Icon for: Steven Hawks

STEVEN HAWKS

University of California at Los Angeles
Years in Grad School: 3

Judges’ Queries and Presenter’s Replies

  • Icon for: Ananth Iyer

    Ananth Iyer

    Judge
    May 20, 2013 | 11:38 a.m.

    Given the ability to reconstruct the J-V curves, what is the impact on the efficiency of the associated solar cells ?

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 20, 2013 | 01:38 p.m.

    Hi Ananth,

    Thanks for the question. I’m assuming you are asking about the impact of (non ideal) recombination on the efficiency of the solar cells? If so, I have not explored that question in detail because altering the recombination kinetics will also alter everything else (e.g., how the excess carrier density scales with voltage, etc.) It is certainly a question worth looking into more, though. In the end, one could always use a drift-diffusion simulator with the empirically measured recombination model implemented and use that to get an idea of how sensitive the efficiency is to changes in recombination.

  • May 20, 2013 | 09:49 p.m.

    What do your studies suggest with respect to finding an even better combination of fullerene or polymer as a replacement for these tried-and-true p-n components?

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 21, 2013 | 11:07 a.m.

    Our results imply that the p-n material morphology determines the electrical properties more than anything else (and quite strongly so), which suggests that research effort should be directed toward understanding and controlling the nanoscale structure of these polymer:fullerene blends. Our work also suggests that for this system the lowest unoccupied molecular orbital (LUMO) offset between the p and n components is nearly optimal — less of an offset would seem to promote more electron back transfer. Understanding why this is is of fundamental importance, because the LUMO-LUMO offset required for splitting excitons is an energy loss process that should be minimized.

  • May 21, 2013 | 04:15 p.m.

    How common is the observation of non-Langevian recombination in organic solar cells?

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 21, 2013 | 06:27 p.m.

    A Langevin “reduction factor” that can range from 1 (pure Langevin) to 10^-3 (“reduced” Langevin) is often used to equate the mobility and recombination constants. However, until now, no one has reported anti-Langevin recombination like we do above. We find that the mobility increases while the recombination simultaneously decreases with extra PCBM content. Typically, people find that the recombination and mobility scale together, but the “reduction factor” is needed to marry the two.

  • May 21, 2013 | 06:56 p.m.

    Thank you, Steven. Your results are truly unique and keep up the good work!

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 21, 2013 | 06:59 p.m.

    Thank you!

  • May 21, 2013 | 09:09 p.m.

    Since nanoscale morphology is so critical, how do es one control it? Interfacial free energy would be a critical parameter that one has little or no control over without altering the composition.

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 21, 2013 | 10:04 p.m.

    Experimentally researchers alter the nanoscale blend morphology by adding in small amounts (<3% v/v) of a solvent additive to the solution before casting and/or using post-production treatments like annealing or exposure to e.g., a solvent rich atmosphere.

    Really, the bane of this field is measuring, controlling, and understanding what determines the film morphology. In the end it is extraordinarily difficult just to “look at” the nanoscale (<10 nm) structure because it is practically all amorphous conjugated carbon. Furthermore, it is difficult to single-out the effects of one particular material property (e.g., interfacial free energy) on device performance since so many things determine device performance. There is presently a significant research effort being directed toward measuring/imaging the morphology so that a detailed understanding of the structure-processing-properties relationships can be achieved.

  • Icon for: Ian Harrison

    Ian Harrison

    Judge
    May 21, 2013 | 11:41 p.m.

    Could you explain the terms geminate and nongeminate recombination and why one type might be preferred for your application?

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 22, 2013 | 01:01 a.m.

    Indeed, apologies for the jargon. Geminate recombination refers to recombination between electron-hole pairs that were created from the same photon — i.e., e-h pairs that never dissociated in the first place. Non-geminate recombination refers to everything else, which includes free-to-free, free-to-trapped, and surface (wrong contact) recombination. However, in the context of my data, we are referring to the former two.

