Controllable Growth of Ga Film Electrodeposited from Aqueous

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Controllable growth of Ga film electrodeposited from aqueous solution and Cu(In,Ga)Se2 solar cells Jinlian Bi, Jianping Ao, Qing Gao, Zhaojing Zhang, Guozhong Sun, Qing He, Zhiqiang Zhou, Yun Sun, and Yi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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ACS Applied Materials & Interfaces

Controllable growth of Ga film electrodeposited from aqueous solution and Cu(In,Ga)Se2 solar cells Jinlian Bi, Jianping Ao*, Qing Gao, Zhaojing Zhang, Guozhong Sun, Qing He, Zhiqiang Zhou, Yun Sun and Yi Zhang* College of Electronic Information and Optical Engineering and Tianjin Key Laboratory of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China Abstract Electrodepositon of Ga film is very challenging due to the high standard reduction potential (-0.53 V vs. SHE for Ga3+). In this study, Ga film with compact structure was successfully deposited on Mo/Cu/In substrate by PCE method using GaCl3 aqueous solution. High deposition rate of Ga3+ and H+ can be achieved by applying large overpotential induced by high pulse current. In the meanwhile, the concentration polarization induced by cations depletion can be minimized by changing the pulse frequency and duty-cycle. Uniform and smooth Ga film was fabricated at high deposition rate with pulse current density 125 mA/cm2, pulse frequency 5 Hz, and duty-cycle 0.25. Ga film was then selenized together with electrodeposited Cu and In films to make CIGSe absorber film for solar cell. The solar cell based on the Ga film presents conversion efficiency of 11.04 %, fill factor of 63.40 % and Voc of 505 mV, which is much better than those based on the inhomogeneous and rough Ga film prepared by DCE method, indicating pulse current electrodeposition process is promising for the fabrication of CIGSe solar cell. Keywords: pulse current electrodeposition, Ga film, hydrogen evolution, Cu(In,Ga)Se2, solar cell *Corresponding author: E-mail: [email protected] (J. P. Ao), [email protected] (Y. Zhang) Tel: +86-22-23508572 Fax: +86-22-23508912

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1. Introduction Cu(In,Ga)Se2 (CIGSe) is the most prospective photovoltaic semiconductor for thin film solar cells with low cost and high efficiency conversion. The optical absorption coefficient ɑ of CIGSe (ɑ>105 cm-1) is high and the band-gap can be tuned to 1.04 eV for CuInSe2 (CISe) and 1.68 eV for CuGaSe2 (CGSe) by adjusting Ga content when appropriate band-gap matches the solar spectrum at which most of the photons can be absorbed.1, 2 It has been reported that the highest conversion efficiency can be obtained when Ga/(In+Ga)≈0.3 for CIGSe solar cells3-5 and 22.6 % has been achieved by co-evaporation process.6 However solution-based approach with metal precursor attracted more attention due to the

prominent advantages, including high deposition rate, material utilization, and feasibility of making large area films.3, 7-10 The metal precursors with stacking structure composing of single or binary elements films can be fabricated by electrodeposition method. Due to the difference of standard reduction potentials among Cu2+/Cu, In3+/In, and Ga3+/Ga (+0.34 V, -0.34 V, and -0.52 V vs. SHE respectively),11 it is difficult to precise control the composition of precursor by electrodepositing binary or ternary alloys simultaneously. One approach is to deposit Cu, In, and Ga individual layer however it is difficult to electro-deposited gallium from water-based solutions due to the low standard reduction potential of Ga in that hydrogen evolution reaction (HER) would occur12 before Ga deposition. To reduce the parasitic influence of HER, pulse current electrodeposition (PCE) method was employed rather than the conventional direct current electrodepsition (DCE) method where the energy produced by the electric discharge can be removed immediately by water, which is very beneficial for those metals with low melting point (such as Ga, melting point 29.8 °C

12, 13

). However, few studies have been carried out to deposit

Ga with PCE method. In this study, Ga film was successfully deposited on Mo/Cu/In substrates from GaCl3 aqueous solution by PCE method. The effects of Cu/In layers, pulse parameters on Ga deposition and hydrogen evolution were investigated. By adjusting pulse parameters, including pulse current density j, pulse frequency f, and duty-cycle γ, compact and homogeneous Ga film can be formed, the hydrogen evolution was reduced significantly. An efficiency of 11.04 % was achieved for CIGSe thin film device with the uniform Ga film made by PCE method. 2


