J Mater Sci: Mater Electron DOI 10.1007/s10854-007-9152-5
Copper phthalocyanine based Schottky diode solar cells Suresh Rajaputra Æ Subhash Vallurupalli Æ Vijay P. Singh
Received: 8 January 2007 / Accepted: 12 February 2007 Springer Science+Business Media, LLC 2007
Abstract Copper phthalocyanine (CuPc)/Aluminum (Al) Schottky diode solar cells were studied. The thickness of the CuPc layer was varied from 15 nm to 140 nm. Short circuit current densities (Jsc) increased with thickness from 0.042 mA/cm2 at 15 nm to 0.124 mA/cm2 at 120 nm reaching saturation at that level. Open circuit voltages (Voc) increased from 220 mV at 15 nm to 907 mV at 140 nm. Analysis of the current-voltage characteristics indicated that tunneling and interface recombination current mechanisms are important components of the current transport at the CuPc/Al junction.
1 Introduction Solar cells based on organic semiconductors are of interest because of their potential as flexible, lightweight and inexpensive devices. One of the promising devices [1–7], involves the heterojunction between copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). Earlier, we reported [1, 3] the highest Voc in a single organic heterojunction solar cell involving CuPc and PTCBI. A high open circuit voltage of 1.125 V was obtained in an ITO-PEDOT:PSS/CuPc/PTCBI/Al device structure with a thin PTCBI layer of 7 nm thickness. Results were interpreted in terms of a modified CuPc-Al Schottky diode for this thin PTCBI case [3]. Also, earlier,
S. Rajaputra S. Vallurupalli V. P. Singh (&) Center for Nanoscale Science and Engineering, Department of Electrical and Computer Engineering, University of Kentucky, 453 Anderson Hall, Lexington, KY 40506, USA e-mail:
[email protected]
Kwong et al. [8] reported a high Voc of 870 mV in a CuPc Shottky diode. To further elucidate the physical processes in our cells, a more detailed investigation of CuPc/Al interface was undertaken. Results are described in this paper.
2 Experimental procedures Glass/ITO substrates of sheet resistance 4–6 W/Square (Delta technologies) were sonicated in acetone, then methanol and then dried under nitrogen flow. A 100 nm thick buffer layer of PEDOT: PSS (Bayer) was spin-coated onto a clean ITO substrate at 2000 rpm and subsequently annealed in vacuum at 100 C for 30 min. Layers of CuPc (99.995% Aldrich) and Al electrode were deposited by vacuum evaporation. Devices with CuPc thickness varying from 15 nm to 140 nm were fabricated; device area was 0.07 cm2.
3 Results and discussion An illustrative scanning electron micrograph showing the morphology of the CuPc film of 100 nm thickness deposited on ITO/PEDOT:PSS is shown in Fig. 1; the grain sizes are in the 20 nm–30 nm range. The surface of this film and films of other thicknesses were all relatively smooth over large areas. Fig. 2 shows the x-ray diffraction pattern taken on the 100 nm thick CuPc film. The peak for the CuPc film is seen at the 2º positions of 6.92, corresponding to the a-form as observed also by Forrest et al. [9]. Additional peaks observed in the pattern were from the ITO substrate. Absorption spectra of CuPc films of varying thicknesses in the ultra-violet to visible (UV-Vis) wavelength range,
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J Mater Sci: Mater Electron Fig. 1 Scanning electron micrographs of a 100 nm thick CuPc film under, (a) 10 K and, (b) 40 K magnification
1600
Intensity (arb.units)
1400 1200 1000 800 600 400 200 0 10
20
30
40
50
60
70
2θ (degrees)
Fig. 2 X-ray diffraction pattern of a 100 nm thick Cu Pc film on ITO
a CuPc layer and an Al electrode is sketched in Fig. 4; note that ITO/PEDOT: PSS makes an ohmic contact to CuPc and the active junction is between CuPc and Al; electron affinity and ionization potential values used in this diagram were obtained from the literature [4, 10]. For the purpose of illustration, an upper bound value of the difference (x) between the Fermi level and the HOMO level of CuPc was calculated from the observed Voc value of 0.907 V. In Fig. 4, the junction barrier from the aluminum side (y), is given by, y = [(3.6 + 1.7 – x) – 4.06] eV, while the junction barrier from the CuPc side (qVbi), is given by, qVbi = y – x = [1.24 – 2x] eV. Further, requiring Vbi to exceed Voc, we must require that x be less than or equal to
shown in Fig. 3 are in conformity with the work in the literature [8]. The long wavelength peak of 700 nm corresponds to an energy gap of 1.7 eV which is the difference between the HOMO and the LUMO levels of CuPc [4]. Energy level diagram [3] of a Schottky diode consisting of
Fig. 3 Absorption spectra of CuPc films of varying thicknesses in the ultra-violet and visible (UV-Vis) wavelength ranges
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Fig. 4 Sketch of the energy level diagram, in equilibrium, of a CuPcAl Schottky diode. For this sketch, which is not drawn to scale, device with 140 nm thick CuPc was used as an example
