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2009, 113, 17608–17612 Published on Web 09/18/2009
Improved Efficiency in Poly(3-hexylthiophene)/Zinc Oxide Solar Cells via Lithium Incorporation Matthew T. Lloyd,* Yun-Ju Lee, Robert J. Davis, Erica Fang, Robert M. Fleming, and Julia W. P. Hsu Sandia National Laboratories, Albuquerque, New Mexico 87185
R. Joseph Kline National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Michael F. Toney Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025 ReceiVed: August 11, 2009; ReVised Manuscript ReceiVed: September 8, 2009
The efficiency of poly(3-hexylthiophene)/zinc oxide planar heterojunction solar cells is on average raised by a factor of 2.9 by incorporating lithium during the sol-gel processing of the metal oxide layer. Studies performed on over 300 diodes show systematic increases in both open-circuit voltage (Voc) and short-circuit current (Jsc) up to an optimum Li concentration between 15 and 20 atom %. Compared to pure ZnO devices, the incorporation of lithium improves the Voc and Jsc by an average of 42 and 90%, respectively. For the best device, the efficiency increases by a factor of 7.5, yielding a power conversion efficiency of 0.44%, which is approaching the efficiency of the state-of-the-art nanostructured hybrid solar cells. Enhancements in Voc are attributed to a larger donor-acceptor energy band edge offset due to a rigid shift of Zn1-xLixO energy levels toward to the vacuum level with Li incorporation. Jsc improvements arise from a higher surface roughness with lithium incorporation and from better P3HT ordering at the heterojunction interface. Polymer-metal oxide or hybrid photovoltaic devices have received considerable attention in recent years due to the potential for scalable, low-cost manufacturing. Low-temperature processing (