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2. Experimental 2.1 Preparation of Ga film and solar cells The detailed preparation and treating process of the Mo substrate, composition of Cu and In solutions, were reported elsewhere.14 Both Cu and In layers were fabricated by PCE method with j=62.50 mA/cm2, f=100 KHz, and γ=0.25. In films were also deposited by DCE method with direct current jd=62.50 mA/cm2. Gallium solution was composed of GaCl3 (AR grade, Tianjin Fengchuan), NaOH (Aladdin), NaCl (Aladdin), sulfamic acid (Aladdin), glucose (Aladdin), and triethanolamine (Aladdin, pH=2). The cyclic voltammetry of Ga were carried out in a three-electrode cell, which consisted of a working electrode (Mo/Cu/In), a counter electrode (Pt), and a reference electrode (Hg/HgCl). All cyclic voltammograms were scanned cathodically from the open-circuit voltage to negative (-2 V) direction and then swept oppositely from -2 V to 0.2 V (s. Pt) with a scanning rate of 10 mV s-1. The direct and pulse current deposition processes of Ga were achieved by applying pulse or constant current. GKPT-FB4-24 V/10 A pulse power (Shenzhen Shicheng, China) was used to offer pulse current for film deposition. The direct current was provided by CHI660C (Shanghai Chenhua) for film deposition and cyclic voltammetry tests. The unit quantity of electricity used for Ga deposition were kept constant at 1.5 C cm-2 during electrodeposition process. Cu(In,Ga)Se2 absorber layers were prepared from PCE Cu/In/Ga stacking precursors according to Ref. 14. The CIGSe absorber films were prepared without KCN etching. 2.2 Characterization The composition of Cu/In/Ga precursors and CIGSe absorbers were determined by XRF calibrated by ICP spectroscopy. Since the standard reduction potential of Cu2+ is greater than H+, the current efficiency of Cu is 100 % without hydrogen evolution. Therefore, the actual charge used for Ga deposition can be calculated based on the total charge used for Cu deposition. The current efficiency of Ga was obtained from the following equation: η=

∗ ∗

(1)

where is the total charge used for Ga deposition, is the total charge used for Cu deposition. 3



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The structure of Cu/In/Ga precursors and CIGSe absorbers were characterized by XRD (Cu Kα). SEM (Hitachi S-4800) was used to characterize the morphologies of the metal precursors and CIGSe films. The light J-V characteristics of the CIGSe devices were measured by solar simulator, which is under the standard AM1.5 spectrum with illumination intensity of 1000 W/m2 at 20 °C calibrated with a standard monocrystalline Si reference solar cell. The dark J-V characteristics was measured under dark environment. The carrier concentrations of the CIGSe films were investigated by Capacitance-Voltage (C-V, Keysight Technologies B1500A Semiconductor Device Analyzer) for the completed solar cells. EQE was analyzed by detecting the short-circuit current with spectrally resolved monochromatic light. 3. Results and Discussion 3.1 Electrodeposition of Ga film on different Cu/In films In PCE process, film electrodeposition can be controlled by adjusting pulse current density j, pulse frequency f, as well duty-cycle γ. f can be defined as the reciprocal of the cycle time (T): 15 f=

=

(2)

where is pulse on time with applied current, and is pulse off time without current.15 γ is the percentage of T and given by:16, 17 γ=

=

(3)

The limiting current density and overpotential in PCE process are higher than those of DCE process,14, 15, 18-20 resulting in increased nucleation rate and therefore more uniform distribution of the particles.18, 21-23 PCE method can also improve the current distribution24-28 and control the concentration polarization.29-31 Thus, much more Ga3+ will be reduced14, 32 and the uniformity of deposits will be improved. Table 1 summaries the compositions of Cu, In, and Ga metal precursors and the current efficiencies of Ga under four circumstances, where Ga film was deposited on Cu/In layer by both DCE and PCE method with current density 125 mA/cm2 (f=1 Hz, γ=0.25 for PCE process). Cu/In substrate layer was also prepared by both DCE and PCE process (noted as DC-Cu/In and PC-Cu/In, respectively). For DCE method, Ga deposition on PC-Cu/In layer ((PC-Cu/In)-(DC-Ga)) is less than that on DC-Cu/In layer ((DC-Cu/In)-(DC-Ga)), indicating Ga deposition on PC-Cu/In 4


