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New technologies for CIGS photovoltaics
Solar Energy 77 (2004) 785–793 www.elsevier.com/locate/solener
New technologies for CIGS photovoltaics Alan E. Delahoy *, Liangfan Chen, Masud Akhtar, Baosheng Sang, Sheyu Guo Energy Photovoltaics, Inc., 276 Bakers Basin Road, Lawrenceville, NJ 08648, USA Received 10 March 2004; received in revised form 14 August 2004; accepted 17 August 2004 Available online 21 September 2004 Communicated by: Associate Editor T.M. Razykov
Abstract This paper describes a new process for forming Cu(In,Ga)Se2 (CIGS) by vacuum processing. It is termed the hybrid process, and it involves thermal delivery of In, Ga, and Se and sputtering of Cu. The optimization of the process is described, followed by its successful application to large area processing of CIGS devices and modules. Other processing issues in module formation are also discussed and analyzed, including deposition of the buffer and window layers, and interconnect resistance. The paper also describes the application of a new sputtering process for compound film formation to the area of CIGS processing. This process is termed reactive environment sputtering, and is based on a hollow cathode discharge. The process is applied to the formation of transparent conducting oxides (TCO) such as ZnO:Al, ZnO:B, In2O3:Mo (IMO) and In2O3:Ti (ITiO), and to their use as window layers for CIGS devices. 2004 Elsevier Ltd. All rights reserved. Keywords: Cu( In,Ga)Se2; CIGS; Hybrid process; Thin films; Photovoltaics; Cu sputtering; Hollow cathode sputtering; Transparent conductors; Window layers; PV modules
1. Introduction While the three leading thin-film PV technologies (Cu(In,Ga)Se2, CdTe, and a-Si:H) have advanced considerably in the last few years, it is fair to say that all have yet to establish a strong commercial base. The driving forces for CIGS are compelling: potentially high efficiency and low specific energy for production. In support of these claims we cite, first, the record thin-film solar cell efficiency of 19.2% (0.41 cm2) achieved by NREL using CIGS formed by a three-stage, co-evapora-
*
Corresponding author. Fax: +1 609 587 5355. E-mail address: [email protected] (A.E. Delahoy).
tion method (Ramanathan et al., 2003). Secondly, the energy consumed in manufacturing CIGS modules is reported to be 11 MJ/Wp (Knapp and Jester, 2000), a figure that appears to be the lowest of any PV technology. To these motivating factors we may add the broadly advantageous properties of thin-film PV relative to wafer-based PV: monolithic design leading to reduced parts handling, low consumption of both direct and indirect materials, and fewer process steps. These features should result in a low cost per watt and per kilowatt-hour of electricity produced. Yet from the limited commercial success it may be concluded that, for each thin film technology, some subset of factors drawn from manufacturing cost, yield, durability, stability, efficiency, investment, and marketing continue to hamper the manufacturing and the establishment of viable mar-
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.08.012
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Nomenclature ALD AR CBD CIGS Eff EVA FF HC ICP IMO ITiO Jsc
atomic layer deposition anti-reflection chemical bath deposition copper indium gallium diselenide efficiency ethylene vinyl acetate fill factor hollow cathode inductively coupled plasma molybdenum-doped indium oxide titanium-doped indium oxide short-circuit current density
ket segments for such products. In the case of CIGS, the science and technology is surprisingly complex, and the industry continues to struggle with economical and reliable means for forming device quality CIGS, for preparing a high quality transparent conducting oxide that can serve as a window layer, and device integration to form modules. In short, the technology is still evolving. For forming the active CIGS layer, vacuum deposition offers a safe and, in principle, controllable method. The NREL process involved evaporation of the elements from point sources. However, predictable, uniform, and fast delivery over large areas of the particular elements involved, conjoined with a satisfactory compound formation process capable of producing high quality CIGS, remains a challenging task. Energy Photovoltaics, Inc. (‘‘EPV’’) has previously developed vacuum equipment designed for heating and coating large area moving substrates. The substrates are typically low cost soda-lime glass, and materials (e.g. Cu, In, Ga, Se) can be supplied to the moving substrates using the novel linear thermal source technology that has been developed at EPV (Delahoy et al., 1996a, 2000). The use of elemental selenium rather than toxic H2Se gas helps make for a safe manufacturing environment. These choices concerning film deposition, substrates, and source materials help to minimize the processing costs of CIGS. In this paper we report the use of planar magnetron sputtering to supply the Cu and, as previously described, linear thermal sources to supply the In, Ga, and Se. The technique is applied to both stationary and moving glass substrates. The motivation for this work was the need to improve the ease and uniformity of Cu delivery (as compared to delivery by linear source evaporation) in the case of large area coating of moving substrates. We term the process of forming CIGS using both thermal evaporation and sputtering the hybrid process. The paper outlines the process and presents improved, although still imperfect, results for the spatial uniformity and compo-
J–V current density–voltage PID proportional-integral-derivative Pmax power at maximum power point Rd dynamic deposition rate RE-HCS reactive environment, hollow cathode sputtering QE quantum efficiency TCO transparent conducting oxide Voc open-circuit voltage ZIS zinc indium selenide g conversion efficiency qc specific contact resistance
sition of large area CIGS as determined by Inductively coupled plasma optical emission spectroscopy (ICP) analysis of hybrid CIGS. The process is then applied to module fabrication and optimization. This paper also discusses the preparation and properties of the window layer used in CIGS devices. The window layer most commonly used is the TCO ZnO:Al. These films can be prepared by RF or pulsed DC sputtering of ceramic targets consisting of ZnO with about 2 wt.% Al2O3 (Delahoy et al., 1996b). However, the targets are costly, and the power density that can be applied is limited. Alternatively, reactive magnetron sputtering from Zn:Al targets can be used, although this needs closed loop control to maintain operation in the metal/ oxide transition mode (Delahoy et al., 1996b; Szyszka et al., 2001). With the goal of avoiding the above shortcomings, we have investigated the production of compound films, and doped TCOs in particular, by reactive environment, hollow cathode sputtering or RE-HCS (Delahoy et al., 2004a). We here report the use of hollow cathode sputtering as a means of TCO preparation and low damage deposition. Using this method, ZnO, ZnO:Al, ZnO:B, In2O3:Mo, and In2O3:Ti were produced and applied to CIGS.
2. Baseline process of the CIGS line at EPV EPVÕs current CIGS module is 965 mm · 445 mm (0.43 m2) and consists of 71 monolithically integrated cells. An elementary cell consists of Mo/CIGS/ CdS/i-ZnO/n-ZnO. The fabrication sequence is the following: • The substrate, consisting of 3 mm thick soda-lime glass, is washed.
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 10 ZIS 503
Voc (mV)
0
-2
Current density (mA cm )
• A layer for enhancing adhesion and a bi-layer Mo back contact are deposited by sputtering. • The back contact is linearly patterned (P1) by laser scribing to define the cell areas. • A CIGS absorber layer is formed by the hybrid process (outlined below). • A post-treatment of the CIGS layer is performed, and a thin film buffer layer consisting of CdS is applied by chemical bath deposition. • A resistive layer of ZnO (i-ZnO) is sputtered. • The CIGS layer is patterned (P2) by mechanical scribing to give access to the Mo for cell interconnection. • A window layer (TCO) consisting of Al-doped ZnO is deposited by sputtering. • The ZnO is separated by mechanical scribing (P3) to complete the cell definition. • The module is completed through edge deletion, bonding of electrical contacts, lamination of a lowiron cover glass using EVA, application of connectors and wires, light soaking, and testing.
787
-2
Jsc (mA cm ) 29.7 FF (%) 67.4 Eff. (%) 10.1
CdS (R&D) CdS (pilot) 569 554 32.3 73.5 13.5
30.7 70.6 12.0
-10
-20
pilot line
evap CIGS/ZIS
hybrid CIGS/CdS R&D
-30
-40 -0.2
0.0
0.2
0.4
0.6
Fig. 1. J–V curves of cells discussed in the text: hybrid CIGS produced in R&D and in pilot line (with CdS buffers), and evaporated CIGS with ZIS buffer.
