**1. Introduction**

Environment and energy issues have been presented as the biggest challenges facing human‐ ity nowadays. Among the various solutions, photocatalysis is a promising approach both for photochemical energy conversion and for photochemical decontamination, hence to fulfill the sustainable energy supply and environment remediation by use of the abundant, natural sunlight [1–5]. To achieve efficient solar energy conversion, the photocatalysts are required to possess excellent light‐harvesting capability, charge transfer efficiency (factors including exciton lifetime, mobility, etc*.*), as well as surface activity (specific surface area, ionic adsorp‐ tion, etc*.*) [6–8]. Most research studies in photocatalysis have been concentrated on the use of inorganic semiconductors, such as TiO2 [9–11], Fe2 O3 [12–14], ZnO [15–17], and Cu2 O [17–19] which mostly suffer from inefficient light absorption and hardness. Strategies to enhance the

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efficiency of these catalysts correlating with band engineering [20, 21], texture modification [16, 22], or configuration organization [23, 24] always involve complicated fabrication pro‐ cesses. All these limit their practical affordable applications. Still, much effort is needed to find other photoactive materials as alternatives for facile preparation and economical applications.

During the last decades, increasing attention has been paid to the field of semiconducting organic materials for optoelectronic applications [25–27]. One of the most important advan‐ tages concerning these organic materials is that their molecular structure and functions can be easily modulated via molecular design and tailoring. Additionally, integration of them into lightweight, large‐area devices can be simply realized through solution processing at low cost. In addition, organic semiconductors, also referred to as π‐conjugated molecules, are characterized by a delocalized π‐electron system that makes them ideal building blocks for the fabrication of advanced functional nanomaterials and nanodevices [28–30]. As a typical representative of π‐conjugated molecules, porphyrins are of particular interest due to some key aspects, such as their excellent light‐harvesting property, p‐type semiconducting behav‐ ior, ease of chemical modification, good supramolecular assembly, and film‐forming features by means of either solution‐based or thermal‐based techniques [31–33]. Coupled with their chemical stability and flexibility, the use of porphyrin in optoelectronics has become a fast‐ growing research focus, and great development has been made in the field of organic solar cells (OSCs) [34, 35], organic field‐effect transistors (OFETs) [36, 37], organic light‐emitting diodes (OLEDs) [38, 39], even in flexible organic semiconductor devices.

As a photocatalyst, porphyrins were first used in homogenous photocatalysis [40]. The prob‐ lem with it is the limited stability of porphyrin molecules and the recovery of them for suc‐ cessive use. Fortunately, this could be circumvented by mobilizing porphyrin molecules on solid supports or assembling them into robust nanostructures [41, 42]. Recently, more efforts have been made on the development of a semiconductor‐based photoelectrochemical (PEC) water splitting device [43, 44], and thus organic photoelectrodes have aroused special atten‐ tion. Relating progresses are dealt with in detail in separate sections. Before that we have a brief introduction of the relation between porphyrin molecular structure and optoelectronic properties. The use of molecular porphyrin as modification of inorganic semiconductors to achieve absorption of visible light is not covered in this chapter.
