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Enhanced thermal stability of reduced graphene oxide-Silicon Schottky heterojunction solar cells via nitrogen doping
Materials Science in Semiconductor Processing 59 (2017) 45–49
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Enhanced thermal stability of reduced graphene oxide-Silicon Schottky heterojunction solar cells via nitrogen doping
crossmark
Min Hana, Beo Deul Ryua, Jung-Hwan Hyungb, Nam Hanc, Young Jae Parka, Kang Bok Koa, ⁎ Ko Ku Kanga, Tran Viet Cuonga,d, , Chang-Hee Honga,⁎⁎ a
School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea Department of Nanosystem Research, National Nano Fab Center(NNFC), Daejeon 305-701, South Korea c Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, South Korea d NTT Hi-Tech Institute, Nguyen Tat Thanh University, 298-300 A Nguyen Tat Thanh Street, Ho Chi Minh City, Vietnam b
A R T I C L E I N F O
A BS T RAC T
Keywords: Reduced graphene oxide Nitrogen doping Solar cells Thermal stability
We investigated the thermal stability and electrical properties of nitrogen-doped reduced graphene oxide (NrGO)/n–type Si solar cells. A NrGO layer was used as a transparent electrode as well as an electron-hole separation layer simultaneously. The effect of doping on the carbon and nitrogen bonding configurations in NrGO was investigated using X–ray photoelectron spectroscopy (XPS). The XPS data indicate that pyridinic–N is the dominant bonding configuration. This bonding configuration leads to a reduction in the power conversion efficiency and a decrease in the short circuit current. However, on being subjected to thermal oxidation, the NrGO/n–type Si solar cells exhibit a smaller variation in series resistance compared to the undoped rGO/Si solar cells. Results of accelerated thermal tests suggest that nitrogen doping prevents re-oxidation of the reduced graphene oxide layer.
1. Introduction In the past few years, graphene oxide (GO) has emerged as an attractive candidate for producing graphene. Exfoliated single layer GO sheets can be well dispersed in different aqueous solutions. Additionally, an insulating film of GO can be easily converted to semi-metallic reduced graphene oxide (rGO) for more efficient carrier transport via chemical and thermal reduction [1]. Electrical and optical properties of rGO are dependent on the method and degree of reduction has the advantage be applied to various devices. Typically, it has applied to a solar cell, light-emitting diode (LED), and sensor. Among the applied devices, rGO layers acts as the transparent conducting layer and hole transport layer on Si or GaN Schottky solar cells [2,3] and helps to enhance the stability of devices to suppress the surface oxidation on Si [4]. Doping is an efficient way of tailoring the electronic properties of GO by modulating its band structure, thereby making it suitable for a wide range of applications. Nitrogen doping in GO has been extensively studied in the past. Although nitrogen, being a group 5 element, is considered to be an n–type dopant, there have been reports where it has been used as p–type as well as n–type dopant depending on the
⁎
bonding configuration of nitrogen in GO [5]. Apart from this flexibility, another advantage of nitrogen doping lies in its enhanced thermal stability. Based on thermo gravimetric analysis, Sandoval et al. demonstrated that nitrogen-doped rGO (NrGO) has a higher thermal stability compared to undoped rGO [6]. In this paper, we used NrGO as the window layer as well as holeelectron separation layer of a solar cell to simplify its structure. The electrical and thermal properties of the NrGO/n–type Si solar cell were investigated. NH3 was used as a nitrogen precursor in our experiments. The C 1 s and N 1 s peaks seen in the X-ray photoelectron spectroscopy (XPS) were used to determine the nitrogen bonding configuration and doping state in NrGO. Current-voltage (I – V) characteristics of the NrGO–based solar cell device were measured in order to determine the type of junctions that NrGO forms with n– and p-type Si substrates. Power conversion efficiency (PCE) of the rGO and NrGO-based solar cells were also obtained from the I – V characteristics measured under illumination. To investigate the thermal stability, we performed accelerated thermal tests in a thermal chamber for varying time periods. Apart from a high PCE, improvement of the thermal stability of the device and development of fabrication processes are equally important issues for the realization of graphene-based solar cells as the
Corresponding author at: School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea. Corresponding author E-mail address: [email protected] (C.-H. Hong).
