At the Junction of Organic Solar Cells: Charge Generation and Recombination at Donor/Acceptor Interfaces
Heterojunction, what’s your function?
In organic electronic devices composed of donor and acceptor semiconductors, the donor/acceptor interface is most typically the site with all the action: i.e. charge-carrier generation and recombination. Developing an understanding of the optimal geometry and energetics at this interface is necessary to optimize the active material and device architecture for their desired application. In this dissertation, we explore the role of morphology and energetics at the donor/acceptor interfaces on photovoltaic performance, but the results can be applied to any device with donor/acceptor heterojunctions.
We begin our investigation by characterizing emission from small-molecule blends. We find a correlation between the emergence of phase separation and crystallinity, electroluminescence from the donor singlet-state, and good photovoltaic performance. Next, upon demonstrating control over the molecular orientation, we then uncover the genuine effects of molecular geometry at the donor/acceptor interface on charge generation and recombination: (i) Face-on devices have a higher open-circuit voltage, due to greater charge transfer state energy and radiative efficiency. (ii) Edge-on devices are more efficient at charge generation, which is attributed to a smaller electronic coupling and a lower activation energy for charge generation.
From the perspective of energetics, we focus on a polymer-fullerene blend system with small energetic offsets. This system has very low potential losses: it achieves a high open circuit voltage relative to the energy of the absorbed photons. We characterize the energetic landscape in this blend and conclude that the blend has very high energetic order, and that potential losses associated with charge transfer have been minimized. Unfortunately, the blend is also characterized by exceptionally fast bimolecular recombination, most likely resulting from a highly-mixed blend morphology and charge-trapping effects. Nonetheless, these results are promising as they suggest that given an optimized morphology, organic solar cells (and other organic electronic devices) have more potential than we had previously believed.