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Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering
Materials Today: Proceedings xxx (xxxx) xxx
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TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering G. Wisz a, P. Sawicka-Chudy a, R. Yavorskyi b,⇑, P. Potera a, M. Bester a, Ł. Głowa a a b
Faculty of Mathematics and Natural Sciences, Rzeszow University, Rejtana 16c 35-959 Rzeszow, Poland Vasyl Stefanyk Precarpathian National University, T. Shevchenko, 57, 76-018, Ukraine
a r t i c l e
i n f o
Article history: Received 13 September 2019 Received in revised form 12 October 2019 Accepted 13 October 2019 Available online xxxx Keywords: Titanium dioxide Cuprous oxide Thin film Photovoltaic cells Solar cells
a b s t r a c t In this work, TiO2/Cu2O heterostructures were obtained in a two-step process with a direct current magnetron sputtering method. We studied the morphological properties and composition of the thin films by scanning electron microscopy. Optical properties and energy bands at the heterojunction were recorded using a spectrophotometer. Additionally, the current–voltage characteristics examined in both total darkness and with illumination were estimated to have an irradiation (radiation flux divided by area) of 1000 W/m2. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.
1. Introduction Solar energy is an important renewable energy source with the potential to ameliorate the global energy crisis [1–2]. Current research is focusing on semiconductors such as titanium oxide [3] and zinc oxide [4,5], as well as copper (II) oxide [6] and copper (I) oxide [7,8]. Cu2O thin film deposited on a TiO2 solar cell has emerged as a promising p–n junction semiconductor for photovoltaic application. Cu2O is a non-toxic and direct energy-gap semiconductor that can be used as an absorbing layer in solar cells [1]. Recently, Rokhmat et al. [9] reported the construction of TiO2/ Cu2O heterojunction solar cells, but the efficiency was only 1.62%. Copper forms two well-known stable oxides - copper (II) oxide (CuO) and copper (I) oxide (Cu2O). The physical properties, i.e., crystalline structure, color, absorption spectrum and electrical properties, differ between these two materials [10,11]. They are natural p-type semiconductors with band gaps of 1.21 to 1.51 eV and 2.10 to 2.60 eV, respectively [12–14]. Titanium dioxide (TiO2), a transition metal oxide, is one of the best studied materials [15]. TiO2 has remarkable optical and electronic properties [16], good stability, and a high refractive index [17]. The natural forms of TiO2 are rutile, anatase, and brookite, ⇑ Corresponding author. E-mail address: [email protected] (R. Yavorskyi).
which are n-type semiconductors with an energy band gap in the range of 3.02–3.25 eV, 3.2–3.26 eV, and 2.1–3.54 eV, respectively. TiO2 is easy to fabricate, has low cost, low toxicity, and longterm stability [18]. Various techniques to deposit the absorber layer onto different types of nanostructures of TiO2/Cu2O thin films have been developed, including chemical bath deposition, metal organic chemical vapor deposition, vapor phase epitaxy, anodic oxidation, and reactive sputtering; and recently, pulsed laser deposition [19–22]. Several groups have reported the construction of CuO (Cu2O)/TiO2 solar cells prepared using various technologies with efficiencies ranging from 0.0005% to 1.62% [23–26]. Here, first described TiO2/Cu2O solar cell, prepared within a direct current (DC) reactive magnetron sputtering technique. Next step is the basic properties analysis of TiO2/Cu2O thin film heterojunctions, paying special attention to the morphological, chemical, and photovoltaic properties. Then can be characterized the current–voltage (I-V) characteristics of the solar cells. Received results indicate that the TiO2/Cu2O structure is a promising solution for photovoltaic applications.
