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Effect of swift heavy ion (SHI) irradiation on transparent conducting oxide electrodes for dye-sensitized solar cell applications
Nuclear Instruments and Methods in Physics Research B 353 (2015) 35–41
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of swift heavy ion (SHI) irradiation on transparent conducting oxide electrodes for dye-sensitized solar cell applications Hemant Kr. Singh a, D.K. Avasthi b, Shruti Aggarwal a,⇑ a b
University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi, India Inter University Accelerator Center, Post Box 10502, New Delhi, India
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 14 April 2015 Accepted 15 April 2015
Keywords: Transparent conducting oxides (TCOs) Indium tin oxide (ITO) Fluorine-doped tin oxide (FTO) Swift heavy ion (SHI) Dye-sensitized solar cell (DSSC)
a b s t r a c t Transparent conducting oxides (TCOs) are used as electrodes in dye-sensitized solar cells (DSSCs) because of their properties such as high transmittance and low resistivity. In the present work, the effects of swift heavy ion (SHI) irradiation on various types of TCOs are presented. The objective of this study is to investigate the effect of SHI on TCOs. For the present study, three different types of TCOs are considered, namely, (a) FTO (fluorine-doped tin oxide, SnO2:F) on a Nippon glass substrate, (b) ITO (indium tin oxide, In2O3:Sn) coated on polyethylene terephthalate (PET) on a Corning glass substrate, and (c) ITO on a Corning glass substrate. These films are irradiated with 120 MeV Ag+9 ions at fluences ranging from 3.0 1011 ions/cm2 to 3.0 1013 ions/cm2. The structural, morphological, optical and electrical properties are studied via X-ray diffraction (XRD), atomic force microscopy (AFM), UV–Vis absorption spectroscopy and four-probe resistivity measurements, respectively. The ITO-PET electrode is found to exhibit superior conductivity and transmittance properties in comparison with the others after irradiation and, therefore, to be the most suitable for solar cell applications. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The modification of the properties of thin films by high-energy particles (such as electrons, protons, and a-rays) has received a great deal of attention in recent years, primarily in regard to the technological requirements of the procedures and the stability of the resulting material when exposed to cosmic radiation. Cosmic radiation comprises a variety of high-energy particles (such as electrons, protons, a-rays, and heavy ions), which may cause the films, and thus the performance of the devices fabricated using those films, to degrade over years of operation. The radiation in extraterrestrial regions is strong and creates a large number of lattice defects in the materials of a solar cell. There are two main types of effects of irradiation: transient effects due to electron–hole pair generation and permanent effects due to changes in the crystal lattice. Dye-sensitized solar cells (DSSCs) represent the future generation of solar cells and one promising alternative to the conventional and energy-intensive silicon-based solar cells. However, reports concerning the effects of irradiation on DSSC components with a focus on parameters such as
⇑ Corresponding author. E-mail address: [email protected] (S. Aggarwal). http://dx.doi.org/10.1016/j.nimb.2015.04.031 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
transmittance, conductivity, and sheet resistance are few, and further investigations into the subject are required [1]. Transparent conducting oxide (TCO) is an important component in DSSC. The properties of TCO play an important role in the performance of a DSSC. Incident light enters the device through the TCO, and the optical and electrical properties of this material have a significant effect on the device. TCO films with low resistivity and high transmittance are desired for solar cell applications. The most commonly used TCO materials are indium tin oxide (ITO, In2O3:Sn, also known as Sn-doped indium oxide) and fluorine-doped tin oxide (FTO, SnO2:F). It is a well-known fact that the high conductivity of a TCO film is attributable to intrinsic defects (oxygen vacancies) and dopants (F, Sn, etc.) [2–7]. Doping induces degeneracy through the substitution of four-valent Sn (tin) at three-valent In (indium) sites in ITO and fluorine at oxygen sites in FTO [8–11]. Although ITO is the most commonly used TCO in most devices, including DSSCs, reports of studies of the defect structure of SHI-irradiated ITO are limited. SHI irradiation offers a broad range of possibilities for modifying the structure and properties of materials [12] because a very high local energy density is deposited into the solid along the paths of the ions. The linear density of energy loss of SHI irradiation is on the order of keV/nm, which results in a very short (several hundreds of picoseconds), very localized (a cylinder of approximately 10 nm in diameter) and very high (approximately 0.1 eV/atom)
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excitation yield of electrons [13]. Under such extreme conditions, non-equilibrium processes are initiated and modify the crystal structures of the TCOs, which, in turn, causes their structural, electrical and optical properties to change. It is also possible to optimize the desired properties at a particular level of irradiation [14–16]. Deshpande et al. [17] have reported that the transmittance and conductivity of ITO thin films can be substantially altered via ion beam irradiation. In that study, the transmittance of the ITO film decreased by 40% and the bulk resistance decreased from 200 ohm to 60 ohm. In another study, Singh et al. [18] reported that the transmittance and sheet resistance increased by 13% and from 8 ohm/sq to 18 ohm/sq, respectively. The reports of Deshpande et al. and Singh et al. are contrary to each other because the films considered for these studies were prepared using different techniques and hence had different microstructures. Deshpande et al. [17] prepared ITO samples via the spray pyrolysis technique, whereas Singh et al. [18] used ITO samples prepared using the RF sputtering method. These initial studies indicate that the microstructure of a film plays an important role in determining the effect of SHI irradiation. A similar study for CdS thin films has also been reported by Ison et al. [19]. Herein, we investigate and report the effect of SHI irradiation on DSSC photo-anodes.
