Thin-film organic solar cells have experienced great development in the past years. Multiple types of solar cell device architectures were developed for efficient conversion from light to electricity. Organic materials have many advantages when compared to the inorganic materials such as flexibility and chemical tunability. Researchers have established reliable and comprehensive theories that guide people to develop highly-efficient materials in the light absorbing layer of the solar cells.

The most important issue for the material development is understanding of their structural-property relationship. By studying the structural-property relationship, people could obtain useful information about how the chemical structures of the materials determine their intrinsic photo-elect...[ Read more ]

Thin-film organic solar cells have experienced great development in the past years. Multiple types of solar cell device architectures were developed for efficient conversion from light to electricity. Organic materials have many advantages when compared to the inorganic materials such as flexibility and chemical tunability. Researchers have established reliable and comprehensive theories that guide people to develop highly-efficient materials in the light absorbing layer of the solar cells.

The most important issue for the material development is understanding of their structural-property relationship. By studying the structural-property relationship, people could obtain useful information about how the chemical structures of the materials determine their intrinsic photo-electronic and other physical properties. By observing the device performance and other characterization results, we could understand more about these kinds of materials and realize how to further modify their structures to obtain better results.

The light absorbing layer (also regarded as “active layer”) of a single junction solar cell comprises an electron donating material and an electron accepting material. Most of the high-performance organic solar cells are based on polymer donors, because they have a relatively high hole mobility and could have a well-controlled blend morphology. The other counterpart, the acceptor, are usually fullerene derivatives typically PC₆₁BM and PC₇₁BM. The polymer/fullerene solar cells currently are holding the record efficiency of organic solar cells which is 11.5%. It’s important to develop high-performance polymer materials to further enhance the power conversion efficiency of organic solar cells.

On the other hand, fullerene acceptors have some drawbacks that urge us to develop alternative new non-fullerene acceptors. The organic solar cells based on small molecular acceptors are the promising next generation and the efficiencies of the non-fullerene solar cells are approaching that of fullerene solar cells.

My major research topics in these four years are about developing novel polymers and small molecular acceptors with high power conversion efficiencies in organic solar cells. Systematic characterizations on these materials have been carried on to explain their structural-property relationship and to provide guidance for the future development of materials with similar structures. All works in this thesis have already been published on research journals with me as the first author (or co-first author).[1-4]

In chapter one, a brief introduction on the energy status, inorganic solar cells and organic solar cells is given to provide some basic information of this research field. In chapter two, a novel polymer PffT2-FTAZ-2DT was designed and synthesized. The non-fullerene polymer solar cell with an efficiency of 7.3% was realized by combining this large bandgap polymer with a small bandgap acceptor IEIC. The complementary absorption properties of donor polymer and small molecule acceptor resulted in a better absorption of the solar radiation which is responsible for its high-performance. This strategy is proved to be successful in the polymer-small molecular solar cell systems and provides guidance for material developments.

In chapter three, we designed a small molecular acceptor TPPz-PDI₄ which have reduced extent of intramolecular twisting compared to other two small molecular acceptors TPE-PDI₄ and TPC-PDI₄. TPPz-PDI₄ exhibits highest aggregation tendency and its electron mobility is the highest among the three small molecular acceptors. Multiple experimental evidences reveal the positive effects of lowering the extent of intramolecular twisting on the device performance. Meanwhile, the aggregation TPPz-PDI₄ is not too strong to form excessive large domains. As a result, TPPz-PDI₄ based solar cell device achieves a highest power conversion efficiency of 7.1%.

In chapter four, we report a large bandgap (1.9 eV) donor-acceptor copolymer (named PffT2-FTAZ) that enables a polymer-fullerene solar cells with a high power conversion efficiency of 7.8%. An important structural feature of the PffT2-FTAZ polymer is a difluorinated donor unit that introduces several surprising and beneficial effects. By comparing PffT2-FTAZ with the analog polymer (PT2-FTAZ) without fluorination on the bithiophene donor unit, it is found that the ffT2 unit effectively lowers the HOMO and LUMO energy levels of the polymer and slightly reduces optical bandgap. It also introduces strong interchain aggregation for the polymer in solution, which leads to a highly crystalline polymer film and reasonably high hole transport mobility. On the other hand, the PffT2-FTAZ:fullerene blend still exhibits a reasonably small polymer domain size suitable for polymer solar cell operation. All these positive factors combined leads to dramatically enhanced performance for the polymer solar cells with the power conversion efficiency increasing from 2.8% for PT2-FTAZ to 7.8% for PffT2-FTAZ.

In chapter five, we report high-performance small molecular acceptor based organic solar cells enabled by the combination of a difluorobenzothiadiazole donor polymer named PffBT4T-2DT and a small molecular acceptor named SF-PDI₂. It is found that SF-PDI₂ matches particularly well with PffBT4T-2DT, and non-fullerene organic solar cell with an impressive V_OC of 0.98 V and a high power conversion efficiency of 6.3% are achieved. Another small molecular acceptor diPDI which is previously reported to have superior performance than SF-PDI₂ was compared with the SF-PDI₂ in the same condition. The results reveal that for the system in which PffBT4T-2DT was donor, SF-PDI₂ can perform better than diPDI. Our study shows that PffBT4T-2DT is a promising donor material for SMA-based OSCs and the selection of a matching SMA is also important to achieve the best OSC performance.

In chapter six, multiple materials containing fluorine atoms are developed and investigated. Among them, 2FID-IDTT and PTFT shown considerably high efficiency in corresponding devices. For PffBT-2DT:2FID-IDTT devices, a PCE of 9.0% could be achieved and for PTFT:ITIC-Th non-fullerene system, a PCE of 9.3% was obtained. It’s found that introducing fluorine atoms in these materials could benefit the device performance in many aspects such as increasing the interchain aggregation and hole mobility, improving the device morphology, etc. This research proves that fluorination is an effective method to further improve the device performance on the basis of the existing high-performance materials.