Implementation of graphene as hole transport electrode in flexible CIGS solar cells fabricated on Cu foil

Solar Energy 162 (2018) 357–363

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Implementation of graphene as hole transport electrode in flexible CIGS solar cells fabricated on Cu foil Jae-Kwan Sima,1, San Kanga,1, R. Nandia, Jun-Yong Joa, Kwang-Un Jeongb, Cheul-Ro Leea,

T

⁎

a

Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Baekje-daero 567, Jeonju 54896, Republic of Korea b Department of Polymer-Nano Science and Technology, Baekje-daero 567, Jeonju 54896, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene Flexible CIGS solar cell Current-voltage Efficiency

Graphene has great potential to be used as electrode in many opto-electronic devices, owing to its superior optical and electrical properties. Graphene films have been employed as window electrode in various thin film solar cells. However, to date graphene film has not been used as hole transport electrode, particularly in CIGS solar cell. In this work, we have demonstrated a novel structure for graphene-based flexible CIGS solar cell, in which graphene film on flexible Cu foil was implemented as hole transport electrode. CIGS solar cells were directly fabricated on the chemical vapor deposited graphene film on Cu foil, without any transfer process. Several techniques, including Raman spectroscopy, X-ray diffraction, scanning electron microscopy, external quantum efficiency and J-V characteristics under illumination, have been used to investigate the device performance. The graphene-based device displayed power conversion efficiency of 9.91 ± 0.89% with a fill factor of 64.75 ± 7.34%, which are substantially higher compared to reference cell fabricated using conventional Mo/ stainless steel electrode. High open circuit voltage together with substantially large fill factor is primarily responsible for high cell efficiency of graphene/Cu foil based device. This study provides a plausible implication of graphene as hole transport electrode in flexible CIGS photovoltaic devices.

1. Introduction Given the worldwide ever growing energy demand, the utilization of photovoltaic devices are increasing rapidly and expected to play an important role in electrical power generation in near future. Cu(In, Ga) Se2 (CIGS) thin-film solar cell is currently the fastest growing photovoltaic technology and considered as most promising candidate for the alternative of well-established crystalline silicon solar cells. It has been demonstrated the highest recorded CIGS cell efficiency is 22.6% in laboratory scale, which is nearly close to the highest efficiency of crystalline silicon solar cells (Jackson et al., 2016). Recently, flexible solar cells have attracted extensive research attention in the fields of space, wearable or portable electronics, sun-powered vehicles, unmanned airplanes, biomedical and mobile applications due to their endurance from various deformations, such as rolling, bending, stretching and folding (Brémaud et al., 2007; Liu et al., 2016; Park et al., 2014; Reinhard et al., 2013). In the last few years, several groups reported the development of CIGS solar cell on flexible substrates, particularly, on polyimide, and metal foils such as stainless steel (STS) and titanium (Hashimoto et al., 2003; Ishizuka et al., 2008; Yagioka and Nakada, ⁎

1

2009; Zhang et al., 2013). Graphene has been proposed as a promising replacement for conventional transparent conducting electrodes in flexible electronic devices due to its superior electrical and optical properties (Gomez De Arco et al., 2010; Zhang et al., 2017). More importantly, graphene film exhibits excellent mechanical flexibility and bending durability, and is thus a suitable candidate for flexible electrodes in flexible electronic devices. Graphene transparent conducting films have been employed as window electrodes for various thin film solar cells in several reports (Bi et al., 2011; Electrodes et al., 2015; Miao et al., 2012; Palma et al., 2016; Shi et al., 2013; Yin et al., 2014a). Graphene films used in these studies were commonly grown on copper (Cu) foils by chemical vapor deposition (CVD) and then transfer onto desired substrates/photovoltaic devices. These transfer processes are very sophisticated and source of several defects and formation recombination centers at graphene-semiconductor interface. Furthermore, in the recent years graphene and its derivatives (graphene oxide and reduced graphene oxides) have been employed as hole transport layers in high performance dye and quantum dot sensitize solar cells, organic solar cells and perovskite solar cells (Bae et al., 2014; Iwan et al., 2017; Jeon et al.,

Corresponding author. E-mail address: [email protected] (C.-R. Lee). Equal Contributor.

