Fabrication of organic photovoltaic cells with double-layer ZnO structure

The fabrication process of a photovoltaic cell with a structure of indium-tin-oxide (ITO)/double ZnO/C₆₀/poly(3-hexylthiophene) (PAT6)/Ag has been investigated. The C₆₀/PAT6 heterojunction of this cell was fabricated by spin-coating a chloroform solution of PAT6 onto the C₆₀ thin film formed on double-layer ZnO-coated ITO. The fabrication of this double-layer ZnO was a new method, which was a composite of a sputtered ZnO layer and oriented zinc oxide nanograins layer fabricated at low temperature (343 K). Insertion of the double-layer ZnO in the photovoltaic cells produced enhanced performance with the power conversion efficiency of 1.31% under AM1.5 illumination.     

Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system

Here, we report the fabrication of high-efficiency poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend photovoltaic device. Process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio and cathodes structures are optimized in making the devices. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, made by mixing 6 wt% glycerol to PEDOT:PSS, is used as a buffer layer. Highest efficiency of 4.64% was obtained for the device made with 1:0.7 ratio of P3HT to PCBM, o-dichlorobenzene:chloroform cosolvent, newly developed slow process and G-PEDOT:PSS. Film morphology is evaluated by atomic force microscopy (AFM). Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements are also performed for the best device.     

Preparation of smooth and dense composite films consisting of MEHPPV and neat C60 by means of electrophoretic deposition

A procedure to prepare flat and dense composite films of a conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) and various amounts of neat C₆₀ by means of electrophoretic deposition from dilute suspensions is reported. It has been observed that deposition from suspensions containing both components results in pinhole-free films, which are applicable to photovoltaic devices, while inhomogeneous films are formed from the suspensions containing only one of the components. The applicability of electrophoretic deposition to obtain polymer-C₆₀ composite films showing photovoltaic effects has been successfully demonstrated.     

Solvent treatment inducing ultralong cycle stability poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) fibers as binding-free electrodes for supercapacitors

As one kind of conducting polymer composite, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) has been widely used as an electrode for energy storage and conversion devices because of its optical transmittance, flexibility, and high electrical conductivity etc. Here, we prepared binding-free PEDOT:PSS fibers (PFs) electrodes with high capacitive performance for supercapacitors via a facile method followed by various solvent treatments. Dimethyl sulfoxide (DMSO)-treated electrodes displayed a better specific capacitance (C_s) of 202 F/g at 0.5 A/g with higher elongation at break, flexibility, and conductivity of 140.7 S/cm, compared to those of pristine PEDOT:PSS materials. More importantly, the DMSO-treated fibers possessed improved stability, which retained 105% of the initial C_s after 22 000 long cycles at 10 A/g. It is believed that the fabricated PFs will be promising organic electrodes for portable supercapacitors and other flexible electronic devices in the near future.     

Poly(3-hexylthiophene)/C60 heterojunction solar cells: Implication of morphology on performance and ambipolar charge collection

The performance of heterojunction organic solar cells is critically dependent on the morphology of the donor and acceptor components in the active film. We report results of photovoltaic devices consisting of bilayers and bulk heterojunctions using poly(3-hexylthiophene) (P3HT) and Buckminsterfullerene C₆₀. White light power efficiencies of η2.2% (bulk heterojunction) and 2.6% (bilayer) were measured after a thermal annealing step on completed devices. Optical and structural investigations on non-annealed bilayer thin films indicated a distinct porosity of the spin-coated polymer, which allows C₆₀ to penetrate the P3HT layer and to touch the anode. This resulted for these bilayer solar cells in the experimental observation that electrons were collected predominantly at the cathode after photo-excitation of P3HT, but predominantly at the anode after C₆₀ excitation. A morphological model to explain the ambipolar charge collection phenomenon is proposed.     

