Acceptor Heterojunction Research Articles

Fabrication and charge/energy-transfer study of 4,7-bis(4-triphenylamino)benzo- 2,1,3-thiadiazole/CuPc composite films

Composite films of 4,7-bis(4-triphenylamino)benzo-2,1,3-thiadiazole (TBT) and copper phthalocyanine (CuPc) are fabricated via protonation-coelectrophoretic deposition from nitromethane solutions of TBT/CuPc mixture in the presence of trifluoroacetic acid as a protonation reagent. A nanospheres–nanowires interpenetrating network structure is obtained when the molar percentage of TBT is 70%. Furthermore, the existence of TBT makes α-phased CuPc be partly transformed into the β-phase, and simultaneously, CuPc disorganizes the TBT unit cells. The blue shift on the absorption edge of TBT and the significant fluorescence quenching in the composite films indicate energy/charge transfer and donor–acceptor (D–A) heterojunction formation. Then these results are proved from another point of view: the mutual overlap of absorption and emission spectra of TBT and CuPc lead to a bidirectional Förster resonance energy transfer at the interface; the molecular energy levels calculated from the results of cyclic voltammetry theoretically determine that there exist a D–A heterojunction and charge transfer from TBT to CuPc. Finally, from the investigation of the field-induced surface photovoltage spectra, it can be concluded that this charge transfer results in efficient dissociation of the photoinduced excitons in the composite films, followed by the generation of a strong photovoltage response.

Organic Photovoltaic Cells Based on Continuously Graded Donor–Acceptor Heterojunctions

We demonstrate enhanced organic photovoltaic cell (OPV) efficiency through the use of continuously graded donor-acceptor (D-A) heterojunctions. Device performance is a strong function of both D-A grading and overall composition ratio. The use of a tunable gradient permits an increase in the D-A interface area for high-exciton diffusion efficiency relative to a planar heterojunction, while also improving the charge collection efficiency relative to a uniform mixture. Using the archetypical D-A pair of copper phthalocyanine and C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> , a power conversion efficiency of η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> = (2.1 ± 0.1)% is realized under 100 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> simulated AM1.5G solar illumination for a graded heterojunction. This represents an improvement in η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> of ~60% relative to a planar heterojunction OPV and ~20% compared to a uniformly mixed heterojunction OPV.

Evaluation of bis-dicyanovinyl short-chain conjugated systems as donor materials for organic solar cells

Three conjugated compounds based on carbazole, cyclopentadithiophene and dithienopyrrole substituted by branched alkyl chains have been synthesized by the Knoevenagel condensation of malonodinitrile with the appropriate dicarboxaldehyde. Electronic properties of the target compounds have been analyzed in solution by UV–vis absorption spectroscopy and cyclic voltammetry and their potentialities as a donor material in donor–acceptor heterojunction solar cells have been evaluated in bilayer devices involving both solution processed or vacuum deposited donor layers and thermally evaporated fullerene C 60 as an acceptor.

On the improvement of the anode/organic material interface in organic solar cells by the presence of an ultra-thin gold layer

The energy conversion efficiency of organic solar cells based on simple electron donor/electron acceptor heterojunction has been studied as a function of the interface anode/electron donor. The anode is fluorine doped tin oxide film. It is shown that the solar cell efficiency increases when a metal ultra thin layer is introduced between the anode and the electron donor. Experimental and theoretical studies show that the work function value of the metal is one of the main factors which allow improving the organic solar cells performances, via better hole collection efficiency. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystalline–Crystalline Donor–Acceptor Block Copolymers

Efficient combination of two polymerization reactions allowed various complex issues in photovoltaic devices, such as light absorption, the presence of a donor–acceptor heterojunction, photoluminescence quenching, crystallinity, and microphase separation, to all be addressed in a single block copolymer (see picture).

