| The growing energy demands facilitate the search of cheap and renewable energy. The vision of organic photovoltaics is that of a low cost solar energy conversion platform that provides lightweight, flexible solar cells that are easily incorporated into existing infrastructure with minimal impact on land usage. Organic solar cells have been a subject of growing research interest over the past quarter century, and are now developed to the point where they are on the verge of introduction into the market. Polymer solar cells provide all important characteristics necessary to fill this niche and power conversion efficiency exceeding 10% was recently achieved. However, such a high efficiencies are obtained using multi-junction solar cells which require additional steps in the manufacturing process and thus contradicts the attractive simplicity of the single step solution processing of the active layer in bulk heterojunction solar cells. A basic introduction to polymer:fullerene solar cells is presented in Chapter 1.;The current dissertation is focused on the recently emerged approach of ternary blend bulk heterojunction solar cells, where three components are mixed in the active layer, providing an alternative route to high efficiencies while preserving the simplicity of the solar cell fabrication. A general overview of this class of solar cells is presented in Chapter 2.;The key component to the efficiency increase in ternary blend bulk heterojunction solar cells is the discovered open-circuit voltage (Voc) tunability in the three component systems, which is introduced in Chapter 3. Together with the short-circuit current density (Jsc) increase obtained using polymers with complementary absorption profiles (discussed in Chapter 4), the efficiencies of the ternary blend bulk heterojunction solar cells are increased beyond that of the corresponding binary blend solar cells, as discussed in Chapter 5. By extension, this result suggests that ternary blends provide a potentially effective route toward maximizing the attainable JscxVoc product (which is directly proportional to the solar cell efficiency) in bulk heterojunction solar cells and that with judicious selection of donor and acceptor components, solar cells with efficiencies exceeding the theoretical limits for binary blend solar cells could be possible without sacrificing the simplicity of a single active layer processing step.;Finally, the formation of an organic alloy in ternary blend bulk heterojunction solar cells, introduced in Chapter 6, unravels and explains the origin of the Voc tunability and simultaneous J sc increase in three component systems. |