1.0 H212-2
To prepare CIGS by the hybrid process, an (InxGa1 x)2Se3 layer is formed by delivering In, Ga, and Se to a heated substrate, and then a Cu layer is stacked by magnetron sputtering. The stacked Cu/ (InxGa1 x)2Se3 layer is treated in a Se atmosphere (selenization) followed by delivery of In, Ga, and Se to complete the CIGS deposition. The formation process is a sequential one. High quality CIGS has been produced by the hybrid process with Cu sputtered both ex situ and in situ. Using a standard R&D scale multisource evaporator and stationary substrates, a cell efficiency of 13.5% (Voc 569 mV; Jsc 32.3 mA/cm2; FF 73.5%) was achieved using the hybrid process with the Cu sputtered ex situ (Delahoy et al., 2003). The J–V curve for this cell is shown in Fig. 1. In this figure, the Jsc values are those obtained by integration of the product of the external quantum efficiency and the AM1.5 global solar photon flux normalized to 1000 W/m2. The measured external quantum efficiency (active area basis) versus wavelength for a hybrid process cell without AR coating is shown in Fig. 2. The device shows very good carrier collection and a high peak QE stemming from the reduced reflection resulting from the roughness of the CIGS. The band gap of the CIGS is about 1.11 eV, as determined by extrapolation of the tangent at the inflection point of the QE curve. By substituting linear source evaporation for open-boat evaporation, the process was successfully implemented on large area (0.43 m2) moving substrates in our CIGS pilot line.
Quantum efficiency
0.9
3. Hybrid process for CIGS formation
0.8
Voltage (V)
0.8 0.7 0.6 0.5
2
Jsc = 33.0 mA/cm
0.4 0.3 0.2 0.1 0.0 400
500
600
700
800
900
1000
1100
1200
Wavelength (nm)
Fig. 2. Quantum efficiency curve for hybrid CIGS cell.
Careful control of composition and of deposition temperature profiles are critical factors in making high quality CIGS films. Therefore, the influences of Cu/ (In + Ga) ratio and selenization temperature on device performance were investigated. A (InxGa1 x)2Se3 layer was deposited onto a 965 mm · 445 mm plate which was cut into several 70 mm · 300 mm strips for the experiment. Two kinds of Cu/(InxGa1 x)2Se3 precursor, one Cu poor (Cu/(In + Ga) = 0.87) and the other Cu rich (Cu/(In + Ga) = 1.27), were prepared using these strips. To complete CIGS formation, the strips were processed at different selenization temperatures, followed by delivery of last stage In, Ga, Se on to all strips except one. The processing of this one piece ended with selenization. Small CIGS cells having an area of 0.2 cm2 were prepared on all strips using standard processing. Listed in Table 1 is a summary of the composition of the Cu/(InxGa1 x)2Se3 precursors and of the CIGS films as measured by ICP, and the I–V parameters of the
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Table 1 Formation and composition of CIGS films and J–V parameters of corresponding devices Sample
Cu/(In + Ga) (precursor)
Selenization temp. [C]
Cu/(In + Ga) (CIGS)
Ga/(In + Ga)
Voc [mV]
FF [%]
Jsc (QE) [mA/cm2]
g [%]
Z1559-2a Z1559-1 Z1559-3 Z1559-5 Z1559-6 Z1559-7
0.87 0.87 0.87 1.27 1.27 1.27
550 550 525 550 525 500
0.86 0.63 0.65 0.72 0.68 0.72
0.31 0.31 0.30 0.28 0.26 0.28
351 483 499 558 529 525
52.4 64.4 62.2 72.7 64.7 68.9
27.2 26.5 26.9 26.9 23.1 22.9
5.0 8.2 8.4 10.9 7.9 8.3
a
Last stage In, Ga, Se omitted.
resulting small area cells. The results in Table 1 clearly show that: (1) CIGS film Z1559-2, completed by selenization and without a last In, Ga, Se delivery, does not yield good devices. Most of the devices in this sample exhibit shunting in their dark and light I–V behavior which implies that a conductive Cu–Se phase might have emerged on the surface and grain boundaries of the CIGS, constituting shunting paths even though the total Cu/(In + Ga) ratio is only 0.86; (2) Devices on CIGS films Z1559-5,6,7 made from precursors that passed through a Cu-rich period (‘‘Cu-rich precursors’’) demonstrate noticeably higher Voc; (3) Devices with Cu-rich precursors selenized at 550 C (Z1559-5) show higher efficiencies than those selenized at 525 and 500 C, while there was no obvious performance difference for different selenization temperatures for Cu-poor precursors (Z1559-1,3). In NRELÕs 3-stage process, achievement of a Cu-rich film at the end of the second stage Cu–Se delivery onto a (InxGa1 x)2Se3 precursor is thought to result in a CuxSe liquid phase that aids grain growth, yielding large and columnar morphologies and better devices (Contreras et al., 1994). This requires a Cu-rich stage before the third stage In, Ga, Se delivery as well as a high substrate temperature (>523 C for a CuxSe liquid phase). Our results obtained with the hybrid process agree well with the ideas and reported results of CIGS growth by a three-stage process. This investigation clearly suggests that a Cu-rich precursor having a Cu ratio 1.2–1.3 after the sputtered Cu step and a high selenization temperature are needed in order to fabricate good devices. As a result of improving the temperature control of the linear sources and of the substrate, and applying the results of the above study concerning composition and deposition temperature effects, we can reproducibly make CIGS films of acceptable to good quality. The J–V curve and PV parameters for a diagnostic cell made using the hybrid process on CIGS from our large area (pilot) line is also shown in Fig. 1. Its efficiency is 12.0% (Voc 554 mV; Jsc 30.7 mA/cm2; FF 70.6%) without a grid or AR coating. This result was obtained by further raising the peak deposition temperature to 580 C. Cells with Voc up to 636 mV have been produced using
the hybrid process. As is well known, a high voltage, low current combination is favorable for module performance.
4. Module processing and diagnostics The performance of small area cells benefits from the layer thicknesses being approximately constant over the dimensions of the cell and from the short distance (1 mm) that current entering the ZnO has to travel to reach a grid line or metallization. In contrast, a successful module fabrication process demands that attention be paid to: (1) the uniformity of each layer; (2) more controllable and repeatable processing; (3) the quality of the ZnO; (4) the scribing processes; and (5) the ZnO/Mo interconnect resistance. CIGS uniformity is a key issue in improving module performance. For the hybrid process, both the thickness of the sputtered Cu and the equivalent thickness of the In and Ga evaporations should be uniform along the source direction. Both types of source are mounted transversely to the direction of motion of the plate. An advantage of the hybrid process is that the thickness of the sputtered Cu is easily and precisely controlled through the scan speed of the plate and the power applied to the target. Moreover, the uniformity of the Cu thickness along the long axis of the cathode is usually quite good. The thickness distribution of a finished CIGS film in the direction of the sources is shown in Fig. 3. A decrease in thickness near the long edges of the plate can be seen. Also shown in Fig. 3 is the thickness distribution obtained for Cu. By analyzing the composition of the CIGS using ICP we obtain the ratios Cu/ (In + Ga) and Ga/(In + Ga) from which the ratios Ga/ Cu and In/Cu pertinent to the Ga and In linear sources can be calculated. These results are shown in Table 2. It is concluded that the CIGS thickness non-uniformity results mainly from the In distribution. The necessary corrective actions are being planned. In the absence of flux monitoring, the predictability and constancy of the In and Ga fluxes depends upon the ability to control the temperature of the thermal
Thickness: CIGS and Cu x 10 (µm)
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 2.5
0
5
10
15
0
Cu (x 10)
789 20
25
30
Top of Plate 0.47
2.0
5
CIGS (Z1542)
0.52 0.51
1.5
0.52 0.51
0.49 0.48 0.50
0.48 0.47
10
0.48
1.0
0.50 0.51
0.49
15
0.5
0.50
0.52
0.0
20 0
5
10
15
20
25
30
35
40
0.51
45
0.51 0.50
Position (cm) 25
Fig. 3. Thickness distribution of a hybrid CIGS film measured along the source direction, and a typical sputtered Cu distribution.
