Mo Interfaces in Solar Cells - American

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Comparison of Ag(In,Ga)Se2/Mo and Cu(In,Ga)Se2/Mo Interfaces in Solar Cells Xianfeng Zhang,*,† Masakazu Kobayashi,‡,§ and Akira Yamada∥ †

International Center for Science and Engineering Programs, ‡Department of Electrical Engineering and Bioscience, and §Kagami Memorial Research Institute for Materials Science, Waseda University, 3-4-1 Ookubou, Shinjyuku-ku, Tokyo 169-8555, Japan ∥ Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 Oookayama, Meguro, Tokyo 152-8552, Japan ABSTRACT: The structural and electrical properties of the junction at Ag(In,Ga)Se2AIGS/ Mo, and Cu(In,Ga)Se2 CIGS/Mo layers were characterized. The region between the CIGS and Mo featured a MoSe2 layer with a layered hexagonal structure and thickness of 10−15 nm. The c-axis of the MoSe2 was oriented perpendicular to the Mo layer, and the c -value was 12.6 Å. However, no such layer was observed at the interface between AIGS and Mo. This result was also confirmed by energy-dispersive X-ray spectrometry and X-ray diffraction measurements of the MoSe2 layer. The CIGS/Mo with a MoSe2 layer formed an ohmic contact, while the AIGS/Mo without the MoSe2 layer formed a Schottky contact. This Schottky contact showed a barrier height of 0.8 ± 0.02 eV, a nonideality factor of 1.5 ± 0.1, and a series resistance of 370 ± 8 Ω. A schematic band diagram of the AIGS/Mo junction was constructed on the basis of the above results. KEYWORDS: interface, ohmic contact, Schottky contact, solar cell, conversion efficiency

1. INTRODUCTION Cu(In, Ga)Se2 (CIGS) is one of the most promising candidates for applications in future commercial photovoltaic devices because of its low fabrication costs and high efficiency. Recently, a high efficiency of 22.6% was reported on a laboratory scale,1 which represents better performance than typical multicrystalline Si devices.2 However, to achieve high solar power conversion efficiencies, greater than 30%, the use of CIGS-based tandem solar cells is necessary. Ag(In,Ga)Se2 (AIGS) has been recognized as an appropriate candidate for the top cell in such devices. The power conversion efficiency of cells based on this material system have been reported to be as high as 10.7%.3 However, the performance of these devices remains lower than that of CIGS devices. In particular, the fill factor (FF) of AIGS cells is typically below 60%, whereas FFs greater than 80% are common in CIGS cells. Typically, chalcopyrite solar cells have a glass/Mo/absorber/CdS/TCO/ front electrode structure. Mo is widely used as the back contact of chalcopyrite solar cells because of its excellent electrical properties.4 One possible reason for the low FF of AIGS devices is the interface between the AIGS and the back contact Mo thin film where a barrier to the flow of holes exists. The barrier height at the interface between CIGS and Mo is about 0.87 eV,5 which may be expected to prevent the flow of holes and form a Schottky contact. However, it has been extensively reported that CIGS/Mo forms an Ohmic contact because of the formation of a MoSe2 layer at the interface between CIGS and Mo.6,7 Figure 1 shows a schematic diagram of the band structure of the CIGS/Mo interface.8 A very thin MoSe2 layer between the CIGS and Mo layers acts as a buffer layer to © XXXX American Chemical Society

Figure 1. Schematic of the band structure at the CIGS/Mo interface.

convert the Schottky contact to a quasi-ohmic contact, owing to tunneling and recombination of electrons at the barrier.9−11 The adhesion of CIGS to Mo is also improved by the MoSe2 layer, which is another benefit of using Mo as the contact. Many groups have studied the electrical effects of MoSe2 on CIGS solar cell performance.12−14 However, the interface between AIGS and Mo has not yet been studied in detail and requires further investigation. In this work, AIGS/Mo and CIGS/Mo structures are fabricated, and the interfaces between AIGS/Mo and CIGS/ Mo are characterized. The composition and structural Received: February 21, 2017 Accepted: April 27, 2017 Published: April 27, 2017 A

DOI: 10.1021/acsami.7b02548 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces configuration at the interfaces are investigated. The electrical properties of the AIGS/Mo interface are characterized, and a band diagram of the contact between AIGS and Mo is constructed. We show a relationship between the contact properties and the existence of the MoSe2 layer. Solar cells based on these structures are fabricated, and their performance characterized to show how the solar cell efficiency depends on the absorbing layer/Mo interface.

