Performance Improvement in Polymer/ZnO Nanoarray Hybrid Solar Cells by Formation of ZnO/CdS-Core/Shell Heterostructures
journal contributionposted on 21.03.2013, 00:00 by Qi Cui, Changwen Liu, Fan Wu, Wenjin Yue, Zeliang Qiu, Hui Zhang, Feng Gao, Wei Shen, Mingtai Wang
In this paper, performance in hybrid solar cells based on ZnO nanorod array (ZnO-NA) is significantly improved by formation of a heterostructured ZnO/CdS-core/shell nanorod array (ZnO-CdS-NA), the CdS shell effects on device performance including charge transport and recombination dynamics are discussed, and a model concerning ineffective polymer phase is proposed for understanding the charge generation upon CdS shell formation. The ZnO-CdS-NAs with varied CdS shell thickness (L) were prepared by depositing CdS quantum dots on the ZnO nanorods in the ZnO-NA. Solar cells were prepared by filling the interspaces between the nanorods in ZnO-NA or ZnO-CdS-NAs with poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Compared to MEH-PPV/ZnO-NA devices, both open-circuit voltage (Voc) and short-circuit current (Jsc) in MEH-PPV/ZnO-CdS-NA solar cells were dramatically improved depending on L, resulting in a peak efficiency of ca. 1.23% under AM 1.5 illumination (100 mW/cm2) with a 7-fold increment for L = 6 nm. In particular, the experimental L-dependence of Jsc agreed with the expectation from the proposed model and the Voc was improved from ca. 0.4 V for ZnO-NA up to around 0.8 V. Results demonstrate that in the MEH-PPV/ZnO-CdS-NA devices, the Jsc correlates mainly with the charge generation subjected to the exciton generation altered by CdS shell formation, in which the polymer absorption is dominantly contributive; however, the Voc is determined by the energy difference between the highest occupied molecular orbital level of MEH-PPV and the conduction band edge of ZnO but significantly correlates with the quasi-Fermi levels of the electrons in ZnO nanorods.