Clay DeHart

Kesterite Successes, Ongoing Work, and Challenges: A Perspective From Vacuum Deposition

Recent years have seen dramatic improvements in the performance of kesterite devices. The existence of devices of comparable performance, made by a number of different techniques, provides some new perspective on what characteristics are likely fundamental to the material. Here, we review progress in kesterite device fabrication, aspects of the film characteristics that have yet to be understood, and challenges in device development that remain for kesterites to contribute significantly to photovoltaic manufacturing. Performance goals, as well as characteristics of midgap defect density, free carrier density, surfaces, grain boundaries, grain-to-grain uniformity, and bandgap alloying are discussed.

Characterization of 19.9%-efficient CIGS absorbers

We recently reported a new record total-area efficiency, 19.9%, for CuInGaSe2 (CIGS)-based thin-film solar cells [1]. Current-voltage analysis indicates that improved performance in the record device is due to reduced recombination. The reduced recombination was achieved by terminating the processing with a Ga-poor (In-rich) layer, which has led to a number of devices exceeding the prior (19.5%) efficiency record. This paper documents the properties of the high-efficiency CIGS by a variety of characterization techniques, with an emphasis on identifying near-surface properties associated with the modified processing.

Enhanced Performance in Cu(In,Ga)Se

Cu(In,Ga)Se2 (CIGS) solar cells fabricated with twostep selenization processes commonly suffer from low open-circuit voltage (Voc). We found that the Voc of solar cells made from selenized Cu/Ga/In stacked metal precursors can be increased by employing a potassium fluoride (KF) postdeposition treatment (PDT). This study presents a comparison of films and resulting devices with and without the KF PDT. By including the KF PDT, an 18.6%-efficient CIGS device with a Voc of 0.709 V was fabricated using a two-step selenization process.

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In the conventional CdS/CdTe device structure, the poly-CdS window layer has a bandgap of /spl sim/2.4 eV, which causes absorption in the short-wavelength region. Higher short-circuit current densities (J/sub sc/) can be achieved by reducing the CdS thickness, but this can adversely impact device open-circuit voltage (V/sub oc/) and fill factor (FF). Also, poly-CdS film has about 10% lattice mismatch related to the CdTe film, which limits the improvement of device V/sub oc/ and FF. In this paper, we report a novel window material: oxygenated amorphous CdS film (a-CdS:O) prepared at room temperature by rf sputtering. The a-CdS:O film has a higher optical bandgap (2.5-3.1 eV) than the poly-CdS film and an amorphous structure. The preliminary device results have demonstrated that J/sub sc/ of the CdTe device can be greatly improved while maintaining higher V/sub oc/ and FF. We have fabricated a CdTe cell demonstrating an NREL-confirmed J/sub sc/ of 25.85 mA/cm/sup 2/ and a total-area efficiency of 15.4%.

Solar Cells Fabricated by the Two-Step Selenization Process With a Potassium Fluoride Postdeposition Treatment

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.

High-efficiency polycrystalline CdTe thin-film solar cells with an oxygenated amorphous cds (a-CdS:O) window layer

The stability of intrinsic and Al-doped single- and bilayer ZnO for thin-film CuInGaSe<inf>2</inf> solar cells, along with Al-doped Zn<inf>1−x</inf>Mg<inf>x</inf>O alloy and Sn-doped In<inf>2</inf>O<inf>3</inf> (ITO) and F-doped SnO<inf>2</inf>, was evaluated by direct exposure to damp heat (DH) at 85°C and 85% relative humidity. The results show that the DH-induced degradation rates followed the order of Al-doped ZnO and Zn<inf>1−x</inf>Mg<inf>x</inf>O ≫ ITO ≫ F:SnO<inf>2</inf>. The degradation rates of Al:ZnO were slower for films of higher thickness, higher substrate temperature in sputterdeposition, and with dry-out intervals. As inferred from the optical micro-imaging showing the initiation and propagation of degrading patterns and regions, the degradation behavior appears similar for all TCOs, despite the obvious difference in the degradation rate. A degradation mechanism is proposed to explain the temporal process involving thermal hydrolysis.

