Incident Photon-to-photocurrent Efficiency Research Articles

Synthesis and Characterization of Supramolecules of Li + @C60 and Fluorinated Porphyrins

Metal endohedral [60]fullerenes (M@C60) have a unique structure encapsulating a metal atom (M) inside the π−conjugated spherical C60 cage. Lithium cation endohedral fullerene (Li + @C60) is a commercially available M@C60 and has a potential for application in nano material fields. The cationic Li+@C60 crystallizes with weakly coordinated anions such as SbCl6 − [1], PF6 − [2], Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion (TFPB−) [3], and bis(trifluoromethanesulfonyl)imide anion (TFSI−) [3]. Due to the inside Li+ cation, the first reduction potential of Li+@C60 is shifted to positive direction by 0.7 eV than that of pristine C60. As a result, Li+@C60 has larger electron acceptability than C60 [1]. Li+@C60 also forms a guest–host type supramolecule with [10]cycloparaphenylene ([10]CPP) [4] and π−π stacking supramolecules with porphyrins or phthalocyanines [5][6]. A supramolecule of Li+@C60 and 5,10,15,20-tetrakis(4–sulfonatophenyl) zinc porphyrin anion (ZnTPPS4 –), [Li+@C60 – ZnTPPS4 –] with the binding constant of ca. 105 M– 1 shows a long−lived electron charge separation state by photoinduced electron transfer and can be used in photovoltaic cells [5]. The [Li+@C60 – ZnTPPS4 –] based photovoltaic cell shows maximum incident photon-to-photocurrent efficiency (IPCE) value of 78% under the certain condition [7]. The performance of the photovoltaic cells would be changed by the chemical modification of porphyrins.Here, we synthesized two kinds of porphyrins to obtain new supramolecules of Li+@C60. The first one is neutral 5,10,15,20-Tetrakis(1,2,4,5-tetrafluorophenyl) porphyrin (H2TTFPP). H2TTFPP was synthesized by Lindsey method and characterized by 1H NMR, 19F NMR, UV-vis, CV, DPV, and XRD. The second one is anionic 5,10,15,20-Tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin tetrabutylammonium (ZnTFPP4TC4 −4TBA+). ZnTFPP4TC4 −4TBA+ was synthesized according to the modified Lourenço method [8] and characterized by 1H NMR, 19F NMR, UV-vis, CV, and DPV.H2TTFPP was confirmed to form supramolecular structure with Li+@C60 in the co-crystal by X-ray structure analysis. ZnTFPP4TC4 − was confirmed to form a supramolecule with Li+@C60 in solution by UV-vis titration experiment. The binding constant of [ZnTFPP4TC4 − − Li+@C60] is significantly larger than that of [H2TTFPP − Li+@C60] because of strong coulomb interactions between anionic ZnTFPP4TC4 − and cationic Li+@C60.

Near-Infrared Photoelectrochemical Conversion via Photoinduced Charge Separation in Supramolecular Complexes of Anionic Phthalocyanines with Li+@C60

Two phthalocyanines possessing carboxylate groups ((TBA)4H2Pc·1 and (TBA)4H2Pc·2) form 1:2 supramolecular complexes with lithium cation-encapsulated C60 (Li(+)@C60) [H2Pc·1(4-)/(Li(+)@C60)2 and H2Pc·2(4-)/(Li(+)@C60)2] in a polar mixed solvent. From the UV-vis spectral changes, the binding constants (K) were estimated as ca. 10(12) M(-2). Upon the photoexcitation of constructed supramolecular complexes, photoinduced electron transfer occurred to form the charge-separated (CS) state. The lifetime of the CS state was determined to be 1.2 ms for H2Pc·2(4-)/(Li(+)@C60)2, which is the longest CS lifetime among the porphyrinoid/fullerene supramolecular complexes. H2Pc·1(4-)/(Li(+)@C60)2 also afforded the long-lived CS state of 1.0 ms. The spin state of the long-lived CS states was determined to be a triplet, as indicated by the EPR signal at g = 4. The reorganization energy (λ) and the electronic coupling term were determined to be λ = 1.70 eV, V = 0.15 cm(-1) from the temperature dependence of the rate constant for the charge recombination of the CS state of H2Pc·1(4-)/(Li(+)@C60)2. The energy of the CS state (0.49 eV) is much smaller than the reorganization energy, indicating that the back-electron-transfer process is located in the Marcus normal region. The small electronic coupling term results from the spin-forbidden back electron transfer due to the triplet CS state. Supramolecular complexes of anionic zinc phthalocyanines with Li(+)@C60 were also prepared and investigated. The ZnPc·4(4-)/Li(+)@C60 supramolecular nanoclusters were assembled on the optically transparent electrode (OTE) of nanostructured SnO2 (OTE/SnO2) to construct the dye-sensitized solar cell. The IPCE (incident photon-to-photocurrent efficiency) values of OTE/SnO2/(ZnPc·4(4-)/Li(+)@C60)n were much higher than the sum of the two IPCE values of the individual systems OTE/SnO2/(Li(+)@C60)n and OTE/SnO2/(ZnPc·4(4-))n, covering the near-infrared region.