2. Methods Heterojunctions of TiO2/Cu2O thin-film solar cells (Fig. 1(a, b)) were prepared using a two-step process. TiO2 thin films were
https://doi.org/10.1016/j.matpr.2019.10.054 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 1. Images of TiO2/Cu2O heterojunctions with contacts: (a) for SC1 – gold contact and (b) for SC2, SC3, SC4 – Cu2O contact.
deposited onto commercially available indium tin oxide (ITO)coated triangular and square glass substrates by DC magnetron sputtering method at a temperature of 423.15 K using the Titarget. The next layer of Cu2O was also deposited on the TiO2 structure using the DC magnetron sputtering method. The process parameters used to obtain thin film heterojunctions are presented in Table 1. The deposited material was identified by energy-dispersive Xray (EDX) spectroscopy, using a SEM microprobe (Bruker Quantax). For I–V characterization of the TiO2/Cu2O heterojunction, electrical contacts were performed. For SC1, two gold contacts were stuck onto the Cu2O using conductive glue (Fig. 1, a). Each contact had a diameter of 1.0 mm. The scheme shown in Fig. 2(a). Additionally, for SC2, SC3, and SC4, copper thin films were deposited on Cu2O layers to improve the electrical connection (Fig. 1, b). Next, 13 13 round silver back contacts were soldered onto the Cu and ITO layers (Fig. 2(b)). The transmission spectrum of thin film on a glass substrate was recorded using a CARY 5000 spectrophotometer. Additionally, the sample was annealed in air for 30 min at 300 °C in a NABERTHERM LH04 furnace. After annealing, was recorded the transmission spectrum.
Table 1 Process parameters. Parameter
TiO2
Cu2O
Sample (SC1) Target purity Target Shape of glass Distance between the source and substrate [mm] Pressure process [mbar] Power [W] Time [min] Oxygen flow rates [cm3/s] Argon flow rates [cm3/s] Final substrate temperature [K]
99.999% Ti Triangle 58 2.41 10–2 110 20 1.5 4.0 423.15
99.995% Cu
Samples (SC2, SC3, SC4) Target purity Target Shape of glass Distance between the source and substrate [mm] Pressure process [mbar] Power [W] Time [min] Oxygen flow rates [cm3/s] Argon flow rates [cm3/s] Final substrate temperature [K]
99.999% Ti Square 58 2.41 10–2 120 30 1.5 5.0 423.15
100 1.73 10–2 80 40 3.0 2.0 423.15 99.995% Cu 26 1.73 10–2 70 10 8.0 4.0 423.15
3. Results and discussion 3.1. Composition Analysis or chemical characterization of TiO2 thin films (SC1, SC2) shown in Fig. 3 and Cu2O thin films (SC1, SC2) in Fig. 4. Composition were performed by EDX spectroscopy, using a SEM microprobe (Bruker Quantax). The unmarked peaks are caused by the substrate, and are of no interest in the present study. Analysis of the atomic concentration [in %] showed that the samples composition in both cases had the correct stoichiometry – for each titanium atom there were two oxygen atoms, confirming that the obtained compound was TiO2. Further is shown only the first sample, as the EDX examination indicated that the second film was too thin (composition was examined using voltage set at 2 kV, and that voltage was sufficient to observe peaks from the substrate). Unexpectedly, the samples composition in both cases was not consistent with the estimated stoichiometry. The atomic concentration values were more consistent with CuO than Cu2O (Fig. 4). The reason is unclear – it may be due deficiencies in the obtaining method or inaccurate determination of the composition. It should be noted, however, that examination was performed using a comparatively low voltage (up to 10 kV) to avoid contamination from the substrate (thin films). A high inaccuracy of the copper concentration (30%) was observed, which surely affected the obtained results. Due to this fact, it is not possible to unequivocally conclude the sample composition based only on the above-mentioned examination.
3.2. I-V characteristics of TiO2/Cu2O Measurement of the I-V characteristics was performed using a Keithley 2200 power supply, which is a professional, programmable source of DC with multimeter functionality. The samples were placed in a solar simulator, a hermetically sealed safe equipped with a halogen light bulb acting as a sunlight source. Dependencies between voltage and current were examined both in total darkness and with illumination, which was estimated to have an irradiation (radiation flux divided by area) of 1000 W/m2. Fig. 5 shows the I-V characteristics of TiO2/Cu2O for samples SC1, SC2, SC3 and SC4. Due to the fact that all samples were obtained using the same processes with identical parameters, the observed differences were subtle and were not due to the obtaining process itself but to naturally occurring fluctuations that changed the properties of the obtained photovoltaic cells. It is well known in the electronics
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 2. Scheme of TiO2/Cu2O heterojunctions for SC1, SC2, SC3 (a) and SC4 (b).
