ZnO nanorod growth and annealing
Large-scale ZnO nanorod arrays on ITO glass substrates were grown by a simple one-step electrodeposition method. A 1.4 × 1.4 cm2 ITO/glass substrate (Praezisions Glas & Optik, GmbH, Iserlahn, Germany; 180 nm ITO on float glass and sheet resistance of ca. 10 Ω/sq.) was used as the working electrode and a 4-cm2 Pt foil as the counter electrode. A 0.01 M Zn(NO3)2 solution was used. Growth was carried out galvanostatically at a constant current density of 0.15 mA cm−2 at 85°C.
The morphology of the electrodeposited ZnO depends on the solution concentration used [23
]. In order to obtain nanorods without the need of a seed layer, a solution of intermediate concentration (0.01 M Zn(NO3
) was used, thus simplifying the device fabrication.The arrays obtained with this concentration are sufficiently dense to ensure that short circuiting in the solar cell devices is prevented. During electrodepositon, a Zn(OH)2
film seed layer is initially obtained. Once this layer is formed, nanorods form by decomposition of this Zn(OH)2
with nucleation believed to occur after hydroxide dehydration [26
]. At the end of electrodeposition, a ZnO nanorod array is obtained, with Zn(OH)2
being present at the surface of the nanorods.
Post-annealing studies were performed on the as-deposited nanorod arrays at either 100°C, 200°C, 300°C, 400°C, or 500°C for 4 h with heating and cooling rates of 1°C min−1 and 3°C min−1, respectively.
The annealing atmosphere was either pure air or air saturated with Zn vapor (formed by wrapping Zn foil around the samples). Annealing in these different atmospheres was performed with the aim of differentiating between the influences of oxygen vacancies and Zn-related defects [27
] on BHJ cell performance. It is well known that Zn annealing can change either the Zn interstitial and/or Zn vacancy concentration, depending on the form of the starting ZnO material and hence the initial defect landscape and also on the presence of H and N impurities [16
To assess defect types and concentrations, PL measurements were performed at room temperature with an ACCENT RPM 2000 compound semiconductor PL system equipped with a Nd:YAG laser of wavelength 266 nm. The area under the visible band emission was calculated in order to estimate changes in the defect concentration.
IR measurements were undertaken to determine the information about the decomposition of Zn(OH)2 present on our nanorod samples. A Bruker 66v IFS spectrometer (Brookline, MA, USA) was used with a KBr beamsplitter, a Globar source, and a DTGS detector. The arrays were grown on quartz substrates onto which ITO was sputtered using a K575 Emitech sputter coater (Ashford, UK), and the samples were analyzed under vacuum. The data were recorded with an instrumental resolution of 2 cm−1 and 512 scans.
Electrical measurements of ZnO nanorods (on ITO on glass) were performed using a two-probe nanomanipulator retrofit inside a JEOL 6701F scanning electron microscope (Akishima, Tokyo, Japan). Current versus voltage curves were acquired by making a contact to the top of a ZnO nanorod with one of the probes, applying a bias between the probe and the substrate and measuring the current flowing through the rod. The current and voltage to the probes and the sample were independently measured and controlled using an Agilent B1500A semiconductor device analyser (Santa Clara, CA, USA). The resistances were determined for several rods at each temperature and the values averaged. For the calculation of the resistivities, a rod length of 800 nm was estimated from SEM images (the deviation from the average being around 5%).
Scanning electron microscopy images were taken using a LEO VP-1530 field emission scanning electron microscope (Peabody, MA, USA).
Photovoltaic cell processing
ZnO nanorod arrays were incorporated in inverted poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction cells. Prior to spin coating of the thin blend, the arrays were annealed in air in a tubular furnace as described above.
Solar cell measurements
Current density-voltage measurements of all devices were performed using a Keithley 2636 source meter (Cleveland, OH, USA) with a custom-made Lab-View program. A Newport Oriel class A solar simulator (Irvine, CA, USA) equipped with AM 1.5 G filters calibrated to a silicon reference diode was used at 100 mW cm−2 intensity. Several cells were studied.
Figure ,b,c shows the scanning electron micrograph (SEM) images of the ZnO nanorods produced. Uniform coverage of the ITO/glass substrate with the nanorod arrays was obtained. The nanorods are 80 to 130 nm in diameter and ca. 800 nm in length. Figure ,e shows cross-sectional images of the solar cell devices produced herein, which will be discussed later.
SEM and cross-sectional images. (a) to (c) SEM images of ZnO nanorod arrays deposited on bare ITO. (d) to (e) Cross-sectional images of ITO/ZnO/P3HT:PCBM/Ag devices.