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Performance improvement of perovskite solar cells using vanadium oxide interface modification layer
Journal Pre-proof Performance improvement of perovskite solar cells using vanadium oxide interface modification layer Tsung-Han Yeh, Hsin-Ying Lee, Ching-Ting Lee PII:
S0925-8388(19)34866-2
DOI:
https://doi.org/10.1016/j.jallcom.2019.153620
Reference:
JALCOM 153620
To appear in:
Journal of Alloys and Compounds
Received Date: 20 September 2019 Revised Date:
12 December 2019
Accepted Date: 30 December 2019
Please cite this article as: T.-H. Yeh, H.-Y. Lee, C.-T. Lee, Performance improvement of perovskite solar cells using vanadium oxide interface modification layer, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153620. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author contributions Section Hsin-Ying Lee planned and proposed the project, and wrote the manuscript. Tsung-Han Yeh and Ching-Ting Lee designed the experiments. Tsung-Han Yeh fabricated the perovskite solar cells with and without VOx layer and measured the characteristics.
Performance improvement of perovskite solar cells using vanadium oxide interface modification layer
Tsung-Han Yeha, Hsin-Ying Leea,*, Ching-Ting Leea,b
a. Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China b. Department of Electrical Engineering, Yuan Ze University, Taoyuan 320, Taiwan, Republic of China
Journal of Alloys and Compounds
Corresponding Author: Professor Hsin-Ying Lee Address: Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China Tel: 886-6-2082368 Email: [email protected]
1
Abstract To improve the performance of perovskite solar cells (PSCs), vanadium oxide (VOx) film was deposited as an interface modification layer (IML) by a radio frequency magnetron sputtering system. The VOx IML was utilized to modify the interface between the indium tin oxide (ITO) anode electrode and the poly(3,4-ethylenedioxythiophene)-poly
(styrene
sulfonate)
(PEDOT:PSS)
hole
transport layer (HTL). The valence band maximum (VBM) of 4.94 eV of the VOx films was measured by an ultraviolet photoelectron spectroscopy (UPS). Using the optical energy bandgap and the VBM of the VOx film, the conduction band minimum (CBM) energy level was 2.12 eV. This phenomenon verified that the VOx IML could be an electron blocking layer and made a more match energy level between the work function of ITO anode electrode and the highest occupied molecular orbital (HOMO) of PEDOT:PSS HTL. Using the measurement of contact angle, the surface energy of PEDOT:PSS HTL spun on VOx IML and ITO anode electrode was evaluated as 47.76 mJ/m2 and 38.21 mJ/m2, respectively. The enhanced surface energy of the PEDOT:PSS HTL spun on VOx IML could improve the adhesion ability of the perovskite absorption layer spun on the PEDOT:PSS HTL. Consequently, the carrier extraction could be enhanced and the leakage current could be reduced by the predominant functions of VOx IML. Therefore, the performances of the PSCs were 2
significantly improved. The power conversion efficiency (PCE) of the PSCs with VOx IML was enhanced from 9.43% to 13.69% in comparison with the conventional PSCs without VOx IML.
Keywords: Interface modification layer; Perovskite solar cells; Radio frequency magnetron sputtering system; Vanadium oxide.
3
1. Introduction In order to solve human energy demand and environmental pollution problems, the urgent development of green renewable energy sources has become an important issue and goal. Among the renewable energy sources, solar energy is the most concerned, because it is a permanent energy source with sufficient energy. Consequently, the solar energy plays as the most potential and promising energy source to replace the current existing energy in the near future [1]. In recent years, organic solar cells have widely attracted attentions due to their inherent advantages, such as simple process, low cost, light weight, large area preparation, and flexibility [2–4]. Among the organic solar cells, in view of high absorbance at visible light, high carrier mobility, low carrier recombination rate, and low processing temperature, the conversion efficiency growth of perovskite solar cells (PSCs) is beyond the previous types of solar cells [5–12]. High
conductive
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) has been widely used as the hole transport layer (HTL) in PSCs. However, the indium tin oxide (ITO) anode electrode of the PSCs is seriously etched by its acidity with pH of 1~2. Consequently, more defects would be resulted at the interface between PEDOT:PSS and ITO. Those phenomena make a reduction of conversion efficiency and less stability of the organic solar cells. In order to overcome 4
those problems, several metal oxide materials, such as molybdenum oxide (MoOx) [13–15], nickel oxide (NiOx) [16–18], tungsten oxide (WOx) [19,20], copper oxide (CuOx) [21,22], iron oxide (Fe3O4) [23], germanium dioxide (GeO2) [24], and vanadium oxide (VOx) [25–29], were used to replace PEDOT:PSS HTL, recently. In this work, to keep the inherent advantages of high conductivity and high transmittance of PEDOT:PSS HTL, the VOx was deposited using a radio frequency (RF) magnetron sputtering system to insert between the ITO anode electrode and the PEDOT:PSS HTL as the interface modification layer (IML) of PSCs. The VOx could match the energy level between ITO and PEDOT:PSS and block the electrons to enhance the stability and photoelectric characteristics of the resulting PSCs.
