Controllable Growth of Ga Film Electrodeposited from Aqueous

Research Article www.acsami.org

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 S Supporting Information *

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 the Mo/Cu/In substrate by the pulse current electrodeposition (PCE) method using GaCl3 aqueous solution. A high deposition rate of Ga3+ and H+ can be achieved by applying a large overpotential induced by high pulse current. In the meanwhile, the concentration polarization induced by cation 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 a CIGSe absorber film for solar cells. 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 the DCE method, indicating the 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 evolution reaction (HER) would occur12 before Ga deposition. To reduce the parasitic influence of HER, the 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 °C12,13). However, few studies have been carried out to deposit Ga with the PCE method. In this study, Ga film was successfully deposited on Mo/Cu/ In substrates from GaCl3 aqueous solution by the 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, and the hydrogen evolution was reduced significantly. An efficiency of 11.04% was achieved for the CIGSe thin-film device with the uniform Ga film made by the PCE method.

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 α of CIGSe (α >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 the 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 cells 3−5 and 22.6% has been achieved by the coevaporation process.6 However, a solution-based approach with a 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 composed of single or binary element films can be fabricated by the 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 precisely control the composition of a precursor by electrodepositing binary or ternary alloys simultaneously. One approach is to deposit Cu, In, and Ga individual layers; however, it is difficult to electrodeposit gallium from water-based solutions because the low standard reduction potential of Ga in the hydrogen © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Ga Film and Solar Cells. The detailed preparation and treating process of the Mo substrate and composition Received: January 27, 2017 Accepted: May 22, 2017 Published: May 22, 2017 18682

DOI: 10.1021/acsami.7b01388 ACS Appl. Mater. Interfaces 2017, 9, 18682−18690

Research Article

ACS Applied Materials & Interfaces

Table 1. Atomic Concentration (at. %) of Cu/In/Ga Metal Precursors and the Current Efficiency of Ga Calculated from XRFa no.

Cu, at. %

In, at. %

Ga, at. %

Ga/(In + Ga)

current efficiency of Ga, %

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

46.71 51.31 38.19 40.29

40.60 43.46 32.54 33.42

12.64 5.18 29.24 26.25

0.24 0.11 0.47 0.44

13.51 5.07 38.28 32.57

a

Ga was deposited on DC-Cu/In and PC-Cu/In layers by direct current and pulse current electrodeposition methods, noted as (DC-Cu/In)−(DCGa) 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). of Cu and In solutions were reported elsewhere.14 Both Cu and In layers were fabricated by the PCE method with j = 62.50 mA/cm2, f = 100 kHz, and γ = 0.25. In films were also deposited by the 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 was 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 to 0.2 V (vs 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 a 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 was kept constant at 1.5 C cm−2 during the 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 was 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=

f=

(2)

where Ton is pulse on time with applied current, and Toff is pulse off time without current.15 γ is the percentage of T and given by16,17 γ=

Ton = Tonf Ton + Toff

(3)

The limiting current density and overpotential in the PCE process are higher than those of the DCE process,14,15,18−20 resulting in increased nucleation rate and therefore more uniform distribution of the particles.18,21−23 The PCE method can also improve the current distribution24−28 and control the concentration polarization.29−31 Thus, much more Ga3+ will be reduced,14,32 and the uniformity of deposits will be improved. Table 1 summarizes the compositions of Cu, In, and Ga metal precursors and the current efficiencies of Ga under four circumstances, where Ga film was deposited on the Cu/In layer by both the DCE and PCE method with current density 125 mA/cm2 ( f = 1 Hz, γ = 0.25 for the PCE process). The Cu/In substrate layer was also prepared by both DCE and PCE processes (noted as DC-Cu/In and PC-Cu/In, respectively). For the DCE method, Ga deposition on the PC-Cu/In layer ((PC-Cu/In)−(DC-Ga)) is less than that on the DC-Cu/In layer ((DC-Cu/In)−(DC-Ga)), indicating Ga deposition on the PC-Cu/In layer is more difficult than on the 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 the (PC-Cu/In)−(DC-Ga) sample (5.07%) is lower than that of the (DC-Cu/In)−(DCGa) sample (13.51%), indicating HER of the (PC-Cu/In)− (DC-Ga) sample is stronger than that of the (DC-Cu/In)− (DC-Ga) sample. Whereas the content of Ga deposited by the PCE method on PC-Cu/In layer ((PC-Cu/In)−(PC-Ga)) is less than that of the (DC-Cu/In)−(PC-Ga) sample, the current efficiency of Ga for the (DC-Cu/In)−(PC-Ga) sample (38.28%) is higher than that of the (PC-Cu/In)−(PC-Ga) sample (32.57%), indicating the Ga deposition on the PC-Cu/ In layer is more difficult than that on the DC-Cu/In layer. Ga/ (In+Ga) for the (DC-Cu/In)−(PC-Ga) sample was 0.47, larger than the (DC-Cu/In)−(DC-Ga) sample (0.24). Therefore, Ga deposition by the PCE method is more efficient than by the DCE method, and the HER in the DCE process is stronger than that in the 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

