Electrophoretic Deposition of CuIn1âxGaxSe2 Thin Films Using

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Electrophoretic Deposition of CuIn1-xGaxSe2 Thin Films Using Solvothermal Synthesized Nanoparticles for Solar Cell Application Alireza Khanaki, Hossein Abdizadeh, and Mohammad Reza Golobostanfard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07300 • Publication Date (Web): 11 Sep 2015 Downloaded from http://pubs.acs.org on September 19, 2015

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The Journal of Physical Chemistry

Electrophoretic Deposition of CuIn1-xGaxSe2 Thin Films Using Solvothermal Synthesized Nanoparticles for Solar Cell Application Alireza Khanaki1, Hossein Abdizadeh1,2 , Mohammad Reza Golobostanfard1,* 1

School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran 2

Center of Excellence for High Performance Materials, University of Tehran, Tehran, Iran

[email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

*

To whom correspondence should be addressed. Telephone: +98-21-82084157. Fax: +98-21-88006076.

[email protected]

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ABSTRACT. CuIn1-x Gax Se2 (CIGS) thin films are successfully prepared by convenient electrophoretic deposition, using colloidal nanoparticles. This is the first report which focuses on the electrophoretic deposition (EPD) of pre-synthesized CIGS nanoparticles directly from their colloid for solar cell application. In this research, CIGS nanoparticles are first synthesized via solvothermal process and then dispersed in a media containing a mixture of ethanol as the solvent and triethylenetetramine as the additive, to be used for the film deposition via electrophoretic method. By simple adjustment of the electrophoretic parameters, including applied voltage, pH, deposition time, and composition of nanoparticles, CIGS thin films with controlled thickness and optoelectronic properties can be fabricated. The highest photovoltaic efficiency of 5.57% is obtained in the CuIn0.75 Ga0.25 Se2 sample. It is believed that this fabrication approach may open up a new gate to reduce the production cost of a highly demanding CIGS absorber layer used in thin film solar cells.

KEYWORDS. CuIn1-x Gax Se2 ; CIGS; Electrophoretic Deposition; Solvothermal Method; Thin Films, Solar Cells.


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INTRODUCTION I-III-VI2 ternary and quaternary compound semiconductors have attracted much attention over the past decades. CuInx Ga1-x (S,Se)2 (CIGS) compound was first introduced by Goodman and Douglas in 1954 1 . Since then, many researchers have focused on this topic to explore its intriguing structural and optoelectronic properties theoretically and experimentally. CIGS materials are known for their exceptionally high optical absorption coefficient (>10 5 cm− 1 ), tunable direct band gap energy (1.05≤ Eg≤1.63 eV) which is well-matched with the solar spectrum, and long-term stability

2-4 ,

which make this

material a promising absorber for solar cells. In contrast with the past, this material is now quite mature in terms of fundamental and experimental studies. Nevertheless, there are two major challenges that have not yet been totally addressed. One challenge is shortage of indium supplies 5 . Some studies have recently proposed using aluminum, zinc, and tin instead of indium, which are much more abundant in the earth’s crust and possess some of the same properties of this material

6-9 .

The next challenge is the production cost of high quality CIGS

thin films when compared with the current silicon thin film technology

10 .

A diverse range of synthesis

methods have been developed to tackle this issue, such as stacked elemental layer deposition laser deposition coating

16 ,

12 ,

sputtering

dip coating

17 ,

13 ,

evaporation of precursor sources

and spin coating

18 .

14 ,

doctor-blade coating

11 , 15 ,

pulsed paste

Although such synthesis methods can effectively fabricate

high quality CIGS thin films, most of them involve either a high-vacuum chamber or atmospherecontrolled post-annealing/selenization steps, all of which result in the increased complexity of fabrication procedure and total cost. On the other hand, the electrophoretic deposition process (EPD) is a well-known, cost-effective process which requires a simple apparatus. It also benefits from high versatility, accepting different materials, a short deposition time, and a vacuum-free deposition method. The EPD process is based on the suspension of the nanoparticles in a liquid media (colloid), which differentiates it from electrodeposition (ELD). In ELD, the deposition media is the solution of salts, i.e. ionic species

19 .

