Photovoltaic Device Based Small-Molecules-Doped Polymer

Compared to traditional inorganic semiconductor materials, organicsemiconductors have advantages of low cost, low pollution, easy preparation, andflexible thin-film devices. However, the biggest problem that limits the applications oforganic materials is the low carrier mobility in organic semiconductors. The lowcarrier mobility will reduce significantly the efficiency of the optoelectronic devices,thus limiting the possible applications. Therefore, to improve the charge transferefficiency in organic materials is one of the key issues for the development of organicoptoelectronics. Using blends of organic semiconductors to construct hetero-junctionstructures is one of the effective and practical approaches. Furthermore, time-resolvedspectroscopy enables understanding of the photophysics related to energy transfer,charge generation, charge transport, charge separation and combination processes.This thesis is based on the blend materials of a small molecule semiconductorperylene N,N9-bis(1-ethylpropyl)-3,4,9,10-perylenebis (dicarboximide)(EPPTC) anda Polyfluorene derivative of Poly(9,9’-dioctylfluorene-co-benzothiadiazole)(F8BT).Bulk hetero-junction structures form in the blend film and photophysical propertieshave been studied using steady-state and time-resolved spectroscopy, as well asmicorscopic characterization. Photovoltaic devices using the blend materials havebeen prepared and the conversion efficiency has been measured at different annealingtemperatures and different composite ratios, where changing the annealingtemperatures or the composite ratio was used to modify the phase-separation, themorphology, and the crystalization properties of the molecules in the blend film. Thethesis is arranged as follows:1. The influence of the annealing temperature on the surface morphology of theF8BT: EPPTC blend film has been characterized by the atomic force microscopy(AFM), where the annealing process modified the phase separation between the smalland the polymer molecules and the crystalization of the EPPTC small molecules.2. The spectroscopic characteristics of the blends of F8BT: EPPTC have beenstudied using the absorption and photoluminescence spectra measured at differentannealing temperatures and different composite ratios between the two kinds ofmolecules. Furthermore, the crystalization properties of EPPTC are found to beenhanced with increasing the annealing temperature and the concentration of EPPTC. 3. By changing the composition ratio of the F8BT: EPPTC blend and theannealing temperature, the external quantum efficiency of the photovoltaic devicebased on the bulk hetero-junction structure was characterized systematically. It isfound that the annealing process and modification of the composite ratio in the blendsare effective methods to address the photophysics in the organic hetero-junctionstructures and to optimize the optoelectronic properties of the photovoltaic devices.