Phase Distribution And Photophysics In Ternary Organic Solar Cells

Excessive exploitation of fossil energy has brought increasingly serious environmental problems.Besides,the overuse of non-renewable energy source makes it in a state of shortage.Therefore,it is very important to find environmentally friendly renewable energy source to replace the non-renewable energy source in order to solve the problems of environmental pollution and energy source shortages.Currently,hydroelectric power,wind power,thermal power and solar power are the popular environmentally friendly renewable energy sources.However,the use of hydroelectric power,wind power and thermal power are limited by some restrictions.Compared to the mentioned renewable energy sources,solar energy shows some advantages.On one hand,we can use the solar energy without any limitations.On the other hand,solar energy is inexhaustible.In the field of photovoltaics,organic solar cells(OSCs)have shown great competitiveness compared to the traditional solar cells because of the advantages of light weight,low-cost,flexibility,and semitransparent as well as the roll-to-roll production method.In general,the bulk heterojunction(BHJ)based OSC has one electron donor and one electron acceptor anside the active layer.Due to the great efforts,the power conversion efficiency(PCE)of sinsle-junction OSC has exceeded 16%.However,further improvement in performance is still needed for its future commercialization.A lot of strategiesd including synthesis of new materials,design and optimization of new device structures,have been used to improve the performance of OSCs.Ternary strategy,which consist of two donors and one acceptor or one donor and two acceptors in a single-heterojunction,has proven to be a simple and effective method to enhance the OSC performance.Besides,some functional OSCs also can be obtained via ternary strategy.However,the introduction of a third component into a binary system means the introduction of more complex phase distribution and physical mechanisms.It is very important to know the phase distribution ofthe third components in the host systems in order to use the advantages of the ternary strategy.In addition,the phase distribution of the third components in the host systems also play important role in the nanomorphology and photophysical processes of the host systems.There are few studies related to the issues mentioned above.Therefore,our research is mainly focus on the above issues.(1)In the first study,an organic small molecule named p-DTS(FBTTH2)2 has introduced into the host system of P3HT:PC7iBM as the third component.The lowest unoccupied molecular orbital(LUMO)and highest occupied molecular orbital(HOMO)levels of p-DTS(FBTTH2)2 are locate between the LUMO and HOMO levels of P3HT and PC71BM which means the formation of energy cascade.Besides,the absorption spectrum of p-DTS(FBTTH2)2 is complementary to the host system.Therefore,the additional p-DTS(FBTTH2)2 can enhance and broaden the absorption spectrum of the host system.The steady PL spectrum of P3HT is well overlapped with the absor:ption spectrum of p-DTS(FBTTH2)2,which means the energy 'of P3HT can be transferred to the p-DTS(FBTTH2)2 through Forster resonance energy transfer(FRET)effect.Most importantly,p-DTS(FBTTH2)2 is locate at the interface of P3HT and PC71BM.With 15 wt%of p-DTS(FBTTH2)2 in the host system,the PCE of the binary OSC can be enhanced 24%.The performance improvement mainly contributed to two aspects.On one hand,the enhancement and broadening of the absorption spectrum of the host system by p-DTS(FBTTH2)2.On the other hand,the p-DTS(FBTTH2)2 locates at the interface of P3HT and PC71BM which can fully exploit the advantages of the charge transfer and energy transfer synergistic effects to improve the performance of the solar cells.The result indicates that it is an effective method to improve the performance of OSC by using an appropriate third component to modify the interface of host donor and acceptor.(2)Based on the study in the previous section,we introduced dual FRET effects into a single BHJ active layer.The dual FRET effects exist between the third component PCDTBT(energy donor)and the host donor PTB7-Th(energy acceptor 1)and the host acceptor ITIC(energy acceptor 2).Due to the similarity of crystallinity and crystal orientation between PCDTBT,PTB7-Th and ITIC,we found that the third component PCDTBT can be uniformly distributed in the host donor PTB7-Th and the host acceptor ITIC.The phase distribution of the third components means PCDTBT can have intimate contact with the host donor PTB7-Th and host acceptor ITIC,which ensuring the existence of dual FRET effects in the single BHJ.PCDTBT reduces the ?-? stacking coherent length of host donor and host acceptor.However,PCDTBT also can decrease the d-spacing of ?-? stacking of the host donor and host acceptor.As a result,the charge transport ability is not decreased obviously with appropriate amount of PCDTBT in the host system.Finally,the short-circuit current density of the binary OSC is increased from 13.89 mA cm-2 to 16.71 mA cm-2 with 30 wt%PCDTBT doped.The PCE of binary solar cell was increased about 15%.This work shows that introducing dual FRET effects into ternary OSC is an effective way to obviously improve photovoltaic performance.(3)The vertical phase distribution of components in bulk heterojunction is diversified in OSCs.The variation of vertical phase distribution significantly influences device performance because of its impact on the charge transport and charge recombination.In the third part,PC71BM was incorporated into the binary system of PBDB-T:ITIC to fabricate ternary OSCs.Depth resolved absorption spectrum was recorded by oxygen plasma to study the distribution of the PC71BM as well as its function in regulating the vertical phase distribution of the host components in the active layer.Through a combination of various measurements,we found that PC71 BM can effectively regulate the phase distribution of PBDB-T and ITIC in vertical direction.which provides favorable vertical phase distribution for charge transport and charge collection.Moreover,the addition of PC71BM can also increase the ?-? stacking coherence length of both donor and acceptor,which facilitates charge transport and reduces the bimolecular recombination.As a result,appropriate amount of PC71BM can obviously improve both fill factor and short-circuit current density of the OSC based on PBDB-T:ITIC while open-circuit voltage reduces only about 0.01 V,which indicates a rational low energy loss.Consequently,the ternary OSC exhibits a best PCE of 11.0%compared to the 9.6%PCE of the binary counterpart.(4)Ultra-low band-gap small molecule IEICO-4F has been employed as a secondary acceptor in both fullerene system of PTB7-Th:PC71BM and non-fullerene based system of PBDB-T:ITIC OSCs to improve device performance by broadening the absorption spectrum and enhancing charge transfer and transport ability.A notable photocurrent and PCE enhancement can be obtained by introducing an appropriate amount of IEICO-4F in ternary blends.A new model of donor/acceptor "quasi-alloy"is proposed here to describe the mechanism operating in these ternary systems.The quasi-alloy model suggests that IEICO-4F can enhance the crystallinity of polymer donors and provide additional channels for charge transfer and transport compared to binary systems,leading to an ultrafast charge transfer and a more effective charge transport process.