Fig. 3. EDX spectrum composition of TiO2 (SC1 (a) and SC2 (b)).
Fig. 4. EDX spectrum composition of Cu2O (SC1 (a) and SC2 (b)).
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 5. I–V characteristics of TiO2/Cu2O for SC1 (a), SC2 (b), SC3 (c) and SC4 (d).
industry that components of the same series can show some properties variability [27,28]. A common feature present in all the graphs is the lack of a characteristic dependency for photovoltaic devices. Typical for these components, no curvature of function in the fourth quarter of the coordinate system was observable. That means these samples are useless for building photovoltaic sources of energy. Moreover, samples labeled 1 and 3 were insensitive to electromagnetic radiation during the experiment – they are suspected to be nonphotosensitive. A single SC3 sample exhibited noticeable photosensitivity, but for the reasons mentioned above, cannot be said that it’s a result of obtaining technique. Therefore, it can be concluded, based on the characteristics shown above, that the method used to photovoltaic cells producing, isn’t efficient for industrial application. In the second phase of the study, the production method was changed and one photovoltaic cell obtained. This time, a trial was performed to obtain a new type of terminal by first depositing a thin film of gold and then soldering a thin gold wire on it. Despite made efforts, the procedure caused a short circuit, which was confirmed by resistance measuring. It can be suspected, that the gold atoms penetrate through the thin layers into the ITO substrate. Afterwards the sample was cut to remove the short circuit
part and the terminals were made according to the traditional method – by pasting terminals to the sample with conductive glue. The following results were obtained. The characteristics shown in Fig. 5(a) confirm photosensitivity of the sample, indicating that made efforts resulted in an improved obtaining method. Still, the expected photovoltaic I-V function was not observed, which disqualifies the applied method for the production of photovoltaic devices production that collect sunlight energy. 3.3. Optical spectra The transmittance spectrum of the TiO2/Cu2O heterojunctions on a glass substrate is shown in Fig. 6(a) with complete light absorption below 500 nm. Above 500 nm, monotonic transmission increases and interference fringes are observed. Annealing the TiO2/Cu2O heterojunctions in air (annealing time 30 min, temperature 300 °C) led to an increase in transmission. The absorption edge of the sample can be approximated by the Tauc relation [28–29]
ðahmÞ
1=n
¼ Bðhm EgÞ;
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 6. Transmission spectra of TiO2/Cu2O (a) and Tauc plot for a direct allowed optical transition with a linear fit to derive an optical band gap of TiO2/Cu2O before and after annealing.
where Eg is the optical band gap, h is Planck’s constant, m is the frequency of incident photons, B is a constant, (ahm)1/n is an index that can have different values (2, 3, 1/2, and 3/2) corresponding to indirect allowed, indirect forbidden, direct allowed, and direct forbidden transitions, respectively. In observed case, the approximation with n = 1/2 was satisfactory (Fig. 6(b)) for the TiO2/Cu2O heterojunction samples before and after annealing, which corresponds the direct allowed transitions. The determined value of the energy gap was 2.18 eV, which is smaller than previously reported (2.48 eV) [30–31]. Annealing in air led to an increase of the band gap energy (2.25 eV). 4. Conclusions Thin films TiO2/p-Cu2O heterojunctions for solar cells were obtained using the DC magnetron sputtering technique on ITO and glass substrates, and the basic material properties were analyzed: chemical composition, morphological properties, and I-V characteristics of TiO2/Cu2O. Received results revealed that: According to the SEM images, grain size differs in the range of 2–100 lm. On the layers surface there are many irregular drops with a diameter of 2–10 lm. Thus, the surface of each TiO2/Cu2O solar cell was multi-faceted. The EDX study revealed that Cu2O thin films have a similar composition, but the composition of TiO2 thin films was totally different. I–V characterization was performed in the dark and under illumination. A single SC4 sample was photosensitive, but showed no photovoltaic activity. Funding Publications are based on the research provided by the grant support of the Ministry of Education and Science of Ukraine for young researches (project No 0119U100062 – 2019). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering G. Wisz a, P. Sawicka-Chudy a, R. Yavorskyi b,⇑, P. Potera a, M. Bester a, Ł. Głowa a a b
Faculty of Mathematics and Natural Sciences, Rzeszow University, Rejtana 16c 35-959 Rzeszow, Poland Vasyl Stefanyk Precarpathian National University, T. Shevchenko, 57, 76-018, Ukraine
a r t i c l e
i n f o
Article history: Received 13 September 2019 Received in revised form 12 October 2019 Accepted 13 October 2019 Available online xxxx Keywords: Titanium dioxide Cuprous oxide Thin film Photovoltaic cells Solar cells
a b s t r a c t In this work, TiO2/Cu2O heterostructures were obtained in a two-step process with a direct current magnetron sputtering method. We studied the morphological properties and composition of the thin films by scanning electron microscopy. Optical properties and energy bands at the heterojunction were recorded using a spectrophotometer. Additionally, the current–voltage characteristics examined in both total darkness and with illumination were estimated to have an irradiation (radiation flux divided by area) of 1000 W/m2. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.
