Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Mechanical flexibility of transparent PEDOT:PSS electrodes prepared by gravure printing for flexible organic solar cells

The mechanical flexibility of transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films printed onto a flexible PET substrate using a gravure printing method was investigated using a lab-made bending test system. Gravure-printed PEDOT:PSS electrodes with a sheet resistance of 359 Ω/square and a transparency of 88.92% showed outstanding flexibility in several types of flexibility tests, including outer/inner bending, twisting and stretching. Notably, the PEDOT:PSS electrode had a constant resistance change (ΔR/R₀) within an outer and inner bending radius of 10 mm. In addition the stretched PEDOT:PSS electrode showed a fairly constant resistance change (ΔR/R₀) up to 4%, which is more stable than the resistance change of conventional amorphous ITO electrode. The twisting test revealed that the resistance of the PEDOT:PSS electrode began to increase at an angle of 36° due to delamination of the film from the PET substrate. Despite the high sheet resistance of the PEDOT:PSS electrode the flexible organic solar cells fabricated on the PEDOT:PSS electrode showed a power conversion efficiency of ∼2% (FF: 44.9%, V_o: 0.495 V and J_sc: 9.1 mA/cm²), indicating the possibility of using gravure printed PEDOT:PSS as a flexible and transparent electrode for printing-based flexible organic solar cells.     

A study on the catalytic hydrogenation of N-ethylcarbazole on the mesoporous Pd/MoO3 catalyst

Mesoporous MoO₃ shows an apparent activity in the catalytic hydrogenation of N-ethylcarbazole (NEC), where a significant amount of tetrahydro-N-ethylcarbazole (4H-NEC) and perhydro-N-ethylcarbazole (PNEC) are detected with the hydrogen uptake of 0.97 wt% after 6 h when the temperature rises to 220 °C. 0.5 wt% Pd/MoO₃ catalyst shows a superior catalytic efficiency than the traditional precious metal catalysts 0.5 wt% Ru/Al₂O₃ and 0.5 wt% Pd/Al₂O₃, especially in the conversion of Octahydro-N-ethylcarbazole (8H-NEC) to PNEC. The hydrogenation mechanism of MoO₃ is completely different from the traditional precious metal catalysts. With the presence of a small amount of Pd, the breaking of HH bond is greatly accelerated, result in the promotion of hydrogen spillover rate and the increase of the concentration of hydrogen molybdenum bronze H_xMoO₃, which improves the catalytic efficiency of the MoO₃ catalyst. Rise the temperature also helps increasing the concentration of H in H_xMoO₃.     

Fe3O4 nanoparticles induced magnetic field effect on efficiency enhancement of P3HT:PCBM bulk heterojunction polymer solar cells

Fe₃O₄ magnetic nanoparticles (mean size of about 10 nm) capped by surfactant oleic acid (OA) were incorporated into P3HT:PCBM BHJ-PSCs by doping in the P3HT:PCBM photoactive layer for the first time. The PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is enhanced by ∼18% at the optimum OA-Fe₃O₄ NPs doping ratio of 1%. The role of the magnetic property of Fe₃O₄ NPs on the PCE of OA-Fe₃O₄:P3HT:PCBM devices was studied, confirming the exclusive contribution of the Fe₃O₄ NPs to the observed enhancement of PCE. The enhancement of the PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is found to be primarily due to the increase of short-circuit current (J_sc) by ∼14%, which is attributed to the magnetic field effect originated from the superparamagnetism of Fe₃O₄ NPs, resulting in the increase of the population of triplet excitons. Finally, the effect of Fe₃O₄ NPs on the enhancement of PCE of OA-Fe₃O₄:P3HT:PCBM device is further investigated by comparing different means of doping in P3HT:PCBM or PEDOT:PSS layer, confirming that such an effect can be achieved only when Fe₃O₄ NPs are doped in the P3HT:PCBM photoactive layer.     

Transparent electrode with ZnO nanoparticles in tandem organic solar cells

The transparent inter-electrodes with the p/n heterojunction consisting of the solution-processible ZnO nanoparticles as the n-type and the conventional hole injection layers (MoO₃ or PEDOT:PSS) as the p-type materials are studied for developing tandem organic solar cells employing different band gap active materials (i.e., P3HT:PCBM blend layer for larger band gap material in the bottom cell and ZnPc/C₆₀ bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the V_OC corresponding to the sum of V_OC’s of the top and bottom unit cells is obtained, denoting that the two unit cells are successfully connected in series. For the ZnO/MoO₃ inter-electrode, the open-circuit voltage (V_OC) of the tandem cell is smaller than the sum of V_OC’s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (∼3 nm) between ZnO and MoO₃ (ZnO/Al/MoO₃) as the recombination center for carriers.     

Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi₂S₃), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi₂S₃ compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi₂S₃ film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi₂S₃/P3OT/Au structure, Bi₂S₃ was established as an electron acceptor. To produce ITO/Bi₂S₃/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi₂S₃ film and a gold film was thermally evaporated. Under 100 mW/cm² tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi₂S₃ film with thickness of 50 nm has the highest open circuit voltage (V_oc) of 440 mV and short-circuit current density (J_sc) of 0.022 mA/cm². The addition of a CdS thin film (90 nm) between ITO and B₂S₃ would increase V_oc to 480 mV, and J_sc to 0.035 mA/cm². The role of surface morphology and optoelectronic properties of the Bi₂S₃ film in the photovoltaic performance of the junction is discussed.     

Sol-gel synthesis of Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 composite membrane for visible-light driving photocatalytic hydrogen generation

By using TiO₂ and Ta₂O₅ colloids, a stable and efficient visible-light driven photocatalyst, Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane, was successfully prepared via sol–gel dip coating method at room temperature. The XRD, FTIR, SEM, TEM and EDX results confirm that approximately spherical Er³⁺:Y₃Al₅O₁₂ nanoparticles were embedded in TiO₂Ta₂O₅ matrix. UV–vis absorption and PL spectra of Er³⁺:Y₃Al₅O₁₂ were also determined to confirm the visible absorption and ultraviolet emission. The photocatalytic hydrogen generation was carried out by using methanol as sacrificial reagent in aqueous solution under visible-light irradiation. Furthermore, some main influence factors such as heat-treated temperature, heat-treated time and molar ratio of TiO₂ and Ta₂O₅ on visible-light photocatalytic hydrogen generation activity of Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane were studied in detail. The experimental results showed that the photocatalytic hydrogen generation activity of Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane heat-treated at 550 °C for 3.0 h was highest when the molar ratio of TiO₂ and Ta₂O₅ was adopted as 1.00:0.50. And that a high level photocatalytic activity can be still maintained after four cycles. In addition, a possible mechanism for the visible-light photocatalytic hydrogen generation of the Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ membrane was proposed based on PL spectra.     

Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules

Solar cells based on nanoporous TiO₂ films with an inverted structure of indium tin oxide (ITO)/TiO₂/copper phthalocyanine (CuPc):fullerene (C₆₀)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO₂ (30 nm)/nanoporous TiO₂ (130 nm)/C₆₀:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO₂ films has better photovoltaic properties compared to those using dense TiO₂ films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C₆₀:CuPc between TiO₂ and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability.     

Morphological and electrical effect of an ultrathin iridium oxide hole extraction layer on P3HT:PCBM bulk-heterojunction solar cells

An ultrathin iridium layer was treated with O₂-plasma to form an iridium oxide (IrO_x), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO_x layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrO_x by the O₂-plasma treatment. The OPV cell with IrO_x (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm⁻² illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.     

Effect of indium and tin contamination on the efficiency and electronic properties of organic bulk hetero-junction solar cells

Organic photovoltaic devices using an electrode of indium tin oxide (ITO) coated with a buffer layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) exposed to controlled humidity during fabrication showed a 65-75% decrease in efficiency and displayed S-shaped J-V curves, changes, which are attributed to different levels of indium and tin migration into the PEDOT:PSS film. A distinct shift in the secondary electron cut-off in the UV Photoelectron spectra (UPS) of ITO/PEDOT:PSS samples exposed to controlled humidity indicate an increase of the dipole at the ITO/PEDOT:PSS interface, which could explain the appearance of S-shaped J-V curves. Additionally, the electron density at low binding energies is reduced for the humidity exposed PEDOT:PSS suggesting a second mechanism for decreased device performance.     