0.52
0.52
0.51 0.50
0.52
0.47 0.48 0.49 0.50 0.51 0.52 0.53
0.49 0.48
30
Fig. 4. CdS/TCO/glass transmission (at 420 nm) mapped over 30 · 30 cm2 plate. Table 2 Composition of CIGS film (Z1542 in Fig. 3) along the source direction Position (cm)
Cu/(In + Ga)
Ga/(In + Ga)
Ga/Cu
In/Cu
7 18 28 38
0.92 0.87 0.84 0.95
0.31 0.28 0.27 0.30
0.33 0.32 0.32 0.32
0.75 0.83 0.86 0.74
sources. The substitution of PID temperature control for the previous simpler control scheme was found to improve the repeatability of our CIGS processing. We have previously reported re-optimization of the CdS buffer layer deposition specifically for hybrid CIGS (Delahoy et al., 2003). New parameters for the Cd salt, S/Cd ratio (thiourea/Cd salt ratio), bath temperature, and time were arrived at. Depositing a uniform CdS buffer layer of the optimum thickness is naturally important for maintaining good device efficiency over a large area module. Since the buffer layer is a thin film of about 50–100 nm in thickness, it is difficult to measure the thickness directly. Therefore, in order to characterize the deposition, we measure the optical transmission of the CdS film deposited on a TCO-coated glass. This gives information about the thickness of the film and the reproducibility of the process. Fig. 4 shows an example of a transmission contour map for a 930 cm2 (1 ft2) CdS/TCO glass sample. Using such maps, we have adjusted the deposition system and deposition conditions to improve the film uniformity. The resistivity and spectral transmittance of the window layer are additional factors that reduce the efficiency of modules below that of small area cells. These effects have been modeled by several authors (see, for example, Burgelman and Niemegeers, 1998). Moreover,
the sheet resistance of ZnO:Al deposited on CdS is found to be higher than that of films co-deposited on glass. The increase may range from 30% to 100%, and seems to depend on the method of ZnO:Al deposition. EPV has very recently improved its large area ZnO:Al, and a sheet resistance of 15 X/sq. can now be obtained on CdS/CIGS while maintaining an average transmission of 88%. On glass, the resistivity is about 9 · 10 4 X cm. Modeling indicates that the power loss (I2R dissipation) in the TCO at 15 X/sq. is about 14%, compared to about 21% at 30 X/sq., and is manifested, of course, in a reduced fill factor for the module relative to that of a small test cell. The three patterning operations P1, P2, P3 are also important in realizing good module performance. Laser scribing of the Mo (P1) should result in effective electrical separation of the resulting Mo pads. Mechanical scribing of the CIGS should leave a clean, well-defined Mo line exposed. We have found that the nature and geometry of the scribing tool and the scribing pressure affect the line quality. Another consideration is minimization of the scribe spacings to reduce area loss. This trivial-sounding exercise can in reality be frustrating once it is recognized that scribes may in practice not be perfectly straight lines and that registration of scribes performed by different techniques can present unforeseen difficulties. An inexpensive digital microscope has recently been installed on the x–y table and is useful in checking scribe spacing. The last element not present in individual devices is the ZnO/Mo interconnection. The value of the specific contact resistance qc for ZnO/Mo is monitored at EPV using both special test structures and directly on modules. Remarkably, qc can vary from 2 · 10 4 to 0.2 X cm2 depending on details of the processing. The most important factors are conductivity of the ZnO
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793
5. Module performance Using the hybrid CIGS process, an efficiency approaching 10% was quickly obtained for a small module fabricated on CIGS cut from a large plate. The I–V curve for this module is shown in Fig. 5. Attention was then turned to re-starting large module fabrication. Fig. 6 shows a histogram of Voc for all cells on a 4300 cm2 plate, the cells being 93.5 cm in length. Good consistency is seen. Fig. 7 shows the I–V curve measured in sunlight for a finished module having an aperture area of 3450 cm2. The sunlight intensity was measured using a CIGS module that had been previously calibrated at NREL. The module produced 26.1 W with an aperture area efficiency of 7.5% and a Voc/cell of 538 mV. The module efficiency was limited mainly by current generation, partly by ZnO sheet resistance, and not at all by interconnect resistance. The factors responsible for limited current generation were an inadequacy in the CIGS red response, optical absorption in the TCO, and a notinsignificant dead area. As previously explained, at the end of the Cu deposition the material is Cu rich. However, the fall-off in indium near the edges of the plate means that these regions are very Cu rich. To ensure that these areas do not remain Cu rich at the end of the process, the amount of In and Ga supplied in the third stage is larger than is desirable for the middle of the plate. This distortion of the ideal recipe is partly responsible for the inferior red response of large area CIGS. In principle, the Voc/cell, FF, and aperture area Jsc obtained for the mini module of Fig. 5 should be maintainable in large modules. In reality, all three parameters for the large module of Fig. 7 were lower than for the mini-module (538 mV versus 553 mV,
0 Z1565-32-C2 (15 cell mini-module) Voc: 8.30V (553 mV/segment) Isc: 82.7 mA (Jsc: 28.15 mA/cm2 ap.) FF: 62.5% Eff: 9.74% (aperture area basis)
Current (mA)
-20
-60
-80
-100 2
4
Module ZH1646; cell length 93.5 cm
600 500 400 300 200 100 0 0
10
20
30
40
50
60
70
Cell number
Fig. 6. Histogram of Voc values for all cells of a large module.
0.2 CIGS module Z1668, outdoor meas. (data normalized to 25oC, 1000 W/m 2) Voc = 38.2 V; # cells = 71 Voc / cell = 538 mV Isc = 1.24 A FF = 55.1 % Pmax = 26.1 W
0.0 -0.2 -0.4 -0.6
A = 0.345 m2 (aperture) Eff. = 7.5 %
-0.8 -1.0 -1.2 -1.4 -10
0
10
20
30
40
50
Voltage (V)
Fig. 7. I–V curve for a large CIGS module made using the hybrid process (Pmax = 26.1 W).
55.1% versus 62.5%, and 25.5 mA/cm2 versus 28.1 mA/ cm2). We are confident that significantly higher module power will be obtained in the near future by improving both the quality of the CIGS and the uniformity of its composition.
6. New processing for the buffer layer and window layer
-40
0
700
Open-circuit voltage (mV)
and quality of the P2 scribe. At EPV, qc values < 6 · 10 4 X cm2 can now be consistently achieved.
Current (A)
790
6
8
10
Voltage (V)
Fig. 5. I–V curve for a small CIGS module with 9.7% aperture area efficiency.
Although CBD CdS continues to be the most reliable buffer layer, other materials and techniques have been explored. Atomic layer deposition (ALD) of the buffer layer, although slow, has given good cell results, especially for In2S3 buffers (Lincot et al., 2000). The success of ALD is usually attributed to its good covering power. At EPV, reasonably good results have been obtained through the use of ZnIn2Se4 (ZIS) and In2S3 as singlesource materials for direct evaporation (Delahoy et al., 2002). Using ZIS, a best cell efficiency of 10.1% was obtained (J–V curve shown in Fig. 1). However, the aver-
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 30 Z1665; CIGS/CdS/In2O3:Ti Voc: 565 mV
Current density (mA/cm2)
20
2
Jsc: 28.4 mA/cm FF: 66.8% Eff: 10.7%
10 0 -10 -20 -30 -0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Fig. 8. I–V curve for one of the worldÕs first CIGS cells having an In2O3:Ti window layer.
