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J. Phys. Chem. C 2007, 111, 7218-7223
Efficient Charge Collection with ZnO Nanorod Array in Hybrid Photovoltaic Devices Kazuko Takanezawa,† Kouske Hirota,†,‡ Qing-Shuo Wei,† Keisuke Tajima,*,† and Kazuhito Hashimoto*,† Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Electronic and Engineered Materials Laboratory, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan ReceiVed: February 19, 2007
Photovoltaic performance of the hybrid devices consisting of polymer/fullerene/ZnO nanorod array was studied. The dependence of the photovoltaic performance on the ZnO nanorod length and the organic layer thickness was investigated, and it is concluded that the ZnO nanorod array plays an important role in collecting photogenerated electrons and acts as a conducting path to the indium tin oxide electrode. Fill factor of the devices increased from 38% to 50% when the array of the ZnO nanorods was introduced, which directly contributed to the improvement of the power conversion efficiencies up to 2.7%. As the peak absorption of the device reaches >97% in a transmitting geometry, the results shown here give us insights toward designing the devices with efficient utilization of the incident light.
Introduction Organic photovoltaic devices are drawing much attention these days because of their potential for the production of flexible and large-area solar cells at dramatically low costs. The most common strategy is so-called bulk heterojunction, in which electron donors such as poly(3-hexylthiophene) (P3HT) and acceptors such as (6,6)-phenyl C61 butyric acid methyl ester (PCBM) are blended to form one mixed layer.1,2 In this system, charge separation of photoinduced excitons is greatly enhanced because of ultrafast electron transfer and large interface between the two components. One of the characteristics in this type of solar cell is that there is an optimal device thickness of typically 100-200 nm, depending on the combination of the materials. This optimal thickness is determined by a tradeoff relationship between absorption of the films and charge carrier transport in the device. It is evident that increasing thickness of the active layer can enhance the absorption and thus the photogeneration of carriers. However, when the thickness exceeds the optimal value, the overall power conversion efficiency starts to fall off. This has been attributed to an increasing series resistance, which is derived from increased charge recombination and/or space charge effects in the devices.3 This “thickness dilemma” could be solved by introducing nanostructures that help charge carrier collection and transport.4,5 One of the possible structures is a vertically oriented array of nanorods with inorganic n-type materials.6-8. Single crystalline nature of the nanorods could be advantageous for smooth electron transport after charge collection by “antennas” of the nanorods. As a result, it might be possible to achieve both large absorption and an efficient charge transport with thicker film devices. ZnO is an attractive material for nanorod formation, suitable to the application for such hybrid photovoltaic devices.9-12 Recently, Olson et al. demonstrated for the first time that hybrid photovoltaic devices * Corresponding authors. E-mail:
[email protected],
[email protected]. † The University of Tokyo. ‡ Mitsui Chemicals, Inc.
Figure 1. Schematic representation and energy diagram of P3HT: PCBM bulk heterojunction photovoltaic device with ZnO nanorod array.
could be fabricated using the combination of a semiconducting polymer and a ZnO nanorod array prepared by a solution process.9 They also reported the introduction of PCBM into the hybrid devices, which significantly improved the efficiency of the devices up to 2.03% under a simulated solar light. Nelson and Peiro´ et al. also reported the hybrid photovoltaic devices using the polymer/ZnO nanorod combinations.10,11 In these reports, however, the open circuit voltages of the devices (
