Growth of TiO2 thin films on chemically textured Si for solar cell applications as a hole-blocking and antireflection layer

Applied Surface Science 418 (2017) 225–231

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Growth of TiO2 thin films on chemically textured Si for solar cell applications as a hole-blocking and antireflection layer Ranveer Singh a,b , Mohit Kumar a,b,1 , Mahesh Saini a,b , Avanendra Singh c , Biswarup Satpati d , Tapobrata Som a,b,∗ a

SUNAG Laboratory, Institute of Physics, Bhubaneswar 751 005, Odisha, India Homi Baba National Institute, Training School Complex, Anushakti Nagar, Mumbai 400085, India c School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar, Jatni 752050, India d Saha Institute of Physics, 1/AF Bidhannagar, Kolkata 700 064, India b

a r t i c l e

i n f o

Article history: Received 14 October 2016 Received in revised form 28 January 2017 Accepted 31 January 2017 Available online 2 February 2017 Keywords: TiO2 thin film txt-Si Hole-blocking Heterojunction Band gap

a b s t r a c t In this work, we investigate the broad-band photoabsorption of an n-TiO2 thin film and its hole-blocking properties when a heterostructure is grown on a chemically textured p-Si substrate. We demonstrate that average specular reflectance of conformally grown TiO2 thin films on chemically prepared pyramidally textured Si substrates can be brought down to ∼0.2% (in the wavelength range of 300–1200 nm), which increases up to ∼0.53% after annealing at 673 K in air for 1 h. X-ray diffraction data reveal the amorphous nature of as-grown TiO2 thin films which undergoes a transition to a crystalline one after annealing. In addition, bulk current-voltage characteristics show that the leakage current increases after annealing which corroborates well a with change in the band gap, as is measured from the optical absorption spectra, due to a transition from amorphous to crystalline (anatase phase) of TiO2 . Moreover, TiO2 /Si heterojunction allows the transport of electrons but blocks the transport of holes. The present results are not only important for the fundamental understanding of the charge transport across TiO2 /Si heterostructures but also to design hole-blocking solar cells. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Transition metal oxides are very promising materials for energy harvesting applications due to their wide band gap, photo-stability, chemical inertness, and physical stability [1,2]. In a solar cell, the efficiency is affected mainly by two reasons: (a) loss due to the surface reflection [3] and (b) recombination of charge carriers [4,5]. To overcome the first issue, surfaces with graded refractive index can be utilized [3]. For instance, by using a TCO (e.g., AZO) on a chemically textured Si substrate, one can govern the light propagation and in turn the anti-reflection (AR) property improves due to the formation of graded refractive index [6]. On the other hand, to get rid of the second problem, a heterojunction can be made between crystalline silicon and a wide band gap semiconductor [7–12]. Out of the commonly used materials, TiO2 /Si heterojunction

∗ Corresponding author at: SUNAG Laboratory, Institute of Physics, Bhubaneswar 751 005, Odisha, India. E-mail addresses: [email protected], [email protected] (T. Som). 1 Present address: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 761 00, Israel. http://dx.doi.org/10.1016/j.apsusc.2017.01.307 0169-4332/© 2017 Elsevier B.V. All rights reserved.

is very important since TiO2 is considered to be a useful material for different applications such as photodiodes, resistive switching memory device, self-cleaning windows, anti-fogging glasses, gas sensors, self-sterilizing, photocatalysis, and solar cells [13–17]. The wide band gap of TiO2 depends on its different crystalline structures. Basically, TiO2 is found in three different phases: anatase, rutile, and brookite [18]. It is known that the anatase and rutile phases crystallize in the tetragonal structure, whereas the brookite phase crystallizes in orthorhombic one [19]. The anatase phase is reported to have band gaps in the range of 3.77 (in case of direct band gap) to 3.85 eV and 3.23 eV (in case of indirect band gap) at room temperature (RT) [20,21]. On the other hand, the rutile phase has the band gap of 3.06 eV and 3.10 eV (at RT) for direct and indirect transition, respectively [22]. Unfortunately, the inherent high band gap limits the optical applications and the above-mentioned properties are strongly influenced by the microstructure and crystalline structure of TiO2 . Due to the large band gap, it is not able to absorb light in a wide range (absorbs light only in the ultra-violet (UV) region). To overcome this problem, effort is on to modify its properties to enhance the light absorption capability from the UV to visible region [23–25]. Generally, TiO2 behaves as an n-type semiconductor due to oxygen vacancies [26], although there are reports where

