Enhancement of photovoltaic characteristics of nanocrystalline 2,3-naphthalocyanine thin film-based organic devices

Applied Surface Science 259 (2012) 600–609

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Enhancement of photovoltaic characteristics of nanocrystalline 2,3-naphthalocyanine thin film-based organic devices A.A.M. Farag a,∗ , W.G. Osiris b , A.H. Ammar a a b

Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Biophysics Department, Faculty of Science, Cairo University, Giza, Egypt

a r t i c l e

i n f o

Article history: Received 13 January 2012 Received in revised form 6 July 2012 Accepted 11 July 2012 Available online 24 July 2012 Keywords: Al/NPC/ITO device Optical band gap Activation energy Thermal annealing Nanostructured film

a b s t r a c t In this work, nanocrystalline thin films of 2,3-naphthalocyanine (NPC) were successfully deposited by a thermal evaporation technique at room temperature under high vacuum (∼10−4 Pa). The crystal structure and surface morphology were measured using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. A preferred orientation along the (0 0 1) direction was observed in all the studied films and the average crystallite size was calculated. Scanning electron miscroscopy (SEM) images of NPC films at different thermal treatment indicated significant changes on surface level patterns and gave clear evidence of agglomeration of nanocrystalline structures. The molecular structural properties of the thin films were characterized using Fourier transform infrared spectroscopy (FTIR), which revealed the stability of the chemical bonds of the compound under thermal treatment. The dark electrical conductivity of the films at various heat treatment stages showed that NPC films have a better conductivity than that of its earlier reported naphthalocyanine films and the activation energy was found to decrease with annealing temperature. The absorption edge shifted to the lower energy as a consequence of the thermal annealing of the film and the fundamental absorption edges correspond to a direct energy gap. The temperature coefficient of the onset and optical band gaps for the film was calculated to be −4.4 × 10−4 and −1.76 × 10−3 eV/K, respectively. The effect of thermal annealing on the photovoltaic properties of Al/NPC/ITO devices was also considered. The as-deposited device showed maximum power conversion efficiency about 0.70% under illumination of 100 mW/cm2 , whereas 2.65% power conversion efficiency was achieved after annealing the samples at 500 K for 1 h. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Organic dye pigments such as phthalocyanines and naphthalocyanines have been widely studied because of their potential use in semiconducting materials [1,2], non-linear optics [3], other optical devices [4] and even in medicine for photodynamic therapy of cancer [5]. Naphthalocyanine is a phthalocyanine derivative with a more extended ␲-electron-delocalized system consisting of four benzoisoindole units [6]. As a result, these materials exhibit strong absorption bands in the near-IR region which together with their photoconductive properties, render them more suitable materials for application in solar energy conversion, laser electrophotography, photoelectrochemical cells, photosensitization, electrocatalysis and electrophotography than the phthalocyanine [6–12]. Moreover, naphthalocyanine and its derivatives have been widely studied in recent years as a functional material owing to its special optical and electrical properties, biological activity,

∗ Corresponding author. Tel.: +20 2 33518705; fax: +20 2 22581243. E-mail address: [email protected] (A.A.M. Farag). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.083

good processability, structural variety and functional adjustment [6,13,14]. Studies on preparation, characterization and properties of naphthalocyanine thin films and its derivatives have attracted much attention in the literature [6–14]. Many applications of naphthalocyanine compounds are based on their properties in the form of thin films. Langmuir–Blodgett (LB) [15], spin-coating [16] and vacuum deposition [17] techniques can be employed to prepare naphthalocyanine films. In the present study, nanocrystalline naphthalocyanine thin films were prepared by a thermal evaporation technique under high vacuum in order to explore and find new applications of this type of films. However, to our knowledge little attention has been paid to naphthalocyanines thin films until now. A few works dealing with photovoltaic studies have been reported for nanocrystalline naphthalocyanine films [18,19]. Further efforts have been taken to report structural and surface morphological studies of these naphthalocyanine thin films. Our goal in the present study was to prepare high quality nanocrystalline naphthalocyanine (NPC) thin films by thermal evaporation. Moreover, structural and optical parameters were calculated and interpreted under the influence of heat treatment. The photovoltaic characteristics of Al/NPC/ITO Schottky diode were

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

Fig. 1. Thermogravimetric analysis (TGA) of NPC film. The inset shows the structural formula of 2,3-naphthalocyanine (NPC).

also considered. Finally, a comparative study between the different nanocrystalline NPC films and other organic conjugated films was done to evaluate their applications in optoelectronic devices. 2. Experimental procedures 2.1. Materials and preparation 2,3-Naphthalocyanine (NPC) was obtained from Sigma–Aldrich Company with purity >97%. The chemical structure of the NPC is shown in the inset of Fig. 1. Thin NPC films were prepared by a conventional thermal evaporation technique, using a high-vacuum coating unit (Edwards type E 306 A, England). The NPC films were deposited onto pre-cleaned ITO-coated glass substrates. The films were vacuum deposited from a quartz crucible source heated by a tungsten coil in a vacuum of 10−4 Pa during deposition. The temperature of the substrates was kept at room temperature (300 K) and the deposition rate was controlled at 2 nm/s using a quartz crystal thickness monitor (Model FTM4, Edwards, England). The measured film thickness of films was approximately 640 ± 10 nm. The films of NPC were heat treated at various temperatures in the range 350–500 K for 1 h. Finally, a mask was used on the prepared films to define devices suitable for electrical measurements. Pure aluminum was used as a rectifying contact with NPC films. The non-ohmic (Al) electrode was evaporated on the NPC using a high vacuum coating unit (Edwards, E-306 A) at room temperature (300 K) during the deposition using a tungsten filament under vacuum better than 2 × 10− 4 Pa. 2.2. Measurements The thermal behavior of the prepared films was examined using a Thermogravimetric Analyzer model Schimadzu TGA-50H (Kyoto, Japan) from 293 to 750 K. A heating rate of 10 ◦ C/min was used under nitrogen atmosphere and at a flow rate of 20 mL/min. Dry sample weighing about 1.596 mg was used. The standard uncertainty of the sample mass measurement was ±1%. The instrument was calibrated using calcium oxalate which was supplied with the instrument. The infrared transmission spectra of the prepared samples were measured at room temperature in the range 4000–400 cm−1 by an infrared spectrophotometer (ATI Mattson Infrared Spectrophotometer) using the KBr disc as reference material. The KBr powder was subjected to a load of 5 tons/cm for 2 min to produce clear homogenous discs. The NPC film was prepared on KBr using the

