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Spectroscopic and microscopic studies of thermally treated Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine thin films
Physica E 44 (2012) 1815–1819
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Spectroscopic and microscopic studies of thermally treated Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine thin films Fakhra Aziz a,b,c, M.H. Sayyad b, Zubair Ahmad a,n, K. Sulaiman a, M.R. Muhammad a, Kh.S. Karimov b a
Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi-23640, Pakistan c Department of Electronics, Jinnah College for Women, University of Peshawar, Peshawar 25120, Pakistan b
H I G H L I G H T S c
c
c
G R A P H I C A L
A B S T R A C T
Optical, morphological and structural properties of VOPcPhO thin films. These properties are significantly influenced by the post-deposition thermal annealing. VOPcPhO has potential for applications such as organic solar cell and photodiode.
a r t i c l e i n f o
abstract
Article history: Received 5 December 2011 Received in revised form 11 April 2012 Accepted 27 April 2012 Available online 11 May 2012
This paper reports the structure, morphology and optical properties of Vanadyl 2,9,16,23-tetraphenoxy29H,31H-phthalocyanine (VOPcPhO) pristine and annealed thin films for photo-devices application. The VOPcPhO thin films have been prepared by spin-coating technique on glass substrates using VOPcPhO solution in chloroform. The UV/vis absorption spectra are used to study the optical properties while atomic force microscopy (AFM) is used to investigate the surface morphology and structure of the thin films. The AFM results show that the surface roughness increases with increase in annealing temperature making the annealed films more prone to high absorption rather than the pristine sample. The results also demonstrate that the thermal annealing processes on the VOPcPhO thin films significantly enhance the features of light absorption and surface morphology. Hence, VOPcPhO can be used in the organic solar cell or photodiode, where such features are of prime importance. & 2012 Elsevier B.V. All rights reserved.
1. Introduction Research on the emerging field of organic materials has extended the capabilities and possibilities of modern electronic devices into unforeseen domain. The motivation of using organic materials in electronic devices arose from their easily tunable electronic and processing properties [1]. Among the organic semiconductors metal phthalocyanines (MPcs) have been widely
n
Corresponding author. Tel.: þ60 16 9582584; fax: þ603 79674146. E-mail address: [email protected] (Z. Ahmad).
1386-9477/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2012.04.025
studied due to their noteworthy chemical and physical properties. The phthalocyanines exhibit very intense absorption in the UV/vis spectral region which make them suitable for the optoelectronic applications [2,3]. The use of VOPcPhO for photovoltaic applications is particularly attractive due to its high solubility in a variety of organic solvents. The photovoltaic response demonstrated for single-layer VOPcPhO based solar cell has shown its potential for fabricating simple, easy and inexpensive devices. Recently, many authors [4–8] reported that the thermal annealing significantly improves the structural, morphological, electrical and optical properties of the phthalocyanines. Thermal annealing refines the structure of materials by making them
F. Aziz et al. / Physica E 44 (2012) 1815–1819
2. Experimental 2.1. Sample preparation VOPcPhO is a macro-cyclic compound which occurs in the form of a green color dye. The structure of VOPcPhO is formed by four isoindole units surrounding the metal atom in the center [10]. The switching of various functional groups in the boundary and different metal atoms in the center can change the properties of the phthalocyanines [11]. The VOPcPhO has a Vanadyl as a central metal atom in core of the macro-cycle. VOPcPhO was purchased from Sigma–Aldrich and used as received. Fig. 1 shows a schematic diagram representing the molecular structure of VOPcPhO. The VOPcPhO was dissolved in chloroform to yield solution of concentration 30 mg/ml, which was used to deposit the films onto the glass substrates. Prior to spin coating, the glass substrates were cleaned by ultrasonic treatment in detergent for 20 min. Later the substrates were rinsed with deionized water, acetone and ethanol sequentially. Finally, the glass substrates were dried up by blowing nitrogen gas. Thin films of VOPcPhO were produced by spin coating the solution on the glass substrates in the presence of nitrogen. The speed of the spin-coater was fixed at 3000 rpm and the time was 20 s to produce VOPcPhO thin films with the thickness of 1407 5 nm. Immediately after spin-coating, the deposited films were subjected to post-annealing procedure. The annealing treatment has been carried out for three samples on hot plates at temperatures of 95 1C, 125 1C and 155 1C for 20 min in air.
P-6 surface profilometer with horizontal and vertical resolutions being 300 mm and 327 mm, respectively. The AFM micrographs were obtained in a tapping mode using Digital Instruments Veeco D3000 microscope. The AFM cantilever was provided by APP NANO. The normal spring constant of the cantilever was 0.03 N/m and the radius of the tip was less than 10 nm.
3. Results and discussion 3.1. UV/vis absorption spectra measurements Fig. 2 shows the UV/vis absorption spectra for the as-cast and the thin films annealed at temperatures 95 1C, 125 1C and 155 1C for a period of 20 min in atmosphere. It is observed that VOPcPhO thin films demonstrate two predominant bands in their absorption spectra. It is evident from Fig. 2(a) that the Q-band, a wellknown band in a phthalocyanine molecule, exists in the visible region between 630 nm and 750 nm and the Soret-band (B-band) is observed in the UV region ranging from 270 nm to 410 nm. The representative curve in Fig. 2(a) shows the inherent bands for the as-cast film of VOPcPhO in the UV and visible spectral regions. It is generally considered that the aromatic cyclic conjugated 18-p electrons system is responsible for the spectral properties of the phthalocyanines. The UV and visible spectra for the metal phthalocyanines emerge from (i) the molecular orbitals contained
2.0
Soret-Band
1.8 Q-Band
1.6 1.4 shoulder
1.2 1.0 0.8 0.6 0.4 0.2 0.0 300
2.2. Sample characterization A JASCO V-570 UV/vis/NIR double beam spectrophotometer was used to perform the optical absorption spectroscopy in the ultraviolet/visible range of the spectrum at room temperature. The thickness measurements were carried out using KLA Tencor
Absorption coeffient (x 105 cm-1)
1.5
125 °C
400
As-cast
1.4
1.3
1.2
155 °C
500 600 Wavelegth (nm) Absorption coeffient (x 10 cm )
homogeneous and trims down the dislocation density and improves film quality [9]. Like most of the metal-substituted phthalocyanines, VOPcPhO behaves as a p-type semiconductor. VOPcPhO does not possess a rigid structure due to which it can be conveniently dissolved in a variety of organic solvents and thin films of VOPcPhO can be easily prepared not only by vacuum evaporation but also by numerous solution processing techniques. As far as we know, very little work has been reported incorporating VOPcPhO therefore, in the present work, an attempt has been made to explore structural, morphological and optical properties of VOPcPhO thin films. The purpose of the present work is to elucidate the influence of postthermal annealing temperatures on the optical, morphological and structural properties of the spin coated VOPcPhO thin films for organic solar cell or photodiode, where such features are of prime importance.
