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J. Phys. Chem. C 2009, 113, 15778–15782
Optimization of ZnO Nanorod Array Morphology for Hybrid Photovoltaic Devices Yun-Ju Lee,* Matthew T. Lloyd,† Dana C. Olson,† Robert K. Grubbs, Ping Lu, Robert J. Davis, James A. Voigt, and Julia W. P. Hsu‡ Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185 ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: July 9, 2009
Hybrid inorganic oxide/conjugated polymer photovoltaic devices using ZnO nanorod arrays (NRAs) instead of planar films as the electron-transport layer exhibit significant improvements in performance that have been attributed to increased heterojunction surface area, although the relationship has not been quantitatively established. Here, we independently measure the surface area of ZnO NRAs and quantify its effect on the performance of ZnO NRA/poly(3-hexylthiophene) (P3HT) photovoltaic devices. We find that a device utilizing a vertically aligned 180 nm ZnO NRA exhibits an ∼2.7× enhancement in the short-circuit current compared with that of a bilayer device, in excellent agreement with the increase in surface area. In addition, we show that a subtle difference in the NRA morphology can impact P3HT crystallinity in the photoactive region. Improved P3HT crystallinity leads to an ∼25% enhancement in the short-circuit current for devices with the same surface area. On the basis of these findings, we modify the NRA growth to introduce more spacing between nanorods and create a ZnO NRA/P3HT device with a high short-circuit current density of 2.91 mA/cm2. These results indicate that, although increased surface area is the most important factor to improving photocurrent and efficiency, other factors, such as ZnO NRA morphology and P3HT crystallinity, also impact the performance of ZnO/P3HT photovoltaic devices. Introduction Hybrid polymer-nanostructured oxide photovoltaic (PV) devices utilizing electron transfer from a conjugated polymer to a nanostructured oxide with controlled morphologies represent a promising route toward low-cost, large surface area energy generation.1 The power conversion efficiency of nanostructured PV devices may be enhanced over that of planar bilayer devices in several ways. First, infiltration of a conjugated polymer into a nanostructured oxide leads to a higher interfacial area, which, in turn, may increase the number of carriers extracted. Second, because oxides are more air stable than organic electron acceptors, such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), hybrid devices utilizing oxides as electron acceptors2,3 or electron-transport layers can exhibit greater stability in performance over time.4-6 Of the oxide materials, ZnO is especially attractive because of the ease of synthesizing crystalline,highsurfaceareananorodarrays(NRAs)atlowtemperatures.7-13 Using ZnO NRAs, various groups have fabricated dye-sensitized solar cells14-16 and hybrid PV devices,2,3,17,18 which generally exhibit a significantly enhanced short-circuit current density Jsc compared with that of bilayer devices. Although it is believed that the enhanced performance comes from increased heterojunction areas, the precise relationship between oxide acceptor morphology and device current density-voltage (J-V) response has not been quantitatively studied. Here, we systematically examine the relationship between ZnO NRA morphology, heterojunction interfacial area, P3HT crystallinity, and PV device performance. By varying seeding and growth parameters, we fabricated ZnO NRAs of different lengths on glass substrates. We * To whom correspondence should be addressed. E-mail:
[email protected]. † Present address: National Renewable Energy Laboratory, Golden, Colorado. ‡ Also at the Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico.
deposited a conformal coating of TiO2 on the ZnO NRAs using atomic layer deposition (ALD)19 and measured the TiO2 absorbance in the UV-visible spectra to determine the surface area enhancement of ZnO NRAs. We then examined the J-V response of corresponding ZnO NRA/P3HT PV devices and found a correlation between NRA surface area and Jsc, suggesting that increased interfacial area in NRAs generally improves performance. However, for ZnO NRAs with similar surface areas, we found that devices made from NRAs grown on thick (25 nm) sol-gel ZnO seed layers consistently exhibit a 25% higher Jsc compared with that of devices made from NRAs on thin (