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it shows p-type conductivity as well due to acceptor-type defects in the form of Ti vacancies [27]. In photovoltaics, TiO2 nanoparticles are used to aid electron injection and it can also be used in multilayered structures to provide a direct electrical pathway for the photogenerated electrons to ease the electron transport by blocking the holes, leading to a higher efficiency of the solar device [28,29]. To meet the above goal, one needs to make a heterojunction with an appropriate combination of materials so that the flow of one type of charge carrier (electron or hole) can be blocked which in turn should help to increase the efficiency of a solar cell [7]. To make a TiO2 /Si heterojunction, TiO2 thin films are prepared by different techniques, viz. sputtering [30], electron beam evaporation [31], metallo-organic chemical vapor deposition [32], pulsed laser deposition [33], and sol-gel method [34]. Among these techniques, the sputtering process provides an easy way to integrate TiO2 thin films, since it offers the advantage of growing high quality films over large areas with tunable physical properties [30]. Many researchers are working on TiO2 thin films from last few decades and have reported on the effect of temperature on structural, optical, and electrical properties. For instance, Mathews et al. [34] have made nanostructured TiO2 thin films on glass substrates by sol-gel dip coating technique and studied the effect of annealing on structural and optical properties of these films. Likewise, Rath et al.[35] have deposited TiO2 thin films on a Si substrate by sol-gel method and studied the dependence of temperature on structural, morphological, and optical properties. In fact, they observe that due to annealing at 673 K the amorphous TiO2 films undergo a transition to a crystalline anatase phase and upon annealing the films at 1073 K, it changes into the rutile phase. Although there are some reports on I–V characteristics of TiO2 /Si heterojunctions [31,36,37], a detail study on the effect of annealing on the broadband AR and hole-blocking properties of an n-TiO2 /p-Si (textured-Si substrate) heterostructure is still lacking. As a matter of fact, this will be important to study following the work of Avasthi et al. [7] where they have reported the hole-blocking nature of an n-TiO2 /p-Si heterojunction (TiO2 grown on commercially available silicon substrate) and its application for photovoltaics. In this work, we investigate the effect of crystallinity on the antireflection property of TiO2 thin films and the hole-blocking nature of a TiO2 /Si heterojunction (TiO2 grown on chemically prepared, pyramidally textured p-Si) using radio-frequency (RF) sputtered deposition technique. Crystalline nature of the samples are characterized by X-ray diffraction (XRD), whereas microstructural analysis are carried out by field emission gunbased scanning electron microscopy (FEGSEM), and transmission electron microscopy (TEM). We show that due to a conformal growth of TiO2 films on pyramidally textured-Si substrates (before annealing), the TiO2 surface reflectance can be brought down to 0.2% which increases up to 0.53% after annealing at 673 K for 1 h in air. In addition, we measure the hole-blocking property of both type of heterojunctions. The present study will be useful for fabrication of solar cells based on a hole-blocking material deposited on a textured surface.

2. Experimental TiO2 thin films were grown on textured p-Si (100) substrates (native oxide covered) at RT by a RF magnetron sputtering setup (Excel Instruments, India) at normal incidence. Before preparing the textured p-Si substrates, a p-Si (100) was cut into slices (1 cm × 1 cm) which were ultrasonically cleaned in trichloroethylene, acetone, propanol, and de-ionized water to remove the organic contaminations and further air dried. Pyramidally textured p-Si substrates were prepared through a chemical route [6]. It may be mentioned that hereinafter chemically prepared pyramidally

Fig. 1. SEM images: (a) a txt-Si substrate and (b) an as-deposited TiO2 film on the txt-Si substrate. The inset in (b) show the magnified image of a single pyramid from the marked region on (b) and a further zoomed image obtained from the apex of this pyramids, confirming the granular nature of TiO2 . (c) Depicts the SEM image of an annealed (673 K for 1 h in air) TiO2 thin film grown on txt-Si substrate. Here the inset shows the magnified image obtained from the marked region (c).