https://doi.org/10.1016/j.solener.2018.01.053 Received 11 October 2017; Received in revised form 8 January 2018; Accepted 17 January 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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(NH4OH), 0.015 M cadmium sulfate (CdSO4) and 1.5 M N-methylthiourea (NH2CSNH3) at 65 °C for 13 min. A ∼300 nm thick transparent conducting electrode (indium tin oxide film) was then deposited on CdS buffer layer by rf magnetron sputtering of indium tin oxide target at room temperature in argon atmosphere. Finally, Ag metal contacts were deposited by magnetron sputtering to complete the fabrication of CIGS solar cells. For the comparison reference CIGS solar cells also have been fabricated under identical growth conditions, but on conventional Mo/ stainless steel (Mo/STS) substrate (Kang et al., 2017; Sim et al., 2016). The area of all the fabricated solar cells is similar and the estimated area is 0.36 cm2. The morphology of the CIGS film was analyzed by using fieldemission scanning electron microscopy (FE-SEM, Hitachi S-7400, Hitachi, Japan) with an operating voltage of 15 kV. Crystal structure and orientation were studied by X-ray diffraction (XRD) measurements using a Rigaku diffractometer equipped with a Cu-Kα radiation source. To study the quality of graphene/Cu foil, Raman spectroscopy was performed using a WiTec Alpha 300R M-Raman system; the excitation wavelength was 532 nm. The morphology of the CIGS/graphene/Cu foil structures was analyzed by cross-sectional HR-TEM (A JEOL JEM 2010) system operated at 200 kV. CIGS solar cells were characterized by current-voltage measurements in dark and under illumination of solar simulator of AM 1.5 (100 mW/cm2) at room temperature. External quantum efficiency (EQE) measurements were performed using EQE system equipped with Xenon lamp, monochromator and Si detector.

2012; Li et al., 2016; Ubani et al., 2016; Xu et al., 2017; Yun et al., 2011). It has also been shown that the graphene-based materials significantly improved the performance and stability of the devices (Agresti et al., 2016; Biccari et al., 2017; Ma et al., 2018; Yin et al., 2014b). However, application of graphene-based transparent conducting electrode in CIGS solar cells is severely limited. Recently, CVD grown graphene used as window electrode for the replacement of Aldoped ZnO films in CIGS solar cell (Yin et al., 2014a). To the best of our knowledge there has been no report on the use of graphene as hole transport electrode in CIGS solar cell. Considering the advantages of graphene, in the present work we have demonstrated a novel structure for graphene-based flexible CIGS solar cell, in which graphene film on flexible Cu foil was used as hole transport electrode. CIGS solar cells were directly fabricated on graphene film synthesized by CVD on flexible Cu foil. The graphene-based device displayed power conversion efficiency of 9.91 ± 0.89% with a fill factor of 64.75 ± 7.34%, which are substantially higher than that of reference cell, fabricated using conventional Mo/stainless steel electrode. 2. Experimental section The entire deposition and fabrication processes of CIGS solar cell on graphene/Cu foil is schematically represented in Fig. 1. A ∼25 μm thick Cu foil (99.8%, Alfa Aesar) was used as substrate for the deposition of graphene films. The Cu foil surface was electrochemically polished in H3PO4 (85%) solution and the root-mean-square surface roughness is less than 15 nm. After polishing, the Cu foil was loaded into a lowpressure chemical vapor deposition (LP-CVD) system and heated at a process temperature of 1050 °C for 60 min under H2 flow at 5 sccm for in situ surface cleaning. The monolayer graphene film was then deposited under a mixture of CH4/H2 (10 and 5 sccm, respectively) with a reactor pressure of ∼90 mTorr for growth duration of 15 min. After graphene growth, the chamber was rapidly cooled down to room temperature in H2 atmosphere. The details of the growth and characterization processes of graphene on Cu foil have been reported earlier (Kang et al., 2015). Subsequently, CIGS absorber layer was deposited on graphene film/Cu foil following the steps as described in reference (Sim et al., 2016). A copper indium gallium (CIG) layer was deposited on the graphene/Cu foil by co-sputtering of Cu0.8Ga0.2 and In (indium) targets using direct current (dc) magnetron system. The sputtering was carried out at room temperature in argon ambient under dc power of 150 W. Next, a thermal evaporation system equipped with effusion cell was used to complete the selenization process of the sputtered CIG layer. The selenium powder was thermally evaporated at 230 °C and the ptype CIGS chalcopyrite structure was obtained from the selenization of CIG layer at substrate temperature of 500 °C for one hour. About 65 nm thick n-type CdS buffer layer was chemically deposited on top of the CIGS absorption layer by the reaction of 28–30% ammonium hydroxide

