Organic Double Heterojunction Solar Cells

Organic solar cells offer the promise of renewable energy capture with an inexpensive, easy to produce, and mechanically flexible technology. Through two decades of research, power conversion efficiencies have risen dramatically. However, the materials used are far from cheap, and high-efficiency cells are difficult to produce at scale. While these challenges appear tractable, nearly 20 years of progress in understanding their underlying photophysics suggests that a radical rethinking of organic solar cell design may produce useful new directions. We present a design for a fully-conjugated block polymer solar cell that we term a double heterojunction. Detailed balance calculations suggest that device efficiencies approaching 30% are possible with exciton binding energies of 0.3 eV or less. The key to the design is a conjugated linker between the electron accepting and electron donating portions of the diblock system, called a bridge. Our calculations show that optimizing the energy level of this component is critical in achieving high efficiency devices. We will provide insight into the disappointing performance of more conventional diblock conjugated polymers and donor-bridge-acceptor dyads. In order to better describe general synthetic guidelines, we have performed self-consistent field calculations which suggests that a system composed in the following way: [D1-A1]-[D1-A2]-[D1-A2] results in optimal energy alignment of the central bridge segment. Although the calculations are relatively crude, the results provide a clear example of why fully conjugated block polymer solar cell materials are worthy of more intense study. Moreover, the synthetic complexity of this system points toward a dire need to leverage more sophisticated computational resources to identify tractable candidate systems. Time permitting, cells with leverage both the double heterojunction design and singlet fission processes will be discussed.