601

thermal evaporation method. The FTIR spectrum was measured immediately after preparing the discs. The cross section and the surface morphology of the prepared films were observed by scanning electron microscopy, SEM (type JEOL-JSM-636 OLA). The structural characterization was investigated by using the obtained X-ray diffraction patterns (XRD). A Philips X-ray diffractometer (model X’ pert) was used for the measurements with utilized monochromatic CuK␣ radiation operated at 40 kV and 25 mA. The diffraction patterns were recorded automatically with a scanning speed of 2◦ /min in the 2 range 5–40◦ . The measurement of the optical absorption spectrum was carried out using a double beam spectrophotometer model (JASCO570-UV-VIS-NIR Spectrophotometer) at normal incidence of light in the wavelength range 200–1200 nm with step of 2 nm. All the measurements were carried out at room temperature. DC electrical conductivity measurements of NPC films were carried out in dark at the temperature range from 300 to 500 K, using a high impedance electrometer (Keithley 617A) with a high impedance of 1014 . The temperature was measured directly by means of chromel–alumel thermocouple connected to hand-held digital thermometer. For the current density–voltage (J–V) measurements at different temperatures, stabilized power supply and high Impedance Electrometer (Keithley 617A) were used. The temperature of the sample was recorded during the electrical measurements by using NiCr–NiAl thermocouple with an accuracy of ±1 K. Photovoltaic characteristics were carried out by using halogen lamp containing iodine vapor and tungsten filament. The intensity of light was measured with a solar power meter (TM-206, Taiwan). 3. Results and discussion 3.1. Thermal analysis characterization of NPC film The thermal stability of NPC films was evaluated by thermogravimetric analysis (TGA) [20] to determine the beginning of the thermal degradation. This was helpful in guiding further phase transition investigations. The TGA thermogram of NPC as shown in Fig. 1 is characterized by the presence of three discrete characteristic decomposition steps. The first decomposition step in NPC was identified in the temperature range of 297–373 K with a percentage loss of 2.72%. This decomposition step had small weight losses that were tentatively ascribed to the evaporation of physically adsorbed water [21]. The second decomposition step in the TGA of NPC was identified in the temperature range 513–652 K. This decomposition step was found to correspond to a percentage loss of 12.86% followed by decomposition in the temperature range 652.5–750 K with a percentage loss of 4.65%. The overall percentage loss of the NPC molecule corresponds to 19.21% up to the maximum heating temperature (750 K) specified by TGA conditions. Seoudi et al. [22] reported that the loss in the second region is attributed to pyrolysis by a minor decomposition reaction. Bilgin et al. [23] attributed the weight loss below 473 K to volatiles evaporating (N2 , O2 , CO2 and H2 O) or low temperature degradation of unstable chemical fragments in the samples. At higher temperatures, above 650 K, the main degradation step is not visible due to the low percent of degradation which give evidence for thermal stability of NPC films over a wide range of temperature. 3.2. Molecular structure characterization of NPC thin films Fig. 2 compares the FTIR spectra in the region of fundamental frequencies of 4000–400 cm−1 for the as-deposited (at

602

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

Fig. 2. FTIR of the as-deposited (at 300 K) and annealed NPC thin films at different annealing temperatures (350–500 K). The corresponding wavenumber of each band is illustrated on the plot.