2. Experiment Fig. 1 shows the schematic configuration of the PSCs with VOx IML. Using vanadium target, a 20-nm-thick VOx film was deposited on a patterned ITO-coated glass (sheet resistance of 7 Ω/ ) using a RF magnetron sputtering system. The VOx film was used as the IML of PSCs. During the deposition process, the RF power applied to the vanadium target was 200 W under the Ar/O2 gas flow rate of 48/2 sccm and the working pressure of 5 mtorr. A 50-nm-thick PEDOT:PSS HTL was spun on the VOx IML using a spin-coating system and was then annealed in N2 ambient at 120 5
°C for 15 min. After preparing the mixed solution of CH3NH3I (0.395 g) and PbI2 (1.157 g) with a solvent of dimethylsulfoxide (DMSO)/γ-butyrolactone (GBL) (1/1 mL), a 300-nm-thick CH3NH3PbI3 perovskite absorption layer was formed by spinning the solution on the PEDOT:PSS HTL. To enhance the crystallinity of the perovskite absorption layer, the sample was subsequently annealed in N2 ambient at 90 oC for 20 min. Then, a 30-nm-thick C70 electron transport layer (ETL), a 10-nm-thick bathocuproine (BCP) hole-blocking layer, and a 100-nm-thick Ag cathode electrode of the PSCs were sequentially deposited on the perovskite absorption layer using a thermal evaporator. The chemical binding energy and the work function of VOx films were analyzed using an X-ray photoelectron spectroscopy (XPS) and an ultraviolet photoelectron spectroscopy (UPS), respectively. The contact angle of PEDOT:PSS HTL respectively spun on VOx film and ITO anode electrode was measured using a contact angle meter. The current density versus voltage (J-V) characteristics of the PSCs were measured using a J-V curve tracer with an AM 1.5G solar simulator with a power density of 100 mW/cm2. For the measurements of the J-V characteristics, the resulting PSCs were illuminated through a shadow mask to overcome the collection of charge carriers from areas surrounding the electrodes of the PSCs. The external quantum efficiency (EQE) was also measured using a chopped calibrated light beam from a 6
xenon lamp combined with a lock-in amplifier.