Figure 2. Schematic diagram of film structure used to measure I−V curves: (a) film structure used to measure CIGS/Mo interface and (b) film structure used to measure AIGS/Mo interface.

2. EXPERIMENTAL METHODS 2.1. Film and Solar Cell Fabrication. Mo films were fabricated by a sputtering method onto 1.25 cm × 2.5 cm × 1.5 mm soda-limeglass (SLG). The substrates were cleaned by ultrasonication in the following sequence: deionized water → ethanol → acetone → ethanol for 10 min in each medium. The substrates were then dried with N2 gas and immediately introduced into the vacuum system of a directcurrent sputtering apparatus. A Mo target (diameter 80 mm, thickness 30 mm, purity greater than 99.9%) was used to deposit the Mo films. The Mo films featured a bilayer structure consisting of a 100 nm lower layer with a porous structure to improve adhesion between Mo and SLG and a dense 600 nm top layer with a high conductivity. The total thickness of the Mo thin film was about 700 nm. The whole deposition process was maintained at room temperature. The CIGS film was fabricated by the well known three-stage method.15 In the first stage, In, Ga, and Se were deposited at a substrate temperature of 350 °C, and then the substrate temperature was increased to 550 °C to deposit Cu and Se. The deposition time was determined by monitoring the composition variation during the process; finally, In, Ga, and Se were deposited again to achieve a stoichiometric composition. AIGS films were deposited by our modified three-stage method.16 In the first stage, In, Ga, and Se were deposited on the substrate at 350 °C, i.e., the same deposition temperature as that used for CIGS. During the second stage, the substrate temperature was increased to 580 °C for deposition of Ag and Se. Unlike the CIGS deposition process where the composition was monitored to determine the end of the stage, the deposition time was set to be the same as that of the first stage. Finally, In, Ga, and Se were deposited once more to adjust the film composition to be Ag deficient. The final thickness of the absorbing layer was about 3 μm in both cases. Other layers of the solar cell were deposited to complete the solar cell structure as follows. A CdS buffer layer with a thickness of 50 nm was deposited by chemical bath deposition. Then ZnO- and B-doped ZnO layers with thickness of 80 and 600 nm, respectively, were deposited on the CdS layer by metal−organic chemical vapor deposition. Finally, an Al grid was deposited on top of the stack by evaporation to form the front contact. 2.2. Characterization Methods. The interface between AIGS and Mo was observed by an H-9000NAR high-resolution transmission electron microscope (TEM) operated at 300 kV. An energy-dispersive spectrometer (EDS) fitted to the TEM was used to analyze the composition of the interface. A Rigaku Hyper-RINT X-ray diffractometer (XRD) with Cuα radiation of 40 kV and 20 mA was used to characterize the crystal phases of the absorbing and Mo layers. Solar cell current−voltage (I−V) curves were measured with a 913 CV type I−V tester under 100 mW/cm2 (AM1.5) illumination provided by an EKO (LP-50B) solar simulator. The I−V properties of Al/ CIGS/Al, Al/AIGS/Al, CIGS/Mo, and AIGS/Mo contacts were also measured with the solar cell evaluation system in the dark. The quantum efficiency (QE) of the solar cells was characterized under monochromatic light ranging from 300 to 1300 nm by a CEP-25MLT QE tester. To measure the I−V curve of CIGS/Mo and AIGS/Mo contacts, Al was deposited on the absorbing layer as the front contact by evaporation. A schematic of the structure is shown in Figure 2. To measure the I−V curve, Mo was used as the ground terminal, and Al was used for the positive terminal. Capacitance−voltage measurements were also performed on this structure to characterize the carrier density in the CIGS and AIGS films with a 913 CV type I−V tester. A

focused ion beam sampling technique with Ga ions as the cutting source was used to prepare the cross-sectional TEM sample. The CIGS and AIGS films were lifted-off from the substrate by a mechanical method for XRD measurements at the interface between the absorbing and Mo layers.