300 K) and four different annealed NPC thin films. In the spectral region from 400 to 1000 cm−1 , the remaining vibrations have main contributions from the dihedral plane deformation involving the aromatic C H groups (wagging, torsion and out-of-plane bending vibrations) [24–29]. The relatively strong absorption at 734 cm−1 could be attributed to vibrations related to pyrrole or isoindole stretching, breathing or deformation [25]. Because of the extended ␲-electronic system of the macrocycles, important C C and C N coupled motions in the 1300–1600 cm−1 region [25–29] are expected. Aromatic in-plane C H bending vibrations are responsible for the FTIR bands of NPC molecules at 1347 cm−1 can be attributed to the pyrrole ring stretching. The band at 1517 cm−1 is due to the coupling of isoindole stretching [26–29]. The band around 2229 cm−1 is due to the CN group [30]. The weak band around 2860 cm−1 was earlier reported to be the C H stretching vibration in similar organic thin films [27]. A weak band around 3060 cm−1 is due to the aromatic C H stretching on the NPC ring [25–29]. The FTIR results revealed the presence of thermal stability of the prepared films as well as the absence of remarkable change in the main structure of NPC compound as a result of annealing. 3.3. Crystalline structure characterization of NPC thin films The XRD spectra of the powder NPC, as-deposited and annealed NPC films are shown in Fig. 3. The XRD of the powder verified the polycrystalline nature (Fig. 4a). The X-ray diffraction patterns of the as-deposited and annealed films were also provided for comparison. The main crystalline peaks of the powder were found to be at 2 = 6.67◦ , 8.69◦ , 12.28◦ and 26.6◦ . The d spacing values along with the (h k l) indices corresponding to these peaks were of ˚ respectively. The lattice plane indices 13.24, 10.16, 7.20 and 3.34 A, were confirmed by the CRYSFIRE computer program [31]. The values of Miller indices, h k l, corresponding to each diffraction line were computed using CHECKCELL program [32] and remarks on each peak are shown in Fig. 3. Furthermore, the structure of the as-deposited and annealed films is shown in Fig. 4(a) and (b). A preferred orientation along the (0 0 1) direction was observed in all the studied films. This clearly indicates that the NPC films have the same structure of the powder. As the annealing temperature increases, the width of the peak corresponding to (0 0 1) orientation decreases as compared with the other peaks. The average

Fig. 3. X-ray diffraction (XRD) patterns of: (a) powder NPC, (b) thin film at 300 K, (c) thin film annealed at 350 K, (d) thin film annealed at 400 K, (e) thin film annealed at 450 K and (f) thin film annealed at 500 K.

crystallite size, D, was calculated using the modified Scherrer’s equation [33]: D=



C

(1)

2 2 ˇsample − ˇsilicon cos 

where  is the X-ray wavelength, C is a constant (∼0.94), ˇsample and ˇsilicon are the width at half maxima of the broadened peaks of the sample and a standard silicon crystal, respectively. The use of silicon defect free crystal measures the instrumental broadening. The low scanning rate of the 2 and neglecting the micro-strain of the nonthermal stressed films enabled us to calculate the crystallite size using Eq. (1). Fig. 4(a) shows the Gaussian fit of the peak for each annealing temperature. As a result, the mean crystallite size can be calculated. Fig. 4(b) shows the obtained crystallite size dependence upon the annealing temperature. The increasing in crystallite size for the annealed films as compared to that for the as-deposited films can be attributed to nucleation and crystallite formation. The obtained crystallite sizes were in the range 20–30.8 nm. 3.4. Morphological characterization of NPC thin films The cross section and the surface morphology of the asdeposited and annealed NPC films at various temperatures in the range 350–500 K were studied by SEM and shown in Fig. 5. The SEM micrographs (Fig. 5(b)-(f)) indicate that the films are uniform and dense on glass and free from cracks, peels and/or other defects. Moreover, the images show a disordered array of “island

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

32

300 K annealed at 350 K annealed at 400 K annealed at 450 K annealed at 500 K

(a)

603

(b) 30

D (nm)

Intensity (a.u.)

28

26

24

22

20 6.0

6.5

7.0

2

7.5

o

300

350

400

450

500

Tannealing (K)

Fig. 4. (a) Gaussian fitting of the X-ray diffraction (XRD) data of the as deposited and annealed thin film of NPC. (b) Crystallite size, D, versus annealing temperature, Tannealing .

aggregation” of NPC distributed over the whole surface. Obviously, the heat treatment led to significant changes of the film structure. The fine-island aggregations on the thin films were transformed to a structure with grosser plate-like islands as a result of coalescence and reorganization of the grains during the heat treatment. Also, the islands became larger and the surface/volume ratio became smaller when the annealing temperature increased. An important measurement was the cross sectional image of a cleaved specimen for the thickness measurement as shown in Fig. 5(a). Using the SEM facility for distance measurements program at the film boundaries, the average film thickness was then calculated and found to be about 644 ± 7 nm.

3.5. Optical characterization of NPC films Fig. 6(a) illustrates the absorption spectra of the as-deposited and annealed NPC thin films in both UV-vis and near-infrared regions up to about 1200 nm. The highly conjugated NPC macrocycle shows intense absorption in Soret band (B), that appears in the wavelength range 270–420 nm and Q-band in the range 650–950 nm. The peaks in UV–vis are generally interpreted in terms of ␲–␲* transition type from the highest occupied molecular orbital (HOMO) to the excited lowest unoccupied molecular orbital (LUMO) [34–38]. The absorption edge can be related to the onset of the fundamental absorption [39]. In the UV region, the absorption maxima are resolved as the Soret (B) band. The present observation of similar structure on the visible and Soret bands is taken as supporting evidence for an explanation of the structure in terms of a molecular vibration [34–39]. It should be emphasized that the absorption spectra of NPC films are broad and cover a wide range of the UV–vis and NIR spectrum up to about 1200 nm. This feature is desirable for the photovoltaic properties of the compounds. The presence of the ␲electron aromatic fused ring system of the molecules contributes to the broadening of their absorption [8]. The absorption coefficient as a function of annealing temperature at wavelengths of 320, 630 and 930 nm is represented in Fig. 6(b). This figure confirms the enhancement of absorption coefficient as a result of heat

treatment for NPC films. The ␲-electron aromatic fused ring system is of interest. The applicability of using the band theory to describe the electronic transition in organic systems was suggested by various authors [39–41]. Thus, for a molecular crystal, the valence band is formed by the combination of the highest occupied molecular orbital (HOMO; -orbital) whereas the lowest unoccupied molecular orbital (LUMO; *-orbitals) contributes to the conduction band. These bands are separated by the band gap. The optical band gap can be calculated by the following relation [42]: (˛h) = A(h − Eg )

