Organic bulk heterojunction (BHJ) solar cells consisting of electron-donating polymers and electron-accepting fullerene derivatives garner interest because they can be manufactured inexpensively at high throughput via solution processing. The power conversion efficiency of BHJ solar cells is now above 11 and 12% in single-junction and tandem architectures, respectively. Much of the recent improvement in device performance is due to (i) the development of low band gap polymers with broad absorption capabilities, (ii) the development of polymers and fullerene derivatives with energy levels optimized for higher open-circuit voltages, and (iii) the use of solvent additives to tailor the BHJ morphology. Despite these improvements, the efficiency of single junction BHJ solar cells must surpass 15% before organic solar cells can compete with inorganic solar cells based on silicon or cadmium telluride. In this doctoral thesis, I examine how the polymer and fullerene morphology affect the performance of BHJ solar cells and determine how the efficiency of these devices can be improved. In Chapter 2, I show that the morphology of polymer-fullerene BHJs consists of three phases: pure polymer aggregates, pure fullerene clusters, and an amorphous phase consisting of polymer and fullerene mixed at the molecular level. The concentration of fullerene in the molecularly mixed phase has a strong influence on device performance. In order to have a fully percolated network of electron transporting fullerene molecules within the mixed regions, at least 20 weight percent fullerene must be mixed with the polymer. Decreasing the concentration of fullerene below this percolation threshold reduces the number of electron transport pathways within the mixed regions and creates morphological electron traps that enhance charge-carrier recombination and decrease device efficiency. In Chapter 3, I discuss how the polymer molecular weight plays a role in determining the final BHJ morphology and device efficiency. BHJs made with low molecular weight polymer have exceedingly large fullerene-rich domains. Increasing the molecular weight of the polymer decreases the size of these domains and significantly improves device efficiency. I show that polymer aggregation in solution affects the size of the fullerene-rich domains and determine that this effect is linked to the dependency of polymer solubility on molecular weight. Due to its poor solubility, high molecular weight polymer quickly aggregates in solution and forms a network that acts as a template and prevents large scale phase separation. Finally, I find that the performance of devices made with low molecular weight polymer can be improved by using solvent additives during processing to force the polymer to aggregate in solution. I examine how the efficiency of organic solar cells can be improved to 15% in Chapter 4. To surpass 15% efficiency, devices likely will need to be 300 nm thick and achieve fill factors near 0.8. Using a numerical device simulator, I show that the key to achieving these performance metrics is a high charge-carrier mobility and a low recombination rate constant. Devices with low charge-carrier mobility (10-2 cm2 V-1 s-1) suffer from high rates of bimolecular recombination because many charge carriers must reside in the device to drive a given drift current. Furthermore, I demonstrate that numerical device simulators are a powerful tool for investigating charge-carrier transport in BHJ devices and are useful for rapidly prototyping BHJ solar cells. To conclude, I discuss how researchers can improve the efficiency of organic solar cells. Researchers should aim to design molecular systems that exhibit high miscibility ( 20 weight percent fullerene in the mixed phase) or immiscibility (H" weight percent fullerene in the mixed phase). Furthermore, the synthesis of new, high molecular weight polymers with exceptionally high charge-carrier mobility and low recombination rate constants is imperative for reaching high device fill factor. With these improvements, the efficiency of organic solar cells can surpass 15%, which would allow these devices to compete with traditional inorganic solar cell technologies.