CCu nGa 3 2 nCu

C0

1 1 = Ton + Toff T

(1)

where C0 is the total charge used for Ga deposition and CCu is the total charge used for Cu deposition. The structure of Cu/In/Ga precursors and CIGSe absorbers was 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 a 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 the 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 18683


Research Article


Figure 1. Cyclic voltammograms of Ga film deposited on the PC-Cu/ In and the DC-Cu/In layers in the GaCl3 electrolyte.

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 and therefore increases the difficulty of the Ga deposition on the PC-Cu/In layer by the DCE method. Figure 2 shows SEM images of both the 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)); however, the film on the 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 the CuGa2 phase appeared in both (DC-Cu/In)−(DC-Ga) and (PC-Cu/In)−(DC-Ga) samples, and the (DC-Cu/In)−(DC-Ga) sample presents extra peaks CuGa2 (102) and CuGa2 (110). To form the CuGa2 phase in Cu/In/Ga stacking precursors, Ga needs to go

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.

through the In layer, and the diffusion will be affected by the compact In layer. As a consequence, the content of Ga deposited on the compact In layer will be low, which is in accordance with the results presented in Table 1, where the content of Ga deposited on the (PC-Cu/In)−(DC-Ga) sample is only 5.18%, indicating Ga diffusion in the PC-Cu/In sample is more difficult than in the DC-Cu/In sample. While the compact In layer will decrease the current efficiency during the Ga electrodeposition 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 the PCE method was

Figure 2. SEM images of DC-Cu/In (a), (c) and PC-Cu/In samples (b), (d). 18684


Research Article


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 each image. The insets in (a2) (b1), and (c2) are an enlarged view of the void marked in red circles.

(PC-Cu/In)−(PC-Ga) and (DC-Cu/In)−(PC-Ga) samples increase as the PCE method is employed. Also in Table 1, the Ga content in the (DC-Cu/In)−(DCGa) sample was high, but this appeared to not be a good method to deposit Ga film. In the PCE process, high cathode polarization means decreased nucleus size, increased nucleation rate, and improved uniformity of the film composition. Besides, Ga3+ migrates from bulk solution to the depleted areas during Toff before the next pulse occurs, and even distributed Ga3+ is available for deposition onto the part,40 which is also in favor of homogeneous deposit formation. Moreover, the PCE method can be employed to improve the current distribution,24−28 and 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 film radial direction and photos of Cu/In/Ga metal precursors of (DC-Cu/In)−(DCGa) and (PC-Cu/In)−(PC-Ga) samples taken by camera. The fluctuation of Ga content along the radial direction in the (DCCu/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 the (PCCu/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 indicate that the Ga film in (DC-Cu/In)−(DC-Ga) (Figure S1(c)) is not very uniform, while the Ga film in the (PC-Cu/In)−(PC-Ga) sample (Figure S1(d)), shows bright and reflective appearance, indicating a smooth and uniform Ga film formed on the PCCu/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 the 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).

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 the PCE method on the PC-Cu/In layer ((PC-Cu/In)−(PCGa)). Obviously, the sample (PC-Cu/In)−(PC-Ga) presents more Ga than the (PC-Cu/In)−(DC-Ga) sample, which implies the increase of current efficiency of Ga deposition. The phenomenon can be explained as follows. First, hydrogen evolution is a complex process involving several steps. In the DCE process and during the Ton of the PCE process, hydrogen is produced through the reduction of H+ as shown in eq 4, creating H2 bubbles and attaching on the film surface.