Photoinduced electron transfer in aqueous carbon nanotube/block copolymer/CdS hybrids: application in the construction of photoelectrochemical cells

Pristine and shorted multi-walled carbon nanotubes (pMWCNT and sMWCNT, respectively) non-covalently modified with a block copolymer (poly[sodium (2-sulfamate-3-carboxylate) isoprene-b-styrene]—abbreviated as CSI) are used for the formation of CdS semiconductor nanohybrids. The CdS nanoparticles are preferentially localized on the surface of the nanotubes due to specific interactions with the polymer chains. In these nanohybrid materials photoinduced electron transfer phenomena are found to occur from the photoexcited CdS nanoparticles to the nanotubes as evidenced by the efficient fluorescence emission quenching of CdS nanoparticles. Nanosecond transient absorption spectroscopy sheds light on the transient species formed by charge separation, namely (pMWCNT)˙−/CSI/(CdS)˙+ and (sMWCNT)˙−/CSI/(CdS)˙+. The MWCNT/CSI/CdS nanohybrid materials are deposited onto ITO electrodes by the drop-casting method. The ITO/sMWCNT/CSI/CdS electrode exhibit an incident photon to photocurrent efficiency (IPCE) of 7% at an applied bias of +0.2 V vs.SCE in a standard three-compartment electrochemical cell. Direct electron injection from the reduced nanotubes to ITO electrode after the photoinduced charge separation is responsible for the photocurrent generation.

Fullerene-Based Supramolecular Nanoclusters with Poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] for Light Energy Conversion

Organized composite molecular nanoassemblies of fullerene and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) prepared in acetonitrile/toluene mixed solvent absorb light over the entire spectrum of visible light. The highly colored composite clusters can be assembled as a three-dimensional array onto nanostructured SnO2 films by electrophoretic deposition approach. The composite cluster films exhibit an incident photon-to-photocurrent efficiency (IPCE) as high as 18%, which is significantly higher than that of a molecular assembly composed of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-21H,23H-porphyrin (H2P) and C60 prepared in the same manner (4%). The maximum IPCE value increases to 25% at an applied bias potential of 0.2 V vs saturated calomel reference electrode (SCE). The power conversion efficiency of a MEH-PPV and C60 assembly-modified electrode is determined to be 0.24%. The photocurrent generation properties observed with MEH-PPV and C60 clusters demonstrate the synergy of these systems towards yielding efficient photoinduced charge separation within these composite nanoclusters.

Photoelectrochemistry of Stacked-Cup Carbon Nanotube Films. Tube-Length Dependence and Charge Transfer with Excited Porphyrin

Photoelectrochemical solar cells are constructed with stacked-cup carbon nanotubes (SCCNT) on optically transparent electrodes (OTE). Three SCCNT samples with different tube lengths (long type, L-SCCNT; medium type, M-SCCNT; and short type, S-SCCNT) were electrophoretically deposited on OTE/SnO2 electrodes to probe the tube-length dependence of the photoelectrochemical behavior. The maximum incident photon-to-photocurrent efficiency (IPCE) of 19% is attained at an applied bias potential of 0.2 V vs SCE in OTE/SnO2/L-SCCNT. The power conversion efficiencies (η) of SCCNT-modified electrodes increase with increasing tube length. The maximum power conversion efficiency (η) of OTE/SnO2/L-SCCNT electrode is determined to be 0.11%, which is about 6 times greater than that of the OTE/SnO2/S-SCCNT electrode (0.018%). Molecular assemblies composed of S-SCCNT and 5,15-bis(3,5-di-tert-butylphenyl)porphyrin (H2P) prepared in acetonitrile/toluene (5/1, v/v) were deposited as three-dimensional arrays onto nanostructured...

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.

Supramolecular Nanostructured Assemblies of Different Types of Porphyrins with Fullerene Using TiO2 Nanoparticles for Light Energy Conversion.

