbital level (LUMO) of rose bengal (RB) dye. Due to this, faster electron transfer from LUMO level of RB dye to CB of CeO₂ is facilitated. Recombination rate of electrons is less in CeO₂ photoanode than that of TiO₂ photoanode. Hence, the lifetime of electrons is more in CeO₂ photoanode. Therefore, we have replaced TiO₂ by ceria (CeO₂) and expensive ruthenium-based dye by a low cost RB dye. In this study, we have synthesized CeO₂ nanoparticles. X-ray diffraction (XRD) analysis confirms the formation of CeO₂ with particle size ～7 nm by Scherrer formula. The bandgap of 2.93 eV is calculated using UV–visible absorption data. The scanning electron microscopy (SEM) images show formation of porous structure of photoanode, which is useful for dye adsorption. The energy dispersive spectroscopy is in confirmation with XRD results, confirming the presence of Ce and O in the ratio of 1:2. UV–visible absorption under diffused reflectance spectra of dye-loaded photoanode confirms the successful dye loading. UV–visible transmission spectrum of CeO₂ photoanode confirms the transparency of photoanode in visible region. The electrochemical impedance spectroscopy analysis confirms less recombination rate and more electron lifetime in RB-sensitized CeO₂ than TiO₂ photoanode. We found that CeO₂ also showed with considerable difference between dark and light DSSCs performance, when loaded with RB dye. The working mechanism of solar cells with fluorine-doped tin oxide (FTO)/CeO₂/RB dye/carbon-coated FTO is discussed. These solar cells show V _OC ～360 mV, J _SC ～0.25 mA cm ^?2 and fill factor ～63% with efficiency of 0.23%. These results are better as compared to costly ruthenium dye-sensitized CeO₂ photoanode.     

Adsorption of indigo carmine and methylene blue dye: Taguchi’s design of experiment to optimize removal efficiency

In the present study, Taguchi’s experimental methodology has been applied to find optimum level (value) of adsorption parameters (factors) for the removal of indigo carmine dye (ICD) and methylene blue dye (MBD) using activated carbon derived from Acacia Nilotica sawdust (ACSA). The effect of significant adsorption parameters, viz. adsorbent dose (m), initial concentration (C₀), temperature (T) and contact time (t), on the adsorption capacity (q_t) of ACSA for each dye has been discussed. Average values and S/N ratio for each parameter at three different levels have been estimated using L₉ orthogonal array (OA). Analysis of variance (ANOVA) has been used to identify the significant parameters and the most favourable optimal conditions for both raw data and S/N data. The study revealed that, for ICD, initial dye concentration is found to be the most significant parameter with 55.8% contribution followed by ACSA dose, temperature and contact time with 35.7%, 5% and 3.4% contribution, respectively. For MBD, the ACSA dose (m) is found to be the most significant parameter with 46.4% contribution followed by initial dye concentration, temperature and contact time with 44%, 5.3% and 4.4% contribution, respectively. The contact time (t) is found to be the least significant parameter in the overall sorption process for both ICD and MBD. The optimized levels of parameters for both dyes are found to be A₁, B₃, C₃ and D₃. The predicted and average confirmatory values of total dye adsorbed (q_t) on ACSA at optimized levels were found to be 31.02 and 31.01 mg/g, respectively, for ICD and 57.35 and 57.36 mg/g, respectively, for MBD. The percentage removal of ICD and MBD at optimized levels was found to be 77.5% and 95.4%, respectively.     

