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.

Flexible and wearable electronics is one major technology after smartphones. It shows remarkable application potential in displays and informatics, robotics, sports, energy harvesting and storage, and medicine. As an indispensable part and the cornerstone of these devices, soft metal electrodes (SMEs) are of great significance. Compared with conventional physical processes such as vacuum thermal deposition and sputtering, chemical approaches for preparing SMEs show significant advantages in terms of scalability, low-cost, and compatibility with the soft materials and substrates used for the devices. This review article provides a detailed overview on how to chemically fabricate SMEs, including the material preparation, fabrication technologies, methods to characterize their key properties, and representative studies on different wearable applications.

No abstract

α‐Formamidinium lead triiodide (α‐FAPbI3) represents the state‐of‐the‐art for perovskite solar cells (PSCs) but experiences intrinsic thermally induced tensile strain due to a higher phase‐converting temperature, which is a critical instability factor. An in situ crosslinking‐enabled strain‐regulating crystallization (CSRC) method with trimethylolpropane triacrylate (TMTA) is introduced to precisely regulate the top section of perovskite film where the largest lattice distortion occurs. In CSRC, crosslinking provides in situ perovskite thermal‐expansion confinement and strain regulation during the annealing crystallization process, which is proven to be much more effective than the conventional strain‐compensation (post‐treatment) method. Moreover, CSRC with TMTA successfully achieves multifunctionality simultaneously: the regulation of tensile strain, perovskite defects passivation with an enhanced open‐circuit voltage (VOC = 50 mV), and enlarged perovskite grain size. The CSRC approach gives significantly enhanced power conversion efficiency (PCE) of 22.39% in α‐FAPbI3‐based PSC versus 20.29% in the control case. More importantly, the control PSCs’ instability factor—residual tensile strain—is regulated into compression strain in the CSRC perovskite film through TMTA crosslinking, resulting in not only the best PCE but also outstanding device stability in both long‐term storage (over 4000 h with 95% of initial PCE) and light soaking (1248 h with 80% of initial PCE) conditions.

such as conducting polymers and carbon nanotubes, metal-the most conventional conductor-still outperforms significantly in terms of the electrical conductivity, device compatibility, materials stability, and cost-effectiveness. [1,[30][31][32][33][34][35][36][37][38] In particular, with the recent understanding of the nanomechanics of the metal and the soft structural design, metal conductors are indispensable for most of the flexible and wearable applications.In the literature, the fabrication of metal conductors typically consists of vacuum deposition of the metal and subsequent patterning with photolithography. Although this fabrication strategy can be directly applied on flexible polymeric substrates, it is often too costly for most flexible applications and may encounter materials instability issues of the flexible substrates at high vacuum or strong solvent. In addition, vacuum technology is not suitable for coating substrates with 3D structures such as foams, fibers, and arbitrary surfaces. This has led to extensive research on developing solution-based metal deposition and printing methods in the past three decades. The most reported solution-based strategy is the so-called "metal ink" method, in which solution-dispersed nanostructured metal particles (including nanoparticles (NPs), nanowires, nanotubes, and nanoplates) or metal precursors are cast or printed onto the flexible substrates and then thermally sintered or chemically reduced to yield the metallic conductors. [39][40][41][42][43] Nevertheless, they are still not widely used because of the following reasons. 1) The metal conductor fabricated through the "metal ink" method shows much lower electrical conductivity compared to their bulk, due to the additives of the ink (such as surfactants, binders, and stabilizers) and the large contact resistance between the NPs. [44,45] 2) The "metal ink" method works very well with noble metals such as Au and Ag, but is not compatible with more frequently used, yet less stable metals such as Cu and Ni. Even though there has been a major shift of research focus from Ag to Cu in recent years, inks for making high-quality Cu or Ni conductors at low-temperature and in the air atmosphere are yet to be developed. 3) The metal conductors made by the "metal ink" method are more suitable for interconnects but are less suitable for electrode applications due to the high roughness and impurity of the metal film. 4) Penetration of the "metal ink" into highly porous substrates or structures with small gaps is The rapid development of flexible and wearable electronics favors low-cost, solution-processing, and high-throughput techniques for fabricating metal contacts, interconnects, and electrodes on flexible substrates of different natures. Conventional top-down printing strategies with metal-nanoparticleformulated inks based on the thermal sintering mechanism often suffer from overheating, rough film surface, low adhesion, and poor metal quality, which are not desirable for most flexible electronic applications. In recent ...