100
Transmittance, refl., abs. (%)
age efficiency and yield of CBD CdS continues to be superior. The window layer most commonly used for CIGS-based solar cells and modules is ZnO:Al. As a transparent conductor, ZnO:Al has limitations in its IR transmission and conductivity. Using reactive environment, hollow cathode sputtering (RE-HCS) in a pulsed power mode we have produced good transparent conductors consisting of ZnO:Al, ZnO:B, and In2O3:Mo (Delahoy et al., 2004a). We have also achieved excellent results using Ti as a dopant in In2O3 (Delahoy et al., 2004b). Such TCOs produced by RE-HCS could in principle be substituted for magnetron-sputtered ZnO:Al. The method of deposition is based on metal sputtering in a hollow cathode configuration with the working gas passing through the cathode at a high flow rate. The sputtered atoms are entrained by the working gas and exit the cathode through an elongated slot. A reactive gas (in this case oxygen) is supplied to the vicinity of the substrate, and is prevented from accessing the target by the high flow of the working gas. We have been developing processes for fabricating CIGS devices using TCOs deposited by RE-HCS. Modest but encouraging efficiencies in the range of 8–11% have been achieved. Some results are shown in Table 3. Using no buffer layer, an efficiency of 7.7% was obtained by direct deposition of ZnO onto CIGS. It may be noted that in hollow cathode sputtering, the CIGS is not immersed in the plasma. In the more conventional structure with CdS, an efficiency of 10.7% was obtained using a ZnO:B window layer prepared by RE-HCS. Such ZnO:B layers have been produced with resistivities of 5.7 · 10 4 X cm (Delahoy et al., 2004a). We also report a 10.7% cell with an In2O3:Ti (ITiO) window layer (see Fig. 8). The potential advantage of an In2O3:Mo or In2O3:Ti window layer relative to a ZnO:Al window layer of comparable sheet resistance resides in its high mobility and near-IR transmittance. Fig. 9 shows the optical transmittance, reflectance, and absorbance of a 450 nm thick, 7.1 X/sq., In2O3:Mo (IMO) film prepared by reactive environment, hollow cathode sputtering. The longwavelength fall off in transmittance does not occur until 1500 nm, compared to about 1000 nm for a 8.0 X/sq. ZnO:Al film sputtered from a ZnO:Al2O3 (2 wt.%) target. Usually, CIGS cells are still responsive in the 1000—1100 nm range, and can therefore benefit from improved window layer transmission in this wavelength
791
In2O3:Mo (IMO); 7.1 ohms/sq. (T,R,A) ZnO:Al; 8.0 ohms/sq. (T only) 80 IMO (T)
60 ZnO:Al (T) 40
IMO (A) 20
IMO (R)
0 0
500
1000
1500
2000
2500
Wavelength (nm)
Fig. 9. Optical spectra of a 7.1 X/sq. In2O3:Mo film produced by reactive-environment hollow cathode sputtering, and transmittance of a 8.0 X/sq. ZnO:Al film for comparison.
range. This particular IMO film had a resistivity of 3.2 · 10 4 X cm and a mobility of 51 cm2/V s. Both IMO and ITiO films have been prepared by RE-HCS at EPV with mobilities up to 80 cm2/V s, and minimum resistivities of 1.6 · 10 4 X cm and 1.8 · 10 4 X cm, respectively (Delahoy et al., 2004b). In contrast, the mobility of doped ZnO generally lies in the range 25–35 cm2/V s.
Table 3 Some device results obtained using hollow cathode deposition of the window layers Buffer
Window layer
i layer
Voc (mV)
Jsc (mA/cm2)
FF (%)
g (%)
None CdS CdS
ZnO ZnO:B In2O3:Ti
No Yes Yes
444 560 565
31.0 28.8 28.4
56.0 66.5 66.8
7.7 10.7 10.7
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Table 4 Comparison of sputtering-based processes for production of ZnO:Al films Method
Target
Substrate temp. (C)
Deposition ˚ s 1) rate (A
Dynamic dep. rate Rd (nm m min 1)
Resistivity (10
RF magnetron (600 W, 38 cm) Pulsed magnetron (2100 W, 56 cm) Hollow cathode (900 W, 10 cm)
Ceramic Ceramic Metal
Unheated 150 90
5.8 42.0 67.0
3.5 25.4 17.0
4.5 12.5 5.7
In Table 4 we assess the RE-HCS process for ZnO:Al relative to two other sputtering-based processes in use at EPV. The pulsed magnetron process utilizes substrate ˚ s 1 deposition rate is the translation, and the 42 A instantaneous rate averaged over the 10 cm target width. The data for the hollow cathode process was obtained with a cavity 10 cm long and 2.5 cm wide, and at a sputtering power of 900 W. The dynamic deposition rate Rd obtained with moving substrates is defined as film thickness multiplied by substrate speed. It can be seen that the hollow cathode process offers high film quality, low target cost, and a dynamic deposition rate for a single cavity of 17 nm m min 1 that approaches that currently used in the pulsed magnetron process (25 nm m min 1) and easily exceeds that of the RF magnetron process (3.5 nm m min 1). EPV is consequently exploring the scale up of this technology.
7. Conclusions Hybrid CIGS processing and reactive environment, hollow cathode sputtering represent two new technologies that have been applied to thin-film CIGS photovoltaics. The hybrid process for CIGS formation involves thermal delivery of In, Ga, and Se and sputtering of Cu. The process shows good reproducibility, and shows promise for being manufacturable. The highest efficiency devices were those for which the precursor passed through a Cu-rich stage and for which the peak processing temperature reached at least 550 C. Champion cell and mini-module efficiencies were 13.5% and 9.7%, respectively. It is not yet known whether the process is capable of producing the highest quality CIGS. The early use of this process in module fabrication has so far yielded a Voc per cell of up to 567 mV and a power of 26.1 W (7.5% aperture area efficiency at 3540 cm2). A ZnO:Al sheet resistance of 15 X/sq. (on CdS) and a specific contact resistivity for the ZnO/Mo interconnect of < 6 · 10 4 X cm2 are routinely achieved. The application of RE-HCS to CIGS technology has just started to be explored. The method uses a linear and scalable hollow cathode sputtering source. The key advantage of this method is the ability to sputter in a fully metallic mode while forming compounds on the substrate. The method utilizes low cost metal targets
4
X cm)
and offers intrinsic process stability. The method was applied to the formation of TCOs, including ZnO:Al, ZnO:B, and high mobility In2O3:Mo (IMO) and In2O3: Ti (ITiO), and to their use as window layers for CIGS devices. A CIGS/CdS/ITiO device was reported for the first time. It is hoped that these and other new technologies will, with further development, revitalize CIGS production.
Acknowledgments We would like to thank J. Cambridge, R. Govindarajan, S. Kleindienst, R. Saramak, and F. Ziobro for technical assistance. The mobility and optical spectra of IMO were measured by Y. Yoshida (Colorado School of Mines) and T.J. Coutts (NREL). ITiO films were characterized by D. Ginley and co-workers (NREL). The CIGS work was supported in part by the U.S. DOE under subcontract No. ZDJ-2-30630-21 with NREL, and the initial development of hollow cathode sputtering by NIST under ATP award 70NANB0H3031.