Organic dyes containing fluorenylidene functionalized phenothiazine donors as sensitizers for dye sensitized solar cells

Two new organic sensitizers featuring fluorenylidene decoration on the phenothiazine donor have been synthesized and characterized as sensitizers for nanocrystalline TiO₂-based dye sensitized solar cells (DSSCs). The dyes possess cyanoacrylic acid as acceptor/anchoring group and a conjugation pathway composed of fluorene and thiophene. Introduction of the fluorenylidene moiety on phenothiazine enhances the optical density of the dyes while the extension of conjugation by thiophene insertion red-shifts the absorption peak originating from the intramolecular charge transfer from phenothiazine to cyanoacrylic acid. The ground and excited state oxidation potentials of the fluorenylidene-containing dyes are upwardly shifted when compared to the parent dyes indicating a π-delocalized donor segment. The electronic properties were supported by density functional theoretical computations. Among the DSSCs fabricated, the dye (5a) having fluorene and thiophene in the spacer resulted higher power conversion efficiency (3.31 %) than the corresponding bithiophene analog (5b, 2.83 %) attributable to the relatively high electron life time and enhanced resistance for recombination in the former.     

Influence of donor substitution at $$\mathrm{D}{-}\uppi {-}\mathrm{A}$$ architecture in efficient sensitizers for dye-sensitized solar cells: first-principle study

Using density functional theory and time-dependent density functional theory, we theoretically studied a new series of five novel metal-free organic dyes, namely D1–D5, for application in dye-sensitized solar cells based on donor–\(\uppi \)-spacer–acceptor (\(\mathrm{D}{-}\uppi {-}\mathrm{A}\)) groups. In this present study, five different donor groups have been designed based on triphenylamine–stilbene–cyanoacrylic acid (TPA–St–CA). The electronic structures, UV–visible absorption spectra and photovoltaic properties of these dyes were investigated. Different exchange-correlation functionals were used to establish a proper methodology procedure for calculation and comparison to experimental results of dye TPA–St–CA. The TD-WB97XD method, which gives the best correspondence to experimental values, is discussed. The calculated results reveal that the donor groups in D2 and D3 are promising functional groups for \(\mathrm{D}{-}\uppi {-}\mathrm{A}\). In particular, the D2 dye showed small energy levels and red-shift, negative \(\Delta {G}_{\mathrm{inject}}\), fastest regeneration and largest dipole moment and exciton binding energy when compared with TPA–St–CA.     

Imine-linked receptors decorated ZnO-based dye-sensitized solar cells

This study reports the synthesis, characterization and photophysical properties of imine-linked receptors decorated ZnO nanoparticles using wet precipitation method. Initially, polymer dye 3 was synthesized using condensation reaction between 2-furancarboxaldehyde 1 and polyethylenimine 2. The decoration of imine-linked receptors on ZnO nanoparticles (sample A) was characterized and investigated by X-ray diffraction, scanning electron microscope and dynamic light scattering spectroscopic studies. Further, polymer dye 3 was added to ruthenium chloride (RuCl₃) to form a polymer–ruthenium-based composite dye-capped ZnO nanoparticles (sample B). The optical properties of sample A were evaluated by fluorescence and UV–Vis spectroscopy. The samples A and B were further processed to dye-sensitized solar cells using wet precipitation method. The results of observations revealed that the addition of ruthenium–polymer dye molecules increased the light harvesting capacity of ZnO-based DSSCs. A maximum solar power to electricity conversion efficiency (η) of 3.83% was recorded for sample B-based DSSCs with ruthenium–metal complex dye as a good photosensitizer. The recorded photovoltaic efficiency of sample B-based DSSCs was enhanced by 1.36% compared to sample A-based DSSCs.

Open image in new window

Graphical Abstract ?

     

Electronic structure and photoluminescence study of silicon doped diamond like carbon (Si:DLC) thin films

We have investigated the electronic and bonding structure using Fourier-transform infra-red (FT-IR) spectra and studied photoluminescence (PL) from micro-Raman spectra analysis of a-C:H:Si (Si:DLC) thin films deposited by plasma enhanced chemical vapour deposition (PECVD) method. Tetramethylsilane [Si(CH₃)₄, TMS] vapour was used as Silicon precursor and a bias voltage of 400 V was applied during deposition. It is observed from FT-IR spectra that with increasing TMS flow rate, the intensity of SiH_n and CH_n modes is increased significantly. PL study indicates that the PL is increased and that the PL peak position is shifted towards lower energy when the TMS flow rate increases gradually during deposition.     