    Preferably one only wants nongeminate recombination since geminate recombination is a unique and additional loss process associated with excitonic materials; that is, materials that don’t create “free” carriers immediately upon photoexcitation. Almost all inorganic semiconductors create free charges upon photoexcitation, whereas all organic semiconductors first create excitons. Our data suggests that geminate recombination plays a minor role in determining the overall power-conversion efficiency of our devices. Instead, excessive free-to-free and/or free-to-trapped recombination are the primary loss processes that limit our (already relatively high) cell efficiencies. Moreover, it appears that the morphology is the dominating factor that governs the two aforementioned processes as well as the equally important charge carrier mobilities.

  • Icon for: Ian Harrison

    Ian Harrison

    Judge
    May 22, 2013 | 09:48 p.m.

    Thank you, that’s interesting.

  • Further posting is closed as the competition has ended.

Presentation Discussion
  • Icon for: Robert Opila

    Robert Opila

    Faculty
    May 23, 2013 | 12:03 a.m.

    This is the type of understanding necessary to make OPV viable. Good job.

  • May 23, 2013 | 11:05 a.m.

    I think solar is great, and hopefully your work will make it a more viable energy source. I take issue with the idea that these can transform “unused land” into energy sources, and you show a photo of an awesome sagebrush-type habitat! I hate to get on my soapbox, but unused land is not often wasted! Why not stress that putting these in urban and suburban areas will reduce infrastructure requirements and save money! Less travel time for the electricity! There are lots of rooftops and parking lots that solar panels can be put on.

  • Icon for: Steven Hawks

    Steven Hawks

    Presenter
    May 23, 2013 | 06:08 p.m.

    Robert: Thanks!

    Maggi: I’m afraid I have to stand by my comment and respectfully disagree with some of your points. First, I agree that less travel-time for the electricity is good and there are lots of prime rooftops, etc. that would be great substrates for solar energy harvesting.

    However, I think maybe my image choice was poor, because there is plenty of uninhabited land that sees lots of sun. Also, solar farms are usually quite small when compared to the surrounding territory. I think in the end one has to weigh the pros/cons of generating megawatts of clean renewable energy for society vs. destroying a very small portion of local habitat (that is preferably hardly inhabited). It seems shortsighted to me to disallow a solar farm in favor of, for example, mining/burning coal for energy.

  • May 23, 2013 | 06:25 p.m.

    Hi Steven, thanks for your response. As a landscape ecologist, I think that it is important to involve ecologists when proposing new energy infrastructure (not what you’re doing in this presentation, but you could do it at some point!). Even if solar farms are small, their construction risks fragmenting otherwise intact habitat (fragmentation results in edge effects, so the impact of the solar farm can actually be much larger than the area of the farm itself). And uninhabited land often provides a number of ecosystem services that have yet to be valued in our economic system, but they may very well overtop the benefits of a solar farm.

    How much land would it take to generate 100MW of solar energy? For a wind farm, the actual area is quite small, but the impact is quite large because of road construction and habitat fragmentation. Since solar can be concentrated, I assume it’s less area. If you do get into promoting solar (and I hope you will!), please keep in mind there is more to this than providing a cleaner source of energy than coal.

    Thank you!

  • Icon for: Robin Garrell

    Robin Garrell

    Faculty
    May 24, 2013 | 12:12 a.m.

    Steven-
    Looks like you’ve gained an important insight that can serve as the basis for optimizing device composition and morphology to improve efficiency.

    RLG

  • May 24, 2013 | 01:41 a.m.

    Hi Steven, i interesting work! I’m curious if you have done these experiments with other materials or if you have an idea of how much these recombination losses are dependent on the specific donor/acceptor pair. Do you think using a different acceptor (or polymer) could reduce these types of recombinations and if so, is there an approach to specifically design/select them? Thanks!

  • Further posting is closed as the competition has ended.