2. Experimental details Three types of TCOs were considered: (1) FTO film on a Nippon glass substrate from NSG/Pilkington, Hong Kong, China; (2) ITOPET [In2O3:Sn with polyethylene terephthalate (PET)] film on a Corning glass substrate from Sigma–Aldrich, Bangalore, India; and (3) ITO film on a Corning glass substrate from Vin Karola Instruments, PVT Ltd, USA. The SHI irradiation of these samples was performed using the 15 UD Pelletron tandem accelerators at the Inter-University Accelerator Centre (IUAC), New Delhi, India. All TCO films were subjected to 120 MeV Ag+9 ion irradiation at five different fluences ranging from 3.0 1011 ions/cm2 to 3.0 1013 ions/cm2 under a 10 6 Torr vacuum, and their structural, morphological, optical and electrical properties were compared. The electronic and nuclear energy loss values for 120 MeV Ag+9 ions in the FTO film were found to be 1.115 102 and 0.52 eV/nm, respectively. These values were calculated using the SRIM simulation software (SRIM-2010) [20,21]. Similarly, these values were found to be 1.513 102 eV/nm and 0.80 eV/nm, respectively, for the two types of ITO samples. These values clearly indicate that the entire passage of the ions through the films is dominated by the electronic energy loss, which is approximately 200 times higher than the nuclear loss. The stopping ranges of Ag ions in the FTO and ITO samples are 19.33 lm and 14.06 lm s, respectively, which are several times greater than the film thicknesses of these materials. Hence, radiation can easily pass through the films, and the effect of ion implantation can be completely
neglected. These values for the TCOs are summarized in Table 1, and the details of the computation are provided elsewhere [20,21]. 3. Results and discussion The structural, morphological, optical and electrical properties of the TCO samples were studied via XRD, AFM, UV–Vis and fourprobe measurements, and analyses of these results are presented in the following sections. 3.1. Structural analysis 3.1.1. XRD study The XRD spectra of the FTO film are given in Fig. 1a. They show that the FTO is polycrystalline in nature, with the dominant plane (200) at 2h = 37.757°. However, the full width at half maximum FWHM (b) of this peak monotonically increases with increasing fluence. The crystallite size, as calculated using Scherrer’s formula [22], decreases from 34 nm to 23 nm with increasing fluence. In the case of the ITO-PET sample, the FWHM of the dominant plane (2 2 2) decreases and crystallite size increases monotonically from 13 nm to 20 nm with increasing fluence, as shown in Fig 1b, which is contrary to what is observed in the FTO. Further, another important plane (4 0 0), associated with oxygen vacancies, is observed to become stronger with increasing fluence. This is caused by an increase in oxygen vacancies, as previously reported in the literature [17,23]. Note that the presence of the diffraction peak is not directly related to the presence of oxygen vacancies; it may appear or disappear because of changes in the structure factor. For pure ITO on a Corning glass substrate, it is observed that the peaks of the pristine samples are similar to those of the ITO-PET samples, i.e., the (2 2 2), (4 0 0), (5 5 1) and (4 4 1) planes have dominant peaks, and the splits in the (2 2 2) peak are also similar, as shown in Fig. 1c. When a sample is irradiated, the peaks become sharper for ITO-PET and broader for ITO on Corning glass. This finding is attributed to the different types of growth that occur in ITO films, which seem to merge into a single peak for the ITO-PET film, indicating that the structure is becoming more cubic. When ITO films are deposited on different substrates (here, two types of substrates are considered), the ion interactions and the thermal dissipation of ion energy differ, although they occur within the same material. Microstructural differences are responsible for these different results observed for the same material (ITO), as reported by Ison et al. [19], and are also reflected in the XRD patterns recorded in the present study. The SHI irradiation produced different crystalline effects in the FTO and ITO films. It was observed that the crystallinity of the FTO decreased with increasing fluence, whereas that of the ITO films improved. The different crystalline behaviors in the FTO and ITO films upon irradiation can be attributed not only to their different microstructures but also to the different thermal conductivity
Table 1 Details of TCOs considered in the study. S. No
TCOs
Material composition
Company
Thickness (nm)
Energy loss for 120 MeV SHI of Ag+9 Sn (eV/ nm)
Se (eV/ nm)
R (lm)
1
FTO
SnO2:F
Kintec, Hong-Kong, China
250
0.52
111.50
19.33
2
ITO-PET
Sigma–Aldrich, India
160
0.80
151.30
14.06
3
ITOcorn
In2O3:Sn + PET (50 nm) In2O3: Sn
Vin Carola Instrument, Pvt. Ltd., USA
160
0.80
151.30
14.06
Inference
Implantation of Ag+9 is ignored Implantation of Ag+9 is ignored Implantation of Ag+9 is ignored
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values of their respective substrates. A material with a higher thermal conductivity K dissipates heat energy more homogeneously during irradiation. The amorphization of the FTO films can be attributed to their lower value of K in comparison with the ITO films. In the literature, there are several reports that discuss the electrical conductivity of TCO films but very few related to their thermal conductivity [24–26]. It has been reported that the electrical conductivity of an ITO thin film is approximately 1.5 104 S/cm [24,25], which is 100 times higher than the conductivity of an FTO thin film, which is on the order of 6.66 102 S/cm [26]. Generally, conducting materials with high electrical conductivity also have high thermal conductivity. Therefore, this difference may be attributable to the thermal conductivity of ITO, which is superior to that of an FTO film and hence results in better heat conduction within the film. This is one possible reason for the increased crystallinity of the ITO films in comparison with the FTO films with increasing fluence. The detailed XRD results for all TCO films along with the corresponding fluences are given in Table 1.
