Ultrafast Spectroscopic Study Of Carrier Loss Channels In Non-fullerene Organic Photovoltaic Blends

Organic solar cells are cost-effective power suppliers with mechanical compliances which are promising for portable optoelectronic devices.The record power-conversion efficiency has already exceeded 18% in single-junction devices benefiting from the rapid development of non-fullerene electron acceptors.In organic photovoltaic(OPV)blends,photocharge generation is enabled in the bulk heterojunction of donor and acceptor where the photo-induced excitons are separated at the interfaces.In typical non-fullerene OPV systems,photocharge generation is frequently observed with near-unit quantum efficiency.It becomes critical to elucidate and suppress the loss channels to further improve the performances of OPV devices.In this thesis,we monitor the dynamics of charge generation and recombination in polymer/small molecule and polymer/polymer combinations of donor/acceptor.We demonstrate the triplet loss suppression through fluorination of acceptor in polymer/small molecule blends and the local state trapping reduction by dielectric screening manipulation in all polymer blends.The major findings are summarized in the following.1)Hole transfer mediated by charge-transfer excitations can be promoted by doping additive to enhance the dielectric screening effect in all-polymer bulkheterojunction.Using transient absorption spectroscopy,we find that doping viscous additive into the all-polymer bulk-heterojunction(TQ-F/N2200)can remarkably promote the hole transfer while electron transfer remains unchanged.Combining the dielectric constant characterization and photoluminescence dynamics of blends,we find that the different additive effects on the two charge transfer channels can be attributed to the different dielectric screening effects for the separation of local and charge-transfer states as the initial states of the electron transfer and hole transfer channels,respectively.These findings have important implications for optimizing the excitation forms and their dynamics in polymers by manipulating the dielectric properties.2)Regulation of triplet energy level alignment by acceptor fluorination can effectively suppress the triplet recombination pathway in polymer/small molecule bulkheterojunctions.We employ transient absorption spectroscopy to investigate the charge generation and recombination dynamics in a series of polymer/non-fullerene blends(PTQ10/IDIC、IDIC-2F、IDIC-4F,PTQ10/MO-IDIC、MO-IDIC-2F、MO-IDIC-4F)with different levels of fluorine substitutions on the acceptors.In the blends without fluorine substituents,we observe the triplet terminal loss channel of the photo charges.The triplet exciton generation is gradually suppressed when increasing the level of fluorine substitutions on the acceptor.From the temperature-dependent carrier dynamics and quantum chemical calculations,we can conclude that the increased energy mismatch between the triplet charge-transfer states and the triplet excitons is responsible for suppressing triplet exciton generation.These results provide an energetic consideration of triplet excited states for optimizing the performance of organic solar cells.