Ag incorporation in low-temperature grown Cu(In,Ga)Se2 solar cells using Ag precursor layers

Solar Energy Materials & Solar Cells 146 (2016) 114–120

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Ag incorporation in low-temperature grown Cu(In,Ga)Se2 solar cells using Ag precursor layers Kihwan Kim a, Joo Wan Park a,b, Jin Su Yoo a, Jun-sik Cho a, Hi-Deok Lee b,n, Jae Ho Yun a,n a b

Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea Department of Electronic Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 1 April 2015 Received in revised form 7 October 2015 Accepted 16 November 2015

In this work, with Ag alloying, we attempted to improve the microstructure and device performance of low-temperature grown Cu(In,Ga)Se2 (CIGS) solar cells. Ag precursors with various thicknesses are deposited onto Mo prior to the CIGS growth step, and absorber films are formed via a single-step coevaporation at a substrate temperature of 350 °C. The addition of Ag in low-temperature grown CIGS films induces significant recrystallization and Na incorporation. Through adjustment of the Ag content of the Ag-alloyed CIGS films, an improved device performance is obtained compared with a CIGS solar cell without Ag alloying. & 2015 Published by Elsevier B.V.

Keywords: Photovoltaics CIGS Thin films Solar cells Chalcopyrite

1. Introduction In the past decade, CIGS-based solar cells have demonstrated that they are one of the most promising and cost effective thinfilm solar cells by demonstrating significant technological advances in manufacturing-scale devices, as well as laboratory-scale devices [1,2]. Typically, highly efficient CIGS solar cells have been fabricated using either a precursor reaction or a three-stage coevaporation process. Both methods have resulted in solar cells with conversion efficiencies in excess of 20% [1,3]. However, these methods typically use high temperatures (4530 °C) and/or delicate processes to obtain high-quality films. Recent efforts to reduce processing costs have led to a wide variety of research studies on low-temperature grown CIGS cells because of the possibility of providing increased flexibility regarding substrate selection, easier substrate handling, and reduced energy consumption. However, using a reduced processing temperature for CIGS growth is known to yield a poorer device performance due to incomplete chalcopyrite conversion [4], the increased number of structural/electronic disorders [5], or insufficient alkali incorporation [6]. Meanwhile, a fine microstructure does not seem to be a crucial factor to obtain a high-quality device. With post-deposition thermal annealing, the device performances of low-temperature grown CIGS cells were significantly improved without noticeable changes in the n

Corresponding authors. E-mail addresses: [email protected] (H.-D. Lee), [email protected] (J.H. Yun).

http://dx.doi.org/10.1016/j.solmat.2015.11.028 0927-0248/& 2015 Published by Elsevier B.V.

CIGS microstructure [5]. Recently, low-temperature grown CIGS solar cells on flexible substrates have exhibited marked progress and demonstrated comparable device performances compared with the conventional CIGS solar cells on soda-lime glass [7]. A crucial factor in obtaining a high efficiency is considered to be the delicate control of the compositional profiles of the CIGS films. Another approach to obtaining controlled device performances from low-temperature grown CIGS film is incorporation of Ag. Alloying CIGS with Ag to produce (Ag,Cu)(In,Ga)Se2 (ACIGS) offers a method not only to widen the bandgap but also to lower the melting temperature [8,9]. A lower melting temperature might enable a reduction of structural defects and electronic defects, and consequently a better device performance. In particular, Hanket et al. reported that co-evaporated ACIGS solar cells exhibited significant device performances in 1.4 eV – bandgap absorbers grown at low temperature [10]. In this work, we attempted to incorporate Ag into co-evaporated CIGS using Ag precursors on SLG/Mo substrates, and we examined the effects of Ag alloying in low-temperature grown CIGS films. The Ag precursors with various thicknesses were deposited onto Mo prior to the elemental co-evaporation. Subsequently, the ACIGS films were fabricated using the single-step evaporation of Cu, In, Ga, and Se on the SLG/Mo/Ag precursor substrates. We characterized the Ag alloying effects on the microstructure of the low-temperature grown ACIGS films, and we investigated the characteristics of the devices using these absorber films.

