The influence of the optoelectronic properties of poly(3-alkylthiophenes) on the device parameters in flexible polymer solar cells

in this report regioregular poly(3-alkylthiophenes) (P3ATs) (P3HT: poly(3-hexylthiophene, P3OT: poly(3-octylthiophene, P3DDT: poly(3-dodecylthiophene) were studied regarding their optical and electrochemical properties and used as electron donors in polymer solar cells. The optical band gap energy for the three polymers amounts to 1.92 eV. With longer side chain length their electrochemical band gaps are slightly increased, whereas the absorption coefficient undergoes a systematic decrease. The absorption spectra of the pristine P3ATs exhibit a distinctive blue shift of the pi-pi* interband transition upon mixing with PCBM 1:3 (as prepared films; P3HT: similar to 45 nm, P3OT: similar to 85 nm, P3DDT: similar to 50 nm). Films based on composites of the three polymers with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) show a distinctive photoluminescence quenching effect. At 77 K two types of light-induced electron spin resonance (LESR) signals were identified, one of polaron (P+) on the polymer chain and one of PCBM- radical anion, which detect the photoinduced charge generation and charge transfer in P3AT/PCBM composites. Photovoltaic devices were prepared on flexible PET-ITO foils on ambient conditions using P3HT/PCBM (1:3 wt.%) with eta(AM1.5) = 1.54%, P3OT/PCBM (1:3 wt.%) with eta(AM1.5) = 1.1% and P3DDT/PCBM (1:3 wt.%) with eta(AM1.5) = 0.59% (A: 0.25 cm(2), P-IN = 100 mW/cm(2)). The serial and parallel resistance increases within the series P3HT:PCBM < P3OT:PCBM < P3DDT:PCBM cells, therefore the short-circuit current decreases and the open-circuit voltage increases in this order. (c) 2005 Elsevier B.V. All rights reserved.