ZnO Nanostructures as Efficient Antireflection Layers in Solar Cells Yun-Ju Lee,* Douglas S. Ruby, David W. Peters, Bonnie B. McKenzie, and Julia W. P. Hsu Sandia National Laboratories, Albuquerque, New Mexico 87185 Received March 4, 2008; Revised Manuscript Received March 26, 2008 ABSTRACT An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution- grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering. Antireflection coatings (ARCs) play a major role in enhanc- ing the efficiency of photovoltaic (PV) devices by increasing light coupling into the active region of the devices. On lithographically patterned Si PV devices, various groups have fabricated surface textured ARCs by anisotropic etching, 1 etching through patterned masks, 2–5 or via other techniques that generate porosity and/or roughness. 6–8 The textured surface traps light, leading to a broadband suppression in reflection. For thin film PV devices (e.g., amorphous Si, CdTe, CdInGaSe 2 ), ARCs generally consist of one or more dielectric layers, either in the form of a quarter wave thickness film that exhibits a wavelength sensitive reduction in reflection due to interference 9–15 or as a nanoporous film that takes advantage of light trapping for a more broadband response. 16,17 Recently, Xi et al. utilized oblique-angle deposition at different angles to create five layers of TiO 2 and SiO 2 nanorods with an optimized overall refractive index gradient, and achieved an extremely low reflectance. 18 In their case, discontinuous refraction index changes between layers with different volume fractions of dielectrics created the desired refractive index profile. In this communication, we report the effects of highly textured ZnO nanorod arrays (NRAs), synthesized via low temperature solution growth, on ARC performance. By changing the growth conditions, we modify the shape of the ZnO nanorod tips, leading to continuously varying refractive index profiles in a single layer. Subtle changes in the nanorod tip shape result in significantly improved antireflection properties, in good agreement with predictions from rigorous coupled wave analysis (RCWA). Because our approach is substrate inde- pendent, these textured ZnO ARCs may be applicable to a wide variety of PV devices and other antireflection applica- tions. ZnO is attractive as a dielectric ARC material because of its good transparency, appropriate refractive index (n ) 2), and ability to form textured coating via anisotropic growth. For example, textured ZnO films deposited on Si via metal organic chemical vapor deposition (MOCVD) demonstrated superior ARC performance compared with a TiO 2 single layer ARC (SLARC). 12 We synthesized ZnO NRA ARCs using a two-step seeding and growth method on n-type Si(100) (Allied Bendix). 19,20 The seeding process was carried out at room temperature at 35% relative humidity, the optimal condition for producing ordered NRAs. 20 Seeded substrates were placed in aqueous solutions containing zinc nitrate (Fisher) and hexamethylene tetraamine (HMT, Fisher) at 92.5 or 60 °C. Several concentrations of 1,3-diaminopropane (DAP, Acros) were also added to control the tapering of nanorods; see Supporting Information for the full list of ZnO NRAs and their synthesis conditions. 21,22 An example of a ZnO NRA in cross section imaged with a scanning electron microscope (SEM) is shown in Figure 1a. For comparison, a 0.55 μm thick porous ZnO film was also deposited on Si via multiple steps of spin coating a sol of 2 M zinc acetate (Aldrich) and 2 M ethanolamine (Aldrich) in 2-methoxy- ethanol (Aldrich) at 2000 rpm followed by heating on a hot plate at 300 °C for 5 min. 23 * Corresponding author. E-mail: ylee@sandia.gov. NANO LETTERS 2008 Vol. 8, No. 5 1501-1505 10.1021/nl080659j CCC: $40.75  2008 American Chemical Society Published on Web 04/17/2008