1/2

(2)

where ˛ is the absorption coefficient, h is the photon energy, A is constant, and Eg is the optical band gap. It is evaluated that the optical band gap of the NPC films has direct optical transition. Fig. 7(a) and (b) shows plots of (˛h)2 versus photon energy (h). Two direct optical band gaps Eg1 and Eg2 were obtained depending on the energy range and listed in Table 1 for the as-deposited and annealed NPC thin films. As expected, the absorption edge shifts to lower energies as the temperature is increased. The determined onset energy gap (Eg1 ) and the fundamental energy gap (Eg2 ) are compared with the published data of the other NPC derivatives [10,43,44] as listed in Table 1. As observed, a common characteristic behavior for NPC films and most derivatives is the direct transitions [10,43,44]. The NPC and its derivatives have validity in the field of bulk heterojunction photovoltaic devices with other low band gap organic materials. Moreover, the relevant energy level positions and the absorption in long wavelength region suggest that these new complexes could be effective in photovoltaic devices [44]. Moreover, the optical band gap decreases with heat treatment and the major contribution to this modification is from a shift in the relative positions of the valence and conduction bands due to temperature dependence of dilatation of the lattice and/or electron–lattice interaction [10]. Yakuphanoglu et al. [44] discussed the temperature affecting the optical band gap by changing atomic distances. They attributed the decrease of optical band gap to the interatomic distances with increasing heat treatment. This change can also arise

604

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

Fig. 5. Scanning electron microscopy (SEM) image of NPC films: (a) cross section view, (b) surface morphology of the film at 300 K, (c) surface morphology of the annealed film at 350 K, (d) surface morphology of the annealed film at 400 K), (e) surface morphology of the annealed film at 450 K, and (f) surface morphology of the annealed film at 500 K.

from the electron–lattice interaction. This dominates at elevated temperatures, in which the band gap energy decreases with temperature. The change in the optical band gap with temperature or heat treatment is quite linear as shown in Fig. 8(a) and (b) and can be expressed as [44] Eg (T ) = Eg0 + ˇT

(3)

where Eg0 is the absolute zero value of the band gap and ˇ is the rate of change of optical band gap with temperature. This temperature dependence of the onset and fundamental energy gaps is shown in Fig. 8(a) and (b). These graphical representations indicate the linear

dependence between annealing temperature and optical band gap energy. The linearity change is valid for thin film studied, as shown in Fig. 8. This dependence was expressed by Mathew et al. [45]. The values Eg01 , Eg02 , ˇ1 and ˇ2 for the low and high energy region were obtained and tabulated in Table 1. The negative ˇ1 and ˇ2 values show that the valence band goes up towards the conduction band. Thus, the decrease in the optical band gap is attributed to shortening of the interatomic distances. These changes are determined by the amplitude decrease of the atomic oscillations. The interatomic distances may be associated with a decrease in the amplitude of atomic oscillations around their equilibrium positions. It is well-known that the major contribution comes from a

Table 1 Some absorption parameters of the as-deposited and annealed samples of NPC thin films compared with those of other conjugated organic films. Compound

Eg1 (eV)

Eg2 (eV)

Eg01 (eV)

Eg02 (eV)

ˇ1 (eV/K)

NPC film TTBNc thin films Tin(II) 2,3-naphthalocyanine Poly(ethylene terepthalate)

1.55 1.46 – –

3.547 2.95 2.98 3.66

1.678 – 3.24 –

4.0832 – – 4.6357

−4.4 × 10−4 – −8.89 × 10−4 −3.15 × 10−3

ˇ2 (eV/K) −1.76 × 10−3 – – –

1

2

Reference

0.0196 – 0.029 0.027

0.049 – – –

Present work [8] [10] [45]

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

605

Fig. 6. (a) Absorption spectra of the as-deposited and annealed NPC thin films at different temperatures. (b) Absorption coefficient spectra, ˛, versus annealing temperature, Tannealing .

shift in the relative positions of the valence and conduction bands due to the temperature-dependence dilatation of the lattice and the temperature-dependent electron–lattice interaction [46]. It is also noted that the decrease in optical band gap increases the width of the energy bands. As a consequence of thermalannealing process, the optical band gap decreases and this decrease in the optical band can be explained by rearrangement during the annealing treatment. The absorption edge may be explained as a consequence of random thermal fluctuations in the band gap energy [44]. Also, the absorption edge can explained as a consequence of random thermal fluctuation in the band gap energy. Based on this model the steepness parameter  is calculated by the following relation [44]:

dEg k =−  dT

(4)

where k is the Boltzmann’s constant. According to energy range the values of  are referred as  1 and  2 and calculated from the slopes of Fig. 8(a) and (b) via Eq. (4) and listed in Table 1. 3.6. Electrical conductivity characteristics of NPC films The dark electrical conductivity of the as-deposited and annealed NPC thin films was studied using the hot probe technique in the temperature range 300–500 K. The thermal activation energy of the films was calculated from the Arrhenius plot (Fig. 9(a)) using the relation [43]:  = A exp

 − E  1

kT

+ B exp

 − E  2

kT

(5)

where E1 and E2 are the thermal activation energies in different linear regions corresponding to trap level impurities of the compound, A and B constants, T is the absolute temperature and  is

Fig. 7. (a) Photon energy dependence of (˛h)2 of the as deposited and annealed NPC thin films (low energy region), (b) photon energy dependence of (˛h)2 of the as deposited and annealed NPC thin films (high energy region).

