We report significant improvement in both the power conversion efficiency and the environmental stability of solution-processed hybrid organic-inorganic solar cells by including a solution-processed ZnO nanocrystal layer between the photoactive layer and the cathode. For devices based on blends of poly(3-hexylthiophene) (P3HT) and mostly-spherical CdSe nanocrystals, incorporation of the ZnO layer leads to an up to 70% increase in the power conversion efficiency. Compared to only a few hours of shelf lifetime for unencapsulated devices with the metal cathode directly deposited on the hybrid active layer, devices with the ZnO layer can retain approximately 70% of the original efficiency when they are exposed to the laboratory ambient without encapsulation for more than two months. We attribute the function of this ZnO nanocrystal layer to a combination of optical, electronic, morphological, and chemical effects, including blocking leakage of photogenerated holes to the cathode, optimizing the optical intensity profile in the hybrid active layer, minimizing recombination or quenching of photogenerated excitons and charge carriers, significantly reducing the transport rate of oxygen and water molecules to the active layer and reducing degradation/oxidation of any low work function layer at the cathode interface.
Bibliographical noteFunding Information:
This work was supported in part by the US Department of Energy Solar Energy Technologies Program (Grant DE-FG36-08GO18020), Army Research Office Grant W911NF-07-1-0545), and the CAREER Program of the National Science Foundation (ECCS 0644690). Y.Z. and J.X. also acknowledge helps from Professor Qiwen Zhan at the University of Dayton for ellipsometry measurements. A.T. is grateful to the Chinese Scholarship Council for financial support. Assistance in data collection and reduction by the Major Analytical Instrumentation Center, particularly by Eric Lambers and Kerry Siebein, is gratefully acknowledged.