Synthesis And Performance Investigation Of Small-Molecule Donor Materials For Organic Solar Cells

Organic solar cells (OSCs) offer great opportunities as renewable energy sources due to their attractive features, including low-cost large area fabrication, light-weight and good mechanical flexibility. Solution-processed small molecules have been emerging in recent years as an attractive alternative to widely studied polymeric counterparts and offered several unique advantages which are well-defined molecular structures, definite molecular weight, higher purity, good batch-to-batch reproducibility. The studies of small molecule organic photovoltaic are mainly designing of new donors and optimizing device fabrication in order to improve power conversion efficiencies (PCEs). However, there are still some fundamental problems to be solved, such as halogen atoms effect on the energy levels, molecular alignment and stacking; influence of chains on thin film morphology; mechanism of conductivity at single molecules level etc..In this thesis, we designed two different donor-acceptor-donor (D-A-D) and acceptor-donor-acceptor (A-D-A) systems, based on photosensitive dye diketopyrrolopyrrole (DPP) framework. By examining physical and chemical properties of these new materials, we investigated the effects of halogen atom and alkyl chain substitutions on DPP small molecules. The main content of this thesis includes the following parts:1. Three D-A-D type small organic donor molecules containing a central DPP segment have been designed and synthesized using F, Cl and alkane as end groups, which were named to FDPP, CDPP and RDPP, respectively. A systematic comparative investigation between the FDPP, CDPP and RDPP helps to unveil the effects of halogenation in the small molecule OSCs. We studied their thermal stability, absorption spectra, the charge carrier mobility, film morphology and PCEs. The results showed that by varying the terminal group can significantly alter the physical and optical properties of the molecule. Introducing F atoms as the end group may exist C-F…H, F…S, and C-F…πF interaction between FDPP molecules, as the result FDPP have greater thermal stability and crystallinity. Furthermore, the greatest electronegative F atom also led FDPP molecule shows lower HOMO and LUMO energy levels. When FDPP were blended with the fullenene derivative PC71BM into bulk heterojunction (BHJ) configuration OSCs devices, they exhibited higher open-circuit voltage (Voc) and higher short-circuit current density (Jsc). The introduction of Cl atom, resulting strong electron delocalization, made the CDPP molecule be a more rigid structure, which led to low solubility and PCEs. Introducing the well soluble alkyl as end group on the molecule made RDPP has better ability to self-assemble, besides their absorption spectrum shows strong bathochromic shift resulting moderate performance.2. A series of linear push-pull molecules (DPP-DTP) with different alkyl groups substituted DTP as electron donor core and DPP as electron acceptor arms have been designed and synthesized. The molecules showed similar thermal stability and light absorption characteristics in solution. However, due to the different alkyl substitution, molecular packing in thin-films is different. Besides, they exhibited different absorbance response when treated with active layers. After solvent vapor annealing, DPP-DTPC6 and DPP-DTPC8B exhibited increased performance. However, DPP-DTPC7 and DPP-DTPC8 exhibited reduced performance.3. We designed and synthesized three organic small molecule containing a central DPP segment with thienyl,2-(methylthio)thiophene and aniline end groups named 3-DPP, DPP-SCH3 and DPP-NH2, respectively. These molecules were self-assemble on gold substrate, and their molecular conductivities were studied by scanning tunneling microscope break junctions. We observed visible light response of these molecules in their single-molecular device. These results may provide new insights to molecular conductance and light response mechanism.4. We synthesized a series of electron donor base on triphenylamine, oligomers and anthracenezodithiophene derivatives, and acceptor segments base on DPP and thieno [3,4-c]pyrrole-4,6-dione structure. The optical absorption of compound TA5 indicated introducing benzothiadiazole framework caused strong intermolecular charge transfer in the triphenylamine segment. These results may expand the scope of the D and A units of OSCs.5. We have demonstrated the highly efficient and green synthesis of benzofurans under aqueous condition through an "intermediate-in-water" heterogeneous reaction strategy. This heterogeneous reaction exhibits many attractive features, including the easy collection and purification of product, wide range of functional group tolerance, thus rendering the methodology as a highly eco-friendly alternative to the existing methods. Further works including expanding the scope of the strategy to other organic semiconductor are undergoing.