Colin A. Wolden

Photovoltaic manufacturing: Present status, future prospects, and research needs

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

On the formation and stability of p-type conductivity in nitrogen-doped zinc oxide

The behavior of nitrogen in ZnO thin films grown by high-vacuum plasma-assisted chemical vapor deposition is examined. Highly oriented (002) films doped with 0–2at.% N were characterized by x-ray photoelectron spectroscopy, x-ray diffraction (XRD), Seebeck, and Hall measurements. XRD measurements revealed that the zinc oxide lattice constant decreased systematically with nitrogen doping. The as-deposited films were p-type at high doping levels, as confirmed by both Seebeck and Hall measurements. However, it was observed that hole conduction decreased and films reverted to n-type conductivity in a period of several days. This change was accompanied by a simultaneous increase in the lattice constant. The transient electrical behavior may be explained by compensation caused either by hydrogen donors or through defect formation processes common to analogous II-VI semiconductors.

A comparison of plasma-activated N2∕O2 and N2O∕O2 mixtures for use in ZnO:N synthesis by chemical vapor deposition

A high-vacuum plasma-assisted chemical-vapor deposition system was used to systematically study ZnO:N thin film synthesis. Nitrogen doping was achieved by mixing either N2O or N2 with O2 in a high-density inductively coupled plasma (ICP) source. In situ diagnostics showed that the ICP composition was predominantly a function of the elemental oxygen to nitrogen ratio, and relatively insensitive to the choice of N2 or N2O as the molecular precursor. Nitrogen incorporation was measured by both x-ray photoelectron spectroscopy and secondary ion mass spectrometry and was found to increase monotonically with both N2O and N2 addition. Nitrogen doping was correlated with systematic shifts in the lattice spacing, electrical conductivity, and optical absorption. Quantitative comparisons between film properties and gas composition suggest that atomic nitrogen is the primary precursor for doping in this system.

Synthesis of Stoichiometric FeS2 through Plasma-Assisted Sulfurization of Fe2O3 Nanorods

Pyrite (FeS(2)) thin films were synthesized using a H(2)S plasma to sulfurize hematite (Fe(2)O(3)) nanorods deposited by chemical bath deposition. The high S activity within the plasma enabled a direct solid-state transformation between the two materials, bypassing S-deficient contaminant phases (Fe(1-x)S). The application of plasma dramatically enhanced both the rate of conversion and the quality of the resulting material; stoichiometric FeS(2) was obtained at a moderate temperature of 400 °C using a chalcogen partial pressure <6 × 10(-5) atm. As the S:Fe atomic ratio increased from 0 to 2.0, the apparent optical band gap dropped from 2.2 (hematite) to ~1 eV (pyrite), with completely converted layers exhibiting absorption coefficients >10(5) cm(-1) in the visible range. Room-temperature conductivity of FeS(2) films was on the order of 10(-4) S cm(-1) and approximately doubled under calibrated solar illumination.

An investigation of annealing on the dielectric performance of TiO2 thin films

The impact of annealing on the dielectric performance of TiO2 thin films synthesized by PECVD was investigated. Films annealed between 500 and 700 °C have an anatase crystal structure, while 800 °C annealed films display the rutile phase. The optimal annealing temperature was 600 °C, which both maximized the dielectric constant and minimized the leakage current density. The intrinsic dielectric constant of TiO2 improved from 82 ± 10 in as-deposited films to 168 ± 30 after annealing. The leakage current of optimized films was superior to the SiO2 control samples over a range of equivalent oxide thickness. Fowler–Nordheim tunnelling and Frenkel–Poole conduction were observed in the optimized films, while Schottky emission dominated leakage current at other conditions.

Infrared detection of hydrogen-generated free carriers in polycrystalline ZnO thin films

The changes in the free-carrier concentration in polycrystalline ZnO films during exposure to H2 and O2 plasmas were studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. The carrier concentration and mobility were extracted from the free-carrier absorption in the infrared using a model for the dielectric function. The electron density in polycrystalline zinc oxide films may be significantly increased by >1019cm−3 by brief exposures to hydrogen plasma at room temperature and decreased by exposure to O2 plasmas. Room-temperature oxygen plasma removes a fraction of the H at donor sites but both elevated temperatures (∼225°C) and O2 plasma were required to remove the rest. We demonstrate that combinations of O2 and H2 plasma treatments can be used to manipulate the carrier density in ZnO films. However, we also show the existence of significant drifts (∼15%) in the carrier concentrations over very long time scales (hours). Possible sites for H incorporation in polycrystal...

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.

CuSbSe2 photovoltaic devices with 3% efficiency

Recent technical and commercial successes of existing thin film solar cell technologies motivates exploration of next-generation photovoltaic (PV) absorber materials. Of particular scientific interest are compounds like CuSbSe

Properties of reactively sputtered oxygenated cadmium sulfide (CdS:O) and their impact on CdTe solar cell performance

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Self-limiting growth of tantalum oxide thin films by pulsed plasma-enhanced chemical vapor deposition

, which do not have the conventional tetrahedral semiconductor bonding. Here, we demonstrate 1.5 {\mu}m thick CuSbSe