606

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

1.56

3.6

(a)

(b)

1.54

3.5

Eg2(eV)

Eg1(eV)

1.52

1.50

3.4

3.3

1.48

3.2

1.46 300

350

400

450

500

T annealing(K)

300

350

400

450

500

T annealing (K)

Fig. 8. (a) Energy gap, Eg1 versus annealing temperature, Tannealing . (b) energy gap, Eg2 versus annealing temperature, Tannealing of NPC thin films.

the dark electrical conductivity. Fig. 9(a) shows that plotting ln  versus 1000/T yields a straight line with the slope can be used to determine the thermal activation energy of the film. Each curve contains two linear regions related to the two activation energies E1 and E2 , respectively. The activation energy corresponding to the higher temperature region ( E1 ) was associated with the resonant energy involved in a short lived excited state (intrinsic generation process) and the lower ones ( E2 ) are associated with a short lived charge transfer between impurities and the complex (impurity conduction) [10]. The values obtained for annealed samples at various temperatures are shown in Fig. 9(b). The activation energy value in the higher temperature region is in good agreement with E1 ≈ 1/2 Eg1 , matching with the results outlined from the optical characteristics in this work as well as those reported by other workers [10,47,48]. The conductivity of NPC film can be attributed to the great number of acceptor/donor states, which inject holes/electrons into the valence band due to the ␲-orbital overlap of neighboring molecules and also the pronounced anisotropy of the lattice due to the low

symmetry of NPC as compared to the metal substituted NPC [10]. Since the organic semiconductors have poor self-organizing properties due to the weak van der Waal intermolecular bonds, the successful model to describe the transport is by thermally activated charge carrier tunneling or via a slow process (hopping) [10]. 3.7. Dark current density voltage characteristics of Al/NPC/ITO devices To study the effect of annealing on junction characteristics, current density–voltage (J–V) characteristics of the as-deposited and annealed Al/NPC/ITO devices were measured and shown in Fig. 10(a). It is clear from this figure that the devices exhibit rectifying characteristics. This behavior can be understood by the formation of a barrier at the interface of Al/NPC that limits the forward and reverse carrier’s flow across the junction, where the built-in potential could be developed [48]. A rectification ratio RR [is calculated as the ratio of the forward current density to the reverse current density at a certain applied voltage, i.e. RR = (JF /JR )v=const ]

Fig. 9. (a) Temperature dependence of dark electrical conductivity of the as-deposited and annealed NPC thin films (b) activation energies versus annealing temperature.

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

-3

10

-4

60

300 K annealed at 350 K annealed at 400 K annealed at 450 K annealed at 500 K

(b)

50

40

Forward 10

-5

10

-6

10

-7

RR

2

J (A/cm )

(a) 10

607

Reverse

30

20

10 -2

-1

0

1

2

300

350

400

450

500

Tannealing (K)

V (Volt)

Fig. 10. (a) Dark current density–voltage characteristics (J–V) of Al/NPC/ITO devices, (b) rectification ratio, RR versus annealing temperature.

is narrow and becomes comparable to the diffusion length of the carriers [49,50]. The current density curve under forward bias, shown in Fig. 10(a), becomes quickly dominated by series resistance from contact wires or bulk resistance of the semiconductor, giving rise to the curvature at high current in the semi-logarithmic J–V plot. The series resistance is a very important parameter of photovoltaic device performance. The resistance of the Schottky diode is the sum total resistance value of the resistors in series and resistance in semiconductor devices in the direction of current flow. In order to estimate the series resistance of the devices, the semi-logarithmic J–V characteristics at room temperature were considered [51]. The simple way to obtain the device series resistance, Rs , is from the fitting of J–V curves recorded in the dark at high forward bias [52]. The dependence of Rs on the annealing temperature is shown in Fig. 11. The series resistance decreases with increasing annealing temperature and then an enhancement of the devices, performance is observed with annealing temperature.

2.4

n Rs

12

1.8

4

2

8 2.0

Rs(k .cm )

2.2

Ideality factor,n

[49] was calculated at ±1 V and shown in Fig. 10(b) as a function of annealing temperature. The tendency of the rectification ratio to increase when the annealing temperature increases from 350 to 500 K may be attributed to the increase in the width of the depletion region formed at the junction between the NPC and Al electrode [49]. The exponential behavior of J–V characteristics depends on the property of active material used in the investigated devices. The slope of J–V characteristics in the exponential region depends on two parameters, the ideality factor, n, and the reverse saturation current density, J0 . The ideality factor gives information about the recombination process taking place in the devices and shape of the interfaces, i.e. the internal bulk morphology for the organic devices [49,50]. The saturation current density gives the numbers of charges capable of overcoming the energetic barrier under reverse bias [49,50]. Theoretically, the ideality factor, n, of the Schottky diode made on Al/NPC is introduced to take into account the deviation of the experimental J–V data from the ideal thermionic emission model and can then be evaluated from the slope of a semi-logarithmic plot of J versus V as shown in Fig. 10(a). However, this curve is clearly non-linear in the relatively high voltage region, indicating that another transport mechanism is present in the device. The ideality factor, n, depends on the current flow mechanism through the junction. It is equal to unity when the diffusion current dominates the carrier transport through the junction and becomes larger than one for the generation-recombination current mechanism. The experimental value of the ideality factor, n is evaluated by fitting equation (n = q/kT[1/(d(ln J/J0 )/dV]) to the experimental J–V characteristics and illustrated in Fig. 11 as a function of annealing temperature. As shown in Fig. 11 the ideality factor, n, changes from 2.42 at 300 K down to 1.49 at annealing temperature of 400 K indicating that the diode current changes its character from the space-charge generation-recombination current to the diffusion current with the increasing annealing temperature. The generation-recombination current dominating at room temperature is a sign of a large number of defect recombination center present in the junction area [49,50]. If there are no defects in the junction, the diode current would be diffusion current and ideality factor would be equal to unity. At enhanced annealing temperatures, the ideality factor of the devices approaches unity, which shows that the width of the depletion layer

