Investigation of novel PCBM analogs containing thienyl groups for organic photovoltaic devices

Novel [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) analogs containing benzo[b]thiophene and thieno[3,2-b]thiophene were used as electron acceptors in bulk heterojunction thin layer organic photovoltaic devices. The solubility of the novel methanofullerenes in dichloromethane significantly varied, and es-TThCBM, which was 1.4 times as soluble as PCBM, exhibited maximum solubility. The morphology of the thin films prepared from the mixtures of these methanofullerenes with regioregular poly(3-hexylthiophene) (P3HT) was investigated by XRD, AFM, and UV–vis absorption spectroscopy. Devices containing the novel methanofullerenes exhibited nearly the same performance as those containing PCBM.     

Photovoltaic properties of benzotriazole containing alternating donor–acceptor copolymers: Effect of alkyl chain length

A series of variable alkyl chain length substituted donor–acceptor (D–A) conjugated polymers with thiophene ring as the donor and benzotriazole moiety as the acceptor has been investigated in bulk heterojunction solar cells. The optical and electrochemical properties showed that the absorption onsets and the energy levels of the copolymers were not affected by alkyl substitution revealing 1.9 eV of optical band gap. The morphologies of the blend film can be fine-tuned by increasing the chain length attached to the benzotriazole unit. Photovoltaic devices were fabricated using (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM) and (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) as the acceptors. The maximum performance was achieved with a V_oc of 0.75 V, J_sc of 3.70 mA/cm², FF of 45% and a PCE of 1.23% for the PTBT-DA10:PC₇₀BM device using 1:4 w/w ratio in chlorobenzene (CB).     

Poly(2,5-dihexyloxyphenylene vinylene-alt-2,2-dibutylpropylenedioxythiophene): Synthesis and characterization for photovoltaic applications

A thiophene/phenylene vinylene-based polymer (P1) was synthesized and its properties compared with those of a previously described analogous polymer (P2-CN) which contains CN substituents in the vinyl moieties. In solution, the P1 polymer absorbed in a range from UV to near 600 nm and, compared to P2-CN, exhibited a band gap energy that was larger, from 1.7 to 2.0 eV, and the energy levels of LUMO and HOMO lower, by 0.2 and 0.5 V, respectively. However, the power conversion efficiencies (PCE, 0.30%) of photovoltaic devices fabricated using a blend of P1 and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) appeared to be slightly higher than the reported PCE (0.15%) for P2-CN devices fabricated in the same configuration.     

Fine tuning the HOMO energy levels of polythieno[3,4-b]thiophene derivatives by incorporation of thiophene-3,4-dicarboxylate moiety for photovoltaic applications

To lower the HOMO (highest occupied molecular orbital) energy level of polythieno[3,4-b]thiophene (∼−4.5 eV), a series of ester-functionalized polythieno[3,4-b]thiophene derivatives (P1–P3) were designed and synthesized by Stille cross coupling reaction. The resulting copolymers exhibited broad and strong absorption bands from visible to near infrared region with low optical band gaps of 1.23–1.42 eV. Through cyclic voltammetry measurements, it was found that the HOMO energy levels of the copolymers gradually decreased with increasing the content of the thiophene-3,4-dicarboxylate moiety, i.e. −4.91 eV for P1, −5.00 eV for P2, and −5.11 eV for P3. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as electron acceptor were investigated. Among the three copolymers, P1 exhibited the best photovoltaic performance with an open circuit voltage (V_oc) of 0.54 V, a short circuit current density (I_sc) of 3.3 mA/cm², a fill factor (FF) of 0.57, and a power conversion efficiency (PCE) of 1.02%. A high V_oc up to 0.71 V was achieved in the solar cell based on a P3:PCBM blend.     

LESR study on PPV–PPE/PCBM composites for organic photovoltaics

Photoinduced charge carrier generation and charge transfer in the new conjugated poly(phenylene vinylene)–poly(phenylene ethynylene) (PPV–PPE) copolymers mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were studied by LESR technique at 77 K. The effective charge separation has been verified by two ESR signals belonging to positive polarons on the polymer chains and to radical anions of PCBM molecules, respectively. The relaxation times T₁ and T₂ of the anion radical of PCBM were estimated by anisotropic linewidth analysis and microwave power saturation studies. The low temperature recombination of long living charge carrier pairs after cessation of photoexcitation was studied and described by a tunnel model in which the recombination time depends on the distances between charge carrier pairs. Results on PPV–PPE/PCBM were compared with those of a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO–PPV)/PCBM composite.     