3. Results and discussion The graphene growth process and corresponding temperature profiles are schematically shown in Fig. 2(a). Fig. 2(b) displays a representative Raman spectrum of graphene film synthesized on Cu foil at temperature of approximately 1050 °C. The recorded Raman spectra exhibit three characteristic peaks of graphene monolayer. The D, G and 2D bands of the graphene are located at 1355 cm−1, 1584 cm−1, and 2673 cm−1, respectively. The G/2D band and full-width at half-maximum (FWHM) of the 2D-band maps in Fig. 2(c) illustrate the uniformity of the graphene films over large areas (∼490 μm2). Graphene sheets deposited on Cu foil primarily exhibit typical monolayer characteristics, as indicated by the IG/I2D ≤ 0.38 and the FWHM of the 2D band of ≤38 cm−1 (Tseng et al., 2016; Wu et al., 2011; Yan et al., 2012). These features in the Raman spectrum suggest the successful growth of uniformly distributed and high quality graphene monolayers on Cu foil by CVD process. However, the presence of a weak D-band in Raman spectrum suggests the presence of some defects such as vacancies, surface dislocations and subdomain boundaries (Banhart et al., 2011; Vicarelli et al., 2015). The electrical properties of the monolayer graphene sheets on Cu foil were analyzed by the circular transfer length method (CTLM) (Kang

Fig. 1. Schematic representation of the deposition and fabrication processes of flexible CIGS solar cells on graphene/Cu foil electrode.

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Fig. 2. (a) Schematic diagram of graphene synthesis process and time dependence of experimental parameters: temperature and gas composition/flow rate. (b) Representative Raman spectrum of the monolayer graphene synthesized at 1050 °C on Cu foil. (c) Raman map of the G/2D-bands (left) and the FWHM of the 2D-band (right) of the monolayer graphene. The scale bars are all 15 μm. (d) Total resistance of graphene as a function of channel spacing (S = 4–1300 μm) before and after applying the correction factors. The error bars represent the standard deviations of the resistance measured for 5 different devices.

et al., 2015). Fig. 2(d) shows the measured resistances (blue2 circles) determined by the CTLM as a function of channel spacing (S) varied from 4 to 1300 μm. The contact parameters were determined by fitting the measured resistances to the following equation (Jacobs et al., 2002; Kang et al., 2015).

R S 1 1 ⎞⎫ RT = ⎛ s ⎞ ⎧ln ⎛1 + ⎞ + LT ⎛ + + r r ( r S) ⎠ ⎬ ⎝ 2π ⎠ ⎨ 1 1 1 ⎠ ⎝ ⎩ ⎝ ⎭ ⎜

⎟

⎜

compositions of Cu, In, Ga and Se. The ratios of Cu/(In+Ga) and Ga/ (Ga+In) in the film are estimated as ∼0.9 and ∼0.2, respectively. These values are quite similar to those reported for conventional CIGS solar cell (Arnou et al., 2017; Ramanujam and Singh, 2017). The XRD pattern of the CIGS absorber layer deposited on graphene/ Cu foil is shown in Fig. 4(a). For the comparison the XRD pattern of the CIGS absorber deposited on Mo thin film on the STS substrate is shown Fig. 4(b). The diffraction peaks at 2θ values of 26.8°, 44.6° and 53° are related to the reflections from (1 1 2), (220/204) and (116/312) planes of CIGS absorber (JCPDS-#35-1102). In addition, diffraction peaks from the metal surfaces such as, Cu, Mo or STS are also seen in the respective samples, as indicated in the figure. The presence of strong and sharp (1 1 2) reflection further suggests the CIGS absorber layers are preferentially (1 1 2) oriented. This result indicates the successful formation of polycrystalline CIGS films without any undesired binary or ternary phases. The morphology and elemental compositions of the CIGS absorber deposited on graphene/Cu foil were further analyzed by cross-sectional transmission electron microscopy (XTEM) and EDS mapping. The high (Fig. 5(a)) and low magnification (Fig. 5(b)) XTEM images reveal the presence of graphene monolayer sheets which were deposited on Cu foil. It is also noticed that the CIGS absorber layer is uniformly deposited on graphene/Cu foil. The EDS elemental mapping of Cu, In, Ga and Se (Fig. 5(c–f)) showed that all the desired elements are uniformly distributed over the entire CIGS absorber. CIGS solar cells have been fabricated in order to investigate the performance of graphene/Cu foil as hole transport electrode in flexible