Absorption coeffient (x 105 cm-1)
1816
700
1.8 125 °C
As-cast
1.6
1.4 155 °C
1.2
1.0 280
300
320
340
360
Wavelegth (nm)
1.1
1.0 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 Wavelegth (nm)
Fig. 1. Molecular structure of VOPcPhO.
Fig. 2. (a) The spectral behavior of the absorption coefficient a for VOPcPhO ascast thin film; (b) Optical absorption spectrum of the VOPcPhO thin films in the Q-band region before and after thermal annealing. Inset shows a spectrum before and after annealing in the Soret-band region.
F. Aziz et al. / Physica E 44 (2012) 1815–1819
a ¼ 2:303nA=d
ð1Þ
where ‘A’ is the measured absorbance of the as-cast and annealed samples and ‘d’ is the thickness of the film. A change in the intensities of the absorption coefficient of the VOPcPhO films after thermal treatment is observed in the curves of Fig. 2(b). It can be observed from Fig. 2(b) that the annealing temperature affects the absorption coefficient and the amount of Davydov splitting. The intensities of the absorption peaks in the lower energy band (Qband) tend to increase with increasing temperature (see Fig. 2(b)). The spectra of the as-cast and annealed samples reveal no distinguishable shift before and after thermal annealing. The increase in the intensities of the absorption peaks in the Soret band (UV spectral region) is presented in the inset of Fig. 2(b). The variation in the maximum absorption coefficient for both Q-band and Soret-band against the annealing temperature of the spincoated VOPcPhO thin films is presented in Fig. 3. The increment in the absorption coefficient may be ascribed to better molecular arrangement in the annealed VOPcPhO films as compared to the as-cast sample. The films annealed at 155 1C causes reduction in the absorption coefficient. This abrupt drop in the absorption coefficient suggests that the VOPcPhO molecules have been disrupted at such high temperatures of 155 1C (and 170 1C, result not shown) in the presence of oxygen (as the films are annealed in air). The results shown in Fig. 2(b) and Fig. 3 indicate that in order to achieve maximum light absorption within the photoactive layer, the VOPcPhO film needs to be annealed at the optimum temperature of about 125 1C.
2.0 Absorption Coefficient (x105 cm-1)
in the 18-p electrons system and (ii) the overlapping orbitals on the metal atom, present in the center of the ring [12]. A close look at the absorption spectra of VOPcPhO thin film in Fig. 2(a) reveals that the Q-band splits out in two characterized peaks. The higher energy peak (strong peak) occurs at 666 nm and the low energy peak (shoulder) appears at 715 nm in the visible region. However, the Soret-band possesses two peaks with one shoulder in the UV region of the absorption spectrum. The high energy peaks are observed at 290 nm and 344 nm while the shoulder exists at 408 nm [13–15]. We attribute both Q-band and Soret-band to the two lowest singlet–singlet electronic transitions of the conjugated system [16,17]. The previous studies have reported that the Q-band, which is quite sensitive to the environment of the molecule, is strongly localized on the Pc-ring [18]. In the visible region (Qband) the excitation takes place from the ground state a1u (p) highest occupied molecular orbital (HOMO) to eg (pn) lowest unoccupied molecular orbital (LUMO) of the phthalocyanine ring, which can be interpreted as a transition between bonding and anti-bonding molecular orbitals [13]. In the visible spectral region, the higher energy peak appearing at 1.88 eV is associated with the first p–pn transition on the phthalocyanine macro-cycle, while the low energy shoulder occurring at 1.73 eV may be attributed to the presence of second p–pn transition, excitonic transition, vibrational interval or surface state [13,19,20]. Nevertheless, the characteristic splitting (Davydov splitting), which results due to the vibronic coupling in the excited state, is observed in the Q-band region. The energy separation value due to the Davydov splitting was found to be 0.15 eV. This value is close to the values reported previously for the other phthalocyanine molecules [13,21]. The difference in the relative orientation of molecules is considered an important factor for the extent of the Davydov splitting. In the near UV region of the spectrum, the Soret band may arise from a2u (p) highest occupied molecular orbital (HOMO) to eg (pn) lowest unoccupied molecular orbital (LUMO) transition. The following equation [22] has been employed to calculate the absorption coefficient for the as-cast and thermally annealed films of VOPcPhO:
1817
1.8 1.6 1.4 1.2 λmax = 344 in UV region
1.0
λmax = 666 in Visible region
0.8 0.6 20
40
60
80 100 120 Temperature (°C)
140
160
180
Fig. 3. Relation between absorption spectra and temperature for UV and visible spectrum regions.