Fig. 1. XRD patterns of pristine and irradiated (Ag+9 SHI at an energy of 120 MeV) TCOs. (a) FTO on Nippon glass: the crystallinity is degraded after irradiation. (b) ITO-PET on Corning glass: the crystallinity is increased after irradiation. (c) ITO on Corning glass: the crystallinity is increased after irradiation.
3.1.2. AFM study: surface morphology The rms roughness values of the pristine and 3 1013 ions/cm2 irradiated samples for the FTO, ITO-Corning and ITO-PET samples are given in Table 2. AFM micrographs of the FTO (Fig. 2a and b), ITO-PET (Fig. 2c and d) and ITO-Corning (Fig 2e and f) samples show that upon irradiation, the rms roughness increases for the FTO and ITO-Corning samples and decreases for the ITO-PET samples. In the case of the FTO, the grain size distribution shifts from a lower to a higher range (200–250 nm to 250–300 nm), whereas for the ITO-PET, the grain size distribution decreases from a higher to a lower range (50–100 nm to 0–50 nm). For the ITO-Corning samples, the grain size distribution remains the same, in the range of 50–100 nm. However, one would expect the grain size distribution of the ITO-Corning films to increase, consistent with their higher roughness after irradiation compared with that of the pristine ITO-PET films. These findings indicate that the same material (ITO, in the present case) can exhibit different grain growth for the same SHI irradiation when deposited on different substrates. The reason for this difference may be the different thermal capacities of the substrates, which would typically dissipate heat in a dissimilar manner and thus cause different grain growth. It should also be noted that the electrical conductivity of the FTO film is lower than that of the ITO film, as mentioned above [24–26]. It is clear from these results that the larger grain size distribution of the FTO films corresponds to a higher rms roughness (8.1–9.5 nm) and the lower grain size distribution of the ITO films corresponds to a smaller value of rms roughness (1.8–3.4 nm). Furthermore, for the ITO-PET samples, the rms roughness
Table 2 Parameters of TCOs affected by 120 MeV Ag+9 radiation. S. No
1 2 3 4 5 6
Fluence (ion/cm2)
Pristine 3.0 1011 1.0 1012 3.0 1012 1.0 1013 3.0 1013
Sample type FTO-Nippon glass
ITO-PET glass
ITO-Corning glass
Crystallinity
T* (%)
Roughness (nm)
Rsh* (X/h)
Crystallinity
T* (%)
Rough ness (nm)
Rsh* (X/h)
Crystallinity
T* (%)
Rough ness (nm)
Rsh* (X/h)
Ref.* Dec* Dec Dec Dec Dec
65 62 55 52 55 53
8.1 –** – – – 9.5
21 26 30 28 27 72
Ref. Inc* Inc Inc Inc Inc
65 65 75 80 76 77
4.1 – – – – 2.8
12 15 16 18 20 20.5
Ref. Inc Inc Inc Inc Inc
80 60 67 66 66 66
1.8 – – – – 3.4
7 7.5 9.5 10.5 11 11.5
Optimum results are obtained at this fluence (3.0 1012) value. * T: transmittance; Rsh: sheet resistance; Ref.: reference; dec: decreases; inc: increases. ** ‘–’ roughness is not observed at these fluences during AFM measurements.
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Fig. 2. AFM micrographs of pristine and irradiated (3 1013 ions/cm2 fluence) TCOs. (a) Pristine FTO on Nippon glass: the rms roughness of the film is 8.1 nm. (b) Irradiated FTO on Nippon glass: the rms roughness of the film (9.5 nm) is increased after irradiation. (c) Pristine ITO-PET on Corning glass: the rms roughness of the film is 3.2 nm. (d) Irradiated ITO-PET on Corning glass: the rms roughness of the film (1.6 nm) is decreased after irradiation. (e) Pristine ITO on Corning glass: the rms roughness of the film is 1.8 nm. (f) Irradiated ITO on Corning glass: the rms roughness of the film (4.1 nm) is increased after irradiation.
decreases by approximately a factor of 2 (i.e., from 4.1 nm to 2.8 nm), whereas for the ITO-Corning samples, it increases by a factor of 2. These results can be attributed to the different microstructures of the same ITO material on the different substrates, as reported by Ison et al. [19]. The details of the AFM measurements (especially the roughness values) for all TCOs and the corresponding fluences are given in Table 2. 3.2. Optical analysis The UV–Vis transmittance spectra of the FTO and ITO films are shown in Fig. 3a–c. The transmittance (T) decreased by 10–15% in the FTO and ITO-Corning films and increased by almost the same
percentage for the ITO-PET samples upon SHI irradiation. This shows that transmittance (T) of ITO-PET samples can be increased by applying SHI irradiation, which is a very useful result because it increases the amount of incident visible radiation that reaches the active region of the solar cell. Fig. 4 shows the transmittances (T) of all TCO samples at a wavelength of 550 nm. It can be observed that the transmittance (T) decreases slightly with increasing fluence for the ITO-Corning samples, whereas it increases for the ITO-PET samples. Furthermore, the transmittance (T) values for the FTO films are lower than those of the ITO films. In the present case, the 3 1012 ions/cm2 fluence is of particular interest, because the results achieved at this fluence are superior compared with the others.