3. Experimental results and discussion To evaluate the chemical binding energy of VOx films, the V 2p core level spectrum was measured using XPS and shown in Fig. 2. The V 2p3/2 core level spectrum was deconvolved into three components of V2O5 (517.4 eV), VO2 (516.3 eV), and V2O3 (515.7 eV) [30]. The V 2p1/2 core level spectrum was also constructed by three components of V2O5 (524.3 eV), VO2 (523.2 eV), and V2O3 (522.6 eV). As shown in Fig. 2, since the intensity of the V2O5 was much larger than that of VO2, and V2O3, it was deduced that the V2O5 was the most stable phase in the VOx films. To study the improvement of the energy level match between the work function of ITO anode electrode and the highest occupied molecular orbital (HOMO) of PEDOT:PSS HTL, the VOx IML was inserted between them. Using the UPS with He I source (photon energy = 21.22 eV), the UPS spectrum of the VOx film at the low-binding energy side and the high-binding energy side are shown in Fig. 3a and Fig. 3b, respectively. As shown in Fig. 3a, the energy difference between the Fermi level (EF) and the valence band maxima (VBM) of the VOx film was 0.34 eV. Furthermore, the cut-off energy (Ecut-off) of 16.62 eV could be obtained as shown in Fig. 3b. The work function (Φ) of the VOx film can be calculated as follows [31,32]: 7
Φ=Es−Ecut-off
(1)
where Es is the photon energy (21.22 eV) of He I source. Therefore, the evaluated work function of the VOx film was 4.60 eV. Using the evaluated work function and the energy difference between the EF and the VBM of the VOx films, the VBM located at the position of 4.94 eV was evaluated and the value was similar with the previous literature [33]. In addition, to evaluate the optical energy bandgap, the transmittance and reflectance spectra of the VOx film were measured by using an UV-visible near infrared spectroscopy. The absorption coefficient (α) can be calculated as follows [34,35]: T=(1−R)2exp(−αd)
(2)
where T, R, and d are the transmittance, the reflectance, and the thickness of the VOx films, respectively. The optical energy bandgap (Eg) of the VOx film can be determined by using Tauc plot and estimated as follows [34,36]:
αhν=A(hν−Eg)1/2
(3)
where A and hν are the constant and the photon energy, respectively. Fig. 3c shows (αhν)2 as a function of photon energy (hν) of the VOx films. The optical energy bandgap of the VOx films was 2.82 eV. Using the optical energy bandgap and the position of VBM, the conduction band minimum (CBM) energy level of the VOx film was 2.12 eV. According the above-mentioned parameters, the energy band diagram of 8
the resulting PSCs was shown in Fig. 3d. Consequently, it is worth noting that the VOx IML could effectively work as an electron blocking layer and could make more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL. It could be expected that the performances of the PSCs were improved due to the reduction of the leakage current and the improvement of the hole transmission efficiency contributed by the VOx IML. Since the adhesion ability of perovskite absorption layer spun on PEDOT:PSS HTL was an important factor for the resulting PSCs, the surface energy of the PEDOT:PSS HTL should be analyzed. To measure the contact angle, glycol and methylene iodide liquids were respectively dropped on the surface of the PEDOT:PSS HTL spun on VOx films (PEDOT:PSS/VOx) and the PEDOT:PSS HTL spun on ITO anode electrode (PEDOT:PSS/ITO). Fig. 4a and Fig. 4b shows the measured contact angle of the PEDOT:PSS/VOx sample and the PEDOT:PSS/ITO sample using the glycol liquid, respectively. It was found that the contact angle of the PEDOT:PSS/VOx sample and the PEDOT:PSS/ITO sample was 18.15o and 44.20o, respectively. Fig. 4c and Fig. 4d shows the measured contact angle of the PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample using the methylene iodide liquid, respectively. The contact angle of the PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample was 63.75o and 46.90o, respectively. Using the Owens-Wendt equation [37], the surface energy of the 9
PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample was determined as 47.76 mJ/m2 and 38.21 mJ/m2, respectively. Compared the determined surface energy between the PEDOT:PSS/VOx and the PEDOT:PSS/ITO, it was worth to note that the inserted VOx IML could enhance the surface energy of the PEDOT:PSS HTL. Therefore, the better adhesion ability of the perovskite absorption layer spun on the PEDOT:PSS/VOx sample could be expected. Consequently, the carrier injection between the PEDOT:PSS HTL and the perovskite absorption layer in the PSCs could be enhanced [38]. Fig. 5a shows the transmittance spectra of the PEDOT:PSS HTL and the PEDOT:PSS HTL/VOx IML. It was found that the transmittance of the PEDOT:PSS HTL/VOx IML was slightly lower than that of the PEDOT:PSS HTL at the short wavelength. However, both the transmittances were larger than 95% at the wavelength range from 300 nm to 800 nm. Consequently, it was deduced that the incident light of the PSCs did not be affected by the inserted VOx IML. The current density-voltage (J-V) characteristics of the PSCs without and with VOx IML are shown in Fig. 5b. The short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) of the PSCs without VOx IML were 18.68 mA/cm2, 0.80 V, 63.10%, and 9.43%, respectively. The JSC, VOC, FF, and PCE of the PSCs with VOx IML were 23.55 mA/cm2, 0.80 V, 10
72.68%, and 13.69%, respectively. By using VOx IML, the JSC, FF, and PCE of the PSCs were significantly improved, while the VOC was kept at the same 0.80 V. Fig. 5c shows the dark J-V characteristics of the PSCs without and with VOx IML. It was worth noting that the dark current density of the PSCs with VOx IML was lower than that of the PSCs without VOx IML. Since the VOC was kept at the same 0.80 V, the significant improvement of JSC, FF, PCE, and the dark current density of the PSCs with VOx IML was attributed to the electron blocking function and the more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL by inserting VOx IML. Moreover, the carrier injection between the PEDOT:PSS HTL and the perovskite absorption layer in the PSCs could be improved by the enhanced surface energy of the PEDOT:PSS HTL by inserting the VOx IML. Consequently, the adhesion ability of the perovskite absorption layer could be enhanced. Fig. 5d shows the external quantum efficiency (EQE) spectra of the PSCs without and with VOx IML. It was found that the overall EQE of the PSCs with VOx IML was larger than that of the PSCs without VOx IML.
