The Effect Of Third Component On The Performance Of Bulk Heterojunction Organic Solar Cells And The Physical Mechanism

Organic solar cells(OSCs)have attracted extensive attention mainly due to their solution processability that enables making inks for printing flexible,lightweight and large area devices via roll-to-roll printing technology.Over the last few decades,heuristic approaches have been successfully used to enhance the performance of OSCs,including synthesis of new donor and acceptor molecules,optimization of the interface and morphology,ternary blending,and device engineering.As a result,power conversion efficiency(PCE)of OSCs has surpassed 15% for single-layer bulk-heterojunction systems.To make organic photovoltaics commercially viable and competitive,researchers have been making efforts on characterizing,understanding and rationally engineering the long-term stability of OSC devices.The main contents of this thesis are as follows:(1)We demonstrate a bulk heterojunction(BHJ)OSCs with the PCE of 6.39% by incorporating a squaraine derivative SQ-3 as the additional donor material into PCDTBT:PC71BM host binary blend.The absorption spectrum of PCDTBT is mainly in the range of 350-650 nm,and SQ-3 can harvest photons at the wavelength of 500-800 nm.Incorporation of SQ-3 improves the photon harvesting of the blend film,which results in an increased short-circuit current density(JSC).We also demonstrate that charge transfer between PCDTBT and SQ-3 increases the exciton dissociation efficiency,which is beneficial to improve the JSC of the device.As a result,the PCE of ternary devices with 10 wt% SQ-3 is about 30% greater than that of PCDTBT:PC71BM based binary OPVs.In order to explore the effect of the third component on the performance of the ternary solar cells and summarize the selection principle of the third component,we compare the performance of PCDTBT:PC71BM:SQ ternary devices incorporated with five different squaraines(SQ-1,SQ-2,SQ-3,SQ-4 and SQ-5).Atomic force microscopy(AFM)and grazing incidence X-ray diffraction(GIXRD)analysis indicate that the different performance of ternary devices did not result from the morphology change of blend film.Through the charge carrier mobility and transient photovoltage(TPV)measurements,it is found that the incorporation of 10% SQ-5 with shallower Highest Occupied Molecular Orbital(HOMO)level hinders holes in transferring from SQ to PCDTBT,which leads to unbalanced charge carrier transport,significant charge recombination,and decrease of performance.Therefore,the third component should have an absorption spectrum complementary to the host material and appropriate energy levels to improve the performance of the organic solar cell.(2)Squaraine is a candidate for the potential high-efficiency device because of its high extinction coefficient,low-cost synthesis and solution casting.However,the charge carrier mobility of squaraine derivative based film is much lower than other effective materials,which leads to the pretty low fill factor(FF).In this study,we enhance the charge carrier transport and improve the photovoltaic performance of LQ-51:PC71BM solar cells by incorporation of PCDTBT.The VOC,JSC and FF of LQ-51:PC71BM binary solar cells are 0.83 V,9.47 m A cm-2,47.77%,respectively,and they increase to 0.85 V?10.86 m A cm-2?51.98%,respectively,by incorporating 70 wt% PCDTBT.Finally,the PCE of 70 wt% PCDTBT ternary device increase from 3.75%(binary device)to 4.80%.The incorporation of PCDTBT can not only increase the photon harvesting but also provide an additional hole transport pathway.Through the charge carrier mobility and transient photovoltage measurements,we find that the ternary system exhibits more balanced charge carrier mobility and increased charge carrier lifetime.Also,we carefully demonstrate that the charge carrier transport follows a parallel-like behavior.(3)Long device lifetime is still a missing key requirement to commercializing OSCs technology.Understanding thermodynamic factors driving morphology degradation or stabilization are correspondingly lacking.We combine thermodynamics with morphology to elucidate the instability of highly efficient PTB7-Th:IEICO-4F binary solar cells and to rationally use PC71 BM in ternary solar cells to reduce the loss in the power conversion efficiency from ~35% to <10% after storage for 90 days and at the same time improve performance.The hypo-miscibility observed for IEICO-4F in PTB7-Th(below the percolation threshold)leads to over-purification of the mixed domains.In contrast,the hyper-miscibility of PC71 BM in PTB7-Th of 48 vol% is well above the percolation threshold.At the same time,PC71 BM is partly miscible in IEICO-4F suppressing crystallization of IEICO-4F.Our work systematically illustrates the origin of the intrinsic degradation of PTB7-Th:IEICO-4F binary solar cells,demonstrates the structure-function relations among thermodynamics,morphology and photovoltaic performance,and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold,yet also needs to be partly miscible with the crystallizable acceptor.