In the present Book, the Bulk Hetro-junction (BHJ) Organic Solar Cells were Designed, Fabricated & characterized of fabricated by different Electron Transport Layer (ETL) Hole Transport Layer (HTL) with very low band gap Donor-Acceptor polymers. This book was carried out with the intention of large area cell fabrication which is desired for mass production of the solar cells. The work focuses on effect of variable active area on the photovoltaic parameters. In summary, the book emphasis on modeling, fabrication & characterization of conventional and inverted BHJ Organic Solar Cells.