Information concerning the optical band gap of materials is important for practical considerations especially in the optoelectronic device fabrication. The optical absorption spectra play a vital role in studying the energy band structure and type of optical transitions. The absorption coefficient (a) can be correlated to the photon energy (hn) according to the following Bardeen relationship [23]. ðahnÞ ¼ a0 ðhnEg Þr
ð2Þ
where a0 is a constant independent of photon energy, Eg is the optical energy gap between localized states and r is an index used to determine the type of transition. For direct allowed and direct forbidden transition, r equals 1/2 and 3/2, respectively. The factor r takes the values 2 and 3 for indirect allowed and indirect forbidden transitions, respectively. In order to ascertain the direct or indirect optical band gap, we rearrange Eq. (2) by taking natural logarithm and derivative as follows: dlnðahnÞ r ¼ dhn hnEg
ð3Þ
The curve of dln(ahn)/dhn versus hn has been plotted in Fig. 4. The energy value for which a peak is observed on the curve presents approximately the optical energy gap (Eg). This particular value of Eg helps determining the value of r. From the slope of ln(ahn) versus ln(hn–Eg) shown in the inset of Fig. 4, the value of r 0.5 was observed, which indicates the presence of direct allowed optical transitions between the intermolecular energy bands in the VOPcPhO thin films. To find out the accurate energy band gaps in the lower energy and higher energy regions of the absorption spectra, a graph between (aE)2 and photon energy (E) was plotted as shown in Fig. 5. The x-axis intercepts, resulting form the extrapolation of the linear absorption, are exploited to obtain the direct energy band gaps for the as-cast and the sample annealed at 125 1C. The results sow hthat the energy gap reduces for the annealed sample in the Q-band region while in the Soret-band (B-band) the energy gaps for the as-cast and the annealed thin films are almost the same. 3.2. AFM images The AFM images of the as-cast and thermally annealed films have been examined to obtain a deep insight into the structural
1818
F. Aziz et al. / Physica E 44 (2012) 1815–1819
12.6
120
12.4 12.2 ln(αhν)
dln(αhν)/dhν
100 80
12.0 11.8 11.6 11.4
60
11.2 11.0 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0
40
ln(hν-Eg)
20 0 1.62
1.64
1.66
1.68
1.70
1.72
1.74
Energy (eV) Fig. 4. Plot of dln(ahn)/dhn versus hn for the as-cast sample. Inset shows a plot of ln(ahn) versus ln(hn–Eg) to determine the type of transition.
5x1011
4x1011 As-cast
(α E)2
Annealed at 125 °C
3x1011
2x1011 As-cast Annealed at 125 °C
1x1011 3.25
3.30
3.35
3.40
3.45
3.50
3.55
3.60
E (eV) Fig. 5. The variation of the absorption coefficient (a) as a function of photon energy before and after annealing of the films in the Soret-band region. Inset shows a Q-band region.
1400 1200 Grain Size (nm2)
changes resulting within the organic material by thermal annealing process. The 2-D and 3-D AFM images of the VOPcPhO thin films in tapping mode are depicted in Fig. 6. It can be observed that the as-cast and thermally annealed films possess different surface topographies. The morphology of the as-cast film (with no thermal treatment) exhibits rather a smooth and uniform surface without distinct features, suggesting some small-sized clusters. The root-mean-square (rms) roughness of the as-cast film is 0.286 nm. The samples that have undergone thermal treatment at 95 1C and 125 1C showed the rms roughness equal to 0.369 nm and 0.500 nm, respectively. The surface of the thin films becomes rougher and non-uniform as the annealing temperature increases from 95 1C to 125 1C. The 3-D AFM micrographs show a considerable change in the texture of the film after thermal treatment at various temperatures. The image in Fig. 6(c) suggests that the film has greater roughness with much coarser texture and sharp peaklike features as compared to the other two films. It can be observed from Fig. 6(d) that the surface of the VOPcPhO thin film starts deteriorating and shows evaporation of some parts of the film due to degradation upon exposure to the annealing temperature of 155 1C and above. The AFM image in Fig. 6(d) verifies that the sudden drop in the intensity of the absorption spectra of
1000 800 600 400 200 0 As-cast
95 °C
125 °C
Fig. 6. AFM images (2-D and 3-D ) of VOPcPhO thin films spun-cast on glass substrates; scan size is 5 mm 5 mm: (a) as-cast and annealed at (b) 95 1C, (c) 125 1C, (d) 155 1C (e) relation between grain size and different annealing temperatures.
the film annealed at 155 1C is the outcome of degraded films due to the high temperature effect. The best surface morphological characteristic is achieved by annealing the VOPcPhO thin film at 125 1C with surface roughness almost doubled as compared to the as-cast film, which is in agreement with the results obtained from
F. Aziz et al. / Physica E 44 (2012) 1815–1819
optical absorption spectrum. In order to confirm our results some of the films were annealed at other temperatures such as 110 1C, 140 1C and 170 1C. However, the results are not presented here but the samples undergoing thermal treatment at 110 1C and 140 1C showed smooth and uniform surfaces as compared to the film annealed at 125 1C. On the other hand, the film annealed at 170 1C confirmed degradation of the film at temperatures above 155 1C. Hence, it can be concluded that annealing beyond 155 1C is quite unsafe. The rough and non-uniform morphology of the VOPcPhO thin films suggests the formation of interpenetrating VOPcPhO molecular network. The coarse and rough surface, obtained by thermal annealing, enhances the ordered structure formation in the thin film. The relation between the device performance and the surface roughness of the films has been studied and it has been reported that the higher roughness of the film results in higher efficiency device [24,25]. The 2-D micrographs in Fig. 6 are used to estimate the grain size of the thin films of VOPcPhO. The as-cast VOPcPhO thin film consists of small grains and has an average grain size of about 651 nm2. On the contrary, the films annealed at temperatures 95 1C and 125 1C show large grain size of 1088 nm2 and 1433 nm2, respectively. It is observed that in our samples the grain size of the films increases with increasing annealing temperatures. The observed increase in the grain size with elevating temperature may be attributed to the fact that atoms move across the grown surface to the sites of low energy. As a result, the grain size of the films increases and has a subsequent influence on the surface roughness [26]. Some of the studies have proven that increased grain size enhances the crystallinity of the material [27]. It may also be assumed that some adjacent grains in the non-annealed film join together during thermal annealing process [28]. The graph of Fig. 6(e) shows the influence of annealing temperature on the average grain size (obtained from the AFM images data) of VOPcPhO thin films. We found that the grain size of VOPcPhO films can be enlarged by increasing the annealing temperature. Thus, we anticipate that the annealing process can be exploited to improve surface morphology thereby enhancing the charge transport properties. Other researchers have discovered that charge transport can be raised by annealing, which in turn gives a better device performance [27,29]. It is generally believed that some films posses increased carrier transport efficiency due to well-ordered structure and/or large grain size. However, the grain size of the degraded films (annealed at 155 1C and 170 1C) is quite large which is unable to be recorded on the graph of Fig. 6(e).