References Burgelman, M., Niemegeers, A., 1998. Calculation of CIS and CdTe module efficiencies. Sol. Energy Mater. Sol. Cells 51, 129–143. Contreras, M.A., Tuttle, J., Gabor, A., Tennant, A., Ramanathan, K., Asher, S., Franz, A., Keane, J., Wang, L., Scofield, J., Noufi, R., 1994. High efficiency Cu(In,Ga)Se2based solar cells: processing of novel absorber structures. In: Proceedings of First WCPEC, Hawaii, pp. 68–75. Delahoy, A.E., Britt, J.S., Gabor, A.M., Kiss, Z.J., 1996a. Large-Area, Thin-film CIS deposition and module fabrication. In: AIP Conf. Proc., vol. 353, pp. 3–11. Delahoy, A.E., Cherny, M., 1996b. Deposition schemes for low cost transparent conductors for photovoltaics. In: Mat. Res. Soc. Symp. Proc., vol. 426, pp. 467–477. Delahoy, A., Bruns, J., Chen, L., Akhtar, M., Kiss, Z., 2000. Advances in large area CIGS technology. In: Proceedings of 28th IEEE PV Specialists Conference, Anchorage, AK, pp. 1437–1440. Delahoy, A.E., Akhtar, M., Cambridge, J., Chen, L., Govindarajan, R., Guo, S., Romero, M.J., 2002. CIGS devices with
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 ZIS, In2S3, and CdS buffer layers. In: Proceedings of 29th IEEE PV Specialists Conference, New Orleans, LA, pp. 640–643. Delahoy, A.E., Chen, L., Akhtar, M., Cambridge, J., Govindarajan, R., Ziobro, F., 2003. Advanced CIGS technology. In: Proceedings of NCPV and Solar Program Review Meeting, Denver, CO, pp. 882–885. Delahoy, A.E., Guo, S.Y., Paduraru, C., Belkind, A., 2004a. Reactive-environment, hollow cathode sputtering: basic characteristics and application to Al2O3, doped ZnO, and In2O3:Mo. J. Vac. Sci. Technol. A 22 (4), 1697–1704. Delahoy, A.E., et al., 2004b. Presented at 19th European PVSEC, Paris, June. Knapp, K.E., Jester, T.L., 2000. Initial empirical results for the energy payback time of photovoltaic modules. In: Proceed-
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ings of 16th European PVSEC, Glasgow, Scotland, pp. 2053–2056. Lincot, D., Yousfi, E.B., Canava, B., Cowache, P., Guillemoles, J.F., Powalla, M., Hariskos, D., Schock, H.-W., 2000. Interfacial engineering of Cu(In,Ga)Se2 thin film solar cells based on atomic layer epitaxy. In: Proceedings of 16th European PVSEC, Glasgow, Scotland, pp. 801–804. Ramanathan, K., Contreras, M., Perkins, C., Asher, S., Hasoon, F., Keane, J., Young, D., Romero, M., Metzger, W., Noufi, R., Ward, J., Duda, A., 2003. Properties of 19.2% efficiency ZnO/CdS/CuInGaSe2 thin-film solar cells. Prog. Photovolt: Res. Appl. 11, 225–230. Szyszka, B., Ho¨ing, T., Jiang, X., Bierhals, A., Malkomes, N., Vergo¨hl, M., Sittinger, V., Bringmann, U., Bra¨uer, G., 2001. In: Proceedings of 44th SVC Annual Technical Conference, p. 272.
New technologies for CIGS photovoltaics Alan E. Delahoy *, Liangfan Chen, Masud Akhtar, Baosheng Sang, Sheyu Guo Energy Photovoltaics, Inc., 276 Bakers Basin Road, Lawrenceville, NJ 08648, USA Received 10 March 2004; received in revised form 14 August 2004; accepted 17 August 2004 Available online 21 September 2004 Communicated by: Associate Editor T.M. Razykov
Abstract This paper describes a new process for forming Cu(In,Ga)Se2 (CIGS) by vacuum processing. It is termed the hybrid process, and it involves thermal delivery of In, Ga, and Se and sputtering of Cu. The optimization of the process is described, followed by its successful application to large area processing of CIGS devices and modules. Other processing issues in module formation are also discussed and analyzed, including deposition of the buffer and window layers, and interconnect resistance. The paper also describes the application of a new sputtering process for compound film formation to the area of CIGS processing. This process is termed reactive environment sputtering, and is based on a hollow cathode discharge. The process is applied to the formation of transparent conducting oxides (TCO) such as ZnO:Al, ZnO:B, In2O3:Mo (IMO) and In2O3:Ti (ITiO), and to their use as window layers for CIGS devices. 2004 Elsevier Ltd. All rights reserved. Keywords: Cu( In,Ga)Se2; CIGS; Hybrid process; Thin films; Photovoltaics; Cu sputtering; Hollow cathode sputtering; Transparent conductors; Window layers; PV modules
1. Introduction While the three leading thin-film PV technologies (Cu(In,Ga)Se2, CdTe, and a-Si:H) have advanced considerably in the last few years, it is fair to say that all have yet to establish a strong commercial base. The driving forces for CIGS are compelling: potentially high efficiency and low specific energy for production. In support of these claims we cite, first, the record thin-film solar cell efficiency of 19.2% (0.41 cm2) achieved by NREL using CIGS formed by a three-stage, co-evapora-
*
Corresponding author. Fax: +1 609 587 5355. E-mail address: [email protected] (A.E. Delahoy).
tion method (Ramanathan et al., 2003). Secondly, the energy consumed in manufacturing CIGS modules is reported to be 11 MJ/Wp (Knapp and Jester, 2000), a figure that appears to be the lowest of any PV technology. To these motivating factors we may add the broadly advantageous properties of thin-film PV relative to wafer-based PV: monolithic design leading to reduced parts handling, low consumption of both direct and indirect materials, and fewer process steps. These features should result in a low cost per watt and per kilowatt-hour of electricity produced. Yet from the limited commercial success it may be concluded that, for each thin film technology, some subset of factors drawn from manufacturing cost, yield, durability, stability, efficiency, investment, and marketing continue to hamper the manufacturing and the establishment of viable mar-
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.08.012
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Nomenclature ALD AR CBD CIGS Eff EVA FF HC ICP IMO ITiO Jsc
atomic layer deposition anti-reflection chemical bath deposition copper indium gallium diselenide efficiency ethylene vinyl acetate fill factor hollow cathode inductively coupled plasma molybdenum-doped indium oxide titanium-doped indium oxide short-circuit current density
ket segments for such products. In the case of CIGS, the science and technology is surprisingly complex, and the industry continues to struggle with economical and reliable means for forming device quality CIGS, for preparing a high quality transparent conducting oxide that can serve as a window layer, and device integration to form modules. In short, the technology is still evolving. For forming the active CIGS layer, vacuum deposition offers a safe and, in principle, controllable method. The NREL process involved evaporation of the elements from point sources. However, predictable, uniform, and fast delivery over large areas of the particular elements involved, conjoined with a satisfactory compound formation process capable of producing high quality CIGS, remains a challenging task. Energy Photovoltaics, Inc. (‘‘EPV’’) has previously developed vacuum equipment designed for heating and coating large area moving substrates. The substrates are typically low cost soda-lime glass, and materials (e.g. Cu, In, Ga, Se) can be supplied to the moving substrates using the novel linear thermal source technology that has been developed at EPV (Delahoy et al., 1996a, 2000). The use of elemental selenium rather than toxic H2Se gas helps make for a safe manufacturing environment. These choices concerning film deposition, substrates, and source materials help to minimize the processing costs of CIGS. In this paper we report the use of planar magnetron sputtering to supply the Cu and, as previously described, linear thermal sources to supply the In, Ga, and Se. The technique is applied to both stationary and moving glass substrates. The motivation for this work was the need to improve the ease and uniformity of Cu delivery (as compared to delivery by linear source evaporation) in the case of large area coating of moving substrates. We term the process of forming CIGS using both thermal evaporation and sputtering the hybrid process. The paper outlines the process and presents improved, although still imperfect, results for the spatial uniformity and compo-
J–V current density–voltage PID proportional-integral-derivative Pmax power at maximum power point Rd dynamic deposition rate RE-HCS reactive environment, hollow cathode sputtering QE quantum efficiency TCO transparent conducting oxide Voc open-circuit voltage ZIS zinc indium selenide g conversion efficiency qc specific contact resistance
sition of large area CIGS as determined by Inductively coupled plasma optical emission spectroscopy (ICP) analysis of hybrid CIGS. The process is then applied to module fabrication and optimization. This paper also discusses the preparation and properties of the window layer used in CIGS devices. The window layer most commonly used is the TCO ZnO:Al. These films can be prepared by RF or pulsed DC sputtering of ceramic targets consisting of ZnO with about 2 wt.% Al2O3 (Delahoy et al., 1996b). However, the targets are costly, and the power density that can be applied is limited. Alternatively, reactive magnetron sputtering from Zn:Al targets can be used, although this needs closed loop control to maintain operation in the metal/ oxide transition mode (Delahoy et al., 1996b; Szyszka et al., 2001). With the goal of avoiding the above shortcomings, we have investigated the production of compound films, and doped TCOs in particular, by reactive environment, hollow cathode sputtering or RE-HCS (Delahoy et al., 2004a). We here report the use of hollow cathode sputtering as a means of TCO preparation and low damage deposition. Using this method, ZnO, ZnO:Al, ZnO:B, In2O3:Mo, and In2O3:Ti were produced and applied to CIGS.
2. Baseline process of the CIGS line at EPV EPVÕs current CIGS module is 965 mm · 445 mm (0.43 m2) and consists of 71 monolithically integrated cells. An elementary cell consists of Mo/CIGS/ CdS/i-ZnO/n-ZnO. The fabrication sequence is the following: • The substrate, consisting of 3 mm thick soda-lime glass, is washed.
