Stephen Glynn

Required material properties for high-efficiency CIGS modules

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt are driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given capital investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key in both reducing cost per watt and differentiating the technology from other thin films. This paper discusses the material properties required of each layer of the CIGS device such that large-area CIGS modules can achieve efficiencies 15%, which is substantially higher than the current industrial state-of-the-art. The sensitivity of module performance to the important material parameters is quantified based on both experimental data and modeling. Necessary performance differences between small-area devices and large-area modules imposed by geometry are also quantified. Potential technical breakthroughs that may relax the requirements for each layer are discussed.

A comparative study of Zn(O,S) buffer layers and CIGS solar cells fabricated by CBD, ALD, and sputtering

Zn(O,S) thin films were deposited by chemical bath deposition (CBD), atomic layer deposition, and sputtering. Composition of the films and band gap were measured and found to follow the trends described in the literature. CBD Zn(O,S) parameters were optimized and resulted in an 18.5% efficiency cell that did not require post annealing, light soaking, or an undoped ZnO layer. Promising results were obtained with sputtering. A 13% efficiency cell was obtained for a Zn(O,S) emitter layer deposited with 0.5%O2. With further optimization of process parameters and an analysis of the loss mechanisms, it should be possible to increase the efficiency.

Beneficial effect of post-deposition treatment in high-efficiency Cu(In,Ga)Se2 solar cells through reduced potential fluctuations

World-record power conversion efficiencies for Cu(In,Ga)Se2 (CIGS) solar cells have been achieved via a post-deposition treatment with alkaline metals, which increases the open-circuit voltage and fill factor. We explore the role of the potassium fluoride (KF) post-deposition treatment in CIGS by employing energy- and time-resolved photoluminescence spectroscopy and electrical characterization combined with numerical modeling. The bulk carrier lifetime is found to increase with post-deposition treatment from 255 ns to 388 ns, which is the longest charge carrier lifetime reported for CIGS, and within ∼40% of the radiative limit. We find evidence that the post-deposition treatment causes a decrease in the electronic potential fluctuations. These potential fluctuations have previously been shown to reduce the open-circuit voltage and the device efficiency in CIGS. Additionally, numerical simulations based on the measured carrier lifetimes and mobilities show a diffusion length of ∼10 μm, which is ∼4 times la...

Fiber-fed time-resolved photoluminescence for reduced process feedback time on thin-film photovoltaics

Fiber-fed time-resolved photoluminescence is demonstrated as a tool for immediate process feedback after deposition of the absorber layer for CuInxGa(1-x)Se2 and Cu2ZnSnSe4 photovoltaic devices. The technique uses a simplified configuration compared to typical laboratory time-resolved photoluminescence in the delivery of the exciting beam, signal collection, and electronic components. Correlation of instrument output with completed device efficiency is demonstrated over a large sample set. The extraction of the instrument figure of merit, depending on both the initial luminescence intensity and its time decay, is explained and justified. Limitations in the prediction of device efficiency by this method, including surface effect, are demonstrated and discussed.

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.

Reflection Optimization for Alternative Thin-Film Photovoltaics

The recent improvements in efficiencies for kesterite (copper zinc tin selenide, CZTS) devices warrant an investigation into how the kesterite device stack can best be capped to minimize losses due to reflection. Additionally, ongoing efforts to replace the cadmium sulfide (CdS) layer in copper indium gallium selenide (CIGS)-based devices, most notably with zinc sulfide (ZnS), need to be accompanied by a similar investigation into how to best finish a CIGS/ZnS stack to minimize reflection losses. An optical analysis of how CZTS/CdS and CIGS/ZnS devices reflect light has been performed for the purpose of optimizing the transparent conducting oxide and antireflection layers for each stack. This research addresses what is similar and what is different between the alternative stacks and the routine CIGS/CdS stack and how to best reduce the reflection losses for each situation.

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.

A study on the humidity susceptibility of thin-film CIGS absorber

The susceptibility of a thermally co-evaporated CuInGaSe2 (CIGS) thin-film absorber to humidity and its consequence on composition, morphology, electrical and electronic properties, and device efficiency was investigated. CIGS films on Mo-coated soda lime glass were degraded either in the ambient at ∼21°C and ∼21% relative humidity (RH) for a period of several months or in damp heat (DH) at 85°C and 85% RH briefly for 15–30 min; then the films were processed simultaneously into devices in a batch that included an unexposed control. In addition to severe delamination on some samples of the absorber films, prolonged ambient exposure resulted in numerous “spot” formations that lost CIGS with scale-like disintegration rippling around the spots and showed a significant presence of Na. Exposure in DH for 5 h was able to reproduce the spot formations on the CIGS films. A significant to large decrease of cell efficiency was observed from 14%–16% for the unexposed control to 8%–11% for the CIGS absorber exposed in DH for 15 and 30 min and 1%–4%% for the ambient-degraded CIGS with high series resistance and very low shunt resistance.

Density profiles in sputtered molybdenum thin films and their effects on sodium diffusion in Cu(In x Ga 1−x )Se 2 photovoltaics

Molybdenum (Mo) thin films were sputtered onto soda lime glass (SLG) substrates. The main variable in the deposition parameters, the argon (Ar) pressure p<inf>Ar</inf>, was varied in the range of 6–20 mTorr. Ex situ spectroscopic ellipsometry (SE) was performed to find out that the dielectric functions ε of the Mo films were strongly dependent on p<inf>Ar</inf>, indicating a consistent and significant decrease in the Mo film density ρ<inf>Mo</inf> with increasing p<inf>Ar</inf>. This trend was confirmed by high-angle-annular-dark-field scanning transmission electron microscopy. ε of Mo was then found to be correlated with secondary ion mass spectroscopy profiles of Sodium (Na) in the Cu(In<inf>x</inf>Ga<inf>1−x</inf>)Se<inf>2</inf> (CIGS) layer grown on top of Mo/SLG. Therefore, in situ optical diagnostics can be applied for process monitoring and optimization in the deposition of Mo for CIGS solar cells. Such capability is demonstrated with simulated optical transmission and reflectance of variously polarized incident light, using ε deduced from SE.