Tanaka and coworkers also studied the OPV-b-PEO aggregation behavior of the block copolymers in a coil selective solution (THF/water) by the photophysical properties of the OPV block [41]. With increasing water concentration, the absorption maximum was blue-shifted; the single-chain emission of molecularly dissolved OPV decreased, but emission from the aggregated OPV increased, as shown in Fig. 11. The three emissions were identified as follows: the 540-nm emission was attributable to the single-chain emission of molecularly dissolved OPV chains, and the emissions at 580 and 625 nm were attributed to the aggregated OPV chains, indicating that the OPV blocks formed H-aggregates in which the OPV blocks aligned in a parallel orientation. A TEM observation revealed that the block copolymers with a lower PEO weight fraction formed long cylindrical aggregates, whereas the block copolymer with a higher PEO exhibited distorted spherical aggregates ( Fig. 12). It was believed that the block copolymer preferred the spherical aggregate for the longest PEO segment in order to decrease the stretching of PEO and the resultant entropy loss.