Molecular Engineering Of Materials For Polymer Solar Cells And Its Effects On Photovoltaic Performance

As a prospective photovoltaic technology for green and sustainable energy,bulk heterojunction?BHJ?polymer solar cells?PSCs?have attracted significant interests because of their remarkable characteristics including bandgap adjustable,light weight,flexibility and low cost.Due to the short diffusion length of exciton?5-20 nm?in organic semiconductor materials,the nanoscale phase separation in the active layer is one of the crucial factors for achieving efficient exciton dissociationt.In addition,charge carrier transport is closely related to molecular stacking and orientation and crystal crystallinity.Among them,the molecular structure of polymer donor materials and film processing method can significantly improve the active layer morphology.Therefore,based on the molecular engineering of materials,we detailed discussed the effect of molecular structure change on aggregate structure and device performance in this work.The main innovation results are as following:?1?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 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:A ratios and the molecular weights,which endow the two samples with entirely different aggregation behaviors and optoelectronic properties.Among them,the polymer samples prepared by using Pd?PPh₃?₄ as a catalyst exhibited remarkable temperature-dependent aggregation effect and more ordered molecular stacking,leading to the superior photovoltaic performance of 8.65%,which was almost twice that of the Pd₂?dba?₃/P?o-tol?₃ catalyst system?4.07%?.The reported findings clearly demonstrate the critical importance of choosing the right catalyst to prepare high performance D-A copolymers and preventing misreading the corresponding photovoltaic performance when an incompetent catalyst is employed?2?To investigate how the molecular conformation variation due to the existence of steric and corresponding PSC performance,we well-designed two isomeric D-A copolymers via interchanging the alkyl side chains positioned on the donor?Terthiophene?and acceptor units?Benzo[c][1,2,5]thiadiazole?using the platform of D-A copolymers,affording two copolymers of PTDTffBT?C6/EH?and PTDTffBT?EH/C6?,respectively.Positioning the alkyl chains of 2-ethylhexyl on the acceptor units and hexyl on the donor units resulted in slightly decreased dihedral angles within the donor unit and slightly increased dihedral angles between the donor and the acceptor units.Such subtle structure perturbation by the backbone steric distortions from the dihedral angles is found to show negligible impact on electronic structures but lead to distinctive film microstructure.Compared with the PTDTffBT?EH/C6?,PTDTffBT?C6/EH?exhibits the apparant coexistence of face-on orientation with improved crystallinity,leading to the superior charge transport for increasing J_scc and FF.Therefore,The PTDTffBT?C6/EH?:PC₇₁BM PSC devices offer a much improved maximum power conversion efficiency?PCE?of 8.24% over 6.13% of the PTDTffBT?EH/C6?device.?3?Besides lowering the HOMO energy level of polymer donors to promote higher open circuit voltages of polymer solar cells,fluorination of their conjugated backbones has been widely demonstrated to improve the overall device performance as well by inducing face-on orientated polymer crystalline lamellae,which is profitable for vertical charge transport.Nevertheless,overfluorination?up to 4 fluorine atoms?causes severe solubility reduction and serious segregation.Guaranteed with sufficient solubility,two D-A copolymers of PDTBBT-3F and PDTBBT-4F with 3 and 4 fluorine substituents on the same backbone,respectively,were therefore designed and synthesized.Interestingly,the molecular packing of PDTBBT-3F and PDTBBT-4F in neat films was found to be diametrically opposite.PDTBBT-4F exhibits a preferred face-on orientation whereas the edge-on orientation is dominant for PDTBBT-3F.When mixed with PC₇₁BM to form BHJ blends,the molecular orientation of PDTBBT-3F crystals was transferred to uniform edge-on/face-on intermixing with better crytal crystallinity.Oppositely,the face-on oriented crystals were suppressed to be comparable with edge-on orientated ones with less crystallinity for PDTBBT-4F from the neat film to the BHJ blend.Such distinctive morphology improved the charge generation efficiency and balanced the charge transport in the PDTBBT-3F:PC₇₁BM device,eventually leading to an approximate 40%increase of the maximum power conversion efficiency?8.33%?in comparison with the PDTBBT-4F blend device?5.92%?,which provides a reference for designing high performance fluorinated polymer photovoltaic materials.?4?With a higher boiling point and much improved solubility for PC₇₁BM compared with 1,8-diiodooctane?DIO?,?,?di?thiophen-2-yl?alkanes?butane,hexane,octane and decane?were applied as processing solvent additives to fabricate PTB7-Th:PC₇₁BM bulk heterojunction?BHJ?polymer solar cells?PSCs?.These solvent additives can provide fine control of the BHJ blend morphology by influencing molecular packing and orientation during film formation.A large gain in photocurrent was achevied for the devices processed with these?,?di?thiophen-2-yl?alkanes as additives.Among all studied additives,1,8-di?thiophen-2-yl?octane results in the most optimized PTB7-Th:PC₇₁BM blend with more uniform polymer crystals of both the IP and OOP?100?lamellae with better crystallinity,thereby leading to the most enhanced power conversion efficiency of 9.87%when compared with the reference device?8.65%?processed with1,8-diiodooctane?DIO?.Furthermore,the lifetime of the device processed with 1,8-di?thiophen-2-yl?octane significantly increased in comparison with that of the DIO-based device in a nitrogen-filled glove box without encapsulation.?5?Since the charge transfer between organic semiconductor molecules is closely related to the molecular stacking and orientation,and the crystal crystallinity,optimizing the BHJ blend film morphology is an effective means for achieving efficient charge transfer and transport by modifying the molecular structure of the additive in film processing.Then,a series of novel additives of?,?-di?5-fluorothiophen-2-yl?alkanes were synthesized by introducing fluorine in the ortho position of thiophene,and were applied to fabricate PTB7-Th:PC₇₁BM bulk heterojunction?BHJ?polymer solar cells?PSCs?.When using the 1,6-di?5-fluorothiophen-2-yl?hexane?DTH-2F?as solvent additive,the device shows the most optimied photovoltaic performance of 10.53%,mainly due to more efficient charge transfer and transport resulted from the highest crystal crrstallinity over the other?,?-di?5-fluorothiophen-2-yl?alkanes.In addition,the device performance is improved by approximately 9%compared with 1,6-di?thiophen-2-yl?hexyl?DTH?.These results advantageously demonstrate that it is also an effective method to adjust the aggregate structure in the active layer by regulating molecule structure of additive.