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In the case of the FTO films, the band edge shifts toward higher wavelengths, whereas for the ITO films, this shift is marginal, as shown in Fig. 3a–c. This illustrates that the creation of defects is insufficient to create an observable change in the case of the ITO films. An increase or decrease in the flat (visible) region of the transmittance (T) of TCO films is predominantly attributable to the surface morphology of the materials, and in the present case, it is substantial (10–15%). The details of the transmittances (T) of all types of TCO films and the corresponding fluences are given in Table 2. 3.3. Electrical analysis
Fig. 3. Plots of transmittance vs. wavelength for the TCOs. (a) FTO on Nippon glass: the transmittance is decreased after SHI irradiation. At a fluence of 3.0 1012 ions/ cm2, it reaches its lowest value of 52%. (b) ITO-PET on Corning glass: the transmittance is increased after SHI irradiation. At a fluence of 3.0 1012 ions/ cm2, it reaches its highest value of 80%. (c) ITO on Corning glass: the transmittance is decreased after SHI irradiation. At a fluence of 3.0 1011 ions/cm2, it reaches its lowest value of 60%.
It is evident from Fig. 5 that the increase in the sheet resistance of the FTO films is marginal up to a fluence of 1.0 1013 ions/cm2 and then rises sharply thereafter, i.e., at 3 1013 ions/cm2. In the case of the ITO-PET and ITO-Corning samples, the sheet resistance increases marginally as the fluence increases and remains within acceptable limits for solar cell applications. For the ITO-Corning samples, the increase in sheet resistance observed after irradiation may be attributable to the dominance of the roughness over the other parameters, such as the crystallinity (which is improving) and the band gap (which is decreasing). In addition, the creation of unfavorable defects may be one possible reason for the increase in the sheet resistance upon irradiation. It is important to note that roughness is not the sole parameter driving the change in the resistivity of the film, as observed in the case of the ITO-PET films, wherein the sheet resistance is monotonically increasing although the roughness is decreasing. Here, the crystallinity is improving and the band gap is also decreasing marginally. The trends of all these parameters suggest a decrease in the sheet resistance of the ITO-PET samples; however, on the contrary, it is increasing. The reason for this phenomenon may lie in certain other dominant effects, such as the creation of defects or trap states, which may be instrumental in enhancing the sheet resistance of the material. Furthermore, in the present case, neither oxygen vacancies nor band gaps play a significant role in modifying the sheet resistance, as reported previously by several other groups [17,23,27]. Therefore, an increase in roughness or other unfavorable defects may be the primary cause of the increasing resistance in both ITO films [28–29]. In the case of the FTO samples, their sheet resistances also follow the same trend as those of the ITO samples up to a fluence of 1 1013 ions/cm2 and rise sharply beyond this point. The steep rise in the resistance observed in the case of the FTO samples at a fluence of 3 1013 ions/cm2 may not be solely attributable to the roughness, considering that the increase in roughness is very small in comparison to the increase in the sheet resistance. A decrease in the crystallinity of an irradiated film is another factor that can lead to an increase in sheet resistance, as is evident from the XRD study, but it cannot explain this sudden increase, as amorphization begins at a lower fluence. Amorphization causes defects to form at the edges of both the valence and conduction bands and thus increases the resistance. One possible reason for the sudden increase in the sheet resistance of the FTO films may be the release of fluorine gas from the films, because the fluorine atoms decrease the concentration of electrons in the conduction band of FTO [8–10]. According to the thermal spike model [30], transient heat may be produced inside the film along the ion tracks, and at higher fluences, columnar defects that overlap each other will form, which will certainly result in the release of large amounts of fluorine gas. Overlapping columnar defects formed at higher fluences will affect the entire sample homogeneously. These results are also summarized in Table 2.
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Fig. 4. A plot of the transmittance vs. fluence curves for the TCO electrodes. The ITO-PET on Corning glass is superior in comparison with the other electrodes after SHI irradiation, as its transmittance has improved substantially.
Fig. 5. A plot of sheet resistance vs. fluence for the TCO films. The sheet resistances of the ITO-PET and ITO on Corning substrates are slightly affected by SHI irradiation, but the FTO films on Nippon substrates are strongly affected, and their sheet resistances increase sharply at fluences of 1.0 1013 ions/cm2 and beyond.
4. Conclusion ’The present work focuses on the possibility of improving the efficiency of DSSCs by modifying the transmittance and conductivity of TCO film substrates by subjecting them to 120 MeV Ag+9 SHI irradiation. Of the three commonly used TCOs considered in this study, it is very clear that ITO-PET electrodes are superior to FTO and ITO-Corning electrodes in terms of both transmittance and conductivity after SHI irradiation. The transmittance of ITO-PET is increased by 15% after irradiation at a fluence of 3.0 1012 ions/cm2, whereas those of FTO and ITO-Corning are decreased by the same percentage. This fluence is very similar to that found in extraterrestrial regions, and
therefore, the results of this study are relevant for space applications. Furthermore, the observed increase in the sheet resistance of the ITO-PET film was marginal, whereas that of the FTO film was substantial. FTO films are prone to radiation damage upon exposure to an ion beam such as that used in the present experiment, whereas the crystallinity of ITO films improves upon ion irradiation. It can be concluded from the above investigation that ITO-PET electrodes are superior to FTO and ITO-Corning electrodes for solar cell applications. Hence, ITO-PET electrodes will serve as a better TCO for DSSCs. Further SHI irradiation at low fluences (3.0 1011 ions/cm2) will have little effect, as at this fluence, the transmittance is nearly constant and the sheet resistance changes only marginally.