3. RESULT AND DISCUSSION Parts a and b of Figure 3 show cross-sectional images of the CIGS/Mo and AIGS/Mo structures, respectively. The thick-

Figure 3. Cross-sectional image of (a) CIGS/Mo structure and (b) AIGS/Mo structure. White circles indicate the positions at which the interface was observed.

ness of the absorber layers was approximately 3 μm in each case. The dark points observed in the cross section were likely Cu−Ga particles in CIGS and Ag−Ga particles in AIGS which formed during the FIB process because Ga ions were used as the cutting source. The grain size could not be identified from these images because of the milling of the surface. However, our scanning electron microscope studies (not shown here) suggested that the grain size was 1−2 μm. The CIGS/Mo and AIGS/Mo interfaces are indicated in the figures, and the areas where the interfaces were observed are identified by white circles. Figure 4a(1) shows a TEM image of the CIGS/Mo interface area. The structure in the figure could be divided into three regions: the top part was inferred to be the CIGS layer, and the lower part showed the typical morphology of crystalline Mo.17 The region between the CIGS and Mo layers featured a layered structure lying mainly parallel to the Mo layer. This layered structure showed the typical morphology of hexagonal MoSe2.18 The TEM image of the AIGS/Mo interface, shown in Figure 4 a(1), could be divided into two regions: the top part is the AIGS layer and the lower part is the Mo layer. However, B


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no MoSe2 layer formed at the junction between Mo and AIGS. This result was consistent with the appearance of the TEM image, as shown in Figure 4b(2). Point f was determined to be the Mo layer because only Mo was detected. To better understand the interface between the absorbing and Mo layers, the contact region between the absorbing and Mo layers was studied at high magnification, as shown in Figure 4a(2),b(2). Figure 4a(2) shows that the MoSe2 layer between the CIGS and Mo possessed Se−Mo−Se sheets perpendicular to the c-axis bonded by van der Waals forces with a c value of 12.9 Å, which agreed well with the lattice constant c = 12.9 Å of a MoSe2 hexagonal structure.19 Figure 4a(3) shows a schematic diagram of the layered MoSe2 structure. The red arrows perpendicular to the Mo surface represent the direction of c axis of the hexagonal MoSe2. In previous reports, the Mo c axis typically orients parallel to the MoSe2 layer in CIGS/Mo interfaces. In our case, the c axis of Mo orienting perpendicular to the MoSe2 can be attributed to low Na-doping concentration during a CIGS deposition process20 because of the low sodium SLG substrates used. The thickness of the MoSe2 layer was approximately 10−15 nm, which is smaller than previously reported values.16−18 In our case, the c axis of MoSe2 was perpendicular to the Mo surface, which suppressed subsequent Se diffusion through the formed MoSe2 film, leading to a thinner MoSe2 layer. The high magnification image of the AIGS/Mo interface (shown in Figure 4b(2)) did not show the presence of a MoSe2 layer between the AIGS and Mo. In the case of CIGS, the absence of the MoSe2 layer leads to formation of a Schottky contact between the CIGS and Mo because of a large energy barrier. Thus, it is likely that the AIGS/Mo interface also could not form an Ohmic contact between the AIGS and Mo because of absence of MoSe2 at the interface. After CIGS and AIGS were mechanically peeled from the Mo substrate, we performed XRD measurements of the Mo surface to determine the crystal phase near the interface. Figure 5

Figure 4. a(1) TEM image of CIGS/Mo interface; a(2) high magnification image of the region between CIGS and Mo contacts; a(3) schematic diagram of layered MoSe2 structure; b(1) TEM image of AIGS/Mo interface area; b(2) high-magnification image of the region between AIGS and Mo contact. Points a, b, and c indicate the CIGS, CIGS/Mo interface, and Mo, respectively. Points d, e, and f indicate the AIGS, AIGS/Mo interface, and Mo, respectively. Compositional analysis was performed at points a, b, c, d, e, and f.

no layered structure was observed between the AIGS and Mo layers. To confirm our assignment of these regions in the images, an EDS fitted to the TEM apparatus was used to determine the elemental composition at points a, b, c, d, e, and f, as marked in the figures. The results are summarized in Tables 1 and 2. The EDS analysis revealed the composition of a Table 1. EDS Analysis of Points a−c in Figure 4a(1) elemental ratio

Cu (%)

In (%)

Ga (%)

Se (%)

a b c

23.13 3.20

18.55 2.43

7.42 0.79

50.90 63.08

Mo (%) 30.5 100

Table 2. EDS Analysis of Points a and b in Figure 4b(1) elemental ratio

Ag (%)