Essentially, this difference enables one to use pre-synthesized nanoparticles and immediately integrate ACS Paragon Plus Environment

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them into a ready-to-use film easily. While there are some reports on ELD deposition of CIGS thin films 20-24 ,

to the best of our knowledge, there is no study that focuses on EPD parameters for CIGS thin films

formation for solar cell application. In the present article, we utilize a convenient electrophoretic deposition to produce CIGS thin films. Simplifying more the whole process and following our previous reports

25,26 ,

we used CIGS

nanoparticles which were pre-synthesized by our newly introduced modified solvothermal method. The prepared CIGS nanoparticles were then dispersed in a specific media, and electrophoretically deposited onto a conductive substrate. We investigated the effects of main process parameters on the structural, morphological, compositional, and optoelectronic properties of CIGS thin films and compared the J-V characteristics of all solution-processed solar cells based on electrophoretically deposited films of CuInx Ga1-x Se2 . MATERIALS AND METHODS

Materials. Copper chloride (CuCl2 , ≥98%, Merck), indium chloride (InCl3, 99.99%, Alfa Aesar), selenium chloride (SeCl4 , 99.5%, Alfa Aesar), sulfur powder (S, 99%, Merck), gallium nitrate (Ga(NO 3 )3 , 98%, Merck), absolute ethanol (EtOH, Merck, 99%), 1-propanol (1PrOH, 99.9%, Merck), deionized water (DIW, 18.2 MΩ), hydrochloric acid (HCl, 32%, Merck), tetrapropyl ortotitanate (TTiP, 98.5%, Merck), cadmium sulfate (3CdSO 4 .8H2 O, 99.5%, Merck), thiourea (NH2 -CS-NH2 , 99%, Merck), ammonia (NH4 OH, 30%, Merck), triethylenetetramine (TETA, 95%, Merck), and fluorinedoped tin oxide transparent conductive substrate (FTO, 15 Ω/sq, Dyesol) were purchased and used as received.

Nanoparticles Preparation. CIGS nanoparticles were pre-synthesized by the afore-mentioned new modified solvothermal method. The growth technique details can be found elsewhere

26 .

CuCl2 (0.2

mmol), InCl3 (0-0.2 mmol), SeCl4 (0.4 mmol), and Ga(NO 3 )3 (0-0.2 mmol) were dissolved in TETA and poured in a lab-constructed autoclave. The solution was processed for 1.5 h, at 220 °C and internal imposed pressure of 400 kPa under continuous stirring to obtain CIGS nanoparticles. After the process, the autoclave was allowed to cool naturally to ambient temperature. The precipitates were collected by ACS Paragon Plus Environment

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centrifugation and washing several times with ethanol to remove the organic residues and then dried at 60 °C for 20 min.

Thin Film Fabrication. For the deposition media, a mixture of ethanol as a solvent and TETA as an additive was made with the volume ratio of 100:4. The mixture was carefully selected to form a relatively stable colloid in a media with high dielectric constant (>10 at 20 °C), low viscosity, and minimum hydrolysis reaction. All these features can potentially lead to a high electrophoretic mobility of nanoparticles and successful electrophoretic deposition

27 .

Right before performing each experiment,

the as-prepared CIGS nanoparticles were added to the solvent mixture, and using an ultrasound probe, dispersed for 15 min to make a blackish colloid. The weight percentage of the nanoparticles in the colloid was 4%. At higher concentrations, the nanoparticles were prone to settling quickly, while at lower concentrations, no significant deposition was found. Various experiments were designed to study the effects of major electrophoretic parameters including applied voltage (95-110 V/cm), deposition time (15-75 min), and gallium substitution with indium (CuIn1-x Gax Se2 with x = 0, 0.25, 0.5, 0.75, and 1). The detailed experimental conditions are listed in Table 1.


