Fabrication And Mechanism Investigation Of Organic Solar Cells Based On Ternary Strategy

With the advantages of low cost,light weight,easy fabrication and good flexibility,bulk heterojunction organic solar cells have become one of the most promising renewable energy technologies.For ternary organic solar cells,the power conversion efficiency(PCE)can be increased due to enhanced photon harvesting of active layers by employing three materials with complementary absorption spectra.At the same time,ternary organic solar cells maintain the simple the fabrication process as single-junction devices,exhibiting the great development and application prospects.Due to different physical and chemical properties of used materials,complex phase separation and carrier kinetic processes exist in ternary active layers.The working mechanism of ternary organic solar cells needs to be further investigated.In this paper,the high-efficiency binary devices are the prerequisite,material selection and morphology optimization are the research method,synergetic improvements of photon harvesting,exciton dissociation,charge transport and collection are the research direction,and high-efficiency ternary organic solar cells are the ultimate goal.In ternary organic solar cells with various systems,the effect of the third component on performance improvement is explored,the working mechanism is investigated,and the relationship among material properties,preparation process,working mechanism and photovoltaic performance is revealed.The main research contents and innovations are summaried as follows:(1)The photon harvesting of active layers can be enhanced by empoloying the dye small molecule DIB-SQ with high extinction coefficient as photosensitizer,which was demonstrated in two ternary systems.For PIDTDTQx:DIB-SQ:PC71BM based ternary polymer solar cells,the PCE was increased from 5.47%to 6.49%.For BTR:DIB-SQ:PC7,BM based ternary small molecule solar cells,the PCE was increased from 9.37%to 10.3%.The devices with only donors were fabricated to study the working mechanism of ternary solar cells.The effective charge transfer occurs between PIDTDTQx and DIB-SQ and the effective energy transfer occurs from BTR to DIB-SQ.In this work,the interaction between the two donors was investigated in depth,thus the working mechanism of ternary devices was further clarified.(2)A new fabsication techonology,up-side-down thermal annealing and solvent vapor annealing,was proposed.The PCE is increased from 8.08%to 9.11%for DRCN5T:PC7,BM based small molecule solar cell by employing the techonology.The performance improvement is attributed to the optimized vertical phase separation and enhanced DRCN5T crystallinity according to the grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy characteristics.Based on high-efficiency binary devices,active layer morphology was further optimized by empolying high-crystallinity liquid crystal small molecule BTR as the third component.The PCE of ternary devices reachs 10.05%by incorporating 1.5 wt%BTR in donors.This work shows that the combination of optimized techonology and ternary strategy is beneficial to maximize photovoltaic performance.At the same time,the work points out that employing high-crystallinity small molecular materials as morphology regulator is an effective method to fabricate high-efficiency ternary small molecular solar cells.(3)In PTB7-Th:BDTThIT-4F:COi8DFIC based ternary system,a new phenomenon was observed that photon harvesting can be enhanced by using the third component to regulate the molecular arrangement.The cut-off filter was used to study the effect of the phenomenon on the photovoltaic performance.The PCE of the optimized ternary devices exceed 13%,and 14%efficiency improvement is obtained compared with the reference device.The tilted-up absorption edge was observed in ternary blend films with BDTThIT-4F incorporation,which enhances light absorption of active layer in the near-infrared wavelength.The tilted-up absorption edge is attributed to the varied COi8DFIC molecular arrangement induced by BDTThIT-4F.The varied COi8DFIC molecular arrangement can be further demonstrated through the femtosecond transient absorption spectra,leading to the varied energy levels.This work provides more insight into the underlying reason on performance improvement of ternary devices.(4)The method is proposed that selecting the two polymer donors with similar molecular structure to design ternary organic solar cells.The PTB7:PBDT-TS1:PC71BM based ternary polymer solar cells were successfully fabricated according to this strategy.Due to the similar molecular structure,two polymer donors show good compatibility.Regardless of the blending content of PTB7 and PBDT-TS1,all ternary devices can maintain high FF,indicating that effective carrier transport channels are formed in ternary active layers.The two donors should work independently with PC71BM to form parallel structure.The optimized ternary devices achieve PCE of 7.91%,28.2%and 12.8%performance improvement are obtained compared with PTB7 and PBDT-TS1 based binary devices.This work indicates it is an effective way that selecting materials with similar molecular structures to fabricate ternary organic solar cells.(5)It was proposed that complementary photovoltaic parameters of two binary devices is an important reference standard to design high-efficiency ternary organic solar cells.According to the reference standard,PBDB-T:ITCPTC:IDT6CN-M and PBDB-T-2F:FTCCN:IT-2F based ternary polymer solar cells were successfully fabricated.By well balancing three photovoltaic parameters,the PCEs of two ternary devices reach 11.92%and 12.99%,respectively.At the same time,the fill factors of two ternary devices exceed 76%due to good compatibility of the used materials,which are among the highest values of reported ternary organic solar cells.In order to accurately study the effect of the third component on the photo harvesting,a special device with the structure of ITO/PEDOT:PSS/PMMA/PDIN/Al was designed to characterize the device parasitic absorption.Then the absorption spectra of active layers were calculated.Compared with the absorption spectra of blend films,the interference effect between the reflected light from A1 electrode and the incident light is investigated in the absorption spectra of active layers.This work points out that combining the advantages of two binary devices into ternary device is an effective strategy to improve photovoltaic performance.