4. Conclusions In this work, using the RF magnetron sputtering system, the VOx film was deposited and inserted between the ITO anode electrode and the PEDOT:PSS HTL as 11
the interface modification layer in the PSCs. According to the UPS spectra and the optical energy bandgap results, the CBM and VBM of the VOx IML were 2.12 eV and 4.94 eV, respectively. Consequently, the VOx IML could work as an electron blocking layer and make more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL. In addition, according to the contact angle measurement and the Owens-Wendt equation, the surface energy of 47.76 mJ/m2 for the PETDOT:PSS HTL spun on the VOx IML was larger than that of 38.21 mJ/m2 for the PETDOT:PSS HTL directly spun on the ITO anode electrode. The carrier injection was improved by the enhanced adhesion ability of the perovskite absorption layer on the PEDOT:PSS HTL induced by the improved surface energy. Consequently, the performances of the PSCs with the VOx IML were significantly improved. The PCE of the PSCs was increase from 9.43% to 13.69% by using the VOx IML. It was expected that the inserted VOx film was a promising material to modify the interface between the ITO anode electrode and the PEDOT:PSS HTL. It is well known that the PCE of the perovskite solar cells is highly determined by the quality of perovskite materials. In this work, since we focused and emphasized to propose the promising function of VOx IML in the perovskite solar cells, the general marketed perovskite materials were used. According to the PCE improvement of the resulting PSCs using the VOx IML, high performance perovskite solar cells could be 12
obtained if high quality materials were used.
Acknowledge This work was performed at Taiwan Semiconductor Research Institute and supported from the Ministry of Science and Technology of the Republic of China under
contract
No.
MOST
107-2221-E-006-144
108-2221-E-006-196-MY3.
13
and
MOST
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Figure captions Figure 1 Schematic configuration of PSCs with VOx IML. Figure 2 XPS spectrum of V 2p core level spectrum for VOx films. Figure 3 UPS spectrum of VOx film (a) at low-binding energy side and (b) at high-binding energy side. (c) (αhν)2 as a function of photon energy of VOx films. (d) Energy band diagram of the resulting PSCs. Figure 4 Contact angle of (a) PEDOT:PSS/VOx and (b) PEDOT:PSS/ITO measured using glycol liquid. Contact angle of (c) PEDOT:PSS/VOx and (d) PEDOT:PSS/ITO measured using methylene iodide liquid. Figure 5 (a) Transmittance spectra of PEDOT:PSS HTL and PEDOT:PSS HTL/VOx IML. (b) Current density-voltage characteristics, (c) dark current density-voltage characteristics, and (d) external quantum efficiency of PSCs without and with VOx IML.
21
Highlights: •
VOx film was deposited as an interface modification layer of perovskite solar cell.
•
VOx IML could be an electron blocking layer and make a more match energy level.
•
The carrier extraction could be enhanced by the predominant functions of VOx IML.
•
VOx IML enhance surface energy to improve the adhesion ability of the active layer.