1. Introduction Solar energy is an important renewable energy source with the potential to ameliorate the global energy crisis [1–2]. Current research is focusing on semiconductors such as titanium oxide [3] and zinc oxide [4,5], as well as copper (II) oxide [6] and copper (I) oxide [7,8]. Cu2O thin film deposited on a TiO2 solar cell has emerged as a promising p–n junction semiconductor for photovoltaic application. Cu2O is a non-toxic and direct energy-gap semiconductor that can be used as an absorbing layer in solar cells [1]. Recently, Rokhmat et al. [9] reported the construction of TiO2/ Cu2O heterojunction solar cells, but the efficiency was only 1.62%. Copper forms two well-known stable oxides - copper (II) oxide (CuO) and copper (I) oxide (Cu2O). The physical properties, i.e., crystalline structure, color, absorption spectrum and electrical properties, differ between these two materials [10,11]. They are natural p-type semiconductors with band gaps of 1.21 to 1.51 eV and 2.10 to 2.60 eV, respectively [12–14]. Titanium dioxide (TiO2), a transition metal oxide, is one of the best studied materials [15]. TiO2 has remarkable optical and electronic properties [16], good stability, and a high refractive index [17]. The natural forms of TiO2 are rutile, anatase, and brookite, ⇑ Corresponding author. E-mail address: [email protected] (R. Yavorskyi).
which are n-type semiconductors with an energy band gap in the range of 3.02–3.25 eV, 3.2–3.26 eV, and 2.1–3.54 eV, respectively. TiO2 is easy to fabricate, has low cost, low toxicity, and longterm stability [18]. Various techniques to deposit the absorber layer onto different types of nanostructures of TiO2/Cu2O thin films have been developed, including chemical bath deposition, metal organic chemical vapor deposition, vapor phase epitaxy, anodic oxidation, and reactive sputtering; and recently, pulsed laser deposition [19–22]. Several groups have reported the construction of CuO (Cu2O)/TiO2 solar cells prepared using various technologies with efficiencies ranging from 0.0005% to 1.62% [23–26]. Here, first described TiO2/Cu2O solar cell, prepared within a direct current (DC) reactive magnetron sputtering technique. Next step is the basic properties analysis of TiO2/Cu2O thin film heterojunctions, paying special attention to the morphological, chemical, and photovoltaic properties. Then can be characterized the current–voltage (I-V) characteristics of the solar cells. Received results indicate that the TiO2/Cu2O structure is a promising solution for photovoltaic applications.
2. Methods Heterojunctions of TiO2/Cu2O thin-film solar cells (Fig. 1(a, b)) were prepared using a two-step process. TiO2 thin films were
https://doi.org/10.1016/j.matpr.2019.10.054 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 1. Images of TiO2/Cu2O heterojunctions with contacts: (a) for SC1 – gold contact and (b) for SC2, SC3, SC4 – Cu2O contact.
