Design And Synthesis Of The Organic Semiconducor Materials And Their Applications In Solar Cells

The photosensitive active layer of organic solar cells is fabricated with organic semiconductors.Organic solar cells have been attracting considerable attentions in last decades due to their low cost,flexibility,light weight,simple process and large area devices making.Related research has achieved great progress and efficiency of devices has been improved.Organic semiconductors as active layer in solar cells are crucial to the photovoltaic performance of devices,which include electron donors and electron acceptors.The rapid development of donor materials,especially polymer donors,continuously enhance the efficiency of organic solar cells.In contrast,fullerene derivatives still dominate in acceptor materials,but more and more attentions are focusing on non-fullerene acceptors.On these topics,at first,we studied the preparation and properties of π-stacking polymer semiconductors;furthermore,the role of donor in low donor content solar cells and the influence of donors’ ionization potential on the performance of devices were investigated;finally,new non-fullerene small molecule acceptors(dimers and 3D structure molecules)were designed,synthesized and applied in bulk heterojunction solar cells.The detailed works are listed as follows:(1)We designed and synthesized two novel polymers(PFMA-Ma and PFMA-Mb)through a new method combining conventional free radical polymerization with Diels-Alder reaction.Different molar percentages of the functionalized pendants in the polymers and two different steric configurations(endo and exo in the D-A adduct structures)are revealed in 1H NMR spectra.The fluorescence intensities of polymer films(PFMA-Ma(x= 0.700)and PFMA-Mb(x= 0.609))become weak and even almost disappear as the annealing temperatures increase,which indicates that thermal annealing is beneficial for π-π stacking and aggregation of the pendants in the polymers.Concurrently,as the molar ratios of pendant moieties in the polymers are increased,characteristic peak of face-to-face stacking appears in the fluorescence spectra of PFMA-Ma and slight red-shift is observed in the fluorescence spectra of PFMA-Mb.These phenomena suggest that high molar percentage of pendants can also incur their π-π stacking and aggregation.(2)Two new low ionization potential small molecule donors(5,5’-Bis{di(4-methylphenyl)amino}-2,2’-bithiophene,DMPA-BT and N-(tert-Butyl)-2,6-bis{di(4-methylphenyl)amino}dithieno[3,2-b;2’,3’-d]pyrrole,DMPA-DTP)were synthesized and studied.To understand the function of ionization potential of donor in low donor content solar cells,these two donors and two commercial available small molecules donor materials(DMFL-TPD and TAPC)with different ionization potential were applied in low donor content solar cells.The molar ratio of donor and acceptor(C60)is 5:95.The photovoltaic performances of these solar cells were measured and compared.The results show that the open circuit voltage,the short circuit current and fill factor of low donor content solar cells decrease synchronously,with the decrease of ionization potential of donor.The drop of Voc could be due to the decrease of the energy difference of ionization potential of donor and electron affinity of acceptor.The decline of Jsc indicates that the donor can not improve the light absorption and enhance Jsc.Low ionization potential donors may hinder the effective separation and transport of exciton,and lower the device performance parameters.(3)A new PDI dimer with single bond C-C linker at the ortho position(o-diPDI)was designed and synthesized.As compared with the previously reported bay-to-bay PDI dimer(b-diPDI),for the o-diPDI the first reduction occurs at slightly more negative potential and calculations suggest that it has a larger twist angle between the perylene planes(dihedral angle:88°)than the b-diPDI(dihedral angle:62°).The photovoltaic performance of o-diPDI as an acceptor was investigated by fabricating the inverted solar cells with structure of ITO/ZnO/PTB7-Th:o-diPDI/MoO3/Ag.An efficiency of 5.16 ± 0.19%(Voc=0.78 ± 0.01 V,Jsc=12.57 ± 0.17 mA/cm2 and FF= 52.36 ± 1.07%)was achieved.In comparison for the b-diPDI case,the efficiency was 5.24 ± 0.14%(with Voc= 0.76 ± 0.00 V,Jsc= 12.57 ± 0.29 mA/cm2 and FF= 55.02 ± 0.75%).(4)Two non-fullerene small molecule acceptors BF-PDI4 and BF-NDI4 have been synthesized by the Suzuki coupling of 2,2’,7,7’-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9’-bifluorenylidene with PDI-Br and NDI-Br,respectively.Theoretical calculations suggest that both BF-PDI4 and BF-NDI4 have 3D structures.The photovoltaic performance of BF-PDI4 and BF-NDI4 as acceptors was investigated by fabricating inverted solar cells.Power-conversion efficiencies of 3.64%and 2.28%were achieved for PTB7-Th:BF-PDI4 and PTB7-Th:BF-NDI4 based solar cells.Moreover,BF-PDI4 based solar cells show better Jsc and FF than BF-NDI4.BF-PDI4 exhibits much stronger visible absorption than BF-NDI4,but this absorption makes a relatively small contribution to the photocurrent.BF-NDI4 based solar cells have a relatively poor performance,partly due to its weak absorption in the visible spectral region,and also may because of the less photocurrent resulting from polymer absorption in the BF-NDI4 based solar cells.