Stability and photodegradation mechanisms of conjugated polymer/fullerene plastic solar cells

Degradation studies of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV), fullerenes ((6,6)-phenyl C₆₁- butyric acid methyl ester (PCBM) and C₆₀), and mixtures, which are the photoactive components in plastic solar cells, are shown. The degradation processes of the individual components and of a 1 : 3 mixture are characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and by current/voltage (I–V) measurements of devices under the influence of light and oxygen. A faster degradation rate was found for the polymer compared with C₆₀. In composites with fullerenes, the stability of MDMO-PPV is enhanced due to the fast electron transfer to C₆₀.     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

Fabricating red-blue-switching dual polymer electrochromic devices using room temperature ionic liquid

In this work, room temperature ionic liquid (RTIL)—1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF₆)—was employed to fabricate dual polymer electrochromic devices (DPECDs). [BMIM]PF₆ was used as the electrolyte both in the electrochemical synthesis of conducting polymers (CPs) and in the fabrication of DPECDs. The electrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methylthiophene) (PMeT) were employed to serve as two complementary coloring electrochromic thin films. Through combining these two electrochromic layers, the assembled DPECDs were found to switch between deep red and deep blue, which are two primary colors for a display. By employing RTIL as electrolyte, the devices retained 65% of their optical contrast and electroactivity after 5×10³ deep double potential steps, showing enhanced stability and durability. The DPECDs also exhibited stable electrochromic performance, with a maximum optical contrast of 26% at 665 nm, and achieved a high coloring efficiency of 460 cm² C^-1.     

Polymer photovoltaic devices using highly conductive poly(3,4-ethylenedioxythiophene-methanol) electrode

In this work, modified poly(3,4-ethylenedioxythiophene) (PEDOT) was used as an anode in polymer photovoltaic devices (PVDs) based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM). We synthesized poly(3,4-ethylenedioxythiophene methanol) (PEDTM) with a transmittance of 87% (at 510 nm) and a conductivity of 700 S/cm. PEDTM was applied in photovoltaic devices as a hole transporting layer on indium-tin oxide (ITO) electrode as well as a direct anode layer. PVDs with PEDTM as hole transporting layers on ITO showed a very high short-circuit density of 14.87 mA/cm² and power conversion efficiency of 2.67% under an illumination of AM 1.5 G (100 mW/cm²). In addition, we also fabricated ITO-free PVDs using PEDTM as an anode, which exhibited a performance of 0.61% with a result of J_sc of 4.48 mA/cm², V_oc of 0.51 V, and FF of 27%.     

Photovoltaic performance and stability of CdTe/polymeric hybrid solar cells using a C60 buffer layer

The influence of C₆₀ as a buffer layer on photovoltaic performance and stability of bulk hetero-junction solar cells using a photoactive layer of CdTe nanocrystals and poly(p-phenylenevinylene) derivative has been investigated. The incident photon-to-current conversion efficiency of the sealed-cells with a C₆₀ buffer layer was about 70% of that of solar cells using a buffer layer of LiF. The introduction of C₆₀ buffer layer gave an improvement in the stability for a light-harvesting test. In a long-term thermal stability test, there was no difference between the solar cells using C₆₀ and LiF. An impedance analysis suggested that the degradation of the performance in the stability tests was associated with a decrease in the conductance of the cells.     

Effect of molecular aggregation by thermal treatment on photovoltaic properties of MEH-PPV: Fullerene-based solar cells

The effect of a thermal annealing treatment at various temperatures on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and buckminsterfullerene (C₆₀) composites was investigated. With the increase in temperature, three stages were observed: from 60 to 120 °C, the photovoltaic performance of the composite devices became better with the rising temperature; from 120 to 180 °C, the photovoltaic performance decreased to a minimum; but, at about 200 °C, a sharp rise occurred, followed by a gradual decline. The obvious red-shift of the photoluminescence (PL) spectra and the new PL peak and shoulder at about 620 and 660 nm, respectively, indicated the formation of molecular aggregation and the lengthened and the ordered conjugated segments. It was suggested that the crystallization of C₆₀ after thermal annealing at 200 °C, which matched the nano-sized ordered domains of MEH-PPV led to a sharp enhancement in photovoltaic performance.     