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Acknowledgments This work was supported by the Inter University Accelerator Center (IUAC), New Delhi, India project # UFUP-43314. The authors SA and HKS are very grateful to Dr. F. Singh and Dr. D.C. Agrawal at IUAC New Delhi for providing valuable suggestions and discussions. The authors are also thankful to Mr. Pawan Kulriya (XRD) and Mrs. Indra Sulania (AFM) for providing valuable support during the measurements at IUAC New Delhi. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of swift heavy ion (SHI) irradiation on transparent conducting oxide electrodes for dye-sensitized solar cell applications Hemant Kr. Singh a, D.K. Avasthi b, Shruti Aggarwal a,⇑ a b
University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi, India Inter University Accelerator Center, Post Box 10502, New Delhi, India
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 14 April 2015 Accepted 15 April 2015
Keywords: Transparent conducting oxides (TCOs) Indium tin oxide (ITO) Fluorine-doped tin oxide (FTO) Swift heavy ion (SHI) Dye-sensitized solar cell (DSSC)
a b s t r a c t Transparent conducting oxides (TCOs) are used as electrodes in dye-sensitized solar cells (DSSCs) because of their properties such as high transmittance and low resistivity. In the present work, the effects of swift heavy ion (SHI) irradiation on various types of TCOs are presented. The objective of this study is to investigate the effect of SHI on TCOs. For the present study, three different types of TCOs are considered, namely, (a) FTO (fluorine-doped tin oxide, SnO2:F) on a Nippon glass substrate, (b) ITO (indium tin oxide, In2O3:Sn) coated on polyethylene terephthalate (PET) on a Corning glass substrate, and (c) ITO on a Corning glass substrate. These films are irradiated with 120 MeV Ag+9 ions at fluences ranging from 3.0 1011 ions/cm2 to 3.0 1013 ions/cm2. The structural, morphological, optical and electrical properties are studied via X-ray diffraction (XRD), atomic force microscopy (AFM), UV–Vis absorption spectroscopy and four-probe resistivity measurements, respectively. The ITO-PET electrode is found to exhibit superior conductivity and transmittance properties in comparison with the others after irradiation and, therefore, to be the most suitable for solar cell applications. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The modification of the properties of thin films by high-energy particles (such as electrons, protons, and a-rays) has received a great deal of attention in recent years, primarily in regard to the technological requirements of the procedures and the stability of the resulting material when exposed to cosmic radiation. Cosmic radiation comprises a variety of high-energy particles (such as electrons, protons, a-rays, and heavy ions), which may cause the films, and thus the performance of the devices fabricated using those films, to degrade over years of operation. The radiation in extraterrestrial regions is strong and creates a large number of lattice defects in the materials of a solar cell. There are two main types of effects of irradiation: transient effects due to electron–hole pair generation and permanent effects due to changes in the crystal lattice. Dye-sensitized solar cells (DSSCs) represent the future generation of solar cells and one promising alternative to the conventional and energy-intensive silicon-based solar cells. However, reports concerning the effects of irradiation on DSSC components with a focus on parameters such as
⇑ Corresponding author. E-mail address: [email protected] (S. Aggarwal). http://dx.doi.org/10.1016/j.nimb.2015.04.031 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
transmittance, conductivity, and sheet resistance are few, and further investigations into the subject are required [1]. Transparent conducting oxide (TCO) is an important component in DSSC. The properties of TCO play an important role in the performance of a DSSC. Incident light enters the device through the TCO, and the optical and electrical properties of this material have a significant effect on the device. TCO films with low resistivity and high transmittance are desired for solar cell applications. The most commonly used TCO materials are indium tin oxide (ITO, In2O3:Sn, also known as Sn-doped indium oxide) and fluorine-doped tin oxide (FTO, SnO2:F). It is a well-known fact that the high conductivity of a TCO film is attributable to intrinsic defects (oxygen vacancies) and dopants (F, Sn, etc.) [2–7]. Doping induces degeneracy through the substitution of four-valent Sn (tin) at three-valent In (indium) sites in ITO and fluorine at oxygen sites in FTO [8–11]. Although ITO is the most commonly used TCO in most devices, including DSSCs, reports of studies of the defect structure of SHI-irradiated ITO are limited. SHI irradiation offers a broad range of possibilities for modifying the structure and properties of materials [12] because a very high local energy density is deposited into the solid along the paths of the ions. The linear density of energy loss of SHI irradiation is on the order of keV/nm, which results in a very short (several hundreds of picoseconds), very localized (a cylinder of approximately 10 nm in diameter) and very high (approximately 0.1 eV/atom)
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excitation yield of electrons [13]. Under such extreme conditions, non-equilibrium processes are initiated and modify the crystal structures of the TCOs, which, in turn, causes their structural, electrical and optical properties to change. It is also possible to optimize the desired properties at a particular level of irradiation [14–16]. Deshpande et al. [17] have reported that the transmittance and conductivity of ITO thin films can be substantially altered via ion beam irradiation. In that study, the transmittance of the ITO film decreased by 40% and the bulk resistance decreased from 200 ohm to 60 ohm. In another study, Singh et al. [18] reported that the transmittance and sheet resistance increased by 13% and from 8 ohm/sq to 18 ohm/sq, respectively. The reports of Deshpande et al. and Singh et al. are contrary to each other because the films considered for these studies were prepared using different techniques and hence had different microstructures. Deshpande et al. [17] prepared ITO samples via the spray pyrolysis technique, whereas Singh et al. [18] used ITO samples prepared using the RF sputtering method. These initial studies indicate that the microstructure of a film plays an important role in determining the effect of SHI irradiation. A similar study for CdS thin films has also been reported by Ison et al. [19]. Herein, we investigate and report the effect of SHI irradiation on DSSC photo-anodes.