Electrical properties of inverted poly(3-hexylthiophene): Methano-fullerene [6,6]-phenyl C71-butyric acid methyl ester bulk hetero-junction solar cell with Cs2CO3 and MoO3 layers

We report the electrical properties of inverted P3HT:PC₇₁BM bulk hetero-junction (IBHJ) with Cs₂CO₃ and MoO₃ interlayers. The current density (J)-voltage (V) characteristics of the inverted P3HT:PC₇₁BM bulk hetero-junction solar cells are studied, where MoO₃ thickness is varied from 0.5 to 2 nm. The temperature independent, symmetric J-V characteristics in dark were found in the low bias voltage region between −0.35 and 0.35 V, and they exhibited the power law relationship J-V^1.2 when the MoO₃ thickness is between 1 and 2 nm. Open-circuit voltages at various light intensities of the IBHJ cell plotted as a function of absolute temperature converge to ∼0.965 V at zero temperature, which is very close to the energy difference between LUMO of PC₇₁BM and HOMO of P3HT.     

The structural,electrical and optical properties of ZnO/Al2O3 multilayer deposited on PET substrates by RF sputtering

In this work ZnO/Al₂O₃ multilayer is deposited on PET(polyethylene terephthalate) substrate by RF sputtering in argon atmosphere and the effect of Al₂O₃ layer on optical, electrical and structural properties of ZnO is investigated. It is observed that the presence of Al₂O₃ layer between ZnO and PET reduce the transmission of light from ZnO/Al₂O₃/PET structure 11% but if a thin layer of SiO₂ ∼100 nm is applied between Al₂O₃ and PET it can be compensated. The XRD results show that the ZnO film had hexagonal wurtzite structure with (002) preferred orientation and the application of Al₂O₃ and SiO₂ layers do not have any effect on its structure. The effect of Al₂O₃ and SiO₂ layers on optical band gap of ZnO has been investigated, also the surface morphology of ZnO film is studied by scanning electron microscopy.     

Low bandgap carbazole copolymers containing an electron-withdrawing side chain for solar cell applications

A new series of low bandgap carbazole copolymers containing an electron-withdrawing moiety as a side chain, via Suzuki, Yamamoto, and Stille polymerization reactions has been synthesized. Their bandgaps and molecular energy levels can be tuned by copolymerizing with different conjugated electron-donating units. The resulting copolymers have low optical and electrochemical bandgaps. The optical bandgaps of the copolymers range from 1.79 to 1.24 eV. In order to investigate their photovoltaic properties, polymer solar cell devices based on low bandgap copolymers were fabricated with a structure of ITO/PEDOT:PSS/copolymers:PCBM/Al, under the illumination of AM 1.5 G, 100 mW/cm². The power conversion efficiencies (PCE) of the polymer solar cells based on these low bandgap copolymers were measured. The best performance was obtained by using PC-CARB as the electron donor and 6,6-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron acceptor. The PCE of the solar cell based on PC-CARB/P₇₁CBM (1:4) was 1.27% with an open-circuit voltage (V_oc) of 0.65 V, and a short-circuit current (J_sc) of 6.69 mA/cm².     

Electrochemistry of LiMn2O4 epitaxial films deposited on various single crystal substrates

LiMn₂O₄ epitaxial thin films were synthesized on SrTiO₃:Nb(1 1 1) and Al₂O₃(0 0 1) single crystal substrates by pulsed laser deposition (PLD) method and the electrochemical properties were discussed comparing with that of amorphous LiMn₂O₄ film on polycrystalline Au substrate. LiMn₂O₄ epitaxial film showed only a single plateau in charge–discharge curves and a single redox peak at the corresponding voltage of cyclic voltammograms. This phenomenon seems to originate from the effect of the epitaxy: the film is directly connected with the substrate by the chemical bond and this connection would suppress the phase transition of Li_xMn₂O₄ film during lithium (de-)intercalation. The discharge voltage of LiMn₂O₄ epitaxial film on SrTiO₃ was lower than that of LiMn₂O₄ film on Al₂O₃. This lowered discharge voltage may be caused by the electronic interaction between LiMn₂O₄ film and SrTiO₃:Nb n-type semiconductor substrate.     

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (J_sc) of 0.93 μ Acm⁻², open-circuit photovoltage (V_oc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm⁻² intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a J_sc value of 2.36 μ Acm⁻², V_oc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm⁻² intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H₂pyp₃p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)₂).     

