The field of Physics Education Research (PER) seeks to investigate how students learn physics and how instructors can help students learn more effectively. The process by which learners create understanding about a complex physics concept is an active area of research. My study explores this process, using solar cells as the context. To understand how a photovoltaic cell works involves drawing knowledge from many different areas of physics, so this provides a fertile area to study how students build understanding of complex ideas. I have used the "knowledge in pieces" theoretical framework to understand how students learn about solar cells by activating cognitive resources. In this framework, we can see learners building understanding out of more basic bits of knowledge, known as resources, that are derived from students' prior experience. This study seeks to learn more about how students combine multiple resources as they construct understanding of a complex physics topic. To achieve this goal, I have created instructional materials and assessment instruments used to collect written and spoken data on students' reasoning. The analysis of this data revealed that students are most likely to successfully build understanding when they activate multiple types of resource simultaneously. I propose possible explanations for this pattern and present ways this finding could impact instruction. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]