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Table 1. Compositions, applied voltages, and deposition times of thin film samples

Samples

Composition

Applied Voltage (V/cm)

Deposition Time (min)

T-CIS 1

CuInSe2

95

45

T-CIS 2

CuInSe2

110

45

T-CIS 3

CuInSe2

125

45

T-CIS 4

CuInSe2

110

15

T-CIS 5

CuInSe2

110

75

T-CIGS

CuIn0.5 Ga0.5 Se2

110

45

T-CGS

CuGaSe2

110

45

For the experimental setup, two 1×3 cm2 FTO glasses were used as both anode and cathode. The FTO glass as a substrate benefits from the flat surface and chemical resistance for EPD. The two electrodes were held 5 mm apart. Although a greater distance between electrodes would allow increased media access for film deposition, the electric field strength would also decay accordingly. For this reason, we prefer a shorter distance and a lower voltage. A DC power supply was used to provide the desired voltages in the range of 95-125 V/cm. Figure 1 shows the schematic representation of the electrophoretic method and related apparatus used in this research. After each deposition, the prepared thin film was immersed in ethanol to wash off the solvent residue, and then dried at 60 °C for 1 min. ACS Paragon Plus Environment

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Figure 1. Schematic representation of electrophoretic deposition and the corresponding apparatus used in this research.

Solar Cell Fabrication. TTiP was dissolved in 1PrOH for 10 min. In another beaker, the solution of 1PrOH, DIW, and HCl was prepared and added drop-wisely to the former solution under vigorous stirring, then stirred for 1 h. The buffer layer was deposited by dip coating of the prepared solution on pre-cleaned FTO substrates and dried at 100 °C for 10 min. The dip coating and drying processes were repeated twice to reach a thickness of about 100 nm. The film was then calcined at 450 °C for 1 h. The CdS n-type window layer was deposited through chemical bath deposition. For this purpose, 0.015 M CdSO 4 solution in 2 ml DIW, 1.5 M thiourea in 1 ml DIW, and 2.6 ml NH4 OH in 15 ml DIW solutions were separately prepared and added before deposition. The substrates were placed in the bath at 80 °C for 10 min. The pale orange CdS deposited substrates were washed and dried. After deposition of CIGS nanoparticles by EPD, the films were washed by dipping in ethanol and dried naturally. The films were annealed at 300 °C for 30 min and then at 500 °C for 1 h under argon/H2 S atmosphere. Finally, the Mo layer with the thickness of about 500 nm was deposited on the film with magnetron sputtering. The solution processed superstrate type CIGS solar cell with Glass/FTO/TiO 2 /CdS/CIGS/Mo configuration is obtained. ACS Paragon Plus Environment

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Characterization. The size, morphology, and crystal structure of the synthesized CIGS nanoparticles were investigated by transmission electron microscopy (TEM) (Philips CM30) operated at 200 kV. After 15 min sonication of the CIS nanoparticles in the deposition media, the size distribution of the nanoparticles was obtained by dynamic light scattering (DLS) at ambient temperature with a Zetasizer (Malvern Instruments, 3000 HAS). The data are the average of three runs of the same sample. All prepared thin films were characterized by X-ray diffraction (XRD) (Philips, PW1730) using Cu Kα radiation (λ=1.5405 Å, 40 kV, 30 mA). The surface morphology, thickness, and chemical composition of the samples were studied by field emission scanning electron microscopy (FESEM) (Hitachi, S4160) equipped with an energy dispersive X-ray analysis (EDX). The current density during the deposition was recorded by a computer connected multimeter (GDM 396, GW Insteck). Optical properties of the samples were measured in the range of 400-2000 nm using UV-Vis-NIR spectroscopy (Cary-214G). Photovoltaic measurements were performed using an AM 1.5 solar simulator (Sharif Solar). The power of simulated light was calibrated to 1000 W/m2 with Si photodiode. J–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Sharif Solar digital source meter. All tests were conducted at room temperature. Two cells with the same conditions for each parameter were fabricated for repeatability test. RESULTS AND DISCUSSION Figure 2 shows the TEM image and the corresponding selected area electron diffraction (SAED) pattern of CIS nanoparticles synthesized by the modified solvothermal method. Figure 2a illustrates CIS nanoparticles with irregular shapes and sizes (diameter) in the range of 10-30 nm. It also reveals an aggregation of nanoparticles that form larger particles due to the surface energy reduction. The rings pattern in Figure 2b exhibits nanoparticles are polycrystalline in nature. The interpreted pattern corresponded to (204/220), (112), and (116/312) crystal planes, specifying that the sample is CIS (bodycentered tetragonal)