2. Experimental details Three types of TCOs were considered: (1) FTO film on a Nippon glass substrate from NSG/Pilkington, Hong Kong, China; (2) ITOPET [In2O3:Sn with polyethylene terephthalate (PET)] film on a Corning glass substrate from Sigma–Aldrich, Bangalore, India; and (3) ITO film on a Corning glass substrate from Vin Karola Instruments, PVT Ltd, USA. The SHI irradiation of these samples was performed using the 15 UD Pelletron tandem accelerators at the Inter-University Accelerator Centre (IUAC), New Delhi, India. All TCO films were subjected to 120 MeV Ag+9 ion irradiation at five different fluences ranging from 3.0 1011 ions/cm2 to 3.0 1013 ions/cm2 under a 10 6 Torr vacuum, and their structural, morphological, optical and electrical properties were compared. The electronic and nuclear energy loss values for 120 MeV Ag+9 ions in the FTO film were found to be 1.115 102 and 0.52 eV/nm, respectively. These values were calculated using the SRIM simulation software (SRIM-2010) [20,21]. Similarly, these values were found to be 1.513 102 eV/nm and 0.80 eV/nm, respectively, for the two types of ITO samples. These values clearly indicate that the entire passage of the ions through the films is dominated by the electronic energy loss, which is approximately 200 times higher than the nuclear loss. The stopping ranges of Ag ions in the FTO and ITO samples are 19.33 lm and 14.06 lm s, respectively, which are several times greater than the film thicknesses of these materials. Hence, radiation can easily pass through the films, and the effect of ion implantation can be completely
neglected. These values for the TCOs are summarized in Table 1, and the details of the computation are provided elsewhere [20,21]. 3. Results and discussion The structural, morphological, optical and electrical properties of the TCO samples were studied via XRD, AFM, UV–Vis and fourprobe measurements, and analyses of these results are presented in the following sections. 3.1. Structural analysis 3.1.1. XRD study The XRD spectra of the FTO film are given in Fig. 1a. They show that the FTO is polycrystalline in nature, with the dominant plane (200) at 2h = 37.757°. However, the full width at half maximum FWHM (b) of this peak monotonically increases with increasing fluence. The crystallite size, as calculated using Scherrer’s formula [22], decreases from 34 nm to 23 nm with increasing fluence. In the case of the ITO-PET sample, the FWHM of the dominant plane (2 2 2) decreases and crystallite size increases monotonically from 13 nm to 20 nm with increasing fluence, as shown in Fig 1b, which is contrary to what is observed in the FTO. Further, another important plane (4 0 0), associated with oxygen vacancies, is observed to become stronger with increasing fluence. This is caused by an increase in oxygen vacancies, as previously reported in the literature [17,23]. Note that the presence of the diffraction peak is not directly related to the presence of oxygen vacancies; it may appear or disappear because of changes in the structure factor. For pure ITO on a Corning glass substrate, it is observed that the peaks of the pristine samples are similar to those of the ITO-PET samples, i.e., the (2 2 2), (4 0 0), (5 5 1) and (4 4 1) planes have dominant peaks, and the splits in the (2 2 2) peak are also similar, as shown in Fig. 1c. When a sample is irradiated, the peaks become sharper for ITO-PET and broader for ITO on Corning glass. This finding is attributed to the different types of growth that occur in ITO films, which seem to merge into a single peak for the ITO-PET film, indicating that the structure is becoming more cubic. When ITO films are deposited on different substrates (here, two types of substrates are considered), the ion interactions and the thermal dissipation of ion energy differ, although they occur within the same material. Microstructural differences are responsible for these different results observed for the same material (ITO), as reported by Ison et al. [19], and are also reflected in the XRD patterns recorded in the present study. The SHI irradiation produced different crystalline effects in the FTO and ITO films. It was observed that the crystallinity of the FTO decreased with increasing fluence, whereas that of the ITO films improved. The different crystalline behaviors in the FTO and ITO films upon irradiation can be attributed not only to their different microstructures but also to the different thermal conductivity
Table 1 Details of TCOs considered in the study. S. No
TCOs
Material composition
Company
Thickness (nm)
Energy loss for 120 MeV SHI of Ag+9 Sn (eV/ nm)
Se (eV/ nm)
R (lm)
1
FTO
SnO2:F
Kintec, Hong-Kong, China
250
0.52
111.50
19.33
2
ITO-PET
Sigma–Aldrich, India
160
0.80
151.30
14.06
3
ITOcorn
In2O3:Sn + PET (50 nm) In2O3: Sn
Vin Carola Instrument, Pvt. Ltd., USA
160
0.80
151.30
14.06
Inference
Implantation of Ag+9 is ignored Implantation of Ag+9 is ignored Implantation of Ag+9 is ignored
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values of their respective substrates. A material with a higher thermal conductivity K dissipates heat energy more homogeneously during irradiation. The amorphization of the FTO films can be attributed to their lower value of K in comparison with the ITO films. In the literature, there are several reports that discuss the electrical conductivity of TCO films but very few related to their thermal conductivity [24–26]. It has been reported that the electrical conductivity of an ITO thin film is approximately 1.5 104 S/cm [24,25], which is 100 times higher than the conductivity of an FTO thin film, which is on the order of 6.66 102 S/cm [26]. Generally, conducting materials with high electrical conductivity also have high thermal conductivity. Therefore, this difference may be attributable to the thermal conductivity of ITO, which is superior to that of an FTO film and hence results in better heat conduction within the film. This is one possible reason for the increased crystallinity of the ITO films in comparison with the FTO films with increasing fluence. The detailed XRD results for all TCO films along with the corresponding fluences are given in Table 1.
