Conjugated polymers with charge transfer characters in a large extend are responsible for the recent advancement in organic photovoltaic (OPV) applications in bulk heterojunction (BHJ) devices. We have carried out collaborative studies on electronic processes and in-situ morphological development of these low bandgap polymers and small molecules using ultrafast optical spectroscopy and in-situ grazing incident X-ray scattering (GIXS). Conventional organic photovoltaic models, in which donor molecules are treated as anonymous electron sources and charge carrier diffusion channels, are challenged by near-infrared transient absorption results of low bandgap polymers indicating strong correlations between intramolecular donor dynamics in < 100 fs and corresponding device power conversion efficiencies. The other conventional model being challenged is the driving force for exciton splitting in the bulk heterojunction environment which has been described by the LUMO-LUMO energy off-set between conjugated polymer electron donor and fullerene derivative electron acceptor. Our study suggests the intramolecular charge transfer characters must be combined with local and global conformations of conjugated polymer chains to achieve the low band gap. Moreover, the morphology of the BHJ films is also investigated by in-situ GIWAS/GISAX methods including the effects of additives which suggest the interplays of the additives and the polymers in solution. The morphology of the heterojunction films has been correlated directly with the yield of the charge separation on time scales from femtosecond to microsecond. In addition, new photophysical studies are also carried out on a series of metal chelating conjugated polymers showing the capability and potential in photocatalytic hydrogen generation.
KEYWORDS: Molecules, Luminescence, Absorption, Picosecond phenomena, Polymers, Upconversion, Electron transport, Solar cells, Quantum efficiency, Energy transfer
In a recent study, it has been shown that organic photovoltaic (OPV) solar cells consisting of polymers with certain
stoichiometric ratios of alkyl thiophene:thieno[3,4-b]thiophene monomeric units in random sequences, when combined
with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), may have potentials for creating more efficient devices. Such a
potential enhancement is mainly due to the light harvesting in most of the visible and near infrared region by these low
band-gap polymers. However, very little is known about the photoinduced energy/electron transfer and transport within
these copolymers. It is important to understand both the ultrafast interactions between these two monomeric units when
they are linked in the copolymers and their interactions with the electron acceptor PCBM in order to determine the
transport mechanisms in these systems, and then to create the architectures that optimize electronic transport properties.
Therefore, three oligomer molecules have been synthesized to model the local interactions in the copolymers, each of
which consists of a thieno[3,4-b] thiophene derivative at its center linked with two alkyl oligothiophene side units. The
alkyl oligothiophene units for the three molecules are 2, 4, or 8 units in length. By performing transient absorption and
fluorescence upconversion measurements, the nature of the early exciton diffusion and energy transfer between these
different units is elucidated.
Photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase
which lead to solar hydrogen generation. Therefore, knowing their structures with atomic resolution and sufficient time
resolution is crucial in correlating structures with molecular properties. Using x-ray transient absorption (XTA)
spectroscopy, transient metal oxidation states, coordination geometry, and atomic rearrangements during photochemical
processes can be probed. Such an approach complements with ultrafast optical laser spectroscopy in obtaining kinetics
and coherence information among different excited states as well as intra- and intermolecular energy/charge transfer
processes associated with solar energy conversion. Excited state structures of transition metal complexes, such as
metalloporphyrins in solution, created by photoexcitation have been studied by XTA combined with optical transient
absorption spectroscopy. Direct evidences of photoinduced redox reactions and coordination geometry changes as well
as electronic configurations of the metals can be observed. These experimental studies are combined with quantum
mechanical calculations to rationalize the evolution of the ultrafast excited state pathways with electronic configuration
changes that may be responsible for the reactivity of the molecules in solar hydrogen generation. Preliminary time-resolved
X-ray absorption near edge structure (XANES) studies on Pt coated TiO2 nanoparticles during photocatalysis
show a significant potential impact of XTA in understanding solar hydrogen production.
Enhancement of the nonresonant second order molecular hyperpolarizabilities y were observed in stacked macrocyclic
molecular systems, previously in a t-oxo silicon phthalocyanme (SiPcO) monomer, dimer and trimer series, and now in
bacteriochiorophyll a (BChla) arrays oflight harvesting (LH) proteins. Compared to monomeric BChla in a tetrahydrofuran
(TifF) solution, the <γ> for each macrocycle was enhanced in naturally occurring stacked macrocyclic molecular systems in the
bacterial photosynthetic LH proteins where BChla 's are arranged in tilted face-to-face arrays. In addition, the γ enhancement
is more significant in B875 of LH1 than in B850 in LH2. Theoretical modeling of the nonresonant γ enhancement using simplified molecular orbitals for model SiPcO indicated that the energy level of the two photon state is crucial to the γ enhancement when a two photon process is involved. Additionally, charge transfer between the monomers may be important if this produces states which are close to one-photon resonance. The calculated results can be extended to γ enhancement in B875 and B850 arrays, suggesting that BChla in B875 are more strongly coupled than in B850. In addition, a 50-160 fold increase in <γ> for the S1 excited state ofrelative to S0 ofbacteriochlorophyll in vivo was observed which provides an alternative method for probing excited state dynamics and a potential application for molecular switching.
A series of new silicon phthalocyanine monomers containing two or more hydroxyl groups has been synthesized by axial etherification of [PcSi(OH)2] with various polyfunctional alcohols, where R is a residue generated from an appropriate alcohol namely, ethylene glycol, glycerol, triethanolamine, triethylene glycol, 1,4-bis(2- hydroxyethyl)piperazine and N,N-bis(2-hydroxyethyl) isonicotinamide. All derivatives are soluble in common organic solvents such as chloroform and dioxane, and exhibit low absorptions in the visible spectral region. The dimer [RO(SiPcO)2R] and trimer [RO(SiPcO)3R] of triethanolamine have also been synthesized to investigate structure-property relationships. The above derivatives can be processes into thin polymeric films either by reacting with a diisocyanate compound or with a reactive polysiloxane. Third- order nonlinear optical properties of the above derivatives are described.
The enhancement of the third order susceptibilities (chi) (3) and second order molecular hyperpolarizabilites < (gamma) > in a series of covanlently stacked dimer and trimer macrocycles (SiPc and SiNc) was observed throught he phase conjugated signals from DFWM measurements at both 1064 and 532 nm. The FWHM of the phase conjugated signals at 532 nm is much smaller than those at 1064 nm and those of the reference compound CS2. The < (gamma) > values at both wavelengths as a function of number of the monomer units (n) have an approximate n3 dependence, differing only in the scaling constant. The origin of such enhancement is discussed and compared to those in 1D (pi) -conjugated systems.
KEYWORDS: Polymers, Chemical species, Four wave mixing, Molecules, Absorption, Energy transfer, Chemistry, Chemical elements, Proteins, Picosecond phenomena
As one of the important elements in natural and artificial electron transfer and energy transfer processes, porphyrin and its derivatives have received much attention in photoelectronics and photoelectronic materials. As our first attempt to relate the (pi) -(pi) electronic couplings between porphyrin macrocycles to apparent third order nonlinear susceptibilities, we measured (chi) (3) for several porphyrin and chlorophyll a derivatives, including dimers with different configurations. Our preliminary results show that the dimers have enhanced (chi) (3) compared to those of the monomer. This enhancement is related to the relative orientations between the two macrocycles in the dimers. The parallel dimers with close face- to-face distances seem to have the highest enhancement in (chi) (3). More model compounds and related theoretical works are in progress before reaching a final conclusion.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.