28 .


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Figure 2. (a) TEM image and (b) SAED pattern of as-synthesized CIS nanoparticles. Figures 3a-c show the stability of the CIS nanoparticles in the dispersion media (EtOH+TETA). The stability conditions were constantly monitored for two hours. Photos were captured for the colloids of as-prepared, 1 h, and 2 h after preparation (Figures 3a-c). It can be seen that the CIS nanoparticles have a good stability up to 1 h. Beyond that time, the settling rate was visually observed to be almost constant, and all the nanoparticles eventually settled after 5 h. The stability of CIS nanoparticles does not change with more sonication time (>15 min) meaning that the nanoparticles should have been large in size

29 .

For better understanding of the nanoparticle size and their distribution through the sample,

DLS measurements were also performed and the results are shown in Figure 3d. According to this figure, there are two size distributions of the nanoparticles: group (1) 120-190 nm, and group (2) 8001300 nm. The size distribution of large nanoparticles, group (2), is quite wider than small nanoparticles, group (1), indicating that nanoparticles in group (1) are more uniform in size. Moreover, group (2) makes up a larger portion (approximately 75%) of the particles and appears to be dominant. Therefore, ACS Paragon Plus Environment

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this group is expected to play a major role in the stability of the colloid as well as the quality of deposited thin films. Nevertheless, since the average size of all nanoparticles is below 1 μm (~827 nm), they can be potentially used for a successful electrophoretic process

29 .

Figure 3. Stability of CIS nanoparticles in the deposition media for (a) as-prepared, (b) after 1 h, (c) after 2 h , and (d) size distribution of nanoparticles through DLS measurement of the same sample.

Effect of Applied Voltage. Figure 4a shows the plot of current density versus time for CIS deposition with three different applied voltages between the cathode and anode (samples T-CIS 1 (95 V/cm), TCIS 2 (110 V/cm), and T-CIS 3 (125 V/cm)). It can be seen that the initial current density increases from 3.4 (T-CIS 1) to 9.8 mA/cm2 (T-CIS 3) by increasing the applied voltage from 95 to 125 V/cm. This is one of the direct results of increasing the electric field strength between the electrodes. In addition to initial current density, the current density range in each sample also increases by increasing the applied voltage. These ranges are 3.4-4.6 mA/cm2 for T-CIS 1, 7.4-9.1 mA/cm2 for T-CIS 2, and ACS Paragon Plus Environment

