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DOI: 10.1002/adfm.201904014 Photovoltaic (PV) devices can directly convert solar energy into electricity, offering a practical, clean, and sustainable solution to address the challenge of the ever-increasing global energy demand. Current research is focused on identifying high efficiency solar cells with low-cost fabrication. Currently, PV devices based on various inorganic materials including silicon (Si), III-V group semiconductors, CdTe, and CIGS dominate the market. However, partially due to the high production cost and related environmental issues, conventional PV technology places obvious constraints on the scaling-up of and expenses related to the manufacturing capacity as well as their wide application. In recent years, there has been growing interest in emerging organic and perovskite-based PV technologies owing to their synthetic variability, low-temperature processing, and the possibility of producing lightweight, flexible, easily manufactured, and inexpensive solar cells using earth-abundant materials. It is our great pleasure to propose this special issue titled “Emerging Photovoltaic Materials and Devices” for Advanced Functional Materials. The issue highlights important aspects in the research of emerging PV materials from both the fundamental research and the commercialization perspectives. We present a collection of 6 reviews and 12 feature articles from this exciting field that covers material science and device engineering for both organic and perovskite solar cells. A fundamental understanding of crystallization mechanisms during film fabrication and its influence on the morphological and optoelectronic properties of the active layer are important to improve the performance of solar cells. The complex crystallization mechanism from perovskite precursors to perovskite is well demonstrated by Anthopoulos, Zhao, Amassian, and coworkers using in situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements (article number 1807544). Direct and complete phase conversion from precursors to lead-based perovskite without the need for thermal annealing is realized for the first time, which could encourage novel and easy fabrication of perovskite solar cells in the future. Chen, Huang, and co-workers and Zhang, Zhu, and co-workers demonstrate the control of crystallization kinetics during solution processing of 2-dimensional (2D) perovskites by tuning the solvent properties or intermediate phase formation (article numbers 1806831 and 1901652). Liu, Han, and co-workers realize sequential crystallization of donors and acceptors during solution-processing organic bulk heterojunction (BHJ) organic thin films, which is critical to the formation of an interpenetrating network and the optimization of vertical phase separation (article number 1807591). Wadsworth and co-workers design a novel organic acceptor of O-IDTBCN with dicyano Dedicated to the 70th anniversary of DICP-CAS