Organic solar cell is the new emerge in photovoltaic technology. This book briefs organic material properties and their working principle explicitly. Among so many organic materials P3HT:PCBM provides the best performance so far. The performance of P3HT:PCBM based solar cells is analyzed in terms of fundamental device model. Same procedure is followed to evaluate the current-voltage characteristics of pBBTDPP2:PCBM, PCDTBT:PCBM materials. To improve power conversion efficiency further, two series connected tandem models are proposed in this book where appreciable value of efficiencies are found.