Constructing Small Molecular Acceptor-based Single-compound Materials And Polymer Acceptors For Efficient And Stable Organic Solar Cells

Solution-processed organic solar cells（OSCs）are appealing candidates in flexibility,semitransparency devices and portable electronic products,owing to their special merits of low-cost and light-weight.Thanks to the innovation of active materials and optimization of device process,the power conversion efficiency（PCE）of OSCs has made a steady breakthrough,and the PCE of a single-junction cell has reached 19.6%.However,the stability of OSCs has not been greatly improved,which is still the core problem limiting the commercialization process of OSCs.Compared with the mainstream small-molecular acceptors,single-component materials and polymer acceptors have inherent advantages in device morphological,photo and thermal stability.However,their efficiencies of devices are hampered by the lack of single-component materials and polymer acceptors,and difficulty of morphology optimization in the early stage.Learning from the considerable body of knowledge accumulated during the past two decades on the small molecular acceptors based OSCs,we utilize high-performance small molecular acceptors to construct new single-component materials and polymer acceptors,and finally obtain efficient and stable OSCs.According to this proposal,the main works were summarized as follows.In the second chapter,we first determined the optimal unit ratio of donor:acceptor in bulk heterojunction（BHJ）OSCs,and then designed and synthesized an oligothiophene-fullerene dyad Rh-PC₇₁BM with balanced D-A backbone.Benefitting from the high exciton dissociation efficiency and relatively balanced electron/hole mobility of Rh-PC₇₁BM single-molecular OSCs（SMOSCs）,the final efficiency of Rh-PC₇₁BM SCOSCs was 3.22%,which was the highest value reported at that time for SMOSCs.The success of Rh-PC₇₁BM proved the feasibility of drawing lessons from the optimization experience of BHJ device to guide the design of one-component materials.Importantly,compared to the corresponding Rh-OH:PC₇₁BM BHJ devices,Rh-PC₇₁BM devices demonstrated excellent thermal stability.After annealing at 85℃for400 h in nitrogen atmosphere,Rh-PC₇₁BM devices exhibited a PCE degradation down to 83%,while Rh-OH:PC₇₁BM devices displayed poorer thermal stability with a PCE loss down to around 16%at the same periods.In the previous chapter,the absorption of one-component material Rh-PC₇₁BM is mainly distributed at 400-700 nm,which coincide low with solar spectrum,resulting in the short circuit current density(J_SC)of the device is only 7.50 m A cm^-2.In the third chapter,to further broaden the absorption of molecular dyads,we introduced a non-fused ring non-fullerene acceptor as the acceptor segments to synthesize two molecular dyads,SW1 and SW2,with different donor units.Notably,the two dyads exhibited panchromatic absorption spectra within 400-850 nm.Impressively,the SW1 device demonstrateed an inspiring PCE of 3.78%with a high J_SC of 9.12 m A cm^-2.We believe this new strategy opens up the possibility for designing high-performance non-fullerene acceptor-based single-component photovoltaic materials.But SW2-based device was extremely inefficient,only 0.25%.Then,we systematically investigated the effect of different donor units on the optoelectronic properties,energy levels,photo-physical processes and morphology of two dyads,which may provide reference value for the selection of donor/acceptor units.In addition,SW1 and SW2-based devices had excellent stability.Both stored in nitrogen atmosphere and continuously light-soaking under one sun illumination,SW1 and SW2 devices can maintain more than 90%of the initial values.In the previous chapter,the acceptor segment is non-fused skeleton,which leads to low electron mobility,serious charge recombination,and low device efficiency.In the fourth chapter,considering that Y series acceptors tend to form a continuous and regular three-dimensional structure,we replaced the non-fused ring acceptor segment with Y series acceptor,and constructed two molecular dyads SM-1Y and SM-2Y with different donor/acceptor unit ratio.Compared to molecular dyad SW1,SM-1Y and SM-2Y have wider absorption with 400-910 nm and higher electron/hole mobility.The SM-1Y-based SMOSC exhibited a PCE of 2.51%.Impressively,the SM-2Y-based device demonstrated a much higher efficiency of 6.22%,which was the highest value reported for SMOSCs so far.It is the first time of the efficiency of non-fullerene acceptor based molecular dyads exceeds the fullerene-based counterparts.At the same time,SM-1Y and SM-2Y devices showed excellent stability.Both stored in nitrogen atmosphere and continuously light-soaking under one sun illumination for 300 h,SM-1Y and SM-2Y devices can maintain more than 85%of the initial values.In the fifth chapter,applying a fused-ring electron acceptor Y5-C20 as the key building block and thiophene as theπ-bridges,we designed and synthesized aπ-conjugated polymer acceptor PYT.PYT possessed broad absorption with a narrow band gap and high absorption coefficient.For fine-tuning the active layer morphology,we synthesized a series of PYT polymer acceptors with controlled M_n values（PYT_L、PYT_M and PYT_H）.When fabricated into all-polymer solar cells with PM6,it was observed a clear molecular weight dependence on device performance.Optimized devices based on PM6:PYT_L,PM6:PYT_M and PM6:PYT_H exhibits PCEs of 12.55%,13.44%and 8.61%,respectively.It is worth pointing out that the values of J_SC and PCE in PM6:PYT_M device are the highest values reported at that time for all-polymer solar cells（all-PSCs）.In addition,we systematically investigate the underlying impact of PYT’s molecular weight on the optoelectronic properties,energy levels,photo-physical processes,morphology and energy loss.In the previous chapter,when PM6 as the donor,the efficiency was sensitive to PYT molecular weight.However,different batches of PYT are difficult to ensure the same molecular weight during the reactions.In the sixth chapter,we set up an in-situ photoluminescence spectroscopy to real-time monitor the change of fluorescence spectrum during PYT polymerization,so as to establish the relationship curves between the characteristic parameters of fluorescence spectrum（peak wavelength、peak intensity and half peak wavelength,etc.）and PYT molecular weight.Then,the molecular weight of PYT can be accurately controlled just by monitoring the change of fluorescence spectrum during polymerization.In addition,the reliability of the established relationship was tested by changing the source of catalyst and monomer batch to interfere with the polymerization process.It was found that the efficiency of obtained PYT fluctuated in a small range,which can enough to meet the product quality requirements of industrialization,proving the feasibility of building a real-time monitoring device to control molecular weight of polymer materials.