8.0x10
-3
6.0x10
-3
4.0x10
-3
2.0x10
-3
-1.8
-1.5
-1.2 -0.9
0.0 -0.6 -0.3 0.0 -2.0x10
Table 1 Device parameters for CuPc Schottky diodes under dark conditions
15nm 60nm 80nm 100nm 120nm 140nm
2
J (Amps/cm )
J Mater Sci: Mater Electron
0.6
0.3
Thickness Series Resistance (Rs) Ideality factor (n) Jo (mA/cm2)
0.9
1.2
1.5
1.8
V( volts)
-3
2
J(Amps/cm )
Fig. 5 I–V curves of ITO/PEDOT:PSS/CuPc/Al devices with varying thicknesses under dark conditions
3.0x10
-4
2.0x10
-4
1.0x10
-4
15 nm 60 nm 80 nm 100 nm 120 nm 140 nm
0.0 -0.4
-0.2
0.0
-1.0x10
-4
-2.0x10
-4
-3.0x10
-4
0.2
0.4
0.6
0.8
1.0
1.2
V (volts)
Fig. 6 I–V curves of ITO/PEDOT:PSS/CuPc/Al devices with varying thicknesses under one sun illumination
{[1.24 – qVoc]/2} eV. Thus, substituting 0.907 V for Voc, we get 0.16 eV as an upper bound value for x. The current-voltage (I–V) characteristics of ITO/PEDOT:PSS/CuPc/Al solar cell devices of varying CuPc thicknesses are shown in Figs. 5 and 6 and in Tables 1 and 2, under dark and ‘‘one sun’’ illumination conditions respectively. Voc increased with increase in CuPc layer thickness reaching a value of 907 mV for a CuPc layer thickness of 140 nm. The overall low values of Jsc are attributed [3] to exciton diffusion length problems in organic semiconductors and to the series resistance of a thin aluminum oxide layer in these devices. As for the variation in short circuit current density with thickness, we observed a rapid increase initially but as the thickness of the CuPc layer reached 120 nm, saturation set in. This saturation is attributed to the inability of the optically Table 2 Device parameters for CuPc Schottky diode solar cells under ‘‘one sun’’ illumination
Rs
15 nm
6.87 kW/cm2
60 nm
2
100 nm
7.10 kW/cm2 2
7.12 kW/cm
2
8.24 kW/cm
2
8.57 kW/cm
7.7
0.121
60 nm
7.86 kW/cm
18.03
0.127
80 nm
8.45 kW/cm2
17.5
0.135
100 nm
8.31 kW/cm2
17.78
0.149
120 nm
9.32 kW/cm2
15.95
0.142
140 nm
9.41 kW/cm2
18.29
0.126
generated excitons to reach the CuPc/Al interface when the CuPc film became too thick. I–V curves were analyzed for extracting the effective values of the diode ideality factor (n) and the reverse saturation current density (J0). These and other important solar cell parameters are tabulated in Tables 1 and 2. In our CuPc/Al cells, the measured value of n is larger than two and therefore tunneling, recombination-generation currents in the depletion region and recombination through interface states at the CuPc/Al junction are expected to play important roles [11]. Also, we see from Tables 1 and 2 that In CuPc devices under study, variations in effective values of n and Jo with thickness are relatively small (less than 40%) while the value of Jsc varies over a much larger range (factor of three). Thus it appears that as the thickness of CuPc layer is increased, the junction transport mechanism (tunneling etc.) is not much altered but the light generated current is. It is likely therefore that the enhancement in the Voc with thickness is primarily due to enhanced Jsc and not due to reduced J0. In other words, the Voc in these solar cells is not being limited by some fundamental junction transport factor and reasonably high Voc values (0.907 V for example) are achievable.
4 Conclusions CuPc/Al Schottky diode solar cells exhibited higher Voc values as the CuPc layer thickness was increased, reaching a value of 907 mV at a CuPc thickness of 140 nm. Jsc, on the other hand saturated at a CuPc thickness of 120 nm. Diode ideality factor values (n) ranged from 7.7 to 18.2.
n
Thickness
80 nm
15 nm
Jo
Voc
Jsc
7.66
0.147 mA/cm2
220 mV
0.042 mA/cm2
19.30
0.110 mA/cm
2
360 mV
0.054 mA/cm2
0.127 mA/cm
2
584 mV
0.094 mA/cm2
0.139 mA/cm
2
770 mV
0.114 mA/cm2
2
879 mV
0.124 mA/cm2
907 mV
0.125 mA/cm2
16.08 17.62
120 nm
2
9.11 kW/cm
16.50
0.118 mA/cm
140 nm
8.97 kW/cm2
16.08
0.109 mA/cm2
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Such high values of n are indicative of the importance of tunneling and interface recombination mechanisms for the current transport at the CuPc/Al junction. Acknowledgements This work was supported in part by a grant from Kentucky Science & Technology Council Inc. (Grant # KSEF 148-502-02-27).
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3. V.P. Singh, B. Parthasarathy, R.S. Singh, A. Aguilera, J. Anthony, M. Payne, Sol. Energy. Mater. Sol. Cells 90, 798 (2006) 4. P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93, 3693 (2003) 5. C.W. Tang, Appl. Phys. Lett. 48, 183 (1986) 6. P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76, 2650 (2000) 7. A. Yakimov, S.R. Forrest, Appl. Phys. Lett. 80, 1667 (2001) 8. C.Y. Kwong, A.B. Djurisic, P.C. Chui, L.S.M. Lam, W.K. Chan, Appl. Phy A Mater Sci Process A77, 555 (2003) 9. J. Xue, B.P. Rand, S. Uchida, S.R. Forrest, Adv. Mater. 17, 66 (2005) 10. I.G. Hill, J. Schwartz, A. Kahn, Org. Electronics 1, 5 (2000) 11. V.P. Singh, J.C. McClure, Sol. Energy. Mater. Sol. Cells 76, 369 (2003)