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layer is more difficult than on DC-Cu/In layer. Ga/(In+Ga) of the (PC-Cu/In)-(DC-Ga) sample is only 0.11, much less than the needed ratio of 0.3. Additionally, the current efficiency of Ga for (PC-Cu/In)-(DC-Ga) sample (5.07 %) is lower than that of (DC-Cu/In)-(DC-Ga) sample (13.51 %), indicating HER of (PC-Cu/In)-(DC-Ga) sample is stronger than that of (DC-Cu/In)-(DC-Ga) sample. Whereas, the content of Ga deposited by PCE method on PC-Cu/In layer ((PC-Cu/In)-(PC-Ga)) is less than that of (DC-Cu/In)-(PC-Ga) sample, the current efficiency of Ga for (DC-Cu/In)-(PC-Ga) sample (38.28 %) is higher than that of (PC-Cu/In)-(PC-Ga) sample (32.57 %), indicating Ga deposition on PC-Cu/In layer is more difficult than that on DC-Cu/In layer. Ga/(In+Ga) for (DC-Cu/In)-(PC-Ga) sample was 0.47, larger than (DC-Cu/In)-(DC-Ga) sample (0.24). Therefore Ga deposition by PCE method is more efficient than by DCE method, the HER in DCE process is stronger than that in PCE process. Figure 1 presents the cyclic voltammograms of PC-Cu/In and DC-Cu/In layers in GaCl3 solution. The current density (negative scanning direction) at the PC-Cu/In electrode increases faster and is larger than that at the DC-Cu/In electrode, indicating hydrogen evolution at the PC-Cu/In electrode is stronger than that at the DC-Cu/In electrode. The fact that no peak was found for the reduction of Ga3+ to Ga implies that the reduction current of Ga3+ is covered by the reduction current of H+.33, 34 This strong hydrogen evolution reduces the current efficiency of Ga, therefore increases the difficulty of the Ga deposition on PC-Cu/In layer by DCE method. Figure 2 shows SEM images of both surface and cross section of the DC-Cu/In and PC-Cu/In samples. The DC-Cu/In sample is covered by island-shaped clusters and the film is not compact along the surface (Figure 2 (a) and (c)), whereas the film on PC-Cu/In sample is more uniform and compact with large grains (Figure 2 (b) and (d)). X-ray diffraction patterns (Figure 3) showed that CuGa2 phase appeared in both (DC-Cu/In)-(DC-Ga) and (PC-Cu/In)-(DC-Ga) samples, and (DC-Cu/In)-(DC-Ga) sample presents extra peaks CuGa2 (102) and CuGa2 (110). To form CuGa2 phase in Cu/In/Ga stacking precursors, Ga needs to go through In layer, and the diffusion will be affected by the compact In layer. As a consequence, the content of Ga deposited on compact In layer will be low, which is in accordance with the results presented in Table 1, where the content of Ga deposited on (PC-Cu/In)-(DC-Ga) sample is only 5.18 %, indicating Ga diffusion in PC-Cu/In sample is more difficult than in DC-Cu/In sample. While compact In layer will decrease the current efficiency during Ga electrodeposition 5



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process, the flat surface would be desired for uniform and compact Ga film deposition. Thus flat and uniform PC-Cu/In layers were used as substrate for Ga deposition and PCE method was

employed for the purpose to increase the current efficiency and control the HER during Ga deposition. Table 1 lists the composition and the current efficiency by PCE method on PC-Cu/In layer ((PC-Cu/In)-(PC-Ga)). Obviously, the sample (PC-Cu/In)-(PC-Ga) presents more Ga than (PC-Cu/In)-(DC-Ga) sample, which implies the increase of current efficiency of Ga deposition. The phenomenon can be explained as following. First, hydrogen evolution is a complex process involving several steps. In the DCE process and during the of the PCE process, hydrogen is produced through the reduction of H+ as Eq. (4), creating H2 bubbles and attaching on the film surface. 2H++2e-→H2

(4)

Once H2 bubble is big enough, it may coalesce and detach from the surface during . 39 Then new bubbles will be formed continuously. In DCE process, these H2 bubbles attached at the active site of the surface which may hinder the Ga deposition. Second, the pulse peak current density used in PCE process for Ga film deposition is 500 mA/cm2, higher than the current density of DCE method (125 mA/cm2). This large pulse peak current during induces a large electrode potential and high cathode polarization, which may change the deposition rate of different ions in the solution. 35-37 Furthermore, the electrode potential shifts to the value that accelerate metal deposition under large applied current. 38 Theoretical analysis38 of the HER during pulse current electrodepositing revealed that HER was weakened by employing PCE method. And third, reactive ions, depleted during , are supplied by mass transfer during ,15, 31 thus the effect of concentration polarization during deposition can be minimized. Compared with the DCE method, the influences of concentration polarization and hydrogen evolution during Ga deposition will be eliminated by PCE method. Therefore, the content and the current efficiency of Ga in (PC-Cu/In)-(PC-Ga) and (DC-Cu/In)-(PC-Ga) samples increase as the PCE method being employed. Also in Table 1, the Ga content in (DC-Cu/In)-(DC-Ga) sample was high, but this appeared to be not a good method to deposit Ga film. In PCE process, high cathode polarization means decreased nucleus size, increased nucleation rate, and improved uniformity of the film 6