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9.8-12.5 mA/cm2 for T-CIS 3. The trend could result from the fact that the current density tends to be more unstable in higher voltages and it takes more time for the current to reach steady state. Interestingly, the current density noticeably increases to a certain point for the applied voltages higher than 110 V/cm and then decreases to the lower values, leaving a peak. Furthermore, this maximum value peak of the current density shifts slightly toward a longer time as the applied voltage increases. Two phenomena could cause the peak to appear in T-CIS 2 and T-CIS 3. Based on the first hypothesis, the migration of free ions brought by TETA might contribute to an increase in the total current density at early stages of film deposition. This could be dominant at early span of deposition time as the amount of free ions is limited. At the same time, current density can also decrease with time due to the formation of more resistive layers onto the substrate. As time goes on, the second effect becomes more dominant due to the increase in the thickness of the deposited film. Therefore, these two naturally opposite occurrences might eventually compromise each other in some level and create a maximum point. The second possible hypothesis refers to the deposition of nanoparticles because of their difference in size (Figure 3d). Besides the fact that small nanoparticles (group (1) in Figure 3d) can acquire higher charge density and electrophoretic mobility compared to large nanoparticles (group (2) in Figure 3d), their lower amounts and narrower size distribution lead them to quickly form a very thin uniform layer over the FTO substrate at the beginning of the EPD process (Supporting Information, Figure S1). Therefore, it is least likely that current density plot is actually altered by the deposition of nanoparticles in group (1). Despite the effect of group (1), the nanoparticles in group (2) have a wider size distribution, contain 75% of the survey, and carry smaller amounts of charge density. As a result, they will be deposited later and cover the substrate non-uniformly at the early minutes of their deposition. This non-uniform deposition gradually turns into having less available area left on the substrate to conduct electricity and causes the current density to increase. However, these conditions can continue until a uniform film is formed from the large nanoparticles and begins to thicken. At this moment, the current density starts falling off due to the considerable increase in the electrical resistivity of the film. The final outcome is a peak in the current density-time plot. Based on this argument and the ACS Paragon Plus Environment

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observed trends in Figure 4a, it is assessed that all these phenomena are likely taken place between the deposition times of 5-20 min in particular for samples deposited at 110 and 125 V/cm. The serrated shape of the plots in Figure 4a is also remarkable. In fact, this was attributed to instantaneous generation and destruction of tiny hydrogen bubbles around the edges of the electrodes. It is believed that the hydrogen bubbles are produced by the hydrolysis reaction of water impurity in TETA, and can be intensified by applying higher voltages. Since the generation of a bubble can decrease the current density in a short time, its destruction might also increase it quickly. Consequently, the result could be reflected as a tooth-like plot of current density versus time. For better understanding of how TETA contributes to the current density of EPD process, four CIS samples were prepared with different amounts of TETA. The purpose was to make samples with a range of different pH. TETA could change the pH of the deposition media in the range of ~10.5-12. Since the current density was changed during the EPD process, its initial value was only recorded. In addition, the data were collected for two different applied voltages of 95 and 125 V/cm. The results are shown in Figure 4b. Interestingly, according to Figure 4b, the trends show that the initial pH of the media and the initial current density of each sample are inversely proportional at the same applied voltage. The possible mechanism behind this observation could be as follows: When TETA is added to the solvent (ethanol), absorption of existing protons (H+) by TETA increases the pH of dispersed media. At this time, majority of dissociated OH- groups can be absorbed on the surface of the nanoparticles. Then, it gives rise to positively charged CIS nanoparticles, i.e. cathodic electrophoretic deposition, and enhances the electrophoretic mobility of nanoparticles. However, adding excessive amounts of TETA to the dispersed media can also suppress its surface ionization

30 ,

which can reduce the surface charge of CIS

nanoparticles and consequently the initial current density. This observation highlights the ability of changing the current density, and thus, the deposition rate by solely adjusting the pH of the deposition media. Although high current densities are desirable for a successful EPD process

31 ,

it can also lead to an

increase in the media temperature and encourage unsolicited reactions. Moreover, since the deposition ACS Paragon Plus Environment

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rate is high in the high current densities, the nanoparticles would have less time to properly find the location to deposit on the substrate. This potentially results in a thin film with non-uniform thickness and morphology. Considering these issues, we decided to use pH around 11.5, and work with moderate applied voltages (e.g. 110 V/cm) to obtain higher current densities.