K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

2. Experimental A Mo back contact with a thickness of 1 μm was deposited onto soda-lime glass (SLG) using DC magnetron sputtering. Next, Ag precursors were deposited on the SLG/Mo substrate at room temperature using thermal evaporation, with the thicknesses of the Ag precursors adjusted from 70 nm to 220 nm. With the single-step co-evaporation of Cu, In, Ga, and Se, 2- to 3-μm-thick ACIGS (or CIGS) films with a Ga/(Inþ Ga) ratio of approximately 0.35 were formed on the Mo/Ag precursor at a substrate temperature of 350 °C. Note that the depositions of the Ag precursors and the CIGS films were separately performed (i.e., an ex-situ process). The flux rates of each element were maintained constant during the co-evaporation; therefore, the ACIGS (or CIGS) films did not have intentional composition gradients. With adjustment of the Cu flux rates, the Ag/(AgþCu) ratios of the ACIGS films were controlled from 0 to 0.63 while maintaining the (Agþ Cu)/(InþGa) ratio in the range of 0.74–0.93. To examine device properties, solar cells with the ACIGS (CIGS) films were also fabricated with a SLG/Mo/ACIGS(or CIGS)/CdS/i–ZnO/ n–ZnO/Al structure. After the ACIGS (or CIGS) absorber layer formation, 60-nm-thick CdS buffer layers were deposited by chemical bath deposition (CBD). The composition of the solution was 0.0015 M

115

CdSO4 and 0.05 M SC(NH2)2 with pH¼11. The pH of the solution was controlled using ammonia. Next, the i-ZnO (50 nm) and n-ZnO (400 nm) TCO layers were prepared by RF magnetron sputtering using a pure ZnO target and an Al2O3/ZnO (2.5/97.5 wt%) alloy target, respectively. Finally, a conventional thermal evaporation was used to deposit the Al top electrode. The thickness of the Al top electrode was approximately 1 μm. The details of the device fabrication are available in our previous study [11]. The deposited ACIGS films were characterized using scanning electron microscope (SEM) imaging, energy-dispersive spectroscopy (EDS), secondary ion mass spectroscopy (SIMS), and X-ray diffraction (XRD). The device characterizations include current–voltage (J–V) (25 °C, AM 1.5G condition), external quantum efficiency (EQE), and temperature-dependent open-circuit voltage (VOC–T) measurements.

3. Results and discussion 3.1. Material characterizations Plan-view and cross-sectional SEM images of ACIGS (or CIGS) films are shown in Fig. 1. The film compositions are also given in

Fig. 1. Plan-view and cross-sectional SEM images of the CIGS and ACIGS films: (a) CIGS (no Ag), (b) ACIGS with Ag/(Ag þ Cu) ¼0.17, (c) ACIGS with Ag/(Ag þCu) ¼ 0.36, and (d) ACIGS with Ag/(Ag þ Cu)¼ 0.63.

K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

0 (control)

(312)/(116)

(220)/(204)

Mo

(112)

region compared with the other ACIGS (or CIGS) films. Therefore, it is reasonable to rule out the formation of the ODC phase from among other possible causes. The second possible cause is a strong Ag gradient through the film. Due to the low substrate temperature, intermixing between Ag and Cu appeared to be limited. Therefore, the ACIGS film is separated into a relatively Cu-rich top region and a Ag-rich bottom region. We believe the latter is the primary reason for the shoulder peak at 26.75°, as will be discussed in more detail in the SIMS analysis section. To investigate the compositional distributions of the ACIGS (or CIGS) films, SIMS analyses were performed. Fig. 3 presents the depth profiles of Cu and Ag (left Y-axis) and the Ga/(InþGa) ratio (right Y-axis) of each sample. Due to the lack of standardization in

Intensity (arb. units)

