Critical Role of Molecular Electrostatic Potential on Charge Generation in Organic Solar Cells

Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non‐fullerene organic solar cells (OSCs) indicates efficient charge separation even with negligible energetic offset between the donor and acceptor materials. These new findings trigger a critical question concerning the charge separation mechanism in OSCs, traditionally believed to result from sufficient energetic offset between the polymer donor and fullerene acceptor. We propose a new mechanism, which involves the molecular electrostatic potential, to explain efficient charge separation in non‐fullerene OSCs. Together with the new mechanism, we demonstrate a record efficiency of ~12% for systems with negligible energetic offset between donor and acceptor materials. Our analysis also rationalizes different requirement of the energetic offset between fullerene‐based and non‐fullerene OSCs, and paves the way for further design of OSC materials with both high photocurrent and high photovoltage at the same time.     

Photoinduced electron transfer from synthetic chlorophyll analogue to fullerene C60 on carbon paste electrode. Preparation of a novel solar cell

Upon a carbon paste electrode, fullerene C60 and successively methyl pyropheophorbide-a (chlorin) were casted to prepare a chlorin-fullerene modified carbon paste electrode (CFE). Photocurrents on the CFE were produced by irradiating of visible lights (> 510 nm) in an aqueous solution of 0.05 M ethylenediaminetetraacetic acid and 0.1 M Na2SO4 at pH 6.7. Larger anodic photocurrent was induced by the CFE than by the carbon paste electrodes modified with either the fullerene or the chlorin. In addition, the photocurrent of the CFE was dependent upon the amount of fullerene casted. The photocurrent action spectra of the CFE (at 300 mV vs. Ag/Ag+) showed that photoinduced electron transfer occurred from the excited state of the chlorin to the fullerene and/or from the chlorin to the photoexcited fullerene, and the electron of the fullerene anion radical produced was then shifted to the carbon paste. Upon irradiation of > 375 nm lights, the anodic photocurrent of the CFE was enhanced by increase in the illuminated light power and reached 0.03 mA cm-2 in the present system.     

Molecular photoelectric switch using a mixed SAM of organic [60]fullerene and [70]fullerene doped with a single iron atom

We describe a photoswitch fabricated on indium tin oxide (ITO) as a self-assembled monolayer (SAM) of two fullerene molecules, a purely organic [60]fullerene that generates an anodic current and a [70]fullerene doped with a single iron atom. This device generates a bidirectional photocurrent upon irradiation at 340 and 490 nm. The new [70]fullerene iron complex bearing three rigid carboxylic acid legs, Fe[C(70)(C(6)H(4)C(6)H(4)COOH)(3)]Cp, generates only a cathodic current upon photoexcitation between 350 and 700 nm, whereas the organic [60]fullerene absorbs at wavelengths shorter than 500 nm. The quantum efficiency of the photocurrent generation by the mixed SAM is comparable to that of a single-component SAM, indicating that the individual diode molecules on ITO generate photocurrents independently with little cross talk.     

Liquid-crystalline bisadducts of [60]fullerene

A second-generation cyanobiphenyl-based dendrimer was used as a liquid-crystalline promoter to synthesize mesomorphic bisadducts of [60]fullerene. Liquid-crystalline trans-2, trans-3, and equatorial bisadducts were obtained by condensation of the liquid-crystalline promoter, which carries a carboxylic acid function, with the corresponding bisaminofullerene derivatives. A monoadduct of fullerene was also prepared for comparative purposes. All the compounds gave rise to smectic A phases. An additional mesophase, which could not be identified, was observed for the trans-2 derivative. The supramolecular organization of the monoadduct derivative is governed by steric constraints. Indeed, for efficient space filling, adequacy between the cross-sectional areas of fullerene (approximately 100 A(2)) and of the mesogenic groups (approximately 22-25 A(2) per mesogenic group) is required. As a consequence, the monoadduct forms a bilayered smectic A phase. The supramolecular organization of the bisadducts is essentially governed by the nature and structure of the mesogenic groups and dendritic core. Therefore, the bisadducts form monolayered smectic A phases. The title compounds are promising supramolecular materials as they combine the self-organizing behavior of liquid crystals with the properties of fullerene.     

Ultrafast exciton dissociation followed by nongeminate charge recombination in PCDTBT:PCBM photovoltaic blends

The precise mechanism and dynamics of charge generation and recombination in bulk heterojunction polymer:fullerene blend films typically used in organic photovoltaic devices have been intensively studied by many research groups, but nonetheless remain debated. In particular the role of interfacial charge-transfer (CT) states in the generation of free charge carriers, an important step for the understanding of device function, is still under active discussion. In this article we present direct optical probes of the exciton dynamics in pristine films of a prototypic polycarbazole-based photovoltaic donor polymer, namely poly[N-11'-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), as well as the charge generation and recombination dynamics in as-cast and annealed photovoltaic blend films using methanofullerene (PC(61)BM) as electron acceptor. In contrast to earlier studies we use broadband (500-1100 nm) transient absorption spectroscopy including the previously unobserved but very important time range between 2 ns and 1 ms, which allows us not only to observe the entire charge carrier recombination dynamics but also to quantify the existing decay channels. We determine that ultrafast exciton dissociation occurs in blends and leads to two separate pools of products, namely Coulombically bound charge-transfer (CT) states and unbound (free) charge carriers. The recombination dynamics are analyzed within the framework of a previously reported model for poly(3-hexylthiophene):PCBM (Howard, I. A. J. Am. Chem. Soc. 2010, 132, 14866) based on concomitant geminate recombination of CT states and nongeminate recombination of free charge carriers. The results reveal that only ~11% of the initial photoexcitations generate interfacial CT states that recombine exclusively by fast nanosecond geminate recombination and thus do not contribute to the photocurrent, whereas ~89% of excitons create free charge carriers on an ultrafast time scale that then contribute to the extracted photocurrent. Despite the high yield of free charges the power conversion efficiency of devices remains moderate at about 3.0%. This is largely a consequence of the low fill factor of devices. We relate the low fill factor to significant energetic disorder present in the pristine polymer and in the polymer:fullerene blends. In the former we observed a significant spectral relaxation of exciton emission (fluorescence) and in the latter of the polaron-induced ground-state bleaching, implying that the density of states (DOS) for both excitons and charge carriers is significantly broadened by energetic disorder in pristine PCDTBT and in its blend with PCBM. This disorder leads to charge trapping in solar cells, which in turn causes higher carrier concentrations and more significant nongeminate recombination. The nongeminate recombination has a significant impact on the IV curves of devices, namely its competition with charge carrier extraction causes a stronger bias dependence of the photocurrent of devices, in turn leading to the poor device fill factor. In addition our results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer:fullerene organic photovoltaic devices.     

Electric field dependent photocurrent generation in a thin-film organic photovoltaic device with a [70]fullerene-benzodifuranone dyad

A [70]fullerene-benzodifuranone acceptor dyad synthesized by a Ag?-mediated coupling reaction was used to construct a thin-film organic solar cell. The fullerene and the benzodifuranone dye in the dyad have close-lying LUMO levels in the range of 3.7-3.9 eV, so that energy transfer from the dye to the fullerene can take place. A p-n heterojunction photovoltaic device consisting of a tetrabenzoporphyrin and a [70]fullerene-benzodifuranone dyad showed a weak but discernible contribution from light absorption of the dyad to the photocurrent under both a positive and a negative effective bias. These results indicate that the benzodifuranone moiety attached to the acceptor contributes to light-harvesting by energy transfer.     

Self-assembly and characterization of a novel hydrogen-bonded nanostructure

A novel hydrogen-bonded supramolecular system of a [60]fullerene derivative with perylene bisimide was synthesized and characterized. 1H NMR spectra confirmed the existence of strong hydrogen-bonding interaction between compounds 1 and 5. Transmission electron microscopy images of 1.5 aggregates showed spherical particles having a mean diameter of 50 nm. The photocurrent response of the film was measured, and a steady and rapid anodic photocurrent response was obtained.     

