CNMS RESEARCH

PS-b-P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

Zhenzhong Sun1, Kai Xiao2, Jong Kahk Keum3, Xiang Yu2, Kunlun Hong1, Jim Browning3, Ilia Ivanov1, Jihua Chen2, Jose Alonzo3, Dawen Li1, Bobby Sumpter2, Andrew Payzant2, Christopher Rouleau2, and David Geohegan2

1-Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, AL
2-Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN
3-Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN

Achievement

This work reports that small amounts (5%) of a tailored diblock copolymer can serve as a compatibilizer to effectively improve the power-conversion efficiency of photovoltaic cells involving blends of polythiophene (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). At optimal concentrations the diblock copolymer (PS-b-P3HT), which includes an insulating polystyrene (PS) block, was found to induce a highly favorable active layer morphology with interpenetrating nanoscale domains, thus enhancing the overall crystallinity and orientation of the P3HT conducting polymer, and facilitating hole transport within the active layer for higher efficiencies. Neutron reflectivity was used to characterize the phase separation in the polymer blends, and quantum density functional theory calculations were used to understand that the PS-b-P3HT diblock copolymer controlled the phase separation of the P3HT and PCBM through strong interactions between the PCBM and the PS block, while the P3HT block induced alignment of the polythiophene.

Significance

This work provides fundamental understanding of a practical pathway to simultaneously improve the crystallinity and nanoscale phase segregation of conducting polymer blends used in polymer-based organic photovoltaics. Understanding how nanoscale morphology and crystallinity affect charge transport and balance in organic electronics is crucial to achieve higher efficiencies required for their practical utilization in solar cells, lighting, and flexible electronics.

Credit

This work was published in Advanced Materials, November 18, 2011 DOI: 10.1002/adma.201103361. The research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. DL and ZS acknowledge partial support provided from the University of Alabama. JA, JC, and BGS acknowledge partial support provided DOE Laboratory Directed Research and Development (LDRD) award (#5388). Neutron reflectometry measurements were performed on the liquids reflectometer at the Spallation Neutron Source, Oak Ridge National Laboratory which is also sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. Computations were performed at the National Center for Computational Sciences.

Citation for Highlight

Zhenzhong Sun, Kai Xiao, Xiang Yu, Kunlun Hong, Jong Kahk Keum, Ilia Ivanov, Jihua Chen, Dawen Li, Bobby Sumpter, Andrew Payzant, Christopher Rouleau, and David Geohegan, “PS-b-P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics,” Advanced Materials (published online Nov. 18, 2011).