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composition. Besides, Ga3+ migrates from bulk solution to the depleted areas during before next pulse occurs, even distributed Ga3+ are available for deposition onto the part,40 which is also in favor of homogeneous deposits formation. Moreover, the PCE method can be employed to improve the current distribution,24-28 uniform current distribution will be also helpful to form homogeneous deposits. Figure S1 shows the Ga content in (DC-Cu/In)-(DC-Ga) and (PC-Cu/In)-(PC-Ga) samples measured by EDS along the Cu/In/Ga films radial direction and photos of Cu/In/Ga metal precursors of (DC-Cu/In)-(DC-Ga) and (PC-Cu/In)-(PC-Ga) samples taken by camera. The fluctuation of Ga content along the radial direction in (DC-Cu/In)-(DC-Ga) sample is greater than that in the (PC-Cu/In)-(PC-Ga) sample. The standard deviation of the (DC-Cu/In)-(DC-Ga) sample is 2.14 %, greater than that of (PC-Cu/In)-(PC-Ga) sample (0.76 %), indicating the (PC-Cu/In)-(PC-Ga) sample`s composition is more uniform than the (DC-Cu/In)-(DC-Ga) sample. The photos of Ga indicates that the Ga film in (DC-Cu/In)-(DC-Ga) (Figure S1 (c)) is not very uniform, while the Ga film in (PC-Cu/In)-(PC-Ga) sample (Figure S1 (d)), shows bright and reflective appearance, indicating a smooth and uniform Ga film formed on PC-Cu/In layer. 3.2 Pulse current electrodeposition of Ga on PC-Cu/In film To improve the quality of Ga film, the growth process of PCE method on PC-Cu/In film was studied in detail. Figure 4 shows the current efficiency of Ga vs. current density and SEM images of Ga film deposited with different parameters (pulse current density j, pulse frequency f, and duty-cycle γ). Figure 4 (a) shows the efficiency of Ga with f=1 Hz, γ=0.25. The peak current density jp is defined as j/γ.14, 15, 41, 42 The ions reduction rate increases with the increase of pulse current density due to the large overpotential,18, 23, 31 and the current efficiency of Ga increased with pulse current density increasing to 187.5 mA/cm2. As mentioned before, H2 bubbles attached on film surface can affect the nucleation and growth processes, which may cause defects (pits) on the film surface (Figure 4 (a1)). Pit size decreases with the increase of current efficiency of Ga and film surface becomes flat when pulse current density reaches 187.5 mA/cm2 (Figure 4 (a2)). Voids came from the coalescence of Ga adatoms during the electrodepositing process were also observed (Figure 4 (a2)). Ga adatoms units contain high energy and became liquid when coalesced. The energy of Ga adatoms can be 7



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removed by water during . As the pulse current density increases to 375 mA/cm2, the pulse peak current density increases to 1500 mA/cm2, the concentration polarization was then occurred due to the depletion of Ga3+ ions in the vicinity of electrode surface, leading to decrease of current efficiency of Ga ( 5 Hz, the mass transfer of Ga3+ from bulk solution to cathode surface area during T would be insufficient with the increase of pulse frequency (concentration polarization), resulting in the decrease of current efficiency and deposition rate of Ga. However, the film surface becomes flat without voids due to strong hydrogen evolution reaction (Figure 4 (b3)). The effect of duty cycle on Ga deposition is similar to pulse current density and pulse frequency (Figure 4 (c)). The pulse peak current density was 2500 mA/cm2 with duty cycle 0.05. Ga3+ and H+ were depleted instantaneously during T , but HER is further induced by the reduction of H2O and the deposition rate of Ga decreased (less than 7 %). Pits induced from the strong hydrogen evolution formed on the film surface (Figure 4 (c1)). Increasing duty-cycle to 0.25, the pulse peak current density reduced to 500 mA/cm. The depletion rates of Ga3+ and H+ decreased and the current efficiency of Ga increased (Figure 4 (c)). With 100 % duty-cycle (direct current deposition), the current efficiency of Ga is less than 5 %. Continuous hydrogen evolution and Ga3+ depletion decreased the deposition rate of Ga. Figure 5 shows the corresponding EDS analysis of Ga content in Cu/In/Ga films, which were performed along the radial direction. The total charges used for Ga deposition are kept the same as stated in experimental. Ga content increased with increasing current efficiency of Ga, ranging 8