2H+ + 2e− → H 2

(4)

Once the H2 bubble is big enough, it may coalesce and detach from the surface during Toff.39 Then new bubbles will be formed continuously. In the 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 the PCE process for Ga film deposition is 500 mA/cm2, higher than the current density of the DCE method (125 mA/cm2). This large pulse peak current during Ton 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 accelerates metal deposition under large applied current.38 Theoretical analysis38 of the HER during pulse current electrodepositing revealed that HER was weakened by employing the PCE method. Third, reactive ions, depleted during Ton, are supplied by mass transfer during Toff;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 the PCE method. Therefore, the content and the current efficiency of Ga in 18685


Research Article


Figure 5. Corresponding EDS analysis of Ga content along the radial direction of the Cu/In/Ga films presented in Figure 4.

Figure 4(a) shows the efficiency of Ga with f = 1 Hz and γ = 0.25. The peak current density jp is defined as j/γ.14,15,41,42 The ion 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 the 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 the film surface becomes flat when pulse current density reaches 187.5 mA/cm2 (Figure 4(a2)). Voids from the coalescence of Ga adatoms during the electrodepositing process were also observed (Figure 4(a2)). Ga adatom units contain high energy and became liquid when coalesced. The energy of Ga adatoms can be removed by water during Toff. As the pulse current density increases to 375 mA/cm2, the pulse peak current density increases to 1500 mA/cm 2 , and the concentration polarization then occurred due to the depletion of Ga3+ ions in the vicinity of the electrode surface, leading to a decrease of current efficiency of Ga ( 5 Hz, the mass transfer of Ga3+ from bulk solution to the cathode surface area during Toff 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 Ton; however, 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 the Experimental Section. Ga content increased with increasing current efficiency of Ga, ranging from 18.4 to 20 at. % as presented in Figure 5(a1) and (a2) and only 11.1 at. % for the 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 18686


Research Article


Figure 6. Corresponding AFM morphologies (scanning area 20 μm × 20 μm) and RMS roughness of the Ga film presented in Figure 4.

(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 the PCE method. 3.3. Cu(In,Ga)Se2 Absorber Layer and Solar Cell. Two Cu/In/Ga films were selected for further study. They were converted to CIGSe chalcopyrite semiconductors by the postselenizing process for solar cells: the CIGSe-DC sample is made by the DCE method with direct current density 125 mA/cm2, and the CIGSe-PC sample is made by the PCE method with pulse current density 125 mA/cm2, pulse frequency 5 Hz, and 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 the CIGSe-DC sample, voids can be found at the Mo/CIGSe interface and grain boundaries, which mainly arose from the nonuniform distribution of metal elements during the selenization process (Figure 7(a)), whereas no void can be found for the CIGSe-PC sample in Figure 7(b). Due to the more uniform Cu/In/Ga precursor deposited by the PCE method, the CIGSe-PC sample presents a much smoother surface than the CIGSe-DC sample.

5(b1) ∼ (b3) and (c1) ∼ (c3)). With pulse current density of 125 mA/cm2, pulse frequency of 5 Hz, and duty cycle of 0.25, the average Ga content increased to 29.2 at. %, and the standard deviation of Ga decreased to 0.51%. With duty cycle of 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 affects 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 was 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 a decrease 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. The AFM image of the Ga film deposited on the Cu/In layer deposited by the DCE method with direct current density 125 mA/cm2 is shown in Figure S2. XRF results indicate that Cu/ 18687


Research Article


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 CIGSeDC (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 compositions 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-DC44,45 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), the 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 diode reverse saturation current to increase. Additionally, the Voc can be described as46

Figure 7. SEM images (a, b) and XRD pattern (c) of the CIGSe-DC and CIGSe-PC absorber layers.

Table 2. Atomic Concentration (at. %) of the CIGSe Films Measured by XRF no.

Cu, at. %

In, at. %

Ga, at. %

Se, at. %

thickness, μm

CIGSe-DC CIGSe-PC

22.19 22.39

18.82 18.91

8.01 7.83

50.98 50.87

1.62 1.61

CIGSe phase formation can be observed from XRD patterns (Figure 7(c)). CIGSe(112) main peaks are observed at 26.93°, in accordance with the database (JCPDS-ICDD 00-035-1102). In general, the crystallization of the CIGSe-PC sample is better than the CIGSe-DC sample due to the higher intensity of CIGSe peaks, which is attributed to the homogeneous metal precursor prepared by the PCE method. Figure 8 compares the performance of solar cells based on CIGSe-DC and CIGSe-PC. An 11.04% power conversion efficiency was achieved for the CIGSe-PC solar cell with Voc = 505 mV, Jsc = 34.47 mA/cm2, and FF = 63.40%, while only 8.18% power conversion efficiency was achieved for the CIGSe-