•
The power conversion efficiency was increase from 9.43% to 13.69% using VOx IML.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34866-2
DOI:
https://doi.org/10.1016/j.jallcom.2019.153620
Reference:
JALCOM 153620
To appear in:
Journal of Alloys and Compounds
Received Date: 20 September 2019 Revised Date:
12 December 2019
Accepted Date: 30 December 2019
Please cite this article as: T.-H. Yeh, H.-Y. Lee, C.-T. Lee, Performance improvement of perovskite solar cells using vanadium oxide interface modification layer, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153620. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author contributions Section Hsin-Ying Lee planned and proposed the project, and wrote the manuscript. Tsung-Han Yeh and Ching-Ting Lee designed the experiments. Tsung-Han Yeh fabricated the perovskite solar cells with and without VOx layer and measured the characteristics.
Performance improvement of perovskite solar cells using vanadium oxide interface modification layer
Tsung-Han Yeha, Hsin-Ying Leea,*, Ching-Ting Leea,b
a. Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China b. Department of Electrical Engineering, Yuan Ze University, Taoyuan 320, Taiwan, Republic of China
Journal of Alloys and Compounds
Corresponding Author: Professor Hsin-Ying Lee Address: Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China Tel: 886-6-2082368 Email: [email protected]
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Abstract To improve the performance of perovskite solar cells (PSCs), vanadium oxide (VOx) film was deposited as an interface modification layer (IML) by a radio frequency magnetron sputtering system. The VOx IML was utilized to modify the interface between the indium tin oxide (ITO) anode electrode and the poly(3,4-ethylenedioxythiophene)-poly
(styrene
sulfonate)
(PEDOT:PSS)
hole
transport layer (HTL). The valence band maximum (VBM) of 4.94 eV of the VOx films was measured by an ultraviolet photoelectron spectroscopy (UPS). Using the optical energy bandgap and the VBM of the VOx film, the conduction band minimum (CBM) energy level was 2.12 eV. This phenomenon verified that the VOx IML could be an electron blocking layer and made a more match energy level between the work function of ITO anode electrode and the highest occupied molecular orbital (HOMO) of PEDOT:PSS HTL. Using the measurement of contact angle, the surface energy of PEDOT:PSS HTL spun on VOx IML and ITO anode electrode was evaluated as 47.76 mJ/m2 and 38.21 mJ/m2, respectively. The enhanced surface energy of the PEDOT:PSS HTL spun on VOx IML could improve the adhesion ability of the perovskite absorption layer spun on the PEDOT:PSS HTL. Consequently, the carrier extraction could be enhanced and the leakage current could be reduced by the predominant functions of VOx IML. Therefore, the performances of the PSCs were 2
significantly improved. The power conversion efficiency (PCE) of the PSCs with VOx IML was enhanced from 9.43% to 13.69% in comparison with the conventional PSCs without VOx IML.
Keywords: Interface modification layer; Perovskite solar cells; Radio frequency magnetron sputtering system; Vanadium oxide.
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1. Introduction In order to solve human energy demand and environmental pollution problems, the urgent development of green renewable energy sources has become an important issue and goal. Among the renewable energy sources, solar energy is the most concerned, because it is a permanent energy source with sufficient energy. Consequently, the solar energy plays as the most potential and promising energy source to replace the current existing energy in the near future [1]. In recent years, organic solar cells have widely attracted attentions due to their inherent advantages, such as simple process, low cost, light weight, large area preparation, and flexibility [2–4]. Among the organic solar cells, in view of high absorbance at visible light, high carrier mobility, low carrier recombination rate, and low processing temperature, the conversion efficiency growth of perovskite solar cells (PSCs) is beyond the previous types of solar cells [5–12]. High
conductive
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) has been widely used as the hole transport layer (HTL) in PSCs. However, the indium tin oxide (ITO) anode electrode of the PSCs is seriously etched by its acidity with pH of 1~2. Consequently, more defects would be resulted at the interface between PEDOT:PSS and ITO. Those phenomena make a reduction of conversion efficiency and less stability of the organic solar cells. In order to overcome 4
those problems, several metal oxide materials, such as molybdenum oxide (MoOx) [13–15], nickel oxide (NiOx) [16–18], tungsten oxide (WOx) [19,20], copper oxide (CuOx) [21,22], iron oxide (Fe3O4) [23], germanium dioxide (GeO2) [24], and vanadium oxide (VOx) [25–29], were used to replace PEDOT:PSS HTL, recently. In this work, to keep the inherent advantages of high conductivity and high transmittance of PEDOT:PSS HTL, the VOx was deposited using a radio frequency (RF) magnetron sputtering system to insert between the ITO anode electrode and the PEDOT:PSS HTL as the interface modification layer (IML) of PSCs. The VOx could match the energy level between ITO and PEDOT:PSS and block the electrons to enhance the stability and photoelectric characteristics of the resulting PSCs.