1.6 0 1.4

300

350

400

450

500

T annealing (K) Fig. 11. Annealing temperature dependence of both the ideality factor and series resistance of Al/NPC/ITO devices.

608

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

Table 2 Photovoltaic parameters of the as-deposited and annealed Al/NPC/ITO devices compared with those of other conjugated organic films. Device structure

Condition

Jsc (mA/cm2 )

Voc (V)

Al/NPC/ITO Al/NPC/ITO Al/NPC/ITO Al/NPC/ITO Al/NPC/ITO Al/Pyronine B/ITO Al/Methyl orange/ITO Al/poly (3-phenyl azo methinethiophene)/ITO Al/polythiophene/ITO

T = 300 K Tannealing = 350 K Tannealing = 400 K Tannealing = 450 K Tannealing = 500 K T = 300 K T = 300 K T = 300 K T = 300 K

0.53 0.94 1.20 1.43 1.69 4.00 14.9 × 10−3 1.1 × 10−3 0.98 × 10−3

0.367 0.417 0.446 0.471 0.512 0.50 0.37 0.85 0.50

3.8. Photovoltaic characteristics of Al/NPC/ITO devices The photovoltaic properties of the as-deposited (at 300 K) and annealed Al/NPC/ITO devices at different annealing temperatures in the range 350–500 K were determined by measuring the J–V characteristics using a load resistance under illumination of 100 mW/cm2 (Fig. 12). The current in the reverse direction is strongly increased by illumination as compared to the forward current. The photocurrent is higher than the dark current at the same bias. This suggests that the light generates carrier contributing photocurrent due to the production of excitons as a result of the light absorption [53]. Under illumination, most of the excitons will be generated at Al/NPC interface and dissociate into free carriers resulting in a higher photocarriers. If excitonic motion is involved, then one has to assume that the excitons diffuse towards the barriers and dissociate into free carriers at the barrier, the resulting electrons and holes move to the NPC side and ITO side, respectively [53]. The plot of output power versus voltage is shown in Fig. 13. The output power increases with bias voltage and reaches a maximum value. The maximum point is called the maximum power point with coordinate Pmax (=Jm × Vm ). Here Jm and Vm are maximum current density and voltage values of maximum power point, respectively. These maximum values represent the condition, where the devices can deliver its maximum power to an external load resistance. The obtained photovoltaic results suggest that the Al/NPC/ITO devices generate moderate photovoltages with low photocurrents.

3

2

300 K annealed annealed annealed annealed

at 350 K at 400 K at 450 K at 500 K

0.70 1.01 1.35 1.84 2.65 0.798 17.2 × 10−4 – –

FF

(%)

Reference

0.278 0.392 0.369 0.366 0.436 0.35 0.310 0.42 0.38

0.70 1.01 1.35 1.84 2.65 0.798 0.00172 0.35 0.00037

Present work Present work Present work Present work Present work [55] [56] [57] [58]

These low photocurrents may be attributed to the recombination of some charged carrier after the dissociation of the excitons and the large value of the series resistance as well as a finite equivalent shunt resistance [54]. The annealing temperature dependence of the photovoltaic parameters like short circuit current density, Jsc , open circuit voltage, Voc , maximum output power, Pmax and fill factor, FF are listed in Table 2. The as-deposited device exhibits a short-circuit current density (Jsc ) of 0.53 mA/cm2 , an open-circuit voltage (Voc ) of 0.367 volt, and a fill factor (FF) of 0.278which lead to a power conversion efficiency,

, of 0.70%. The maximum power conversion efficiency ( ) of 2.65% is achieved for the device annealed at 500 K as shown in Fig. 13. The key parameters of the devices for different annealing conditions are compared to conjugated organic devices in the literatures [55–58] and listed in Table 2. The power conversion efficiency ( ) and the fill factor of the devices were calculated using the relations [59]:

=

 V J FF  oc sc

and FF =

(6)

Pin

J V  m m

(7)

Jsc Voc

where Pin is the incident light power density. The fill factor (FF) increases gradually from 0.278 to 0.436 with increasing annealing temperatures. Moreover, open-circuit voltage (Voc ) changes from 0.367 to 0.512 V with increasing annealing temperature from 350 to 500 K. The open-circuit voltage mostly depends upon the energy gap between the highest occupied molecular orbital (HOMO) level of donor material and the lowest unoccupied molecular orbital (LUMO) level of the acceptor material [60,61]. Annealing temperatures also affect the short-circuit current density (Jsc ), which reaches a maximum of 1.69 mA/cm2 at 500 K. The

2.5 1 2.0

300 K annealed at 350 K annealed at 400 K annealed at 450 K annealed at 500 K

2

Pout(mW/cm )

2

J (mA/cm )

Pmax (mW/cm2 )

0

-1

1.5 1.0 0.5

-2 0.0

0.1

0.2

0.3

0.4

0.5

V (Volt) Fig. 12. Current density–voltage characteristics under illumination of 100 mW/cm2 of the as deposited and annealed NPC thin films.

