Efficient Photoinduced Charge Transfer Research Articles

Efficient Solar Cells from Layered Nanostructures of Donor and Acceptor Conjugated Polymers

Efficient photoinduced charge transfer and separation accompanied by large photovoltaic power conversion efficiencies were observed in layered nanostructures of acceptor poly(benzimidazobenzophenanthroline ladder) (BBL) and donor poly(p-phenylenevinylene) (PPV) or poly(2-methoxy-5(2‘-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH−PPV) at practical sunlight intensities. Nearly complete quenching of photoluminescence and large shortening of fluorescence decay lifetimes of donors PPV and MEH−PPV evidenced fast efficient photoinduced charge transfer and separation in PPV/BBL and MEH−PPV/BBL bilayers. ITO/PPV(60 nm)/BBL(60 nm)/Al photovoltaic cells had an external power conversion efficiency of 1.5% and 4.6% under AM1.5 white light illumination intensities of 80 and 1 mW/cm2, respectively, in ambient air. Under single-wavelength (460 nm) 0.5 mW/cm2 illumination, the same PPV/BBL diodes had a power conversion efficiency of 5.4% and incident photon-to-current conversion efficiency of 62%. ITO/MEH−PPV(60 nm)/BBL(60 ...

Donor-acceptor polymers: a conjugated oligo(p-phenylene vinylene) main chain with dangling perylene bisimides.

Two new donor-acceptor copolymers that consist of an enantiomerically pure oligo(p-phenylene vinylene) main chain with dangling perylene bisimides have been synthesized by using a Suzuki cross-coupling polymerization. Absorption and circular dichroism spectroscopy revealed that the transition dipole moments of the donor in the main chain and the dangling acceptor moieties of the copolymers are coupled and in a helical orientation in solution, even at elevated temperatures. A strong fluorescence quenching of both chromophores indicates an efficient photoinduced charge transfer after photoexcitation of either donor or acceptor. The formation and recombination kinetics of the charge-separated state were investigated in detail with femtosecond and near-steady-state photoinduced absorption spectroscopy. The charge-separated state forms within 1 ps after excitation, and recombination occurs with a time constant of 45-60 ps, both in solution and in the solid state. These optical characteristics indicate a short distance and appreciable interaction between the electron-rich donor chain and the dangling electron-poor acceptor chromophores.

Hybrid nanocrystalline TiO2 solar cells with a fluorene–thiophene copolymer as a sensitizer and hole conductor

We report the effects of layer thickness, interface morphology, top contact, and polymer–metal combination on the performance of photovoltaic devices consisting of a fluorene–bithiophene copolymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed in this system, while charge recombination is relatively slow (∼100 μs–10 ms). External quantum efficiencies of 13% and monochromatic power conversion efficiencies of 1.4% at a wavelength of 440 nm are achieved in the best device reported here. The device produced an open-circuit voltage of 0.92 V, short-circuit current density of about 400 μA cm−2, and a fill factor of 0.44 under simulated air mass 1.5 illumination. We find that the short-circuit current density and the fill factor increase with decreasing polymer thickness. We propose that the performance of the indium tin oxide/TiO2/polymer/metal devices is limited by the energy step at the polymer/metal interface and we investigate this situation using an alternative fluorene-based polymer and different top contact metals.

Carbon Nanotube/CdS Core–Shell Nanowires Prepared by a Simple Room‐Temperature Chemical Reduction Method

A facile solution‐phase reaction has been developed to fabricate carbon nanotube/CdS core–shell nanowires (see Figure). The thickness of the CdS sheath can be controlled by varying the concentration of the precursors. The obtained heterostructures demonstrate an enhanced surface photovoltage response, which is associated with the efficient photoinduced charge transfer from the CdS shell to the carbon nanotube core.

Photoinduced Electron Transfer and Photovoltaic Response of a MDMO‐PPV:TiO2 Bulk‐Heterojunction

Combining conjugated poylmers and TiO2 in hybrid bulk‐heterojunctions is a promising method for producing novel solar cells. Here TiO2 is introduced into a poly(p‐phenylene vinylene) layer. Nanometer‐scale phase separation allows efficient photoinduced charge transfer between the two components. The films can be used as the active layer in a photovoltaic cell and provide external quantum efficiencies of up to 11 % (see Figure).