deposited onto commercially available indium tin oxide (ITO)coated triangular and square glass substrates by DC magnetron sputtering method at a temperature of 423.15 K using the Titarget. The next layer of Cu2O was also deposited on the TiO2 structure using the DC magnetron sputtering method. The process parameters used to obtain thin film heterojunctions are presented in Table 1. The deposited material was identified by energy-dispersive Xray (EDX) spectroscopy, using a SEM microprobe (Bruker Quantax). For I–V characterization of the TiO2/Cu2O heterojunction, electrical contacts were performed. For SC1, two gold contacts were stuck onto the Cu2O using conductive glue (Fig. 1, a). Each contact had a diameter of 1.0 mm. The scheme shown in Fig. 2(a). Additionally, for SC2, SC3, and SC4, copper thin films were deposited on Cu2O layers to improve the electrical connection (Fig. 1, b). Next, 13 13 round silver back contacts were soldered onto the Cu and ITO layers (Fig. 2(b)). The transmission spectrum of thin film on a glass substrate was recorded using a CARY 5000 spectrophotometer. Additionally, the sample was annealed in air for 30 min at 300 °C in a NABERTHERM LH04 furnace. After annealing, was recorded the transmission spectrum.
Table 1 Process parameters. Parameter
TiO2
Cu2O
Sample (SC1) Target purity Target Shape of glass Distance between the source and substrate [mm] Pressure process [mbar] Power [W] Time [min] Oxygen flow rates [cm3/s] Argon flow rates [cm3/s] Final substrate temperature [K]
99.999% Ti Triangle 58 2.41 10–2 110 20 1.5 4.0 423.15
99.995% Cu
Samples (SC2, SC3, SC4) Target purity Target Shape of glass Distance between the source and substrate [mm] Pressure process [mbar] Power [W] Time [min] Oxygen flow rates [cm3/s] Argon flow rates [cm3/s] Final substrate temperature [K]
99.999% Ti Square 58 2.41 10–2 120 30 1.5 5.0 423.15
100 1.73 10–2 80 40 3.0 2.0 423.15 99.995% Cu 26 1.73 10–2 70 10 8.0 4.0 423.15
3. Results and discussion 3.1. Composition Analysis or chemical characterization of TiO2 thin films (SC1, SC2) shown in Fig. 3 and Cu2O thin films (SC1, SC2) in Fig. 4. Composition were performed by EDX spectroscopy, using a SEM microprobe (Bruker Quantax). The unmarked peaks are caused by the substrate, and are of no interest in the present study. Analysis of the atomic concentration [in %] showed that the samples composition in both cases had the correct stoichiometry – for each titanium atom there were two oxygen atoms, confirming that the obtained compound was TiO2. Further is shown only the first sample, as the EDX examination indicated that the second film was too thin (composition was examined using voltage set at 2 kV, and that voltage was sufficient to observe peaks from the substrate). Unexpectedly, the samples composition in both cases was not consistent with the estimated stoichiometry. The atomic concentration values were more consistent with CuO than Cu2O (Fig. 4). The reason is unclear – it may be due deficiencies in the obtaining method or inaccurate determination of the composition. It should be noted, however, that examination was performed using a comparatively low voltage (up to 10 kV) to avoid contamination from the substrate (thin films). A high inaccuracy of the copper concentration (30%) was observed, which surely affected the obtained results. Due to this fact, it is not possible to unequivocally conclude the sample composition based only on the above-mentioned examination.
3.2. I-V characteristics of TiO2/Cu2O Measurement of the I-V characteristics was performed using a Keithley 2200 power supply, which is a professional, programmable source of DC with multimeter functionality. The samples were placed in a solar simulator, a hermetically sealed safe equipped with a halogen light bulb acting as a sunlight source. Dependencies between voltage and current were examined both in total darkness and with illumination, which was estimated to have an irradiation (radiation flux divided by area) of 1000 W/m2. Fig. 5 shows the I-V characteristics of TiO2/Cu2O for samples SC1, SC2, SC3 and SC4. Due to the fact that all samples were obtained using the same processes with identical parameters, the observed differences were subtle and were not due to the obtaining process itself but to naturally occurring fluctuations that changed the properties of the obtained photovoltaic cells. It is well known in the electronics
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Fig. 2. Scheme of TiO2/Cu2O heterojunctions for SC1, SC2, SC3 (a) and SC4 (b).
Fig. 3. EDX spectrum composition of TiO2 (SC1 (a) and SC2 (b)).
