The era of emerging photovoltaics has been comprehensively developed. For the perovskite solar cells, the power conversion efficiency of the single-junction devices has reached more than 26%, as well as the lifetime is improved. In the next stage, the large-scale fabrication and issue of stability are two main factors should be focus. For the organic solar cells, its power conversion efficiency has also reached up to 20%. Design of novel materials, especially donor and acceptor in active layer, essentially contributes to the improvement of device performance. On the other hand, ternary device structure commonly has higher PCE than binary device. Among the third components, fullerene derivatives are promising candidates not only due to its facile synthesis and lower cost than other small...[ Read more ]

The era of emerging photovoltaics has been comprehensively developed. For the perovskite solar cells, the power conversion efficiency of the single-junction devices has reached more than 26%, as well as the lifetime is improved. In the next stage, the large-scale fabrication and issue of stability are two main factors should be focus. For the organic solar cells, its power conversion efficiency has also reached up to 20%. Design of novel materials, especially donor and acceptor in active layer, essentially contributes to the improvement of device performance. On the other hand, ternary device structure commonly has higher PCE than binary device. Among the third components, fullerene derivatives are promising candidates not only due to its facile synthesis and lower cost than other small molecule acceptors, but also its adoptability in diverse system. This thesis is constituted of five chapters:

Chapter I provides the introduction of emerging photovoltaic technologies, including the organic solar cells and perovskite solar cells, along with their progress of high efficiencies, the device structures and working principle. Furthermore, the key materials used in this devices are also discussed in this section.

I develop a new methodology for synthesis of 1,4-bisbenzylfullerene adducts in Chapter II. By screening of reaction solvents, catalytical system, ratio of substrates, and reaction time, we improve the reaction yield up to 75%. Furthermore, facile purification by Soxhlet extraction also contribute to its large-scale production. The spectra of UV-vis absorption show a unique broad peak around 450nm which is concerned as the result of 1,4-addition of fullerene instead of 1,2-additon. The higher LUMO energy level of this 1,4-diadducts is detected by cycle voltammetry measurement. The upshifted LUMO is predictable since it disconnects more π-conjugated systems than 1,2-addition of fullerene, and thus the LUMO levels of such 1,4-diadducts is higher owing to their lower electron affinity. Additionally, fullerene derivatives with high LUMO are imperative since current high-performance non-fullerene acceptors (such as L8-BO and BTP-eC9) have a similar LUMO level compared with widely used PC61BM. By adding the YF1 into the PM6:L8-BO system, delivered an improved maximum PCE of 18.8%. The remarkable raise of efficiency reveal that YF1 is a powerful guest acceptor for ternary organic solar cells.

In Chapter III, fullerene acceptors typically possess excellent electron-transporting properties and can work as guest components in ternary organic solar cells to enhance the charge extraction and efficiencies. However, conventional fullerene small molecules typically suffer from undesirable segregation and dimerization, thus limiting their applications in organic solar cells. Herein we report the use of a poly(fullerene-alt-xylene) acceptor (PFBO-C12) as guest component enables a significant efficiency to increase from 16.9% for binary cells to 18.0% for ternary all-polymer solar cells. Ultrafast optic and optoelectronic studies unveil that PFBO-C12 can facilitate hole transfer and suppress charge recombination. Morphological investigations show that the ternary blends maintain a favorable morphology with high crystallinity and smaller domain size. Meanwhile, the introduction of PFBO-C12 reduces voltage loss and enables all-polymer solar cells with excellent light stability and mechanical durability in flexible devices. This work demonstrates that introducing polyfullerenes as guest components is an effective approach to achieving highly efficient ternary all-polymer solar cells with good stability and mechanical robustness.

Fullerene and its derivatives are used as electron materials in perovskite solar cells. In Chapter IV, a new type of ETMs is designed and synthesized by polymerizing C60 fullerene with an aromantic linker unit. The resultant polyfullerene (PFBS-C12) not only maintains the good optoelectronic properties of fullerenes, but also can address the aggregation problem of PCBM. The polyfullerene-based blade-coated cells exhibit a high efficiency of 23.2% and good device stability that maintain 96% of initial efficiency after > 1,300-hour light soaking. An aperture efficiency of 18.9% is also achieved on a 53.6-cm2 perovskite mini-module. This type of poly fullerene provides a new strategy for designing ETMs that retain the key figure-of-merits of conventional fullerene molecules and enable more stable perovskite solar devices simultaneously.