Developing clean and low-cost photovoltaic technologies is an important means and technological guarantee to achieve China's carbon peaking and carbon neutrality goal. In recent years, new types of solar cells, represented by thin-film photovoltaics, have developed rapidly. Among them, organic solar cells and perovskite solar cells have attracted worldwide attention in the academic and industrial communities due to their low cost, solution processibility, flexibility and lightweight characteristics. In these two photovoltaic technologies, small molecule fullerene materials such as Phenyl-C61-butyric acid methyl ester (PCBM) have played an important role. In the field of organic photovoltaics, PCBM works as an electron acceptor material and forms a heterojunction with polymer donor materials to increase the efficiency of exciton separation. In the field of perovskite photovoltaics, PCBM has excellent electron extraction properties and is an irreplaceable electron transporting material in the current inverted (or p-i-n) structure perovskite solar cells. However, PCBM tends to aggregate and diffuse under photothermal conditions, which can significantly reduce its charge transfer ability and affect the stability of thin-film solar cells. However, designing fullerene small molecule substitutes with excellent electron transporting performance and good stability is a difficult task and one of the current challenges in the field of new thin-film photovoltaics.
Figure 1. Synthesis route of PFBO-C12和PFBS-C12 polyfullerenes.
To this end, Prof. Shangshang Chen group at School of Chemistry and Chemical Engineering, Nanjing University, develop a new type of conductive polymers—Polyfullenes (Figure 1). This type of fullerene polymers are synthesized via atom transfer radical polymerization reaction, which proceeds at mild condition and gives good reaction yield. This type of polyfullerenes have good solubility in solvents and exhibit comparable light absorption, energy levels and electron-transporting properties to PCBM.
Figure 2. Chemical structures and optoelectrical properties of organic solar cell materials.
When introducing the polyfullerene into the all-polymer solar cells as a guest acceptor component (Figure 2), PFBO-C12, on the one hand, can strengthen the absorption and aggregation of pristine binary components. The hole transfer from polymer acceptor to polymer donor is also effectively promoted by PFBO-C12. As a result, both photocurrent generation and fill factor have been significantly improved, and a power conversion efficiency of 18% is realized (Figure 3). On the other hand, PFBO-C12 can effectively improve the device stability and mechanical robustness due to its stable polymeric backbone and resistance to aggregation/diffusion.
Figure 3. Characterizations of PFBO-C12-based all-polymer solar cells.
Figure 4. Characterizations of PFBS-C12-based inverted perovskite solar cells and modules.
Besides, Prof. Shangshang Chen group also develop a polyfullerene electron-transporting material named PFBS-C12, and introduce polyfullerene materials into perovskite solar devices for the 1st time. It is found that PFBS-C12-based perovskite solar cells can achieve 23.2%-efficiency inverted perovskite solar cells with better device yield than PCBM counterpart. In the stability test, the PFBS-C12-based perovskite solar cells can retain 96% of initial efficiency after 1,300-hour light soaking, while the PCBM-based cells lost 66% of its efficiency, confirming the good stability of PFBS-C12 as the electron-transporting layer in inverted perovskite solar cells. In addition to small-area solar cells, PFBS-C12 can also be scaled up to large-area film via blade-coating method. The perovskite solar modules (53.6 cm2) based on PFBS-C12 realize power conversion efficiency approaching 19%, which is one of the highest efficiencies for the perovskite solar modules based on solution-processed electron-transporting materials.
The related works entitled “Improved Photovoltaic Performance and Robustness of All-Polymer Solar Cells Enabled by A Polyfullerene Guest Acceptor” and “High-Performance Inverted Perovskite Solar Devices Enabled by a Polyfullerene Electron Transporting Material” have been published in Nature Communications (https://www.nature.com/articles/s41467-023-37738-9) and Angewandte Chemie (https://onlinelibrary.wiley.com/doi/10.1002/anie.202210610), respectively. The first and co-first authors are Xiaoyu Shi, Junli Yin and Han Yu. Prof. Shangshang Chen (Nanjing University), Prof. Zonglong Zhu (City University of Hong Kong), Prof. Kam Sing Wong (Hong Kong University of Science and Technology) and Prof. He Yan (Hong Kong University of Science and Technology) are corresponding authors of these works. Both works are financially supported by National Natural Science Foundation of China, Fundamental Research Funds for the Central University, State Key Laboratory of Coordination Chemistry and MOE Key Laboratory of High Performance Polymer Materials & Technology. In addition, Prof. Shangshang Chen has filed a patent covering polyfullerene materials, and a chemical company is scaling up the polyfullerene materials, which have passed the pre-tests of a few photovoltaic companies. If anyone is interested, please contact Prof. Chen (schen@nju.edu.cn).