The influence of the terminal acceptor and oligomer length on the photovoltaic properties of A–D–A small molecule donors

Four new A–D–A small molecules (denoted as FG1–4) have been designed and synthesized. The compounds have cyclopentadithiophene-vinylene (CPDTV) oligomers of different lengths as the central donor core linked with different terminal acceptor units (3-ethylrhodanine or dicyanomethylene-3-ethylrhodanine). The effects that conjugation length and terminal acceptor units have on the optical and electrochemical properties were investigated. These small molecules were used as donors in conjunction with PC71BM as an acceptor in the bulk heterojunction active layer for the fabrication of solution-processed organic solar cells. Solvent vapor annealing treatment improved the crystallinity and the interpenetrating networks of donor and acceptor phases for exciton dissociation and charge transfer, thus leading to significant improvements in the overall power conversion efficiency (PCE) of the organic solar cells. The PCE values for the organic solar cells based on the optimized FG1:PC71BM, FG2:PC71BM, FG3:PC71BM and FG4:PC71BM active layer were 5.58%, 6.99%, 7.51% and 8.43%, respectively. These results indicate that an enhancement in the PCE of small molecule organic solar cells can be achieved by an increase in conjugation-length and variation of terminal acceptor units in the molecular backbone of small molecules and optimization of the crystallinity and nanoscale interpenetrating morphology by appropriate solvent vapor annealing treatment

Formation and Photoinduced Electron Transfer in Porphyrin- and Phthalocyanine-Bearing N-Doped Graphene Hybrids Synthesized by Click Chemistry

Graphene doped with heteroatoms such as nitrogen, boron, and phosphorous by replacing some of the skeletal carbon atoms is emerging as an important class of two-dimensional materials as it offers the much-needed bandgap for optoelectronic applications and provides better access for chemical functionalization at the heteroatom sites. Covalent grafting of photosensitizers onto such doped graphenes makes them extremely useful for light-induced applications. Herein, we report the covalent functionalization of N-doped graphene (NG) with two well-known electron donor photosensitizers, namely, zinc porphyrin (ZnP) and zinc phthalocyanine (ZnPc), using the simple click chemistry approach. Covalent attachment of ZnP and ZnPc at the N-sites of NG in NG−ZnP and NG−ZnPc hybrids was confirmed by using a range of spectroscopic, thermogravimetric and imaging techniques. Ground- and excited-state interactions in NG−ZnP and NG−ZnPc were monitored by using spectral and electrochemical techniques. Efficient quenching of photosensitizer fluorescence in these hybrids was observed, and the relatively easier oxidations of ZnP and ZnPc supported excited-state charge-separation events. Photoinduced charge separation in NG−ZnP and NG−ZnPc hybrids was confirmed by using the ultrafast pump-probe technique. The measured rate constants were of the order of 1010 s,−1 thus indicating ultrafast electron transfer phenomena

Triplet photosensitizer-nanotube conjugates: synthesis, characterization and photochemistry of charge stabilizing, palladium porphyrin/carbon nanotube conjugates

The ability of a triplet photosensitizer to generate long-lived charge separated states, in contrast to traditionally used singlet photosensitizers, in covalently functionalized single-walled carbon nanotube hybrids has been investigated. Enriched single-walled carbon nanotubes with two diameters, namely (6,5) and (7,6), were covalently modified to carry a charge-stabilizing triplet photosensitizer derived from a palladium porphyrin. The nanohybrids were fully characterized and the presence of intramolecular interactions between the porphyrin and nanotubes was established from various spectroscopic, imaging, electrochemical and thermochemical studies. Photoluminescence of palladium porphyrin was found to be quantitatively quenched in the presence of covalently appended SWCNTs and this quenching is due to excited state charge separation and has been established by femtosecond transient absorption studies. Owing to the presence of the triplet photosensitizer, the charge separated states lasted over 3 ns, i.e., much longer than those reported earlier for singlet photosensitizer-derived nanotube hybrids. The nanohybrids also exhibited efficient photocatalytic behavior in experiments involving electron pooling of oneelectron reduced methyl viologen in the presence of a sacrificial electron donor. Higher yields of photoproducts were achieved from the present donor–acceptor nanohybrids when compared with those of singlet photosensitizer-derived nanohybrids, more so for (6,5) nanotube derived hybrids compared to (7,6) nanotube derived hybrids. The present findings highlight the importance of triplet photosensitizer derived nanohybrids in artificial photosynthesis of charge separation and photocatalytic applicatons

N-Doped graphene/C60 covalent hybrid as a new material for energy harvesting applications

N-Doped graphene (N-G)was chemically functionalized byN-alkylationwith the well-known electron acceptor C60. The degree of functionalization and the key structural features of the N-G/C60 hybrid were systematically investigated by a number of techniques including thermogravimetric analysis, X-ray photoelectron and Raman spectroscopies and transmission electron and atomic force microscopies. Absorption and electrochemical studies revealed interactions between the N-G and C60 while the fluorescence of C60 within the hybrid was found to be fully quenched. Evidence for the occurrence of excited state charge transfer from the singlet excited C60 to N-G in the hybrid was obtained from femtosecond transient absorption studies covering the visible–near-IR regions. Electron-pooling experiments performed in the presence of a sacrificial electron donor and a second electron acceptor, methyl viologen, revealed the accumulation of the one-electron reduced product of methyl viologen upon continuous irradiation of the N-G/C60 nanohybrid, thus revealing the utility of this material in photocatalytic energy harvesting applications

Sc3N@Ih-C80 based donor–acceptor conjugate: role of thiophene spacer in promoting ultrafast excited state charge separation

Light induced charge separation in a newly synthesized triphenylamine–thiophene-Sc3N@Ih-C80 donor–acceptor conjugate and its C60 analog, triphenylamine–thiophene-C60 conjugate is reported, and the significance of the thiophene spacer in promoting electron transfer events is unraveled

Ultrafast Electron Transfer in All-Carbon-Based SWCNT-C60 Donor-Acceptor Nanoensembles connected by Poly(phenylene-ethynylene) Spacers

Building all-carbon based functional materials for light energy harvesting applications could be a solution to tackle and reduce environmental carbon output. However, development of such all-carbon based donor-acceptor hybrids and demonstration of photoinduced charge separation in such nanohybrids is a challenge since in these hybrids part of the carbon material should act as an electron donating or accepting photosensitizer while the second part should fulfil the role of an electron acceptor or donor. In the present work, we have successfully addressed this issue by synthesizing covalently linked all-carbon-based donor-acceptor nanoensembles using single-walled carbon nanotubes (SWCNTs) as the donor and C60 as an acceptor. The donor-acceptor entities in the nanoensembles were connected by phenylene-ethynylene spacer units to achieve better electronic communication and to vary the distance between the components. These novel SWCNT-C60 nanoensembles have been characterized by a number of techniques, including TGA, FT-IR, Raman, AFM, absorbance and electrochemical methods. The moderate number of fullerene addends present on the side-walls of the nanotubes largely preserved the electronic structure of the nanotubes. The thermodynamic feasibility of charge separation in these nanoensembles was established using spectral and electrochemical data. Finally, occurrence of ultrafast electron transfer from the excited nanotubes in these donor-acceptor nanohybrids has been established from femtosecond transient absorption studies, signifying their utility in building light energy harvesting devices