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 10 ZIS 503
Voc (mV)
0
-2
Current density (mA cm )
• A layer for enhancing adhesion and a bi-layer Mo back contact are deposited by sputtering. • The back contact is linearly patterned (P1) by laser scribing to define the cell areas. • A CIGS absorber layer is formed by the hybrid process (outlined below). • A post-treatment of the CIGS layer is performed, and a thin film buffer layer consisting of CdS is applied by chemical bath deposition. • A resistive layer of ZnO (i-ZnO) is sputtered. • The CIGS layer is patterned (P2) by mechanical scribing to give access to the Mo for cell interconnection. • A window layer (TCO) consisting of Al-doped ZnO is deposited by sputtering. • The ZnO is separated by mechanical scribing (P3) to complete the cell definition. • The module is completed through edge deletion, bonding of electrical contacts, lamination of a lowiron cover glass using EVA, application of connectors and wires, light soaking, and testing.
787
-2
Jsc (mA cm ) 29.7 FF (%) 67.4 Eff. (%) 10.1
CdS (R&D) CdS (pilot) 569 554 32.3 73.5 13.5
30.7 70.6 12.0
-10
-20
pilot line
evap CIGS/ZIS
hybrid CIGS/CdS R&D
-30
-40 -0.2
0.0
0.2
0.4
0.6
Fig. 1. J–V curves of cells discussed in the text: hybrid CIGS produced in R&D and in pilot line (with CdS buffers), and evaporated CIGS with ZIS buffer.
1.0 H212-2
To prepare CIGS by the hybrid process, an (InxGa1 x)2Se3 layer is formed by delivering In, Ga, and Se to a heated substrate, and then a Cu layer is stacked by magnetron sputtering. The stacked Cu/ (InxGa1 x)2Se3 layer is treated in a Se atmosphere (selenization) followed by delivery of In, Ga, and Se to complete the CIGS deposition. The formation process is a sequential one. High quality CIGS has been produced by the hybrid process with Cu sputtered both ex situ and in situ. Using a standard R&D scale multisource evaporator and stationary substrates, a cell efficiency of 13.5% (Voc 569 mV; Jsc 32.3 mA/cm2; FF 73.5%) was achieved using the hybrid process with the Cu sputtered ex situ (Delahoy et al., 2003). The J–V curve for this cell is shown in Fig. 1. In this figure, the Jsc values are those obtained by integration of the product of the external quantum efficiency and the AM1.5 global solar photon flux normalized to 1000 W/m2. The measured external quantum efficiency (active area basis) versus wavelength for a hybrid process cell without AR coating is shown in Fig. 2. The device shows very good carrier collection and a high peak QE stemming from the reduced reflection resulting from the roughness of the CIGS. The band gap of the CIGS is about 1.11 eV, as determined by extrapolation of the tangent at the inflection point of the QE curve. By substituting linear source evaporation for open-boat evaporation, the process was successfully implemented on large area (0.43 m2) moving substrates in our CIGS pilot line.
Quantum efficiency
0.9
3. Hybrid process for CIGS formation
0.8
Voltage (V)
0.8 0.7 0.6 0.5
2
Jsc = 33.0 mA/cm
0.4 0.3 0.2 0.1 0.0 400
500
600
700
800
900
1000
1100
1200
Wavelength (nm)
Fig. 2. Quantum efficiency curve for hybrid CIGS cell.
Careful control of composition and of deposition temperature profiles are critical factors in making high quality CIGS films. Therefore, the influences of Cu/ (In + Ga) ratio and selenization temperature on device performance were investigated. A (InxGa1 x)2Se3 layer was deposited onto a 965 mm · 445 mm plate which was cut into several 70 mm · 300 mm strips for the experiment. Two kinds of Cu/(InxGa1 x)2Se3 precursor, one Cu poor (Cu/(In + Ga) = 0.87) and the other Cu rich (Cu/(In + Ga) = 1.27), were prepared using these strips. To complete CIGS formation, the strips were processed at different selenization temperatures, followed by delivery of last stage In, Ga, Se on to all strips except one. The processing of this one piece ended with selenization. Small CIGS cells having an area of 0.2 cm2 were prepared on all strips using standard processing. Listed in Table 1 is a summary of the composition of the Cu/(InxGa1 x)2Se3 precursors and of the CIGS films as measured by ICP, and the I–V parameters of the
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Table 1 Formation and composition of CIGS films and J–V parameters of corresponding devices Sample
Cu/(In + Ga) (precursor)
Selenization temp. [C]
Cu/(In + Ga) (CIGS)
Ga/(In + Ga)
Voc [mV]
FF [%]
Jsc (QE) [mA/cm2]
g [%]
Z1559-2a Z1559-1 Z1559-3 Z1559-5 Z1559-6 Z1559-7
0.87 0.87 0.87 1.27 1.27 1.27
550 550 525 550 525 500
0.86 0.63 0.65 0.72 0.68 0.72
0.31 0.31 0.30 0.28 0.26 0.28
351 483 499 558 529 525
52.4 64.4 62.2 72.7 64.7 68.9
27.2 26.5 26.9 26.9 23.1 22.9
5.0 8.2 8.4 10.9 7.9 8.3
a
Last stage In, Ga, Se omitted.
resulting small area cells. The results in Table 1 clearly show that: (1) CIGS film Z1559-2, completed by selenization and without a last In, Ga, Se delivery, does not yield good devices. Most of the devices in this sample exhibit shunting in their dark and light I–V behavior which implies that a conductive Cu–Se phase might have emerged on the surface and grain boundaries of the CIGS, constituting shunting paths even though the total Cu/(In + Ga) ratio is only 0.86; (2) Devices on CIGS films Z1559-5,6,7 made from precursors that passed through a Cu-rich period (‘‘Cu-rich precursors’’) demonstrate noticeably higher Voc; (3) Devices with Cu-rich precursors selenized at 550 C (Z1559-5) show higher efficiencies than those selenized at 525 and 500 C, while there was no obvious performance difference for different selenization temperatures for Cu-poor precursors (Z1559-1,3). In NRELÕs 3-stage process, achievement of a Cu-rich film at the end of the second stage Cu–Se delivery onto a (InxGa1 x)2Se3 precursor is thought to result in a CuxSe liquid phase that aids grain growth, yielding large and columnar morphologies and better devices (Contreras et al., 1994). This requires a Cu-rich stage before the third stage In, Ga, Se delivery as well as a high substrate temperature (>523 C for a CuxSe liquid phase). Our results obtained with the hybrid process agree well with the ideas and reported results of CIGS growth by a three-stage process. This investigation clearly suggests that a Cu-rich precursor having a Cu ratio 1.2–1.3 after the sputtered Cu step and a high selenization temperature are needed in order to fabricate good devices. As a result of improving the temperature control of the linear sources and of the substrate, and applying the results of the above study concerning composition and deposition temperature effects, we can reproducibly make CIGS films of acceptable to good quality. The J–V curve and PV parameters for a diagnostic cell made using the hybrid process on CIGS from our large area (pilot) line is also shown in Fig. 1. Its efficiency is 12.0% (Voc 554 mV; Jsc 30.7 mA/cm2; FF 70.6%) without a grid or AR coating. This result was obtained by further raising the peak deposition temperature to 580 C. Cells with Voc up to 636 mV have been produced using
the hybrid process. As is well known, a high voltage, low current combination is favorable for module performance.
