Wide band-gap organic molecules containing benzodithiophene and difluoroquinoxaline derivatives for solar cell applications

Abstract

Two new semiconducting organic small molecules, namely BDTQ-BDT(EH) and BDTQ-BDT(OC), were prepared by attaching electron accepting 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (DTQ) unit on 2,6-position of electron donating 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(EH)) and 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(OC)) units. Molecule BDTQ-BDT(EH) showed higher thermal stability (5% weight loss temperature, T_d “349 ^оC), slightly lower band-gap (E_g “2.10?eV) and deeper highest occupied molecular orbital energy level (HOMO “–5.36?eV) level compared to those (T_d “336 ^оC, E_g “2.11?eV, and HOMO “–5.30?eV, respectively.) of the molecule BDTQ-BDT(OC). The organic solar cells (OSCs) made with the synthesized molecules as an electron donor and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₀BM) as an electron acceptor gave a maximum power conversion efficiency (PCE) of 1.20% and 0.83%, respectively, for BDTQ-BDT(EH) and BDTQ-BDT(OC). This study confirmed that the substituents attached on the 4,8-position of BDT unit greatly alter the properties of the resulting molecules.     

Synthesis of A-D-A type quinoxaline-based small molecules for organic photovoltaic cells

Abstract

We have synthesized two acceptor-donor-acceptor (A-D-A) type quinoxaline-based small molecules for organic photovoltaic cells (OPVs). To construct the A-D-A architecture, the electron-accepting quinoxaline derivative (Qx) was attached on both ends of the electron-donating benzodithiophene (BDT) unit via the Stille coupling reaction. After the confirmation of the structural features, the optical and electrochemical characteristics and the photovoltaic properties of the two small molecules were investigated. A typical inverted-type device with the configuration of ITO/ZnO/small molecules: PC₇₁BM/MoO₃/Ag was fabricated and examined. It has been observed that the performances of OPVs were improved by the incorporation of the additional thiophene linkage between the BDT and Qx moieties. Therefore, this study can provide insights into the design and structure–property correlation of quinoxaline-based organic small molecules for OPVs     

Small molecular electrolytes as the interlayer for enhanced performance of organic solar cellsExperimental

Abstract

New non-conjugated small molecules (C4-OH and C6-OH) were synthesized and applied to the inverted organic solar cells (OSCs) as the cathode interlayer (CIL). The power conversion efficiencies (PCEs) of 9.35 and 9.21% were obtained in the device based on ZnO/C4-OH and ZnO/C6-OH, which are better than those of pristine ZnO (8.74%). The work function of ZnO/C4-OH and ZnO/C6-OH are also decreased from ?4.5?to ?4.27?eV and ?4.37?eV, respectively. Consequently, OSCs with two small molecules exhibited enhanced photovoltaic properties due to the formation of interface dipole.     

Organic dyes incorporating 9,10-dihydrophenanthrene moiety for dye-sensitized solar cells

Abstract

A series of new dipolar organic dyes containing 9,10-dihydrophenanthrene has been synthesized as sensitizers for the application in dye-sensitized solar cells. These new materials containing less rigid π–linker as sensitizers exhibit good performance with conversion efficiencies ranging from 1.58% to 6.21% reaching 11?～?83% of the ruthenium dye N719-based cell under the same condition.     

Design and synthesis of small molecules with difluoroquinoxaline units for OSCs

Abstract

We designed and synthesized three novel small molecule donors (SM1, SM2 and SM3) that consist of thiophene as the electron-donating end group and 6,?7-?difluoroquinoxaline moiety as a novel electron-withdrawing core group. The organic low band gap molecules with 6,?7-?difluoroquinoxaline and thiophene units were synthesized using Stille coupling to generate SM1, SM2 and SM3. The absorption of SM2 and SM3 in solution was red shifted due to increased conjugation length of added thiophene units. In case of SM2 and SM3, introduction of hexyl chain in terminal thiophene units improve solubility in organic solvent. The maximum absorption peaks of SM1, SM2 and SM3 in solid thin films were at 482, 505 and 518?nm, respectively. SM3 was red shifted as compare to SM2 due to increased π-π stacking of electron donor materials.     