Table 1. The CIGS film without an Ag precursor (i.e., the control) exhibited a relatively small microstructure, while the ACIGS films with Ag precursors exhibited larger grain sizes. When the Ag/(Ag þCu) ratio was greater than 0.36, further grain growth of the ACIGS films was not clearly evident. In particular, the ACIGS sample with Ag/(Ag þ Cu) ¼0.63 exhibited a relatively nodular morphology. The spot EDS analyses revealed that the nodule region (marked by “P1” in Fig. 1(d)) had a greater Ag content than the background region with a finer microstructure (marked by “P2” in Fig. 1(d)). We believe that the highly Ag-incorporated grains grew more rapidly, and thus, it might prohibit grain growth of low Ag-incorporated grains, which is similar to an abnormal grain growth phenomenon [12]. Highly Ag-incorporated ACIGS films (i.e., Ag/(Ag þCu) ¼ 0.63) also exhibited voids at the Mo/ACIGS interface. The origin of the voids has not yet been confirmed; however, it is assumed that the Ag ions migrated to react with other elements during the deposition and consequently left voids in the original Ag locations. Fig. 2(a) shows broad (step size ¼ 0.05°) XRD patterns of the ACIGS (or CIGS) films. The CIGS film (i.e., the control) exhibited a (112) preferred orientation, yielding a (112)/[(220) þ(204)] intensity ratio of approximately 5.3 (cf., (112)/[(220) þ(204)] intensity ratio of CIGS powder 1.6) [13]. However, as the degree of Ag alloying increased, the ACIGS films exhibited even greater (112)/[(220) þ (204)] intensity ratios (i.e., stronger (112) preferred orientation). In chalcopyrite, the (112) plane is known to have the lowest surface energy [14]; thus, if the ions are sufficiently mobile and thus grains grow significantly, an ACIGS film may have a greater chance to exhibit a strong (112) preferred orientation to reduce its surface energy [15]. Fine scans (step size: 0.01°) of the (112) peaks of each sample are presented in Fig. 2(b). The (112) peaks shifted to a lower angle as the Ag/(Agþ Cu) ratio increased. This shift resulted from the greater ionic radius of Ag than Cu. Note that the CIGS film with Ag/(Ag þCu) ¼ 0 (i.e., the control) exhibited a broad feature at 2θ ¼ 25.38°, noted with *, while the films with Ag alloying did not. We believe that this feature is associated with stacking faults [16,17]. This observation suggests that the ACIGS film exhibited improved recrystallization compared with the CIGS film with Ag/(Ag þCu) ¼ 0. The elimination of stacking faults in the ACIGS films appears to be attributed to enhanced ion movements via Ag alloying. Note that the (112) peaks did not have a noticeable shoulder for the Ag/(Ag þCu) ratio of r0.36; however, the sample with Ag/(Ag þCu) ¼0.63 exhibited a distinctive shoulder at approximately 26.75°. The observation of the shoulder peak is possibly due to the two causes, described as follows. The first possible cause is the formation of a secondary ordered defect compound (ODC; I1–III5–VI8) [18]. However, in the present study, this formation is considered to be very unlikely, on the basis of an observation that the ODC phase is found in the specific composition range of 0.5 oAg/(Ag þCu) o 1 and 0.5 oGa/(Ga þ In)o 1. The ACIGS films in this study have a Ga/(Ga þ In) ratio of approximately 0.3–0.4 according to EDS analysis. Furthermore, the ODC phase should have a smaller Se content than the I1–III1–VI2 chalcopyrite phase [18]. However, the Se SIMS profile (not shown here) of the ACIGS film with Ag/(Ag þCu) ¼ 0.63 does not exhibit a Se-poor

0.17

0.36

0.63

20

30

40

50

60

o

2θ ( ) 0.63

Normalized Intensity (arb. units)

116

0.36 0.17

0

* 25

26

27

28

o

2θ ( ) Fig. 2. (a) Broad and (b) (112) fine XRD patterns of the CIGS and ACIGS films in Fig. 1.