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from 18.4 and 20 at. % as presented in Figure 5 (a1) and (a2), and only 11.1 at. % for sample shown in Figure 5 (a3). The calculated standard deviations of Ga content are 0.92 %, 0.62 %, and 1.18 % respectively (Figure 5 (a1)-(a3)). Ga distribution is improved with increasing current efficiency of Ga. The influences of the pulse frequency and the duty-cycle on the composition and uniformity of Ga are similar to the pulse current density (Figure 5 (b1)~(b3) and (c1)~(c3)). With pulse current density 125 mA/cm2, pulse frequency 5 Hz, and duty-cycle 0.25, the average Ga content increased to 29.2 at. %, and the standard deviation of Ga decreased to 0.51 %. With duty-cycle 100 % (DCE method), the average Ga content is only 3.7 at. % (Figure 5 (c3)), which is much less than the stoichiometric ratio. The fact that standard deviation of Ga increased to 1.73 % (Figure 5 (c3)) implies the uniformity of Ga distribution decreasing. The strong hydrogen evolution affacts the deposition of Ga and uniformity of the film. The stronger the hydrogen evolution, the more difficult the deposition of Ga. Figure 6 shows the corresponding AFM morphologies of the deposits (scanning area 20 µm×20 µm) as presented in Figure 4. The root mean square (RMS) roughness of the films (in nm) decreases with increasing current efficiency of Ga when it was over 20 %. The RMS roughness presented to be the lowest at 21.02 nm for the film fabricated with j= 125 mA/cm2, f= 5 Hz, and γ= 0.25 (Figure 6 (b2)). Whereas the RMS roughnesses decreased with decreasing of current efficiency of Ga when it was less than 15 % (Figure 6 (c3)). The reason is that a number of H2 bubbles attached on the substrate surface that impeded Ga deposition, resulting in a smoother surface. AFM image of the Ga film deposited on Cu/In layer deposited by DCE method with direct current density 125 mA/cm2 is shown in Figure S2. XRF results indicate Cu/(In+Ga) and Ga/(In+Ga) ratios of the film are 0.8 and 0.3. Ga content increased by extending the deposition time, however, the film surface roughness increased due to hydrogen evolution. The RMS roughness of the film is 42.4 nm, much higher than the films deposited by PCE method. 3.3 Cu(In,Ga)Se2 absorber layer and solar cell Two Cu/In/Ga films were selected for further stydy. They were converted to CIGSe chalcopyrite semiconductors by post-selenizing process for solar cells: CIGSe-DC sample is the one made by DCE method with direct current density 125 mA/cm2, and CIGSe-PC sample is made by PCE method with pulse current density 125 mA/cm2, pulse frequency 5 Hz, and 9



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duty-cycle 0.25. Figure 7 shows cross-section SEM images (a, b) and XRD patterns (c) of the CIGSe-DC and CIGSe-PC absorber layers. The composition of the selenized films measured by XRF is listed in Table 2. As can be seen from SEM images, the grain sizes of both CIGSe films are large. For CIGSe-DC sample, voids can be found at the Mo/CIGSe interface and grain boundaries, which was mainly arising from the nonuniform distribution of metal elements during selenization process (Figure 7 (a)), whereas no void can be found for CIGSe-PC sample in Figure 7 (b). Due to the more uniform of Cu/In/Ga precursor deposited by PCE method, CIGSe-PC sample presents much smoother surface than CIGSe-DC sample. CIGSe phase formation can be observed from XRD patterns (Figure 7 (c)). CIGSe(112) main peak are observed at 26.93°, in accordance with the database (JCPDS-ICDD 00-035-1102). In general, the crystallization of CIGSe-PC sample is better than CIGSe-DC sample due to the higher intensity of CIGSe peaks, which is attributed to the homogeneous metal precursor prepared by PCE method. Figure 8 compares the performance of solar cells based on CIGSe-DC and CIGSe-PC. 11.04 % power conversion efficiency was achieved for CIGSe-PC solar cell with Voc=505 mV, Jsc=34.47 mA/cm2 and FF=63.40 %, while only 8.18 % power conversion efficiency for CIGSe-DC solar cell with Voc=476 mV, Jsc=31.66 mA/cm2 and FF=54.29 % (Figure 8 (a)). It is supposed that the inhomogeneous diffusion of Ga and voids at the Mo/CIGSe interface and grain boundaries are the major causes for the decrease of Voc and FF. The EQE response of CIGSe-PC is higher than that of CIGSe-DC (Figure 8 (b)), indicating the carrier transition in the space charge region of CIGSe-PC is better than that of CIGSe-DC. The absorber energy band gaps (Eg) derived from EQE data are very close, 1.02 eV for CIGSe-DC and 1.01 eV for CIGSe-PC, which implies Ga composition are the same in the absorber layer. Figure 8 (c) shows the diode reverse saturation current density of the two samples. CIGSe-PC presents a better current collection than CIGSe-DC,44, 45 since it presents much lower current density (1.5×10-5 A cm-2 vs. 2.3×10-6 A cm-2 for CIGSe-PC). The straight-line portion of the logarithmic J-V curve represents the exponential behavior of an ideal diode. As shown in Figure 8 (c), diode current behavior has much weaker voltage dependence at lower voltages, which may arise from the shunt leakage current. Figure S3 shows the shunt and diode current of J-V curves of the two samples. The total current can be obtained from the intersection of shunt leakage current and diode current. Large shunt leakage current leads the 10