⎟

(1)

where RT is the measured total resistance between the channel spacing of the contact pads, Rs is the sheet resistance of the graphene films, r1 is the radius of the inner circular contact and LT is the transfer length. From the linear fit (red line), the estimated sheet resistance and transfer length of the graphene films are Rs ≈ 490 O/sq and LT ≈ 13.97 μm, respectively. These results implied that the graphene sheets synthesized on Cu foil are of high crystalline quality monolayers. Fig. 3(a,c) and (b,d) show the cross-sectional and top-down FE-SEM images of CIGS absorber layers deposited on graphene/Cu foil and Mo/ STS, respectively. The CIGS absorber layers are compact, densely packed morphology and uniformly deposited on both graphene/Cu foil and Mo/STS. The elemental compositions of CIGS absorber layer are obtained from the energy dispersive X-ray spectroscopy (EDS) and the results are presented in Table 1. CIGS absorber layers deposited on both the graphene/Cu foil and Mo/STS substrates exhibit nearly similar 2 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.

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Fig. 3. Cross-sectional and top-down FE-SEM images of CIGS absorber layers deposited on graphene/Cu foil (a,c) and Mo/STS (b,d).

condition. Under illumination electron-hole pairs are generated in the p-type CIGS absorber layer and the electrons move in direction of the pn-junction and holes move to the back graphene contact (Azimi et al., 2014). Fig. 6(b) illustrates the current density vs bias voltage (J – V) characteristic curves of the CIGS solar cells fabricated with graphene/ Cu foil and Mo/STS electrodes. The photovoltaic device parameters of both the CIGS solar cells are summarized in Table 2. It is clear that the CIGS solar cells fabricated with graphene/Cu foil electrode display superior photovoltaic performance and exhibits high power conversion efficiency (PCE) of 9.91%. From Table 2, it is realized that the cell efficiency is primarily dependent on the changes of open circuit voltage (Voc) and fill factor (FF). To analyze the superior photovoltaic performance of graphene/Cu foil based solar cells, the diode parameters such as reverse saturation current density (Jo), diode ideality factor (A), shunt resistance (Rsh) and series resistance (Rs) for both the CIGS cells are obtained from the corresponding dark J – V characteristics and the values are also summarized in Table 2. The relation between Voc and Jo can be expressed as following (Zhang et al., 2013).

Table 1 Elemental composition of CIGS absorber layers deposited on graphene/Cu foil and Mo/ STS.

CIGS/graphene/Cu foil CIGS/Mo/STS

Cu (%)

In (%)

Ga (%)

Se (%)

Cu/(In +Ga)

Ga/(Ga +In)

24.1

20.37

5.33

50.2

0.94

0.21

23.3

20.06

5.74

50.9

0.90

0.22

Voc =

AkT ⎛ Jsc ln + 1⎞ q ⎝ Jo ⎠ ⎜

⎟

(2)

where k is the Boltzmann constant, q is the electron charge and Jsc is the short circuit current density. Obviously, Voc increases with decrease in Jo, and should be as low as possible. It is noticed that the reverse saturation current of graphene/Cu foil based solar cell is substantially lower, along with large Rsh, smaller Rs and better diode ideality factor compare to those of Mo/STS based solar cell. Thus the high Voc obtained from graphene/Cu foil based solar cell is originated from the significantly lower Jo. In addition, large Rsh and smaller Rs resulted in considerably high fill factor (64.75%). Therefore, the high open circuit voltage together with substantially large fill factor is primarily responsible for high PCE of graphene/Cu foil based device. However, a marginally increased Jsc is observed in this case, which may be attributed to the better collection of carriers at the graphene/Cu electrode. Fig. 7 displays the external quantum efficiency (EQE) curves for both the solar cells. The EQE remained similar for the both devices in the wavelength range of 300–520 nm. In contrast, a marginal (5%)