Figure 4. (a) Plot of current density versus time for CIS deposition with three different applied voltages (samples T-CIS 1 (95 V/cm), T-CIS 2 (110 V/cm), and T-CIS 3 (125 V/cm)) and (b) plot of initial current density versus initial pH for two different applied voltages (95 and 125 V/cm). Note that the data shown at pH 11.5 are for samples T-CIS 1 and T-CIS 3. Figure 5 shows the XRD patterns of the samples prepared with the three different applied voltages (samples T-CIS 1 (95 V/cm), T-CIS 2 (110 V/cm), and T-CIS 3 (125 V/cm)). According to Figure 5, although both FTO glass and CIS have a peak at the same diffraction angle of 26.58°, which makes them indistinguishable, CIS tetragonal structure (JCPDS card no. 40-1487) can be detected in XRD ACS Paragon Plus Environment

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patterns. It can be seen that the relative intensity of (220)/(204) and (312)/(116) crystal planes increases with increasing applied voltage from 95 to 125 V/cm. This is attributed to the possible higher thickness of the thin film samples at higher applied voltages.

Figure 5. XRD patterns of prepared CIS samples with different applied voltages (T-CIS 1 (95 V/cm), TCIS 2 (110 V/cm), and T-CIS 3 (125 V/cm)) along with FTO glass substrate. Figure 6 shows the SEM images including the top and cross sectional views of the samples for different applied voltages at the same deposition time (45 min). As it is shown in Figure 6, the thickness of the samples increases from 3.1 μm for sample T-CIS 1 with 95 V/cm (Figure 6b) to 9.5 μm for sample T-CIS 3 with 125 V/m (Figure 6f). The change in the thickness is in accordance with our results in the XRD patterns (Figure 5). If, however, higher voltages were applied, there would have been no more significant change in the thickness as all the nanoparticles have already incorporated in the thin film formation during 45 min of deposition time. The surface morphology of the samples is also changed by increasing the applied voltage. According to the top view SEM image of the samples with low applied voltages of 95 V/cm (Figure 6a), it can be seen that a layer of small nanoparticles is deposited prior to deposition of the large nanoparticles. Based on the discussion in Figure 3d, these small and large nanoparticles should belong to group (1) and group (2), respectively (Supporting


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Information, Figure S1). Those large nanoparticles can cover more parts of the substrate and hide the layer of small nanoparticles by applying higher voltages (up to 125 V/cm, Figures 6c,e). Since high voltages can also lead to porosity of the thin films due to instability of the current density and/or increasing the bubbles bursting rate, using moderate applied voltages (e.g. 110 V/cm) is recommended (Supporting Information, Figure S2).

Figure 6. Top and cross-sectional SEM images of the CIS samples for different applied voltages (a,b) TCIS 1 (95 V/cm), (c,d) T-CIS 2 (110 V/cm), and (e,f) T-CIS 3 (125 V/cm). SP and LP in (a,c) stand for small nanoparticles and large nanoparticles, respectively. Figure 7 shows the EDX analysis of sample T-CIS 2 (110 V/cm, 45 min) along with the individual elemental mapping of Cu, In, and Se atoms. The measured percentage for each atom is also shown on the onset of Figure 7a. According to Figure 7a, the calculated molar ratio for Cu:In:Se are 1.00:0.97:2.16, which matched well with the nominal stoichiometry of CIS materials (1:1:2). The peaks for oxygen (O) and tin (Sn) belong to the FTO substrate. Moreover, the images of elemental mapping (Figure 7b) and considering nearly the same coverage of copper, indium, and selenium atoms


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throughout sample T-CIS 2 support the idea that it is least likely to have other secondary phases such as common copper or indium rich phases in the sample

32,33 .

Figure 7. (a) EDX analysis and (b) corresponding elemental mapping of sample deposited by 110 V/cm (T-CIS 2). UV-Vis-NIR spectroscopy measurement was conducted on sample deposited by 110 V/cm (T-CIS 2) and the obtained transmission spectrum is shown in Figure 8. The spectral transmittance indicates that the sample has low level of transmission in the visible region (

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