Efficient organic photovoltaics from soluble discotic liquid crystalline materials

Two different types of soluble discotic liquid crystalline materials and a crystalline perylene dye have been used to create, directly from solution, photovoltaic devices which are compared in this work. Self-organisation of the soluble electron-accepting perylene derivative and the soluble liquid crystalline (LC) discotic material which is stable in a LC phase at room temperature (HBC-PhC 12) leads to segregated structures optimised for charge separation and transport in photovoltaic device structures. High external quantum efficiencies up to 34% near 490 nm have been reached. The high efficiencies result from efficient photo-induced charge transfer between the materials as well as effective transport of electrons and holes to the cathode and anode through segregated perylene and the discotic peri-hexabenzocoronene p-system. Atomic force microscopy and device characteristics suggest that the driving force for phase separation and surface energy effects during spin coating of the HBC-PhC 12:perylene blend result in a spontaneous vertical segregation of the HBC and the perylene normal to the plane of the spun film. This represents a nearly ideal, self-organised structure in which vertical segregation of charge transport layers coexist with a high interfacial area between the two charge transfer components. This vertical segregation has not been observed in the spin-coated blends where the HBC-PhC 12 is replaced by HBC- C 8 ∗ . One probable reason for this may be the different phase stability of the LC phase in the HBCs, which leads to different film-forming properties and film morphologies.

Optical and xerographic properties of phthalocyanine codeposited composite film and ultrathin multilayered structure

The optical and xerographic properties of the phthalocyanine codeposited composite (cDC) films and ultrathin multilayered (UTML) structures have been studied. Observed UV-visible absorption spectra indicate that the Q-band absorption of the cDc films is different from that of the single component films, and from that of the UTML structures. The absorption peaks are shifted with the number and thickness of the layers. This spectral variation is most likely to be due to the dipole–dipole and charge transfer interactions among different molecular assemblies. The photosensitivity of the cDC films is superior to that of the single component films. This improvement is attributed to the efficient photoinduced charge transfer and charge separation among the composites. Moreover, the photosensitivity of the UTML structures is over tenfold better than that of the cDC films, probably due to the ultrafast photoinduced charge transfer and separation and the unipolar transport of the photogenerated holes.

SYNTHESIS AND PROPERTIES OF [60]FULLERENE-POLYVINYLPYRIDINE CONJUGATES FOR PHOTOVOLTAIC DEVICES

A two-phase radical polymerization was applied for the synthesis of fullerene-containing poly(4-vinylpyridine), which were used as polyelectrolytes in the fabrication of photovoltaic devices. Two-phase radical polymerization resulted in the products of poly(4-vinyl pyridinated) fullerenes (PVPyF) containing a high fullerene content and poly(4-vinylpyridine) arms of short length. These polyelectrolytes with a C60 content of up to 35% by weight are soluble in organic solvents and water at a low pH. High efficient photoinduced charge transfer in samples of PVPyF and polyanionic polythiophene derivatives fabricated by the electrostatic layer-by-layer deposition technique in producing self-assembled multilayers (SAMs) was demonstrated. As a result, high response to the increase of photoinduced conductivity was observed on the 75 bilayer device of PVPyF-3+/H-PURET− upon irradiation. Photoconductivity was found to increase in more than one order of magnitude from 0.024 to 0.31 S/cm at an applied voltage of 1.0 volt. The sign of short-circuit photocurrent was consistent with electrons flowing from Al electrode towards the ITO modified substrate. †Deceased.

Self-organized discotic liquid crystals for high-efficiency organic photovoltaics.

Self-organization of liquid crystalline and crystalline-conjugated materials has been used to create, directly from solution, thin films with structures optimized for use in photodiodes. The discotic liquid crystal hexa-peri-hexabenzocoronene was used in combination with a perylene dye to produce thin films with vertically segregated perylene and hexabenzocoronene, with large interfacial surface area. When incorporated into diode structures, these films show photovoltaic response with external quantum efficiencies of more than 34 percent near 490 nanometers. These efficiencies result from efficient photoinduced charge transfer between the hexabenzocoronene and perylene, as well as from effective transport of charges through vertically segregated perylene and hexabenzocoronene pi systems. This development demonstrates that complex structures can be engineered from novel materials by means of simple solution-processing steps and may enable inexpensive, high-performance, thin-film photovoltaic technology.