4. Conclusion The optical properties, surface morphology and structure of solution processed VOPcPhO thin films are significantly influenced by the post-deposition thermal annealing process. The absorption intensities of the film annealed at 125 1C has increased, in both UV and visible regions, due to the increased number of absorption sites as compared to the as-cast film. However, increasing the temperature beyond 155 1C has disrupted the film formation, thereby reducing the light absorption. It has been observed from the AFM topographic images that the annealing process considerably enhances the rms
1819
roughness and the grain size of the VOPcPhO thin film annealed at 125 1C. The features of light absorption and surface morphology of the films can be improved by conducting this simple thermal annealing process on the spin-coated VOPcPhO thin films. The present study on VOPcPhO may find suitability in developing low cost organic solar cells or photodiodes, where such features are of prime importance.
Acknowledgment This work is supported by the Ministry of Higher Education Malaysia through Long Term Research Grant Scheme (LRGS-P2) under project number LR003-2011A. Ms. Fakhra Aziz highly acknowledges the financial support provided by Higher Education Commission, Pakistan through ‘‘International Research Support Initiative Program’’. References [1] Z. Ahmad, P.C. Ooi, K.C. Aw, M.H. Sayyad, Solid State Communications 151 (2011) 297. [2] M. El-Nahass, K. Abd-El-Rahman, A. Darwish, Materials Chemistry and Physics 92 (2005) 185. [3] M. El-Nahass, S. Yaghmour, Applied Surface Science 255 (2008) 1631. [4] S. Karan, B. Mallik, Solid State Communications 143 (2007) 289. [5] T.D. Anthopoulos, T.S. Shafai, Journal of Physics and Chemistry of Solids 64 (2003) 1217. [6] H.X. Wei, J. Li, Z.Q. Xu, Y. Cai, J.X. Tang, Y.Q. Li, Applied Physics Letters 97 (2010) 083302. [7] R. Ben Chaabane, A. Ltaief, C. Dridi, H. Rahmouni, A. Bouazizi, H. Ben Ouada, Thin Solid Films 427 (2003) 371. [8] J. Kim, S. Yim, Journal of Nanoelectronics and Optoelectronics 6 (2011) 249. [9] L. Wang, Y. Pu, W. Fang, J. Dai, C. Zheng, C. Mo, C. Xiong, F. Jiang, Thin Solid Films 491 (2005) 323. [10] B. Derkowska, M. Wojdy"a, R. Czaplicki, W. Ba"a, B. Sahraoui, Optics Communications 274 (2007) 206. [11] S.V. Rao, D.N. Rao, Journal of Porphyrins and Phthalocyanines 6 (2002) 233. [12] E.A. Ough, M.J. Stillman, K.A.M. Creber, Canadian Journal of Chemistry 71 (1993) 1898. [13] A.T. Davidson, Journal of Chemical Physics 77 (1982) 168. [14] M. El-Nahass, K. Abd-El-Rahman, A. Al-Ghamdi, A. Asiri, Physica B: Condensed Matter 344 (2004) 398. [15] M. Wojdyla, B. Derkowska, Z. Lukasiak, W. Bala, Materials Letters 60 (2006) 3441. [16] Y. Pan, Y. Wu, L. Chen, Y. Zhao, Y. Shen, F. Li, S. Shen, D. Huang, Applied Physics A 66 (1998) 569. [17] M. Sayyad, Z. Ahmad, K.S. Karimov, M. Yaseen, M. Ali, Journal of Physics D: Applied Physics 42 (2009) 105112. [18] Q. Chen, D. Gu, J. Shu, X. Tang, F. Gan, Materials Science and Engineering B 25 (1994) 171. [19] M. El-Nahass, H. Zeyada, M. Aziz, N. El-Ghamaz, Optical Materials 27 (2004) 491. [20] S. Senthilarasu, R. Sathyamoorthy, S. Lalitha, A. Subbarayan, K. Natarajan, Solar Energy Materials and Solar Cells 82 (2004) 179. [21] B. Schechtman, W. Spicer, Journal of Molecular Spectroscopy 33 (1970) 28. [22] F.F. Muhammad, A.I. Abdul Hapip, K. Sulaiman, Journal of Organometallic Chemistry (2010). [23] K. Krishnakumar, C. Menon, Journal of Solid State Chemistry 128 (1997) 27. [24] G. Li, V. Shrotriya, Y. Yao, Y. Yang, Journal of Applied Physics 98 (2005) 043704. [25] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nature Materials 4 (2005) 864. [26] A. Moses Ezhil Raj, S.G. Victoria, V.B. Jothy, C. Ravidhas, J. Wollschl¨ager, M. Suendorf, M. Neumann, M. Jayachandran, C. Sanjeeviraja, Applied Surface Science 256 (2010) 2920. [27] S. Karak, S. Ray, A. Dhar, Solar Energy Materials and Solar Cells 94 (2010) 836. [28] T. Ahn, H. Jung, H.J. Suk, M.H. Yi, Synthetic Metals 159 (2009) 1277. [29] S. Tatemichi, M. Ichikawa, T. Koyama, Y. Taniguchi, Applied Physics Letters 89 (2006) 112108.