Effects of n-type perylene derivative as an additive on the power conversion efficiencies of polymer solar cells

The synthesized n-type perylene derivative, N,N′-bis-(5-bromopyridine-2-yl)-1,6,7,12-tetrakis(4-n-butoxyphenoxy)-3,4,9,10-perylene tetracarboxydiimide (PPyB), was applied to polymer solar cells as an additive in P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C₆₁-butyric acid methyl ester] blend films. The addition of 1 wt% PPyB into a P3HT:PCBM layer led to a considerable improvement of over 70% in power conversion efficiency due to the enhanced short circuit current when compared with that of reference cell without an additive. Photoluminescence, X-ray measurements and carrier mobility studies revealed that the perylene additives increased electron mobility and contributed to more effective exciton dissociation due to the presence of the additive, PPyB, in the interfaces between the P3HT and PCBM domains.     

ESR studies of ambipolar charge carriers in metal–insulator–semiconductor diodes of regioregular poly(3-hexylthiophene)/PCBM composites

We have applied field-induced electron spin resonance (FI-ESR) method to organic metal–insulator–semiconductor (MIS) diodes using regioregular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) composites as the active layer, Al₂O₃ as the gate insulator and gold (Au) or aluminum (Al) as the top electrode. In the case of Al electrode, clear FI-ESR signals are observed with g ～ 2.002 and 1.999 for negative and positive gate voltage (V_G), respectively, demonstrating that these signals with former and the latter g-values correspond to field-induced positive polarons of RR-P3HT and PCBM radical anions, respectively. Both RR-P3HT and PCBM signals monotonically increase as |V_G| increases. The threshold voltages (V_th) for field-induced ambipolar charge carriers coincide well with those of capacitance measurements with hysteresis behaviors. On the other hand, in the case of Au electrode, only positive polarons signals are clearly observed and PCBM signals are nearly undetected. These different behaviors of FI-ESR signals between Al and Au top electrodes are discussed in relation to the difference of work functions.     

Synthesis,characterization, and photovoltaic properties of diketopyrrolopyrrole-oligothiophene/fullerene dyads

Low-band-gap dyad molecules containing diketopyrrolopyrole (DPP) chromophore and hexathiophene were designed and synthesized in combination with C₆₀ and C₇₀ derivatives as the electron acceptors. The effect of alkyl side chains in the donor parts on film morphology, thermal properties, and optical properties were investigated by X-ray diffraction analysis, differential scanning calorimetry, UV–vis absorption spectroscopy, and fluorescence spectroscopy. The donor part with a shorter alkyl side chain showed better packing of the π-plane and an enhancement of absorption in the longer-wavelength region, which led to higher J_SC of organic solar cells. The use of C₇₀ as the electron acceptor led to stronger absorption in the visible region, resulting in improved external quantum efficiency and J_SC compared with those of the C₆₀ derivatives. The single-component solar cell of the dyad with the methyl side chains and C₇₀ derivative showed the power conversion efficiency of 0.84% with a relatively high J_SC of 4.4 mA cm⁻² under AM 1.5 illumination (100 mW cm⁻²).     

Transparent polymer cathode for organic photovoltaic devices

We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6], [6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 10²–10³ S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.     

Different solvents effect on the performance of the solar cells based on poly(3-hexylthiophene):methanofullerenes

This paper reports the effect on the performance of the solar cells based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with different casting solvents. These blend films are characterized by UV–vis absorption spectra, photoluminescence spectra, charge-transport dark J–V curve, X-ray diffraction pattern curve, and AFM images. The results indicate that high boiling point solvent leads to an enhanced self-organization of P3HT in the active layer, which causes an increased charge transport. Increased incident light absorption and higher carrier mobility in the active layer contribute to the enhancement in the device performance, the power conversion efficiency of 3.69% and fill factor up to 65.3% are achieved with 1,2,4-trichlorobenzene as casting solvent without further heat treatment under Air Mass 1.5, 100 mW/cm².     