Fig. 1. XRD patterns of pristine and irradiated (Ag+9 SHI at an energy of 120 MeV) TCOs. (a) FTO on Nippon glass: the crystallinity is degraded after irradiation. (b) ITO-PET on Corning glass: the crystallinity is increased after irradiation. (c) ITO on Corning glass: the crystallinity is increased after irradiation.
3.1.2. AFM study: surface morphology The rms roughness values of the pristine and 3 1013 ions/cm2 irradiated samples for the FTO, ITO-Corning and ITO-PET samples are given in Table 2. AFM micrographs of the FTO (Fig. 2a and b), ITO-PET (Fig. 2c and d) and ITO-Corning (Fig 2e and f) samples show that upon irradiation, the rms roughness increases for the FTO and ITO-Corning samples and decreases for the ITO-PET samples. In the case of the FTO, the grain size distribution shifts from a lower to a higher range (200–250 nm to 250–300 nm), whereas for the ITO-PET, the grain size distribution decreases from a higher to a lower range (50–100 nm to 0–50 nm). For the ITO-Corning samples, the grain size distribution remains the same, in the range of 50–100 nm. However, one would expect the grain size distribution of the ITO-Corning films to increase, consistent with their higher roughness after irradiation compared with that of the pristine ITO-PET films. These findings indicate that the same material (ITO, in the present case) can exhibit different grain growth for the same SHI irradiation when deposited on different substrates. The reason for this difference may be the different thermal capacities of the substrates, which would typically dissipate heat in a dissimilar manner and thus cause different grain growth. It should also be noted that the electrical conductivity of the FTO film is lower than that of the ITO film, as mentioned above [24–26]. It is clear from these results that the larger grain size distribution of the FTO films corresponds to a higher rms roughness (8.1–9.5 nm) and the lower grain size distribution of the ITO films corresponds to a smaller value of rms roughness (1.8–3.4 nm). Furthermore, for the ITO-PET samples, the rms roughness
Table 2 Parameters of TCOs affected by 120 MeV Ag+9 radiation. S. No
1 2 3 4 5 6
Fluence (ion/cm2)
Pristine 3.0 1011 1.0 1012 3.0 1012 1.0 1013 3.0 1013
Sample type FTO-Nippon glass
ITO-PET glass
ITO-Corning glass
Crystallinity
T* (%)
Roughness (nm)
Rsh* (X/h)
Crystallinity
T* (%)
Rough ness (nm)
Rsh* (X/h)
Crystallinity
T* (%)
Rough ness (nm)
Rsh* (X/h)
Ref.* Dec* Dec Dec Dec Dec
65 62 55 52 55 53
8.1 –** – – – 9.5
21 26 30 28 27 72
Ref. Inc* Inc Inc Inc Inc
65 65 75 80 76 77
4.1 – – – – 2.8
12 15 16 18 20 20.5
Ref. Inc Inc Inc Inc Inc
80 60 67 66 66 66
1.8 – – – – 3.4
7 7.5 9.5 10.5 11 11.5
Optimum results are obtained at this fluence (3.0 1012) value. * T: transmittance; Rsh: sheet resistance; Ref.: reference; dec: decreases; inc: increases. ** ‘–’ roughness is not observed at these fluences during AFM measurements.
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Fig. 2. AFM micrographs of pristine and irradiated (3 1013 ions/cm2 fluence) TCOs. (a) Pristine FTO on Nippon glass: the rms roughness of the film is 8.1 nm. (b) Irradiated FTO on Nippon glass: the rms roughness of the film (9.5 nm) is increased after irradiation. (c) Pristine ITO-PET on Corning glass: the rms roughness of the film is 3.2 nm. (d) Irradiated ITO-PET on Corning glass: the rms roughness of the film (1.6 nm) is decreased after irradiation. (e) Pristine ITO on Corning glass: the rms roughness of the film is 1.8 nm. (f) Irradiated ITO on Corning glass: the rms roughness of the film (4.1 nm) is increased after irradiation.
decreases by approximately a factor of 2 (i.e., from 4.1 nm to 2.8 nm), whereas for the ITO-Corning samples, it increases by a factor of 2. These results can be attributed to the different microstructures of the same ITO material on the different substrates, as reported by Ison et al. [19]. The details of the AFM measurements (especially the roughness values) for all TCOs and the corresponding fluences are given in Table 2. 3.2. Optical analysis The UV–Vis transmittance spectra of the FTO and ITO films are shown in Fig. 3a–c. The transmittance (T) decreased by 10–15% in the FTO and ITO-Corning films and increased by almost the same
percentage for the ITO-PET samples upon SHI irradiation. This shows that transmittance (T) of ITO-PET samples can be increased by applying SHI irradiation, which is a very useful result because it increases the amount of incident visible radiation that reaches the active region of the solar cell. Fig. 4 shows the transmittances (T) of all TCO samples at a wavelength of 550 nm. It can be observed that the transmittance (T) decreases slightly with increasing fluence for the ITO-Corning samples, whereas it increases for the ITO-PET samples. Furthermore, the transmittance (T) values for the FTO films are lower than those of the ITO films. In the present case, the 3 1012 ions/cm2 fluence is of particular interest, because the results achieved at this fluence are superior compared with the others.
