Organized assemblies of single wall carbon nanotubes and porphyrin for photochemical solar cells: charge injection from excited porphyrin into single-walled carbon nanotubes

Photochemical solar cells have been constructed from organized assemblies of single-walled carbon nanotubes (SWCNT) and protonated porphyrin on nanostructured SnO2 electrodes. The protonated form of porphyrin (H4P2+) and SWCNT composites form 0.5-3.0 microm-sized rodlike structures and they can be assembled onto nanostructured SnO2 films [optically transparent electrode OTE/SnO2] by an electrophoretic deposition method. These organized assemblies are photoactive and absorb strongly in the entire visible region. The incident photon to photocurrent efficiency (IPCE) of OTE/SnO2/SWCNT-H4P2+ is approximately 13% at an applied potential of 0.2 V versus saturated calomel electrode. Femtosecond pump-probe spectroscopy experiments confirm the decay of the excited porphyrin in the SWCNT-H4P2+ assembly as it injects electrons into SWCNT. The dual role of SWCNT in promoting photoinduced charge separation and facilitating charge transport is presented.     

Photovoltaic cells using composite nanoclusters of porphyrins and fullerenes with gold nanoparticles

Novel organic solar cells have been prepared using quaternary self-organization of porphyrin (donor) and fullerene (acceptor) units by clusterization with gold nanoparticles on nanostructured SnO2 electrodes. First, porphyrin-alkanethiolate monolayer-protected gold nanoparticles (H2PCnMPC: n is the number of methylene groups in the spacer) are prepared (secondary organization) starting from the primary component (porphyrin-alkanethiol). These porphyrin-modified gold nanoparticles form complexes with fullerene molecules (tertiary organization), and they are clusterized in acetonitrile/toluene mixed solvent (quaternary organization). The highly colored composite clusters can then be assembled as three-dimensional arrays onto nanostructured SnO2 films to afford the OTE/SnO2/(H2PCnMPC+C60)m electrode using an electrophoretic deposition method. The film of the composite clusters with gold nanoparticle exhibits an incident photon-to-photocurrent efficiency (IPCE) as high as 54% and broad photocurrent action spectra (up to 1000 nm). The power conversion efficiency of the OTE/SnO2/(H2PC15MPC+C60)m composite electrode reaches as high as 1.5%, which is 45 times higher than that of the reference system consisting of the both single components of porphyrin and fullerene.     

Generation and decay dynamics of triplet excitons in Alq3 thin films under high-density excitation conditions

We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.     

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.     

五甲川菁染料敏化p-NiO纳米结构电极的光电化学

研究了NiO纳米结构电极及五甲川菁染料敏化NiO纳米结构电极的光电化学行为。结果表明,NiO纳米结构电极在光照下产生阴极光电流,为p-型半导体,其禁带宽度为2.8eV,使用五甲川菁染料敏化可以显著地提高NiO纳米结构电极的阴极光电流,使NiO纳米结构电极吸收波长红移至可见光区,光电转换效率得到明显改善。研究了OTE/TiO~2/Ru(bpy)~2(SCN)~2和OTE/NiO/PMC作为光阳极和光阴极组成电池的电池特性,结果表明复合电池的光电压提高,但光电流的大小受光电流小的电极限制。     

Polymer‐coated Boron Doped Diamond Optically Transparent Electrodes for Spectroelectrochemical Sensors

Spectroelectrochemical sensors combine electrochemistry, spectroscopy, and partitioning into a film to provide improved selectivity for the target analyte. The sensor usually consists of an optically transparent electrode (OTE) coated with a charge selective polymer film. The polymer film is chosen to pre‐concentrate analyte at the OTE surface to improve the sensitivity and provide selectivity against like charged interferences. OTEs such as Indium Tin Oxide (ITO) have been used extensively for spectroelectrochemical sensors, but little is known about the applicability of such sensors using other OTE materials, such as Boron Doped Diamond (BDD). One distinct advantage of BDD OTEs over ITO OTEs is their significant increase in sensitivity for organic compounds, such as 4‐aminophenol and hydroquinone. We have developed absorption and fluorescence‐based sensing methods with a BDD OTE coated with a sulfonated ionomer film, Nafion. This is demonstrated with tris(2,2′‐bipyridyl)ruthenium(II) ion [Ru(bpy)₃²⁺] using an attenuated total reflectance (ATR) flow cell setup for both absorption and fluorescence. With a Nafion coated BDD optically transparent thin layer electrode (OTTLE), we developed a fluorescence based sensor for a common polyaromatic hydrocarbon (PAH), 1‐hydroxypyrene (1‐pyOH), achieving a detection limit of 80 nM (17 ppb). This work manifests new sensing applications while broadening the use of spectroelectrochemistry, OTEs, and BDD as an electrode material.     

