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thinner and more fragile, inkjet-patterned contacts are ideal as they are applied without direct contact to the cell surface. Silver lines 250 μm wide and 10 μm thick were inkjet-printed on SiNx-coated Si ribbon p/n junctions provided by Evergreen Solar. Al back contacts of 1 μm thickness were deposited by e-beam evaporation. The two contacts were co- fired in a single annealing step at 850oC for 10 min in air, forming a solar cell with 8% efficiency, Voc=0.529 V, Jsc= 22.67 mA, and a fill factor of 0.65. In this experiment, the ohmic contact between Ag and Si was formed through the SiNx layer without the use of glass frits. The high temperature and long time required for the penetration of the Ag through the AR coating can be detrimental for the junction. Facilitating the process of burning through the AR coating is desirable to lower the temperature and time of annealing for inkjet-printed contacts. 2.2 Thin-Film Polycrystalline Compounds The project leads the development of thin-film CdTe, CIGS, and related materials for use in high- performance and stable single-junction solar cells. The objectives are to support near-term manufacturing, build the knowledge and technology base for future manufacturing improvements, and sustain innovation that supports progress toward the future and the long- term Solar Program goal of 15%-efficient commercial modules. Over the past decade, we have steadily improved the quality of the layers in the CdTe device and achieved a world-record efficiency of 16.5%. Our work in CIGS set the world record at 19.5%. Our polycrystalline tandem solar cells of semi-transparent CdTe on top of CIS set the record at 15.3%, exceeding the FY 2006 milestone (15%) in DOE/NCPV’s High- Performance PV Project. The price of indium has risen sharply in recent times, and the concern about using it in large quantities resurfaces from time to time. Reducing the thickness of the absorbers from 2.5 μm to ≤1 μm reduces the quantity of In used in the cell. This must be done without adversely affecting the efficiency. This aspect has been studied previously by the group at Upsala University in Sweden, who showed a decrease in efficiency when the absorber thickness is reduced below 1μm. We have revisited the growth of thin absorbers by using a modified three-stage process and a co-evaporation process. To apply the three- stage process for thinner films, the deposition rates of all the elements were reduced in the first two stages, and the heating rates were also Photovoltaic R&D Fundamental Research adjusted. Compositional monitoring was still possible in spite of the reduced film thickness. In the co-evaporation process, film growth was initiated in a Cu-rich CuGaSe2 layer, and the overall composition was converted to device quality, Cu-poor Cu(InGa)Se2. This type of growth often resulted in some voids and poor adhesion at the Mo interface. However, compositional uniformity was easily achieved, and the deposition time was considerably shorter (5–10 min) than that used for thicker, record-efficiency cells. With the latter, it was possible to produce dense and smooth thin films. The distribution of Ga through the depth of the film was governed by the ramp rates and the kinetics of the reaction between the binary selenides. Hence, we observed a difference in the solar cell properties fabricated from absorbers made by the two methods. The following table shows the properties of the best solar cells fabricated as a function of absorber thickness. Properties of Thin CIGS Solar Cells Thickness (μm) 1.0 (3-stg) 1.0 (codep) 0.75 (codep) 0.5 (codep) 0.4 (3-stg) Voc (V) 0.654 0.699 0.652 0.607 0.565 Jsc 2 FF Eff. (mA/cm ) (%) (%) 31.6 78.3 16.2 30.6 75.4 16.1 26.0 74.0 12.5 23.9 60.0 8.7 21.3 75.7 9.1 18 The project also investigates several other materials systems and device concepts. We lead the development of thin-film CdTe solar cells. During the year, these devices have been adapted to serve as the top cell of high-performance tandem thin-film solar cells. We have successfully applied a CuxTe back contact to fabricate high- efficiency transparent CdTe cells, as required to pass lower energy light through to the CIS bottom cell. In the past, almost all R&D activities in this area focused on developing a transparent back contact with Eg larger than the Eg of the top cell, such as ZnTe:Cu or ZnTe:N with Eg of ~2.26 eV, or ITO with Eg of ~3.9 eV. The best result is a 10.1%-efficient CdTe cell with a ZnTe:Cu back contact that has a 60%–85% film transmission in the near-infrared (NIR) region. However, we exploited a thinner CuxTe back contact and modified device structure to fabricate high- efficiency poly-CdTe thin-film solar cells with higher NIR transparency. We fabricated several CTO/ZTO/nano-CdS:O/CdTe/CuxTe/ITO/Ni-Al grid cells with efficiencies of more than 13% by this technique. The best cell has an NREL-confirmed,PDF Image | DOE Solar Energy Technologies Program
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