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Spectroscopic and microscopic studies of thermally treated Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine thin films Fakhra Aziz a,b,c, M.H. Sayyad b, Zubair Ahmad a,n, K. Sulaiman a, M.R. Muhammad a, Kh.S. Karimov b a
Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi-23640, Pakistan c Department of Electronics, Jinnah College for Women, University of Peshawar, Peshawar 25120, Pakistan b
H I G H L I G H T S c
c
c
G R A P H I C A L
A B S T R A C T
Optical, morphological and structural properties of VOPcPhO thin films. These properties are significantly influenced by the post-deposition thermal annealing. VOPcPhO has potential for applications such as organic solar cell and photodiode.
a r t i c l e i n f o
abstract
Article history: Received 5 December 2011 Received in revised form 11 April 2012 Accepted 27 April 2012 Available online 11 May 2012
This paper reports the structure, morphology and optical properties of Vanadyl 2,9,16,23-tetraphenoxy29H,31H-phthalocyanine (VOPcPhO) pristine and annealed thin films for photo-devices application. The VOPcPhO thin films have been prepared by spin-coating technique on glass substrates using VOPcPhO solution in chloroform. The UV/vis absorption spectra are used to study the optical properties while atomic force microscopy (AFM) is used to investigate the surface morphology and structure of the thin films. The AFM results show that the surface roughness increases with increase in annealing temperature making the annealed films more prone to high absorption rather than the pristine sample. The results also demonstrate that the thermal annealing processes on the VOPcPhO thin films significantly enhance the features of light absorption and surface morphology. Hence, VOPcPhO can be used in the organic solar cell or photodiode, where such features are of prime importance. & 2012 Elsevier B.V. All rights reserved.
1. Introduction Research on the emerging field of organic materials has extended the capabilities and possibilities of modern electronic devices into unforeseen domain. The motivation of using organic materials in electronic devices arose from their easily tunable electronic and processing properties [1]. Among the organic semiconductors metal phthalocyanines (MPcs) have been widely
n
Corresponding author. Tel.: þ60 16 9582584; fax: þ603 79674146. E-mail address: [email protected] (Z. Ahmad).
1386-9477/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2012.04.025
studied due to their noteworthy chemical and physical properties. The phthalocyanines exhibit very intense absorption in the UV/vis spectral region which make them suitable for the optoelectronic applications [2,3]. The use of VOPcPhO for photovoltaic applications is particularly attractive due to its high solubility in a variety of organic solvents. The photovoltaic response demonstrated for single-layer VOPcPhO based solar cell has shown its potential for fabricating simple, easy and inexpensive devices. Recently, many authors [4–8] reported that the thermal annealing significantly improves the structural, morphological, electrical and optical properties of the phthalocyanines. Thermal annealing refines the structure of materials by making them
F. Aziz et al. / Physica E 44 (2012) 1815–1819
2. Experimental 2.1. Sample preparation VOPcPhO is a macro-cyclic compound which occurs in the form of a green color dye. The structure of VOPcPhO is formed by four isoindole units surrounding the metal atom in the center [10]. The switching of various functional groups in the boundary and different metal atoms in the center can change the properties of the phthalocyanines [11]. The VOPcPhO has a Vanadyl as a central metal atom in core of the macro-cycle. VOPcPhO was purchased from Sigma–Aldrich and used as received. Fig. 1 shows a schematic diagram representing the molecular structure of VOPcPhO. The VOPcPhO was dissolved in chloroform to yield solution of concentration 30 mg/ml, which was used to deposit the films onto the glass substrates. Prior to spin coating, the glass substrates were cleaned by ultrasonic treatment in detergent for 20 min. Later the substrates were rinsed with deionized water, acetone and ethanol sequentially. Finally, the glass substrates were dried up by blowing nitrogen gas. Thin films of VOPcPhO were produced by spin coating the solution on the glass substrates in the presence of nitrogen. The speed of the spin-coater was fixed at 3000 rpm and the time was 20 s to produce VOPcPhO thin films with the thickness of 1407 5 nm. Immediately after spin-coating, the deposited films were subjected to post-annealing procedure. The annealing treatment has been carried out for three samples on hot plates at temperatures of 95 1C, 125 1C and 155 1C for 20 min in air.
P-6 surface profilometer with horizontal and vertical resolutions being 300 mm and 327 mm, respectively. The AFM micrographs were obtained in a tapping mode using Digital Instruments Veeco D3000 microscope. The AFM cantilever was provided by APP NANO. The normal spring constant of the cantilever was 0.03 N/m and the radius of the tip was less than 10 nm.
3. Results and discussion 3.1. UV/vis absorption spectra measurements Fig. 2 shows the UV/vis absorption spectra for the as-cast and the thin films annealed at temperatures 95 1C, 125 1C and 155 1C for a period of 20 min in atmosphere. It is observed that VOPcPhO thin films demonstrate two predominant bands in their absorption spectra. It is evident from Fig. 2(a) that the Q-band, a wellknown band in a phthalocyanine molecule, exists in the visible region between 630 nm and 750 nm and the Soret-band (B-band) is observed in the UV region ranging from 270 nm to 410 nm. The representative curve in Fig. 2(a) shows the inherent bands for the as-cast film of VOPcPhO in the UV and visible spectral regions. It is generally considered that the aromatic cyclic conjugated 18-p electrons system is responsible for the spectral properties of the phthalocyanines. The UV and visible spectra for the metal phthalocyanines emerge from (i) the molecular orbitals contained
2.0
Soret-Band
1.8 Q-Band
1.6 1.4 shoulder
1.2 1.0 0.8 0.6 0.4 0.2 0.0 300
2.2. Sample characterization A JASCO V-570 UV/vis/NIR double beam spectrophotometer was used to perform the optical absorption spectroscopy in the ultraviolet/visible range of the spectrum at room temperature. The thickness measurements were carried out using KLA Tencor
Absorption coeffient (x 105 cm-1)
1.5
125 °C
400
As-cast
1.4
1.3
1.2
155 °C
500 600 Wavelegth (nm) Absorption coeffient (x 10 cm )
homogeneous and trims down the dislocation density and improves film quality [9]. Like most of the metal-substituted phthalocyanines, VOPcPhO behaves as a p-type semiconductor. VOPcPhO does not possess a rigid structure due to which it can be conveniently dissolved in a variety of organic solvents and thin films of VOPcPhO can be easily prepared not only by vacuum evaporation but also by numerous solution processing techniques. As far as we know, very little work has been reported incorporating VOPcPhO therefore, in the present work, an attempt has been made to explore structural, morphological and optical properties of VOPcPhO thin films. The purpose of the present work is to elucidate the influence of postthermal annealing temperatures on the optical, morphological and structural properties of the spin coated VOPcPhO thin films for organic solar cell or photodiode, where such features are of prime importance.
