Characterization of Photocurrent Generation Dynamics in Polymer Solar Cells Based on ZnO/CdS-Core/Shell Nanoarrays by Intensity Modulated Photocurrent Spectroscopy: Theoretical Modeling

A theoretical model is developed for the dynamic characterization of hybrid polymer-based solar cells (HPSCs) based on vertically aligned ZnO/CdS-core/shell nanorod arrays (ZC-NAs) by intensity modulated photocurrent spectroscopy (IMPS). The model describes the effects of CdS shell formation on charge generation and transport dynamics. Particularly, an analytical expression for the ineffective polymer phase model in nanoarray solar cells is developed and introduced into IMPS model for the first time. The main expectations of the IMPS model are confirmed by the experimental data of the polymer/ZC-NA cells with the CdS shell thickness (<i>L</i>) of 3–8 nm. It is shown that the contributions from CdS absorption (<i>f</i>₁) and polymer absorption (<i>f</i>₂) to charge generation are determined by the core/shell nanoarray structure and the intrinsic polymer property, while the optimal CdS shell thickness (<i>L</i>_opt) depends on the interspacing between ZnO core nanorods and the exciton diffusion length of the polymer. The photocurrent generation is dominantly the competitive results of <i>f</i>₁ and <i>f</i>₂ contributions subjected to the change in <i>L</i>, with the polymer as a dominant absorption material. Fittings of the measured IMPS responses to the IMPS model reveal that the <i>L</i>-dependence of photocurrent generation dominantly originates from <i>f</i>₁, <i>f</i>₂, and the polymer exciton dissociation rate <i>S</i> at the polymer/CdS interface. Moreover, the first-order rate constants for the surface defects to trap and detrap the injected electrons in ZnO core nanorods are found to decrease with CdS shell growth and become saturated at <i>L</i>_opt. Furthermore, it is demonstrated that the effective electron diffusion coefficient <i>D</i>_e in the ZnO nanorods reaches a peak value at <i>L</i>_opt as the result of the largest photogenerated electron density in conduction band. Those results provide a guide to the design of efficient HPSCs based on the core/shell nanoarrays with complementary properties

Performance Improvement in Polymer/ZnO Nanoarray Hybrid Solar Cells by Formation of ZnO/CdS-Core/Shell Heterostructures

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 (<i>L</i>) 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 (<i>V</i>_oc) and short-circuit current (<i>J</i>_sc) in MEH-PPV/ZnO-CdS-NA solar cells were dramatically improved depending on <i>L</i>, resulting in a peak efficiency of ca. 1.23% under AM 1.5 illumination (100 mW/cm²) with a 7-fold increment for <i>L</i> = 6 nm. In particular, the experimental <i>L</i>-dependence of <i>J</i>_sc agreed with the expectation from the proposed model and the <i>V</i>_oc 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 <i>J</i>_sc 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 <i>V</i>_oc 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