Organic-Inorganic Hybrid Solar Cells Based on ZnO Nanorods and Conjugated Polymer
Date Issued
2009
Date
2009
Author(s)
Chou, Chen-Yu
Abstract
Solar cells attract great attention owing to the growing need for renewable energy. Polymer solar cells offer the potential for large-scale power generation based on materials that provide flexibility, light weight, low-cost production and low-temperature fabrication. The most common and efficient material system for polymer devices is thus so far the one consisting of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). However, the conventional bulk-heterojunction (BHJ) architecture has limitations in device stability because of the acidic, hygroscopic nature of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and oxidation of the Al electrode. Moreover, there exist problems of inherently poor polymer properties, such as the short exciton diffusion length and the relatively low carrier mobility, which limit the usefulness of thick film. A possible approach to overcome the above difficulties is to use inverted configuration with ZnO nanorod arrays. The inverted structure has the advantage of improved stability by replacing the low work function metal cathode and PEDOT:PSS. The ZnO nanorods provide the possibility of thick active layer for light harvesting without sacrificing the charge transport due to their excellent electron mobility and the long diffusion length. Moreover, ZnO nanorod arrays using solution processing and Ag anode allowing the use of the nonvacuum printing technique provide a route for printed solar cells. The aim of this work is to realize a low-cost and high-efficiency inverted polymer solar cells hybridized with ZnO nanorod arrays by the use of optimized protocols and the introduction of solution-processed interlayer. Our investigation shows that the annealing-free approach, slow drying, improves device performance where thermal annealing is either ineffective or undesirable. As the polymer solidification time is lengthened by lowering the spin-coating rate of the photoactive layer, the photoactive layer becomes thickened, and the polymer chains have enough time to self-organize and effectively infiltrate into ZnO nanorod spacing. While the thickness of the photoactive layer is increased to 400 nm accompanying self-organized polymer, the power conversion efficiency of the device is improved to 3.58% with an enhanced fill factor of 58%. The 400 nm film is composed of the highly ordered polymer and the ZnO nanorod arrays, resulting in increased light harvesting without decreasing the possibility for charge transport. On the other hand, the power conversion efficiency is improved to 3% by the introduction of the solution-processed V2O5 interlayer due to the efficient suppression of the leakage currents at the organic/metal interface. The devices in this study are fabricated not only in an atmosphere environment due to no need of thermal annealing but also without the use of vacuum-deposited interlayer. These are very important for commercial realization of low-cost and large-area printed solar cells.
Subjects
Polymer Solar Cells
ZnO Nanorod
Inverted Structure
Slow Drying
Solution Processing
Interlayer
SDGs
Type
thesis
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