2. Experiment Fig. 1 shows the schematic configuration of the PSCs with VOx IML. Using vanadium target, a 20-nm-thick VOx film was deposited on a patterned ITO-coated glass (sheet resistance of 7 Ω/ ) using a RF magnetron sputtering system. The VOx film was used as the IML of PSCs. During the deposition process, the RF power applied to the vanadium target was 200 W under the Ar/O2 gas flow rate of 48/2 sccm and the working pressure of 5 mtorr. A 50-nm-thick PEDOT:PSS HTL was spun on the VOx IML using a spin-coating system and was then annealed in N2 ambient at 120 5
°C for 15 min. After preparing the mixed solution of CH3NH3I (0.395 g) and PbI2 (1.157 g) with a solvent of dimethylsulfoxide (DMSO)/γ-butyrolactone (GBL) (1/1 mL), a 300-nm-thick CH3NH3PbI3 perovskite absorption layer was formed by spinning the solution on the PEDOT:PSS HTL. To enhance the crystallinity of the perovskite absorption layer, the sample was subsequently annealed in N2 ambient at 90 oC for 20 min. Then, a 30-nm-thick C70 electron transport layer (ETL), a 10-nm-thick bathocuproine (BCP) hole-blocking layer, and a 100-nm-thick Ag cathode electrode of the PSCs were sequentially deposited on the perovskite absorption layer using a thermal evaporator. The chemical binding energy and the work function of VOx films were analyzed using an X-ray photoelectron spectroscopy (XPS) and an ultraviolet photoelectron spectroscopy (UPS), respectively. The contact angle of PEDOT:PSS HTL respectively spun on VOx film and ITO anode electrode was measured using a contact angle meter. The current density versus voltage (J-V) characteristics of the PSCs were measured using a J-V curve tracer with an AM 1.5G solar simulator with a power density of 100 mW/cm2. For the measurements of the J-V characteristics, the resulting PSCs were illuminated through a shadow mask to overcome the collection of charge carriers from areas surrounding the electrodes of the PSCs. The external quantum efficiency (EQE) was also measured using a chopped calibrated light beam from a 6
xenon lamp combined with a lock-in amplifier.
3. Experimental results and discussion To evaluate the chemical binding energy of VOx films, the V 2p core level spectrum was measured using XPS and shown in Fig. 2. The V 2p3/2 core level spectrum was deconvolved into three components of V2O5 (517.4 eV), VO2 (516.3 eV), and V2O3 (515.7 eV) [30]. The V 2p1/2 core level spectrum was also constructed by three components of V2O5 (524.3 eV), VO2 (523.2 eV), and V2O3 (522.6 eV). As shown in Fig. 2, since the intensity of the V2O5 was much larger than that of VO2, and V2O3, it was deduced that the V2O5 was the most stable phase in the VOx films. To study the improvement of the energy level match between the work function of ITO anode electrode and the highest occupied molecular orbital (HOMO) of PEDOT:PSS HTL, the VOx IML was inserted between them. Using the UPS with He I source (photon energy = 21.22 eV), the UPS spectrum of the VOx film at the low-binding energy side and the high-binding energy side are shown in Fig. 3a and Fig. 3b, respectively. As shown in Fig. 3a, the energy difference between the Fermi level (EF) and the valence band maxima (VBM) of the VOx film was 0.34 eV. Furthermore, the cut-off energy (Ecut-off) of 16.62 eV could be obtained as shown in Fig. 3b. The work function (Φ) of the VOx film can be calculated as follows [31,32]: 7
Φ=Es−Ecut-off
(1)
where Es is the photon energy (21.22 eV) of He I source. Therefore, the evaluated work function of the VOx film was 4.60 eV. Using the evaluated work function and the energy difference between the EF and the VBM of the VOx films, the VBM located at the position of 4.94 eV was evaluated and the value was similar with the previous literature [33]. In addition, to evaluate the optical energy bandgap, the transmittance and reflectance spectra of the VOx film were measured by using an UV-visible near infrared spectroscopy. The absorption coefficient (α) can be calculated as follows [34,35]: T=(1−R)2exp(−αd)
(2)
where T, R, and d are the transmittance, the reflectance, and the thickness of the VOx films, respectively. The optical energy bandgap (Eg) of the VOx film can be determined by using Tauc plot and estimated as follows [34,36]:
αhν=A(hν−Eg)1/2
(3)
