Zhenan Bao, Department of Chemical Engineering, Stanford University; Alán Aspuru-Guzik, Chemistry and Chemical Biology, Harvard University; and Michael Toney, Stanford Synchrotron Radiation Laboratory, SLAC National Accelerator Laboratory

Objective

The goal of this project is to advance the fundamental understanding of materials design rules for charge and exciton transport, both of which are crucial for organic solar cell applications.  To achieve this goal, powerful theoretical tools and characterization techniques will be combined to develop a rational design methodology for novel materials.  A feedback loop combining theory, synthesis and characterization will accelerate the discovery of novel photovoltaic materials (Figure 1). This approach will allow enable development of a systematic path toward identifying promising molecules that will greatly improve the performance and efficiency of organic solar cells.

Background

Research suggests that thin film solar cells made of copper indium gallium selenide, cadmium telluride and similar compounds will eventually be three-to-five times cheaper to build than conventional solar cells - in part because thin film cells require less material than solid photovoltaic devices and can be easily interconnected on the same substrate.  However, there is growing concern that relatively scarce elements, such as indium and tellurium, will be depleted as demand increases for renewable sources of electricity to reduce greenhouse emissions.  Organic photovoltaic cells, which can be inexpensively manufactured in high-throughput roll-to-roll coating machines similar to those used to print newspapers, may enable large-scale solar cell deployment for electricity production.

Despite the economic and processing advantages of organic solar cells, laboratory experiments have only recently come close to achieving the 15% efficiency target needed for commercial viability.  Creating high-performance organic photovoltaics will require semiconductors with low energy gaps, high exciton diffusion lengths and high charge carrier mobility.  A key challenge is how to blend all of the requirements in materials with optimized device performance.

Approach

This project will address this challenge from a new angle by combining molecular design and device fabrication, theoretical simulation and structural characterization with the large distributed computing power of IBM's World Community Grid (WCG).  The WCG distributed computer network permits an 8-hour single-processor calculation to be submitted every 6 seconds - the equivalent of having a 4000-node cluster dedicated to this project (Figure 2).

Instead of testing and manufacturing potential molecular materials, researchers will use the WCG to set up massive numbers of computational calculations to scan through a huge collection of molecular candidates.  The results of these theoretical calculations will help determine whether a particular molecule is a suitable match for use in organic solar cells.  Specifically, the distributed network will facilitate the prediction of packing structures by molecular dynamics methods, as well as the computation of transfer integrals and reorganization energies for a large number of molecular structures¹.

Figure 2. Screenshot from the Clean Energy Project on the World Community Grid distributed computing website developed by the Aspuru-Guzik laboratory in collaboration with IBM (http://cleanenergy.harvard.edu). The large computational resource provided by the volunteer screensavers of the WCG users will be employed to carry out molecular dynamics calculations aimed at finding the global minimum in the lattice energy of all possible packing arrangements of a given molecular candidate.

Promising candidates derived from the extensive computational search will be synthesized and characterized by measurements of their thin film packing and charge transport.  The results derived from the theoretical calculations will be compared against the experimental measurements of both molecular packing and thin film transistor mobilities^{2, 3}. These experimental results will be used to fine-tune the theoretical methodologies. Initial theory and experiment feedback loop allowed designing of a new high performance organic semiconductor, showing promise of the above approach.⁵

Following the experimental characterization, promising molecular structures will be integrated into organic solar cell devices to assess their efficiencies.  Finally, a database of predicted bandgaps and charge transport parameters for all of the molecules investigated will be created.  This information will be publicly available for data mining to select appropriate candidates for organic solar cells or other applications.

References

• [1] Y. Shao et al., Phys Chem Chem Phys 8, 3172 (2006)
• [2] S. C. B. Mannsfeld et al., Advanced Materials, 20, 2294-2298, (2009)
• [3] Q. Yuan et al., J. Am. Chem. Soc. 130, 3502 (2008)
• [4] R. Olivares-Amaya et al, Energy Environ. Sci., 4, 4849-4861 (2011)
• [5] A.N. Sokolov et al, Nature Communication, 2:437. DOI: 10.1038/ncomms1451 (2011)