Effect of the regioregularity of poly(3‐hexylthiophene) on the performances of organic photovoltaic devices

BACKGROUND: The highest efficiencies of bulk‐heterojunction solar cells from poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) reported so far are close to 6%. Phenomena occurring during the photovoltaic process, such as the creation, diffusion and separation of excitons, as well as charge carrier transport, are governed by the active layer morphology. The latter phenomenon, which depends on the self‐organization of P3HT, can be influenced by its degree of regioregularity. The aim of this work is to clarify the relationship between the regioregularity of P3HT, the composition of P3HT/PCBM blends and the performances of photovoltaic devices. RESULTS: Two types of P3HTs with different degrees of regioregularity have been synthesized and used as active layers with PCBM in photovoltaic cells. The higher performances in photovoltaic devices are obtained for high‐regioregular P3HT and can be explained considering the self‐organizing properties of high‐regioregular P3HT, leading to higher sunlight absorption and higher hole mobilities. In addition, this report demonstrates the importance of the ratio of P3HT versus PCBM in correlation with the regioregularity of P3HT on the optical properties, charge transport and characteristics of photovoltaic cells. CONCLUSION: We have investigated the dependence of the photovoltaic properties of P3HT/PCBM blend‐based photovoltaic devices on the degree of regioregularity of P3HT. We find that the best performance is exhibited by devices based on highly regioregular P3HT. Also, the best performances are not obtained for the same P3HT:PCBM weight ratios for high‐regioregular P3HT (1:0.8) and low‐regioregular P3HT (1:3). Copyright © 2007 Society of Chemical Industry     

Improving the performance of inverted organic solar cells by embedding silica‐coated silver nanoparticles deposited by electron‐beam evaporation

Embedding metallic nanoparticles (MNPs) in organic solar cells (OSCs) is proposed as one of the promising strategies to enhance their photovoltaic performance owing to localized surface plasmon resonance, light scattering effects or a synergy of both effects derived from the MNPs. However, it has been demonstrated that MNPs wrapped by a thin dielectric silica shell can lead to better photovoltaic yield than bare MNPs due to the presence of the dielectric shell which avoids direct contact between the active layer and the MNPs, reducing the charge recombination and the exciton quenching loss at the metal surface. In this study, we report an alternative solution using an ultrathin dielectric layer coating silver nanoparticles (Ag NPs) for improving the performance of plasmonic inverted OSCs instead of the use of metal–dielectric core–shell NPs. A silica (SiO₂) layer 5 nm thick coating evaporated Ag NPs with an average size of 60 nm is deposited on top of the zinc oxide (ZnO) layer used as the electron transport layer, leading to a significant improvement in the short‐circuit current density (J_sc) and the power conversion efficiency (PCE) of the inverted OSCs. The electron‐beam evaporation method is employed for controlled deposition of Ag NPs and SiO₂ on the ZnO layer. The plasmonic devices resulted in an 18% and 14.1% enhancement of the J_sc and PCE, respectively, compared to reference devices. This increase of the photoelectric parameters in plasmonic devices is attributed not only to the plasmonic effects originating from the Ag NPs but also to the ultrathin silica layer which can contribute to facilitating charge extraction. © 2019 Society of Chemical Industry     

BaCl2:Er3+—A High Efficient Upconversion Phosphor for Broadband Near‐Infrared Photoresponsive Devices

Utilization of photons with subband‐gap energy, mostly near‐infrared (NIR) photons, is highly desirable for photovoltaic cells; which can be achieved by adding an upconversion layer at the rear face of photovoltaic cells. Here, we study the upconversion luminescence properties of BaCl₂:Er³⁺ phosphors and hexagonal NaYF₄:Er³⁺ phosphors upon excitation of incoherent NIR sunlight with wavelength λ > 800 nm. Higher efficacious upconversion emissions of BaCl₂:Er³⁺ phosphors have been observed in comparison with the well‐known hexagonal NaYF₄:Er³⁺ phosphors. We demonstrate that the photocurrent response from the thin‐film‐hydrogenated amorphous silicon solar cell attached with the BaCl₂:Er³⁺ phosphor is notably enhanced under irradiation of incoherent NIR sunlight with wavelength λ > 800 nm. This judicious design may be envisioned to shorten the distance for the remarkable improvement of the power conversion efficiency of the next‐generation photovoltaic cells and suggests a promising application for other NIR photoresponsive devices.     

