Distinction between PTB7-Th samples prepared from Pd(PPh3)4 and Pd2(dba)3/P(: O -tol)3 catalysed stille coupling polymerization and the resultant photovoltaic performance
State-of-the-art polymer donors for bulk heterojunction (BHJ) polymer solar cells (PSCs) are mostly alternating donor-acceptor (D-A) copolymers prepared from palladium catalysed Stille cross-coupling condensations. The structural variation of D-A copolymers, such as conjugated backbones, alkyl side chains and positions, substituents and molecular weights, has been proven to significantly impact the energy levels, intermolecular interactions and molecular packing, which hereafter synergistically determine the performance of corresponding BHJ PSCs. For a given D-A copolymer, the alternation of D and A units does not always proceed as intended when a specific catalyst is employed. The actual D:A ratios and the molecular weights would most likely be inconsistent as well when catalyzed differently in preparation. To clarify the impact of the catalysts employed for polymerizations on the structure of the resultant polymers and on the corresponding photovoltaic performance, a comprehensive investigation was conducted on the distinction between two PTB7-Th samples prepared from Stille coupling polymerization with the classic palladium catalysts Pd2(dba)3/P(o-tol)3 and Pd(PPh3)4, respectively. The structural variation between the two PTB7-Th samples is discovered to be distinct with respect to both the actual D: ratios and the molecular weights, which endow the two samples with entirely different aggregation behaviors and optoelectronic properties. The optimized polymer:PC71BM BHJ PSC device demonstrates a normal PCE of 8.65% with the PTB7-Th sample catalyzed by Pd(PPh3)4 as reported and a deteriorated PCE of 4.07% with the PTB7-Th sample catalyzed by Pd2(dba)3/P(o-tol)3. A significant morphological evolution between the two PTB7-Th samples from the neat film to the BHJ film was clarified. This wealth of information on the strong correlation among the variations in the chemical structure, the morphology and the device performance allows the establishment of guidance on the selection of the appropriate catalyst to obtain high-performance PSC polymers