Voc =

⎞ nKT ⎛ Jsc ln⎜⎜ + 1⎟⎟ e ⎠ ⎝ J0

(5)

where n represents the diode ideality factor; J0 is the diode reverse saturation current density; T is the temperature; and 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

Figure 8. (a) Light J−V curves, (b) external quantum efficiencies (EQE), (c) dark J−V curves, and (d) carrier concentration vs distance to the junction of CIGSe-DC and CIGSe-PC devices. 18688



■

defect concentration increase as well as recombination loss.50,51 Therefore, the current density of CIGSe-PC is larger than that of the CIGSe-DC sample. The diode reverse saturation current of the CIGSe-PC sample decreases with uniform Ga distribution. In Figure 8(d), the space charge region of CIGSe-PC (541 nm) is wider than that of CIGSe-DC (504 nm). There is no recombination loss if carriers are collected in the space charge region; therefore, the wider the space charge region, the better the carrier collection. Since the CIGSe-PC sample presents better carrier collection, the CIGSe-PC device exhibited larger current density than the CIGSe-DC device (Figure 8(a) and (d)). Therefore, surface morphology of the electrodeposited stacked metallic layers is critical for 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

ACKNOWLEDGMENTS 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)

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01388. Ga content in (a) (DC-Cu/In)−(DC-Ga) and (b) (PCCu/In)−(PC-Ga) samples measured by EDS along the Cu/In/Ga film radial direction and photos of Cu/In/Ga precursors of (c) (DC-Cu/In)−(DC-Ga) and (d) (PCCu/In)−(PC-Ga) samples; AFM morphological image of the Ga film on the Cu/In layer deposited by the DCE method with direct current density of 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 (PDF)

■

REFERENCES

(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 SolutionProcessed Chalcogenide Solar Cells- from Single Functional Layers Towards a 13.8% Efficient CIGS Device. Adv. Funct. Mater. 2015, 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, USA, 1999. (12) Steichen, M.; Thomassey, M.; Siebentritt, S.; Dale, P. J. Controlled Electrodeposition of Cu-Ga from a Deep Eutectic Solvent for Low Cost Fabrication of CuGaSe2 Thin Film Solar Cells. Phys. Chem. Chem. Phys. 2011, 13 (10), 4292−4302. (13) Liu, W.; Cheng, L.; Zhang, Y.; Wang, H.; Yu, M. The Physical Properties of Aqueous Solution of Room-Temperature Ionic Liquids based on Imidazolium: Database and Evaluation. J. Mol. Liq. 2008, 140 (1−3), 68−72. (14) Bi, J.; Yao, L.; Ao, J.; Gao, S.; Sun, G.; He, Q.; Zhou, Z.; Sun, Y.; Zhang, Y. Pulse Electro-deposition of Copper on Molybdenum for Cu(In,Ga)Se2 and Cu2ZnSnSe4 Solar Cell Applications. J. Power Sources 2016, 326, 211−219. (15) Chandrasekar, M. S.; Pushpavanam, M. Pulse and Pulse Reverse Plating-Conceptual, Advantages and Applications. Electrochim. Acta 2008, 53 (8), 3313−3322. (16) Beattie, S.; Dahn, J. Single bath, Pulsed Electrodeposition of Copper-Tin Alloy Negative Electrodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2003, 150 (7), A894−A898. (17) Masuko, N.; Osaka, T.; Ito̅, Y. Electrochemical technology: innovation and new developments; Gordon and Breach: Amsterdam, 1996; pp 265. (18) Xuetao, Y.; Yu, W.; Dongbai, S.; Hongying, Y. Influence of Pulse Parameters on the Microstructure and Microhardness of Nickel Electrodeposits. Surf. Coat. Technol. 2008, 202 (9), 1895−1903.

4. CONCLUSIONS Ga film with smooth and compact structure was successfully fabricated in this work and electrodeposited from GaCl3 aqueous solution by the pulse current electrodeposition method. The composition distribution of Ga film and current efficiency of Ga deposition were greatly improved using the 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%.