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diode reverse saturation current to increase. Additionally, the Voc can be described as:46 !" =

# $

(

ln ( )* + 1) (

(5)

where n represents the diode ideality factor, J0 is the diode reverse saturation current density, T is the temperature, K is the Boltzmann Constant. It is clear from Eq. (5) that Voc decreases when large diode reverse saturation current

occurs. Moreover, the defect concentration of the absorber film depends strongly on Ga content.47-49 Inhomogeneous Ga distribution induces the defect concentration increasing as well as recombination loss.50,

51

Therefore, the current density of CIGSe-PC is larger than that of

CIGSe-DC sample. The diode reverse saturation current of CIGSe-PC sample decreases with uniform Ga distribution. In Figure 8 (d), space charge region of CIGSe-PC (541 nm) is wider than that of CIGSe-DC (504 nm). There are no recombination loss if carriers are collected in the space charge region, therefore the wider space charge region, the better carrier collection. Since CIGSe-PC sample presents better carrier collection, CIGSe-PC device exhibited larger current density than CIGSe-DC device (Figure 8 (a) and (d)). Therefore, surface morphology of the electrodeposited stacked metallic layers is critical for the the distribution of metallic element during the annealing process at high temperature. Homogeneous and flat Ga film leads to uniform distribution of Ga elements, which can reduce the recombination loss. 47-49 4 Conclusions Ga film with smooth and compact structure was successful fabricated in this work, electrodeposited from GaCl3 aqueous solution by pulse current electrodeposition method. The composition distribution of Ga film and current efficiency of Ga deposition were greatly improved using PCE method. Concentration polarization of Ga deposition can be eliminated, and the parasitic reaction of hydrogen evolution during Ga depositition was minimized with optimized parameters: pulse current density 125 mA/cm2, duty-cycle 0.25, and pulse frequency 5 Hz. Cu(In,Ga)Se2 solar cells made wih the CIGSe absorbers presented improved efficiency over 11 %. Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. 11



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Ga content in (a) (DC-Cu/In)-(DC-Ga) and (b) (PC-Cu/In)-(PC-Ga) samples measured by EDS along the Cu/In/Ga films radial direction and photos of Cu/In/Ga precursors of (c) (DC-Cu/In)-(DC-Ga) and (d) (PC-Cu/In)-(PC-Ga) samples; AFM morphological image of the Ga film on Cu/In layer deposited by DCE method with direct current density 125 mA/cm2 and with a scanning area of 20 µm×20 µm in size. Cu/(In+Ga) and Ga/(In+Ga) ratios of the film are 0.8 and 0.3 measured by XRF; Shunt and diode current of dark J-V curves of (a) CIGSe-DC and (b) CIGSe-PC samples. Acknowledgements This work was supported by the National Science Foundation of China (51572132, 61674082),YangFan Innovative and Entepreneurial Research Team Project (2014YT02N037), and Tianjin Natural Science Foundation of Key Project (16JCZDJC30700) Reference (1) Jackson, P.; Hariskos, D.; Wuerz, R.; Kiowski, O.; Bauer, A.; Friedlmeier, T. M.; Powalla, M. Properties of Cu(In,Ga)Se2 Solar Cells with New Record Efficiencies up to 21.7%. Phys. Status Solidi RRL 2015, 9 (1), 28-31. (2) Chiang, C.-Y.; Hsiao, S.-W.; Wu, P.-J.; Yang, C.-S.; Chen, C.-H.; Chou, W.-C. Depth-Profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se)2 Thin-Film Solar Cell. ACS Appl. Mater. Interfaces 2016, 8 (36), 24152-24160. (3) Saji, V. S.; Lee, S.-M.; Lee, C.-W. CIGS Thin Film Solar Cells by Electrodeposition. J. Korean Electrochem. Soc. 2011, 14 (2), 61-70. (4) Niki, S.; Contreras, M.; Repins, I.; Powalla, M.; Kushiya, K.; Ishizuka, S.; Matsubara, K. CIGS Absorbers and Processes. Prog. Photovoltaics 2010, 18 (6), 453-466. (5) Gütay, L.; Bauer, G. Spectrally Resolved Photoluminescence Studies on Cu(In, Ga)Se2 Solar Cells with Lateral Submicron Resolution. Thin Solid Films 2007, 515 (15), 6212-6216. (6) Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of Heavy Alkali Elements in Cu(In,Ga)Se2 Solar Cells with Efficiencies up to 22.6%. Phys. Status Solidi RRL 2016, 10 (8), 583-586. (7) Romanyuk, Y. E.; Hagendorfer, H.; Stücheli, P.; Fuchs, P.; Uhl, A. R.; Sutter-Fella, C. M.; Werner, M.; Haass, S.; Stückelberger, J.; Broussillou, C.; Grand, P.-P.; Bermudez, V.; Tiwari, A. N. All Solution-Processed Chalcogenide Solar Cells- from Single Functional Layers Towards a 13.8 % Efficient CIGS Device. Adv. Funct. Mater. 2014, 25(1), 12-27. (8) Bhattacharya, R. N. CIGS-based Solar Cells Prepared from Electrodeposited Stacked Cu/In/Ga Layers. Sol. Energy Mater. Sol. Cells 2013, 113, 96-99. (9) Saji, V. S.; Choi, I.-H.; Lee, C.-W. Progress in Electrodeposited Absorber Layer for CuIn1-xGaxSe2 (CIGS) Solar Cells. Sol. Energy 2011, 85 (11), 2666-2678. (10) Dini, J. W. Electrodeposition-the Materials Science of Coatings and Substrates; Noyes Publications: New Jersey, USA, 1993; pp 331-339. (11) Lange, N.; Dean, J. Lange’s Chemistry Handbook Version 15th; McGraw-Hill: Knoxville, 12


