This is a fabulous time for me to write two prefaces for my first two books in my life! Time goes really fast. I think a lot about the old days, the choices, and interesting experiences. When I finished three years in Chemistry Department of Anhui University, I needed to choose my major, and there were three majors: chemical analysis, organic synthesis, and polymer science. I chose polymer, because I thought polymer would be more fantastic and useful; that was 1987. The last semester I chose lecturer Wei Yang (now emeritus professor in SCUT) as my thesis supervisor to do copolymerization aiming to obtain materials with good mechanical properties. The time with supervisor Wei Yang built my preliminary experience on chemicals, reaction, and polymerization. During 1990–1998, I did my MS, PhD, postdoc work on solid polymer electrolytes and electron transfer dynamics, in Changchun Institute of Applied Chemistry (CIAC), which was the biggest institute in Chinese Academy of Sciences. For my PhD period, I was a joint student in CIAC. My supervisors, Professors Zinan Zhou, Yunqing Lin (CIAC), Dezhu Ma, Chengzong Yang (USTC), Shaojun Dong, and Erkang Wang (CIAC), stimulated my spirit and kindly helped me, and these will stay in my memory forever. During my postdoc time, I really got interested in optoelectronic devices based on conjugated polymers. In October of 1997, Professor Olle Inganäs interviewed me in a hotel in Wangfujing, Beijing; we talked around 25 min, then Olle kindly invited me to a small restaurant nearby for lunch. Before Christmas I received the postdoc offer from Olle. Without any experimental background on devices, I did learn a lot from those old group members in Linköping University on how to cut ITO glass, etching, spin coating, vacuum evaporation, measurements, etc. All new to me. I quickly learned how to fabricate polymer solar cells and polymer light‐emitting diodes. That time, I made solar cells with a trilayer structure (Synthetic Metals 2000, 110, 133–140). Sweden, a small kindom, stays really special, and a very beautiful land, people like Olle doing cool stuff there everyday, serious, quiet, and outstanding! Later, I moved to the United States and worked with Robert Haddon on CNTs, Frank Karasz on PLEDs and organic solar cells (OSCs), Mike Durstock and Liming Dai on OSCs, and Tom Russell on photoactive polymers, block copolymers and phase separation. I worked in Konarka Lowell as a senior scientist for one year on flexible polymer solar cells, doing blade coating, materials screening, and device optimization. It was cool to learn the structure, running and commercial operation of a facing‐future high‐tech company. In 2010, I came back to China and joined National Center for Nanoscience and Technology as a full professor. My research project was on OSCs. We have invented many building blocks and high‐performance materials such as copolymer donors, non‐fullerene small molecular acceptors, and fullerene acceptors. We have pushed the PCE record for OSCs from 14% to 18%. This jumping just took two years, really awesome! We hold the dream that one day single‐junction OSCs will deliver PCEs over 20%. I think it is the right time to deliver a book on OSCs, a way to sum up the past and to face the future, to challenge the PCE limit and those commercialization issues. All the contributing authors have done excellent jobs in their respective direction. The topic goes from polymer donors, p‐type materials, fullerene acceptors, non‐fullerene small molecular acceptors, ladder‐type heteroacenes, chlorination, all‐polymer solar cells, single component, tandem structure, large‐area fabrication, indoor application, interfacial design, morphology control, stability, to voltage loss, covering almost all research hotspots. This book would help those people who have interests in OSCs or who are working in the same field, no matter students or colleagues, academic or industry. Special thanks to Dr. Shaoyu Qian, without her trust and invitation, this won't happen! I would thank Kat Wong and Farhath Fathima for their nice assistance and patience. I appreciate the help from my team members: Ling Liu, Ke Jin, Xiaoyan Du, Zuo Xiao, Chuantian Zuo, Jiamin Cao and Shan Chen.

At last, I thank my mom for her eternal love.

Beijing, 28 September 2021

Liming Ding