where A and hν are the constant and the photon energy, respectively. Fig. 3c shows (αhν)2 as a function of photon energy (hν) of the VOx films. The optical energy bandgap of the VOx films was 2.82 eV. Using the optical energy bandgap and the position of VBM, the conduction band minimum (CBM) energy level of the VOx film was 2.12 eV. According the above-mentioned parameters, the energy band diagram of 8
the resulting PSCs was shown in Fig. 3d. Consequently, it is worth noting that the VOx IML could effectively work as an electron blocking layer and could make more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL. It could be expected that the performances of the PSCs were improved due to the reduction of the leakage current and the improvement of the hole transmission efficiency contributed by the VOx IML. Since the adhesion ability of perovskite absorption layer spun on PEDOT:PSS HTL was an important factor for the resulting PSCs, the surface energy of the PEDOT:PSS HTL should be analyzed. To measure the contact angle, glycol and methylene iodide liquids were respectively dropped on the surface of the PEDOT:PSS HTL spun on VOx films (PEDOT:PSS/VOx) and the PEDOT:PSS HTL spun on ITO anode electrode (PEDOT:PSS/ITO). Fig. 4a and Fig. 4b shows the measured contact angle of the PEDOT:PSS/VOx sample and the PEDOT:PSS/ITO sample using the glycol liquid, respectively. It was found that the contact angle of the PEDOT:PSS/VOx sample and the PEDOT:PSS/ITO sample was 18.15o and 44.20o, respectively. Fig. 4c and Fig. 4d shows the measured contact angle of the PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample using the methylene iodide liquid, respectively. The contact angle of the PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample was 63.75o and 46.90o, respectively. Using the Owens-Wendt equation [37], the surface energy of the 9
PEDOT:PSS/VOx sample and PEDOT:PSS/ITO sample was determined as 47.76 mJ/m2 and 38.21 mJ/m2, respectively. Compared the determined surface energy between the PEDOT:PSS/VOx and the PEDOT:PSS/ITO, it was worth to note that the inserted VOx IML could enhance the surface energy of the PEDOT:PSS HTL. Therefore, the better adhesion ability of the perovskite absorption layer spun on the PEDOT:PSS/VOx sample could be expected. Consequently, the carrier injection between the PEDOT:PSS HTL and the perovskite absorption layer in the PSCs could be enhanced [38]. Fig. 5a shows the transmittance spectra of the PEDOT:PSS HTL and the PEDOT:PSS HTL/VOx IML. It was found that the transmittance of the PEDOT:PSS HTL/VOx IML was slightly lower than that of the PEDOT:PSS HTL at the short wavelength. However, both the transmittances were larger than 95% at the wavelength range from 300 nm to 800 nm. Consequently, it was deduced that the incident light of the PSCs did not be affected by the inserted VOx IML. The current density-voltage (J-V) characteristics of the PSCs without and with VOx IML are shown in Fig. 5b. The short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) of the PSCs without VOx IML were 18.68 mA/cm2, 0.80 V, 63.10%, and 9.43%, respectively. The JSC, VOC, FF, and PCE of the PSCs with VOx IML were 23.55 mA/cm2, 0.80 V, 10
72.68%, and 13.69%, respectively. By using VOx IML, the JSC, FF, and PCE of the PSCs were significantly improved, while the VOC was kept at the same 0.80 V. Fig. 5c shows the dark J-V characteristics of the PSCs without and with VOx IML. It was worth noting that the dark current density of the PSCs with VOx IML was lower than that of the PSCs without VOx IML. Since the VOC was kept at the same 0.80 V, the significant improvement of JSC, FF, PCE, and the dark current density of the PSCs with VOx IML was attributed to the electron blocking function and the more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL by inserting VOx IML. Moreover, the carrier injection between the PEDOT:PSS HTL and the perovskite absorption layer in the PSCs could be improved by the enhanced surface energy of the PEDOT:PSS HTL by inserting the VOx IML. Consequently, the adhesion ability of the perovskite absorption layer could be enhanced. Fig. 5d shows the external quantum efficiency (EQE) spectra of the PSCs without and with VOx IML. It was found that the overall EQE of the PSCs with VOx IML was larger than that of the PSCs without VOx IML.