Synthesis, crystal structure and photoconductivity of the first [60]fullerene complex with metal diethyldithiocarbamate: {CuII(dedtc)2}2.C60

The first molecular complex of fullerene C60 with metal dithiocarbamate, namely, {CuII(dedtc)2}2.C60(dedtc: diethyldithiocarbamate) (1) was obtained as single crystals. Butterfly-shaped CuII(dedtc)2 molecules efficiently co-crystallized with spherical fullerene molecules to form a layered structure, in which closely packed hexagonal C60 layers alternate with the layers composed of CuII(dedtc)2 dimers. The formation of the complex with C60 changes geometry and the EPR spectrum of starting CuII(dedtc)2. Magnetic susceptibility of 1 follows the Curie-Weiss law in the 300-1.9 K range with the negative Weiss constant of -2.5 K showing a weak antiferromagnetic interaction between CuII centers in the dimers. The crystals of 1 have low dark conductivity of 10(-11) S cm-1, which is consistent with a neutral ground state of the complex. Illumination of the crystals by white light increases the photocurrent by 20-50 times. The photoconductivity spectrum of 1 has a maximum at 470 nm showing that both intermolecular charge transfer between neighboring C60 molecules and photoexcitation of CuII(dedtc)2 can contribute to photogeneration of free charge carriers. The effect of a weak magnetic field with Bo<0.5 T on the photoconductivity of 1 has been found.     

Effect of multiple adduct fullerenes on charge generation and transport in photovoltaic blends with poly(3‐hexylthiophene‐2,5‐diyl)

The effect of replacing [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) by its multiadduct analogs (bis‐PCBM and tris‐PCBM) in bulk heterojunction organic solar cells with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is studied in terms of blend film microstructure, photophysics, electron transport properties, and device performance. Although the power conversion efficiency of the blend with bis‐PCBM is similar to the blend with PCBM, the performance of the devices with tris‐PCBM is considerably lower as a result of small photocurrent. Despite the lower electron affinity of the fullerene multiadducts, μs‐ms transient absorption measurements show that the charge generation efficiency is similar for all three fullerenes. The annealed blend films with multiadducts show a lower degree of fullerene aggregation and lower P3HT crystallinity than the annealed blend films with PCBM. We conclude that the reduction in performance is due largely to poorer electron transport in the blend films from higher adducts, due to the poorer fullerene network formation as well as the slower electron transport within the fullerene phase, confirmed here by field effect transistor measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Hydrogen bonding effect on photocurrent generation in porphyrin-fullerene photoelectrochemical devices

A hydrogen bonding effect on photocurrent generation has been evaluated successfully in a mixed film of porphyrin and fullerene with hydrogen bonding on an ITO electrode, which exhibits efficient cathodic photocurrent generation as compared to the reference system without hydrogen bonding.     

Photocurrent-generating properties of organometallic fullerene molecules on an electrode

Compact, rigid, five-legged fullerene derivatives C60R5Me and M(C60R5)Cp (M = Fe and Ru; R = C6H4COOH, C6H4C6H4COOH, and CH2COOH) were synthesized and arrayed on an indium-tin oxide (ITO) surface. These devices exhibit a respectable quantum yield with photocurrent generation up to 18%, and, more importantly, the direction of the photocurrent can be changed not only by the molecular structure itself but also by changing the geometric configuration of the photoactive acceptor (fullerene) and donor (metal atom) on the ITO surface.     

Photoinduced electron transfer in self-assembled monolayers of porphyrin-fullerene dyads on ITO

Two porphyrin-fullerene dyads were synthesized to form self-assembled monolayers (SAMs) on indium-tin oxide (ITO) electrode, with either ITO-porphyrin-fullerene or ITO-fullerene-porphyrin orientations. The dyads contain two linkers for connecting the porphyrin and fullerene moieties and enforcing them essentially to similar geometries of the donor-acceptor pair, and two linkers to ensure the attachment of the dyads to the ITO surface with two desired opposite orientations. The transient photovoltage responses (Maxwell displacement charge) were measured for the dyad films covered by insulating LB films, thus ensuring that the dyads interact only with the ITO electrode. The direction of the electron transfer was from the photoexcited dyad to ITO independent of the dyad orientation. The response amplitude for the ITO-fullerene-porphyrin structure, where the primary intramolecular electron-transfer direction coincides with the direction of the final electron transfer from the dyad to ITO, was 25 times stronger than that for the opposite ITO-porphyrin-fullerene orientation of the dyad. Static photocurrent measurements in a liquid electrochemical cell, however, show only a minor orientation effect, indicating that the photocurrent generation is controlled by the processes at the SAM-liquid interface.     

