Optimizing Morphology of Bulk Heterojunction Polymer Solar Cells

The performance of bulk heterojunction polymer solar cells is profoundly influenced by the spatial arrangements of microstructure at various length scales in its photo-active layer, referred to as morphology. Due to their complex chemical structures, polymers usually exhibits low crystallinity and carrier mobility, leading to a limited thickness ~100 nm of the active layer for a typical polymer solar cell. Such thin films are incompatible with the prevailing large-area coating techniques, thus increasing the difficulty to realize the high-throughput production of polymer-based photovoltaics in industry. On the other hand, for most high-performance low-band-gap polymers, during their film-casting process, processing solvent additives are usually essential for morphology optimization, which help boost device efficiency. However, most commonly-used solvent additives such as 1, 8-Diiodooctane (DIO), are disturbingly reactive to oxygen or water in air, leading to deteriorated performance of devices made under the ambient environment. Therefore, fabrication processes involving DIO have to be limited to an air-free environment, which is quite unfavorable for large-area fabrication techniques, as majority of them are carried on under the ambient environment. Therefore, an efficient air-stable solvent additive would be greatly appreciated in terms of OPV industrialization. As a result, in order to achieve thick active layers as well as to find an air-stable alternative additive for industrial applications, a thorough and systematic study on morphology is necessitated.First, via rational modification of polymer chemical structure(fine-tuning on side chains), new polymers with enhanced structure order (e.g., crystallite size increases from 35 � to 53 �) and higher hole mobility (from ~10-5 to ~10-4 cm2/(V*s)) are obtained, enabling thicker optimum active layers ~200 nm with a larger thickness tolerance up to ~350 nm for the corresponding bulk heterojunction devices. This result is of great potential for relaxing the required level of precision in active layer thickness, which has important industrial implications for large-area film deposition. Second, through examining those solvents with a great potential to satisfy the criteria for efficient additives, a new efficient air-stable solvent additive -1,2-dichlorobenzene (DCB) was successfully found for the Diketopyrrolopyrrole-based narrow bandgap polymer under investigation in this work, with a much larger working operation window (up to 80%) and higher device efficiency than DIO. The reason for improved performance lies in higher hole mobility due to polymer crystallinity enhancement in films cast from solution processed by both additives, as demonstrated by Transmission Electron Microscopy (TEM), photoluminescence (PL) and Grazing Incident Wide Angle X-ray Scattering (GIWAXS) results. Small Angle Neutron Scattering (SANS) and UV-visible absorption spectroscopy were also conducted on polymer structures in solution, and their results revealed a novel working mechanism of DCB for morphology control, which involves the modified solution-stage polymer conformations due to the polymer-additive interaction. Upon incorporating DCB into blend solution, the resultant polymer configurations in solution would have a high tendency to preserve into crystalline regions in the as-cast films and this unique way of tuning thin-film morphology via altering polymer conformations in solution has established a new guide for future additive selection in other polymer systems. Results of this manuscript will resolve the current obstacle for high-throughput process in industry and should be of great potential to contribute to practical OPV applications in the near future.