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(30) Chin, D. T. Mass Transfer and Current ‐ Potential Relation in Pulse Electrolysis. J. Electrochem. Soc. 1983, 130 (8), 1657-1667. (31) Ghaemi, M.; Binder, L. Effects of Direct and Pulse Current on Electrodeposition of Manganese Dioxide. J. Power Sources 2002, 111 (2), 248-254. (32) Simons, W.; Gonnissen, D.; Hubin, A. Fundamental Aspects of Electrochemical Deposition and Dissolution Including Modeling; Paunovic, M.; Datta, M.; Matosz, T.; Talbot, J. B., Eds.; ECS Proceedings Series; Journal of the Electrochemical Society: Paris, France, 1998; pp 124. (33) Lai, Y.; Liu, F.; Zhang, Z.; Liu, J.; Li, Y.; Kuang, S.; Li, J.; Liu, Y. Cyclic Voltammetry Study of Electrodeposition of Cu(In,Ga)Se2 Thin Films. Electrochim. Acta 2009, 54 (11), 3004-3010. (34) Lai, Y.; Liu, J.; Yang, J.; Wang, B.; Liu, F.; Zhang, Z.; Li, J.; Liu, Y. Incorporation Mechanism of Indium and Gallium during Electrodeposition of Cu (In, Ga)Se2 Thin Film. J. Electrochem. Soc. 2011, 158 (12), D704-D709. (35) Yin, K.-M.; Jan, S.-L.; Lee, C.-C. Current Pulse with Reverse Plating of Nickel-Iron Alloys in a Sulphate Bath. Surf. Coat. Technol. 1997, 88 (1), 219-225. (36) Peng, Y.; Zhu, Z.; Chen, J.; Ren, J.; Han, T. Research on Pulse Electrodeposition of Fe-Ni Alloy. AIP Adv. 2014, 4 (3), 031301. (37) Hansal, W. E. G.; Tury, B.; Halmdienst, M.; Varsányi, M. L.; Kautek, W. Pulse Reverse Plating of Ni-Co Alloys: Deposition Kinetics of Watts, Sulfamate and Chloride Electrolytes. Electrochim. Acta 2006, 52 (3), 1145-1151. (38) Yin, K.-M.; White, R. E. A mathematical Model of Pulse Plating on a Rotating Disk Electrode. AIChE J. 1990, 36 (2), 187-196. (39) Liu, Z.; Zheng, M.; Hilty, R. D.; West, A. C. Effect of Morphology and Hydrogen Evolution on Porosity of Electroplated Cobalt Hard Gold. J. Electrochem. Soc. 2010, 157 (7), D411-D416. (40) Kelly, J. J.; Bradley, P. E.; Landolt, D. Additive Effects during Pulsed Deposition of Cu-Co Nanostructures. J. Electrochem. Soc. 2000, 147 (8), 2975-2980. (41) Jadhav, H. S.; Kalubarme, R. S.; Ahn, S.; Yun, J. H.; Park, C.-J. Effects of Duty Cycle on Properties of CIGS Thin Films Fabricated by Pulse-Reverse Electrodeposition Technique. Appl. Surf. Sci. 2013, 268 (0), 391-396. (42) Devaraj, G.; Guruviah, S.; Seshadri, S. K. Pulse Plating. Mater. Chem. Phys. 1990, 25 (5), 439-461. (43) Abdulin, V.; Chernenko, V. Current Yield and Mechanical Properties of Nickel Deposited in Pulsed Conditions. Prot. Met. 1983, 18 (6), 777-779. (44) Contreras, M. A.; Mansfield, L. M.; Egaas, B.; Li, J.; Romero, M.; Noufi, R.; Rudiger-Voigt, E.; Mannstadt, W. Improved Energy Conversion Efficiency in Wide Bandgap Cu(In,Ga)Se2 Solar Cells, IEEE Photovoltaic Specialists Conference, Seatle, June 19-24, 2011; IEEE: U. S. A., 2011, 37, 000026 (45) Zhang, R.; Hollars, D. R.; Kanicki, J. CIGS Solar Cell on Flexible Stainless Steel Substrate Fabricated by Sputtering Method: Simulation and Experimental Results, Active-Matrix Flatpanel Displays and Devices, 19th International Conference onWeb Services, Honolulu, Hawaii, June 24-29, 2012; Goble, C., Chen, P., Zhang, J., Eds.; IEEE: U. S. A., 2012, 289-292. (46) Hegedus, S. S.; Shafarman, W. N. Thin-Film Solar Cells: Device Measurements and Analysis. 14