⁎⁎
http://dx.doi.org/10.1016/j.mssp.2016.11.021 Received 9 May 2016; Received in revised form 25 October 2016; Accepted 9 November 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 59 (2017) 45–49
M. Han et al.
Fig. 1. (a) XPS survey-scan spectra of rGO sheets annealed under different NH3 flow rate conditions. XPS C 1 s of the GO films before (b) and after (c) the thermal reduction. (d) Highresolution C 1 s spectrum of a typical rGO film annealed at an NH3 flow rate of 8 slm indicating the C–N bond formation.
Fig. 2. (a) XPS spectra of the GO and rGO films under H2 and different NH3 molar flow rates. (b) C–N bonding area and nitrogen atomic percentage with variation of NH3 flow in XPS C 1 s peak. (c) XPS N 1 s spectra of NrGO films with 4, 6, and 8 slm of NH3 flow rates, respectively. (d) Schematic graphics of N atom incorporation with C atom.
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deposition (CVD) reactor. Herein, prepared sample was then annealed at 800 °C for 10 min under NH3/H2 atmosphere with different NH3 flow rates for the simultaneous reduction and nitrogen doping of the GO layer to yield NrGO. Finally, prepared the NrGO was cut into an area of 1×1 cm2, and used in the fabrication of Schottky heterojunction solar cells, as illustrated in Fig. 1. The front and back contacts were produced via E-beam evaporation of 100 nm-thick Au, and eutectic
2. Experimental method A GO aqueous solution with a concentration of 0.025 mg/ml, synthesized using a modified Hummer's method, was spray-coated on <100> n-type Si substrate and then loaded into a chemical vapor 46
Materials Science in Semiconductor Processing 59 (2017) 45–49
M. Han et al.
Fig. 3. (a) I–V characteristics of rGO/n–Si and NrGO/n–Si solar cells device under dark state with different NH3 flow. (b) Variation of VOC and ISC graph under illumination with different NH3 flow.
has more defect sites than CVD–grown graphene. Thus, in GO synthesized using modified Hummer's method, the N atoms preferentially localize at the defect sites, thereby leading to a pyridinic–N type bonding configuration, rather than replace the C atoms inside the GO lattice that would have resulted in a graphitic–N type bonding configuration. This tunability of the nitrogen bonding configuration is an important finding and could be crucial for the fabrication of various optoelectronic devices since N-doped GO exhibits either p– or n–type semiconducting behavior depending on the type of nitrogen bonding configuration. In order to investigate the electrical properties of NrGO film, GO aqueous solutions were coated on n– and p–type Si substrates, and then annealed in NH3 atmosphere under identical conditions. Fig. 3(a) exhibits typical current-voltage curves of NrGO coated on n– and p– type Si substrates. It is clearly seen that NrGO coated on a n–type and a p-type Si substrate exhibited a rectifying, and an ohmic behavior, respectively. Thus, our investigation of the current-voltage characteristics of the heterojunction between NrGO and n–/p–type silicon substrates indicated that the NrGO films are p–type semiconductors. Our results are in agreement with earlier reports of Schiros where the author presented theoretical as well as experimental proof that pyridinic–N doped graphene exhibits p–type semiconducting behavior [8]. Using this result, we have developed a prototype of a Schottky solar cell device based on the heterojunction between NrGO and n–type Si substrates, and tested it under simulated AM 1.5 G illumination. Fig. 3(b) shows the photovoltaic performance of the NrGO films synthesized using different NH3 flow rates. The PCE exhibited a decrease with an increase in the NH3 flow rate. This result differs from earlier reports that observed a significant improvement of the PCE in NrGO. Specifically, it was reported that NrGO exhibits n–type semiconductor properties and the nitrogen doping induces a change in the work function of the rGO sheets, thereby increasing the PCE [9,10]. However, Fig. 3(b) and Eq. (1) show that a change in the NH3 flow rate does not affect the open circuit voltage (VOC). In other words, N– doping in rGO does not affect its work function as evidenced by an unchanged VOC. The VOC is expressed as
gallium-indium pasted on Cu foil, respectively. The thermal degradation tests were performed by placing the devices in an oven at 120 °C in air, and measuring their efficiencies at set intervals of time. 3. Results and discussion Fig. 2(a) shows the XPS of the GO films after thermal reduction using different NH3 flow rates. Appearance of N 1 s peak at 400 eV was expected when the chamber was filled with NH3; on the other hand, in the absence of NH3, the N 1 s peak is absent. This feature is the first spectroscopic evidence of incorporation of N atoms in GO aided by NH3. Thus, annealing in NH3 atmosphere converted the GO layers to NrGO. Fig. 2(b) shows the XPS C 1 s peak results of all samples which include the C-N bonding area and nitrogen atomic percentage with variation of NH3 flow. It demonstrates that the C-N bonding area and nitrogen atomic percentage in the XPS C 1 s spectra increased with the increasing NH3 flow. Furthermore, it is observed that the intensity of the N 1 s peak increases with an increase in the NH3 flow rate, thus indicating a corresponding increase in the N-doping concentration. For a more detailed investigation of the doping mechanism and type of bonding between the N and C atoms in NrGO, high-resolution XPS with special focus on the N 1 s core-level was performed. Fig. 2(c) shows the evaluation of the N 1 s core-level spectra of the annealed samples as a function of different NH3 flow rates. The N 1 s spectrum consists of three components, referred to as pyridinic–N (398.2 eV), pyrrolic–N (399.5 eV), and graphitic–N (401.4 eV). The highest intensity peaks at 398.2 eV correspond to pyridinic–N [7]. These peaks have been attributed to N atoms that are located at the edge of defect sites in the rGO lattice with a π conjugated system, and which contribute one ρ–orbital to the π–system. A schematic graphics of N atom incorporation with C atom was shown in Fig. 2(d). It shows that the incorporation of N atoms into sp2-bonded carbon network can be classified into three different N functionalities; (i) pyridinic–N (refers to N atoms at the edge of graphene planes, each of which is bonded to two carbon atoms and donates one p-electron to the aromatic π– system), (ii) pyrrolic–N (refers to N atoms that are bonded to two carbon atoms and contribute to the π system with two p-electrons), and (iii) graphitic–N (refers to substitutional N atoms at the carbon sites). Our XPS data indicate that pyridinic–N type is the dominant bonding configuration when GO is thermally reduced in NH3 atmosphere. This result is completely different from what has been observed in N-doped graphene synthesized using CVD where graphitic-N type of bonding configuration plays a predominant role. This difference could arise from that fact that GO synthesized using modified Hummer's method
VOC =
kT ⎡ (NA+∆n )∆n ⎤ ⎥ ln ⎢ q ⎣ ni2 ⎦
(1)
where k is the Boltzmann constant, q is the electron charge, NA is the doping concentration, Δn is the excess carrier concentration and ni is the intrinsic carrier concentration [11]. We measured the carrier concentration of our samples by the Hall measurement. In the result, all samples indicate p–type properties and the carrier concentration is 47
Materials Science in Semiconductor Processing 59 (2017) 45–49
M. Han et al.
Fig. 4. (a) The normalized PEC of rGO/n–Si and NrGO/n–Si solar cells with different intentionally thermal oxidation time. (b) Variation of series resistance with different intentionally thermal oxidation time in rGO/n–Si and NrGO/n–Si solar cell.
~ 3.3×1016 cm−3 for rGO, ~ 3.3×1016 cm−3 for rGO with NH3 2 slm, ~ 3. ×1016 cm−3 for rGO with NH3 4 slm, ~ 3.3×1016 cm−3 for rGO with NH3 6 slm, and ~ 3.3×1016 cm−3 for rGO with NH3 8 slm. It means that the variation of carrier concentration was not significantly changed with increasing NH3 flow. We think that the pyridinic–N incorporation in rGO was not affected to the electric states of rGO sheet. As the results, the Δn in rGO with increasing NH3 flows rarely change, thus the VOC does not change. Furthermore, Fig. 3(b) reveals a decrease in the short circuit current (ISC) with an increase in the NH3 flow rate. This indicates that the primary reason for the observed decrease in the PCE can be attributed to the degradation of the ISC. As has been discussed earlier in relation to Fig. 2(c), pyridinic–N bonding configuration corresponds to N atoms incorporated at defect sites. The defects in rGO affect the recombination process of the solar cell devices [4]. In other words, the N atoms act as localized defect sites. These defect sites can trap or scatter the carriers generated by the illumination, thereby resulting in a degradation of the ISC. Our results, thus, establish that an increase in the NH3 flow rate causes a degradation of the ISC, but does not affect the VOC. Although a higher nitrogen doping concentration leads to a lower ISC, it is expected to prevent surface oxidation at the NrGO/Si interface. This is because a physisorbed oxygen have a small adsorption energy and a long distance from the graphene oxide plane. [12]. Thus, NrGO is expected to act as a passivation layer that suppresses oxidation and enhances the lifetime of the NrGO/n–type Si Schottky solar cells. The synthesized NrGO/n–type Si Schottky solar cell devices having different nitrogen concentrations have been subjected to elevated temperatures at 120 °C in air to investigate the correlation between N–doping induced surface modification and suppression of surface oxidation. The thermal stability of the NrGO/n–type Si Schottky solar cells are shown in Fig. 4(a) where the performances of both N–doped and undoped rGO–based devices are presented for comparison. Our accelerated thermal tests revealed a more rapid decrease of the normalized PCE of the undoped rGO–based solar cells compared to that of the NrGO–based solar cells. Increasing the amount of NH3 leads to smaller reductions of the normalized PCE in the N–doped solar cell samples. To quantify the results of the accelerated thermal tests, we calculated the series resistance of each sample in dark state. The variation of the series resistance (ΔRS) as a function of oxidation time is shown in Fig. 4(b). ΔRS of the undoped rGO-based solar cell (72.25 Ω) was larger than ΔRS of the NrGO-based solar cells. This higher value of ΔRS of the undoped rGO-based solar cell corresponds to a larger reduction of the normalized PCE compared to that of the NrGO-based solar cells on being subjected to accelerated thermal tests.
Thus, the NrGO-based solar cells are more resistant to accelerated thermal tests. This confirms that the process of nitrogen doping causes a passivation effect by suppressing surface oxidation. Thus, although nitrogen doping in rGO sheets serves to reduce the PCE by acting as trapping and scattering centers for the carriers, it significantly improves the thermal stability of the device. 4. Conclusion In summary, we have investigated the thermal stability and electrical properties of NrGO/n–type Si solar cells. NrGO was found to form a Schottky junction with n–type Si substrate. This indicated that NrGO has p–type semiconductor properties. XPS data revealed pyridinic–N to be the predominant type of bonding configuration in NrGO that also explained the presence of p–type semiconductor properties. Photovoltaic characteristics of the solar cells recorded under illumination revealed a decrease in the PCE of the NrGO/n– type Si solar cells with an increase in the nitrogen doping concentration. The PCE decreased from ~ 4% in the undoped rGO/n–type Si solar cells to ~ 2.5% in the NrGO/n–type Si solar cells. This decrease in the PCE in the NrGO/n–type Si solar cells was attributed to the pyridinic–N bonding configuration that acted as scattering centers for the carriers. However, accelerated thermal tests revealed that the NrGO/n–type Si solar cells are thermally more stable compared to the undoped rGO/n–type Si solar cells. The enhanced thermal stability arises from the passivation effect that suppresses physisorption of oxygen atoms, and hence surface oxidation. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(Grant No. 2013R1A1A2013044) and by the Transfer machine specialized lighting core technology development professional manpower training project(Project NO: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea). References [1] S. Pei, H.M. Cheng, Carbon 50 (2012) 3210–3228. [2] P. Mahala, A. Kumar, S. Nayak, S. Behura, C. Dhanavantri, O. Jani, Superlattices Microstruct. 92 (2016) 366–373. [3] S.K. Behura, S. Nayak, I. Mukhopadhyay, O. Jani, Carbon 67 (2014) 766–774. [4] B.D. Ryu, J.H. Hyung, M. Han, G.S. Kim, N. Han, K.B. Ko, K.K. Kang, T.V. Cuong,
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(2012) 4025–4031. [9] M. Yun, J.S. Yeo, J.W. Kim, H.G. Jeong, D.Y. Kim, Y.J. Noh, S.S. Kim, B.C. Ku, S.I. Na, Adv. Mater. 23 (2011) 4923–4928. [10] X. Miao, S. Tongay, M.K. Petterson, K. Berke, A.G. Rinzler, B.R. Appleton, A.F. Hebard, Nano Lett. 12 (2012) 2745–2750. [11] R.A. Sinton, A. Cuevas, Appl. Phys. Lett. 69 (1996) 2510–2512. [12] J. Dai, J. Yuan, Phys. Rev. B 81 (2009) 165414.
C.H. Hong, RSC Adv. 6 (2016) 72342–72350. [5] T.F. Yeh, C.Y. Teng, S.J. Chen, H. Teng, Adv. Mater. 26 (2014) 3297–3303. [6] S. Sandoval, N. Kumar, A. Sundaresan, C.N.R. Rao, A. Fuertes, G. Tobias, Chem. Eur. J. 20 (2014) 11999–12003. [7] X. Li, H. Wang, J.T. Robinson, H. Sanchez, G. Diankov, H. Dai, J. Am. Chem. Soc. 131 (2009) 15939–15944. [8] T. Schiros, D. Nordlund, L. Pálová, D. Prezzi, L. Zhao, K.S. Kim, Nano Lett. 12
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Enhanced thermal stability of reduced graphene oxide-Silicon Schottky heterojunction solar cells via nitrogen doping
crossmark
Min Hana, Beo Deul Ryua, Jung-Hwan Hyungb, Nam Hanc, Young Jae Parka, Kang Bok Koa, ⁎ Ko Ku Kanga, Tran Viet Cuonga,d, , Chang-Hee Honga,⁎⁎ a
School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea Department of Nanosystem Research, National Nano Fab Center(NNFC), Daejeon 305-701, South Korea c Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, South Korea d NTT Hi-Tech Institute, Nguyen Tat Thanh University, 298-300 A Nguyen Tat Thanh Street, Ho Chi Minh City, Vietnam b
A R T I C L E I N F O
A BS T RAC T
Keywords: Reduced graphene oxide Nitrogen doping Solar cells Thermal stability
We investigated the thermal stability and electrical properties of nitrogen-doped reduced graphene oxide (NrGO)/n–type Si solar cells. A NrGO layer was used as a transparent electrode as well as an electron-hole separation layer simultaneously. The effect of doping on the carbon and nitrogen bonding configurations in NrGO was investigated using X–ray photoelectron spectroscopy (XPS). The XPS data indicate that pyridinic–N is the dominant bonding configuration. This bonding configuration leads to a reduction in the power conversion efficiency and a decrease in the short circuit current. However, on being subjected to thermal oxidation, the NrGO/n–type Si solar cells exhibit a smaller variation in series resistance compared to the undoped rGO/Si solar cells. Results of accelerated thermal tests suggest that nitrogen doping prevents re-oxidation of the reduced graphene oxide layer.
1. Introduction In the past few years, graphene oxide (GO) has emerged as an attractive candidate for producing graphene. Exfoliated single layer GO sheets can be well dispersed in different aqueous solutions. Additionally, an insulating film of GO can be easily converted to semi-metallic reduced graphene oxide (rGO) for more efficient carrier transport via chemical and thermal reduction [1]. Electrical and optical properties of rGO are dependent on the method and degree of reduction has the advantage be applied to various devices. Typically, it has applied to a solar cell, light-emitting diode (LED), and sensor. Among the applied devices, rGO layers acts as the transparent conducting layer and hole transport layer on Si or GaN Schottky solar cells [2,3] and helps to enhance the stability of devices to suppress the surface oxidation on Si [4]. Doping is an efficient way of tailoring the electronic properties of GO by modulating its band structure, thereby making it suitable for a wide range of applications. Nitrogen doping in GO has been extensively studied in the past. Although nitrogen, being a group 5 element, is considered to be an n–type dopant, there have been reports where it has been used as p–type as well as n–type dopant depending on the
⁎
bonding configuration of nitrogen in GO [5]. Apart from this flexibility, another advantage of nitrogen doping lies in its enhanced thermal stability. Based on thermo gravimetric analysis, Sandoval et al. demonstrated that nitrogen-doped rGO (NrGO) has a higher thermal stability compared to undoped rGO [6]. In this paper, we used NrGO as the window layer as well as holeelectron separation layer of a solar cell to simplify its structure. The electrical and thermal properties of the NrGO/n–type Si solar cell were investigated. NH3 was used as a nitrogen precursor in our experiments. The C 1 s and N 1 s peaks seen in the X-ray photoelectron spectroscopy (XPS) were used to determine the nitrogen bonding configuration and doping state in NrGO. Current-voltage (I – V) characteristics of the NrGO–based solar cell device were measured in order to determine the type of junctions that NrGO forms with n– and p-type Si substrates. Power conversion efficiency (PCE) of the rGO and NrGO-based solar cells were also obtained from the I – V characteristics measured under illumination. To investigate the thermal stability, we performed accelerated thermal tests in a thermal chamber for varying time periods. Apart from a high PCE, improvement of the thermal stability of the device and development of fabrication processes are equally important issues for the realization of graphene-based solar cells as the
Corresponding author at: School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, South Korea. Corresponding author E-mail address: [email protected] (C.-H. Hong).
⁎⁎
http://dx.doi.org/10.1016/j.mssp.2016.11.021 Received 9 May 2016; Received in revised form 25 October 2016; Accepted 9 November 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 59 (2017) 45–49
M. Han et al.
Fig. 1. (a) XPS survey-scan spectra of rGO sheets annealed under different NH3 flow rate conditions. XPS C 1 s of the GO films before (b) and after (c) the thermal reduction. (d) Highresolution C 1 s spectrum of a typical rGO film annealed at an NH3 flow rate of 8 slm indicating the C–N bond formation.
Fig. 2. (a) XPS spectra of the GO and rGO films under H2 and different NH3 molar flow rates. (b) C–N bonding area and nitrogen atomic percentage with variation of NH3 flow in XPS C 1 s peak. (c) XPS N 1 s spectra of NrGO films with 4, 6, and 8 slm of NH3 flow rates, respectively. (d) Schematic graphics of N atom incorporation with C atom.
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deposition (CVD) reactor. Herein, prepared sample was then annealed at 800 °C for 10 min under NH3/H2 atmosphere with different NH3 flow rates for the simultaneous reduction and nitrogen doping of the GO layer to yield NrGO. Finally, prepared the NrGO was cut into an area of 1×1 cm2, and used in the fabrication of Schottky heterojunction solar cells, as illustrated in Fig. 1. The front and back contacts were produced via E-beam evaporation of 100 nm-thick Au, and eutectic
2. Experimental method A GO aqueous solution with a concentration of 0.025 mg/ml, synthesized using a modified Hummer's method, was spray-coated on <100> n-type Si substrate and then loaded into a chemical vapor 46
Materials Science in Semiconductor Processing 59 (2017) 45–49
M. Han et al.
Fig. 3. (a) I–V characteristics of rGO/n–Si and NrGO/n–Si solar cells device under dark state with different NH3 flow. (b) Variation of VOC and ISC graph under illumination with different NH3 flow.
has more defect sites than CVD–grown graphene. Thus, in GO synthesized using modified Hummer's method, the N atoms preferentially localize at the defect sites, thereby leading to a pyridinic–N type bonding configuration, rather than replace the C atoms inside the GO lattice that would have resulted in a graphitic–N type bonding configuration. This tunability of the nitrogen bonding configuration is an important finding and could be crucial for the fabrication of various optoelectronic devices since N-doped GO exhibits either p– or n–type semiconducting behavior depending on the type of nitrogen bonding configuration. In order to investigate the electrical properties of NrGO film, GO aqueous solutions were coated on n– and p–type Si substrates, and then annealed in NH3 atmosphere under identical conditions. Fig. 3(a) exhibits typical current-voltage curves of NrGO coated on n– and p– type Si substrates. It is clearly seen that NrGO coated on a n–type and a p-type Si substrate exhibited a rectifying, and an ohmic behavior, respectively. Thus, our investigation of the current-voltage characteristics of the heterojunction between NrGO and n–/p–type silicon substrates indicated that the NrGO films are p–type semiconductors. Our results are in agreement with earlier reports of Schiros where the author presented theoretical as well as experimental proof that pyridinic–N doped graphene exhibits p–type semiconducting behavior [8]. Using this result, we have developed a prototype of a Schottky solar cell device based on the heterojunction between NrGO and n–type Si substrates, and tested it under simulated AM 1.5 G illumination. Fig. 3(b) shows the photovoltaic performance of the NrGO films synthesized using different NH3 flow rates. The PCE exhibited a decrease with an increase in the NH3 flow rate. This result differs from earlier reports that observed a significant improvement of the PCE in NrGO. Specifically, it was reported that NrGO exhibits n–type semiconductor properties and the nitrogen doping induces a change in the work function of the rGO sheets, thereby increasing the PCE [9,10]. However, Fig. 3(b) and Eq. (1) show that a change in the NH3 flow rate does not affect the open circuit voltage (VOC). In other words, N– doping in rGO does not affect its work function as evidenced by an unchanged VOC. The VOC is expressed as
gallium-indium pasted on Cu foil, respectively. The thermal degradation tests were performed by placing the devices in an oven at 120 °C in air, and measuring their efficiencies at set intervals of time. 3. Results and discussion Fig. 2(a) shows the XPS of the GO films after thermal reduction using different NH3 flow rates. Appearance of N 1 s peak at 400 eV was expected when the chamber was filled with NH3; on the other hand, in the absence of NH3, the N 1 s peak is absent. This feature is the first spectroscopic evidence of incorporation of N atoms in GO aided by NH3. Thus, annealing in NH3 atmosphere converted the GO layers to NrGO. Fig. 2(b) shows the XPS C 1 s peak results of all samples which include the C-N bonding area and nitrogen atomic percentage with variation of NH3 flow. It demonstrates that the C-N bonding area and nitrogen atomic percentage in the XPS C 1 s spectra increased with the increasing NH3 flow. Furthermore, it is observed that the intensity of the N 1 s peak increases with an increase in the NH3 flow rate, thus indicating a corresponding increase in the N-doping concentration. For a more detailed investigation of the doping mechanism and type of bonding between the N and C atoms in NrGO, high-resolution XPS with special focus on the N 1 s core-level was performed. Fig. 2(c) shows the evaluation of the N 1 s core-level spectra of the annealed samples as a function of different NH3 flow rates. The N 1 s spectrum consists of three components, referred to as pyridinic–N (398.2 eV), pyrrolic–N (399.5 eV), and graphitic–N (401.4 eV). The highest intensity peaks at 398.2 eV correspond to pyridinic–N [7]. These peaks have been attributed to N atoms that are located at the edge of defect sites in the rGO lattice with a π conjugated system, and which contribute one ρ–orbital to the π–system. A schematic graphics of N atom incorporation with C atom was shown in Fig. 2(d). It shows that the incorporation of N atoms into sp2-bonded carbon network can be classified into three different N functionalities; (i) pyridinic–N (refers to N atoms at the edge of graphene planes, each of which is bonded to two carbon atoms and donates one p-electron to the aromatic π– system), (ii) pyrrolic–N (refers to N atoms that are bonded to two carbon atoms and contribute to the π system with two p-electrons), and (iii) graphitic–N (refers to substitutional N atoms at the carbon sites). Our XPS data indicate that pyridinic–N type is the dominant bonding configuration when GO is thermally reduced in NH3 atmosphere. This result is completely different from what has been observed in N-doped graphene synthesized using CVD where graphitic-N type of bonding configuration plays a predominant role. This difference could arise from that fact that GO synthesized using modified Hummer's method
VOC =
kT ⎡ (NA+∆n )∆n ⎤ ⎥ ln ⎢ q ⎣ ni2 ⎦
(1)
where k is the Boltzmann constant, q is the electron charge, NA is the doping concentration, Δn is the excess carrier concentration and ni is the intrinsic carrier concentration [11]. We measured the carrier concentration of our samples by the Hall measurement. In the result, all samples indicate p–type properties and the carrier concentration is 47
Materials Science in Semiconductor Processing 59 (2017) 45–49
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Fig. 4. (a) The normalized PEC of rGO/n–Si and NrGO/n–Si solar cells with different intentionally thermal oxidation time. (b) Variation of series resistance with different intentionally thermal oxidation time in rGO/n–Si and NrGO/n–Si solar cell.
~ 3.3×1016 cm−3 for rGO, ~ 3.3×1016 cm−3 for rGO with NH3 2 slm, ~ 3. ×1016 cm−3 for rGO with NH3 4 slm, ~ 3.3×1016 cm−3 for rGO with NH3 6 slm, and ~ 3.3×1016 cm−3 for rGO with NH3 8 slm. It means that the variation of carrier concentration was not significantly changed with increasing NH3 flow. We think that the pyridinic–N incorporation in rGO was not affected to the electric states of rGO sheet. As the results, the Δn in rGO with increasing NH3 flows rarely change, thus the VOC does not change. Furthermore, Fig. 3(b) reveals a decrease in the short circuit current (ISC) with an increase in the NH3 flow rate. This indicates that the primary reason for the observed decrease in the PCE can be attributed to the degradation of the ISC. As has been discussed earlier in relation to Fig. 2(c), pyridinic–N bonding configuration corresponds to N atoms incorporated at defect sites. The defects in rGO affect the recombination process of the solar cell devices [4]. In other words, the N atoms act as localized defect sites. These defect sites can trap or scatter the carriers generated by the illumination, thereby resulting in a degradation of the ISC. Our results, thus, establish that an increase in the NH3 flow rate causes a degradation of the ISC, but does not affect the VOC. Although a higher nitrogen doping concentration leads to a lower ISC, it is expected to prevent surface oxidation at the NrGO/Si interface. This is because a physisorbed oxygen have a small adsorption energy and a long distance from the graphene oxide plane. [12]. Thus, NrGO is expected to act as a passivation layer that suppresses oxidation and enhances the lifetime of the NrGO/n–type Si Schottky solar cells. The synthesized NrGO/n–type Si Schottky solar cell devices having different nitrogen concentrations have been subjected to elevated temperatures at 120 °C in air to investigate the correlation between N–doping induced surface modification and suppression of surface oxidation. The thermal stability of the NrGO/n–type Si Schottky solar cells are shown in Fig. 4(a) where the performances of both N–doped and undoped rGO–based devices are presented for comparison. Our accelerated thermal tests revealed a more rapid decrease of the normalized PCE of the undoped rGO–based solar cells compared to that of the NrGO–based solar cells. Increasing the amount of NH3 leads to smaller reductions of the normalized PCE in the N–doped solar cell samples. To quantify the results of the accelerated thermal tests, we calculated the series resistance of each sample in dark state. The variation of the series resistance (ΔRS) as a function of oxidation time is shown in Fig. 4(b). ΔRS of the undoped rGO-based solar cell (72.25 Ω) was larger than ΔRS of the NrGO-based solar cells. This higher value of ΔRS of the undoped rGO-based solar cell corresponds to a larger reduction of the normalized PCE compared to that of the NrGO-based solar cells on being subjected to accelerated thermal tests.
Thus, the NrGO-based solar cells are more resistant to accelerated thermal tests. This confirms that the process of nitrogen doping causes a passivation effect by suppressing surface oxidation. Thus, although nitrogen doping in rGO sheets serves to reduce the PCE by acting as trapping and scattering centers for the carriers, it significantly improves the thermal stability of the device. 4. Conclusion In summary, we have investigated the thermal stability and electrical properties of NrGO/n–type Si solar cells. NrGO was found to form a Schottky junction with n–type Si substrate. This indicated that NrGO has p–type semiconductor properties. XPS data revealed pyridinic–N to be the predominant type of bonding configuration in NrGO that also explained the presence of p–type semiconductor properties. Photovoltaic characteristics of the solar cells recorded under illumination revealed a decrease in the PCE of the NrGO/n– type Si solar cells with an increase in the nitrogen doping concentration. The PCE decreased from ~ 4% in the undoped rGO/n–type Si solar cells to ~ 2.5% in the NrGO/n–type Si solar cells. This decrease in the PCE in the NrGO/n–type Si solar cells was attributed to the pyridinic–N bonding configuration that acted as scattering centers for the carriers. However, accelerated thermal tests revealed that the NrGO/n–type Si solar cells are thermally more stable compared to the undoped rGO/n–type Si solar cells. The enhanced thermal stability arises from the passivation effect that suppresses physisorption of oxygen atoms, and hence surface oxidation. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(Grant No. 2013R1A1A2013044) and by the Transfer machine specialized lighting core technology development professional manpower training project(Project NO: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea). References [1] S. Pei, H.M. Cheng, Carbon 50 (2012) 3210–3228. [2] P. Mahala, A. Kumar, S. Nayak, S. Behura, C. Dhanavantri, O. Jani, Superlattices Microstruct. 92 (2016) 366–373. [3] S.K. Behura, S. Nayak, I. Mukhopadhyay, O. Jani, Carbon 67 (2014) 766–774. [4] B.D. Ryu, J.H. Hyung, M. Han, G.S. Kim, N. Han, K.B. Ko, K.K. Kang, T.V. Cuong,
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