Design of Multilayered Nanostructures and Donor–Acceptor Interfaces in Solution‐Processed Thin‐Film Organic Solar Cells

AbstractMultilayered polymer thin‐film solar cells have been fabricated by wet processes such as spin‐coating and layer‐by‐layer deposition. Hole‐ and electron‐transporting layers were prepared by spin‐coating with poly(3,4‐ethylenedioxythiophene) oxidized with poly(4‐styrenesulfonate) (PEDOT:PSS) and fullerene (C60), respectively. The light‐harvesting layer of poly‐(p‐phenylenevinylene) (PPV) was fabricated by layer‐by‐layer deposition of the PPV precursor cation and poly(sodium 4‐styrenesulfonate) (PSS). The layer‐by‐layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor–acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best‐performance device with a triple‐layered structure of PEDOT:PSS|PPV|C60, where the thickness of the PPV layer was 11 nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of −3 V. The power conversion efficiency of the triple‐layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100 mW cm−2 in air.

Ionic Space Charge Driven Organic Photovoltaic Devices

Most all-organic solar cells rely on undoped electron donor–acceptor heterojunctions. Power-conversion efficiencies depend critically on the photoinduced charge generation at these interfaces such as the charge transport through the layers and collection at the electrodes. Hence, the ability to regulate and control these processes would offer advanced device functionality. Mobile ions are able to create internal electric fields similar to conventional, electronic p-n junctions without having the inconvenience of doping, which often leads to carrier recombination and excited state quenching. We demonstrate that at organic heterointerfaces these ionic junctions can shift the electronic orbital energy level, which allows the direction of electron transfer processes to be controlled. Cationic cyanine dyes offer a convenient model system to study the effect of mobile ions systematically. In conjunction with usually strong electron acceptors such as the Buckminsterfullerene C60, and donors such as the poly(p-phenylenevinylene) derivative MEH-PPV, we fabricated bilayer photovoltaic devices to reveal exciting effects due to ionic interfacial space charge. For example, we show that C60 can be turned into an electron donor. Furthermore, oxidative or reductive electron transfer processes can simply be switched on and off with an applied bias, thereby drastically altering device performance and spectral sensitivity.

Solar cells utilizing small molecular weight organic semiconductors

AbstractIn this review, we focus on the field of organic photovoltaic cells based on small molecular weight materials. In particular, we discuss the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance. These include the donor–acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterojunction, and the stacked or tandem cell. We show how the choice of materials with known energy levels and absorption spectra affect device performance, particularly the open‐circuit voltage and short‐circuit current density. We also discuss the typical materials and growth techniques used to fabricate devices, as well as the issue of device stability, all of which are critical for the commercialization of low‐cost and high‐performance organic solar cells. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Molecular Engineering of Coaxial Donor−Acceptor Heterojunction by Coassembly of Two Different Hexabenzocoronenes: Graphitic Nanotubes with Enhanced Photoconducting Properties

Coassembly of two different hexa-peri-hexabenzocoronenes (HBC), one possessing an electron-accepting trinitrofluorenone (TNF) unit (1) and the other without TNF (2), takes place to give graphitic nanotubes with varying acceptor concentrations on their surface. While the diameters of the homotropic nanotubes of 1 and 2 differ from one another, the coassembled nanotubes change their dimensions from one to the other as a function of the molar ratio of 1 to 2. The photoconductivity of the coassembled nanotubes shows a bell-shaped dependency on the molar ratio, where the maximum photoconductivity arises at around a 75% mole fraction of 1. Because of a possible excitation energy migration in the graphitic layer of π-stacked HBC, an efficient HBC-to-TNF electron transfer, leading to charge separation, is realized even when the surface concentration of TNF is low. On the other hand, a high concentration of TNF on the nanotube surface hardly results in promoting the anticipated charge recombination, thanks to the ...

Nanostructured Organic Layers via Polymer Demixing for Interface-Enhanced Photovoltaic Cells

Significant progress is being made in the photovoltaic energy conversion using organic semiconducting materials. One of the focuses of attention is the morphology of the donor−acceptor heterojunction at the nanometer scale, to ensure efficient charge generation and loss-free charge transport at the same time. Here, we present a method for the controlled, sequential design of a bilayer polymer cell architecture that consists of a large interface area with connecting paths to the respective electrodes for both materials. We used the surface-directed demixing of a donor conjugated/guest polymer blend during spin coating to produce a nanostructured interface, which was, after removal of the guest with a selective solvent, covered with an acceptor layer. With use of a donor poly(p-phenylenevinylene) derivative and the acceptor C60 fullerene, this resulted in much-improved device performance, with external power efficiencies more than 3 times higher than those reported for that particular material combination s...