High-entropy transition metal diborides by reactive and non-reactive spark plasma sintering: A comparative investigation

The direct synthesis and consolidation by SPS (1950 °C, 20 min, 20 MPa) of high-entropy (Hf_0.2Mo_0.2Zr_0.2Nb_0.2Ti_0.2)B₂ from elemental powders resulted in a multiphase product. An increase of the heating rate determined a change of the mechanism governing the synthesis reaction from gradual solid-state diffusion to rapid combustion regime, while the final conversion degree was 67 wt.%. The sintered product displayed a non-uniform microstructure with the presence of 10–15 μm sized pores, due to volatilization phenomena occurring during the combustion synthesis reaction. In contrast, when the SPS process was preceded by powder synthesis via SHS, a homogeneous single-phase ceramic was obtained. Clear benefits are derived by the use of SHS, able to provide very shortly powders with elemental species very well intermixed, so that the obtainment of (Hf_0.2Mo_0.2Zr_0.2Nb_0.2Ti_0.2)B₂ during the subsequent SPS stage is strongly promoted. The resulting 92.5% dense product shows superior oxidation resistance with respect to individual borides prepared with the same method.     

Ga-doped ZnO transparent electrodes with TiO<Subscript>2</Subscript> blocking layer/nanoparticles for dye-sensitized solar cells

Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs]. The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO₂ blocking layer was deposited on the GZO surface. Then, TiO₂ nanoparticles were coated on the blocking layer, and dye was attached for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO₂ glasses showed an enhanced conversion efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge recombination and series resistance.     

Tetramethylammonium based lead free perovskite active layer for solar cell application

Here we report the synthesis of a novel lead free organic-inorganic halide perovskite layer, tetramethylammonium tin tri-iodide (TMASnI₃), by simple and cost-effective chemical synthesis technique. The microstructural and optical studies confirm the formation of hexagonal perovskite structure with optical band gap ∼2.44 eV. A solar cell structure is fabricated by depositing the perovskite layer on zinc oxide thin film to demonstrate stable photovoltaic response under solar spectrum, where a thin layer of graphene oxide flakes on top of the perovskite layer acts as the charge transport layer. The open-circuit voltage (V_oc) and short circuit current density (J_sc) for this cell as extracted from the current-voltage measurement under one sun illumination of AM1.5 solar radiation are 0.60 V and 8.65 mA/cm², respectively. Without using any conventional hole transport layer, the power conversion efficiency (η) has been obtained as 1.92% which indicates the suitability of this perovskite material as an active layer for perovskite solar cell.     

Synthesis and photocatalytic activity of BiFeO3 and Bi/BiFeO3 cubic microcrystals

BiFeO₃ and Bi/BiFeO₃ cubic microcrystals were synthesized in this work. The phase, microstructure, optical and photo electrochemical properties, as well as the photocatalytic activities in photocatalytic hydrogen generation were investigated. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results demonstrate the successful synthesis of BiFeO₃ and Bi/BiFeO₃. The scanning electron microscope (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) results give the evidence of cubic morphology and the deposition of metal Bi on the surface of BiFeO₃. The absorption spectra show that Bi/BiFeO₃ has longer absorption edge and stronger absorption capability to visible light. The photocurrent curves, emission spectra, and electrochemical impedance spectroscopy (EIS) spectra demonstrate that Bi/BiFeO₃ has higher efficiency of electron-hole separation and charge transfer, as well as longer lifetime of the charge carriers. These benefit to the enhancement of activity in photocatalytic hydrogen generation.     

Flexible photoanodes of TiO2 particles and metallic single-walled carbon nanotubes for flexible dye-sensitized solar cells

Recent advance in flexible electronics demands development of flexible energy sources. Of particular interests are flexible dye-sensitized solar cells (DSCs). However, a brittle nature of TiO₂ materials is one of hurdles to realize flexible DSCs. Here we synthesized flexible photoanodes of TiO₂ particles and single-walled carbon nanotubes (SWNTs). Metallic SWNTs provided a greater photovoltaic conversion efficiency than semiconducting SWNTs due to the more efficient electron transport. The metallic SWNTs also constructed effective mechanical network among TiO₂ particles providing flexibility and durability. The photoanode was transferred on an indium tin oxide (ITO)-coated polyethylene terephthalate film and characterized for front-illuminated DSCs under the AM 1.5 simulated sunlight. There was only a small decrease in photovoltaic conversion efficiency with bending which was primarily caused by cracking of the ITO layer. Due to this limitation, the TiO₂–metallic SWNT photoanode was transferred on a Ti foil and went through up to 1000 bending cycles. The cycled photoanode was assembled for back-illuminated DSCs due to the non-transparent Ti foil. There was no decrease in photovoltaic conversion efficiency even after 1000 bending cycles demonstrating excellent flexibility and durability.     

