Solution-processed solar cells are appealing because of the low manufacturing cost, the good compatibility with flexible substrates, and the ease of large-scale fabrication. Whereas solution-processable active materials have been widely adopted for the fabrication of organic, dye-sensitized, and perovskite solar cells, vacuum-deposited transparent conducting oxides (TCOs) such as indium tin oxide, fluorine-doped tin oxide, and aluminum-doped tin oxide are still the most frequently used transparent electrodes (TEs) for solar cells. These TCOs not only significantly increase the manufacturing cost of the device, but also are too brittle for future flexible and wearable applications. Therefore, developing solution-processed TEs for solar cells is of great interest. This paper provides a detailed discussion on the recent development of solution-processed TEs, including the chemical synthesis of the electrode materials, the solution-based technologies for the electrode fabrication, the optical and electrical properties of the solution-processed TEs, and their applications on solar cells.

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Compared with inorganic two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, organic 2D materials are believed to possess more interesting chemical and biological properties for certain applications, such as separation membranes, smart surfaces, sensors, catalysis and drug delivery. However, the study of organic 2D materials is largely hindered because of the lack of effective methods to produce them. This paper presents a new type of organic 2D material, namely 'polymer@graphene 2D objects', that can be synthesized via a simple and scalable chemistry. Polymer@graphene 2D objects are made of functional polymer brushes that tether one end of the polymer chain on the surface of graphene sheets via non-covalent p-p stacking interactions. These materials are transparent, freestanding, lightweight, flexible, transferable to various substrates with good stability and are patternable into different structures. Their functionality can be tailored by changing the polymer brushes that are immobilized. In this paper, we demonstrate the applications of these 2D objects in the smart control of surface wettability and DNA biosensors. INTRODUCTION Two-dimensional (2D) inorganic materials including graphene 1-3 and various transition metal dichalcogenides 4-6 have received tremendous attention in the past decade because of their unique properties and wide applications in optics and electronics. These remarkable phenomena have recently attracted research interest in 2D organic materials, including 2D polymers 7-10 and quasi-2D polymers. [11][12][13][14][15] The focus has mainly been on their chemical and biological properties and their applications as separation membranes, smart surfaces and sensors, and for catalysis and drug delivery. However, the 2D polymers reported to date are only limited to very specific building blocks and microscale sizes because of the lack of robust methods to produce them. On the other hand, the preparation of quasi-2D polymers either requires the undesirable crosslinking of the materials, using expensive and opaque novel metal supports, or high-energy electron beams. 16,17 Therefore, the investigation of the fundamental properties as well as practical applications of these organic 2D materials have remained largely hindered.In this paper, we report the development of a new type of functional organic 2D material, namely 'polymer@graphene 2D objects' , which are prepared in a cost-effective and high-throughput manner. Polymer@graphene 2D objects are made of functional