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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2016)

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.

    Cover page: At the Junction of Organic Solar Cells: Charge Generation and Recombination at Donor/Acceptor Interfaces
    • Article
    • Peer Reviewed
    Materials Sciences (2019)
    The performance of organic solar cells (OSCs) depends crucially on the morphology in bulk heterojunctions (BHJs), including the degree of crystallinity of the polymer and the amount of each material phase: aggregated donor, aggregated acceptor, and molecular mixed donor : acceptor phase. In this paper, we report the BHJ morphology of as-cast blend films incorporating the polymer PIPCP as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor. Tracking the scattering intensity of PC61BM as a function of PC61BM concentration shows that PC61BM aggregates into donor-rich domains and there is little to no phase where the PC61BM and PIPCP are intimately mixed. We further find that on blending the scattering peak due to PIPCP ordering along the backbone increases with decreasing PIPCP fraction, which is attributed to improved ordering of PIPCP due to the presence of PC61BM. Our results suggest that the improved ordering of PIPCP along the backbone (consistent with an increased conjugation length) with blending contributes to the observed low open-circuit voltage energy loss.
      Cover page: Fullerene derivative induced morphology of bulk heterojunction blends: PIPCP:PC 61 BM
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2017)
      A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss.
        Cover page: Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency
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