Fig. 4. XRD patterns of the CIGS absorber layers deposited on (a) graphene/Cu foil and (b) Mo/STS.

photovoltaic devices. A typical device structure of the fabricated CIGS solar cells is schematically shown in Fig. 6(a). As a reference, we have fabricated CIGS solar cells with Mo/STS electrode under identical experimental conditions. Fig. 6(a) displayed a schematic band diagram of CIGS solar cells fabricated on graphene/Cu foil under zero-bias voltage 360

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Fig. 5. (a) High magnification and (b) low magnification XTEM images of CIGS absorber layer deposited on graphene/Cu foil. The corresponding EDS elemental mapping of (c) Cu, (d) In, (e) Ga and (f) Se.

Fig. 6. (a) Schematic design of the CIGS solar cells fabricated with graphene/Cu foil; Schematic band diagram of CIGS/graphene solar cells under zero-bias voltage condition displaying the generation and collection of charge carriers and (b) J - V characteristics of the CIGS solar cell fabricated on graphene/Cu foil compared with those of the reference cell fabricated on Mo/STS (inset shows optical image of the flexible device).

Table 2 Photovoltaic and diode parameters of CIGS solar cells fabricated with graphene/Cu foil and Mo/STS electrodes. Devices

Voc (V)

Jsc (mA/cm2)

FF (%)

η (%)

Jo (mA/cm2)

A

Rsh (Ω cm2)

Rs (Ω cm2)

CIGS/Graphene/Cu foil CIGS/Mo/STS

0.531 ± 0.005 0.470 ± 0.008

28.84 ± 1.98 28.16 ± 1.86

64.75 ± 7.34 53.64 ± 3.21

9.91 ± 0.89 7.10 ± 0.59

(4.52 ± 0.45) × 10−3 (7.79 ± 0.32) × 10−2

2.01 ± 0.13 2.52 ± 0.12

823 ± 45 313 ± 36

3.5 ± 0.5 5.2 ± 0.5

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Fig. 7. EQE curves of CIGS solar cells fabricated with graphene/Cu foil (red dots) and Mo/STS electrodes (black dots). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

improvement in EQE can be observed in the wavelength range of 520–1100 nm for the device fabricated with graphene/Cu foil electrode. The improvement of EQE is associated with the enhanced carrier generation rate in CIGS absorber layer and better carrier collections at the graphene/Cu electrode. The results are in accordance with the significant improvement in device performance by means of higher power conversion efficiency. 4. Conclusion In summary, this study demonstrated a novel structure for graphene-based flexible CIGS solar cells, in which graphene film on Cu foil was used as hole transport electrode. The CIGS solar cells were directly fabricated on the chemical vapor deposited graphene film on flexible Cu foil. Synthesized graphene film on Cu foil exhibits typical high quality monolayers and uniformly distributed over large area. The CIGS solar cells fabricated with graphene/Cu foil hole transport electrode displayed power conversion efficiency of 9.91% with Jsc of 28.84 mA/cm2, Voc of 0.531 V and FF of 64.75%, respectively. The power conversion efficiency is also found to be substantially higher compared to reference cell, fabricated using a conventional Mo/stainless steel electrode. The high power conversion efficiency of graphene/Cu foil based device is primarily attributed to the high open circuit voltage and substantially large fill factor. In spite of the low efficiency compared to the existing CIGS solar cells on flexible substrates, this work opens window for the implication of graphene as hole transport electrode in flexible photovoltaic devices without involving any sophisticated transfer process. It is envisioned that the performance of the CIGS solar cells fabricated on graphene/Cu foil can be further improved by optimizing the quality, composition and structural properties of CIGS absorber layer. Competing interests The authors declare no competing financial interest. Conflicts of interest There are no conflicts of interest to declare. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A2A01002877) and (No. 2015R1A4A1042417). 362

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