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Table 1 The atomic concentration (at.%) of Cu/In/Ga metal precursors and the current efficiency of Ga calculated from XRF (Ga was deposited on DC-Cu/In and PC-Cu/In layer by direct current and

pulse

current

electrodeposition

method,

noted

as

(DC-Cu/In)-(DC-Ga)

and

(PC-Cu/In)-(DC-Ga), (DC-Cu/In)-(PC-Ga) and (PC-Cu/In)-(PC-Ga), respectively. Experimental parameters: temperature 20 °C, current density 125 mA/cm2, pulse frequency 1 Hz, duty-cycle 0.25) No.

Cu,

In,

Ga,

at. %

at. %

at. %

Ga/(In+Ga)

Current Efficiency of Ga, %

(DC-Cu/In)-(DC-Ga)

46.71

40.60

12.64

0.24

13.51

(PC-Cu/In)-(DC-Ga)

51.31

43.46

5.18

0.11

5.07

(DC-Cu/In)-(PC-Ga)

38.19

32.54

29.24

0.47

38.28

(PC-Cu/In)-(PC-Ga)

40.29

33.42

26.25

0.44

32.57

Table 2 The atomic concentration (at.%) of the CIGSe films measured by XRF No.

Cu, at. %

In, at. %

Ga, at. %

Se, at. %

Thickness, µm

CIGSe-DC

22.19

18.82

8.01

50.98

1.62

CIGSe-PC

22.39

18.91

7.83

50.87

1.61

16


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Figure Captions Figure 1 Cyclic voltammograms of Ga film deposited on the PC-Cu/In and the DC-Cu/In layers in the GaCl3 electrolyte Figure 2 SEM images of DC-Cu/In (a) (c) and PC-Cu/In samples (b) (d). Figure 3 (a) XRD patterns of the (DC-Cu/In)-(DC-Ga) and the (PC-Cu/In)-(DC-Ga) samples, (b) Reference pattern of CuGa2. Figure 4 Current efficiency of Ga (a, b, and c) and SEM images (a1-a3, b1-b3, and c1-c3) of Ga film deposited with different pulse current density, pulse frequency, and duty cycle. Pulse parameters are shown underneath of each image. The insets in (a2) (b1), and (c2) are enlarged view of the void marked in red circles. Figure 5 Corresponding EDS analysis of Ga content along the radial direction of the Cu/In/Ga films presented in Figure 4. The standard deviation of Ga content are shown underneath of each image. Figure 6 Corresponding AFM morphologies (scanning area 20 µm×20 µm) and RMS roughness of the Ga film presented in Figure 4. Figure 7 SEM images (a, b) and XRD pattern (c) of the CIGSe-DC and CIGSe-PC absorber layers. Figure 8 (a) light J-V curves, (b) external quantum efficiencies (EQE), (c) dark J-V curves, (d) carrier concentration vs. distance to the junction of CIGSe-DC and CIGSe-PC devices.

17



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Fig. 1

18


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Fig. 2

19



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Fig. 3

20


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Fig. 4

21



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Fig. 5

22


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Fig. 6

23



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Fig. 7

24


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Fig. 8

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TOC

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Controllable Growth of Ga Film Electrodeposited from Aqueous

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