Optical properties of TiO2 and disperse red-19 composite

Characteristic absorption and photoluminescence (PL) of TiO₂ and disperse red-19 (DR-19) composite have been investigated. Two step synthetic processes were employed to incorporate the DR-19 to the TiO₂ sol–gel. Firstly, urethane bonds between the DR-19 (OH) and 3-isocyanatopropyl triethoxysilane (ICPTES, NCO) were fabricated (ICPDR) prior incorporation to the TiO₂ sol–gel. Secondary, hydrolysis of the ethoxy group from the ICPDR and condensation reaction between silanol groups from ICPDR and TiO₂ sol–gel were performed by adding ICPDR to the TiO₂ sol–gel and aged for several days at room temperature (ICPDRTiO₂). There was no absorption peak shift with increasing the DR-19 concentration in methanol. However, UV–visible absorption band was shifted toward red approximately 0.09 eV for the ICPDRTiO₂ film, which indicated the formation of dimmer or more aggregates. The PL peaks of ICPDRTiO₂ were red-shifted compared with DR-19 in methanol (0.12 eV) and ICPDR film (0.09 eV). The relatively large emission peak shift toward red could be due to the fluorescence resonance energy transfer (FRET) between DR-19 and TiO₂ in ICPDRTiO₂ matrix.     

TiO2 coated urchin-like SnO2 microspheres for efficient dye-sensitized solar cells

Urchin-like SnO2 microspheres have been grown for use as photoanodes in dye-sensitized solar cells （DSSCs）. We observed that a thin layer coating of TiO2 on urchin-like SnO2 microsphere photoanodes greatly enhanced dye loading capability and light scattering ability, and achieved comparable solar cell per- formance even at half the thickness of a typical nanocrystalline TiO2 photoanode. In addition, this photoanode only required attaching -55% of the amount of dye for efficient light harvesting compared to one based on nanocrystalline TiO2. Longer decay of transient photovoltage and higher charge recombination resistance evidenced from electrochemical impedance spectroscopy of the devices based on TiO2 coated urchin-like SnO2 revealed slower recombination rates of electrons as a result of the thin blocking layer of TiO2 coated on urchin- like SnO2. TiO2 coated urchin-like SnO2 showed the highest value （76.1 ms） of electron lifetime （＇r） compared to 2.4 ms for bare urchin-like SnO2 and 14.9 ms for nanocrystalline TiO2. TiO2 coated SnO2 showed greatly enhanced open circuit voltage （Voc）, short-circuit current density （Jsc） and fill factor （FF） leading to a four-fold increase in efficiency increase compared to bare SnO2. Although TiO2 coated urchin-like SnO2 showed slightly lower cell efficiency than nanocrystalline TiO2, it only used a half thickness of photoanode and saved -45% of the amount of dye for efficient light harvesting compared to normal nanocrystalline TiO2.     

Effect of Ba/Y Molar Ratio on Superconducting Properties of BaTiO<Subscript>3</Subscript>-Doped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Films

YBa₂Cu₃ O _7?δ (YBCO) films with BaTiO₃ (BTO) nanostructures were prepared by using the precursor solutions with different cationic molar ratios of Y:Ba:Cu = 1.0:1.6–2.0:3.0 in the TFA-MOD process. These YBCO films were deposited on (00 l)-oriented LaAlO₃ single-crystal substrates using a spin coater. The high superconducting critical current density (J _C) (77 K, self-filed) of more than 10 MA/cm² for the final BTO-doped YBCO film was obtained. Moreover, the effect of different Ba/Y molar ratios in the precursor solution on superconducting properties of BTO-doped YBCO films was investigated. Compared with the BTO-doped YBCO film deposited by using the precursor solutions with Ba/Y molar ratio of 2.0, an enhancement of J _C in a magnetic field for the film from the solution with Ba/Y molar ratio of 1.9 was achieved. For Ba/Y molar ratios of 1.6 and 1.7, a reduction of J _C in a magnetic field occurred. The J _C enhancement may be mainly ascribed to the enhanced flux pinning by the Y₂Cu₂ O ₅ nanostructures with the optimal number dispersing in YBCO matrix.     

