Maher Al-Ibrahim

Effects of solvent and annealing on the improved performance of solar cells based on poly(3-hexylthiophene): Fullerene

Polymer solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) were fabricated using two different solvents. P3HT:PCBM films casted from chlorobenzene solution absorb more red light than the films casted from chloroform solution. After thermal annealing, the films casted from chloroform show higher absorption than the films casted from chlorobenzene. Solar cells made from P3HT:PCBM chlorobenzene solution show no change in the white light power conversion efficiency (2.2%) after annealing. Solar cells processed from P3HT:PCBM chloroform solution show a white light power conversion efficiency of 1.5% without thermal annealing and 3.4% after the thermal annealing. The stated efficiencies are not corrected for the spectral mismatch.

Efficient large-area polymer solar cells on flexible substrates

Polymer solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]:[6,6]-phenyl-C61-butyric acid methyl ester (1:3wt%) were fabricated using flexible 5×5cm2 polyester foils. These cells show a white-light power conversion efficiency of 3% comparable to smaller cells on glass published by other authors. Ten cells were arranged on one substrate with an active area of 25 and 175mm2 demonstrating a remarkably high reproducibility. It was found that the open-circuit voltage does not depend on the device area. An increase of the cell area caused a slightly lower short-circuit current, lower fill factor, and lower white-light power conversion efficiency.

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.