Table 1 Compositional values of the wide-area and spot-EDS measurements taken from Fig. 1. (Standard deviation of the measurement  0.5 at%). Sample

Ag

Cu

In

Ga

Se

Ag/(In þ Ga)

(Ag þ Cu)/(In þGa)

Ga/(Ga þ In)

(a) (b) (c) (d) (d) p1 spot (d) p2 spot

0.0 3.9 8.6 13.6 16.2 11.5

23.3 19.3 15.5 7.9 7.0 8.4

17.5 15.9 15.7 18.3 16.9 17.9

9.6 11.3 10.1 10.6 8.4 11.9

49.6 49.6 50.2 49.7 51.5 50.4

0.00 0.17 0.36 0.63 0.71 0.58

0.86 0.85 0.93 0.74 0.92 0.67

0.35 0.42 0.39 0.37 0.33 0.40

K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

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Fig. 3. Ag and Cu SIMS profiles (left Y-axis) and Ga/(Inþ Ga) ratio (right Y-axis) of the ACIGS (CIGS) films with various Ag/(Ag þCu) ratios: (a) 0, (b) 0.17, (c) 0.36, and (d) 0.63.

aðnmÞ ¼ 0:5618 þ0:032w þ 0:0121ð1  xÞ þ 0:0035w2 þ 0:0054ð1  xÞ2  0:0245wð1 xÞ þ 0:0257w2 ð1  xÞ 2

2

2

þ 0:0126wð1  xÞ  0:0215w ð1  xÞ ;

ð1Þ

26.14

Normalized Intensity (arb. units)

the Ag in CIGS, the Cu and Ag profiles are given in units of counts/s. Each interface in the samples is indicated with dashed lines. Because there was no intentional compositional gradient during the film deposition, the control sample (i.e., without the Ag precursor) did not exhibit a significant compositional change through the film thickness. The ACIGS film with Ag/(AgþCu)¼0.17 also exhibited a similar trend to that of the control sample; only the Ag profile exhibited a tendency to increase as it approached the Mo back contact (Fig. 3(b)). The ACIGS films with Ag/(AgþCu)¼0.36 and 0.63 exhibited relatively sinuate compositional profiles in both Ag count rates and Ga/(GaþIn) ratios. It is assumed that the growth temperature of 350 °C might not be sufficient to induce complete intermixing of each element when Ag/(Agþ Cu)Z0.36. However, note that the Ga/(GaþIn) ratios in each film fluctuated near 0.35 throughout the film thickness. Based on this observation, it is reasonable to assume that the Ag/(AgþCu) ratios, when Ag/(AgþCu)40.36, may mainly result in the shoulder peak near 26.75° (note that it does not indicate the broad feature marked as “*” when Ag/(Agþ Cu)¼0) shown in Fig. 2(b). From Figs. 2 and 3, the ACIGS films with Ag/(AgþCu)Z0.36 exhibited structural and compositional non-homogeneity. In particular, the (112) peak of the ACIGS film with Ag/(Agþ Cu)¼0.63 could be mathematically separated into two peaks at 26.14° and 26.75°, as shown in Fig. 4. The Ga/(Gaþ In) SIMS profile of the ACIGS film in Fig. 3(d) could be approximated to 0.35, even though it had some inflection points. According to Avon's work [19], the lattice parameters (a and c) in the (Agw,Cu1 w)(In1 x,Gax)Se2 system could be estimated using the following empirical equations:

Peak 1 Peak 2 Measured Cumulative peak fit

26.75

25

26

27

28

o

2θ ( ) Fig. 4. Mathematical peak fit of the (112) reflection of the ACIGS film with Ag/(Ag þ Cu)¼ 0.63.

cðnmÞ ¼ 1:1022 þ 0:047w þ 0:0748ð1  xÞ 0:0130w2  0:0186ð1  xÞ2  0:0230wð1  xÞ þ 0:0098w2 ð1  xÞ þ 0:0726wð1  xÞ2  0:0383w2 ð1 xÞ2 :

ð2Þ

The interplanar spacing of the tetragonal system is given in the following simple form: a ðTetragonalÞdhkl ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi: 2 2 2 h þk þ l a2 =c2

ð3Þ

118

K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

10

TCO

0.7

ACIGS (CIGS )

CdS

(c) 0.36 (b) 0.17

10

0.6 0.5 0.4

ACIGS(CIGS)/Mo

10

0.3 35

2

JSC (mA/cm )

Intensity (c/s)

VOC (V)

(d) 0.63 10

(a) 0 (control) 10

10

0

500

1000

1500

2000

2500

3000

3500

Sputtering Time (s)