In (%)

Ga (%)

Se (%)

d e f

21.96 10.60

4.45 3.24

22.36 13.65

51.23 60.05

Mo (%) 12.46 100

Figure 5. XRD of CIGS/Mo and AIGS/Mo interface.

standard CIGS film with Cu/III ≈ 0.89 and In/III ≈ 0.29 at point a. At point b, the composition deviated from the stoichiometric composition of CIGS and a strong Mo signal was detected. The atomic ratio of Se/Mo was found to be approximately 2:1, suggesting the presence of a MoSe2 layer. At point c, only Mo was detected, indicating the Mo layer by EDS. The EDS results from points d−f in Figure 4b(1) (shown in Table. 2) suggested that point d had the typical composition of an AIGS film with Ag/III ≈ 0.82 and Ga/III ≈ 0.83, and at point e both AIGS and Mo were detected. However, the atomic ratio of Se/Mo was about 5:1. This ratio deviated considerably from the stoichiometric composition of MoSe2, indicating that

shows XRD patterns of the CIGS/Mo and AIGS/Mo interfaces after the absorbing layer was peeled from the substrates. The (110) peak of Mo was observed at 2θ = 40.4° for both the CIGS/Mo and AIGS/Mo interfaces, indicating that the Mo layer featured a cubic structure with a preferential growth orientation along the [110] direction.21 Two peaks at 2θ = 31.6° and 2θ = 55.6° were also observed for the CIGS/Mo interface, which corresponded to 101̅0 and 102̅0 of hexagonal MoSe2;22 however, no such peaks were observed at the AIGS/ Mo interface, confirming the absence of the MoSe2 phase. C


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was comparable to that of the Schottky contact, which means that

To further explore the dependence of the absorbing layer/ Mo contact behavior on the presence of MoSe2, I−V measurements were performed on both the CIGS/Mo and AIGS/Mo contacts. The device structure for measuring the I− V characteristics is shown in Figure 2, with an Al contact area of 0.2 cm2. Figure 6 shows I−V curves measured from the CIGS/

V = ΔVdiode + ΔVresistor

(1)

The voltage drop over Schottky and ohmic contacts can be calculated from the standard expressions for the I−V curves of Schottky and Ohm’s law under forward bias as24 V=

nkT ⎛ J + J0 ⎞ ⎟⎟ + JR ln⎜⎜ e ⎝ J0 ⎠

(2)

Here, n is the diode’s dimensionless nonideality factor, R is the resistance of the resistive component, J is the current density, and V is the applied voltage. J0 is the saturation current density and is given by ⎛ −eϕB ⎞ J0 = A*T 2 exp⎜ ⎟ ⎝ kT ⎠

(3)

where A* is the effective Richardson constant calculated as Figure 6. I−V curves of CIGS/Al and AIGS/Al contacts.

A* = Al and AIGS/Al structures. In the measurement, two Al grid electrodes on the surface of the CIGS and AIGS layer were used as positive and negative terminals. Thus, the current flowed in the following direction: Al/CIGS (AIGS)/Al. The current increased linearly as the applied voltage was increased, indicating that both CIGS/Al and AIGS/Al formed Ohmic contacts. Parts a and b of Figure 7 show the I−V characteristics

4πqm*k 2 h3

For free electrons (m* = m0), the Richardson constant is 120 A/cm2·K2 25). The effective mass (m*) of AIGS has been predicted to be approximately 0.39m0 (for AgInSe2) and 0.73m0 (AgGaSe2).26 Considering the Ga/III atomic ratio of AIGS film to be 0.83, we can calculate the effective mass of AIGS to be 0.67m0. Thus, the Richardson constant of AIGS was found to be 80 A/cm2·K2. Through a least-squares fit to the experimental data using eq 2, we obtained a correspondent fitting line, as indicated in Figure 7b. The barrier height ϕBP between the Mo and AIGS, the series resistance, and the nonideality factor could be determined from the fitting. Considering that the system error of the measuring apparatus was 2%, the barrier height was found to be 0.8 ± 0.02 eV. The nonideality factor was 1.5 ± 0.1 and series resistance obtained from the fitting was 370 ± 8 Ω, which represents the total resistance of bulk AIGS and the contact resistance of the AIGS/Al contact. A schematic band diagram of AIGS/Mo is shown in Figure 8 to summarize these findings. The bandgap of AIGS was