Singlet and triplet excited-state interactions and photochemical reactivity of phenyleneethynylene oligomers

The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances have made them useful as molecular linkers in the light energy harvesting assemblies and molecular electronics devices. These linker molecules themselves possess rich photochemistry as evident from the relatively large yields of the excited singlet (0.5-0.66) and triplet (0.4-0.5) states of two model oligomers, 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). In particular, the long-lived triplet excited state is capable of undergoing deactivation by self-quenching processes such as ground-state quenching and triplet-triplet (T-T) annihilation. The T-T annihilation occurs with a nearly diffusion-controlled rate (approximately 2 x 10(9) M(-1) s(-1)), and ground-state quenching occurs with a rate constant of approximately 6 x 10(7) M(-1) s(-1). The electron transfer from the excited OPE-1 and OPE-2 to benzoquinone as characterized from the transient absorption spectroscopy illustrates the ability of these molecules to shuttle the electrons to acceptor moieties. In addition, pulse radiolysis experiments confirm the spectroscopic fingerprint of the cation radical (or "trapped hole") with absorption bands in the 500-600 nm region.     

Excited-state symmetry and reorientation dynamics of perylenes in liquid solutions: time-resolved fluorescence depolarization studies using one- and two-photon excitation

The excited-state symmetry and molecular reorientation of perylene, 1,7-diazaperylene, and 2,5,8,11-tetra- tert-butylperylene have been studied by different fluorescence depolarization experiments. The first excited electronic singlet state was reached through one-photon excitation (OPE) and two-photon excitation (TPE). A 400 and 800 nm femtosecond laser pulse was used for this purpose, and data were collected by means of the time-correlated single-photon counting technique. It is found that the rotational correlation times for each perylene derivative are very similar in the OPE and TPE depolarization experiments. For the determination of the two-photon absorption tensor, a recently described theoretical model has been applied (Ryderfors et al. J. Phys. Chem. A 2007, 111, 11531). It was found that the two-photon process can be described by a 2 x 2 absorption tensor for which the components are solvent dependent and exhibit mixed vibronic character. In the dipole approximation this is compatible with a parity-forbidden two-photon absorption into the first excited singlet state.     

Photoinduced electron injection from Ru(dcbpy)2(NCS)2 to SnO2 and TiO2 nanocrystalline films

Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from approximately 1 ps (2.5 ps for SnO2) to approximately 50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.     

Efficient singlet fission discovered in a disordered acene film

Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPT's pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet fission yield, with 1.22 triplets being created per excited singlet. This triplet production occurs over two principal time scales, with ~50% of the triplets appearing within 1 ps after photoexcitation followed by a slower phase of triplet growth over a few hundred picoseconds. To fit these kinetics, we have developed a model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission. Singlet excitons directly excited at these sites can undergo fission rapidly, while singlet excitons created elsewhere in the film must diffuse to these sites to fission.     

Synthesis and photoinduced electron-transfer process of a novel triphenylamine-substituted polyfluorene-C60 triad

The photoinduced electron-transfer process of a newly prepared, soluble, pi-conjugated poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] (PDPAF), covalently bridged, C60 triad (C60-PDPAF-C60) is described. The molecular orbital calculations revealed that the majority of the highest occupied molecular orbital (HOMO) is located on the polyfluorene entity, while the lowest unoccupied molecular orbitals (LUMO) are found to be entirely on the C60 entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as by transient absorption techniques in toluene and benzonitrile. By excitation of the polyfluorene moiety, fluorescence quenching of the singlet excited state of polyfluorene moiety was observed. The nanosecond transient spectra in near-IR region revealed the charge-separation process from the polyfluorene moieties to the C60 moiety through the excited singlet states of polyfluorene. The lifetimes of the charge separated states were evaluated to be 20-50 ns, depending on the solvent polarity.     

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.     

A functionalized noncovalent macrocyclic multiporphyrin assembly from a dizinc(II) bis-porphyrin receptor and a free-base dipyridylporphyrin

The bis-porphyrin system ZnP(2), in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis DPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP(2)(4'-cis DPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6x10(8) M(-1) was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98 % (k(en)=2x10(10) s(-1) in toluene, ambient temperature) with a mechanism consistent with a dipole-dipole process with a low orientation factor.     

