Energy Relaxation On The Excited Ststes In MEH-PPV Investigated With Femtosecond Transient Grating

Organic photoelectric functional material,which has attracted great attention since its birth, has become the hot topic of physics, chemistry, material science etc. It can be divided into organic polymer and organic molecules according to the molecular size and structure, in which organic conjugated polymer has caused most interests because of its good film forming property and electric transport characteristics, low cost, high photoelectric conversion efficiency. At present, the organic conjugated polymer has already been used in the actual device, but some of its physical and chemical processes were not clearly revealed, such as its light-emitting energy level structure, excited state energy relaxed channels and the time scale, etc. In this article, time-resolved fluorescence spectra and transient grating spectrum were taken from experiment to theory to reveal the above processes in MEH-PPV.Firstly, the fluorescence lifetime measurement of MEH-PPV was taken. The fluorescence decay curve puts up two different lifetime, and the corresponding energy levels were calculated by Gaussian fitting to steady state fluorescence spectrum.Secondly, the transient grating measurement of MEH-PPV was maken by using broadband probe pulse. The kinetic curve of the signal was studied. The energy relaxation way and quantum beat were analysed. Dispersion correction reveals a steep rising edge of both two signal peaks'kinetic curves. And it is concluded that the origin of population on the two emission level is the same. Based on the above analysis, the schematic drawing of MEH-PPV energy level was depicted.Finally, the theoretical simulation of transient grating spectrum was calculated. The origin of decay processes was proved through the comparison of theoretical and experimental results, in which the quick process was resulted from dephasing of chemical bonds coherent vibration, and the slow process was resulted from dissipation of chemical bonds vibration energy. The real vibration levels were calculated to be 29cm-1 for P1 and 36cm-1 for P2.