Synthesizing high α-phase Si3N4 powders containing sintering additives

Fine grain α-phase silicon nitride (Si₃N₄) ceramic powders were produced via carbothermic reduction of colloidal SiO₂, which contained pre-mixed additives of sintering aids primarily consisting of oxides such as MgO and Y₂O₃. The powders that were pre-mixed in the starting reactants were chosen based on the final powder composition and on type and amount of the secondary phases desired for sintering. After synthesizing, powder properties were examined using standard characterization techniques (XRD, SEM, BET, etc). This technique of ceramic synthesis has advantages in providing nitride-based ceramic powders, which contain secondary in situ phases that are distributed as sintering additives. Silicon nitride ceramic powders synthesized using this method might therefore be readily sintered because the homogeneously distributed sintering additives were present in the starting materials. In this work, the processing parameters are described in terms of the synthesis conditions.     

Efficiency of bulk-heterojunction organic solar cells

During the last years the performance of bulk heterojunction solar cells has been improved significantly. For a large-scale application of this technology further improvements are required. This article reviews the basic working principles and the state of the art device design of bulk heterojunction solar cells. The importance of high power conversion efficiencies for the commercial exploitation is outlined and different efficiency models for bulk heterojunction solar cells are discussed. Assuming state of the art materials and device architectures several models predict power conversion efficiencies in the range of 10–15%. A more general approach assuming device operation close to the Shockley–Queisser-limit leads to even higher efficiencies. Bulk heterojunction devices exhibiting only radiative recombination of charge carriers could be as efficient as ideal inorganic photovoltaic devices.     

Recyclable 0D/2D ZnFe2O4/Bi5FeTi3O15 S-scheme heterojunction with bismuth decoration for enhanced visible-light-driven tetracycline photodegradation

Practical application of photocatalysis is often challenged by some intrinsic issues such as recombination of photogenerated charge carriers, stability and separation, etc. Herein, bismuth decorated 0D/2D ZnFe₂O₄/Bi₅FeTi₃O₁₅ (Bi/ZF/BFT) step-scheme (S-scheme) heterojunction was fabricated by an in-situ method. Due to the advantages of structure and composition, the Bi/ZF/BFT with the desired proportion (Bi/ZF/BFT-35) exhibits favorable photocatalytic performance towards tetracycline (TC) degradation. Compared with the pure ZF, the nanohybrid shows superior stability after 5 times cycle tests. Moreover, Bi/ZF/BFT-35 is convenient to be separated from the reaction system due to its magnetic nature. As identified by ESR measurement, ?O₂^? and ?OH radicals were involved in the photodegradation of TC, which supports that the S-scheme is successfully prepared. Also, the Bi/ZF/BFT-35 shows great ability of chemical oxygen demand (COD) removal in the practical wastewater as well. Importantly, antibacterial activity against E. coli test indicates that photodegraded TC has lower biotoxicity. The present work demonstrates that cocatalyst Bi modified ZF/BFT S-scheme can not only significantly improve its stability with good recyclability from the reaction system, but also inhibits the recombination of charge carriers, giving insight on the strategy of fabricating a promising photocatalyst for practical wastewater treatment.     

Triphenylamine-based organic dyes containing benzimidazole derivatives for dye-sensitized solar cells

The synthesis and application to dye-sensitized solar cells of two new triphenylamine-based organic dyes containing benzimidazole derivatives as secondary donors together with a simple triphenylamine derived dye for the purpose of comparison is reported. The photophysical and electrochemical properties of the dyes were investigated by UV–vis spectroscopy and cyclic voltammetry. The introduction of benzimidazole derivatives in the phenyl ring of the triphenylamine core increases the molar extinction coefficients and λ_max because of the extension of the π-conjugation structures of the dyes. Overall conversion efficiencies of ∼2.5% under full sunlight (AM 1.5G, 100 mW cm⁻²) irradiation were obtained for DSSCs based on these new dyes, under the same conditions, the reference dye and di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) (N719) gave overall conversion efficiencies of 1.23% and 5.61%, respectively. Our findings demonstrate that the introduction of benzimidazole derivatives as secondary donors in triphenylamine-based dye can improve their photovoltaic performance compared to the unsubstituted reference dye in DSSCs.     

