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 photomduced 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.