Effect of Thermal Annealing on Aggregations in MEH-PPV Films

Absorption and photoluminescence spectra at various temperatures below 200 K had been used to study the optical properties of poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) films. In the films without annealing, the PL emission was dominated by intrachain excitons with J-aggregate-like properties, where 0-0/0-1 PL ratio decreases with increasing temperatures, although it was obvious that interchain coupling of Delta E was stronger in film from chlorobenzene solution (Delta E similar to 7.5 meV) than that from chloroform solution (Delta E similar to 2 meV). For MEH-PPV films annealed at 420 and 520 K, the Delta E was enhanced dramatically to 76 and 89 meV, respectively. Thus, the interchain interaction (H-aggregate-like behavior) dominated PL properties in annealed films, resulting in the enhancement of 0-0/0-1 PL ratio with increasing temperatures. Our work experimentally proved that HJ-aggregate model may be proper to describe the photophysical properties of pi-conjugated polymer films using adjustable Delta E.