Fig. 4. EDX spectrum composition of Cu2O (SC1 (a) and SC2 (b)).
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Fig. 5. I–V characteristics of TiO2/Cu2O for SC1 (a), SC2 (b), SC3 (c) and SC4 (d).
industry that components of the same series can show some properties variability [27,28]. A common feature present in all the graphs is the lack of a characteristic dependency for photovoltaic devices. Typical for these components, no curvature of function in the fourth quarter of the coordinate system was observable. That means these samples are useless for building photovoltaic sources of energy. Moreover, samples labeled 1 and 3 were insensitive to electromagnetic radiation during the experiment – they are suspected to be nonphotosensitive. A single SC3 sample exhibited noticeable photosensitivity, but for the reasons mentioned above, cannot be said that it’s a result of obtaining technique. Therefore, it can be concluded, based on the characteristics shown above, that the method used to photovoltaic cells producing, isn’t efficient for industrial application. In the second phase of the study, the production method was changed and one photovoltaic cell obtained. This time, a trial was performed to obtain a new type of terminal by first depositing a thin film of gold and then soldering a thin gold wire on it. Despite made efforts, the procedure caused a short circuit, which was confirmed by resistance measuring. It can be suspected, that the gold atoms penetrate through the thin layers into the ITO substrate. Afterwards the sample was cut to remove the short circuit
part and the terminals were made according to the traditional method – by pasting terminals to the sample with conductive glue. The following results were obtained. The characteristics shown in Fig. 5(a) confirm photosensitivity of the sample, indicating that made efforts resulted in an improved obtaining method. Still, the expected photovoltaic I-V function was not observed, which disqualifies the applied method for the production of photovoltaic devices production that collect sunlight energy. 3.3. Optical spectra The transmittance spectrum of the TiO2/Cu2O heterojunctions on a glass substrate is shown in Fig. 6(a) with complete light absorption below 500 nm. Above 500 nm, monotonic transmission increases and interference fringes are observed. Annealing the TiO2/Cu2O heterojunctions in air (annealing time 30 min, temperature 300 °C) led to an increase in transmission. The absorption edge of the sample can be approximated by the Tauc relation [28–29]
ðahmÞ
1=n
¼ Bðhm EgÞ;
Please cite this article as: G. Wisz, P. Sawicka-Chudy, R. Yavorskyi et al., TiO2/Cu2O heterojunctions for photovoltaic cells application produced by reactive magnetron sputtering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.054
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Fig. 6. Transmission spectra of TiO2/Cu2O (a) and Tauc plot for a direct allowed optical transition with a linear fit to derive an optical band gap of TiO2/Cu2O before and after annealing.
where Eg is the optical band gap, h is Planck’s constant, m is the frequency of incident photons, B is a constant, (ahm)1/n is an index that can have different values (2, 3, 1/2, and 3/2) corresponding to indirect allowed, indirect forbidden, direct allowed, and direct forbidden transitions, respectively. In observed case, the approximation with n = 1/2 was satisfactory (Fig. 6(b)) for the TiO2/Cu2O heterojunction samples before and after annealing, which corresponds the direct allowed transitions. The determined value of the energy gap was 2.18 eV, which is smaller than previously reported (2.48 eV) [30–31]. Annealing in air led to an increase of the band gap energy (2.25 eV). 4. Conclusions Thin films TiO2/p-Cu2O heterojunctions for solar cells were obtained using the DC magnetron sputtering technique on ITO and glass substrates, and the basic material properties were analyzed: chemical composition, morphological properties, and I-V characteristics of TiO2/Cu2O. Received results revealed that: According to the SEM images, grain size differs in the range of 2–100 lm. On the layers surface there are many irregular drops with a diameter of 2–10 lm. Thus, the surface of each TiO2/Cu2O solar cell was multi-faceted. The EDX study revealed that Cu2O thin films have a similar composition, but the composition of TiO2 thin films was totally different. I–V characterization was performed in the dark and under illumination. A single SC4 sample was photosensitive, but showed no photovoltaic activity. Funding Publications are based on the research provided by the grant support of the Ministry of Education and Science of Ukraine for young researches (project No 0119U100062 – 2019). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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