Absorption coeffient (x 105 cm-1)
1816
700
1.8 125 °C
As-cast
1.6
1.4 155 °C
1.2
1.0 280
300
320
340
360
Wavelegth (nm)
1.1
1.0 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 Wavelegth (nm)
Fig. 1. Molecular structure of VOPcPhO.
Fig. 2. (a) The spectral behavior of the absorption coefficient a for VOPcPhO ascast thin film; (b) Optical absorption spectrum of the VOPcPhO thin films in the Q-band region before and after thermal annealing. Inset shows a spectrum before and after annealing in the Soret-band region.
F. Aziz et al. / Physica E 44 (2012) 1815–1819
a ¼ 2:303nA=d
ð1Þ
where ‘A’ is the measured absorbance of the as-cast and annealed samples and ‘d’ is the thickness of the film. A change in the intensities of the absorption coefficient of the VOPcPhO films after thermal treatment is observed in the curves of Fig. 2(b). It can be observed from Fig. 2(b) that the annealing temperature affects the absorption coefficient and the amount of Davydov splitting. The intensities of the absorption peaks in the lower energy band (Qband) tend to increase with increasing temperature (see Fig. 2(b)). The spectra of the as-cast and annealed samples reveal no distinguishable shift before and after thermal annealing. The increase in the intensities of the absorption peaks in the Soret band (UV spectral region) is presented in the inset of Fig. 2(b). The variation in the maximum absorption coefficient for both Q-band and Soret-band against the annealing temperature of the spincoated VOPcPhO thin films is presented in Fig. 3. The increment in the absorption coefficient may be ascribed to better molecular arrangement in the annealed VOPcPhO films as compared to the as-cast sample. The films annealed at 155 1C causes reduction in the absorption coefficient. This abrupt drop in the absorption coefficient suggests that the VOPcPhO molecules have been disrupted at such high temperatures of 155 1C (and 170 1C, result not shown) in the presence of oxygen (as the films are annealed in air). The results shown in Fig. 2(b) and Fig. 3 indicate that in order to achieve maximum light absorption within the photoactive layer, the VOPcPhO film needs to be annealed at the optimum temperature of about 125 1C.
2.0 Absorption Coefficient (x105 cm-1)
in the 18-p electrons system and (ii) the overlapping orbitals on the metal atom, present in the center of the ring [12]. A close look at the absorption spectra of VOPcPhO thin film in Fig. 2(a) reveals that the Q-band splits out in two characterized peaks. The higher energy peak (strong peak) occurs at 666 nm and the low energy peak (shoulder) appears at 715 nm in the visible region. However, the Soret-band possesses two peaks with one shoulder in the UV region of the absorption spectrum. The high energy peaks are observed at 290 nm and 344 nm while the shoulder exists at 408 nm [13–15]. We attribute both Q-band and Soret-band to the two lowest singlet–singlet electronic transitions of the conjugated system [16,17]. The previous studies have reported that the Q-band, which is quite sensitive to the environment of the molecule, is strongly localized on the Pc-ring [18]. In the visible region (Qband) the excitation takes place from the ground state a1u (p) highest occupied molecular orbital (HOMO) to eg (pn) lowest unoccupied molecular orbital (LUMO) of the phthalocyanine ring, which can be interpreted as a transition between bonding and anti-bonding molecular orbitals [13]. In the visible spectral region, the higher energy peak appearing at 1.88 eV is associated with the first p–pn transition on the phthalocyanine macro-cycle, while the low energy shoulder occurring at 1.73 eV may be attributed to the presence of second p–pn transition, excitonic transition, vibrational interval or surface state [13,19,20]. Nevertheless, the characteristic splitting (Davydov splitting), which results due to the vibronic coupling in the excited state, is observed in the Q-band region. The energy separation value due to the Davydov splitting was found to be 0.15 eV. This value is close to the values reported previously for the other phthalocyanine molecules [13,21]. The difference in the relative orientation of molecules is considered an important factor for the extent of the Davydov splitting. In the near UV region of the spectrum, the Soret band may arise from a2u (p) highest occupied molecular orbital (HOMO) to eg (pn) lowest unoccupied molecular orbital (LUMO) transition. The following equation [22] has been employed to calculate the absorption coefficient for the as-cast and thermally annealed films of VOPcPhO:
1817
1.8 1.6 1.4 1.2 λmax = 344 in UV region
1.0
λmax = 666 in Visible region
0.8 0.6 20
40
60
80 100 120 Temperature (°C)
140
160
180
Fig. 3. Relation between absorption spectra and temperature for UV and visible spectrum regions.
