R. Sundaramoorthy

Stability of TCO window layers for thin-film CIGS solar cells upon damp heat exposures: part III

Various transparent conducting oxide (TCO) films are used as window layers to permit light transmission, photocurrent generation, and for electrical current collection in thin-film photovoltaic technologies. Bilayer composites (BZO) of intrinsic ZnO (i-ZnO) and Al-doped ZnO (AZO) have been used on NRELs high-efficiency CuInGaSe2 (CIGS) solar cells. Previously, we demonstrated that, when tested in damp heat (DH) condition at 85°C and 85% relative humidity (RH), the stability trend of some TCOs was in decreasing order of SnO2:F > In2O3:SnO2 (ITO) > ZnO-based films of AZO, BZO and Al-doped Zn1-xMgxO (ZMO). We also observed that the degradation rate of AZO, BZO, and ZMO was influenced by additional factors such as film thickness, deposition conditions, and exposure history. This work continued our efforts in searching for a high-performance and high-stability window layer TCO, as well as in finding mitigation methods to protect the ZnO layer, either i-ZnO or BZO, for use on the CIGS solar cells. The current study, which involved the third experimental set of TCOs deposited on glass, further examined in DH test conditions the thickness effect on single-layer AZO films, the glass substrate effect on BZO, the stability and protective effect of amorphous In2O3:ZnO (InZnO or IZO) as a conducting window layer for the underlying i-ZnO, and the stability and protective power of a protective transparent metal oxide (PTMO) coating for all three types of ZnO (AZO, BZO, and i-ZnO). The samples were periodically characterized with optical, electrical, and structural measurements during the course of DH exposure. The results show that the DH stability of AZO increased as the film thickness increased, BZO on Corning® Eagle 2000™ glass degraded somehow faster than on Corning® 7059™, and both the IZO and PTMO showed generally high DH stability and good protective power for the ZnO layers underneath. However, the results of decreased (002) peak intensity of ZnO from X-ray diffraction analysis indicated that both IZO and PTMO still allowed certain levels of moisture penetration.

Imaging characterization techniques applied to Cu(In,Ga)Se2 solar cells

The authors present examples of imaging characterization on Cu(In,Ga)Se2 (CIGS) solar cell devices. These imaging techniques include photoluminescence imaging, electroluminescence imaging, illuminated lock-in thermography, and forward- and reverse-bias dark lock-in thermographies. Images were collected on CIGS devices deposited at the National Renewable Energy Laboratory with intentional spatial inhomogeneities of the material parameters. Photoluminescence imaging shows brightness variations, which correlate to the device open-circuit voltage. Photoluminescence and electroluminescence imaging on CIGS solar cells show dark spots that correspond to bright spots on images from illuminated and forward-bias lock-in thermography. These image-detected defect areas are weak diodes that conduct current under solar cell operating conditions. Shunt defects are imaged using reverse-bias lock-in thermography. The authors show how imaging can be used to detect structural defects detrimental to solar cell performance. T...

Sodium-doped molybdenum targets for controllable sodium incorporation in CIGS solar cells

The efficiency of Cu(In, Ga)Se2 (CIGS) solar cells is enhanced when Na is incorporated in the CIGS absorber layer. This work examines Na incorporation in CIGS utilizing Na-doped Mo sputtered from targets made with sodium molybdate-doped (MONA) powder. Mo:Na films with varying thicknesses were sputtered onto Mo-coated borosilicate glass (BSG) or stainless steel substrates for CIGS solar cells. By use of this technique, the Na content of CIGS can be varied from near-zero to higher than that obtained from a soda-lime glass (SLG) substrate. Targets and deposition conditions are described. The doped Mo films are analyzed, and the resulting devices are compared to devices fabricated on Mo-coated SLG as well as Mo-coated BSG with NaF. Completed devices utilizing MONA exceeded 15.7% efficiency without anti-reflective coating, which was consistently higher than devices prepared with the NaF precursor. Strategies for minimizing adhesion difficulties are presented.

Correlations of Cu(In, Ga)Se2 imaging with device performance, defects, and microstructural properties

Camera imaging techniques have been used for the characterization of Cu(In,Ga)Se2 (CIGS) solar cells. Photoluminescence (PL) imaging shows brightness variations after the deposition of the CIGS layer that persist through CdS deposition and subsequent processing steps to finish the devices. PL and electroluminescence imaging on finished cells show a correlation to the devices’ corresponding efficiency and open-circuit voltage (VOC), and dark defect-related spots correspond to bright spots on images from illuminated lock-in thermography (LIT) and forward-bias dark LIT. These image-detected defect areas are weak diodes and shunts. Imaging provides locations of defects detrimental to solar cell performance. Some of these defects are analyzed in more detail by scanning electron microscopy using cross-sectional views.

Amorphous transparent conductors for PV applications

Transparent conducting oxides (TCOs) with qualitatively better resistance to humidity than ZnO, the traditional Cu(In,Ga)Se2 (CIGS) TCO, are needed to reduce the water-induced degradation of CIGS photovoltaics (PV). Amorphous In-Zn-O (a-IZO) is found both to act as a water vapor transport barrier and to be essentially inert in damp heat testing at 85°C, 85%RH (85/85). In particular, no significant reduction in conductivity or transparency was observed after 40 days at 85/85. In initial PV application testing, a-IZO-finished CIGS solar cells have demonstrated 16.4% efficiency, essentially equal to equivalent CIGS cells finished with Al-doped ZnO.