3.2. Device characterizations Solar cells with CIGS and ACIGS films were fabricated, and their J–V characteristics are presented in Fig. 6 and Table 2. From Ag/(Ag þCu) ¼0 to 0.17, there was a significant improvement in the device performance; however, above this point (i.e., Ag/(Ag þCu) Z0.36), the device performances of the ACIGS solar cells decreased and became scattered. First, the increased Na incorporation by Ag alloying discussed in the SIMS analyses appeared to be connected with the improvement in the device performances

Fill Factor (%)

With Eqs. (1)–(3) and Ga/(Ga þIn) 0.35 (i.e., x  0.35), the (112) reflections at 26.75° and 26.14° yielded Ag/(Ag þCu) (¼ w) ratios of 0.3 and 0.9, respectively. The calculation indicates that intermixing between Ag and Cu was significantly limited during the deposition; thus, this ACIGS film can be considered to have two distinguishable layers in terms of composition. The area ratio between the peaks at 26.14° and 26.75° is 2.8:1. Thus, when considering an ACIGS film thickness of approximately 2.6 μm, a 0.7-μm-thick top part of the ACIGS film appeared to exhibit a relatively low Ag composition. Again, the SEM image in Fig. 1 (d) also revealed that the ACIGS film consisted of a top finegrained part (low Ag content, top 0.7–0.8 μm) and a bottom largegrained part (high Ag content, bottom 1.8–1.9 μm), which is consistent with the SIMS and XRD analyses. Fig. 5 shows the Na SIMS profiles of each sample in Figs. 1–3. Due to the different thicknesses of each sample, the ACIGS (or CIGS)/Mo interfaces of each sample are given separately with dashed bars. The Na incorporation is thought to offer several beneficial effects to chalcopyrite-based solar cells [6,20]. The Na profiles were significantly affected by the Ag content of each film: the control sample exhibited approximately 1016 counts/s in the film bulk, while the ACIGS samples exhibited significantly higher Na content. In particular, the ACIGS film with Ag/(Ag þCu) ¼0.63 exhibited in excess of 1019 counts/s in the film bulk, which is comparable to the Na content of a CIGS film with a substrate temperature of 550 °C [15]. Chen et al. [13] found that Na incorporation did not have noticeable correlations with the Ag alloying of three-stage co-evaporated CIGS (or ACIGS) films, which is inconsistent with the present study. We believe that their results might be attributed to the high substrate temperature (i.e., greater than 580 °C), and thus, the Na ions appeared to readily diffuse into the CIGS (or ACIGS) films through sufficient thermal activation. However, under an insufficient thermal activation condition, such as that in the present study, the Na diffusion appeared to be affected by the presence of Ag due to the increased ion mobilities (i.e., reduced melting point of ACIGS).

25 20 15 80

Efficiency (%)

Fig. 5. Na SIMS profiles of the ACIGS (CIGS) films with various Ag/(Ag þ Cu) ratios: (a) 0, (b) 0.17, (c) 0.36, and (d) 0.63.

30

70 60 50 40 30 14 12 10 8 6 4 2

0.0

0.2

0.4

0.6

Ag/(Ag+Cu) ratio Fig. 6. J–V characteristics of the ACIGS (CIGS) cells with various Ag/(Ag þ Cu) ratios. The error bars are the standard deviations of each value.

by Ag alloying [6]. Furthermore, reduced structural disorders by Ag alloying appear to provide a beneficial effect, as discussed in the XRD results. Although the ACIGS solar cells with Ag/(Ag þCu) Z0.36 exhibited higher Na contents and enhanced recrystallization, the ACIGS solar cells yielded lower VOC values than the ACIGS solar cells with Ag/(Ag þ Cu) ¼0.17. However, more importantly, the data obtained from Ag/(Ag þCu) Z0.36 exhibited significant scatter. Presumably, a compositional non-uniformity causes this wide distribution. Fig. 7(a) shows the external quantum efficiency (EQE) curves of the CIGS and ACIGS solar cells in Fig. 6. The CIGS solar cell (i.e., the control) exhibited relatively poor charge collection efficiency for wavelengths above 600 nm, which indicates that the CIGS film might have poor bulk properties. However, with Ag alloying, the spectral responses of the ACIGS cells were improved. Currently, two possible explanations for the improved spectral response can be considered. The first explanation is that the enhanced recrystallization by Ag alloying might improve the bulk properties by reducing the structural and electronic defects. The second explanation is related to the effects of the reduced charge carrier densities of ACIGS solar cells [21] because the Ag alloying of CIGS is known to induce lowering of the charge carrier density, thereby increasing the depletion region, which might help to collect the electrons/holes generated deep in the bulk of the film. The first derivatives of the EQE curves are also presented in Fig. 7(b). Typically, by obtaining the local minima where a bandgap is expected to be found, the bandgaps of a solar cell can be approximated [22]. However, if multiple local minima are found, it strongly indicates that the solar cell has spatial bandgap nonuniformity. In CIGS (or ACIGS) solar cells, the spatial bandgap non-