Figure 7. I−V characteristics of (a) CIGS/Mo contact and (b) AIGS/ Mo contacts.

of the CIGS/Mo and AIGS/Mo layers, respectively. The I−V curve of the CIGS/Mo contact showed clear ohmic behavior because the current flow in the system varied linearly with the applied bias. Conversely, the AIGS layer showed asymmetric diode-like behavior (as shown by the open cycles in Figure 7b), indicating the presence of a Schottky barrier in the system. The diode-like behavior did not follow the ideal exponential behavior of a Schottky diode.23 Under low forward bias, the current varied almost exponentially as the bias was changed. However, as the forward bias increased, the I−V behavior of the contact became linear, indicating a resistive component in series with the AIGS/Mo Schottky contact. In this work, we attribute the resistive component to the AIGS film itself because of its high resistivity and the contact resistance between AIGS and Al. The voltage drop over the resistive component

Figure 8. Schematic band diagram of the AIGS/Mo interface.

determined to be approximately 1.70 eV. The barrier between AIGS and Mo will prevent the flow of holes from AIGS to Mo, leading to recombination at the back contact. As a result, the fill factor (FF) and short circuit current (Jsc) of solar cells based on this structure may be expected to be low. The CIGS and AIGS samples that were used for TEM measurements were also fabricated into complete solar cell structures. Figure 9 shows I−V curves of the CIGS and AIGS solar cells. The photovoltaic characteristics of solar cells are summarized in Table 3. The CIGS solar cell showed an efficiency of 17.4%, with Voc = 0.64 V, Jsc = 37.5 mA/cm2, FF = D


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Figure 10. Quantum efficiency of CIGS and AIGS solar cells.

Figure 9. I−V curves of CIGS and AIGS solar cells.

respectively. The bandgaps of AIGS and CIGS were calculated to be 1.70 and 1.15 eV, respectively.

72.4% for a device with an area of 0.20 cm2. The AIGS solar cell showed an efficiency of 5.5%, with Voc = 0.95 V, Jsc = 13.2 mA/ cm2, FF = 43.6 for a device of the same area. The FF of the AIGS solar cell was much smaller than that of the CIGS solar cell, indicating a higher leakage current and greater recombination than that of the CIGS solar cell. The absence of the MoSe2 layer at the Mo/AIGS interface likely contributed to the low FF owing to the formation of the Schottky contact. The shunt resistance (Rsh) of the CIGS solar cell was 10.2 kΩ cm2, which is much higher than that of the AIGS solar cell (i.e., 0.4 kΩ cm2). However, the series resistance (Rs) of the AIGS device was 27.2 Ω cm2, which was 20 times larger than that of CIGS solar cell (1.3Ω cm2). This difference likely contributed to the low FF of the AIGS solar cell. The large Rs of AIGS was caused by a combination of the contact and bulk resistances of each layer, and the barrier between AIGS and Mo contributed to the contact resistance. Moreover, a clear rollover of the I−V curve was observed near the open-circuit region. This phenomenon may be attributed to the large back-contact barrier.27 However, some other factors that may lead to this rollover phenomenon are currently under investigation. Umehara et al.28 indicated that a low hole concentration in AIGS could contribute to rollover phenomenon. To determine if this was a factor in our case, we performed C−V measurements of the carrier density of CIGS and AIGS films. The hole concentration was found to be 4.5 × 1016 cm−3 for CIGS and 1.7 × 1013 cm−3 for AIGS. Thus, the hole concentration in CIGS was several hundred times higher than that in AIGS, and this low hole concentration is likely another factor contributing to the rollover phenomenon. Figure 10 shows quantum efficiency (QE) curves of the AIGS and CIGS solar cells. The average QE of the CIGS solar cell in the visible range of sunlight spectrum was over 90%; however, that of the AIGS solar cell was 60%, indicating greater recombination in the AIGS solar cell. A clear drop in the QE drop was observed at ∼390 and ∼500 nm in both curves, corresponding to absorption of sunlight by the ZnO and CdS layers. At the infrared region, the CIGS and AIGS solar cells showed a QE drop at approximate ∼730 and ∼1080 nm, indicating the absorption edges of AIGS and CIGS,