Efficiency boost significantly of ternary organic solar cells by doping low bandgap polymer

Ternary blends composed of two donor absorbers and a complementary absorbing material provide an opportunity to enhance the short-circuit current and thus improved the power conversion efficiency (PCE) of polymer solar cells (PSCs). The used of PBT-T-DPP as the complementary electron donor in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyricacid methyl ester (PC₆₁BM) blend to construct the ternary PSCs. The PCE of the optimized ternary blend PSCs (with 5 wt% PBT-T-DPP) reached 3.65%, which was 15.51% higher than that of the PSCs with binary blends of P3HT:PC₆₁BM. The ternary blend with 5 wt% PBT-T-DPP exhibited well-developed morphology while the blend with 15 wt% PBT-T-DPP showed phase separation with large-sized domains. Improved photovoltaic performance of ternary PSCs was mainly due to complementary absorption of two donors as well as facilitating charge separation and transport while suppressing charge recombination through a combination of cascade energy levels and optimized device morphology.     

Solution-processed small-molecule organic solar cells based on diketopyrrolopyrrole and perlyene derivatives with coplanar structures

ABSTRACT

A series of small-molecules (SMs) based on coplanar perylene unit coupled with diketopyrrolopyrrole chromophoric core exhibit broad absorption in the range of 500–800 nm and low bandgap energise of 1.6–1.7 eV. The power conversion efficiency approximately reach at 0.4% under 1.5 G illumination is achieved for organic solar cells based on a small-molecule bulk heterojunction system consisting of SMs as a donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor.     

Enhanced photovoltaic performance of quinoxaline-based small molecules through incorporating trifluoromethyl substituents

Abstract

Two D-A-D type quinoxaline-based small molecules have been synthesized by Suzuki coupling reaction for solution-processable organic photovoltaic cells (OPVs). The electron-donating triphenylamine was connected to both ends of an electron-withdrawing 2,3-diphenylquinoxaline core through thiophene bridge to produce QxTPA. In addition, QxCF₃TPA was formed by introducing strong electron-withdrawing trifluoromethyl (CF₃) groups into the para-position of the phenyl ring at the 2,3-position of quinoxaline core in QxTPA to explore their effect on the various properties of small molecules. The finding revealed the significant contributions of CF₃ substituents in enhancing the photovoltaic performances of OPVs with QxCF₃TPA compared to the reference case with QxTPA.     

Enhancement of organic thin-film solar cells by incorporating hybrid Au nanospheres and Au nanorods on a metallic grating surface

Abstract

In this study, we demonstrate the fabrication of hybrid plasmonic solar cells using gold nanoparticles (AuNPs). Two types of AuNPs, gold nanospheres (AuNSs) and gold nanorods (AuNRs), were incorporated in a hole transport layer (HTL) (PEDOT:PSS) on a metallic grating electrode. The organic solar cells (OSCs) structure comprised an indium-tin-oxide (ITO)-coated glass substrate/PEDOT:PSS:AuNSs:AuNRs/P3HT:PCBM/Al grating electrode. Adding AuNPs induced localized surface plasmon resonance (LSPR), while grating structured Al at the interface with a photoactive layer excited the propagating surface plasmons. Compared with a flat reference device, the proposed OSCs exhibited improved photovoltaic properties by increasing both the short-circuit current density (J_SC) and the power conversion efficiency (PCE) with large enhancements of 16.23% and 14.06%, respectively. The efficiency improvement was attributed to increased broadband absorption and improved electrical properties inside the thin-film devices.     

A ZnO/TiO2 composite nanorods photoanode with improved performance for dye‐sensitized solar cells

A dye‐sensitized solar cell (DSSC) based on ZnO/TiO₂ composite nanorods (NRs) photoanode is fabricated. The power conversion efficiency (PCE) of the ZnO/TiO₂ composite NRs film DSSC is 4.36%, which is obviously higher than that of DSSCs based on pure TiO₂ NRs (0.6%) and ZnO NRs (3.10%). The enhanced performance of ZnO/TiO₂ composite NRs film DSSC can be attributed to the combined effects of ZnO and TiO₂ NRs. In this architecture, the thick ZnO NRs overlayer offers a large surface area for enough dye absorption, while the thin TiO₂ NRs underlayer not only offers a direct and quick pathway for photoinjected electron transfer along the photoanode but also acts as a blocking layer, which effectively hinders the direct contact between the substrate and the electrolyte resulting in lower carrier recombination.     

