Gerald Zerza

Tracing photoinduced electron transfer process in conjugated polymer/fullerene bulk heterojunctions in real time

Abstract Ultrafast spectroscopic studies using an optical excitation of a conjugated polymer by sub-10-fs pulses are reported. Phonon modes which are strongly coupled to the electronic excitation of the conjugated polymer are directly observed as coherent oscillations during the pump–probe experiment, mirroring the resonant/nonresonant Raman spectrum of the conjugated polymer. In composites of a conjugated polymer with a fullerene the primary photoexcitation is found to be an ultrafast photoinduced electron transfer. We are able to time resolve for the first time the kinetics of this charge transfer process with a forward transfer time of around τ ct ∼45 fs.

Ultrafast charge transfer in conjugated polymer-fullerene composites

Phonon modes, which are strongly coupled to the electronic excitation of the conjugated polymer are coherently excited by sub-10-femtosecond laser pulses. Pump probe experiments show vibrational modulations as fast as 25 fsec. The Fourier transformed spectrum displays the non-resonant Raman spectrum. Addition of fullerenes into the conjugated polymer matrix quenches the vibrational coherence of the polymer due to a strong interaction of the excited polymer with the fullerene moeity. Charge separation times of less than 100 fsec are reported.

Electropolymerization and spectroscopic properties of a novel double-cable polythiophene with pendant fullerenes for photovoltaic applications

A novel double-cable polythiophene carrying fullerene side groups has been prepared electrochemically. Electrochemical and spectroscopic data show that this bipolar material, where phase separation cannot occur, retains the favorable optoelectronic properties of polythiophene/fullerene blends. Excited state spectroscopy reveals clear evidence of a photo-induced electron transfer from the donor cable (polythiophene) to the pendant acceptor cable (fullerene substituents).

Photoinduced electron transfer in solid C60 donor/acceptor complexes

Light-induced ESR spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C 60 donor/acceptor complexes: TMPD.C 60 (1), LCV.C 60 .C 6 H 5 Cl (2), and (BA) 2 C 60 (3). LESR signals from C 60 with g=1.9984-1.9992 and ΔH=3-6G (100K) for 1-3 as well as the LESR signal from TMPD + for 1 were observed. The LESR signals have two components: a long-lived persistent component and a prompt one, which disappears immediately after light is off. All PIET processes are temperature activated with E a =23, 43 and 49 meV for 2, 3, and 1, respectively. The temperature dependence of spin susceptibility in 1 can be described by the Bonner-Fisher model, while in 2 it corresponds to a paramagnetic behavior of isolated spins.

A novel polythiophene with pendant fullerenes: toward donor/acceptor double-cable polymers

The synthesis of a novel bithiophene with a pendant fullerene substituent and its electrochemical polymerisation are reported; light induced ESR measurements on the electrodeposited films reveal a photoinduced electron transfer from the donor cable (polythiophene) to the pendant acceptor cable (fullerene moieties).

Photoinduced Electron Transfer in Solid C60 Donor/Acceptor Complexes Studied by Light-Induced Electron-Spin Resonance

ABSTRACT Light-induced ESR (LESR) spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C60 donor/acceptor complexes. High-field LESR signals attributed to (g = 1.9984−1.9992 and ΔH = 3−6 G at 100 K) were observed in fullerene complexes with Leuco Crystal Violet (LCV·C60·C6H5Cl, 1), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA·C60, 2), and bianthrone ((BA)2C60, 3), whereas a low-field LESR signal in 2 was attributed to TMPDA•+. Microwave power studies reveal two independent spins in 2 and show high relaxation rates for photoinduced spins on . The LESR signals consist of a major long-lived “persistent” component and a minor “prompt” one, which disappears immediately after light is off. The maximal intensity of the LESR signals was observed for the excitation of 1–3 below 2 eV. This range of photon energy corresponds to direct intermolecular charge transfer from donor to C60 molecules. PIET is temperature activated with Ea = 23, 43 and 49 meV for 1, 2, and 3, respectively. Photoinduced spin susceptibility attains the maximum at 30 K in 1 and decreases with temperature down to 4 K indicating a possible antiferromagnetic interaction of spins, while in 2 it increases with the temperature decrease down to 4 K.

Direct observation of the ultrafast electron transfer process in a polymer/fullerene blend

Optical studies on conjugated polymer-fullerene blends are performed with sub-10-fs temporal resoln. The photoinduced electron transfer process is directly monitored in the time domain, obtaining a forward electron transfer time const. of 45 fs. [on SciFinder(R)]

Investigation of Photoinduced Charge Transfer in Composites of a Novel Precursor PPV Polymer and Fullerenes

Homogenous blends of a processable methanofullerene, [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), with poly (p-phenylene vinylene) (PPV) made from a novel nonionic precursor route were investigated by optical spectroscopy and by photocurrent measurements. The conversion process of the precursor polymer to the PPV in the blends with PCBM was monitored by IR absorption. In composites of PPV/PCBM strong quenching of the PPV luminescence was observed. LESR (light induced electron spin resonance) and PIA (photoinduced absorption) studies confirmed the occurrence of photoinduced electron transfer from the PPV to PCBM. Photovoltaic devices made from PPV/PCBM blends showed monochromatic power conversion efficiencies of app. 0.3%. The spectral photocurrent was observed to follow the absorption profile of the PPV.

Fullerenes as functional moieties in conjugated polymers: Towards donor-acceptor double cable polymeric materials

The properties of a novel poly(bithiophene-fulleropyrrolidine), which belongs to the class of so called “double cable” polymers, are studied. The polymer consists of covalently linked electron acceptor moieties to an electron donor polymeric backbone. Photoinduced charge transfer is studied by photoinduced absorption spectroscopy and by light induced ESR spectroscopy.

Photoinduced Charge Carriers in a Donor-Acceptor Double-Cable Polythiophene with Covalently Bound Tetracyanoanthraquinodimethane Moieties

The preparation of double-cable polymers, which consist of a hole conducting conjugated chain carrying pendant electron accepting/conducting moieties, is a promising strategy to prevent donor/acceptor phase segregation and to achieve defined microscopic structure in organic bulk heterojunction solar cells. In this paper we report the electrochemical synthesis and the investigation of a double-cable consisting of a polythiophene backbone bearing, via covalent bonds, electron accepting tetracyanoanthraquinodimethane (TCAQ) type moieties. Electrochemical studies and UVVIS absorption spectroscopy reveal, that in dark, the polythiophene chain and the TCAQ moieties retain their individual ground-state properties. Upon illumination photoinduced electron transfer occurs, which is studied by photoinduced VIS-NIR absorption spectroscopy.