■

Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. P. Ao). *E-mail: [email protected] (Y. Zhang). Tel.: +86-2223508572. Fax: +86-22-23508912. ORCID

Yi Zhang: 0000-0001-9083-0611 Notes

The authors declare no competing financial interest. 18689


Research Article


(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, Seattle, 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 on Web Services, Honolulu, Hawaii, June 24−29, 2012; Goble, C., Chen, P., Zhang, J., Eds.; IEEE: U. S. A., 2012; pp 289−292. (46) Hegedus, S. S.; Shafarman, W. N. Thin-Film Solar Cells: Device Measurements and Analysis. Prog. Photovoltaics 2004, 12 (2−3), 155− 176. (47) Malm, U.; Edoff, M. Simulating Material Inhomogeneities and Defects in CIGS Thin-Film Solar Cells. Prog. Photovoltaics 2009, 17 (5), 306−314. (48) Hanna, G.; Jasenek, A.; Rau, U.; Schock, H. W. Influence of the Ga-Content on the Bulk Defect Densities of Cu(In,Ga)Se2. Thin Solid Films 2001, 387 (1−2), 71−73. (49) Eisenbarth, T.; Unold, T.; Caballero, R.; Kaufmann, C. A.; Abou-Ras, D.; Schock, H. W. Origin of Defects in CuIn1‑xGaxSe2 Solar Cells with Varied Ga Content. Thin Solid Films 2009, 517 (7), 2244− 2247. (50) Rega, N.; Siebentritt, S.; Albert, J.; Nishiwaki, S.; Zajogin, A.; Lux-Steiner, M. C.; Kniese, R.; Romero, M. J. Excitonic luminescence of Cu(In,Ga)Se2. Thin Solid Films 2005, 480−481, 286−290. (51) Baier, R.; Lehmann, J.; Lehmann, S.; Rissom, T.; Alexander Kaufmann, C.; Schwarzmann, A.; Rosenwaks, Y.; Lux-Steiner, M. C.; Sadewasser, S. Electronic Properties of Grain Boundaries in Cu(In,Ga)Se2 Thin Films with Various Ga-Contents. Sol. Energy Mater. Sol. Cells 2012, 103, 86−92.

(19) Landolt, D.; Marlot, A. Microstructure and Composition of Pulse-Plated Metals and Alloys. Surf. Coat. Technol. 2003, 169−170, 8−13. (20) Chene, O.; Landolt, D. The Influence of Mass Transport on the Deposit Morphology and the Current Efficiency in Pulse Plating of Copper. J. Appl. Electrochem. 1989, 19 (2), 188−194. (21) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrocrystallization: Nucleation and Growth Phenomena. Electrochim. Acta 2000, 45 (15− 16), 2559−2574. (22) Kwon, Y. H.; Kim, S. K.; Kim, S. W.; Cho, H. K. Artificially Controlled Two-Step Electrodeposition of Cu and Cu/In Metal Precursors with Improved Surface Roughness for Solar Applications. J. Electrochem. Soc. 2014, 161 (9), D447−D452. (23) Paunovic, M.; Schlesinger, M. Fundamentals of Electrochemical Deposition; John Wiley & Sons: New York, 2006; Vol.45. (24) Puippe, J.-C.; Leaman, F. Theory and Practice of Pulse Plating; AESF Publication: Orlando, 1986. (25) Chene, O.; Datta, M.; Landolt, D. Copper Deposition by Pulse Plating: Effect of Mass Transport on Morphology and Uniformity of Deposits. Oberflache/Surface 1985, 26 (2), 45−49. (26) Wan, H. H.; Chang, R. Y.; Yang, W. L. Current Distribution in a Jet Through-Hole System during Periodic Electrolysis. J. Electrochem. Soc. 1993, 140 (5), 1380−1387. (27) Kwak, S.-I.; Jeong, K.-M.; Kim, S.-K.; Sohn, H.-J. Current Distribution and Current Efficiency in Pulsed Current Plating of Nickel. J. Electrochem. Soc. 1996, 143 (9), 2770−2776. (28) Pesco, A. M.; Cheh, H. Y. The Current Distribution within Plated Through-Holes II. The Effect of Periodic Electrolysis. J. Electrochem. Soc. 1989, 136 (2), 408−414. (29) Cheh, H. Electrodeposition of Gold by Pulsed Current. J. Electrochem. Soc. 1971, 118 (4), 551−557. (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. 18690


Controllable Growth of Ga Film Electrodeposited from Aqueous

Recommend Documents