Information concerning the optical band gap of materials is important for practical considerations especially in the optoelectronic device fabrication. The optical absorption spectra play a vital role in studying the energy band structure and type of optical transitions. The absorption coefficient (a) can be correlated to the photon energy (hn) according to the following Bardeen relationship [23]. ðahnÞ ¼ a0 ðhnEg Þr
ð2Þ
where a0 is a constant independent of photon energy, Eg is the optical energy gap between localized states and r is an index used to determine the type of transition. For direct allowed and direct forbidden transition, r equals 1/2 and 3/2, respectively. The factor r takes the values 2 and 3 for indirect allowed and indirect forbidden transitions, respectively. In order to ascertain the direct or indirect optical band gap, we rearrange Eq. (2) by taking natural logarithm and derivative as follows: dlnðahnÞ r ¼ dhn hnEg
ð3Þ
The curve of dln(ahn)/dhn versus hn has been plotted in Fig. 4. The energy value for which a peak is observed on the curve presents approximately the optical energy gap (Eg). This particular value of Eg helps determining the value of r. From the slope of ln(ahn) versus ln(hn–Eg) shown in the inset of Fig. 4, the value of r 0.5 was observed, which indicates the presence of direct allowed optical transitions between the intermolecular energy bands in the VOPcPhO thin films. To find out the accurate energy band gaps in the lower energy and higher energy regions of the absorption spectra, a graph between (aE)2 and photon energy (E) was plotted as shown in Fig. 5. The x-axis intercepts, resulting form the extrapolation of the linear absorption, are exploited to obtain the direct energy band gaps for the as-cast and the sample annealed at 125 1C. The results sow hthat the energy gap reduces for the annealed sample in the Q-band region while in the Soret-band (B-band) the energy gaps for the as-cast and the annealed thin films are almost the same. 3.2. AFM images The AFM images of the as-cast and thermally annealed films have been examined to obtain a deep insight into the structural
1818
F. Aziz et al. / Physica E 44 (2012) 1815–1819
12.6
120
12.4 12.2 ln(αhν)
dln(αhν)/dhν
100 80
12.0 11.8 11.6 11.4
60
11.2 11.0 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0
40
ln(hν-Eg)
20 0 1.62
1.64
1.66
1.68
1.70
1.72
1.74
Energy (eV) Fig. 4. Plot of dln(ahn)/dhn versus hn for the as-cast sample. Inset shows a plot of ln(ahn) versus ln(hn–Eg) to determine the type of transition.
5x1011
4x1011 As-cast
(α E)2
Annealed at 125 °C
3x1011
2x1011 As-cast Annealed at 125 °C
1x1011 3.25
3.30
3.35
3.40
3.45
3.50
3.55
3.60
E (eV) Fig. 5. The variation of the absorption coefficient (a) as a function of photon energy before and after annealing of the films in the Soret-band region. Inset shows a Q-band region.
1400 1200 Grain Size (nm2)
changes resulting within the organic material by thermal annealing process. The 2-D and 3-D AFM images of the VOPcPhO thin films in tapping mode are depicted in Fig. 6. It can be observed that the as-cast and thermally annealed films possess different surface topographies. The morphology of the as-cast film (with no thermal treatment) exhibits rather a smooth and uniform surface without distinct features, suggesting some small-sized clusters. The root-mean-square (rms) roughness of the as-cast film is 0.286 nm. The samples that have undergone thermal treatment at 95 1C and 125 1C showed the rms roughness equal to 0.369 nm and 0.500 nm, respectively. The surface of the thin films becomes rougher and non-uniform as the annealing temperature increases from 95 1C to 125 1C. The 3-D AFM micrographs show a considerable change in the texture of the film after thermal treatment at various temperatures. The image in Fig. 6(c) suggests that the film has greater roughness with much coarser texture and sharp peaklike features as compared to the other two films. It can be observed from Fig. 6(d) that the surface of the VOPcPhO thin film starts deteriorating and shows evaporation of some parts of the film due to degradation upon exposure to the annealing temperature of 155 1C and above. The AFM image in Fig. 6(d) verifies that the sudden drop in the intensity of the absorption spectra of
1000 800 600 400 200 0 As-cast
95 °C
125 °C
Fig. 6. AFM images (2-D and 3-D ) of VOPcPhO thin films spun-cast on glass substrates; scan size is 5 mm 5 mm: (a) as-cast and annealed at (b) 95 1C, (c) 125 1C, (d) 155 1C (e) relation between grain size and different annealing temperatures.
the film annealed at 155 1C is the outcome of degraded films due to the high temperature effect. The best surface morphological characteristic is achieved by annealing the VOPcPhO thin film at 125 1C with surface roughness almost doubled as compared to the as-cast film, which is in agreement with the results obtained from
F. Aziz et al. / Physica E 44 (2012) 1815–1819
optical absorption spectrum. In order to confirm our results some of the films were annealed at other temperatures such as 110 1C, 140 1C and 170 1C. However, the results are not presented here but the samples undergoing thermal treatment at 110 1C and 140 1C showed smooth and uniform surfaces as compared to the film annealed at 125 1C. On the other hand, the film annealed at 170 1C confirmed degradation of the film at temperatures above 155 1C. Hence, it can be concluded that annealing beyond 155 1C is quite unsafe. The rough and non-uniform morphology of the VOPcPhO thin films suggests the formation of interpenetrating VOPcPhO molecular network. The coarse and rough surface, obtained by thermal annealing, enhances the ordered structure formation in the thin film. The relation between the device performance and the surface roughness of the films has been studied and it has been reported that the higher roughness of the film results in higher efficiency device [24,25]. The 2-D micrographs in Fig. 6 are used to estimate the grain size of the thin films of VOPcPhO. The as-cast VOPcPhO thin film consists of small grains and has an average grain size of about 651 nm2. On the contrary, the films annealed at temperatures 95 1C and 125 1C show large grain size of 1088 nm2 and 1433 nm2, respectively. It is observed that in our samples the grain size of the films increases with increasing annealing temperatures. The observed increase in the grain size with elevating temperature may be attributed to the fact that atoms move across the grown surface to the sites of low energy. As a result, the grain size of the films increases and has a subsequent influence on the surface roughness [26]. Some of the studies have proven that increased grain size enhances the crystallinity of the material [27]. It may also be assumed that some adjacent grains in the non-annealed film join together during thermal annealing process [28]. The graph of Fig. 6(e) shows the influence of annealing temperature on the average grain size (obtained from the AFM images data) of VOPcPhO thin films. We found that the grain size of VOPcPhO films can be enlarged by increasing the annealing temperature. Thus, we anticipate that the annealing process can be exploited to improve surface morphology thereby enhancing the charge transport properties. Other researchers have discovered that charge transport can be raised by annealing, which in turn gives a better device performance [27,29]. It is generally believed that some films posses increased carrier transport efficiency due to well-ordered structure and/or large grain size. However, the grain size of the degraded films (annealed at 155 1C and 170 1C) is quite large which is unable to be recorded on the graph of Fig. 6(e).