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

High-throughput computational and experimental techniques have been used in the past to accelerate the discovery of new promising solar cell materials. An important part of the development of novel thin film solar cell technologies, that is still considered a bottleneck for both theory and experiment, is the search for alternative interfacial contact (buffer) layers. The research and development of contact materials is difficult due to the inherent complexity that arises from its interactions at the interface with the absorber. A promising alternative to the commonly used CdS buffer layer in thin film solar cells that contain absorbers with lower electron affinity can be found in β-In2S3. However, the synthesis conditions for the sputter deposition of this material are not well-established. Here, In2S3 is investigated as a solar cell contact material utilizing a high-throughput combinatorial screening of the temperature-flux parameter space, followed by a number of spatially resolved characterization techniques. It is demonstrated that, by tuning the sulfur partial pressure, phase pure β-In2S3 could be deposited using a broad range of substrate temperatures between 500 °C and ambient temperature. Combinatorial photovoltaic device libraries with Al/ZnO/In2S3/Cu2ZnSnS4/Mo/SiO2 structure were built at optimal processing conditions to investigate the feasibility of the sputtered In2S3 buffer layers and of an accelerated optimization of the device structure. The performance of the resulting In2S3/Cu2ZnSnS4 photovoltaic devices is on par with CdS/Cu2ZnSnS4 reference solar cells with similar values for short circuit currents and open circuit voltages, despite the overall quite low efficiency of the devices (∼2%). Overall, these results demonstrate how a high-throughput experimental approach can be used to accelerate the development of contact materials and facilitate the optimization of thin film solar cell devices.

Damp-heat induced degradation of transparent conducting oxides for thin-film solar cells

The reliability of ZnO-based window layer for CuInGaSe2 (CIGS) solar cells was investigated. Samples of RF magnetron-sputtered, single-layer intrinsic and Al-doped ZnO and their combined bilayer on glass substrates were exposed in a weatherometer (WOM) and damp heat (DH) conditions with or without acetic acid vapor. Some preliminary samples of single-layer Al-doped Zn1-xMgxO (ZMO) alloy, a potential replacement for Al:ZnO with a wider bandgap, were also evaluated in the DH. The Al-doped ZnO and ZMO films showed irreversible loss in the conducting properties, free carrier mobility, and characteristic absorption band feature after <500-h DH exposure, with the originally clear transparent films turned into white hazy insulating films and the degradation rate follows the trend of (DH + acetic acid) > DH > WOM. The degradation rate was also reduced by higher film thickness, higher deposition substrate temperature, and dry-out intervals. The results of X-ray diffraction analysis indicate that the ZnO-based films underwent structural degeneration by losing their highly (002) preferential orientation with possible transformation from hexagonal into cubic and formation of Zn(OH)2. Periodic optical micro-imaging observations suggested a temporal process that involves initial hydrolysis of the oxides at sporadic weak spots, swelling and popping of the hydrolyzed spots due to volume increase, segregation of hydrolyzed regions causing discontinuity of electrical path, hydrolysis of the oxide-glass interface, and finally, formation of insulating oxides/hydroxides with visible delamination over larger areas.

Combinatorial Reactive Sputtering of In2S3 as an Alternative Contact Layer for Thin Film 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.

Degradation of ZnO-based window layers for thin-film CIGS by accelerated stress exposures

Cu diffusion from a ZnTe:Cu/Ti back contact onto CdS/CdTe thin-film solar cells is studied. We find if Cu diffusion is insufficient, the entire CdTe layer is depleted. However, if Cu diffusion is excessive, the depletion width can become too narrow to provide optimum current collection. This analysis suggests that most contact processes used for CdS/CdTe devices are optimized (often unknowingly) to result in a depletion width that extends just far enough into the CdTe to yield the highest possible field in the region where light absorption occurs. Analysis of the samples with very high Cu concentration also suggests that Cu doping of CdS may affect carrier collection from the CdS.