A computer simulation study of the ordered phases of some mesogenic fullerene derivatives

We present a Monte Carlo simulation study of the phase behaviour and molecular organization of a system of fullerene-based mesogens, represented by a three-site model composed of a fullerene sphere and two mesogenic moieties rigidly attached to it. It is shown that a combination of suitably modified Lennard-Jones and Gay–Berne attractive–repulsive potentials allows a satisfactory qualitative modeling of the interactions between the fullerene derivatives under investigation. Indeed, simulation results show that, despite the crude representation of the molecular structure, our model generates nematic and smectic phases, thus accounting qualitatively for the basic experimental observations on the class of compounds considered.     

Fabrication and Characterization of a New Self-Assembled Monolayer of Fullerene Dicarboxylic Acid Derivative and Its Photoelectric Conversion Property

A new fullerene self-assembled monolayer (SAM) which has the property of photoelectric conversion is reported here. The SAM was fabricated on hydrophilic substrates by an esterification reaction. The SAM is characterized by contact angle, AFM, UV spectrum, and cyclic voltammetry. A cathodic photocurrent of 226 nA/cm(2) was obtained. Copyright 2000 Academic Press.     

Transient photocurrents in a charge transfer complex of trinitrofluorenone with a carbazole substituted discotic liquid crystal

We describe transient photoconduction studies on a well known triphenylene discotic liquid crystal molecule modified to incorporate a single carbazole moiety on one side chain and doped with the acceptor, trinitrofluorenone. This material is found to show both electron and hole photocurrents when the transient photoconduction study is carried out in the time of flight geometry although no time of flight transits are observed. Of interest here is the fact that the temperature dependence of the hole and electron photocurrents are radically different, the electron photocurrent being strongly activated while the hole photocurrent decreases with increasing temperature. We suggest a mechanism whereby the electron photocurrent controls the hole photocurrent through recombination. Furthermore, the electron activation energy varies with the applied electric field, indicating the influence of the Poole Frenkel barrier lowering mechanism in the production of the charge carriers.     

磷酸修饰的RGO-BiOBr纳米复合体的制备及其光电化学性质研究

本文通过水热法一步合成了还原氧化石墨烯(RGO)-BiOBr纳米复合体,并进一步对其进行磷酸修饰. 主要研究了所获得的纳米复合体的光电化学性质. 结果表明,与纯BiOBr相比,RGO-BiOBr复合体的光电流密度明显提高. 适量磷酸修饰后,其光电流密度进一步得到提高. 基于羟基自由基等测试结果,分析认为磷酸修饰的RGO-BiOBr纳米复合体光电流密度的提高主要归因于两方面：一是复合的还原氧化石墨烯能够接受光生电子,加快电子的转移,进而促进光生电荷的有效分离;二是复合体表面修饰的磷酸在溶液中电离形成负场,能够起到诱导光生空穴的作用,进一步促进了光生电荷的分离.     

Mass spectrometric characterization of 3'-imino[60]fulleryl-3'-deoxythymidine by collision-induced dissociation

