The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four-month research fellowship by Dr. Jason J. Amsden to work with Dr. ChangHee Lee at Seoul National University in Seoul, South Korea.
Organic photovoltaic (OPV) devices offer an intriguing alternative to silicon based devices due to their ease of processing, mechanical flexibility, and low cost. However, at present the power conversion efficiency of OPV is still too low to compete with silicon based devices. Two possibilities for improving the power conversion efficiency in OPV include use of new low band gap high mobility polymers and use of hybrid materials incorporating semiconducting polymers and semiconductor nanocrystals. Polymers with a low band gap more efficiently collect solar radiation and semiconductor nanocrystals such as zinc oxide are advantageous because of their high electron mobility compared to organic molecules, solution processing, and tunable band gap. This research studies for the first time the morphology and charge transport properties of zinc oxide nanocrystal and low band gap polymer hybrid photovoltaic devices. The project will include two main components: The first objective is characterization of the interaction and charge transport properties between low band gap polymers and zinc oxide nanocrystals in various morphologies using various types of spectroscopy and microscopy. The second parallel objective is to investigate the photovoltaic performance of hybrid devices using the insights gained from spectroscopy and microscopy.
In addition to the obvious benefit of increased knowledge about organic photovoltaic materials for solar energy conversion, this research will help develop new international collaboration between the PI and research groups at Seoul National University in Seoul, South Korea (Prof. ChangHee Lee and Prof. Do Yeung Yoon), at the Max Plank Institute for Polymer Research in Mainz, Germany (Prof. Klaus Müllen), and at Boston University in Boston, USA (Prof. Kenneth J. Rothschild).
The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. For my award, titled "Zinc Oxide/Low Band gap Polymer Hybrid Photovoltaic Devices", I spent 2 years at Seoul National University in Seoul, South Korea in the laboratory of Professor Changhee Lee evaluating the use of ZnO nanoparticles in hybrid bulk heterojunction polymer nanoparticle solar cells. Organic photovoltaic (OPV) devices offer an intriguing alternative to silicon based devices due to their ease of processing, mechanical flexibility, and low cost. However, at present the power conversion efficiency of OPV is still too low to compete with silicon based devices.1 Most organic photovoltaic devices are composed of a semiconducting polymer material called the "donor" and another small organic molecule based on a fullerene called the "acceptor." Two possibilities for improving the power conversion efficiency in OPV include use of new polymers called low band gap polymers for the donor material that are able to absorb a wider spectrum of light from the sun and use of hybrid materials incorporating these polymers and semiconductor nanocrystals like ZnO for the acceptor material. ZnO nanoparticles are an intriguing material for use as acceptors in OPV devices because they are relatively inexpensive and easy to synthesize when compared with fullerenes, and they are non-toxic when compared to other nanoparticles such as CdSe. Previous research using ZnO nanoparticles in OPV devices focused on polymers such as MDMO-PPV2 which do not absorb as much solar radiation as new low band gap polymers.3 In addition, devices made with nanoparticles suffer from relatively poor performance as compared to devices made with the more commonly used fullerenes, due to poor mixing of the polymer with ZnO nanoparticles. 2 First, we developed a new surfactant to improve the quality of mixing between the polymer and ZnO nanoparticles. Our surfactant is unique in that it is a semiconductor. Most other nanoparticle surfactants are insulators and therefore not ideal for use in an electrical device. Our new semiconducting surfactant improved the power conversion efficiency of solar cells based on ZnO nanoparticles and MDMO-PPV from 0.75% to more than 1.3%.4 Furthermore, we used ultrafast laser spectroscopy to compare the function of devices with different types of surfactants just after absorption of light to confirm the reason our semiconducting surfactant improves the power conversion efficiency is faster charge transfer between the polymer and nanoparticle.5 Improving the power conversion efficiency from 0.75% to 1.3% is a relatively large improvement. However some of the best performing OPV devices have power conversion efficiencies around 9%6 because they use special low band gap polymers. We tried several low band gap polymers with ZnO nanoparticles hoping to achieve good power conversion efficiencies. Unfortunately, we found that there is an unfavorable interaction between the nanoparticles and the low band gap polymers that does not occur with polymers like MDMO-PPV and our devices had power conversion efficiencies of <1%. We hope that future work with improved surfactants will prevent this unfavorable interaction and improve the performance of the devices using ZnO nanoparticles and low band gap polymers. 1. (a) Shrotriya, V., Organic photovoltaics: Polymer power. Nat Photon 2009, 3 (8), 447-449; (b) Li, G.; Zhu, R.; Yang, Y., Polymer solar cells. Nat Photon 2012, 6 (3), 153-161. 2. Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J., Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Adv. Mater. 2004, 16 (12), 1009-1013. 3. Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J., Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10 % Energy-Conversion Efficiency. Adv. Mater. 2006, 18 (6), 789-794. 4. Park, I.; Lim, Y.; Noh, S.; Lee, D.; Meister, M.; Amsden, J. J.; Laquai, F.; Lee, C.; Yoon, D. Y., Enhanced photovoltaic performance of ZnO nanoparticle/poly(phenylene vinylene) hybrid photovoltaic cells by semiconducting surfactant. Org. Electron. 2011, 12 (3), 424-428. 5. Meister, M.; Amsden, J. J.; Howard, I. A.; Park, I.; Lee, C.; Yoon, D. Y.; Laquai, F., Parallel Pool Analysis of Transient Spectroscopy Reveals Origins of and Perspectives for ZnO Hybrid Solar Cell Performance Enhancement Using Semiconducting Surfactants. The Journal of Physical Chemistry Letters 2012, 2665-2670. 6. He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y., Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photon 2012, 6 (9), 593-597.