Broadband NIR photoelectronic performance for sunlight‐induced photocurrent from (NaYF4:Yb‐Er)/BiOI hybrid films

The fabrication and near‐infrared optoelectronic performance of the (NaYF₄:Yb‐Er)/BiOI upconversion phosphor/semiconductor hybrid films are reported. The composite films were fabricated by a simple electro‐deposition and successive ionic layer adsorption and reaction (SILAR) method. We observed efficient upconversion emissions in the film of NaYF₄:Yb‐Er upon irradiation by near‐infrared part of simulated sunlight, and the UC emission is drastically quenched in the composite film (NaYF₄:Yb‐Er)/BiOI. More importantly, this composite film displays unusual negative photoresponse to near‐infrared part of simulated sunlight when used in a standard electrochemical cell where the composite film serves as the photoanode. This result may pave the way for potential application of this hybrid films for the development of novel photovoltaic and infrared optoelectronic devices.     

New diketo-pyrrolo-pyrrole (DPP) sensitizer containing a furan moiety for efficient and stable dye-sensitized solar cells

A new metal-free organic sensitizer containing a furan moiety as the π-spacer based on the diketo-pyrrolo-pyrrole unit was synthesized through simple synthetic routes and with low cost for the application of dye-sensitized solar cells. Two corresponding dyes with benzene and thiophene spacers were also synthesized for the purpose of comparison. On the basis of optimized DSSC test conditions, the sensitizer containing the furan shows prominent solar energy conversion efficiency (η) of 5.65% (J_sc = 15.96 mA cm⁻², V_oc = 541 mV, ff = 0.65) under simulated full sunlight irradiation. The dyes were also tested in a solvent-free ionic liquid electrolyte devices and the stability of devices was performed over 2000 h at full sunlight. The sensitizer containing the furan moiety exhibited good stability and better photovoltaic performance of up to 4.41% power conversion efficiency.     

Effects of the film thickness and poling electric field on photovoltaic performances of (Pb,La)(Zr,Ti)O3 ferroelectric thin film-based devices

The ferroelectric photovoltaic (FPV) effect obtained in inorganic perovskite ferroelectric materials has received much attention because of its large potential in preparing FPV devices with superior stability, high open-circuit voltage (V_oc) and large short-circuit current density (J_sc). In order to obtain suitable thickness for the ferroelectric thin film as light absorption layer, in which, the sunlight can be fully absorbed and the photo-generated electrons and holes are recombined as few as possible, we prepare Pb_0.93La_0.07(Zr_0.6Ti_0.4)_0.9825O₃ (PLZT) ferroelectric thin films with different layer numbers by the sol-gel method and based on these thin films, obtain FPV devices with FTO/PLZT/Au structure. By measuring photovoltaic properties, it is found that the device with 4 layer-PLZT thin film (~300 nm thickness) exhibits the largest V_oc and J_sc and the photovoltaic effect obviously depends on the value and direction of the poling electric field. When the device is applied a negative poling electric field, both the V_oc and J_sc are significantly higher than those of the device applied the positive poling electric field, due to the depolarization field resulting from the remnant polarization in the same direction with the built-in electric field induced by the Schottky barrier, and the higher the negative poling electric field, the larger the V_oc and J_sc. At a -333 kV/cm poling electric field, the FPV device exhibits the most superior photovoltaic properties with a V_oc of as high as 0.73 V and J_sc of as large as 2.11 μA/cm². This work opens a new way for developing ferroelectric photovoltaic devices with good properties.     

The influence of the synthesis route on the final properties of SnO2-based varistors

A comparative study of two customary routes of ceramics processing applied to the synthesis of SnO₂-based varistors is reported in this paper. Devices of equivalent composition were prepared through the Pechini method and through directly mixing the oxides without the addition of anti-agglomerants or binders. The microstructures of the sintered samples were characterised with X-ray diffraction and scanning and transmission electron microscopies. The electrical behaviour of the devices was studied on the basis of the current density versus electric field (J–E) characteristics and impedance spectroscopy measurements. The Pechini method ensures the homogeneity in the distribution of the additives in the tin oxide matrix but the formation of secondary phases seems to be independent of the synthesis route. Devices with similar non-linear coefficients of 18 and 21 were obtained through the mixed oxides route and the Pechini method, respectively.     