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In the case of the FTO films, the band edge shifts toward higher wavelengths, whereas for the ITO films, this shift is marginal, as shown in Fig. 3a–c. This illustrates that the creation of defects is insufficient to create an observable change in the case of the ITO films. An increase or decrease in the flat (visible) region of the transmittance (T) of TCO films is predominantly attributable to the surface morphology of the materials, and in the present case, it is substantial (10–15%). The details of the transmittances (T) of all types of TCO films and the corresponding fluences are given in Table 2. 3.3. Electrical analysis
Fig. 3. Plots of transmittance vs. wavelength for the TCOs. (a) FTO on Nippon glass: the transmittance is decreased after SHI irradiation. At a fluence of 3.0 1012 ions/ cm2, it reaches its lowest value of 52%. (b) ITO-PET on Corning glass: the transmittance is increased after SHI irradiation. At a fluence of 3.0 1012 ions/ cm2, it reaches its highest value of 80%. (c) ITO on Corning glass: the transmittance is decreased after SHI irradiation. At a fluence of 3.0 1011 ions/cm2, it reaches its lowest value of 60%.
It is evident from Fig. 5 that the increase in the sheet resistance of the FTO films is marginal up to a fluence of 1.0 1013 ions/cm2 and then rises sharply thereafter, i.e., at 3 1013 ions/cm2. In the case of the ITO-PET and ITO-Corning samples, the sheet resistance increases marginally as the fluence increases and remains within acceptable limits for solar cell applications. For the ITO-Corning samples, the increase in sheet resistance observed after irradiation may be attributable to the dominance of the roughness over the other parameters, such as the crystallinity (which is improving) and the band gap (which is decreasing). In addition, the creation of unfavorable defects may be one possible reason for the increase in the sheet resistance upon irradiation. It is important to note that roughness is not the sole parameter driving the change in the resistivity of the film, as observed in the case of the ITO-PET films, wherein the sheet resistance is monotonically increasing although the roughness is decreasing. Here, the crystallinity is improving and the band gap is also decreasing marginally. The trends of all these parameters suggest a decrease in the sheet resistance of the ITO-PET samples; however, on the contrary, it is increasing. The reason for this phenomenon may lie in certain other dominant effects, such as the creation of defects or trap states, which may be instrumental in enhancing the sheet resistance of the material. Furthermore, in the present case, neither oxygen vacancies nor band gaps play a significant role in modifying the sheet resistance, as reported previously by several other groups [17,23,27]. Therefore, an increase in roughness or other unfavorable defects may be the primary cause of the increasing resistance in both ITO films [28–29]. In the case of the FTO samples, their sheet resistances also follow the same trend as those of the ITO samples up to a fluence of 1 1013 ions/cm2 and rise sharply beyond this point. The steep rise in the resistance observed in the case of the FTO samples at a fluence of 3 1013 ions/cm2 may not be solely attributable to the roughness, considering that the increase in roughness is very small in comparison to the increase in the sheet resistance. A decrease in the crystallinity of an irradiated film is another factor that can lead to an increase in sheet resistance, as is evident from the XRD study, but it cannot explain this sudden increase, as amorphization begins at a lower fluence. Amorphization causes defects to form at the edges of both the valence and conduction bands and thus increases the resistance. One possible reason for the sudden increase in the sheet resistance of the FTO films may be the release of fluorine gas from the films, because the fluorine atoms decrease the concentration of electrons in the conduction band of FTO [8–10]. According to the thermal spike model [30], transient heat may be produced inside the film along the ion tracks, and at higher fluences, columnar defects that overlap each other will form, which will certainly result in the release of large amounts of fluorine gas. Overlapping columnar defects formed at higher fluences will affect the entire sample homogeneously. These results are also summarized in Table 2.
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Fig. 4. A plot of the transmittance vs. fluence curves for the TCO electrodes. The ITO-PET on Corning glass is superior in comparison with the other electrodes after SHI irradiation, as its transmittance has improved substantially.
Fig. 5. A plot of sheet resistance vs. fluence for the TCO films. The sheet resistances of the ITO-PET and ITO on Corning substrates are slightly affected by SHI irradiation, but the FTO films on Nippon substrates are strongly affected, and their sheet resistances increase sharply at fluences of 1.0 1013 ions/cm2 and beyond.
4. Conclusion ’The present work focuses on the possibility of improving the efficiency of DSSCs by modifying the transmittance and conductivity of TCO film substrates by subjecting them to 120 MeV Ag+9 SHI irradiation. Of the three commonly used TCOs considered in this study, it is very clear that ITO-PET electrodes are superior to FTO and ITO-Corning electrodes in terms of both transmittance and conductivity after SHI irradiation. The transmittance of ITO-PET is increased by 15% after irradiation at a fluence of 3.0 1012 ions/cm2, whereas those of FTO and ITO-Corning are decreased by the same percentage. This fluence is very similar to that found in extraterrestrial regions, and
therefore, the results of this study are relevant for space applications. Furthermore, the observed increase in the sheet resistance of the ITO-PET film was marginal, whereas that of the FTO film was substantial. FTO films are prone to radiation damage upon exposure to an ion beam such as that used in the present experiment, whereas the crystallinity of ITO films improves upon ion irradiation. It can be concluded from the above investigation that ITO-PET electrodes are superior to FTO and ITO-Corning electrodes for solar cell applications. Hence, ITO-PET electrodes will serve as a better TCO for DSSCs. Further SHI irradiation at low fluences (3.0 1011 ions/cm2) will have little effect, as at this fluence, the transmittance is nearly constant and the sheet resistance changes only marginally.
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Acknowledgments This work was supported by the Inter University Accelerator Center (IUAC), New Delhi, India project # UFUP-43314. The authors SA and HKS are very grateful to Dr. F. Singh and Dr. D.C. Agrawal at IUAC New Delhi for providing valuable suggestions and discussions. The authors are also thankful to Mr. Pawan Kulriya (XRD) and Mrs. Indra Sulania (AFM) for providing valuable support during the measurements at IUAC New Delhi. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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