The primary structure of 3'-imino[60]fulleryl-3'-deoxythymidine ions is studied using mass spectrometry both in the positive and negative modes. Interaction between the subunits is discussed using collision-induced dissociation (CID) spectra. Collisional activation with argon of the sodiated cations leads to the cleavage of the glycosidic bond and the transfer of a radical hydrogen from the deoxyribose to the thymine. The sodiated thymine is the only fragment observed for low collision energies in the positive mode. In the negative mode, two different ionization mechanisms take place, reduction and deprotonation in the presence of triethylamine. The 2.7 eV electron affinity of C60 and its huge cross section compared to the small cross section and predicted 0.44 eV electron affinity of the thymidine subunit most likely localize the radical electron on the fullerene. On the other hand, deprotonation of the 3'-azido-3'-deoxythymidine (AZT) is known to occur in N-3, the most acidic site of the nucleobase. Consequently, deprotonation causes the negative charge to be initially localized on the thymine. Both types of parent anions give the radical anion C60*- as fragment. The other fragments detected are the dehydrogenated 3'-imino[60]fulleryl-3'-deoxyribose anion, C60NH2-, C60N- and C60H-. Since in negative ion mass spectrometry all fragments include the [60]fullerene unit, this suggests that the fragmentation is driven by the electron affinity of the [60]fullerene, likely responsible for a charge transfer between the deprotonated thymine and the C60.     

Nanostructured thin films of C60-aniline dyad clusters: electrodeposition, charge separation, and photoelectrochemistry

Clusters of C60-aniline dyads are deposited as thin films on nanostructured SnO2 electrodes under the influence of an electric field. At low applied DC voltage (<5 V) the clusters in toluene/acetonitrile (1:3) mixed solvent grow in size (from 160 nm to approximately 200 nm in diameter) while at higher voltages (>50 V) they are deposited on the electrode surface as thin films. The C60- aniline dyad cluster films when cast on nanostructured SnO2 films are photoelectrochemically active and generate photocurrent under visible light excitation. These nanostructured fullerene films are capable of delivering relatively large photocurrents (up to approximately 0.2 mA cm(-2), photoconversion efficiency of 3-4%) when employed as photoanodes in photoelectrochemical cells. Both luminescence and transient absorption studies confirm the formation of charge transfer product (C60 anion) following UV/Vis excitation of these films. Photo-induced charge separation in these dyad clusters is followed by the electron injection from C60-anion moiety into the SnO2 nanocrystallites. The oxidized counterpart is reduced by the redox couple present in the electrolyte, thus regenerating the dyad clusters. The feasibility of casting high surface area thin fullerene films on electrode surfaces has opened up new avenues to utilize dyad molecules of sensitizer bridge donor type in light energy conversion devices, such as solar cells.     

Unusually high performance photovoltaic cell based on a [60]fullerene metal cluster-porphyrin dyad SAM on an ITO electrode

A self-assembled monolayer (SAM) of a C60-triosmium cluster complex Os3(CO)7(CNR)(CNR')(mu3-eta2:eta2:eta2-C60) (ZnP-C60; R = (CH2)3Si(OEt)3, R' = ZnP) on an ITO surface exhibits ideal electrochemical responses as well as remarkable enhancement of the photocurrent generation. The diazabicyclooctane (DABCO) binding (ZnP)-C60/ITO/AsA/Pt cell shows the highest photocurrent generation quantum yield (19.5%) ever reported for the molecular photovoltaic cells based on the covalently linked donor-acceptor dyad structures. The high efficiency in photocurrent generation is ascribed to an efficient electron transfer from photoexcited porphyrin to fullerene, revealed by fluorescence lifetime measurements and transient absorption decay profiles. These results provide valuable information on the new strategy for the construction of molecular photonic devices and artificial photosynthetic systems on ITO electrodes.     

Effects of fullerene substituents on structure and photoelectrochemical properties of fullerene nanoclusters electrophoretically deposited on nanostructured SnO2 electrodes

Two kinds of fullerene derivatives have been designed to examine the effect of the fullerene substituents on the structure and photoelectrochemical properties of fullerene clusters electrophoretically deposited on nanostructured SnO(2) electrodes. The cluster sizes increase and the incident photon-to-current efficiencies decrease with introduction of large substituents into C(60). The trend for photocurrent generation efficiency as well as surface morphology on the electrode can be explained by the steric bulkiness around the C(60) molecules. A C(60) molecule with two alkoxy chains is suggested to give a bilayer vesicle structure, irrespective of the hydrophobic nature of both the C(60) and alkoxy chain moieties. Such information will be valuable for the design of photoactive molecules, which are fabricated onto electrode surfaces to exhibit high energy conversion efficiency.