Soft-chemical derived Bi2O3–ZnO–Ta2O5 nanopowder and ceramics

In this paper, the synthesis of bismuth zinc tantalate pyrochlore (Bi_1.5ZnTa_1.5O₇, α-BZT) by a soft-chemical method, based on the polymeric precursors, was investigated in order to obtain chemically homogeneous powders. The pyrochlore phase was investigated by X-ray diffraction, UV–vis and Raman spectroscopy. The study of α-BZT formation reveals that, at 600 °C, a single-phased nanopowder was obtained without any detectable intermediary or secondary phases. The morphology of the powders was examined by scanning electron microscope; the mean size distribution was measured by low angle laser light scattering. Both measurements showed a nanoscaled powder with large soft agglomerated clusters. The optical gap, obtained from the UV–vis spectra, ranged from 2.5 to 3.2 eV for the powders treated from 500 to 900 °C, respectively.     

Enhanced electron collection efficiency of nanostructured dye‐sensitized solar cells by incorporating TiO2 cubes

Herein, enhancement of dye‐sensitized solar cell (DSC) performance is reported by combining the merits of the dye loading of TiO₂ nanoparticles and light scattering, straight carrier transport path, and efficient electron collection efficiency of TiO₂ cubes. We fabricate DSC devices with various arrangement styles and compositions of the electrodes in the forms of monolayer and double layer films. For this purpose, the solvothermal synthesized TiO₂ cubic particles (100‐600 nm) are employed as the scattering layer, whereas TiO₂ nanoparticles (15‐30 nm) synthesized via a combination of solvothermal and sol‐gel routes are used as the active layer of devices. We improve the photovoltaic characteristics of DSCs by two mechanisms. First, the light harvesting of DSC devices made of nanoparticles is improved by controlling the thickness of monolayer films, reaching the highest efficiency of 7.0%. Second, the light scattering and electron collection efficiency are enhanced by controlling the composition of double layer films composed of mixtures of TiO₂ nanoparticles and cubes, obtaining the maximum efficiency of 8.21%. The enhancements are attributed to balance between charge transfer resistance and charge recombination of photo‐generated electrons as well as dye loading and light scattering.     

Study of photovoltaic performance of host‐guest system comprising photoluminescent polyurethane and rhodamine B dye

The photovoltaic property of Rhodamine B dye embedded into Poly(tolyl‐1,1′‐binaphthyl carbamate) (PU₁) and poly(hexamethylene‐1,1′‐binaphthyl carbamate) (PU₂) matrices have been evaluated using host‐guest approach. The photoactive layer comprising photoluminescent polymer matrix (PU₁ or PU₂), Rhodamine B and TiO₂ nanoparticles were prepared by spin casting method. The power conversion efficiency (PCE) the photovoltaic devices based on PU₁ and PU₂ matrices were found to be 0.043% and 0.029%, respectively. PCE of the photovoltaic devices were limited due to low lying highest occupied molecular orbital of PU₁ and PU₂ polymers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Al-doped ZnO nanostructured powders by emulsion detonation synthesis – Improving materials for high quality sputtering targets manufacturing

Emulsion detonation synthesis method was used to produce undoped and Al-doped ZnO nanostructured powders (0.5–2.0 wt.% Al₂O₃). The synthesized powders present a controlled composition and a morphology which is independent on the doping level. The XRD results indicate wurtzite as the single phase for undoped ZnO and the presence of gahnite as secondary phase for Al-doped ZnO powders. The sintering behavior of each powder was studied based on their linear shrinkage and shrinkage rate curves, showing the high sinterability of the powders. Activation energies for densification in the earlier stage were calculated for all compositions and possible sintering mechanisms are suggested depending on the doping level. The high chemical homogeneity and sinterability and the lower electrical resistivity of the bulk Al-doped sintered samples demonstrates the feasibility of emulsion detonation synthesis for the production of high quality Al-doped ZnO powders to be used in ceramic sputtering targets manufacture.