K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

119

Table 2 Light JV characteristics of the ACIGS and CIGS solar cells (best device, without anti-reflection coating, and active area¼0.44 cm2) in Fig. 6. Average values with their standard deviations are also given in the parentheses. Sample (Ag/(Ag þ Cu) ratio)

VOC (V)

JSC (mA/cm2)

Fill factor (%)

0 0.17 0.36 0.63

0.546 0.594 0.600 0.530

27.1 29.2 30.0 28.7

54.1 70.0 57.5 50.4

(0.5427 0.007) (0.5877 0.006) (0.568 70.040) (0.461 70.106)

(26.47 0.5) (29.37 0.7) (29.47 2.3) (25.77 3.4)

1.3

100

0.17

80

1.1 1.0

60

VOC (V)

EQE (%)

0.63 0

(51.6 72.6) (67.8 7 1.4) (55.6 74.0) (48.0 7 4.0)

8.0 12.2 10.4 7.7

(7.4 7 0.5) (11.7 7 0.5) (9.3 7 1.1) (5.7 7 1.4)

Ag/(Ag+Cu) ratio -3 VOC(T) = 0.96 - 1.39*10 T 0 :

1.2

0.36

Efficiency (%)

0.17 :

VOC(T) = 1.22 - 2.10*10

-3

T

0.36 :

VOC(T) = 1.12 - 1.72*10

-3

T

0.63 :

VOC(T) = 1.14 - 1.94*10

-3

T

0.9 0.8

40

0.7 20

0

0.6 0.5

400

600 800 1000 Wavelength (nm)

1200

0

50

100

150

200

250

300

Temperature (K) Fig. 8. VOC–T plot of the ACIGS (CIGS) cells with various Ag/(Ag þ Cu) ratios.

0.2

dEQE/d λ (%/nm)

0.0 -0.2

0.36

-0.4 -0.6

0.17

-0.8

0.63 0

-1.0 -1.2

700

800

900 1000 Wavelength (nm)

1100

1200

Fig. 7. (a) EQE and (b) dEQE/dλ curves of the ACIGS (CIGS) cells with various Ag/ (Ag þ Cu) ratios.

uniformity is closely related to the spatial compositional nonuniformity. Bandgaps in the (Agw,Cu1  w)(In1  x,Gax)Se2 system can be estimated using the following empirical equation [10]: Eg ðeVÞ ¼ 0:25w2 þ 0:03w þ 0:08x2 þ 0:61x  0:11xw þ 1:01:

ð4Þ

The cells with Ag/(Ag þCu) ¼0 and Ag/(Ag þCu) ¼0.17 are expected to have bandgaps of approximately 1.2 eV according to Eq. (4), and their first derivatives should also exhibit strong inflection points at approximately 1020 nm (i.e.,  1.2 eV). However, the cells with Ag/(Ag þ Cu) ¼0.36 and 0.63 had multiple distinctive inflection points (i.e., local minima), which indicates that the absorber films had compositional non-uniformity. Based on Eq. (4), the cell with Ag/(Ag þCu) ¼0.63 might yield a Eg of 1.33 eV (¼930 nm). However, the most distinctive inflection point of the cell was found at 1.25 eV ( ¼995 nm), and a relatively large discrepancy was observed. However, as discussed regarding

Fig. 4, if an ACIGS film consists of two parts in terms of composition, then this phenomenon can be explained. The upper part of the ACIGS film (i.e., Ag/(Ag þCu) 0.3 and Ga/(Ga þIn)  0.35, estimated thickness  0.7 μm) yielded a Eg of 1.25 eV (¼995 nm, estimated using Eq. (4)), which is consistent with the dEQE/dλ result in Fig. 7(b). However, an inflection by the bottom part (i.e., Ag/(Ag þCu)  0.9 and Ga/(Ga þ In) 0.35) of the absorber film is not evident, even though this part has a greater thickness. Only a small inflection was observed near 1.43 eV ( ¼870 nm, marked with a gray arrow in Fig. 7(b)). It appears that the region with Ag/(Ag þCu) 0.3 and Ga/(Ga þ In)  0.35 was placed at the top part, and it might absorb a considerable amount of incoming photons [15]. Again, when the Ag/(Ag þCu) ratio was greater than 0.17, the ACIGS films began to exhibit compositional and, consequently, bandgap non-uniformity. Temperature-dependent VOC (VOC–T) measurements were performed to determine the activation energy of the dominant recombination mechanism in each cell. The relationship between JSC and VOC is given as follows [23,24]:   Ea AkT J ln 00 ; ð5Þ V OC ¼  q q J SC where J0 is the diode saturation current density, J00 is the temperature-dependent prefactor, and Ea is the activation energy of recombination. Extrapolating the measured VOC values to T¼ 0 K, the activation energy of recombination can be approximated. Typically, if a CIGS solar cell is subjected to a Na deficiency, the activation energy of recombination is smaller than its bandgap [25]. Fig. 8 presents the VOC–T plots of the CIGS and ACIGS solar cells in Fig. 6. The CIGS solar cell (i.e., Ag/(AgþCu)¼0) had an activation energy of 0.9 eV for recombination, which was significantly smaller than its bandgap estimated from the QE results in Fig. 7. This result strongly suggests that the device performance was limited by interface recombination that originated from the Na deficiency. However, the ACIGS solar cells with Ag/(AgþCu)¼ 0.17 exhibited a greater activation energy

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K. Kim et al. / Solar Energy Materials & Solar Cells 146 (2016) 114–120

(Ea ¼1.22 eV) than the CIGS solar cell (i.e., Ag/(AgþCu)¼0), and the value of 1.22 eV was close to the estimated bandgap obtained from the QE results in Fig. 7(b). This observation identifies bulk recombination as being dominant rather than the interface recombination in the ACIGS cell with Ag/(Agþ Cu)¼0.17. The VOC–T analyses also support the Na diffusion enhancement via Ag alloying. However, despite the greater bandgap and sufficient Na incorporation, the ACIGS solar cells with Ag/(AgþCu)Z0.36 had smaller activation energies than the cell with Ag/(AgþCu)¼0.17. A clear explanation regarding this behavior has not yet been determined, but it appears to be associated with the compositional non-uniformity in the ACIGS films.

4. Conclusions In this work, we attempted to improve the microstructure and device performance of low-temperature grown CIGS solar cells using Ag alloying. Ag precursors with various thicknesses were deposited onto Mo prior to the CIGS growth. Next, the ACIGS (or CIGS) absorbers were formed on Mo/Ag using a single-step co-evaporation at a substrate temperature of 350 °C. The surface morphologies, microstructures, and Na incorporation of the ACIGS films were significantly affected by the Ag alloying. In particular, the Ag alloying appeared to significantly enhance the recrystallization and Na diffusion. Devices were also fabricated using the ACIGS films to elucidate the effects of the Ag alloying on the device performance. A CIGS cell without Ag alloying exhibited an efficiency of up to 8.0%; in contrast, an ACIGS cell with Ag/(AgþCu)¼0.17 exhibited improved device performances with an efficiency of up to 12.2% with an improved VOC. This improvement using Ag alloying appeared to be associated with the improved film bulk properties and enhanced Na incorporation.

Acknowledgments This research was financially supported by the Framework of the Research and Development Program of the Korea Institute of Energy Research (Grant no. B5-2419), the Cooperative R&D Program of the National Research Council of Science and Technology (Grant no. B42207), and the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (Grant no. 20138520011120).

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