Zinc(II)-operated intramolecular charge transfer fluorescence emission in pyrene-2,2′-bipyridine conjugated molecular rods

The design and synthesis of two conjugated pyrene-2,2′-bipyridine linear rods is described. Zn(II) ion complexation allows the formation of a highly polar emissive excited state through efficient photoinduced intramolecular charge transfer.

Grafting of buckminsterfullerene onto polythiophene: novel intramolecular donor-acceptor polymers

Conjugated polymer-fullerene composites have shown efficient photoinduced charge transfer. The attachment of a fullerene moiety to the conducting polymer backbone is expected to lead to materials with more intimate association of the donor/acceptor sites. Two approaches to the attachment of fullerenes onto polythiophene derivatives have been explored. In the first case, fullerene has been bonded to a bithiophene derivative which can be electropolymerized. In the second, solvent processable polythiophene copolymers were prepared and functionalized with fullerene. Both these systems exhibit electrochemical and optoelectrochemical properties for fullerene and the conducting polymer.

Photochemical energy storage in a spatially organized zeolite-based photoredox system

Zeolites are often employed as organizational media or supports for entrapped or adsorbed transition-metal catalysts and photo-catalysts1–6. In such applications, the individual catalytic species have been associated with the framework structure of the zeolite in a purely statistical (randomized) arrangement. A synthetic strategy developed recently7 has shown how a much higher level of organization can be obtained, so pointing the way to the generation of systems in which two or more active components can be arranged—both spatially and in terms of reactivity— within the zeolite host to enhance the efficiency of a desired catalytic reaction. Here we describe an application of this approach to photochemical storage of light energy. Such an application requires efficient photoinduced charge transfer between donor and acceptor molecules to form long-lived charge-separated states: the competing thermal back electron transfer reaction must be minimized. This is achieved in our system by arranging the active components (donor, acceptor and a 'sensitizing' intermediate molecule) such that they occupy adjacent cages within the zeolite framework, and results in unprecedented levels of net charge-separation efficiency.

Infrared photoexcitation spectra of conducting polymer/methanofullerene films

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)-methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index, ΔN = Δn(ω)+iΔϰ(ω), are obtained. The results indicate that the PIA spectra are dominated by Δϰ(ω), while the PIR spectra are dominated mainly by Δn(ω). The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials.

Photoinduced absorption and photoinduced reflectance in conducting polymer/methanofullerene films: Nonlinear-optical changes in the complex index of refraction.

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)/methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index \ensuremath{\Delta}N=\ensuremath{\Delta}n(\ensuremath{\omega})+i\ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}) are obtained. The results indicate that the PIA spectra are dominated by \ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}); the subgap electronic absorptions peak at approximately 0.1 and 1.6 eV. The PIR spectra are dominated mainly by \ensuremath{\Delta}n(\ensuremath{\omega}), which exhibits a derivative spectral shape with zero crossing at 0.1 eV. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear-optical applications of these materials. \textcopyright{} 1996 The American Physical Society.

Enhanced nonlinear absorption and optical limiting in semiconducting polymer/methanofullerene charge transfer films

Nonlinear optical absorption in solid films of poly(3-octylthiophene) (P3OT) sensitized with methanofullerene was investigated for wavelengths from 620 to 960 nm. The nonlinear absorption is enhanced over that in either of the component materials by more than two orders of magnitude at 760 nm. The large nonlinearity results from efficient photoinduced intermolecular charge transfer from P3OT to methanofullerene, followed by absorption in the charge separated excited state. P3OT/fullerene films are promising as optical limiters; the transmission clamps at an average fluence of approximately 0.1 J/cm2. The damage threshold was 15 μJ/pulse (≊1 J/cm2 in average fluence), above which there is a permanent change in the linear transmission.

Stable photoinduced charge separation in layered viologen compounds

SOLAR energy can be used and stored by the efficient production of long-lived photoinduced charge separation1–3—a state achieved in photosynthetic systems by the formation of a long-lived radical pair4–7. A number of artificial systems have been reported that efficiently undergo photochemical charge transfer8–10; unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. Here we report the photochromic behaviour of novel layered zirconium phosphonate/viologen compounds which show very efficient photo-induced charge transfer, and form a charge-separated state which is long-lived and stable in air. Spectroscopic studies indicate that the photoproduct is the dialkyl viologen radical cation, produced in the inter lamellar region of the zirconium phosphonate. We suggest that the remarkable stability of the charge-separated state arises from structural features which allow for stabilization of the radicals by delocalization and shielding from molecular oxygen.