Synthesis And Photovoltaic Properties Of Small Molecular Donors And Acceptors Materials

BHJ OSCs have been considered a green and effective technology to harvest sun-light and generate electricity because of the potential for low cost manufacturing of large area solar cells through the roll-to-roll coating technique on various substrates. Compared to the polymer-based OSCs(PSCs), small-molecule-based OSCs(SMSCs) possess numerous advantages, including a relatively simple synthesis and purification process, monodispersity, a well-defined structure, no end-group contaminants, less batch-to-batch variation, a versatile molecular structure, easier band structure control, a high charge carrier mobility, etc. In the field of BHJ OPVs, fullerenes and their derivatives are an important class of n-type electron acceptor materials. Nevertheless, they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth, difficulty in variably tuning the band gap, and poor solubility, which severely limit the fullerene-based materials as an acceptor material more widely used in organic solar cells. Compared to the fullerene derivatives, non-fullerene OSMAs are much easier to be synthesized and purified with lower processing costs and more excellent solubility, and their band gaps could be finely adjusted by proper molecular design.The studies demonstrated in the thesis include two parts: the first is that two small molecules with the same conjugated backbone in a A-π-D-π-A framework comprised of an IDT core as the donor, two BT units as the acceptor, and thiophene π bridges were designed and synthesized as the platform for investigating the effect of the different side chains. The second part is the development of a series of acceptors which provide us with a platform to deeply understand the property-structure relationships and to screen for better non-fullerene acceptors materials.In chapter 2, two IDT-core-containing molecules were successfully synthesized and characterized for use in solution-processed small molecular BHJ solar cells. These two molecules employ the same conjugated backbone but different side chains of rigid and bulky aromatic 4-hexylphenyls and flexible aliphatic n-dodecyls with the same number of carbon atoms. Such a structural difference in the side chains combined with the central IDT moiety leads to different intermolecular interactions, self-aggregation tendency, film morphology, charge transport ability, and thus device performance. Compared with the n-dodecyl side chains, the introduction of 4-hexylphenyl side chains onto the IDT unit significantly improves the solar light absorption and gives a favourable blend film morphology for exciton separation and charge transport, leading to a PCE of 4.72%.In chapter 3, three small molecules were synthesized and characterized as the acceptors for P3HT-based BHJ OSCs. They behave a broad absorption range from 400 to 700 nm and relatively low LUMO energy levels of-3.86,-3.81 and-3.99 e V which approach the energy level of PC61 BM. As well as effective PL quenching was noticed when P3 HT is blended with the three small molecules, showing efficient charge/energy transfer happened. The acceptor based on a PDI core leads to a comparatively high PCE of 0.82%.In chapter 4, two novel perylene diimide molecules were designed, synthesized and characterized, with fused aromatic pyrene rings on the perylene core. These materials possess interesting optical and structural features, revealing extremely weak π-π interactions between the perylene planes, which is an interesting feature, since PDIs are well known for their strong intermolecular packing properties. And that effective PL quenching was noticed when PBDTTT-C-T or PTB7 is blended with the PDI-based molecules, showing efficient charge/energy transfer happened. P1 obtains a profitable film morphology with PTB7 as the donor for efficient exciton separation and charge transport, generating to a properly high PCE of 1.35%.In chapter 5, two novel star-shaped II acceptor materials(P1 and P2) with TPA and phenyl cores were explored. These materials exhibited a 3D structure, weak intermolecular interactions and molecular aggregation, a strong absorption in the visible region, and appropriate energy levels matched with the general polymer donor P3 HT. Solution-processed OSCs based on a P3HT:P1(1:1, w/w) blend film exhibited PCE as high as 0.81%.In chapter 6, four A-D-A structured novel organic molecules were designed, synthesized and characterized with isoindigo as terminal acceptor units, with different aromatic rigid planar cores such as indacenodithiophene, dithienosilole, anthracene, and pyrene as donor units. These materials exhibit rather high-lying LUMO levels, which is beneficial for achieving high Voc. And effective PL quenching was observed when P3 HT is blended with acceptor molecules, showing efficient charge/energy transfer happened. Compared with P2, P3 and P4, a PCE of 1.39% was achieved for the OSCs based on P1 as the acceptor material and P3 HT as the donor material without additive and post-deposition annealing of the active layers, which is the highest value for the polymer BHJ OSCs based on isoindigo small molecules as non-fullerene acceptors.