0.0 0.0

0.1

0.2

0.3

0.4

0.5

V (Volt) Fig. 13. Output power of the as deposited and annealed NPC thin films under illumination of 100 mW/cm2 .

A.A.M. Farag et al. / Applied Surface Science 259 (2012) 600–609

photovoltaic properties confirm the enhancement of the performance of Al/NPC/ITO devices as a result of annealing process. 4. Conclusion NPC thin films were prepared by thermal evaporation technique and the thermal stability was checked by thermogravimetric analysis, TGA. The crystalline nature of the as-deposited and heat treated thin films was examined by XRD suggesting a possibility of stacking arrangements commonly seen in NPC structures, while its spherical nanograins were disclosed in the SEM image. Two optical energy gaps with direct transitions were obtained for the asdeposited and annealed NPC films. Moreover, the optical band gaps were reduced linearly with annealing temperature. The activation energy from the conductivity studies showed a decreasing tendency with annealing temperature. Moreover, one found that there was a significant effect of annealing on the photovoltaic properties of Al/NPC/ITO devices. Photovoltaic characteristics with a power conversion efficiency of 2.65% under illumination of 100 mW/cm2 were demonstrated for the 500 K annealed device. The open-circuit voltage, short-circuit current density, maximum output power and the power conversion efficiency of the devices were found to be improved on annealing the sample at 500 K for 1 h. Acknowledgment This work was carried out through the collaboration between Department of Physics, Faculty of Education, Ain Shams University and Biophysics Department, Faculty of Science, Cairo University, Egypt. References [1] J. Janczak, R. Kubiak, Journal of Molecular Structure 516 (2000) 81. [2] Y. Park, Kyu S. Han, B.H. Lee, S. Cho, K.H. Lee, S. Im, M.M. Sung, Organic Electronics 12 (2011) 348. [3] S.H. Jang, A.K.Y. Jen, Comprehensive Nanoscience and Technology 1 (2011) 143. [4] A. Gunsel, M. Kandaz, F. Yakuphanoglu, W.A. Farooq, Synthetic Metals 161 (2011) 1477. [5] T. Lopez, E. Ortiz, M. Alvarez, J. Navarrete, J.A. Odriozola, F. Martinez-Ortega, E.A. Paez-Mozo, P. Escobar, K.A. Espinoza, I.A. Rivero, Nanomedicine: Nanotechnology, Biology and Medicine 6 (2010) 777. [6] N. Pana, L. Rintoul, D.P. Arnold, J. Jiang, Polyhedron 21 (2002) 1905. [7] T. Rawling, A.M. McDonagh, S.B. Colbran, Inorganica Chimica Acta 361 (2008) 49. [8] I. Dhanya, C.S. Menon, Journal of Non-Crystalline Solids 357 (2011) 3631. [9] E. Kymakis, G.A.J. Amaratunga, Solar Energy Materials and Solar Cells 80 (2003) 465. [10] N.S. Panicker, T.G. Gopinathan, I. Dhanya, C.S. Menon, Physica B 405 (2010) 4556. [11] C. Fabris, M. Soncin, G. Miotto, L. Fantetti, G. Chiti, D. Dei, G. Roncucci, G. Jori, Journal of Photochemistry and Photobiology B 83 (2006) 48. [12] X. Zhang, Y. Zhang, J. Jiang, Journal of Molecular Structure 673 (2004) 103. [13] M.E. El-Khouly, L.M. Rogers, M.E. Zandler, G. Suresh, M. Fujitsuka, O. Ito, F. D’Souza, ChemPhysChem 4 (5) (2003) 474. [14] K.P. Unnikrishnan, J. Thomas, V.P.N. Nampoori, C.P.G. Vallabhan, Optics Communication 204 (2002) 385. [15] S. Casilli, M. De Luca, C. Apetrei, V. Parra, Á.A. Arrieta, L. Valli, J. Jiang, M.L. Rodríguez-Méndez, J.A. De Saja, Applied Surface Science 246 (2005) 304. [16] B. Wang, X. Zuo, X. Cheng, Y. Wu, Thin Solid Films 517 (2008) 937. [17] T.V. Dubinina, S.A. Trashin, N.E. Borisova, I.A. Boginskaya, L.G. Tomilova, N.S. Zefirov, Dyes and Pigments 93 (2012) 1471. [18] Y. Feng, X. Zhang, W. Feng, Organic Electronics 11 (2010) 1016.