Morphology and fluorescence quenching in photovoltaic samples containing fullerene and poly(p-phenylene-vinylene) derivatives

We have studied the morphology of different heterogeneous samples containing poly[2-methoxy,5-(3′,7′-dimethyl-octyloxy)]-p-phenylene-vinylene (MDMO-PPV) and either fullerene (C₆₀) or 6,6-phenyl C₆₁-butyric acid methyl ester (PCBM) using atomic force microscopy in the shear force detection mode or pulsed-force mode and near-field scanning optical microscopy. These investigations confirm the large influence of preparation conditions on the morphology. Systematic variation of the layer thicknesses in two-layer samples indicate that fluoresecence quenching is restricted to an interface layer with a thickness of about 40 nm.     

Electrical, magnetic, and magnetoelectric characterization of fine-grained Pb(Zr0.53Ti0.47)O3-(Ni0.5Zn0.5)Fe2O4 composite ceramics

Fine-grained Pb(Zr_0.53Ti_0.47)O₃-(Ni_0.5Zn_0.5)Fe₂O₄ (PZT-NZFO) magnetoelectric (ME) composite ceramics were fabricated by a modified hybrid process at a low sintering temperature of 900 °C. Well-controlled crystallized grain size and homogeneous microstructure with a good mixture of two phases were observed in the ceramics. The ceramics show coexistence of ferrimagnetic and ferroelectric phases with well-formed ferromagnetic and ferroelectric hysteresis loops at room temperature. A significant ME effect was observed with a ME coefficient of 0.537 V cm⁻¹ Oe⁻¹ in the vicinity of electromechanical resonance. In addition, high capacitance can be obtained at low frequency, and magnetic properties in the ceramics can be tailored by the grain size of the ferromagnetic particles in a simple and flexible way.     

Impact of selective thermal annealing on rubrene–C60 heterojunction solar cells

The influence of thermal annealing at different fabrication stages on rubrene–C₆₀ heterojunction organic solar cells is presented. Devices grown at room temperature showed s-shaped current density–voltage (J–V) characteristic. A pre- or post-fabrication thermal treatment step is found effective in elimination of s-shaped J–V. An exclusive gain in open-circuit voltage was measured when rubrene–C₆₀ layer was fabricated at elevated substrate temperature of 80 °C. The results indicate that introduction of temperature at various stage of device fabrication allows selective control over organic layer growth and hence organic–organic interface formation. The 133 mV increase in open-circuit voltage in solar cells fabricated at substrate temperature is due to temperature induced molecular conformation change at the light in-coupling ITO/PEDOT interface.     

Growth and spectroscopic properties of Nd-doped Na2Gd4(MoO4)7 crystal

This paper reports the growth and spectral properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystals. An Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal with dimensions of Ø20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal were investigated. The Judd-Ofelt theory was applied to calculate the spectral parameters. The polarized absorption cross-sections of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal are 4.25 × 10⁻²⁰ cm² with full width at half maximum (FWHM) of 14.6 nm for the π-polarization and 2.87 × 10⁻²⁰ cm² with FWHM of 16.2 nm for the σ-polarization, respectively. The emission cross-sections are 10.0 × 10⁻²⁰ cm² at 1060 nm for π-polarization and 13.6 × 10⁻²⁰ cm² at 1067 nm for σ-polarization, respectively. The fluorescence quantum efficiency has been estimated to be 90.0%. Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal may be considered as a potential laser gain medium for the diode laser pumping.     

Enhancement of device efficiency in CuPc/C60 based organic photovoltaic cells by inserting an InCl3 layer

An InCl₃ dipole layer is inserted into a copper phthalocyanine (CuPc) and fullerene (C₆₀) based organic photovoltaic cell (OPV) to modify the indium-tin-oxide (ITO) anode surface. The work function of the ITO is improved from 4.63 eV to 5.47 eV. In addition, a 30% enhancement in absorption coefficient is achieved due to the strong interaction between CuPc and InCl₃ molecules, which induces a configuration change of the CuPc stacks from perpendicular to parallel along the ITO substrate. Therefore, the power conversion efficiency of the OPV devices has a 30% improvement because of the improved work function of the ITO anode and the enhanced absorption coefficient of the devices.     

Transport properties of sealed MgB2/Fe/Ni multifilamentary wires heat treated in air

Ni sheathed multifilamentary MgB₂ wires with Fe barrier and Cu stabilizer were prepared by the in situ Powder-In-Tube (PIT) method. After rolling, the ends of the wires were sealed by a simple capping technique and the wires were directly heat treated in air, without vacuum or any inert atmosphere. The quality of the wires was assessed by analysing the phase assemblage and measurement of superconducting properties such as R-T, J_C-T and J_C-H. Phase analysis revealed that only traces of MgO was formed in the superconductor core. Typical multifilamentary wires prepared by this method showed a T_C ≈ 38.5 K and ΔT_C ≈ 1 K and J_C of the order of 10⁵ A/cm² at 6 K (0 T) and 10⁴ A/cm² at 4.2 K (6 T) respectively. These values are quite comparable with the values obtained for wires heat treated in inert atmosphere.     