Synthesis and characterization of a novel fullerene derivative for use in organic solar cells

A novel fullerene derivative with an N-hexylphenothiazine moiety, PTZ-C_60, was synthesized and characterized. The new synthesized fullerene showed good solubility in common organic solvents such as toluene, chlorobenzene and 1, 2 dichlorobenzene. The synthetic product PTZ-C₆₀ was characterized by ¹H and ¹³C NMR, FT-IR and UV-vis spectroscopy. Photovoltaic devices were fabricated using the new fullerene derivative as the electron acceptor and P3HT as the electron donor. The configuration of the device was as follows: ITO/PEDOT:PSS/active layer/LiF/Al. The weight ratios of the electron donor to the acceptor in the active layer were 1:0.5, 1:0.7, and 1:1. The open-circuit voltage (V_oc) of the fabricated devices was found to be higher than that of devices based on C₆₀ because the LUMO energy level of the new fullerene derivative was higher than that of C₆₀. Further, the power conversion efficiency (PCE) of these devices was observed to be high when annealing was carried out at 150 °C for 5 min and the thickness of the active layer was 80 nm. The maximum V_oc, short-circuit current density, and PCE of the best device were 0.608 V, 4.393 mA/cm², and 1.29%, respectively.     

The effect of C60 on the ZnO-nanorod surface in organic-inorganic hybrid photovoltaics

The electrical property of the organic-inorganic heterojunctions is one of the important factors affecting the performance of hybrid photovoltaic devices. We introduce a good electron conductor, C₆₀, to modify the surface of ZnO-nanorod arrays in a ZnO-nanorod/poly(3-hexylthiophene):(6,6)-phenyl C₆₁ butyric acid methyl ester (ZnO/P3HT:PCBM) hybrid photovoltaic structure. We found that there is ∼40% and ∼15% increment in the short circuit current (J_SC) and open circuit voltage (V_OC), respectively, for the device after C₆₀ modification. By probing the carrier dynamics and the surface property of ZnO-nanorods, the presence of the C₆₀ layer assists the exciton separation and passivates part of the defect states on the ZnO-nanorod surface. The charge transport property at the ZnO/polymer blend interface is, therefore, improved. As a result, higher charge concentration can transfer from the polymer blend to the ZnO-nanorods more effectively and subsequently travel to electrodes leading to the improved performance in the photovoltaic device.     

Photoelectric investigations of charge-transferring metal-doped [60] fullerenes

Physical Jet Deposition method was used to form the C₆₀Ni films, one of the metal-doped [60] fullerenes films, which were made into the Al/C₆₀Ni/ ITO layered structure photovoltaic cells. The photovoltaic effect and the current–voltage characteristic in dark with the devices were taken. Compared with the Al/C₆₀/ITO layer structure devices, the enhancement of photovoltage and rectification rate of C₆₀Ni devices was found. Possible reasons were discussed.     

Photovoltaic properties of polymer based organic solar cells adapted for non-transparent substrates

This paper explains a semi-transparent photovoltaic device structure using polymer based materials for light harvesting. Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV:PCBM) as photoactive nano-layers were utilized and semi-transparent cells were compared with reference cells. Photoelectrical properties of developed devices were investigated. Also influencing factors of power conversion efficiency of devices were determined and possible application areas including solar harvesting textiles were discussed.     

Study of the effect of annealing process on the performance of P3HT/PCBM photovoltaic devices using scanning-probe microscopy

We have studied the effect of annealing process on the performance of photovoltaic devices based on the bulk heterojunction of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁ butyric acid methyl ester (P3HT/PCBM). By means of atomic force microscopy (AFM) and scanning of near-field microscopy (SNOM), we can observe the morphology evolution of the annealed P3HT/PCBM composite films. We also studied the changes of optical properties by absorption spectroscopy and the changes of composition distribution of annealed composite films. The results indicate the P3HT in the composite film gradually becomes an ordered structure with annealing. The ordered P3HT facilitates the charge transport. However, the film exhibits a large-scale (1 μm) PCBM aggregation after annealing for an extended period of time. The disrupted bi-continous phase retards the charge transport. Thus, the device efficiency reaches the highest (2.308%) after annealing at 140 °C for 30 min but decreases to 0.810% after 60 min annealing.     