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Supramolecular nanostructured assemblies of different types of porphyrins with fullerene using TiO 2 nanoparticles for light energy conversion

TiO 2 nanoparticles were modified with porphyrin derivatives, 5-[4-benzoic acid]-10,15,20-tris[3,5-di- tert-butylphenyl]-21 H,23 H-porphyrin ( Ar– H 2P – COOH ), 5-[4-benzoic acid]-10,20-tris[3,5-di- tert-butylphenyl]-21 H,23 H-porphyrin ( H– H 2P – COOH ), and 5,10,15,20-tetra[4-benzoic acid]-21 H,23 H-porphyrin ( H 2P – 4COOH ). The porphyrin-modified TiO 2 nanoparticles were deposited on nanostructured OTE/SnO 2 electrode together with nanoclusters of fullerene (C 60) in acetonitrile–toluene (3/1, v/v) using an electrophoretic deposition technique to afford the porphyrin-modified TiO 2 composite electrode denoted as OTE/SnO 2/(porphyrin-modified TiO 2 nanoparticle+C 60) n . The porphyrin-modified TiO 2 composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I 3 − / I − redox couple. The incident photon-to-photocurrent efficiency (IPCE) values of supramolecular nanostructured electrodes of porphyrin-modified TiO 2 nanoparticles with fullerene [OTE/SnO 2/( Ar– H 2P – COO– TiO 2 +C 60) n , OTE/SnO 2/( H– H 2P – COO– TiO 2 +C 60) n , and OTE/SnO 2/( H 2P – 4COO– TiO 2 +C 60) n ] are much larger than those of the reference systems of porphyrin-modified TiO 2 nanoparticles without C 60 [OTE/SnO 2/( Ar– H 2P – COO– TiO 2 ) n , OTE/SnO 2/( H– H 2P – COO– TiO 2 ) n , and OTE/SnO 2/( H 2P – 4COO– TiO 2 ) n ]. In particular, the maximum IPCE value (41%) is obtained for OTE/SnO 2/( H– H 2P – COO– TiO 2 +C 60) n under the bias potential of 0.2 V versus SCE. This indicates that the formation of supramolecular complexes between porphyrins and fullerene on TiO 2 nanoparticles plays an important role in improvement of the light energy conversion properties.

Organization of supramolecular assemblies of fullerene, porphyrin and fluorescein dye derivatives on TiO 2 nanoparticles for light energy conversion

TiO 2 nanoparticles were modified with a porphyrin derivative, 5-[4-benzoic acid]-10,15,20-tris[3,5-di- tert-butylphenyl]-21 H,23 H-porphyrin ( H 2 P- COOH), and fluorescein derivatives, 9-[2-(3-carboxy-9,10-diphenyl)anthryl]-2,7-difluoro-6-hydroxy-3 H-xanthen-3-one ( DPAX- COOH) and 2′,7′-difluorofluorescein ( FL- COOH). The dye-modified TiO 2 nanoparticles were deposited on nanostructured OTE/SnO 2 (OTE: optically transparent electrodes) together with nanoclusters of fullerene (C 60) from acetonitrile/toluene (3:1, v/v) using an electrophoretic deposition technique. The dye-modified TiO 2 composite electrodes [OTE/SnO 2/(dye + C 60) n ] have broad as well as high absorbance properties in the visible region, exhibiting the photo response under visible light excitation using I 3 - / I - redox couple. The incident photon to photocurrent efficiency (IPCE) for these electrodes increases in order: OTE/SnO 2/( H 2 P) n < OTE/SnO 2/( H 2 P- COO– TiO 2 ) n < OTE/SnO 2/( H 2 P- COO– TiO 2 + C 60). The IPCE value can be further improved by replacing H 2 P- COOH with a fluorescein derivative containing an electron donor moiety: DPAX- COOH ( DPA: diphenylanthracene). The maximum IPCE value (42%) is obtained for OTE/SnO 2/( DPAX- COO– TiO 2 + C 60) n under the bias of 0.2 V vs. SCE.

Design of high-efficiency solid-state dye-sensitized solar cells using coupled dye mixtures

Abstract A solid-state dye-sensitized solar cell comprising dye mixtures of [Ru(2,2-bpy-4,4′-dicarboxylic acid)(NCS) 2 ] and [Ru(4,4′,4″-tricarboxy-2,2;6,2″-terpy)(NCS) 3 ] on TiO 2 thin film was fabricated. The different optical properties of dyes results in increased photocurrent and incident photon to photocurrent efficiency (IPCE). The multiple dye system showed the short circuit current ( I sc ) of 10.2 mA/cm 2 and a cell efficiency ( η ) of 2.8 while broadening the spectral sensitivity of the cell. When a single dye is used, I sc of 6 and 5 mA/cm 2 and cell efficiency of 1.7 and 1.2 were observed for [Ru(4,4-bis(carboxy)-bpy) 2 (NCS) 2 ] (dye 1) and [Ru(2,2′,2″-(COOH) 3 -terpy)(NCS) 3 ] (dye 2), respectively. Additionally, the resulting IPCE for the solar cell consisting of dye mixture was 50% at wide wavelength range from 530 to 650 nm while for the dye 1, 32% IPCE was observed at 535 nm while for the dye 2, highest IPCE value observed was 20% at 620 nm.

