Investigation Of Charge Carrier Generation And Recombination Behaviors Of Organic Photovoltaic Materials

With the development and utilization of new organic photovoltaic materials,the performance of organic photovoltaic devices has achieved remarkable development in the field of organic solar cell research during the recent years.In the development of organic photovoltaic materials and devices efficiency optimization,the introduction of solvent additives and third components to the active layer materials were proved to be effective means to improve the power conversion efficiency?PCE?of the organic photovoltaic devices?OPVs?.The PCE of the organic photovoltaic devices are closely related to the generation and recombination behavior of the charge carriers inside the active layer material:under illumination conditions,organic photovoltaic materials need to first generate photo-generated excitons.The excitons dissociate at the interface between the donor material and the acceptor material to form charge carriers.The charge carriers need to overcome the effects of geminate recombination and non geminate recombination and transport to the electrode.Finally the organic photovoltaic devices output the current and power to the external environment.Understand the active layer optimization strategy,that is,adding the solvent additive and introducing the third component,the effect of these strategies on the basic photoelectric conversion process in the active layer material,especially the charge carrier generation and recombination behavior,is beneficial to the optimization of conversion efficiency.In this work,Poly[[4,8–bis[?2–ethylhexyl?oxy]benzo[1,2–b?4,5–b 0]dithiophene–2,6–diyl][3–fluoro-2-[?2-ethylhexyl?carbonyl]thieno[3,4-b]thiophenediyl]]?PTB7?and fullerene material phenyl-[6,6]-c71-butyric acid methyl ester(PC₇₁BM)are donor materials and acceptor materials of the active layer of organic photovoltaics?OPVs?,the influence of solvent additive 1,8-diiodooctane?DIO?and third component BTA2on the charge charge carrier generation and recombination behavior in the active layer materials was investigated.1.The influence of the solvent additive DIO on the charge carrier recombination behaviors of PTB7:PC₇₁BM organic photovoltaics?OPVs?were investigated.The measurement of transient photocurrent?TPC?implies that the devices processed with3%DIO have less charge carrier recombination loss than the devices without DIO.The photo-induced charge extraction by linearly increasing voltage?Photo-CELIV?technique shows that the device with DIO has less charge carrier trap population and thus the charge trap-limited bimolecular recombination can be reduced.The modified Onsager–Braun model with bimolecular recombination rate?_?74?,calculated from the Photo-CELIV,indicated that by adding DIO,the binding energy of the charge-transfer exciton at the interface of PTB7 and PC₇₁BM reduced,thus the geminate recombination loss decreased.Atomic Force Microscope?AFM?images show that DIO can optimize the morphology of the active layer,which is beneficial to the exciton dissociation and the reduction of the charge carriers trapping.The reduction of the charge carrier recombination including geminate recombination and non-geminate recombination lead to the large increase of the short current density(J_SC),the fill factor?FF?,and the power conversion efficiency?PCE?of PTB7:PC₇₁BM OPVs.2.Bulk-heterojunction?BHJ?organic solar cells?OSCs?based on PTB7:PC71BM system with an additive of small molecular and the influence of the third component on the conversion of solar photons to electrons are investigated by experiments and modeling.Under the addition of the fill factor?FF?is still a bottleneck in the ternary OSCs,the short-circuit current density?Jsc?of the cells with the addition of BTA2 can be enhanced significantly with a complementary absorption spectrum.The optical simulation of transfer matrix optical model shows that the active layer of ternary OSCs exhibits higher internal absorption spectra than that of traditional OSCs,which result in the more photons generating excitons in the active layer.The Photo-CELIV investigation indicates that the ternary OSCs suffer a lower charge transport and higher charge recombination for the reason of morphology change.It indicated by the Onsager–Braun model that despite the charge collection efficiency in the ternary OSCs is lower,the optimized photon absorption and exciton generation process ensure the increase of the photo-current and thus the power conversion efficiency?PCE?is improved.