Synthesis and reactivity of aluminized fluorinated acrylic (AlFA) nanocomposites

Fluorinated materials, such as poly(tetrafluoroethylene) and its co-polymers, have attracted significant interest throughout the energetic materials community due to their strong reactivity with aluminum powders. Herein, we report the synthesis of a novel composite material produced through the in situ polymerization of 1H,1H,2H,2H-perfluorodecyl methacrylate in the presence of aluminum nanoparticles which have been previously functionalized with phosphoric acid 2-hydroxyethyl methacrylate ester to promote chemical integration into the polymer matrix. These materials, which we have termed aluminized fluorinated acrylic (AlFA) composites, have been prepared with particle contents ranging from 10% to 70% by weight. At particle loadings of 60 wt.% or less, the AlFA composites exhibited thermoplastic behavior and were able to be processed by melt extrusion. The AlFA-50 composite demonstrated the highest reactivity (most intense flame and shortest time to achieve complete deflagration) during air combustion experiments performed on consolidated pellets. Chemical analysis of the char indicated the presence of AlF₃, in addition to Al₂O₃, Al₄C₃ and residual Al, indicating that reaction with the fluoropolymer matrix does result in fluorination of the aluminum during the deflagration, however, this mechanism competes kinetically with air oxidation and carbide formation at higher particle loadings.     

Interface modification to optimize charge separation in cyanine heterojunction photovoltaic devices

We investigate heterojunction photovoltaic devices using the carbocyanine 1,1′-diethyl-3,3,3′,3′-tetramethylcarbocyanine perchlorate (Cy5) as donor and buckminsterfullerene (C₆₀) as acceptor. We find that photocurrent generation occurs at the interface between CY5 and indium tin oxide (ITO) as well as at the organic heterointerface. By analyzing the spectral dependence of the photocurrent as a function of applied voltage, we were able to demonstrate that poly(3,4-ethylenedioxythiophene) (PEDOT) inhibits electron injection from the cyanine into ITO. Since the photocurrent generation at the ITO electrode is opposite to the one generated at the organic heterojuncion, the use of PEDOT leads to increased short-circuit current and open-circuit voltage.     

Higher fullerenes as electron acceptors for polymer solar cells: A quantum chemical study

In the present study, we have used quantum chemical methods to study the energy levels of the frontier orbitals of higher fullerene derivatives (from C₇₀ to C₈₄ and having the same addend as in [6,6]-phenyl C₆₁-butyric acid methyl ester) with the aim to understand if they can be used as electron acceptors in bulk heterojunction polymer–fullerene solar cells. Higher fullerenes have a stronger and broader absorption compared to C₆₀ and they can improve the current output of the corresponding devices. The geometries of all the compounds were optimized with the density functional theory at the B3LYP/3-21G* level of calculation. The lowest unoccupied molecular orbital (LUMO) levels of the investigated compounds correlate well with the reduction potentials (obtained by cyclic voltammetry) of the already prepared species. We found that the LUMO level depends not only on the fullerene size (number of carbons of the cage) and constitutional isomer, but also on the position and, in some cases, the addend orientation. This issue should be considered because for a proper device operation, a well-defined LUMO is required. The position of the LUMO level of some higher fullerene derivatives can be suitable for low-bandgap polymers.     

Enhancing the short-circuit current and efficiency of organic solar cells using MoO3 and CuPc as buffer layers

Efficient bulk-heterojunction (BHJ) (regioregular poly (3-hexylthiophene) (P3HT): (6, 6)-phenyl C₆₁ butyric acid methyl ester (PCBM)) solar cells were fabricated with molybdenum trioxide (MoO₃) and copper phthalocyanine (CuPc) as buffer layers. The insertion of MoO₃ layer was found to be critical to the device performance, effectively extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy levels. The introduction of CuPc buffer layer was observed to be ameliorative for device performance, further enlarging the visible absorption spectra range of the devices. The effect of the MoO₃ and CuPc layer thickness on device performance was studied. The optimized thickness was achieved when MoO₃ layer was 12 nm and CuPc layer was 6 nm, resulting in optimized power conversion efficiency (PCE) of 3.76% under AM1.5G 100 mW/cm² illumination.