New configuration of optical photosensitizers for dye-sensitized solar cells: combination of carbazole and xantone

Novel organic dyes system containing responsible group for using in dye-sensitized solar cells and effective anchoring group for improving interaction between dye (photosensitizers) and nanolayer were designed and prepared. All intermediates and organic dyes were purified and analyzed using analytical techniques. The results confirmed the structure of intermediates and organic dyes. Dye-coated TiO₂ and ZnO semiconductors were used for the preparation of the photoanode. The synthesized dyes were trapped in J-aggregation, reflected J-aggregation, and red shift in UV–Vis spectra of dyes in solution relative to the coated on nanomaterials. Cyclic voltammetry and DFT methods were used to evaluate the energy levels of the synthesized compounds. Dye-sensitized solar cells were also prepared and evaluated under standard conditions, using the synthesized compounds. The efficiency of Dye 8 as photosensitizer on TiO₂ and ZnO was 6.65% and 6.37%, respectively.

     

Encapsulation of Coumarin 151 into the mesopores of modified rodlike SBA-15

Dye Coumarin 151 was postgrafted into the rodlike SBA-15 mesoporous materials, which were synthesized by a direct hydrothermal synthesis method and further modified by an organic silane with a terminal amino group. Characterization by powder X-ray diffraction, N₂ adsorption-desorption, transmission electron microscopy, photoluminescence and scanning electron microscopy were carried out. Small-angle X-ray diffraction and N₂ adsorption-desorption characterizations showed that these dye containing materials remained as ordered mesostructures and the pore size was from 6 nm for blank sample to 3.6 nm for postgrafting sample. PL characterization of composite samples exhibited optical properties with different dye concentrations. The characterization showed the existence of Coumarin 151 in the channels of SBA-15 and the composite materials with novel optical properties enabled possible applications in optical sensing and electron acceptors.     

Improving photochemical properties of Ipomea pescaprae,Imperata cylindrica (L.) Beauv,and Paspalum conjugatum Berg as photosensitizers for dye sensitized solar cells

This paper reports on the improving photochemical properties of natural dye using acid treatment, coadsorber addition, and dye combination. The natural pigments have been extracted from Ipomea pescaprae, Imperata cylindrica (L.) Beauv, and Paspalum conjugatum Berg containing quercetin 3-O-β-d-glucofuranoside, graminone B, and chlorophyll a. The dyes have been successfully sensitized on TiO₂ anatase nanoparticles as photoelectrodes for the dye-sensitized solar cell. The cells were illuminated under 100 mW/cm² AM 1.5 condition. It is investigated that the acidic treatments have been successfully improving the cell efficiencies of I. pescaprae dye from 0.45 to 0.53 %, I. cylindrica dye from 0.44 to 0.48 %, and P. conjugatum dye from 0.50 to 0.69 %. The acidic treatment leads to broaden the dye spectrum absorption and cell efficiency. Moreover, the addition of coadsorber into I. pescaprae dye is able to improve its cell efficiency from 0.53 to 0.55 % with J _sc of 3.738 mA/cm², V_oc of 0.393 V, and the FF of 0.377. The combination of dyes from I. pescaprae with efficiency of 0.27 % and P. conjugatum with efficiency of 0.55 % could be improved to be 0.76 %. In conclusion, we prove that the acidic treatment, coadsorber addition, and dyes combination play a significant role in improving the photochemical properties of natural dye sensitized solar cell.