4. Conclusion The optical properties, surface morphology and structure of solution processed VOPcPhO thin films are significantly influenced by the post-deposition thermal annealing process. The absorption intensities of the film annealed at 125 1C has increased, in both UV and visible regions, due to the increased number of absorption sites as compared to the as-cast film. However, increasing the temperature beyond 155 1C has disrupted the film formation, thereby reducing the light absorption. It has been observed from the AFM topographic images that the annealing process considerably enhances the rms
1819
roughness and the grain size of the VOPcPhO thin film annealed at 125 1C. The features of light absorption and surface morphology of the films can be improved by conducting this simple thermal annealing process on the spin-coated VOPcPhO thin films. The present study on VOPcPhO may find suitability in developing low cost organic solar cells or photodiodes, where such features are of prime importance.
Acknowledgment This work is supported by the Ministry of Higher Education Malaysia through Long Term Research Grant Scheme (LRGS-P2) under project number LR003-2011A. Ms. Fakhra Aziz highly acknowledges the financial support provided by Higher Education Commission, Pakistan through ‘‘International Research Support Initiative Program’’. References [1] Z. Ahmad, P.C. Ooi, K.C. Aw, M.H. Sayyad, Solid State Communications 151 (2011) 297. [2] M. El-Nahass, K. Abd-El-Rahman, A. Darwish, Materials Chemistry and Physics 92 (2005) 185. [3] M. El-Nahass, S. Yaghmour, Applied Surface Science 255 (2008) 1631. [4] S. Karan, B. Mallik, Solid State Communications 143 (2007) 289. [5] T.D. Anthopoulos, T.S. Shafai, Journal of Physics and Chemistry of Solids 64 (2003) 1217. [6] H.X. Wei, J. Li, Z.Q. Xu, Y. Cai, J.X. Tang, Y.Q. Li, Applied Physics Letters 97 (2010) 083302. [7] R. Ben Chaabane, A. Ltaief, C. Dridi, H. Rahmouni, A. Bouazizi, H. Ben Ouada, Thin Solid Films 427 (2003) 371. [8] J. Kim, S. Yim, Journal of Nanoelectronics and Optoelectronics 6 (2011) 249. [9] L. Wang, Y. Pu, W. Fang, J. Dai, C. Zheng, C. Mo, C. Xiong, F. Jiang, Thin Solid Films 491 (2005) 323. [10] B. Derkowska, M. Wojdy"a, R. Czaplicki, W. Ba"a, B. Sahraoui, Optics Communications 274 (2007) 206. [11] S.V. Rao, D.N. Rao, Journal of Porphyrins and Phthalocyanines 6 (2002) 233. [12] E.A. Ough, M.J. Stillman, K.A.M. Creber, Canadian Journal of Chemistry 71 (1993) 1898. [13] A.T. Davidson, Journal of Chemical Physics 77 (1982) 168. [14] M. El-Nahass, K. Abd-El-Rahman, A. Al-Ghamdi, A. Asiri, Physica B: Condensed Matter 344 (2004) 398. [15] M. Wojdyla, B. Derkowska, Z. Lukasiak, W. Bala, Materials Letters 60 (2006) 3441. [16] Y. Pan, Y. Wu, L. Chen, Y. Zhao, Y. Shen, F. Li, S. Shen, D. Huang, Applied Physics A 66 (1998) 569. [17] M. Sayyad, Z. Ahmad, K.S. Karimov, M. Yaseen, M. Ali, Journal of Physics D: Applied Physics 42 (2009) 105112. [18] Q. Chen, D. Gu, J. Shu, X. Tang, F. Gan, Materials Science and Engineering B 25 (1994) 171. [19] M. El-Nahass, H. Zeyada, M. Aziz, N. El-Ghamaz, Optical Materials 27 (2004) 491. [20] S. Senthilarasu, R. Sathyamoorthy, S. Lalitha, A. Subbarayan, K. Natarajan, Solar Energy Materials and Solar Cells 82 (2004) 179. [21] B. Schechtman, W. Spicer, Journal of Molecular Spectroscopy 33 (1970) 28. [22] F.F. Muhammad, A.I. Abdul Hapip, K. Sulaiman, Journal of Organometallic Chemistry (2010). [23] K. Krishnakumar, C. Menon, Journal of Solid State Chemistry 128 (1997) 27. [24] G. Li, V. Shrotriya, Y. Yao, Y. Yang, Journal of Applied Physics 98 (2005) 043704. [25] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nature Materials 4 (2005) 864. [26] A. Moses Ezhil Raj, S.G. Victoria, V.B. Jothy, C. Ravidhas, J. Wollschl¨ager, M. Suendorf, M. Neumann, M. Jayachandran, C. Sanjeeviraja, Applied Surface Science 256 (2010) 2920. [27] S. Karak, S. Ray, A. Dhar, Solar Energy Materials and Solar Cells 94 (2010) 836. [28] T. Ahn, H. Jung, H.J. Suk, M.H. Yi, Synthetic Metals 159 (2009) 1277. [29] S. Tatemichi, M. Ichikawa, T. Koyama, Y. Taniguchi, Applied Physics Letters 89 (2006) 112108.