4. CONCLUSION The interfaces at AIGS/Mo and CIGS/Mo were compared in this work. TEM investigations confirmed the presence of a layered hexagonal MoSe2 structure with its c axis perpendicular to the Mo layer and a c value of 12.9 Å between the CIGS and Mo layers. However, a MoSe2 layer was not observed between the AIGS and Mo layers. These results were confirmed by EDX measurements and XRD characterization. The I−V characteristics of the contacts suggested that the MoSe2 layer at the CIGS/Mo interface contributed to formation of an Ohmic contact, while its absence resulted in a Schottky contact. The I−V curves of the AIGS/Mo contact were fit based on an equivalent circuit analysis using a least-squares method to determine the barrier height between AIGS and Mo to be approximately 0.8 eV. A schematic band diagram of the AIGS/ Mo junction was constructed on the basis of the obtained results. The CIGS and AIGS films used for analysis in this work were applied to solar cells. The CIGS solar cells showed an efficiency of 17.4% (Voc: 0.64 V, Jsc: 37.5 mA/cm2, FF: 72.4%, area: 0.2 cm2), while the AIGS solar cell showed an efficiency of 5.5% (Voc: 0.95 V, Jsc: 13.2 mA/cm2, FF: 43.6%, area: 0.2 cm2). The Rs of CIGS was much smaller than that of AIGS, while the Rsh was much larger. The absence of MoSe2 at the AIGS/Mo interface may contribute to the roll-over phenomenon observed in the I−V curve of the AIGS solar cell.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xianfeng Zhang: 0000-0003-1040-9283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was performed while X.F.Z. was a postdoc at Tokyo Tech. It was partially supported by the New Energy and

Table 3. Photovoltaic Parameters of CIGS and AIGS Solar Cells

CIGS AIGS

Voc (V)

Jsc (mA/cm2)

FF (%)

η (%)

Rsh (kΩ cm2)

Rs (Ω cm2)

0.64 0.95

37.5 13.2

72.4 43.6

17.4 5.5

10.2 ± 0.5 0.4 ± 0.02

1.3 ± 0.1 27.2 ± 2.0

E


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(19) Agarwal, K. M.; Talele, T. Growth Conditions and Structural Characterization of Molybdenum Sulphoselenide Single Crystals: (MoSxSe2‑x, 0 ≤ x ≤ 2). Mater. Res. Bull. 1985, 20, 329−336. (20) Yoon, J.; Kim, J.; Kim, W.; Park, J.; Baik, Y.; Seong, T.; Jeong, J. Electrical Properties of CIGS/Mo Junctions as a Function of MoSe2 Orientation and Na Doping. Prog. Photovoltaics 2014, 22, 90−96. (21) Scofield, J. H.; Duda, A.; Albin, D.; Ballard, L. B.; Predecki, K. P. Sputtered Molybdenum Bilayer Back Contact for Copper Indium Diselenide-based Polycrystalline Thin-Film Solar Cells. Thin Solid Films 1995, 260, 26−31. (22) Abou-Ras, D.; Kostorz, G.; Bremaud, D.; Kälin, M.; Kurdesau, F. V.; Tiwari, N. A.; Döbeli, M. Formation and Characterisation of MoSe2 for Cu(In, Ga)Se2 Based Solar Cells. Thin Solid Films 2005, 480−481, 433−438. (23) Sze, S. M. Physics of Semiconductor Devices, 3rd ed.; John Wiley and Sons: Hoboken, 2007. (24) : Sze, S. M. Semiconductor Devices: Physics and Technology, 2nd ed.; John Wiley and Sons: Hoboken, 2002. (25) Crowell, C. R. The Richardson Constant for Thermionic Emission in Schottky Barrier Diodes. Solid-State Electron. 1965, 8, 395−399. (26) Márquez, R.; Rincón, C. Defect Physics of Ternary Chalcopyrite Semiconductors. Mater. Lett. 1999, 40, 66−70. (27) Hsiao, K. J.; Liu, J. D.; Hsieh, H. H.; Jiang, T. S. Electrical Impact of MoSe2 on CIGS Thin-Film Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 18174−18178. (28) Umehara, T.; Nakada, K.; Yamada, A. Impact of Roll-overshaped Current−Voltage Characteristics and Device properties of Ag(In,Ga)Se2 Solar Cells. Jpn. J. Appl. Phys. 2017, 56, No. 012302.

Industrial Technology Development Organization (NEDO), Japan.

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REFERENCES

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Mo Interfaces in Solar Cells - American

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