Synthesis And Photovoltaic Properties Of Small Molecules And Polymers Based On Porphyrin

With the advent of the global energy crisis, looking for new and non-polluting renewableenergy has become a worldwide issue. Solar energy has attracted increasing attention due toits advantages. Organic solar cells have attracted extensive attention due to the potentialadvantages, including simple preparation (such as spin-coating, inkjet-printing, etc), low cost,light weight, and fast/cheap roll-to-roll production. Porphyrins have extensively π-conjugatedsystems, are amenable to fast electron transfer to acceptors, absorb light with high molarabsorption coefficients, and their properties can be tuned easily via synthetic modifications ofthe periphery or by metal insertion into the cavity. These physical and chemical propertiesmake porphyrins suitable for OPV studies, and significant efforts have been devoted toporphyrin-based organic solar cells (OSCs) since in the last century. And indeed porphyrinshave been successfully employed as organic dyes in dye-sensitized solar cells (DSSCs), andgained high power conversion efficiency (PCE>12%). However, the studies on porphyrinsbulk heterojunction (BHJ) OPVs, which show many advantages over DSSCs, have been lesssuccessful, especially the power conversion efficiency based on the BHJ OPVs based onsolution-processed porphyrin are still very low.In this thesis, we designed and synthesized small molecules and polymers based onporphyrin for application in solution-processed BHJ photovoltaic cells. The π-conjugation ofthe whole molecule are expanded, the optical bandgaps and energy levels are tuned, and thecharge mobilities are enhanced by introducing electron-withdrawing units and acetylenebonds between the zinc porphyrin unit and electron-withdrawing units. Thus, they exhibitedsuperior photovoltaic performance based on these molecules devices.In the second chapter, the steric hindrance between the zinc porphyrin unit andelectron-withdrawing unit is effectively decreased after the introduction of ethynylenelinkages. The whole molecule became planar and π-conjugated. The π-electrons weredelocalized in the whole molecule. The absorption spectra of the materials were effectivelyexpanded and the carrier mobilities were improved. Meanwhile, the energy levels were tunedby changing the strength of the electron-withdrawing group. Among them, the BHJ OSCs fabricated by solution spin-coating with5,15-bis(7-(4-hexyl-thiophen-2-yl)-2,1,3-benzothiadiazole-4-yl)-10,20-bis(3,5-di(dodecyloxy)-phenyl)-porphyrin zinc (DHTBTEZP), in which the2,1,3-benzothiadiazole was used as theelectron-withdrawing group as the donor and PC71BM as the acceptor, showed a PCE of4.02%. This is the highest PCE of BHJ OSCs based on the solution-processed porphyrin smallmolecules as the donor and PC71BM as the acceptor reported so far.In the third chapter, we synthesized A-π-D-π-A type small molecules, by introducingthienyl group between zinc porphyrin alkyne unit and electron-withdrawing unit. Theinteraction of between both was effectively decreased. The HOMO and LUMO energy levelswere tuned by this method. It solved the problem of the mismatch energy in the secondchapter. Additionally, the energy levels were further tuned by changing the strength of theelectron-withdrawing group. Among them, the BHJ OSCs fabricated by solution spin-coatingwith5,15-bis(3-(thiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione)-10,20-bis(3,5-di(dodecyloxy)-phenyl)-porphyrin zinc (DTDPPTEZP), inwhich the2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dionewas used as the electron-withdrawing group, as the donor and PC61BM as the acceptorshowed a PCE of4.78%. This is the highest PCE of BHJ OSCs based on thesolution-processed porphyrin small molecules as the donor and PC61BM as the acceptorreported so far.In the fourth chapter, we used the electron-withdrawing units and triphenylamine as thenuclear and the zinc porphyrin alkyne as arms and synthesized linear and star type D-A-Dsmall molecules, respectively. The results showed that these small molecules possessed goodsolubility and film-forming properties. The absorption spectra of the materials wereeffectively expanded by introducing the stronger electron-deficient ability ofelectron-withdrawing unit or reducing the content of the zinc porphyrin alkyne in the wholemolecule.In the fifth chapter, we synthesized a series of D-A type porphyrin copolymers by theintroduction of the ethynylene and electron-withdrawing unit simultaneously. With theincreasing strength of the electron-deficient units, the absorption spectra of the copolymers were gradually red-shift, and the energy levels were tuned. Among them, the BHJ OSCsfabricated by solution spin-coating with PEZPEBTffand PEZPETDPPT(O) as the donor andPC61BM as the acceptor both showed a PCE of0.41%, respectively. These PCE still higherthan the majority of OSCs based on the solution-processed porphyrin polymers as the donorand PC61BM as the acceptor reported so far. Therefore, it is a good way to effectively improvethe PCE of the OSCs based on porphyrin polymer by the introduction of the ethynylene andelectron-withdrawing unit simultaneously.