Theoretical and Experimental Studies on Photophysical Characteristics of Low Bandgap Polymers

The basic photophysical characteristics of low bandgap polymer poly{2,7＇-9,9-dioctylfluorene-alt-5- diethylhexyl-3,6-bis（5-bromothiophen-2-yl）pyrrolo[3,4-c]pyrrole-1,4-dione}（PDPP-F） have been systematically in- vestigated by means of theoretical and experimental methods. The quantum chemical calculations clarify the molecular structure and electronic transition properties of PDPP-F. The transient absorption data were used to compare the relaxation dynamics of PDPP-F in chlorobenzene and solid film. It is observed that the dynamics process of simulated emission relaxes much faster in comparison with that of excited state absorption in both the solution and solid film. Moreover, the excitation intensity-dependent dynamics of PDPP-F confirms that the interaction among intrachain excitons may occur under photoexcitation in the solution and solid film.     

Photodecomposition of peroxides containing a 1,4-bis(phenylethynyl)benzene chromophore

The photodecomposition dynamics of 1,4-bis(2-[4-tert-butylperoxycarbonylphenyl]ethynyl)benzene (1) have been compared with those of model compounds in the picosecond and nanosecond time domains by various photophysical techniques. Ultrafast visible transient absorption spectrometry revealed the singlet excited state of 1,4-bis(4-phenylethynyl)benzene (BPB) depopulates radiatively with a rate of 1.75 x 10(9) s(-1) and 95% efficiency. Phenyl ester moieties attached to the BPB core accelerate intersystem crossing (k = 2.8 x 10(8) s(-1)) and reduce the fluorescence quantum yield (phi(FL) = 0.82). The peroxide oxygen-oxygen bond of 1 cleaves (k = 3.6 x 10(11) s(-1)) directly from the singlet excited state (60% efficiency) causing a highly reduced fluorescence yield and leading to formation of aroyloxyl radicals. The next reaction step involves decarboxylation of the aroyloxyl radicals. Transient absorption signals in the MID IR region correspond to CO2 with the formation rate (2.5 x 10(6) s(-1)) as measured by nanosecond transient IR experiments. The transient IR spectra of the excited state of BPB, as well as of the aroyloxyl radical, evidenced a red shift in the acetylene triple bond absorption indicative of a decrease in the bond order. This clearly shows that delocalization of excitation energy over the BPB chromophore induces significant structural changes. The proposed mechanism is based on the rates of photophysical and photochemical channels and involves an additional population channel of the BPB triplet excited state from the upper singlet states.     

PbS/RL~2(NCS)~2复合敏化SnO~2纳米多孔膜电极的光电化学研究

用光电化学方法和UV-vis吸收光谱研究了PbS和Rul~2(NCS)~2(L=2,2'--bipydine--4,4'-dicarboxylicacid)复合敏化SnO~2纳米晶多孔膜电极的光电化学行为.实验表明,复合敏化比用PbS或RuL~2(NCS)~2分别单独敏化的效果好,不仅使电极的光吸收拓展到可见光区,而且复合敏化显著提高了光电转换效率,并讨论了复合敏化电极的电荷传输机理。     

Sub‐Picosecond Singlet Exciton Fission in Cyano‐Substituted Diaryltetracenes

Thin films of 5,11‐dicyano‐6,12‐diphenyltetracene ( TcCN ) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T₁)?E(S₁)=?0.17 eV), where S₁ and T₁ are singlet and triplet excitons, respectively. As a result of tuning the triplet‐state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.     

五甲川菁染料敏化SnO_2纳光结构多孔膜电极的光电化学研究

研究了五甲川菁敏化SnO_2纳米结构电极的光电化学行为.结合循环伏安曲线图及五甲川菁的光吸收阈值,初步确定五甲川菁染料电子基态和激发态能级位置.结果表明,五甲川菁染料电子激发态能级位置能与SnO_2纳米粒子导带边位置相匹配,因而使用该染料敏化可以显著地提高SnO_2纳米结构电极的光电流,使SnO_2纳米结构电极吸收波长红移至可见光区和近红外区,光电转换效率得到明显改善,IPCE值(单色光的转换效率)最高可达45.7％.     

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.     

Preparation and photophysical and photoelectrochemical properties of a covalently fixed porphyrin-chemically converted graphene composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl-functionalized CCG and porphyrin boronic ester. The covalently linked CCG-porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady-state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin-linked CCG revealed a short-lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO(2) electrode coated with the porphyrin-linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π-conjugated systems in the excited and ground states.