Photoelectrochemical solar cells: Stabilization of small band GAP semiconductor in aqueous solution by surface-attached organic conducting polymer

Surface-attached polypyrrole films have been shown to produce a marked improvement in the stability of n-type single-crystal and polycrystalline Si against oxidation in an aqueous electrolyte. The current production of n-type polycrystalline Si coated with polypyrrole deteriorates less than 30% during 122 hr of irradiation whereas the unprotected bare electrode stops producing photocurrent within 30 s. The polymer protection of the n-type single-crystal Si is significantly less than that of the polycrystalline material because of differences in the adhesion of the polymer film to the electrode surfaces. The adhesion strength is shown to depend on various surface properties of Si and other electrode materials. Moreover, the surface morphology of the electrode affects the topography of the growing surface of the polypyrrole film. Requirements are discussed for the applications of organic conducting polymers to photoelectrochemical devices utilized for solar energy conversion.     

Solution-processible Cd-doped ZnO nanoparticles as an electron transport layer to achieve high performance polymer solar cells through improve conductivity and light transmittance

Abstract

In this work, electron transport layers (ETLs) with high charge transfer ability were prepared by doping ZnO nanoparticles with different concentrations of cadmium(Cd). The inverted polymer solar cell based on PTB7-Th: PC₇₁BM as active layer and various concentrations Cd-doped ZnO (CZO) as ETLs were fabricated. The PCE of the device with optimized Cd content in the ZnO film was about 14.7% larger than that of the pure ZnO-based cells. The cadmium-doped ZnO(CZO) is a good candidate to be used as a high-quality transparent electrode in solar cell applications.     

Microstructural and optoelectrical properties of transparent conducting ZnO:Al thin films for organic solar cells

The Al‐doped zinc oxide (ZnO:Al) thin films were grown on glass substrates by the magnetron sputtering technique. The films were characterized with X‐ray diffractometer, four‐point probe and optical transmission spectroscopy, respectively. The dependence of microstructural, electrical and optical properties on deposition temperature was investigated. The results show that all the films have hexagonal wurtzite structure with highly c‐axis orientation. And the microstrural and optoelectrical properties of the films are observed to be subjected to the deposition temperature. The ZnO:Al film prepared at the deposition temperature of 650 K possesses the best optoelectrical properties, with the lowest electrical resistivity (6.1×10⁻⁴ Ω·cm), the highest average visible transmittance (85.3%) and the maximum figure of merit (0.41 Ω⁻¹). The optical energy gap of the films was estimated from Tauc's law and observed to be an increasing tendency with the increment of the deposition temperature. Furthermore, the refractive index of the films was determined by the optical characterization methods and the dispersion behavior was studied by the single electronic oscillator model.     

Tunable synthesis of hierarchical TiO2 spheres consisting of rutile nanowires for dye‐sensitized solar cells and photocatalysis

Hierarchical TiO₂ spheres consisting of rutile nanowires (HTS) are successfully prepared in the bulk solution via a non‐polar solvent/polar solution interfacial syntheses strategy. The effect of reaction temperature on the morphology, size, BET surface area and monodispersion of HTS is studied systematically. Increasing reaction temperature decreases the diameter of spheres in micro‐scale whereas increases the diameter of nanowires in nano‐scale for these HTS. Monodisperse HTS are formed at 230 °C for 20 h, which can suitably be served as light scattering structure for dye‐sensitized solar cell with the additional advantage of rapid electron transport rate. A high efficiency of 8.54% was achieved for the composite DSSC consisting of nanoparticles and monodispersed HTS (synthesized at 230 °C), showing 31.38% improvement compared with that (6.5%) of the DSSCs made from pure P25 nanoparticles at the similar thickness. Furthermore, the photocatalytic performances of HTS for the degradation of methyl orange were also investigated. Contrary to the effect on the performance of DSSC, the HTS synthesized at 150 °C show higher photocatalytic degradation efficiency due to larger BET surface area.     

Structure, thermal behavior, and IR investigation of a new organic sulfate

[1-(2-ammoniumethyl) piperazinium] sulfate denoted PIPS has a monoclinic unit cell. The parameters are: a = 6.6521(3), b = 7.8756(5), c = 19.197(1) Å, = 94.43(1)° and the space group is P2₁/n. The preparation, thermal analysis, and IR spectrometric investigation are described. The PIPS structure exhibits a complex three-dimensional network of H-bonds connecting all its components.     