Drastic Difference in Lifetimes of the Charge-Separated State of the Formanilide−Anthraquinone Dyad versus the Ferrocene−Formanilide−Anthraquinone Triad and Their Photoelectrochemical Properties of the Composite Films with Fullerene Clusters

A long-lived charge-separated (CS) state, which can be observed even at 900 mus after laser excitation, has been attained in the formanilide-anthraquinone dyad (FA-AQ) in dimethyl sulfoxide, whereas the CS lifetime is shortened significantly to 20 ps in the ferrocene-formanilide-anthraquinone triad (Fc-FA-AQ). Such a drastic decrease in the CS lifetime by the addition of a ferrocene moiety to the FA-AQ dyad is ascribed to a decrease in the driving force of back electron transfer and an increase in the reorganization energy of electron transfer despite the longer charge-separation distance. The FA-AQ dyad and the Fc-FA-AQ triad have been employed as components of photovoltaic cells, where composite molecular nanoclusters of the FA-AQ dyad or the Fc-FA-AQ triad with fullerene (C60) are assembled onto a SnO2 electrode using an electrophoretic method. The composite films of the Fc-FA-AQ triad exhibit 10 times smaller values of an incident photon-to-photocurrent efficiency (IPCE) as compared with those of the FA-AQ dyad in accordance with a drastic decrease of the CS lifetime by addition of a ferrocene moiety to the FA-AQ dyad.

Enhancement of Light-Energy Conversion Efficiency by Multi-Porphyrin Arrays of Porphyrin−Peptide Oligomers with Fullerene Clusters

Organic photovoltaic cells using supramolecular complexes of porphyrin-peptide oligomers (porphyrin-functionalized alpha-polypeptides) with fullerene demonstrate remarkable enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and near-infrared regions by increasing the number of porphyrin units in alpha-polypeptide structures. A high power conversion efficiency (eta) of 1.3% and a maximum incident photon-to-photocurrent efficiency (IPCE) of 42% were attained using composite clusters of porphyrin-peptide octamer and fullerene. These results clearly show that the formation of a molecular assembly between fullerene and multi-porphyrin arrays with a polypeptide backbone controls the electron transfer efficiency in the supramolecular complex, which is essential for the light-energy conversion.

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.

Light Energy Conversion Using Mixed Molecular Nanoclusters. Porphyrin and C60Cluster Films for Efficient Photocurrent Generation

Composite molecular nanoclusters of fullerene and porphyrin prepared in acetonitrile/toluene mixed solvent absorb light over the entire spectrum of visible light. Upon slow evaporation of the solvent on the copper grid, these mixed nanoclusters undergo close-packed stacking to produce either tubular- or square-shaped microcrystallites and differ from those obtained from single-component clusters. The highly colored composite clusters can be assembled as 3-dimensional arrays onto nanostructured SnO2 films using an electrophoretic deposition approach. The composite cluster films exhibit an incident photon-to-photocurrent efficiency (IPCE) as high as 17% at an applied potential of 0.2 V vs SCE, which is significantly higher than the additive effect observed from either porphyrin (IPCE = 1.6%) or fullerene clusters (IPCE = 5.0%) under similar photoelectrochemical conditions. The high IPCE values observed with porphyrin and C60 clusters demonstrate the synergy of these systems toward yielding efficient photoinduced charge separation within these composite nanoclusters.

Photoelectrochemical Behavior of Bi2S3 Nanoclusters and Nanostructured Thin Films

Quantized semiconductor nanoclusters of Bi2S3 are prepared in acetonitrile by reacting BiI3 with H2S or Na2S. Both preparation methods yield stable colloids with particle diameters of ≤5 nm. Excitation with a 355-nm laser pulse results in transient bleaching in the 400−500-nm region. This process is followed by the formation of S-surf with a difference absorption maximum around 540 nm. This we attribute to the chemical changes associated with the hole-trapping process. A composite thin film electrode comprised of SnO2/Bi2S3 nanocrystallites has been prepared by sequential deposition of SnO2 and Bi2S3 films onto an optically transparent electrode, and its photoelectrochemical behavior has been studied. The thin film is photoactive in the visible and near-IR and exhibits an incident photon to photocurrent efficiency (IPCE) of ∼15% at 400 nm.