**3. Common building blocks for BHJ solar cell polymers**

Common monomer building blocks to construct conjugated polymer for solar cells have been summarized in Chart 1. They are categorized by number of rings and way of linkage. Due to the space limit, we will only discuss monomers that are commonly encountered in the literature and the property of their representative polymers. Some important building blocks, even though not commonly used for PSC polymer, are also included for comparison.