Comparison of electrical characteristics for p-type and n-type organic thin film transistors using copper phthalocyanine

The p-type and n-type organic thin film transistors (OTFTs) were fabricated in the same experimental conditions by using hexadecahydro copper phthalocyanine (H₁₆CuPc) and hexadecafluoro copper phthalocynine (F₁₆CuPc) molecules, respectively. The mobilities of H₁₆CuPc and F₁₆CuPc-based OTFT devices in saturation region were measured to be ∼1.22 × 10⁻³ cm²/V s and ∼1.04 × 10⁻³ cm²/V s, respectively. The temperature dependence of the mobility and activation energy (E_a) for both OTFTs were measured in saturation and linear regions of the drain-source current. We found that the E_a of the F₁₆CuPc-based OTFTs was lower than that of H₁₆CuPc-based ones. The gate voltage (V_g) dependence of the field-effect mobility measured in linear region for the F₁₆CuPc-based OTFTs was more stable, i.e., weaker variation of the field-effect mobility with increasing V_g, than that of the H₁₆CuPc-based ones. The high electron affinity of the hexadecafluorine (F₁₆) in CuPc contributed to the effective electron accumulation in the active channel.     

Effect of spin ½ radicals on the ultrafast photoexcitation dynamics in RR-P3HT/PCBM blends for photovoltaic applications

We report the effect of spin ½ radical impurity, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of regio-regular poly(3-hexylthiophene) (RR-P3HT)/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) blend. The addition of Galvinoxyl radical impurities to the blend reduces the geminate recombination rate of photogenerated polaron pairs. Consequently organic photovoltaic solar cells made from the radical/blend mixture as the active layer show increased short-circuit current (J_sc), fill-factor, and power conversion efficiency. We speculate that the spin–spin interaction of the radical impurity with the electron and hole forming the polaron pairs is the main mechanism responsible for the obtained reduced recombination rate.     

Effect of UV–ozone treatment on ITO and post-annealing on the performance of organic solar cells

In this paper, the influence of the ultraviolet (UV)–ozone treatment of indium tin oxide (ITO) surface and the active layer post-annealing treatment on the performance of organic solar cells were investigated. Bulk heterojunction organic solar cells based on the blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) were fabricated. It is found that the devices with the UV–ozone treatment for 5 min on ITO substrates show the better performance, compared with the devices without this treatment. The results demonstrate that the short-circuit current density (J_sc) and fill factor (FF) could be improved by the post-annealing treatment. The devices with both treatments together show the best performance, with the increase of J_sc from 2.68 mA/cm² to 4.13 mA/cm² and the enhancement of FF from 32.2% to 38.8%. Therefore, the power conversion efficiency is improved from 0.62% to 1.08%. The morphology of the active layers with and without the post-annealing treatment was characterized by atomic force microscopy.     

Low band gap copolymer containing naphthalene-1,4,5,8-tetracarboxylic bisimide: Synthesis,properties and organic solar cell applications

A new alternating donor–acceptor copolymer, poly{[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-4,7-(2,1,3-benzothiadiazole)]-co-[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracarboxylic-N,N′-di(2-ethylhexyl)imide]} (PFTBTN), was synthesized for the use of photovoltaic cells. The copolymer containing fluorene, as the donor segment and naphthalene bisimide, dithienylbenzothiadiazole, as the acceptor segment was polymerized via Suzuki couplings to achieve a polymer with a narrow band gap. The band gap values of the copolymer film determined from optical and electrochemical measurements were 1.69 and 2.06 eV, respectively. The optical absorption spectrum revealed two broad bands in the range of 300–750 nm. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high open-circuit voltage when fullerene derivative ([6,6]-phenyl-C₆₁ butyric acid methyl ester) was used as an electron acceptor. Bulk heterojunction photovoltaic cells were fabricated in the device configuration of ITO/PEDOT:PSS/PFTBTN:PCBM/TiO_x/Al. Open-circuit voltage reached 0.74 V with the maximum energy conversion efficiency of 0.40% under the illumination of AM 1.5 (100 mW/cm²).