8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
High Rectification Ratio in Nanostructured Organic-Inorganic Photodiodes
Santanu Karan and Biswanath Mallik*
Department of Spectroscopy, Indian Association for the Cultivation of Science,
2A & 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata-700 032, India.
High quality organic-Inorganic heterojunction photodiodes based on nanostructured copper (II) phthalocyanine (CuPc) and intrinsic zinc oxide (i-ZnO) have been fabricated. The i-ZnO thin films/ layers were grown by RF magnetron sputtering on clean ITO coated glass substrate. These films have been characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). CuPc thin films deposited at room temperature on i-ZnO have exhibited change in their surface morphology with the post-deposition annealing temperature under normal atmosphere. The electrical dark and photoconductivity of ITO/i-ZnO/CuPc/Au sandwich structure have been measured under various photoexcitation intensities using Xenon light source. The devices have shown excellent reproducibility of their electrical characteristics and high rectification ratio. The rectification ratio is nearly 831 calculated above the threshold voltage at room temperature. The effects of annealing temperature on the surface morphology and rectification ratio have been discussed. -1.5-1.0-0.50.00.51.01.50246810111315171678 0.00.51.01.50.00.51.01.52.0876541Total Current (
mA/cm2 )
Biased Voltage (Volt)Total Current (mA/cm2 ) Biased Voltage (Volt)
(a)
(b)
(c)
(d)
Fig. 1 (a) FESEM images of the CuPc thin filmdeposited at room temperature on i-ZnOsurface. Inset shows the morphology of i-ZnOsurface deposited on ITO. FESEM images of theCuPc thin film annealed at 100 (b), 200 (c), and250 oC (d), respectively. Insets show the surfaceat higher magnification.
Fig.2 The I-V characteristics for the devices under dark and different intensity of photoexcitation: The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. The inset refers to the magnified view in forward
After deposition of CuPc thin films on i-ZnO layer at room temperature, the deposited thin films were annealed at various temperatures. The FESEM images of the CuPc thin films annealed at different temperatures are shown in Figure 1. Figure 1a shows the FESEM image of as deposited CuPc film at room temperature. Most of the particles are almost spherical in shape and the average size is nearly 30-40 nm. The inset of Figure 1a shows the morphology of i-ZnO deposited on ITO. Figure 1b shows the surface morphology of the CuPc film annealed at 100 oC. Clearly, the aggregation of the CuPc nanoparticles is observed. Figure 1c shows the surface morphology of the CuPc film annealed at 200 oC. Here the film has been found to be formed uniformly with aggregated nanoparticles and some nanorod like structure. Figure 1d represents comparatively smoother surface for a CuPc film annealed at 250 oC. The surface shows the compactness of the nanorod like structures. Inset of Figure 1d shows the nanorod composed of small nanoparticles of size below 10 nm. The high tendency of the self-ordering of phthalocyanine molecules could be one of the main reasons for the change in film morphology depending on the annealing condition. The I-V characteristics for Pc 250 devices under different intensity of photoexcitation are shown in Figure 2. From this figure it has been found that all the devices responded to photoexcitation and gave rise to photocurrent. The RR under different intensity of photoexcitation has been calculated from Figure 2 and is listed in Table 1.
RR for the Devices
Intensity (mW/cm2 )
Pc 30
Pc 100
Pc 200
Pc 250
0
5
6.16
7.51
14.37
44.03
1.28
2.90
831.54
5.27
10
5.01
36.10
2.63
9.42
15
5.99
29.48
2.55
12.90
30
7.17
27.28
3.11
30.37
45
7.78
25.28
3.41
54.01
70
8.35
23.34
3.50
126.37
100
11.73
21.24
3.12
356.23 -1.6-1.2-0.8-0.40.0-32-24-16-8087654321Total Current (
μA/cm2 )
Biased Voltage (Volt)
TABLE 1: Rectification Ratio (RR)Calculated above the Threshold Voltage ofthe Devices under Different Intensity ofPhotoexcitation.
Fig. 3 Reverse biased I-V characteristics in dark and under photoexcitation of different intensities for the device Pc 250. The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. Inset shows the photovoltaic performance of the devices.
Typical I-V characteristics of the photodetector at reverse bias from the devices, under 5-100 mW/cm2 photoexcitation range are shown in Figure 3. I–V response under dark condition is also shown in the figure. The plot shows that upon increasing intensity of photoexcitation, the amplitude of device current at any voltage increases. At higher photoexcitation intensities, in addition to more photo generated excitons and hence more dissociated carriers, higher carrier mobility due to increase in the density of carriers may be the cause of photocurrent. At any particular photoexcitation intensity, the magnitude
of increase is also more at higher bias amplitudes. Field dependent dissociation of excitons at the interfaces can result in higher photocurrent at higher bias amplitudes. Higher bias may also pull out more carriers out of the device and leaves space for more exciton dissociation. At lower bias, charge confinement may hinder exciton dissociation.
In conclusion, for the growth of CuPc films on i-ZnO by vacuum evaporation, surface morphology of the thin films are influenced very strongly on the post-deposition annealing temperature. Good rectification and photosensitivity was observed for the devices. Higher rectification ratio was observed for the device having CuPc layer annealed at 250 oC. For the device having CuPc layer annealed at 200 oC the value of ISC is higher and RR is very small. Above the annealing temperature of 200 oC, i.e. when β-phase occurs, the value of ISC again becomes less and RR becomes higher. The results regarding effects of annealing temperature on CuPc deposited on i-ZnO may be extended to structures involving the growth of multilayer photodiodes.
Corresponing author: spbm@mahendra.iacs.res.in;
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