R. Singh et al. / Applied Surface Science 418 (2017) 225–231

textured-Si substrates will be called as txt-Si substrates. Commercially available 99.99% pure TiO2 target (50 mm dia × 6.2 mm thick) from Testbourne Ltd., UK was used for depositing TiO2 thin films in a vacuum chamber having a base pressure of 2 × 10-7 mbar. Ultrapure (99.99%) argon gas was injected into the deposition chamber (base pressure: 1 × 10−7 mbar) with a flow rate of 30 sccm to maintain the working pressure of 5 × 10-3 mbar during deposition. An RF power of 100 W (Cesar, Advanced energy, USA) was supplied to the target. The substrate was rotated at a speed of 3 rpm for achieving a uniform film thickness where the target-to-substrate distance was kept as 80 mm. Thickness of the as-grown TiO2 film was measured to be 25 ± 1 nm using a surface profilometer (Ambios, XP-200, USA). Five films were grown simultaneously to check the uniformity in film thickness before making the device. The films were annealed at 673 K for 1 h in air. Surface morphology of the txt-Si substrates and the TiO2 films (before and after annealing) was examined by FEGSEM using 5 KeV electrons (Carl-Zeiss, Germany) under the planer and planand cross-sectional views. For each sample, several images were collected from randomly chosen regions to check the uniformity and average grain size. Phase identification and nature of crystallinity of the films were measured by XRD (D8 Advance, Bruker, Germany) under the Bragg-Brentano geometry using a Cu-K␣ radiation (␭ = 1.54 Å) over a 2␪ scan range of 30◦ –75◦ . For local microstructural analysis, cross-sectional transmission electron microscopy (XTEM) and high-resolution transmission electron microscopy (HRTEM) (FEI, Tecnai G2 F30, S-Twin microscope operating at 300 kV and equipped with a GATAN Orius CCD camera) were used. For electrical measurements, silver paste was used to make electrical contacts on the top of TiO2 films and the back side of the substrate. The formation of Ag/TiO2/Si/Ag heterostructure diodes was checked by carrying out the I–V measurements using a source meter (Keithley, 2410) setup. The specular reflectance and transmittance were examined by UV–vis-NIR spectrophotometer (Shimadzu-3101PC, Japan) using unpolarized light. 3. Results and discussion Fig. 1(a) shows the formation of pyramidal structures on a chemically prepared txt-Si substrate. It may be mentioned that these pyramids are formed along the {111} plane because the {100} plane of Si has a higher etching rate than that of the {111} plane [38]. In addition, height (d) of the pyramids and the distance (a) between two consecutive pyramids (the ratio of d and a is known as the aspect ratio) play an important role in governing the surface reflectance. For instance, the value of d and a for a pyramid should

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Fig. 2. (a) and (b) show the XRD patterns of as-deposited and annealed TiO2 thin films grown on txt-Si substrates respectively.

be ∼2–2.5 ␮m to achieve a minimal reflection [39]. However, a random distribution of h and a (which depend on the etching time and temperature [40]) is obvious, although one can control the average aspect ratio of these pyramids. Fig. 2(a) shows the XRD pattern of an as-deposited TiO2 thin film grown on a txt-Si substrate which clearly reveals its amorphous nature [30]. On the other hand, Fig. 2(b) depicts the XRD pattern of the film annealed at 673 K in air. From this figure, it is evident that the annealed TiO2 film becomes crystalline in nature and the peaks (2␪ = 44.26◦ and 64.49◦ ) match well with that of the anatase phase

Fig. 3. XTEM images: (a) depicts an as-deposited TiO2 thin films on txt-Si substrate. The inset in (a) show FFT image from the marked region on (a), confirming the amorphous nature of TiO2 . (b) Shows an image of an annealed (673 K for 1 h in air) TiO2 thin films on txt-Si substrate. Here the inset shows the HRTEM image obtained from the marked region on (b), which confirms crystalline nature of TiO2 thin films.