Effect of Au,Ag and Cu thin films' thickness on the electrical parameters of metal-porous silicon direct hydrogen fuel cell

The effect of the thickness of the gold, silver and cupper films on the electrical properties such as open circuit voltage (V_oc) and short circuit current (I_sc), in the direct hydrogen fuel cell, which uses water as a source of hydrogen, is studied by fabricating Metal/Porous Silicon/n-Silicon/Indium structures. The Porous Silicon (PS) layer on n-type (111) oriented silicon wafers were prepared by anodization. The thin films of Au or Ag or Cu with different thicknesses between 120 and 600 nm were deposited onto the PS surface by the electron-beam technique. The obtained results indicated that V_oc and I_sc, strongly depend on the Au, Ag and Cu layer thicknesses. The Au/PS/n-Si structure generated highest V_oc and I_sc values for all thicknesses of Au film. The best values of V_oc and I_sc were obtained at 325 nm as 0.89 V and 0.021 mA for Au, at 350 nm as 0.75 V and 0.017 mA for Ag, at 350 nm as 0.50 V and 0.010 mA, respectively.     

An amorphous LiCo1/3Mn1/3Ni1/3O2 thin film deposited on NASICON-type electrolyte for all-solid-state Li-ion batteries

Amorphous LiCo_1/3Mn_1/3Ni_1/3O₂ thin films were deposited on the NASICON-type Li-ion conducting glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 130 °C. The amorphous films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiCo_1/3Mn_1/3Ni_1/3O₂/Au all-solid-state cells were fabricated to investigate the electrochemical performance of the amorphous films. It was found that the low-temperature deposited amorphous cathode film shows a high discharge voltage and a high discharge capacity of around 130 mAh g⁻¹.     

Improvement of polymer/fullerene solar cells by controlling geometry of the ITO substrate surface

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO/glass substrate. We prepared an ITO surface with an average roughness, R_a, of 0.7–11 nm by chemical etching. At first J_sc increases with the increase in ITO surface roughness and then gradually decreases. The maximum performance was obtained at R_a≈4 nm. J_sc is also high with a very flat surface of R_a=0.7 nm. This feature can be attributed to the trade-off between the increase in absorption light path length and film-quality deterioration.     

Preparation of flexible conductive composite electrode film of PEDOT:PSS/Aramid nanofibers via vacuum-assisted filtration and acid post-treatment for efficient solid-state supercapacitor

A highly conductive and flexible composite film of Poly (3,4-ethylenedioxythiophene):polystyrene sulfonate and aramid nanofiber (PEDOT:PSS/ANFs) is prepared by vacuum-assisted filtration and post-treated by acid. The composite displays excellent mechanical integrity under bending together with flexibility, properties being attributed to the strong attachment of PEDOT:PSS onto the surface of the ANFs via hydrogen bonding and the ANF structure, respectively. The conductivity of the prepared composite is progressively enhanced by the post-treatment using sulfuric acids (1 M H₂SO₄ and 1.5 M H₂SO₄), reaching 20–25 times higher than that of untreated film. This enhancement is traced to the removal of the insulating PSS group together with an analyzable change in crystallization of the PEDOT:PSS component. However, excessive use of acid treatment is seen to reduce the mechanical strength, and, thus, ultimate loss of conductivity after frequent bending (up to 1000 times), only having 59% conductivity retention with high concentration acid treatment (1.5 M H₂SO₄) compared to a high conductivity retention of 95% with 1.0 M H₂SO_4. Adopting the relatively weaker acid enables a balance to be reached between these crucial factors of electrical conductivity versus mechanical integrity. The prepared film of PEDOT: PSS/ANFs treated by acid as an electrode of supercapacitor shows good electrochemical performances, including good volumetric specific capacitance (83.5 F/cm³ with 1.0 M H₂SO₄ and 75 F/cm³ with 1.5 M H₂SO₄ at 0.5 A/cm^?3), cycle stability and capacitance retention of 83.3% and 87.5% after 2000 cycles, respectively. Furthermore, a solid flexible supercapacitor is finally assembled by the post-treatment of relatively low concentration acid with 1.0 M H₂SO_4. The configured supercapacitor displays excellent volumetric energy density of 23.44 mW h/cm² (power density of 399.95 mW/cm²) at a very wide operating potential window of 0–1.6 V and cycle stability. Therefore, it is quite feasible method to fabricate a highly conductive and flexible composite film using PEDOT:PSS and ANFs by vacuum filtration and acid post-treatment, which expects to be a promising flexible composite electrode material applied in the preparation of energy storage devices.