Structure of “bis(methoxymagnesium)diselenide”: A reagent for the introduction of selenium into organic molecules

The reaction of magnesium and selenium in dry methanol has been reported to yield a reagent tentatively identified as bis(methoxymagnesium) diselenide. The dark red-brown solution obtained from this reaction yielded red crystals that crystallized in the monoclinic space group C2/c, a=17.391(4) ?, b=15.823(3) ?, c=21.626(4) ?, β=110.71(3)°, V=5566.5(19) ?³, Z=4, R=0.0350, wR₂=0.0850 for 4930 reflections. X-ray crystallographic analysis of this reagent showed it to be dodecamethanol-tetramethoxy-di(μ₄-hydroxy)tetra(μ₃-methoxy)hexamagnesium hexaselenide, Mg₆(μ₄-OH)₂(μ₃-OCH₃)₄(OCH₃)₄(CH₃OH)₁₂]Se₆·2CH₃OH (1), not the bis(methoxymagnesium) diselenide as previously described. The structure of 1 is composed of six magnesium ions and six bridging oxygen-containing ligands in a face-sharing cubic arrangement.     

Characterization of thin‐film a‐Si:H/µc‐Si:H tandem solar cells on glass substrates

Techniques, such as photoluminescence (PL) and electron‐beam‐induced current (EBIC), have already proven their effectiveness and applicability for solar materials. Although, the methods are standard techniques for multicrystalline Si PV, their application to thin films is challenging and requires special adjustment and a careful selection of the measuring parameters. Here we report on the investigation of thin‐film tandem solar cells consisting of hydrogenated amorphous (a‐Si:H) and microcrystalline silicon (µc‐Si:H) on glass substrates. We observe a homogeneous spatial distribution of the PL signals caused by the dominance of the surface recombination. A typical PL spectrum exhibits sub‐band gap features of a‐Si:H. We relate the sub‐band‐gap spectral features mainly to transitions of carriers trapped in deep states. Observations on partially processed stacks support this supposition. PL is only detectable on the a‐Si:H layer, while EBIC signal is generated mainly in the µc‐Si:H layer. It is found out that the luminescence features of the thin a‐Si:H layer resemble those on bulk a‐Si:H.     

Studies on the fabrication and characteristics of photoelectrochemical cells using IrO2-coated TiO2 photoanode for Z-scheme water splitting and perovskite solar cell bias

ABSTRACT

In the present work, to build a water splitting system using only a neutral electrolyte solution and light source, we investigated the photoelectrochemical properties and interface resistances of an IrO₂-coated TiO₂ photoanode in Z-scheme configuration. The photoelectrochemical cell was connected to the perovskite solar cell with to realize the system with no need for an external electrical bias. Photoanodes with IrO₂ were found to support hole transport and to reduce the overpotential, therefore increasing the current density of the system consisting of photoelectrochemical cells. A maximum solar-to-hydrogen efficiency of 8.2% was achieved in a neutral electrolyte.     

Soft organic and metal-organic frameworks with porous architecture: From wheel-and-axle to ladder-and-platform design of host molecules

The family of wheel-and-axle host molecules is reviewed and the approaches to the creation of new host geometries for supramolecular materials based on weaker interactions are discussed. The combination of bulky groups (or platforms) and spacers yields various host geometries including humming-top molecules (Werner complexes, metal dibenzoylmethanates and other bis-chelates, dimeric metal carboxylates and macrocyclic complexes), wheel-and-axle and dumb-bell shaped molecules and their modifications, ladder-and-platform, shish-kebab, multi-decker, stair-case and double-strand ladder-and-platform oligomeric and polymeric structures. The host shapes are compared with cyclic and trifoil (trityl) host types. The use of metal centers, chelating and macrocyclic ligands (β-diketones, carboxylic acids, Schiff bases, porphyrins, phthalocyanines, corroles, annulenes and their analogs), bridging ditopic ligands and other building elements in the engineering of host molecules is illustrated. The role of such factors as size and nuclearity of the platforms, rigidity/flexibility of the spacers and hydrogen bonding is discussed. The relation between porosity and packing efficiency, predisposition to self-inclusion, parallel alignment and interdigitation, dimensionality and secondary assembly of host molecules in the crystal is examined. Clathration and sorption abilities and other important qualities and functionalities of the new host materials are elucidated.