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[41]. Such a structural phase transition in TiO2 under annealing was reported earlier [35]. It is known that electrical transport property in any energy storage device is strongly influenced by the crystalline nature of the materials used in the same. Thus, in order to check the same, XTEM and HRTEM measurements are performed on our samples. Fig. 3(a) and (b) depict the XTEM images of as-deposited and annealed TiO2 thin films grown on txt-Si substrate, respectively. The inset in each case shows the corresponding HRTEM image. From the fast Fourier transform (FFT) image [inset, Fig. 3(a)] obtained from the marked region on Fig. 3(a), it is confirmed that the as-deposited film is amorphous in nature [42], whereas after annealing it becomes crystalline [as is seen from the inset of Fig. 3(b)] and the d-spacing (0.35 nm) obtained from the lattice image matches well with the (101) plane of the anatase phase [43]. These results corroborate well with our XRD results described above. Since the optical properties of a material strongly depend on its crystalline structure, we have carried out the surface reflectance and transmittance measurements on TiO2 samples before and after annealing. Specular reflectance is measured at an incident angle of 45◦ with respect to the surface normal. Fig. 4(a) shows the specular reflectance of as-deposited and annealed TiO2 thin films grown on txt-Si substrates in the wavelength range of 300–1200 nm. It is interesting to note that the average specular reflectance of the as-deposited TiO2 film is 0.2% less than that of the txt-Si substrate, whereas upon annealing the film at 673 K in air for 1 h it increases up to 0.53%. This low reflectance of the as-deposited film is caused by its conformal growth on highly dense Si pyramids. This whole combination (granular TiO2 on the txt-Si substrate) creates a graded refractive index medium (from the apex to the base of the Si pyramids), leading to the reduced surface reflectance [44]. In fact, the aspect ratio of the pyramids also plays an important role (as discussed above) in reducing the surface reflectance (like in nanowires) [39]. Thus, after annealing the surface reflectance increases up to 0.53% due to an overall decrease in the aspect ratio of the pyramids and their relatively smoother surfaces which becomes evident from the corresponding SEM image (image of the as-annealed txt-Si substrate not shown). In order to measure the optical band gap and transmittance, TiO2 thin films are simultaneously deposited on glass substrates. Fig. 4(b) shows the transmittance spectra of TiO2 thin films under consideration (asdeposited and annealed). The as-deposited TiO2 thin film shows ∼80% transmittance which reduces to ∼50% and the absorption edge gets marginally shifted towards longer wavelength [45]. In addition, there is also a sharp fall in the film transmittance in the UV region, which corresponds to the optical band gap [38]. This can happen due to annealing of the TiO2 thin film, due to a change in the film density because of the variation in oxygen vacancies in the form of defects [46]. These changes are also significant due to the structural phase transition as is observed from the XRD patterns [Fig. 2]. The optical band-gap of TiO2 thin films is calculated from the Tauc’s equation [45]:



˛=

k hv − Eg hv

n ,

(1)

where ␣ is the optical absorption coefficient, k is a constant, hv is the photon energy in (eV), and n may have different values. By extrapolating the straight line portion of (˛hv)1/2 versus hv plot, one can find the value of the optical band gap. Fig. 4(c) shows the optical band gap of as-deposited and annealed TiO2 films. It is evident that as-deposited TiO2 thin film has a band gap of 3.21 eV which decreases to 3.03 eV after annealing at 673 K for 1 h in air [34]. This decrease in the optical band gap of the film after annealing can be attributed to the change in the film density caused due to the structured phase transition [45]. As a matter of fact, the calculated value

Fig. 4. (a) Shows the reflectance (R%) spectra of txt-Si substrate, TiO2 thin film deposited on txt-Si substrate before and after annealing at 673 K in air, (b) Shows the transmittance (T%) spectra of TiO2 thin film deposited on glass substrate before and after annealing at 673 K, (c) Shows tauc’s plot to determine optical band gap of TiO2 thin films before and after annealing at 673 K in air for 1 h.

matches very well with the band gap of anatase phase of TiO2 thin film which is also consistent our with XRD and HRTEM results. In order to investigate the diode characteristics of n-TiO2 /pSi heterojunction for both as-deposited and annealed samples, current-voltage (I–V) measurements are carried out at RT in dark condition. Based on the Anderson model, the ideal energy-band diagram of the n-TiO2 /p-Si heterojunction is schematically shown in Fig. 5(a) [47]. For TiO2 , electron affinity (␹) and ionization energy are at about 4.30 eV and 7.45 eV, respectively below the vacuum level. On the other hand, for Si, ␹ and ionization energy are at about 4.05 eV and 5.17 eV, respectively below the vacuum level. At the same time, the band gap (Eg ) of TiO2 and Si are 3.15 eV and 1.12 eV, respectively. From the above values, the energy barrier for an electron, Ec = ␹(TiO2 ) – ␹(Si) = 4.30 – 4.05 = 0.25 eV and that for a hole,

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Fig. 5. (a) Shows the energy-band diagram, based on Anderson model, of the nanostructure n-TiO2/p-Si heterojunction diode, (b) Shows schematic diagram to measure I–V characteristic, (c) Shows the I–V plot of Ag/n-TiO2 /p-txt-Si/Ag sample before and after annealing at 673 K in air as measured at room temperature.