VARIATIONS in damp heat-induced degradation behavior of sputtered ZnO window layer for CIGS solar cells

This paper presents our recent observations on variations in properties and damp heat (DH)-induced degradation behavior for single-layer 2%Al-doped ZnO (AZO) and bilayer ZnO (BZO), which comprises 0.1-μm intrinsic ZnO (i-ZnO) and AZO, deposited on glass substrates using the same sputtering system and essentially identical deposition conditions. BZO films with 0.12-μm AZO have been used on the National Renewable Energy Laboratorys (NRELs) high-efficiency CuInGaSe2 (CIGS) solar cells for years. For the as-deposited BZO films, the most apparent variations appeared in notable peak shift in transmittance and reflectance spectra and ZnO (002) peak intensity and peak position in X-ray diffraction. Location of substrates placed on the substrate holder platform contributed partly to the variations. For the DH-degraded AZO and BZO, earlier films became highly resistive, porous, and 10~20 X thicker and showed flattened transmittance spectra caused by a loss of free-carrier absorption. However, recent DH-exposed AZO and BZO films also became highly resistive but exhibited only small changes in transmittance spectra, while the columnar grain structure and film thickness remained nearly unchanged without porous features, but with granular particles formed on the surfaces that increased in size with lengthening DH exposure time.

Damp-heat instability and mitigation of ZnO-based thin films for CuInGaSe 2 solar cells

From our investigation of damp heat (DH)-induced degradation of the main component materials and complete CIGS devices in recent years, this paper summarizes the results on the (1) DH stability of several transparent conducting oxides deposited on glass substrates, including ZnO-based thin films, Sn-doped In2O3 (ITO), and InZnO, and (2) effectiveness of physical and chemical mitigations for ZnO. The electrical results showed that the DH-induced degradation rates of i-ZnO, AZO, their bilayer (BZO), and Al-doped Zn1−xMgxO are significantly greater than those of ITO and InZnO. Thicker AZO films are more stable than thinner ones. Structurally, upon DH exposures, the hexagonal ZnO-based thin films are transformed into highly resistive Zn(OH)2 and/or cubic ZnO with increased transmittance and substantial morphological changes. In the physical mitigation approach, plasma-enhanced chemical vapor-deposited SiOxNy and sputter-deposited InZnO are employed separately as moisture barriers to protect the underlying i-ZnO, AZO, and/or BZO with good results. However, the SiOxNy films required working with chemical treatments to improve adhesion to the BZO surfaces. In the chemical mitigation method, simple wet-solution treatments using special formulations are found effective to protect BZO from DH attack.

Preliminary damp-heat stability studies of encapsulated CIGS solar cells

We observed a large variation in the damp heat (DH) durability of many of the unencapsulated CIGS devices fabricated at NREL. Some devices failed within the first few hours of DH exposure; others failed within a few hundred hours while some lasted for 1000 h. The initial degradation often showed a 50% decrease in efficiency in the first few hundred hours; The premature device failures often correspond to the degradation of the ZnO window layer, the peeling of molybdenum (Mo) from the soda-lime glass (SLG), or both. Repeated J-V measurements lead to significant damage of the contact pads, which provide additional path for moisture ingress. To better understand the onset of degradation and the cause of initial decrease in performance and to minimize the damage caused to the contact pads, we designed an encapsulation scheme to control the moisture ingress by laminating the CIGS devices with a combination of different backsheets having different water vapor transmission rates. The encapsulation provided external contacts which solved the damage caused to the pads. This approach facilitates a way to slow down DH-induced degradation of the CIGS device for a more detailed study.

Influence of damp heat on the electrical, optical, and morphological properties of encapsulated CuInGaSe 2 devices

CuInGaSe2 (CIGS) devices, encapsulated with different backsheets having different water vapor transmission rates (WVTR), were exposed to damp heat (DH) at 85°C and 85% relative humidity (RH) and characterized periodically to understand junction degradation induced by moisture ingress. Performance degradation of the devices was primarily driven by an increase in series resistance within first 50 h of exposure, resulting in a decrease in fill factor and, accompanied loss in carrier concentration and widening of depletion width. Surface analysis of the devices after 700-h DH exposure showed the formation of Zn(OH)2 from hydrolysis of the Al-doped ZnO (AZO) window layer by the moisture, which was detrimental to the collection of minority carriers. Minority carrier lifetimes observed for the CIGS devices using time resolved photoluminescence (TRPL) remained relatively long after DH exposure. By etching the DH-exposed devices and re-fabricating with new component layers, the performance of reworked devices improved significantly, further indicating that DH-induced degradation of the AZO layer and/or the CdS buffer was the primary performance-degrading factor.

The stability and performance of amorphous-InZnO within CIGS devices

NREL CIGS devices with up to 20% efficiency are prepared using a three-stage process for the CIGS layer with the last step of an intrinsic ZnO and conductive ZnO:Al bilayer. This work outlines the efficiency and performance parameters for these CIGs devices when this bilayer is replaced with indium zinc oxide (a-InZnO), an amorphous metal oxide. It is well known that metal oxides can serve a variety of important functions in thin film photovoltaics such as transparent electrical contacts (TCOs), antireflection coatings and chemical barriers. In the case of a-InZnO, we have reported on the determination of the relative roles of metals and oxygen stoichiometries on the opto-electronic properties of a-InZnO thin films as well as the stability of those films in damp heat. Since InZO has a tunable conductivity based on the amount of oxygen introduced during deposition, it can be used as both the intrinsic and TCO layers. We were able to establish preliminary metrics for an all InZnO bilayer whose performance was comparable to a common CIGs device.