4. Module processing and diagnostics The performance of small area cells benefits from the layer thicknesses being approximately constant over the dimensions of the cell and from the short distance (1 mm) that current entering the ZnO has to travel to reach a grid line or metallization. In contrast, a successful module fabrication process demands that attention be paid to: (1) the uniformity of each layer; (2) more controllable and repeatable processing; (3) the quality of the ZnO; (4) the scribing processes; and (5) the ZnO/Mo interconnect resistance. CIGS uniformity is a key issue in improving module performance. For the hybrid process, both the thickness of the sputtered Cu and the equivalent thickness of the In and Ga evaporations should be uniform along the source direction. Both types of source are mounted transversely to the direction of motion of the plate. An advantage of the hybrid process is that the thickness of the sputtered Cu is easily and precisely controlled through the scan speed of the plate and the power applied to the target. Moreover, the uniformity of the Cu thickness along the long axis of the cathode is usually quite good. The thickness distribution of a finished CIGS film in the direction of the sources is shown in Fig. 3. A decrease in thickness near the long edges of the plate can be seen. Also shown in Fig. 3 is the thickness distribution obtained for Cu. By analyzing the composition of the CIGS using ICP we obtain the ratios Cu/ (In + Ga) and Ga/(In + Ga) from which the ratios Ga/ Cu and In/Cu pertinent to the Ga and In linear sources can be calculated. These results are shown in Table 2. It is concluded that the CIGS thickness non-uniformity results mainly from the In distribution. The necessary corrective actions are being planned. In the absence of flux monitoring, the predictability and constancy of the In and Ga fluxes depends upon the ability to control the temperature of the thermal
Thickness: CIGS and Cu x 10 (µm)
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 2.5
0
5
10
15
0
Cu (x 10)
789 20
25
30
Top of Plate 0.47
2.0
5
CIGS (Z1542)
0.52 0.51
1.5
0.52 0.51
0.49 0.48 0.50
0.48 0.47
10
0.48
1.0
0.50 0.51
0.49
15
0.5
0.50
0.52
0.0
20 0
5
10
15
20
25
30
35
40
0.51
45
0.51 0.50
Position (cm) 25
Fig. 3. Thickness distribution of a hybrid CIGS film measured along the source direction, and a typical sputtered Cu distribution.
0.52
0.52
0.51 0.50
0.52
0.47 0.48 0.49 0.50 0.51 0.52 0.53
0.49 0.48
30
Fig. 4. CdS/TCO/glass transmission (at 420 nm) mapped over 30 · 30 cm2 plate. Table 2 Composition of CIGS film (Z1542 in Fig. 3) along the source direction Position (cm)
Cu/(In + Ga)
Ga/(In + Ga)
Ga/Cu
In/Cu
7 18 28 38
0.92 0.87 0.84 0.95
0.31 0.28 0.27 0.30
0.33 0.32 0.32 0.32
0.75 0.83 0.86 0.74
sources. The substitution of PID temperature control for the previous simpler control scheme was found to improve the repeatability of our CIGS processing. We have previously reported re-optimization of the CdS buffer layer deposition specifically for hybrid CIGS (Delahoy et al., 2003). New parameters for the Cd salt, S/Cd ratio (thiourea/Cd salt ratio), bath temperature, and time were arrived at. Depositing a uniform CdS buffer layer of the optimum thickness is naturally important for maintaining good device efficiency over a large area module. Since the buffer layer is a thin film of about 50–100 nm in thickness, it is difficult to measure the thickness directly. Therefore, in order to characterize the deposition, we measure the optical transmission of the CdS film deposited on a TCO-coated glass. This gives information about the thickness of the film and the reproducibility of the process. Fig. 4 shows an example of a transmission contour map for a 930 cm2 (1 ft2) CdS/TCO glass sample. Using such maps, we have adjusted the deposition system and deposition conditions to improve the film uniformity. The resistivity and spectral transmittance of the window layer are additional factors that reduce the efficiency of modules below that of small area cells. These effects have been modeled by several authors (see, for example, Burgelman and Niemegeers, 1998). Moreover,
the sheet resistance of ZnO:Al deposited on CdS is found to be higher than that of films co-deposited on glass. The increase may range from 30% to 100%, and seems to depend on the method of ZnO:Al deposition. EPV has very recently improved its large area ZnO:Al, and a sheet resistance of 15 X/sq. can now be obtained on CdS/CIGS while maintaining an average transmission of 88%. On glass, the resistivity is about 9 · 10 4 X cm. Modeling indicates that the power loss (I2R dissipation) in the TCO at 15 X/sq. is about 14%, compared to about 21% at 30 X/sq., and is manifested, of course, in a reduced fill factor for the module relative to that of a small test cell. The three patterning operations P1, P2, P3 are also important in realizing good module performance. Laser scribing of the Mo (P1) should result in effective electrical separation of the resulting Mo pads. Mechanical scribing of the CIGS should leave a clean, well-defined Mo line exposed. We have found that the nature and geometry of the scribing tool and the scribing pressure affect the line quality. Another consideration is minimization of the scribe spacings to reduce area loss. This trivial-sounding exercise can in reality be frustrating once it is recognized that scribes may in practice not be perfectly straight lines and that registration of scribes performed by different techniques can present unforeseen difficulties. An inexpensive digital microscope has recently been installed on the x–y table and is useful in checking scribe spacing. The last element not present in individual devices is the ZnO/Mo interconnection. The value of the specific contact resistance qc for ZnO/Mo is monitored at EPV using both special test structures and directly on modules. Remarkably, qc can vary from 2 · 10 4 to 0.2 X cm2 depending on details of the processing. The most important factors are conductivity of the ZnO
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793
5. Module performance Using the hybrid CIGS process, an efficiency approaching 10% was quickly obtained for a small module fabricated on CIGS cut from a large plate. The I–V curve for this module is shown in Fig. 5. Attention was then turned to re-starting large module fabrication. Fig. 6 shows a histogram of Voc for all cells on a 4300 cm2 plate, the cells being 93.5 cm in length. Good consistency is seen. Fig. 7 shows the I–V curve measured in sunlight for a finished module having an aperture area of 3450 cm2. The sunlight intensity was measured using a CIGS module that had been previously calibrated at NREL. The module produced 26.1 W with an aperture area efficiency of 7.5% and a Voc/cell of 538 mV. The module efficiency was limited mainly by current generation, partly by ZnO sheet resistance, and not at all by interconnect resistance. The factors responsible for limited current generation were an inadequacy in the CIGS red response, optical absorption in the TCO, and a notinsignificant dead area. As previously explained, at the end of the Cu deposition the material is Cu rich. However, the fall-off in indium near the edges of the plate means that these regions are very Cu rich. To ensure that these areas do not remain Cu rich at the end of the process, the amount of In and Ga supplied in the third stage is larger than is desirable for the middle of the plate. This distortion of the ideal recipe is partly responsible for the inferior red response of large area CIGS. In principle, the Voc/cell, FF, and aperture area Jsc obtained for the mini module of Fig. 5 should be maintainable in large modules. In reality, all three parameters for the large module of Fig. 7 were lower than for the mini-module (538 mV versus 553 mV,
0 Z1565-32-C2 (15 cell mini-module) Voc: 8.30V (553 mV/segment) Isc: 82.7 mA (Jsc: 28.15 mA/cm2 ap.) FF: 62.5% Eff: 9.74% (aperture area basis)
Current (mA)
-20
-60
-80
-100 2
4
Module ZH1646; cell length 93.5 cm
600 500 400 300 200 100 0 0
10
20
30
40
50
60
70
Cell number
Fig. 6. Histogram of Voc values for all cells of a large module.
0.2 CIGS module Z1668, outdoor meas. (data normalized to 25oC, 1000 W/m 2) Voc = 38.2 V; # cells = 71 Voc / cell = 538 mV Isc = 1.24 A FF = 55.1 % Pmax = 26.1 W
0.0 -0.2 -0.4 -0.6
A = 0.345 m2 (aperture) Eff. = 7.5 %
-0.8 -1.0 -1.2 -1.4 -10
0
10
20
30
40
50
Voltage (V)
Fig. 7. I–V curve for a large CIGS module made using the hybrid process (Pmax = 26.1 W).
55.1% versus 62.5%, and 25.5 mA/cm2 versus 28.1 mA/ cm2). We are confident that significantly higher module power will be obtained in the near future by improving both the quality of the CIGS and the uniformity of its composition.
6. New processing for the buffer layer and window layer
-40
0
700
Open-circuit voltage (mV)
and quality of the P2 scribe. At EPV, qc values < 6 · 10 4 X cm2 can now be consistently achieved.
Current (A)
790
6
8
10
Voltage (V)
Fig. 5. I–V curve for a small CIGS module with 9.7% aperture area efficiency.
