**1. Introduction**

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Due to the rising concerns over the exhaust of fossil fuel and the associated environmental consequence of the carbon emission problem, search for renewable energy has become a hot research topic worldwide. Organic semiconducting materials based photovoltaic (PV) technology developments have attracted tremendous attention from both the academic community and the industry. In principle, the organic solar cells employ organic material based light absorbing functional layer to convert sunlight to electricity. Typically the light absorbing layer is made of a blend of donor material and a fullerene based acceptor material. The observation of photovoltaic effect on organic materials began in 1986 with the "Tang cell" [1], which was a two-layer device with a structure of ITO/copper phthalocyanine/perylene diimide/gold. Later in the early 1990s, the discovery of ultrafast charge transfer from polymer to fullerene [2] initiated the research field of bulk hetero-junction (BHJ) solar cells. Over the past two decades, substantial research progress have been made in the development of more robust light harvesting materials, the further modification of the modelling theory of the OPV physics, better understanding and elucidation of the light-to-electricity process and the continuous optimization of the device fabrication process with new strategies employed. This process can be witnessed by the fast growing efficiency data of the OPV cells (Figure 1) and the vast amount of literatures published annually on the topic of OPV technology. Concur‐ rently, the industrial attention is mainly focused on development of robust materials with long lifetime and good efficiency in large scale application, production technique optimization and market exploration for OPV technology. The fast growing research activity on OPV technology involves the collaborative consolidation of knowledge from synthetic chemistry, especially on π-chemistry, semiconductor physics and device engineering. Figure 1 [3] depicts the certified record PCE data of various types of PV technologies that have been continuously optimized

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in the past decade. Compared with inorganic PV technologies, organic solar cells have achieved magnificent improvement in terms of efficiency. Currently the record holder is Mitsubishi Chemicals who has demonstrated a reproducible 10.7 ± 0.3% organic solar cell.

Compared with the existing mature inorganic based photovoltaic technology, a list of pro‐ posed advantages of OPV technology should be mentioned which include 1) short energy payback time [4]; 2) lower production cost compared with inorganic PVs; 3) potential fabrica‐ tion via continuous printing tools; 4) new market opportunities, such as flexible PV, wearable PV, semitransparent PV window, etc.; 5) low weight and easy integration of the organic PV products. Nevertheless, inorganic based PV technologies, such as silicon, cadmium telluride (CdTe), III-V group semiconductors and copper indium gallium selenide (CIGS), are still dominating the PV market.[5] The reasons why the OPV are short of the market are mainly due to the inferior power conversion efficiency (PCE) and the poor stability of the organic solar cell devices compared with their inorganic analogs. As the ultimate goal of materials research is to apply the material based science and technology into material based products, the applicability and the competitiveness of the technology should always be buried in mind. The economic aspects of the OPV technology have attracted more attention as many companies start to step into the OPV market and create opportunities. The optimization of the OPV device, both in terms of device stability and power conversion efficiency, has become a synergistic work between the academia and the industries. Moreover, the industrial production of OPV modules, which is very different from the lab-scale production step, is also being optimized. All these efforts will be highlight in this Chapter.

With both the advantages and existing disadvantages of OPV technology in mind, we plan to give an overview of how to transform a molecule to a material and finally to a product for OPV technology and we organize this Chapter in the following way. Firstly we will summarize the existing strategies to prepare new and better light harvesting materials by synthetic chemists. Due to the limited space, we focus on the most commonly used polymeric donor-acceptor (D-A) type materials. In the following part, we summarize the recently developed device engineering methods to improve the performance of the OPV materials. This part deserves its own merit because the OPV device engineering process is essential to demonstrate the full potential of a new polymer molecule as a functional light harvesting material. Then we will summarize some aspects of OPV materials which are important for the production develop‐ ment, such as the lifetime/stability of the material and production techniques. Finally we will end up our discussion with a summary and perspective on the future research. There have been many excellent summary works dealing with various aspects of OPV technology, such as the working mechanism and physics of the OPV device [6-9], design principles and synthesis of new light absorbing materials [9-19], thin film morphology control and characterization [20-21], new device architecture development [22-24], interface engineering [25], quantum chemical calculations [26], economical aspects [27-29] and many insightful overviews and perspectives [30-42]. There are also a series of *Photovoltaics Literature Survey* papers by Santosh Shrestha [43] in *Progress in Photovoltaics: Research and Applications* which are useful for readers to catch up current research progress in various aspects regarding PV technology.

**Figure 1.** Best Research Cell Efficiencies for all types of PV technology. Data from National Renewable Energy Labora‐ tory (NREL).[3]