Ev = Eg (TiO2 ) + Ec – Eg (Si) = 3.15 + 0.25 – 1.12 = 2.28 eV [Fig. 5(a)] [36]. On the basis of these calculations, one can infer that there is a large barrier in the valance band which is capable to block the transport of holes from Si to TiO2 , whereas there is a small barrier in the conduction band which allows the transport of electrons from Si to TiO2 . Fig. 5(b) shows the I–V characteristics of the n-TiO2 /p-Si heterojunction before and after annealing. From this figure, it is clear that no current flows for small biases (<0.3 V) for both heterojunctions, revealing that the TiO2 film works as a hole-blocking layer in both configurations, albeit it is more prominent for the as-deposited case. This can happen since the band gap of TiO2 decreases from 3.21 eV to 3.03 eV (due to annealing), indicating a reduction in the barrier height for holes. It may be mentioned that the concentration of oxygen vacancies can strongly modify the Fermi level which in turn changes the optical band gap of TiO2 [31]. So the hole-blocking nature in TiO2 is definitely affected by the crystalline nature of TiO2 films. In addition, Fig. 5(b) shows a rectifying behavior. To find the diode parameters, I–V characteristics are used. As a matter fact, series resistance (Rs ) and ideality factor () of a diode can be obtained from the following equation [12]: dV kT = IRs + , q d (ln I)

(2)

where V is the applied bias, k is the Boltzmann constant and T is the absolute temperature (in K). The slope of dV/d(lnI) versus I plot

Table 1 Following parameters were determined for TiO2 /txt-Si heterojunctions: Ideality factor (), series resistance (Rs ), turn on potential (V) from current-voltage (I–V) curve; average value of h (␮m) and a (␮m) from SEM; average surface reflectance from reflectance curve and band-gap from optical absorption.

n Rs (k) Turn-on potential (V) Avg. value of h (␮m) Avg. value of a (␮m) Avg. Surface reflectance, R (%) Band gap, Eg (eV)

Before Annealing TiO2 /txt-Si

After Annealing TiO2 /txt-Si

7.18 3.18 1.83 1.80 0.50 0.20 3.21

8.6 1.83 1.80 1.75 0.17 0.53 3.03

provides Rs and the y-axis intercept of this plot gives (kT/q). The values of  and Rs are calculated and summarized in Table 1. It is observed that after annealing, both Rs and  increase. The high value of  may be attributed to the surface defects and native oxide layer on Si as is seen from the HRTEM images [36]. On the other hand, the reason for having the high Rs value can be attributed to the interfacial layer and a reduction in the carrier density due to trapping of the electrons at grain boundaries [27]. Such heterojunctions, based on the wide band-gap semiconductors, having the ability to block one type of carrier can be widely used in future photovoltaic deices.

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4. Conclusions In conclusion, a structural phase transition from amorphous to crystalline anatase-TiO2 thin film (deposited on txt-Si substrate) upon annealing at 673 K for 1 h in air has been demonstrated. Field emission gun scanning electron microcopy measurements show a random distribution of chemically prepared pyramidal structures and the granular nature of TiO2 thin films under both configurations (as-deposited and annealed). Specular reflectance for the as-deposited TiO2 film (in the range of 300–1200 nm) grown on a txt-Si is found to be 0.2% which increases up to 0.53% after annealing, indicating that these samples can be used for antireflection coating for energy harvesting. In addition, we show that the optical bandgap of TiO2 thin film can be tuned from 3.21 eV to 3.03 eV by annealing it at 673 K in air for 1 h which in turn not only tunes the transmittance from 80% to 50% but also leads to a change in the hole-blocking ability of the film. This study will be important for use of TiO2 thin films in solar cell applications as an antireflecting and the hole-blocking layer.

Acknowledgement The authors would like to acknowledge Dr. Pratap K. Sahoo from National Institute of Science Education and Research (NISER), Bhubaneswar for extending the SEM facility.

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