Solar energy is the largest source of clean and renewable energy. Compared to inorganic counterparts, organic solar cells hold great promise because they are flexible and lightweight, and can be produced via low-cost processing. One of the biggest obstacles of organic solar cell towards commercialization is the cost of the electron acceptors, which are typically fullerene derivatives. This thesis describes the exploration of non-fullerene acceptors, and studies the structure-property relationship of the materials. Both polymers and small molecules are synthesized. The study of electron deficient polymers revealed that in addition to controlling the energy levels, internal polarization is another parameter that should be addressed. Small molecule studies are based on perylene diimide (PDI) building block. By removing the steric hindrance between β-hydrogen and β-substituents on the PDI, thus reducing the backbone distortion of the PDI, small molecule electron acceptors consisted of ɑ-substituted PDIs outperform their β-substituted PDI counterparts. Oxidative cyclization at bay-positon of PDIs provides another way to eliminate steric hindrance, and also provides a method to tune the open circuit voltage value of the material. Further enhancement of power conversion efficiency (PCE) can be realized by expanding PDI dimers into tetramer clusters. Tetramer of bay-cyclized PDI acceptors could achieve higher than 7% PCE, and tetramer of ɑ-substituted PDIs showed higher PCE value of over 8%.