Positional disorder enhancement of exciton dissociation at donor∕acceptor interface

We investigate the dissociation of a Coulomb bounded electron-hole geminate pair at a disordered donor∕acceptor (D-A) heterojunction by extending a previous proposal in the literature [Arkhipov et al., Appl. Phys. Lett. 82, 4605 (2003)] and using Monte Carlo simulations. We show that the presence of a layer of randomly distributed dipoles at the D-A interface creates both a potential well that confines the hole and a repulsive barrier that prevents the geminate pair recombination, even when the effective mass of the hole is around the electron rest mass. Our calculations depend strongly on the heterojunction morphology. However, contrary to what is generally believed, we find that the disorder in the position of the dipoles along the D-A interface axis enhances the pair dissociation. Inhomogeneities in the acceptor concentration at the heterojunction can then create highly efficient centers for exciton dissociation. The model explains recent experimental results for organic D-A heterojunctions and has important consequences on the design of organic photovoltaic devices.

Pathways for Photoinduced Charge Separation and Recombination at Donor−Acceptor Heterojunctions: The Case of Oligophenylenevinylene−Perylene Bisimide Complexes

Semiempirical Hartree-Fock techniques have been applied to assess the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The corresponding rates have been estimated in the framework of the Marcus-Levich-Jortner formalism for different geometries of the complexes. The results indicate that dissociation pathways involving the lowest two charge transfer excited states contribute significantly to the dynamics of the whole process. The rates are found to be strongly sensitive to the relative position of the donor and acceptor units and can be rationalized in terms of symmetry arguments applied to relevant electronic levels.

A novel donor–acceptor heterojunction from single-walled carbon nanotubes functionalized by erbium bisphthalocyanine

Organic functionalization of single-walled carbon nanotubes (SWNTs) was realized by chemical reaction of octaamino substituted erbium bisphthalocyanine (OAErPc 2) and SWNTs with carboxyl groups, and SWNTs covalently bonded by OAErPc 2 via amide bond as a donor–acceptor (D–A) heterojunction were obtained. FTIR and thermal analysis indicated that the amide bond formed as a bridge between OAErPc 2 (as a donor) and SWNTs (as an acceptor). Strong intramolecular interaction in this D–A heterojunction was found from UV–vis absorption and charge transfer from the phthalocyanine rings of OAErPc 2 to SWNTs was observed from the Raman shift, which was expected to facilitate the photoinduced charge transfer and the development of photodetectors.

Controlled growth of a molecular bulk heterojunction photovoltaic cell

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor–acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons1. Further efficiency increases have been achieved in both polymer2,3 and small-molecular-mass4 organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD5), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.

Charge transport, injection, and photovoltaic phenomena in oligo(phenylenevinylene) based diodes

AbstractWe report on the charge transport and injection phenomena of (E,E,E,E)‐1,4‐bis[(4‐styryl)styryl]‐2‐methoxy‐5‐(2′‐ethylhexoxy)benzene (MEH‐OPV5) sandwiched between asymmetric contacts. The hole mobility of MEH‐OPV5 was determined by means of transient electroluminescence. The steady‐state current was injection‐limited. The electric field and temperature dependence of the current were quantitatively described by a phenomenological injection model of thermally assisted charge‐carrier tunneling in a one‐dimensional chain of hopping sites. Furthermore, we report on the photovoltaic properties of thin‐film photovoltaic cells on the basis of donor–acceptor heterojunctions. MEH‐OPV5 and buckminster fullerene were used as the donor and acceptor materials, respectively. The emphasis was on the role of morphology in such devices. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2665–2673, 2003

Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices

AbstractWe report on solution‐processed hybrid solar cells consisting of a nanocrystalline inorganic semiconductor, CuInS2, and organic materials. Synthesis of quantized CuInS2 nanoparticles was performed using a colloidal route, where the particle surface was shielded by an organic surfactant. First attempts were made to use nanocrystalline CuInS2 with fullerene derivatives to form flat‐interface donor–acceptor heterojunction solar cells. We investigated also bulk heterojunctions by replacing the CuInS2 single layer by a blend of CuInS2 and p‐type polymer (PEDOT:PSS; poly(3,4‐ethylenedioxythiophene:poly(styrene sulfonic acid) in the same cell configuration. Bulk heterojunction solar cells show better photovoltaic response with external quantum efficiencies up to 20 %.

Realization of large area flexible fullerene — conjugated polymer photocells: A route to plastic solar cells

Bulk donor — acceptor heterojunctions between conjugated polymers and fullerenes have been utilized for photovoltaic devices with quantum efficiencies of around 1%. These devices are based on the photoinduced, ultrafast electron transfer between non degenerate ground state conjugated polymers and fullerenes. In this work we present efficiency data of solar cells based on a soluble alkoxy poly( para phenylenevinylene) (MDMO-PPV) and a highly soluble fullerene derivative (PCBM). Small area (mm 2) photovoltaic devices show energy conversion efficiencies η e > 1% and charge carrier collection efficiencies η c20%. We present efficiency and stability studies on large area (6 cm x 6 cm) flexible solar cells and compare them with small area devices. The process of large scale coating using printing techniques demands excellent Theological properties of the solution and the compound. We studied the influence of conventional polymers as additives for the photoactive compounds in order to improve homogenous film formation. Addition of small amounts (11 wt%) of conventional polymers does not alter the device characteristics. At higher concentrations of additives conversion efficiency drop down quickly.

Photovoltaic properties of conjugated polymer/methanofullerene composites embedded in a polystyrene matrix

Bulk donor–acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic energy conversion efficiencies above 1%. The photovoltaic response of these devices is based on the ultrafast, photoinduced electron transfer from the conjugated polymer to the fullerene [N. S. Sariciftci and A. J. Heeger, Handbook of Organic Conductive Molecules and Polymers, (Wiley, New York, 1997), pp. 413–455]. In this work we present efficiency data of solar cells based on a soluble derivative of p-phenylene vinylene (PPV), poly [2-methoxy, 5-(3′,7′-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV), and a highly soluble methanofullerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), embedded into a conventional polymer, polystyrene (PS). By the blending of the optimized donor–acceptor components into the conventional polymer matrix, the percolation threshold for photovoltaic response of the three component systems is found to be determined by percolation of the methanofullerene in the polymer matrix. We present current/voltage data of PS–MDMO-PPV–PCBM devices with various PS concentrations as well as photoinduced absorption studies in the infrared [(PIA) Fourier transform infrared] and light induced electron spin resonance studies on the electron transfer in these composites. At low light intensities, the monochromatic power conversion efficiency ηe and the photon carrier collection efficiency ηc of the PS free device are calculated with 1.5% and 18%, respectively.

Laminated fabrication of polymeric photovoltaic diodes

Photoexcited electron transfer between donor and acceptor molecular semiconductors provides a method of efficient charge generation following photoabsorption, which can be exploited in photovoltaic diodes1,2,3. But efficient charge separation and transport to collection electrodes is problematic, because the absorbed photons must be close to the donor–acceptor heterojunction, while at the same time good connectivity of the donor and acceptor materials to their respective electrodes is required. Mixtures of acceptor and donor semiconducting polymers3,4 (or macromolecules5) can provide phase-separated structures which go some way to meeting this requirement, providing high photoconductive efficiencies. Here we describe two-layer polymer diodes, fabricated by a lamination technique followed by controlled annealing. The resulting structures provide good connectivity to the collection electrodes, and we achieve a short-circuit photovoltaic quantum efficiency of up to 29% at optimum wavelength, and an overall power conversion efficiency of 1.9% under a simulated solar spectrum. Given the convenience of polymer processing, these results indicate a promising avenue towards practical applications for such devices.