4. Conclusions In this work, using the RF magnetron sputtering system, the VOx film was deposited and inserted between the ITO anode electrode and the PEDOT:PSS HTL as 11
the interface modification layer in the PSCs. According to the UPS spectra and the optical energy bandgap results, the CBM and VBM of the VOx IML were 2.12 eV and 4.94 eV, respectively. Consequently, the VOx IML could work as an electron blocking layer and make more energy level match between the work function of ITO anode electrode and the HOMO of PEDOT:PSS HTL. In addition, according to the contact angle measurement and the Owens-Wendt equation, the surface energy of 47.76 mJ/m2 for the PETDOT:PSS HTL spun on the VOx IML was larger than that of 38.21 mJ/m2 for the PETDOT:PSS HTL directly spun on the ITO anode electrode. The carrier injection was improved by the enhanced adhesion ability of the perovskite absorption layer on the PEDOT:PSS HTL induced by the improved surface energy. Consequently, the performances of the PSCs with the VOx IML were significantly improved. The PCE of the PSCs was increase from 9.43% to 13.69% by using the VOx IML. It was expected that the inserted VOx film was a promising material to modify the interface between the ITO anode electrode and the PEDOT:PSS HTL. It is well known that the PCE of the perovskite solar cells is highly determined by the quality of perovskite materials. In this work, since we focused and emphasized to propose the promising function of VOx IML in the perovskite solar cells, the general marketed perovskite materials were used. According to the PCE improvement of the resulting PSCs using the VOx IML, high performance perovskite solar cells could be 12
obtained if high quality materials were used.
Acknowledge This work was performed at Taiwan Semiconductor Research Institute and supported from the Ministry of Science and Technology of the Republic of China under
contract
No.
MOST
107-2221-E-006-144
108-2221-E-006-196-MY3.
13
and
MOST
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Figure captions Figure 1 Schematic configuration of PSCs with VOx IML. Figure 2 XPS spectrum of V 2p core level spectrum for VOx films. Figure 3 UPS spectrum of VOx film (a) at low-binding energy side and (b) at high-binding energy side. (c) (αhν)2 as a function of photon energy of VOx films. (d) Energy band diagram of the resulting PSCs. Figure 4 Contact angle of (a) PEDOT:PSS/VOx and (b) PEDOT:PSS/ITO measured using glycol liquid. Contact angle of (c) PEDOT:PSS/VOx and (d) PEDOT:PSS/ITO measured using methylene iodide liquid. Figure 5 (a) Transmittance spectra of PEDOT:PSS HTL and PEDOT:PSS HTL/VOx IML. (b) Current density-voltage characteristics, (c) dark current density-voltage characteristics, and (d) external quantum efficiency of PSCs without and with VOx IML.
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Highlights: •
VOx film was deposited as an interface modification layer of perovskite solar cell.
•
VOx IML could be an electron blocking layer and make a more match energy level.
•
The carrier extraction could be enhanced by the predominant functions of VOx IML.
•
VOx IML enhance surface energy to improve the adhesion ability of the active layer.
•
The power conversion efficiency was increase from 9.43% to 13.69% using VOx IML.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: