Si Schottky-junction solar cells encapsulated with another graphene layer

Accepted Manuscript Ag-nanowires-doped graphene/Si Schottky-junction solar cells encapsulated with another graphene layer Jong Min Kim, Sang Woo Seo, Dong Hee Shin, Ha Seung Lee, Ju Hwan Kim, Chan Wook Jang, Sung Kim, Suk-Ho Choi PII:

S1567-1739(17)30140-2

DOI:

10.1016/j.cap.2017.05.002

Reference:

CAP 4509

To appear in:

Current Applied Physics

Received Date: 10 February 2017 Revised Date:

11 April 2017

Accepted Date: 6 May 2017

Please cite this article as: J.M. Kim, S.W. Seo, D.H. Shin, H.S. Lee, J.H. Kim, C.W. Jang, S. Kim, S.H. Choi, Ag-nanowires-doped graphene/Si Schottky-junction solar cells encapsulated with another graphene layer, Current Applied Physics (2017), doi: 10.1016/j.cap.2017.05.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT

Ag-nanowires-doped graphene/Si Schottky-junction solar cells encapsulated with another graphene layer

RI PT

Jong Min Kim†, Sang Woo Seo†, Dong Hee Shin†, Ha Seung Lee, Ju Hwan Kim, Chan Wook Jang, Sung Kim, and Suk-Ho Choi* Department of Applied Physics, College of Applied Science, Kyung Hee University, Yongin 17104, Korea

SC

ABSTRACT

Graphene/silver nanowires (Ag NWs)-doped graphene stacks are employed for Si

M AN U

Schottky-junction solar cells as transparent conductive electrodes (TCEs). The doping of graphene by Ag NWs decreases the series resistance of the solar cells and enhances the electrical conductivity of the graphene TCEs, resulting in remarkable improvements of the diode properties of the solar cells. In addition, the Ag NWs on the graphene reduces the

TE D

reflectance of the solar cells as well as the transmittance of the graphene TCEs. This trade-off correlation makes the power-conversion efficiency maximized to 3.51 % at concentration of Ag NWs (nA) = 0.1 wt%. The long-term stabilities of the photovoltaic properties are greatly

EP

improved by the encapsulation of the Ag NWs/graphene TCEs with another graphene because of the excellence of graphene as a gas-barrier. These and other nA-dependent

AC C

behaviors of Raman spectra, work function, sheet resistance, external quantum efficiency, and DC conductivity/optical conductivity ratio are discussed to explain the photovoltaic properties of the solar cells.

Keywords: graphene, Schottky junction, Si, solar cell, stability, Ag nanowire Pacs: 72.40.+w, 78.67.Wj, 78.67.Uh, 73.50.Pz †

These authors have contributed equally to this study. Corresponding author: [email protected]

*

ACCEPTED MANUSCRIPT 1. Introduction Graphene has received tremendous attention due to its unique properties, such as extremely-high transmittance and excellent thermal/chemical/mechanical stabilities [1,2].

RI PT

This prompted many researchers to have a growing interest in combining graphene with silicon for developing photovoltaic devices [3]. In graphene/Si solar cells, graphene does not only allow light transmission into Si substrate but also form a Schottky-type junction at its

SC

interface with Si, useful for the separation of electron-hole pairs [4]. For the first time, Li, et al. successfully fabricated 1.5 % graphene/Si solar cells in 2010 [5]. The optoelectronic

M AN U

properties of graphene, including its transparence and sheet resistance, are very crucial for the performance improvement of graphene/Si solar cells [6]. Currently, high sheet resistance of graphene films cannot meet the requirement for high-efficiency solar cells. Thus, various methods have been proposed to improve the electrical conductivity of graphene films, such as

electric field doping [16].

TE D

chemical doping [7-10], formation of metal nanoparticles and nanowires (NWs) [11-15], and

Among the various candidate materials, silver (Ag) NWs are attractive as a dopant for

EP

graphene, especially in graphene/Si solar cells, because Ag NWs reduce the reflectance as well as enhance the absorption [13-15]. Light absorption by the Ag NWs randomly oriented

AC C

on graphene is particularly strong near 370 nm due to the plasmonic effect of the nanosized Ag material, resulting in broad spectral and angular responses, useful for photovoltaic applications. [17,18] However, Ag NWs are easily oxidized when they are exposed to air, thereby increasing the electrical resistivity and the junction resistance of the Ag NWs due to the formation of silver oxides on the surface of the Ag NWs [15]. Therefore, it is required to develop another approach to protect the Ag NWs on graphene for enhancing the long-term stabilities of Ag NWs/graphene/Si solar cells. Recently, Ag NWs-doped graphene transparent conductive electrodes (TCEs) protected by another graphene layer have exhibited highly-

ACCEPTED MANUSCRIPT enhanced electrical properties and long-term stabilities, resulting from the role of the top graphene encapsulation layer as an outstanding gas barrier [13,15]. In this work, we first employ the encapsulated graphene/Ag NWs/graphene TCEs for Schottky-type Si

RI PT

heterojunction solar cells and study their photovoltaic properties by varying the concentration (nA) of Ag NWs. The Ag NWs-doped graphene/Si solar cells show maximum PCE of 3.51% at nA = 0.1 wt%, where the optical transmittance and the sheet resistance of the encapsulated

SC

graphene TCE are 89.4% at 550 nm and 101±20 Ω/sq, respectively. The long-term stabilities of the solar cells are greatly improved by using the encapsulated graphene TCEs. We also

M AN U

systematically analyze the effect of nA on the transmittance/sheet resistance/work function of the graphene TCEs and the resulting solar-cell performances.

2. Experimental

TE D

Graphene, Ag NWs, and another graphene were sequentially stacked on Si (n-type, 15 Ω-cm) and quartz substrates. Monolayer graphene film was produced by 70-µm-thick Cufoils (Wacopa, 99.8 purity)-catalyzed chemical vapor deposition at 1000 °C under flow of

EP

CH4 (30 sccm) as the carbon source and H2 (10 sccm) as the reduction gas at a pressure of 10 Torr. After the growth of graphene on the Cu foil, a poly(methyl methacrylate) (PMMA)

AC C

solution was drop-coated onto the graphene sheet/Cu foil and then annealed at 180 oC for 1 min. The Cu foil was etched away using an FeCl3 etchant (Sigma-Aldrich) for 2 h to obtain a PMMA-coated graphene film. Then, the PMMA layer was removed in an acetone bath for 1 h to obtain a graphene sheet. Ag NWs powder was dissolved in isopropyl alcohol to prepare Ag NWs solution, whose nA was varied from 0.05 to 0.3 wt%. For the fabrication of the solar cells, the solution was coated on graphene/Si substrate at 1500 rpm for 1 min, and then was dried on a hot plate at 100 oC for 2 min. Finally, another graphene layer was transferred onto the Ag

ACCEPTED MANUSCRIPT NWs/graphene/Si to complete the encapsulated sandwich-type graphene/Ag NWs/graphene TCEs on Si wafers. Al and InGa films were deposited on the top of graphene and the bottom of Si substrate as the electrodes by using a shadow mask to open 14 mm2 windows as the

RI PT

illumination area of solar cells. The morphologies on the surface of the TCE structures were analyzed by field emission scanning electron microscopy (FE-SEM) (Carl Zeiss, model LEO SUPRA 55). The

SC

atomic bonding states of the Ag NWs/graphene were characterized by X-ray photoelectron spectroscopy (XPS) using Al ka line of 1486.6 eV. The reflectance/transmittance, sheet

M AN U

resistance, and work function of the TCEs were measured by ultraviolet-visible-near-infrared optical spectrometer (Varian, model cary 5000), 4 probe measurement (Dasol eng, model FPP-HS8-40K), and Kelvin-probe force microscopy (Park systems, model XE 100), respectively. To confirm the charge transfer between the graphene and dopants, the sample

TE D

was analyzed by Raman spectroscopy with a laser excitation energy of 532 nm (2.33 eV). Current density-voltage (J–V) characteristics of the Si solar cells were taken with a Keithley 2400 source meter under illumination of 1 Sun (100 mWcm-2 AM 1.5G) in air. The external

EP

quantum efficiency (EQE) was measured under short circuit conditions while the cells were

AC C

illuminated by a light source system with monochromator.

3. Results and discussion The FE-SEM images, as shown in Figure 1, show the effect of nA on the surface

morphological variations of the Ag NWs-doped graphene sheets. The networking of the Ag NWs are more dense and complicated on the graphene surface at larger of nA, as shown in Figures 1(b)-(e). Figure 2 shows cross-sectional-view FE-SEM images of the Ag NWs/graphene TCEs before and after the transfer of another graphene encapsulation layer.

ACCEPTED MANUSCRIPT The percolation network of the Ag NWs is fully embedded between top and bottom graphene layers. The formation of the Ag NWs on the graphene surface was further confirmed by XPS analysis. Figure 1(f) shows XPS spectra of C 1s, Ag 3d, and O 1s core levels for Ag NWs-

RI PT

doped graphene sheets at nA = 0.1 wt%, demonstrating successful doping of Ag NWs on the graphene surface.

Figure 3(a) shows Raman spectra of bare graphene, Ag NWs/graphene, and graphene/Ag NWs/graphene, where the G/2D peaks are observed at 1586/2681 cm-1, and

SC

1582/2679 cm-1, and 1578/2676 cm-1, respectively. As a result, the G/2D peaks of the Ag

M AN U

NWs/graphene and graphene/Ag NWs/graphene are red-shifted by 4/2 and 8/5 cm-1, respectively, compared to the bare graphene, which can be attributed to the change of the electronic configurations of graphene by Ag NWs or/and another graphene layer. It has been previously reported that the G and 2D bands of bilayer [19] or n-type doped graphene [20,21]

TE D

are red-shifted compared to monolayer graphene. Randomly-oriented regions of bilayer graphene are formed in the graphene/Ag NWs/graphene stack because hollow area can exist sporadically in the percolation network composed of Ag NWs, thereby allowing direct

EP

contact between top and bottom graphene layers. Therefore, the graphene/Ag NWs/graphene stack may possess electronic characteristics of bilayer graphene. The G/2D bands intensity

AC C

ratio increases from 0.47 to 0.79 in the order of bare graphene, Ag NWs/graphene, and graphene/Ag NWs/graphene, possibly because of the change in the electron bands caused by the interactions between Ag NWs and graphene or/and between graphene layers [22]. These results clearly confirm that the encapsulation graphene layer was successfully transferred onto the Ag NWs/graphene, leading to the formation of the sandwiched stack. The value of the doped graphene for a TCE can be estimated by calculating DC conductivity/optical conductivity ratio (σDC/σop) from the sheet resistance (Rs) and transmittance (T), based on the following equation [23]. T = {1 + (Zo/2Rs)(σop/σDC)}-2, where

ACCEPTED MANUSCRIPT Zo is the impedance of free space, 377 [23]. Figure 3(b) summarizes the shifts of the σDC/σop as functions of nA for the Ag NWs/graphene and graphene/Ag NWs/graphene TCEs. As estimated from Rs and T, as shown in Figure 4(a) and (b) (see the Supporting Information,

RI PT

Figure S1 for the Ag NWs/graphene), the σDC/σop of the both TCEs gradually increases to the maxima at nA = 0.1 wt%, and then decreases above nA = 0.1 wt%. The maximum σDC/σop of the graphene/Ag NWs/graphene is ~ 43, much larger than that of the Ag NWs/graphene as

SC

well as the minimum industry standard (σDC/σop = ~ 35) [23]. The trade-off correlation between Rs and T depending on nA results in the maximum σDC/σop of the both TCEs at nA =

for its use as a TCE for solar cells.

M AN U

0.1 wt%. These results suggest that the graphene/Ag NWs/graphene stack is very promising

With increasing nA, the reflectance is reduced in the full range of wavelength, as shown in Figure 4(c). Especially, the noticeable enhancement of absorption near 370 nm is known to

TE D

be due to the surface plasmonic absorption of the nano-sized Ag material [15]. As shown in Figure 4(d), the work function of the graphene/Ag NWs/graphene stack monotonically decreases from ~4.64 to ~ 4.49 eV with increasing nA from 0 to 0.3 wt% (see the Supporting

EP

Information, Table S1). Since the work function of Ag (4.21 eV) is smaller than that of bilayer graphene (4.64 eV), the electron transfer from the Ag NWs to the graphene sheets

AC C

occurs during the doping process, thereby inducing the n-type doping. Figure 5(a) shows typical J-V curves under dark and illumination for nA = 0 and 0.1

wt%. (see the Supporting Information, Figure S2 for the J-V curves at other nA) Table 1 summarizes resulting photovoltaic parameters as functions of nA. The bilayer graphene/n-Si solar cell exhibits the lowest PCE (2.12 %) due to its relatively-high sheet resistance (~660 ohm/sq). The PCE increases by locating the Ag NWs between the graphene layers due to the increase in the conductivity of the TCE. The best solar cell device shows 0.307 V opencircuit voltage (Voc), 23.11 mA·cm-2 short-circuit current density (Jsc), 49.34 % fill factor

ACCEPTED MANUSCRIPT (FF), and 3.51 % PCE at nA = 0.1 wt%. However, the PCE is slightly reduced at nA ≥ 0.2 wt% because the degradation of the transmittance is larger than the improvement of the sheet resistance for nA ≥ 0.2 wt%. The nA-dependent behaviors of the PCE are consistent with those

RI PT

of the σDC/σop, as shown in Fig. 3(b). The nA-dependent statistical deviations of the average PCE for 20 solar cell devices also show that the highest PCE of 3.24 ± 0.27 % are achieved at nA = 0.1 wt% (see also the Supporting Information, Figure S3).

SC

In Table 1, Voc monotonically decreases with increasing nA, which can be attributed to the decreased work function of graphene, as shown in Fig. 4(d). Given the relationship for

M AN U

barrier height: ΦB = WG - χSi, where WG is graphene work function and χSi is electron affinity of Si. The doping-induced decrease of the graphene work function reduces ΦB, resulting in smaller Voc in the solar cell [7,8]. The change of WG by 0.15 V with increasing nA from 0 to 0.3 wt%, as shown above, is consistent with that of Voc by 0.17 V. Due to the decrease of

TE D

WG, the band bending at the interface is reduced (see the Supporting Information, Fig. S4), resulting in the decrease of the carriers collected on the graphene/Si sides. In contrast, the doping increases the conductance as well as decreases the reflectance. Due to this trade-off,

EP

the PCE is maximized at nA = 0.1 wt%, consistent with the above arguments. Figure 5(b) shows similar nA dependences of the PCE irrespective of the encapsulation (see also the

AC C

Supporting Information, Figure S5 for the J-V curves of the solar cells without the encapsulation).

Figure 5(c) shows EQE spectra for various nA. The EQE is greatly enhanced at nA =

0.05 wt% over a broad wavelength ranging from 300 to 1100 nm, but nA > 0.05 wt%, it gradually decreases with increasing nA, possibly due to the decrease of the transmittance. The optical transmittance of the graphene significantly affect the EQE of graphene/Si solar cells because the light should passes through the graphene electrode to reach the Si active layer.

ACCEPTED MANUSCRIPT These behaviors are consistent with the Jsc, as shown in Table 1, resulting from the strong correlation of Jsc with the integration value of EQE [24]. From the dark J-V curves, as shown in the Figure 5(d), the series/shunt resistance

RI PT

values were obtained and summarized in Table 1. The doping-induced decrease of the graphene work function reduces ΦB, and thus the recombination of carriers at the interface increases, leading to the doping-dependent decrease in the shunt resistance, which seems to

SC

limit the enhancement of the FF. In contrast, the reduction of the series resistance by doping leads to the enhancement of the photovoltaic parameters, consistent with the previous reports

M AN U

on the doping [7,8]. Due to the doping-dependent trade-off of the shunt/series resistances, the FF seems to be maximized at 0.2 wt%, as shown in Table 1.

We also studied the long-term stabilities of the solar cells with Ag NWs/graphene and graphene/Ag NWs/graphene TCEs for nA = 0.1 wt% by measuring the PCEs for 30 days, as

TE D

shown in Figure 6. The environmental temperature and humidity for the measurements were typically 25 °C and 50%, respectively. It was found that the encapsulated Ag NWs/graphene/Si solar cell lost only 6.4 % of its original PCE value (absolutely from 3.51 to

EP

3.28 %) after 30 days, much smaller compared to the solar cell with Ag NWs/graphene TCE (degradation ratio: 25.8 %, absolutely from 3.31 to 2.45 %). These results suggest that the

AC C

encapsulation graphene layer well prevents the Ag NWs from being oxidized in air, thereby enhancing the long-term stabilities of the solar cells.

4.

Conclusion

We successfully fabricated Ag NWs-doped graphene/Si solar cells with/without another graphene encapsulation layer. Due to n-type doping of graphene by Ag NWs, the graphene/Ag NWs/graphene TCEs showed sheet resistance/series resistance of ~ 101 ohm-sq1

/3.53 Ω-cm2 at nA = 0.1 wt%, much smaller than those of the TCEs without Ag NWs. The

ACCEPTED MANUSCRIPT sheet resistance and the transmittance of the Ag NWs-doped graphene TCEs with/without the encapsulation layer monotonically decreased with increasing nA, thereby maximizing the σDC/σop at nA = 0.1 wt%. The reflectance of the solar cells also monotonically decreased with

RI PT

increasing nA. As a result, the PCE of the graphene/Ag NWs/graphene/Si solar cells exhibited highest PCE of 3.51 % at nA = 0.1 wt%. The Ag NWs/graphene/Si solar cells lost only 6.4 % of its original PCE value after 30 days by employing the encapsulation layer, much less

SC

compared to the solar cells without the encapsulation.

M AN U

Acknowledgments

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future

AC C

EP

TE D

Planning (NRF-2017R1A2B3006054).

ACCEPTED MANUSCRIPT References [1] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Roll-to-roll

RI PT

procution of 30-inch graphene films for transparent electrodes, Nat. nanotechnol. 5 (2010) 574-578.

[2] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, R. S.

electrodes, Nano lett. 9 (2009) 4359-4363.

SC

Ruoff, Transfer of large-area graphene films for high-performance transparent conductive

M AN U

[3] Y. Ye, L. Dai, Grpahene-based Schottky junction solar cells, J. Mater. Chem. 22 (2012) 24224-24229.

[4] Y. Lin, X. Li, D. Xie, T. Feng, Y. Chen, R. Song, H. Tian, T. Ren, M. Zhong, K. Wang, H. Zhu, Graphene/semiconductor heterojunction solar cells with modulated antireflection and

TE D

graphene work function, Energy Environ. Sci. 6 (2013) 108-115.

[5] X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Jia, Z. Li, X. Li, D. Wu, Graphene-onsilicon Schottky junction solar cells Adv. Mater. 22 (2010) 2743-2748.

EP

[6] X. An, F. Liu, S. Kar, Optimizing performance parameters of graphene-silicon and thin transparent graphite-silicon heterojunction solar cells, Carbon 57 (2013) 329-337.

AC C

[7] X. Miao, S. Tongay, M. K. Petterson, K. Berke, A. G. Rinzler, B. R. Appleton, A. F. Hebard, High Efficiency Graphene Solar Cells by Chemical Doping, Nano lett. 12 (2012) 2745-2750.

[8] E. Shi, H. Li, L. Yang, L. Zhang, Z. Li, P. Li, Y. Shang, S. Wu, X. Li, J. Wei, K. Wang, H. Zhu, D. Wu, Y. Fang, A Cao, Colloidal antireflection coating improves graphene-silicon solar cells, Nano lett. 13 (2013) 1776-1781. [9] T. Cui, R. Lv, Z.-H. Huang, S. Chen, Z. Zhang, X. Gan, Y. Jia, X. Li, K. Wang, D. Wu, F. Kang, Enhanced efficiency of graphene/silicon heterojunction solar cells by molecular

ACCEPTED MANUSCRIPT doping, J. Mater. Chem. A 1 (2013) 2736-2740. [10] C. Xie, X. Zhang, K. Ruan, Z. Shao, S. S. Dhaliwal, L. Wang, Q. Zhang, X. Zhang, J. Jie, High-efficiency, air stable graphene/Si micro-hole array Schottky junction solar cells, J.

RI PT

Mater. Chem. A 1 (2013) 15348-15354. [11] J. Wei, Z. Zang, Y. Zhang, M. Wang, J. Du, X. Tang, Enhanced performance of lightcontrolled conductive switching in hybrid cuprous oxide/reduced graphene oxide (Cu2O/rGO)

SC

nanocomposites, Opt. Lett. 42 (2017) 911-914.

[12] X. Liu, X. W. Zhang, Z. G. Yin, J. H. Meng, H. L. Gao, L. Q. Zhang, Y. J. Zhao, H. L.

M AN U

Wang, Enhanced efficiency of graphene-silicon Schottky junction solar cells by doping with Au nanoparticles, Appl. Phys. Lett. 105 (2014) 183901

[13] D. Lee, H. Lee, Y. Ahn, Y. Lee, High-performance flexible transparent conductive film based on graphene/AgNW/graphene sandwich structure, Carbon 81 (2015) 439-446.

TE D

[14] D. Lee, H. Lee, Y. Ahn, Y. Jeong, D.-Y. Lee, Y. Lee, Highly stable and flexible silver nanowire-graphene hybrid transparent conducting electrodes for emerging optoelectronic devices, Nanoscale 5 (2013) 7750-7755.

EP

[15] J. Lee, A. Lee, H. K. Yu, Graphene protected Ag nanowires: blocking of surface migration for thermally stable and wide-range-wavelength transparent flexible electrodes,

AC C

RSC Adv. 6 (2016) 84985-84989.

[16] X. Yu, L. Yang, Q. Lv, M. Xu, H. Chen, D. Yang, The enhanced efficiency of graphenesilicon solar cells by electric field doping, Nanoscale 7 (2015) 7072-7077. [17] M. Burresi, F. Pratesi, K. Vynck, M. Prasciolu, M. Tormen, and D. S. Wiersma, Twodimensional disorder for broadband, omnidirectional and polarization-insensitive absorption, Opt. Express 21 (2013) 268-275 [18] F. Pratesi, M. Burresi, F. Riboli, K. Vynck, D. S. Wiersma, Disordered photonic structures for light harvesting in solar cells, Opt. Express 21 (2013) 460-468

ACCEPTED MANUSCRIPT [19] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P. C. Eklund, Raman scattering from highfrequency phonons in supported n-graphene layer films, Nano lett. 6 (2006) 2667-2673. [20] D. H. Shin, K. W. Lee, J. S. Lee, J. H. Kim, S. Kim, S.-H. Choi, Enhancement of the

RI PT

effectiveness of graphene as a transparent conductive electrode by AgNO3 doping, Nanotechnology 25 (2014) 125701.

[21] H.-J. Shin, W. M. Choi, D. Choi, G. H. Han, S.-M. Yoon, H.-K. Park, S.-W. Kim, Y. W.

SC

Jin, S. Y. Lee, J. M. Kim, J.-Y. Choi, Y. H. Lee, Control of electronic structure of graphene by various dopants and their effects on a nanogenerator, J. Am. Chem. Soc. 132 (2010) 15603-

M AN U

15609.

[22] B.-T. Liu, H.-L. Kuo, Graphene/silver nanowire sandwich structures for transparent conductive films, Carbon 63 (2013) 390-396.

[23] S. De, J. N. Coleman, Are there fundamental limitations on the sheet resistance and

TE D

transmittance of thin graphene films?, ACS Nano 4 (2010) 2713-2720. [24] J. D. Servaites, M. A. Ratner, T. J. Marks, Practical efficiency limits in organic photovoltaic cells: Functional dependence of fill factor and external quantum efficiency, Appl.

AC C

EP

Phys. Lett. 95 (2009) 163302.

ACCEPTED MANUSCRIPT Table 1. Photovoltaic parameters and series resistance of graphene/Ag NWs/graphene/Si solar cells for various nA. Voc Jsc FF PCE Series resistance Shunt resistance nA (wt%) (V) (mA/cm2) (%) (%) (Ω-cm2) (Ω-cm2) 0.322

16.49

40.02

2.12

6.76

2437

0.05

0.316

24.04

43.80

3.32

4.48

2182

0.1

0.307

23.11

49.34

3.51

3.53

1947

0.2

0.306

22.19

49.75

3.37

3.49

1799

0.3

0.305

19.18

48.92

2.86

3.51

1692

AC C

EP

TE D

M AN U

SC

RI PT

0

ACCEPTED MANUSCRIPT Figure captions

Figure 1. FE-SEM images of (a) pristine graphene and (b)-(e) Ag NWs-doped graphene for various nA. (f) XPS spectra of Ag NWs-doped graphene for nA = 0 and 0.1 wt%. The scale

RI PT

bars all indicate 1 µm. Figure 2. Cross-sectional-view FE-SEM images of the Ag NWs/graphene TCEs (a) before and (b) after the transfer of another graphene encapsulation layer. The scale bars all indicate

SC

100 nm.

Figure 3. (a) Raman spectra of bare graphene, Ag NWs/graphene, and graphene/Ag

M AN U

NWs/graphene at a fixed nA of 0.1 wt%. (b) σDC/σop as functions of nA for the Ag NWs/graphene and graphene/Ag NWs/graphene TCEs, respectively.

Figure 4. (a) and (b) Transmittance spectra/sheet resistance of graphene/Ag NWs/graphene TCEs, respectively for various doping concentrations of Ag NWs. (c) Reflectance spectra of graphene/Ag NWs/graphene TCEs for various nA. (d) Work function of graphene/Ag

TE D

NWs/graphene TCEs as a function of nA.

Figure 5. (a) Typical J-V curves of graphene/Ag NWs/graphene/Si solar cells under dark and illumination for nA = 0 and 0.1 wt%. (b) PCEs of Ag NWs/graphene and graphene/Ag

EP

NWs/graphene solar cells as functions of nA. (c) EQE spectra of graphene/Ag NWs/graphene/Si solar cells for various nA. (d) dV/d(ln J)-vs-J plots of graphene/Ag

AC C

NWs/graphene/Si solar cells for various nA. Figure 6. Degradation of PCEs during 30 days under 25 °C and 50% humidity for Ag NWs/graphene and graphene/Ag NWs/graphene solar cells at a fixed nA of 0.1 wt%.

SC

RI PT

ACCEPTED MANUSCRIPT

TE D

M AN U

Figure 1

EP

Figure 2

(b)

Graphene Ag NWs-graphene Graphene-Ag NWs-graphene

1200

Figure 3

σDC/ σ op

I(G/2D) 0.47

40 30 20

0.52

0.79

1600 2000 2400 -1 Raman shift (cm )

Ag NWs/graphene Graphene/Ag NWs/graphene

50

AC C

Intensity (arb.units)

(a)

10

2800

0

0.05

0.1 0.2 nA (wt%)

0.3

AC C

EP

Figure 4

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT (b)

15 0

2 1

-15 -30

3

0.0

0.1 0.2 0.3 Voltage (V)

0

0.4

Ag NWs/graphene Graphene/Ag NWs/graphene

0

0.05

(d)

60

400

0.3

0.0 0.00

1000

TE D

Graphene/Ag NWs/Graphene

1.00 0.95

EP

0.90 0.85 0.80

Ag NWs/Graphene

AC C

0.75 0.70

0

SC

0.2 0.1

Figure 5

Normalized PCE (%)

0.3

M AN U

20

nA (wt%) 0 0.05 0.1 0.2 0.3 600 800 Wavelength (nm)

d(V)/d(lnJ)

EQE (%)

40

Figure 6

0.1 0.2 nA (wt%)

0.5 Series resistance (Ω-cm2) 0.4

0

RI PT

(c)

4

0 wt% Dark 0 wt% Light 0.1 wt% Dark 0.1 wt% Light

30

PCE (%)

2

Current density (mA/cm )

(a)

6 12 18 24 Storing time (day)

30

0.03

nA (wt%) 0 0.05 0.1 0.2 0.3

0.06 0.09 2 J (A/cm )

0.12

ACCEPTED MANUSCRIPT

Supporting Information for

RI PT

Ag-nanowires-doped graphene/Si Schottky-junction solar cells encapsulated with another graphene layer

SC

Jong Min Kim†, Sang Woo Seo†, Dong Hee Shin†, Ha Seung Lee, Ju Hwan Kim, Chan Wook Jang, Sung Kim, and Suk-Ho Choi*

AC C

EP

TE D

M AN U

Department of Applied Physics, College of Applied Science, Kyung Hee University, Yongin 17104, Korea

†

These authors contributed equally to this work. To whom correspondence should be addressed. E-mail: [email protected]

*

ACCEPTED MANUSCRIPT Table S1. Work function of single and double graphene monolayers as functions of doping concentration. 0.05

0.1

0.2

0.3

Ag NWs/G

4.69 ± 0.033

4.61 ± 0.027

4.52 ± 0.021

4.49 ± 0.025

4.46 ± 0.027

G/Ag NWs/G

4.64 ± 0.030

4.59 ± 0.022

4.56 ± 0.020

4.51± 0.023

4.49 ± 0.025

AC C

EP

TE D

M AN U

SC

RI PT

0

ACCEPTED MANUSCRIPT

SC

RI PT

Figures for Supporting Information

AC C

EP

TE D

M AN U

Figure S1. (a) and (b) Transmittance spectra/sheet resistance of Ag NWs/graphene TCEs, respectively, for various doping concentrations of Ag NWs.

ACCEPTED MANUSCRIPT

30 0.05 wt% Dark Light

15

-15 -30

0.0

0.1 0.2 0.3 Voltage (V)

Dark Light

15

-15 -30

0.0

0.1 0.2 0.3 Voltage (V)

2

30 0.3 wt% Dark Light

15 0 -15 -30

0.0

M AN U

0

(c) Current density (mA/cm )

SC

30 0.2 wt%

0.4

TE D

2

Current density (mA/cm )

(b)

0.4

RI PT

0

EP

2

Current density (mA/cm )

(a)

0.1 0.2 0.3 Voltage (V)

0.4

AC C

Figure S2. J-V curves of graphene/Ag NWs/graphene Si solar cells under dark and illumination for nA = 0.05, 0.2, and 0.3 wt%.

4 3 2 1 0

0

0.05

0.1 0.2 nA(wt%)

RI PT

Average efficiency (%)

ACCEPTED MANUSCRIPT

0.3

TE D

M AN U

SC

Figure S3. Average PCEs of 20 graphene/Ag NWs/graphene/Si solar cells as a function of nA.

EP

16 Ag NWs/graphene 0 wt% 8 0.05 wt% 0.1 wt% 0 0.2 wt% 0.3 wt% -8

AC C

2

Current density (mA/cm )

Figure S4. Band diagrams of graphene/Si solar cell before and after doping of graphene.

-16 -24

0.0

0.1 0.2 Voltage (V)

0.3

0.4

Figure S5. J-V curves of Ag NWs/graphene/Si solar cells under illumination for various nA

ACCEPTED MANUSCRIPT Highlights •

We first employ encapsulated graphene/Ag nanowires/graphene transparent conductive electrodes for Si solar cells. Maximum power-conversion efficiency of 3.51 % is obtained at doping concentration of

RI PT

•

Ag nanowires = 0.1 wt%. •

Long-term stabilities of the photovoltaic properties are greatly improved by the use of the

The effect of the doping concentration on the electrical/optical properties of the graphene

EP

TE D

M AN U

electrodes is systematically correlated with the resulting solar-cell performances.

AC C

•

SC

graphene electrodes.