Although CBD CdS continues to be the most reliable buffer layer, other materials and techniques have been explored. Atomic layer deposition (ALD) of the buffer layer, although slow, has given good cell results, especially for In2S3 buffers (Lincot et al., 2000). The success of ALD is usually attributed to its good covering power. At EPV, reasonably good results have been obtained through the use of ZnIn2Se4 (ZIS) and In2S3 as singlesource materials for direct evaporation (Delahoy et al., 2002). Using ZIS, a best cell efficiency of 10.1% was obtained (J–V curve shown in Fig. 1). However, the aver-
A.E. Delahoy et al. / Solar Energy 77 (2004) 785–793 30 Z1665; CIGS/CdS/In2O3:Ti Voc: 565 mV
Current density (mA/cm2)
20
2
Jsc: 28.4 mA/cm FF: 66.8% Eff: 10.7%
10 0 -10 -20 -30 -0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Fig. 8. I–V curve for one of the worldÕs first CIGS cells having an In2O3:Ti window layer.
100
Transmittance, refl., abs. (%)
age efficiency and yield of CBD CdS continues to be superior. The window layer most commonly used for CIGS-based solar cells and modules is ZnO:Al. As a transparent conductor, ZnO:Al has limitations in its IR transmission and conductivity. Using reactive environment, hollow cathode sputtering (RE-HCS) in a pulsed power mode we have produced good transparent conductors consisting of ZnO:Al, ZnO:B, and In2O3:Mo (Delahoy et al., 2004a). We have also achieved excellent results using Ti as a dopant in In2O3 (Delahoy et al., 2004b). Such TCOs produced by RE-HCS could in principle be substituted for magnetron-sputtered ZnO:Al. The method of deposition is based on metal sputtering in a hollow cathode configuration with the working gas passing through the cathode at a high flow rate. The sputtered atoms are entrained by the working gas and exit the cathode through an elongated slot. A reactive gas (in this case oxygen) is supplied to the vicinity of the substrate, and is prevented from accessing the target by the high flow of the working gas. We have been developing processes for fabricating CIGS devices using TCOs deposited by RE-HCS. Modest but encouraging efficiencies in the range of 8–11% have been achieved. Some results are shown in Table 3. Using no buffer layer, an efficiency of 7.7% was obtained by direct deposition of ZnO onto CIGS. It may be noted that in hollow cathode sputtering, the CIGS is not immersed in the plasma. In the more conventional structure with CdS, an efficiency of 10.7% was obtained using a ZnO:B window layer prepared by RE-HCS. Such ZnO:B layers have been produced with resistivities of 5.7 · 10 4 X cm (Delahoy et al., 2004a). We also report a 10.7% cell with an In2O3:Ti (ITiO) window layer (see Fig. 8). The potential advantage of an In2O3:Mo or In2O3:Ti window layer relative to a ZnO:Al window layer of comparable sheet resistance resides in its high mobility and near-IR transmittance. Fig. 9 shows the optical transmittance, reflectance, and absorbance of a 450 nm thick, 7.1 X/sq., In2O3:Mo (IMO) film prepared by reactive environment, hollow cathode sputtering. The longwavelength fall off in transmittance does not occur until 1500 nm, compared to about 1000 nm for a 8.0 X/sq. ZnO:Al film sputtered from a ZnO:Al2O3 (2 wt.%) target. Usually, CIGS cells are still responsive in the 1000—1100 nm range, and can therefore benefit from improved window layer transmission in this wavelength
791
In2O3:Mo (IMO); 7.1 ohms/sq. (T,R,A) ZnO:Al; 8.0 ohms/sq. (T only) 80 IMO (T)
60 ZnO:Al (T) 40
IMO (A) 20
IMO (R)
0 0
500
1000
1500
2000
2500
Wavelength (nm)
Fig. 9. Optical spectra of a 7.1 X/sq. In2O3:Mo film produced by reactive-environment hollow cathode sputtering, and transmittance of a 8.0 X/sq. ZnO:Al film for comparison.
range. This particular IMO film had a resistivity of 3.2 · 10 4 X cm and a mobility of 51 cm2/V s. Both IMO and ITiO films have been prepared by RE-HCS at EPV with mobilities up to 80 cm2/V s, and minimum resistivities of 1.6 · 10 4 X cm and 1.8 · 10 4 X cm, respectively (Delahoy et al., 2004b). In contrast, the mobility of doped ZnO generally lies in the range 25–35 cm2/V s.
Table 3 Some device results obtained using hollow cathode deposition of the window layers Buffer
Window layer
i layer
Voc (mV)
Jsc (mA/cm2)
FF (%)
g (%)
None CdS CdS
ZnO ZnO:B In2O3:Ti
No Yes Yes
444 560 565
31.0 28.8 28.4
56.0 66.5 66.8
7.7 10.7 10.7
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Table 4 Comparison of sputtering-based processes for production of ZnO:Al films Method
Target
Substrate temp. (C)
Deposition ˚ s 1) rate (A
Dynamic dep. rate Rd (nm m min 1)
Resistivity (10
RF magnetron (600 W, 38 cm) Pulsed magnetron (2100 W, 56 cm) Hollow cathode (900 W, 10 cm)
Ceramic Ceramic Metal
Unheated 150 90
5.8 42.0 67.0
3.5 25.4 17.0
4.5 12.5 5.7
In Table 4 we assess the RE-HCS process for ZnO:Al relative to two other sputtering-based processes in use at EPV. The pulsed magnetron process utilizes substrate ˚ s 1 deposition rate is the translation, and the 42 A instantaneous rate averaged over the 10 cm target width. The data for the hollow cathode process was obtained with a cavity 10 cm long and 2.5 cm wide, and at a sputtering power of 900 W. The dynamic deposition rate Rd obtained with moving substrates is defined as film thickness multiplied by substrate speed. It can be seen that the hollow cathode process offers high film quality, low target cost, and a dynamic deposition rate for a single cavity of 17 nm m min 1 that approaches that currently used in the pulsed magnetron process (25 nm m min 1) and easily exceeds that of the RF magnetron process (3.5 nm m min 1). EPV is consequently exploring the scale up of this technology.
7. Conclusions Hybrid CIGS processing and reactive environment, hollow cathode sputtering represent two new technologies that have been applied to thin-film CIGS photovoltaics. The hybrid process for CIGS formation involves thermal delivery of In, Ga, and Se and sputtering of Cu. The process shows good reproducibility, and shows promise for being manufacturable. The highest efficiency devices were those for which the precursor passed through a Cu-rich stage and for which the peak processing temperature reached at least 550 C. Champion cell and mini-module efficiencies were 13.5% and 9.7%, respectively. It is not yet known whether the process is capable of producing the highest quality CIGS. The early use of this process in module fabrication has so far yielded a Voc per cell of up to 567 mV and a power of 26.1 W (7.5% aperture area efficiency at 3540 cm2). A ZnO:Al sheet resistance of 15 X/sq. (on CdS) and a specific contact resistivity for the ZnO/Mo interconnect of < 6 · 10 4 X cm2 are routinely achieved. The application of RE-HCS to CIGS technology has just started to be explored. The method uses a linear and scalable hollow cathode sputtering source. The key advantage of this method is the ability to sputter in a fully metallic mode while forming compounds on the substrate. The method utilizes low cost metal targets
4
X cm)
and offers intrinsic process stability. The method was applied to the formation of TCOs, including ZnO:Al, ZnO:B, and high mobility In2O3:Mo (IMO) and In2O3: Ti (ITiO), and to their use as window layers for CIGS devices. A CIGS/CdS/ITiO device was reported for the first time. It is hoped that these and other new technologies will, with further development, revitalize CIGS production.
Acknowledgments We would like to thank J. Cambridge, R. Govindarajan, S. Kleindienst, R. Saramak, and F. Ziobro for technical assistance. The mobility and optical spectra of IMO were measured by Y. Yoshida (Colorado School of Mines) and T.J. Coutts (NREL). ITiO films were characterized by D. Ginley and co-workers (NREL). The CIGS work was supported in part by the U.S. DOE under subcontract No. ZDJ-2-30630-21 with NREL, and the initial development of hollow cathode sputtering by NIST under ATP award 70NANB0H3031.
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