609

[19] L. El Chaar, L.A. lamont, N. El Zein, Renewable and Sustainable Energy Reviews 15 (2011) 175. [20] A. Skreiberg, O. Skreiberg, J. Sandquist, L. Sorum, Fuel 90 (2011) 2182. [21] S. Fu, C. Du, M. Zhang, A. Tian, X. Zhang, Progress in Organic Coatings 73 (2012) 149. [22] R. Seoudi, G.S. El-Bahy, Z.A. El Sayed, Journal of Molecular Structure 753 (2005) 119. [23] A. Bilgin, C. Yagcı, U. Yıldız, E. Ozkazanc, E. Tarcan, Polyhedron 28 (2009) 2268. [24] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish, Spectrochimica Acta, Part A 69 (2008) 205. [25] F. Lu, M. Bao, C. Ma, X. Zhang, D.P. Arnold, J. Jiang, Spectrochimica Acta, Part A 59 (2003) 3273. [26] E.A.F. Carrasco, M. Campos-Vallette, M.S. Saavedra, G.F. Diaz, R.E.J. Clavijo, V. Garcıˇıa-Ramos, S. Sanchez-Cortes, Vibrational Spectroscopy 26 (2001) 201. [27] J.P. Wang, X.P. Zhao, H.L. Guo, Q. Zheng, Optical Materials 30 (2008) 1268. [28] P. Dumas, L. Miller, Vibrational Spectroscopy 32 (2003) 3. [29] J. Guo, X. Zhang, Carbohydrate Research 339 (2004) 1421. [30] W. Wan, Y.Z. Meng, Q. Zhu, S.C. Tjong, A.S. Hay, Polymer 44 (2003) 575. [31] R. Shirley, The CRYSFIRE System for Automatic Powder Indexing: User’s Manual, The Lattice Press, Guildford, Surrey GU2 7NL, England, 2000. [32] J. Laugier, B. Bochu, LMGP-Suite of programs for the interpretation of Xray experiments, ENSP/Laboratories des Materiaux et du Genie Physique, BP46.38042, Saint Martin d’Heres, France, 2000. [33] M. Abdel Rafea, H. Farid, Materials Chemistry and Physics 113 (2009) 868. [34] F. Cheng, G. Fang, X. Fan, N. Liu, N. Sun, P. Qin, Q. Zheng, J. Wan, X. Zhao, Solar Energy Materials and Solar Cells 95 (2011) 2914. [35] M.M. El-Nahass, K.F. Abd-El-Rahmana, A.A. Al-Ghamdi, A.M. Asiri, Physica B 344 (2004) 398. [36] M.T.M. Choi, P.P.S. Li, K.P.Ng. Dennis, Tetrahedron 56 (2000) 3881. [37] S. Tai, N. Hayashi, Journal of the Chemical Society, Perkin Transactions 2 (1991) 1275. [38] M.M. El-Nahass, A.A.M. Farag, A.A. Atta, Synthetic Metals 159 (2009) 589. [39] S. Ambily, C.S. Menon, Thin Solid Films 347 (1999) 284. [40] B. Bialek, I.G. Kim, J.I. Lee, Synthetic Metals 129 (2002) 151. [41] M.M. El-Nahass, A.A.M. Farag, K.F. Abd El-Rahman, A.A.A. Darwish, Optics & Laser Technology 37 (2005) 513. [42] A.A.M. Farag, I.S. Yahia, Optics Communication 283 (2010) 4310. [43] X. Jiang, F. Chen, H. Xu, L. Yang, W. Qiu, M. Shi, M. Wang, H. Chen, Solar Energy Materials and Solar Cells 94 (2011) 338. [44] F. Yakuphanoglu, M. Arslan, M. Kucukislamoglu, M. Zengin, Solar Energy 79 (2005) 96. [45] X. Mathew, N.R. Mathews, P.J. Sebastian, Solar Energy Materials and Solar Cells 70 (2001) 277. [46] B.S. Li, Y.C. Liu, Z.Z. Zhi, D.Z. Shen, Y.M. Lu, J.Y. Zhang, X.W. Fan, Journal of Crystal Growth 240 (2002) 497. [47] S.M.S. Haggag, E.I. Fathallah, M.E. Mahmoud, M. Abdel Rafea, A.A.M. Farag, Polyhedron 30 (2011) 1752. [48] K.N. Narayanan Unni, C.S. Menon, Journal of Materials Science Letters 20 (2001) 1207. [49] A.A.M. Farag, A. Ashery, E.M.A. Ahmed, M.A. Salem, Journal of Alloys and Compounds 495 (2010) 116. [50] A.A.M. Farag, I.S. Yahia, T. Wojtowicz, G. Karczewski, Journal of Physics D: Applied Physics 43 (2010) 215102. [51] A.A.M. Farag, A. Ashery, F.S. Terra, M. Nasr, Journal of Optoelectronics and Advanced Materials: Rapid Communications 12 (2010) 2413. [52] F. Chen, Journal of Physics D: Applied Physics 43 (2010) 025104. [53] G.D. Sharma, V.S. Choudhary, S.K. Sharma, M.S. Roy, Journal of Physics and Chemistry of Solids 69 (2008) 2639. [54] H.M. Zeyada, M.M. El-Nahass, E. El-Menyawy, Solar Energy Materials and Solar Cells 92 (2008) 1586. [55] A.A.M. Farag, W.G. Osiris, E.A.A. El-Shazly, Journal of Alloys and Compounds 509 (2011) 6467. [56] A.A.M. Farag, A.M. Mansour, M. Abdel-Rafea, A.H. Ammar, Synthetic Metals 161 (2011) 2135. [57] G.D. Sharma, S.K. Sharma, M.S. Roy, Thin Solid Films 468 (2004) 208. [58] C.O. Too, G.G. Wallace, A.K. Burrell, G.E. Collis, D.L. Officer, E.W. Boge, S.G. Brodi, E.J. Evans, Synthetic Metals 123 (2001) 53. [59] S. Karak, S.K. Ray, A. Dhar, Solar Energy Materials and Solar Cells 94 (2010) 836. [60] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Advanced Functional Materials 11 (2001) 374. [61] M.C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Advanced Materials 18 (2006) 789.