Uladzimir Zhokhavets

Optical and ESR studies on poly(3-alkylthiophene)/fullerene composites for solar cells

Optical measurements (absorption coefficients, photoluminescence) and light-induced electron spin resonance ( LESR ) technique were used to study photoinduced charge generation and charge transfer in conjugated polymer / fullerene composites. The quenching of fluorescence by C 60 gives a first indication of an effective charge transfer from P3DDT to C 60 . At 77 K two kinds of LESR signals were identified, one of polaron (P +. ) on the polymer chain and one of radical anion (C 60 -.) of fullerene. The relaxation rates of generated paramagnetic centres were estimated by microwave power saturation studies. Moreover for the first centre (P +. ) two contributions to the LESR signal were observed: a permanent one and a photoinduced one. The light intensity dependence of the photoinduced P +. and C 60 -. ESR signals is of bimolecular type (I 0.5 ). Solar cells based on P3DDT/ C 60 and P3DDT/ PCBM are prepared and characterised.

Characterization of potential donor acceptor pairs for polymer solar cells by ESR, optical, and electrochemical investigations

In this report new PPV-PPE copolymers DE 69, DE 11 were compared with the state of the art materials MDMO-PPV and poly(3-alkylthiophenes) (P3DDT, P3OT). The optical band gap energy of the two copolymers DE 69, DE11 is somewhat higher than that one of MDMO-PPV. The electrochemical band gap was found to be lower for DE 69, DE11 related to for MDMO-PPV. The absorption coefficient of the new PPV-PPE copolymers is higher than for MDMO-PPV but in the same order of magnitude. Films from composites of MDMO-PPV/PCBM and DE69 or DE11 with PCBM show a clear photoluminescence quenching effect. At 77 K two kinds of LESR signals were identified, one of polarons (P+.) and one of radical anions of fullerenes. The LESR results show strong differences in kinetics between the separation and recombination processes of photoexcited charge carriers. The relaxation rates of paramagnetic centers were estimated by microwave power saturation experiments. Photovoltaic devices were prepared under ambient conditions on flexible PET-ITO foils with MDMO-PPV/ PCBM (1:3 wt. %) with ηAM1.5 = 2.4% and DE 69/ PCBM (1:3 wt. %) with ηAM1.5 = 1.75% (A=0.25 cm2, Pin = 100 mW/cm2).

Anisotropic optical properties of conjugated polymer and polymer/fullerene films

Abstract We have studied the optical properties of thin films of substituted polyphenylene ethynylene/polyphenylene vinylene copolymers. Two novel materials of this type were investigated: poly[1,4-phenyleneethylene-1,4-(2,5-didodecyloxylphenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-didodecyloxyphenylene)ethene-1,2-diyl] (DE15) and poly[1,4-phenyleneethylene-1,4-(2,5-di(2-ethylhexyloxy)phenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-di(2-ethylhexyloxy)phenylene)ethene-1,2-diyl] (DE42). Both pure and fullerene-doped films were prepared by spin coating onto silicon substrates. Their optical properties were obtained by determination of the dielectric function by means of spectroscopic ellipsometry. A large anisotropy of the dielectric function was found both for the pure materials and for their composites with 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM) with the optical axis normal to the film surface. The origin of this phenomenon and the influence of the anisotropy on the performance of plastic solar cells are discussed. The optical properties of such DE15/PCBM and DE42/PCBM composites are successfully described in terms of the Bruggemann effective medium approximation.

Excited state spectroscopy in polymer fullerene photovoltaic devices under operation conditions

Abstract The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes led to intensive spectroscopic investigations as well as to high efficiency photovoltaic devices. Up to now, most spectroscopic investigations were performed on thin films, whereas little is known about the behavior of the charge carriers under working conditions of the device with an applied electric field. In this work, we investigated thin film photovoltaic devices by means of photoinduced absorption spectroscopy. Significant changes were observed for the polaron signal as well as the lifetime of the positive polaron on the conjugated polymer backbone upon changing the applied voltage on the device. The results are discussed in comparison with current–voltage characteristics of the devices.