The Preparation And Morphology Control Of Organic Solar Cells Based On Photovoltaic Poly (Rod-Coil) Polymers

Solution-processed organic solar cells?OSCs?have attracted considerable attention due to their unique advantages : light weight,mechanical flexibility,low cost,and adaptability to roll-to-roll large area fabrication.To date,the power conversion efficiency?PCE?of state-of-the-art single-junction OSCs has reached over 10%.In general,photovoltaic materials can be divided into two main classes: ?-conjugated polymers and small moleculars.Conjugated polymers usually have a good film formation potential and processability.However,conjugated polymers that are mixtures of homologues,often suffered a severe molecular weight-dependent photovoltaic behavior.The average molecular weight and polydispersity make their photovoltaic properties vary from batch to batch.In contrast,small molecular have a definite chemical structure and can be purified straightforwardly,However,since they usually have a large rigid ?-conjugated core,they are prone to aggregate or crystallize in solid state which makes them hard to form a well-qualified film.To solve these problems,we combined the advantages of polymers and small molecules and proposed a new type of polymer materials named poly?rod-coil?polymers,which backbones are composed of definite conjugated segments and non-conjugated segments in an alternative fashion.We expect that they show optoelectrponic properties less dependent on their molecular weight and also have a good film-forming potential.In the first chapter of this paper,we present out effort in investigation on structure-property relationships of poly?rod-coil?polymers mainly by changing the alkyl side chains of the conjugated photoelectric segments and the non-conjugated flexible connection groups to study the influence of the structure to their basic properties and photovoltaic performance.Our group have designed and synthesized five kinds of polyester poly?rod-coil?polymers.The skeleton of rigid conjugated segments takes an A–D–A structure based on benzodithiophene?BDT?and benzothiadiazole?BT?,and the flexible non-conjugated segments possess different dicarboxylate-linked alkyl segments.Through their basic properties and device active layers systematically optimization research,we found that different dicarboxylate linking units in non-conjugated segments have little influence on material optical properties,HOMO and LUMO energy levels,and material bandgap.In contrast,alkyl chains on conjugated segments have been demonstrated to have significant impacton the above film-related properties.The best PCE based on polyester poly?rod-coil?polymers and PC61 BM is 1.09%.In the second part of the work,we were endeavored to improve PCE of poly?rod-coil?polymers and broaden their applications.We replaced the conjugated segments with a diketopyrrolopyrrole?DPP?-cored D-A-D segments and adopt linear alkyl linkers as non-conjugated segments.This new poly?rod-coil?polymers was named as PDPP-6TH-C₁₆.Meanwhile,a small molecular compound named DPP-6TH-C₁₆ possessing identical conjugated segments to PDPP-6TH-C₁₆ was synthesized as a reference for property comparisons.After optimization on device active layers,the best PCEs of the devices based on DPP-6TH-C₁₆/PC61 BM,DPP-6TH-C₁₆/PC71 BM,PDPP-6TH-C₁₆/PC61 BM and PDPP-6TH-C₁₆/PC71 BM were 4.09%,3.21%,1.53% and 1.87% respectively.Obviously,the devices based on poly?rod-coil?polymer,PDPP-6TH-C₁₆,displayed inferior performance than DPP-6TH-C₁₆-based cells.It was found that the account would be mainly ascribed to the poor film morphology and low hole mobility for PDPP-6TH-C₁₆-based cells.Besides their individual performance,we also investigated the use of poly?rod-coil?polymers as additive to tune the morphology of small molecule-based bulk heterojunction?BHJ?films and their ternary BHJ solar cells.We found that the best PCE of DPP-6TH-C₁₆-based cells can be improved into 4.32% when a small amount of PDPP-6TH-C₁₆ was blended together.This result proved that poly?rod-coil?polymers could be a good additive to control active layer morphology of small molecular organic solar cells and improve their photovoltaic performance.