The easy fabrication process and low material usage make solution-processed organic solar cells (OSCs) very attractive for electricity generation. The production of these devices on a commercial scale has been slow due to their relatively low power conversion efficiency and stability problem. It is expected that these obstacles can be surmounted with rigorous research studies actively being done in this field. Tris (8-hydroxyquinoline) metals (Mq3) are well known in the fabrication of organic light emitting diodes (OLED)s and also for their unique optoelectronic properties. Nevertheless, their employment in solution-processed organic solar cells is still rare. Little attention has been paid on the impact of these materials when applied in organic solar cells. Hence, benefiting from the properties of Mq3 and easy fabrication of solution-processed organic solar cell, the current book is focused on the studies that have been performed to utilize Alq3 and Gaq3 in solution-processed OSCs.