Organic photovoltaics (OPVs) have attracted lots of attention from the research community due to their several promising features, such as mechanical flexibility, good processibility with roll-to-roll printing, low cost of raw materials, and good tunability of bandgap. The light-absorbing layer of a typical OPV device contains an electron donor and an electron acceptor. Historically, most donor materials are conjugated polymers, and most acceptors are fullerene derivatives, while fullerene acceptors have several disadvantages, such as low light absorption coefficient, limited chemical tunability, and costly production. Therefore, researchers are dedicated to design and synthesize non-fullerene acceptors to replace the fullerene derivatives.

Lots of work has demonstrated that the chemica...[ Read more ]

Organic photovoltaics (OPVs) have attracted lots of attention from the research community due to their several promising features, such as mechanical flexibility, good processibility with roll-to-roll printing, low cost of raw materials, and good tunability of bandgap. The light-absorbing layer of a typical OPV device contains an electron donor and an electron acceptor. Historically, most donor materials are conjugated polymers, and most acceptors are fullerene derivatives, while fullerene acceptors have several disadvantages, such as low light absorption coefficient, limited chemical tunability, and costly production. Therefore, researchers are dedicated to design and synthesize non-fullerene acceptors to replace the fullerene derivatives.

Lots of work has demonstrated that the chemical structure of organic materials has a strong impact on the device performance because the chemical structure modification can modulate the crystallinity, energy levels, and light absorption ability. Thus, it is important for us to study and understand the structure-property relationships of OPV materials. In this thesis, I will talk about my research work in the past four years, which includes material design and synthesis, device engineering, and photophysical and morphology characterization for efficient OPVs. We successfully developed several high-performance OPV materials and try to provide some intrinsic understanding about the structure-property relationships of OPV materials.

In Chapter II, we concentrate on the chemical structure design and device optimization of IDT-series polymer acceptors for efficient all-polymer solar cells (all-PSCs). All-PSCs exhibit a much better mechanical stability than OPVs that contains small molecular acceptors (SMA), but the efficiencies of all-PSCs are lower than that of polymer-SMA solar cells. The low performance of traditional all-PSCs is attributed to the undesirable morphology and low absorption coefficient of the polymer acceptor (N2200). Thus, Systematic comparisons between two polymer donors (PBDBT, PM6) are made, which provides insights into the effect of different polymer donors on the performance. On the one hand, the more crystalline polymer donor, PM6, tends to form a more favorable morphology, including the larger crystal size, closer molecular stacking, and higher domain purity, which results in higher mobility and lower recombination in the corresponding blend film. On the other hand, PM6 has a better matched energy-level alignment than PBDBT, and this small energy offset in PM6:PFBDT-IDTIC causes a higher V_OC. Besides, adding a small amount of additive (chloronaphthalene) can assist the crystallization and promote phase separation during the film processing. As a result, a high efficiency of 10.3% was achieved by PM6:PFBDT-IDTIC blend film.

Traditionally, it is believed that a large energy offset between donor and acceptor is needed for efficient charge separation. But the emergence of Y6-series SMAs demonstrated that even with a negligible energy offset, OPV devices can exhibit a high external quantum efficiency higher than 80%. Considering the unique property of Y6, in Chapter III, we focus on the exploration of the structure-property-morphology-performance relationship of Y6-series SMAs. The impact of alkoxy chain substitution on linear alkyl chains was systematically investigated, and it is found that the introduced alkoxy chains can significantly enhance the aggregation properties and reduce the solubility due to the conformational locking effect which is originated from the produced hydrogen bond between oxygen and hydrogen atoms, planar conformation, and higher dipole moment. Furthermore, a strategy of asymmetric alkyl and alkoxy substitution was carried out which can fine-tune the properties of Y6-series SMAs to enhance the aggregation properties while maintaining a suitable solubility. Consequently, the asymmetric SMA achieved a higher FF and PCE of 16.1%.

In Chapter IV, we discussed the design rules of high-performance indoor OPVs, which have been considered as a promising candidate to power kinds of tiny indoor electronics, such as wireless sensor and bluetooth beacons, since they can effectively convert the indoor light into electronics. In this work, we successfully designed a high-performance large-bandgap non-fullerene SMA named FCC-Cl. The important design rationale of FCC-Cl is the combination of a weak electron-donating core and a moderate electron-withdrawing end group, which leads to a needed bandgap and high crystallinity. As a result, the corresponding device, D18:FCC-Cl achieves an impressive PCE of over 29.4% was achieved by D18:FCC-Cl at 1000 lux which is one of the highest reported indoor performances for OPVs. Furthermore, it is found that the device performance is insensitive to the active-layer thickness under indoor illuminations which is different from that under one-sun condition due to the negligible impact of series resistance and bimolecular recombination under indoor environment.