Sunday, May 20, 2012

Computational Design and Selection of Optimal Organic Photovoltaic Materials

Noel M. O’Boyle, Casey M. Campbell and Geoffrey R. Hutchison Journal of Physical Chemistry C 2011, 115, 16200 (Paywall)

Quantum chemistry for high throughput screening
In this work over 90,000 pi-conjugated copolymer were computationally screened for new and efficient organic photo-voltaics. Not being an expert in organic photo-voltaics I highlight this paper as a very interesting example of what I believe is an important emerging trend in the use of quantum chemistry: efficient high through-put screening of molecules for desirable properties.  Computers and software have now reached a point where this is computationally feasible to perform computations on thousands of molecules and the challenge is now to make it practically possible, i.e. to find the right combinations of methods and automate their use.

How does one construct 90,000 geometries?  Cheminformatics meets quantum chemistry
OpenBabel is used to construct the 3D structure of each polymer, starting from its SMILES string, and to find the lowest energy conformation using a weighted rotor-search and the MMFF94 force field.  This lowest energy conformation is then minimized with PM6 and used to compute energies and oscillator strengths of the 15 lowest- energy electronic transitions with ZINDO/S, which forms the basis for estimating the energy conversion efficiency (see the paper for more details).  The required CPU time is  8-10 minutes per polymer on a single core.  To put that number in perspective: using 100 cores, 100,000 polymers can be screened in roughly a week.

Starting from 131 different monomers, all possible (19,701) dimers are made and these dimers are then used to construct the corresponding 58,707 tetramers. (A brief description on how exactly this was scripted would have been a welcome addition to the supplementary materials).  The energy conversion efficiency was computed for all these dimer and tetramers.  These results were used to calibrate a genetic search algorithm that was used to identify hexamers and octamers with high energy conversion efficiency without doing an exhaustive search.

Promising candidates and new strategies
The current state-of-the-art is ca 8% energy conversion efficiency.  This study found 621 polymers (mostly hexamers and octamers) with greater than 9%  and 2 with greater than 11 % energy conversion efficiency.  (As the authors point out these polymers still need to be filtered for solubility, crystal packing, and other factors.)  Just as interestingly new design strategies emerged:
"Our analysis of component monomers, dimers, and the copolymer sequence demonstrates important design rules for copolymer photovoltaics. Most importantly, the conventional picture of combining a strong donor and strong acceptor into an alternating copolymer is found to frequently yield poor energy- level alignment. Instead, our top hexamers and octamers reflect a decreased optical band gap due to high coupling between the two-component monomers, not solely due to particular HOMO or LUMO energies of the monomers themselves."

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This work is licensed under a Creative Commons Attribution 3.0 Unported License


  1. Yes, the interesting part of this paper was not that we did it. It's that the process turned up a number of new design rules for these types of copolymers:
    1) Sequence matters -- the conventional alternating polymers rarely did well, but complex sequences boosted both the transition dipole moment (and thus absorption extinction coefficient -- good news for solar cells) and computed efficiencies.
    2) Donor-donor combinations. Most synthetic groups have attempted to optimize band gap with the use of donor/acceptor combinations. The genetic algorithm suggested this is less efficient than finding two electron-rich monomers with high delocalization (the Hab or Beta integral).

    From the synthetic side, #2 would make life a lot easier -- forming bonds with electron-poor acceptor monomers is hard! They're really less reactive.

    As far as the enumeration of compounds, you have to:
    1) Consider whether any monomers are asymmetric, since you'll then have A and A' monomers, yielding AB, A'B, BA, and B'A combinations.
    2) Consider how the dimers will couple into tetramers, e.g., ABBA, ABAB, BABA, BAAB, A'BBA', A'BBA, etc. It's mostly a matter of combinatorics.

    I'll dig up the code and post to GitHub.

    1. Are you collaborating with people who can evaluate some of your top hits? It would be interesting to get some experimental verification.

      Great news about the code. Do let me know when it's up and I'll add a link in the post.

    2. We're synthesizing a few things in my group and are working with others on more complicated monomers. (The bifuran noted in the article is particularly challenging.)

    3. I am a student at the Loudoun County of Science, and I know I'm not a scientist yet, but I'm working on it. For research I am looking at improving the efficiency of an organic photovoltaic cell by introducing graphene nanostructures into the polymer. My problem is I have no clue what polymer to use. Can anybody help me?

    4. Edit: Loudoun County Academy of Science

  2. @Keith Funny, I was just visiting some friends in Sterling over Memorial Day weekend. NIce ideas. My suggestion would be to start with P3HT:PCBM solar cells. The materials are fairly easy to purchase and use without further purification. Most people can usually make a decent cell with P3HT:PCBM with ~1% performance and not need specialized equipment. (Getting better performance requires a lot of purification, a glove box, and practice.) Anyway, this system is heavily used -- everybody tries to tweak it and see if you get better efficiency. BTW, if you're doing experiments, you're a scientist! You don't need a degree, only the desire to work and some good ideas. Good luck!