Improved Efficiency in Poly(3-hexylthiophene)/Zinc Oxide Solar Cells via Lithium Incorporation

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 ( V oc ) and short-circuit current ( J sc ) up to an optimum Li concentration between 15 and 20 atom %. Compared to pure ZnO devices, the incorporation of lithium improves the V oc and J sc 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 V oc are attributed to a larger donor−acceptor energy band edge offset due to a rigid shift of Zn 1− x Li x O energy levels toward to the vacuum level with Li incorporation. J sc improvements arise from a higher surface roughness with lithium incorporation and from better P3HT ordering at the heterojunction interface.