Energy is a prerequisite for creating and sustaining life. The need for energy increases globally as the world's population and economy grow. However, conventional energy sources—fossil fuels—generate carbon dioxide and contribute to global warming, perhaps the most serious environmental problem of our time. Carbon dioxide-free energy is required to stop global warming. Polymer solar cells have been attracting a great deal of interest as a source of renewable energy with a great potential for low cost. Polymer bulk heterojunction (BHJ) solar cells have been greatly improved; the power conversion efficiency is already up to 9.2% making the future of the polymer solar cell very promising.

This thesis is a study of the morphology of polymer:fullerene BHJ, one of the most critical and challenging parts of high efficiency polymer solar cells. To discover the morphology, cross-section as well as top-down transmission electron microscopy were used. The contrast was achieved by utilizing phase contrast microscopy. Thermal annealing, dependence of BHJ thickness, processing additives, solution sequential process and solution sequential process with the use of cosolvent that affects/controls the BHJ morphology are studied in detail.